The Preparation and Characterisation of Gallium Nitride and Group III-V Related Nitrides

THE PREPARATION & CHARACTERISATION OF GALLIUM NITRIDE AND GROUP III-V NITRIDES (DRAFT EDITION) Author: Dean Russell CONTENTS LIST OF FIGURES............................................................................................... 2 1

GROUP-III NITRIDE SEMICONDUCTORS ........................................... 4 1.1 APPLICATIONS OF GAN ................................................................................ 4 1.1.1 GaN within society as LEDs and LDs. ................................................ 4 1.1.2 Overview of GaN growth methods on substrate .................................. 5 1.1.3 Basic structure and substrate related defects..................................... 7 1.1.4 Use of GaN powders for analysis ........................................................ 8

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EXPERIMENTAL THEORY ...................................................................... 9 2.1 EXAFS ........................................................................................................ 9 2.2 X-RAY POWDER DIFFRACTION................................................................... 11 2.2.1 Off-line Powder Diffraction. ............................................................. 11 2.3 POWDER DIFFRACTION TECHNIQUES .......................................................... 13 2.4 OVERVIEW OF EXPERIMENTAL HIGH PRESSURE TECHNIQUES. .................... 14 2.5 PHOTOLUMINESCENCE................................................................................ 17

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HIGH PRESSURE RESEARCH ............................................................... 20 3.1 USE OF COMBINED HIGH PRESSURE TECHNIQUES. ..................................... 20 3.1.1 Overview of Combined high pressure studies ................................... 20 3.1.2 The Hi-Prexx Facility ........................................................................ 21 3.2 HIGH-PRESSURE RESEARCH INTO GAN ....................................................... 25

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INVESTIGATIONS INTO GROUP III-N SYNTHESIS ........................ 28 4.1 SYNTHESIS OF GALLIUM NITRIDE............................................................... 28 4.1.1 Recent research into GaN powder/crystals ....................................... 28 4.1.2 The Solid State Metathesis technique. ............................................... 30 4.1.2.1 Overview of the technique ............................................................ 30 4.1.2.2 GaN Synthesis using SSM method. .............................................. 32 4.1.3 GaN synthesis using Flowing Ammonia Technique. ......................... 40 4.1.3.1 Overview of technique and recent research .................................. 40 4.1.3.2 GaN synthesis using Flowing Ammonia method .......................... 42 4.1.3.3 Results from the Flowing Ammonia method ................................ 45 4.2 INDIUM NITRIDE SYNTHESIS....................................................................... 47 4.2.1 Overview of recent research into indium nitride synthesis ............... 47 4.2.2 Synthesis of Indium Nitride using Flowing Ammonia Technique ..... 48 4.2.2.1 Indium nitride results from Flowing Ammonia Method. .............. 50 4.3 OVERVIEW OF III-N ALLOYS. ..................................................................... 53 4.3.1 Reasons for Investigating III-N alloys............................................... 53 4.3.2 Overview of III-N Material Properties .............................................. 56 4.3.3 Overview SSM method to produce Alloy Powders. ........................... 57 4.3.3.1 Results from SSM method for Alloy Powders. ............................. 58

1 4.4 OVERVIEW OF ETCHING OF GALLIUM NITRIDE POWDERS ............................ 60 4.4.1 Previous research .............................................................................. 60 5

RESULTS ..................................................................................................... 61 5.1 INITIAL DATA - AN OVERVIEW. .................................................................. 61 5.2 INITIAL DATA INVESTIGATING COMMERCIAL POWDERS. ............................. 64 5.3 HI-PREXX INVESTIGATIONS INTO GAN SYNTHESISED USING THE FLOWING AMMONIA TECHNIQUE. ....................................................................................... 66

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CONCLUSIONS AND RECOMMENDATIONS .................................... 70

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List of Figures FIGURE 1-1: OPTICAL ALIGNMENT SCHEMATIC OF HI-PREXX FACILITY USED FOR HIGH PRESSURE COMBINED STUDIES. ................................................................ 1

FIGURE 2-1: DIAGRAMS OF GOLD CUPS DURING FABRICATION ............................... 32 FIGURE 2-2: DIAGRAM OF FIRED OCTOHEDRA WITH HOLE DRILLED THROUGH FACES. ....................................................................................................................... 33 FIGURE 2-3: DIAGRAM OF CONSTITUENT PARTS OF OCTAHEDRA AND SYNTHESIS COMPONENTS.................................................................................................. 34

FIGURE 2-4: INSIDE VIEW OF FULLY FABRICATED OCTAHEDRA WITH GOLD CUP AND SAMPLE.

......................................................................................................... 35

FIGURE 2-5: PHOTOGRAPH OF THE HIGH PRESSURE PRESS USED TO TAKE CELL TO REQUIRED PRESSURES. .................................................................................... 35

FIGURE 2-6: X-RAY DIFFRACTOGRAMS FOR GALLIUM NITRIDE SYNTHESISED USING SSM METHOD. ................................................................................................ 38 FIGURE 2-7: DIAGRAM OF EQUIPMENT USED FOR FLOWING AMMONIA TECHNIQUE USED FOR GALLIUM NITRIDE SYNTHESIS. ........................................................ 42

FIGURE 2-8: DIFFRACTOGRAM OF GALLIUM NITRIDE PRODUCED USING THE FLOWING AMMONIA METHOD ......................................................................................... 45

FIGURE 2-9: ENERGY DISPERSIVE X-RAY ANALYSIS OF INDIUM NITRIDE PRODUCED USING FLOWING AMMONIA METHOD. .............................................................. 51

FIGURE 2-10: GRAPH IDENTIFYING THE VARYING BAND-GAPS OF III-N MATERIALS. ....................................................................................................................... 54 FIGURE 3-1: X-RAY DIFFRACTOGRAM FOR COMMERCIAL GALLIUM NITRIDE POWDER OBTAINED FROM ALFA CHEMICALS. ................................................. 61

FIGURE 3-2: X-RAY DIFFRACTOGRAM FOR GALLIUM NITRIDE PRODUCED USING THE SOLID STATE METATHESIS METHOD ................................................................ 62

FIGURE 3-3: X-RAY DIFFRACTOGRAM FOR GALLIUM NITRIDE PRODUCED USING THE FLOWING AMMONIA METHOD. ........................................................................ 62

FIGURE 3-4: EARLY PHOTOLUMINESCENCE SPECTRA FOR COMMERCIAL AND SSM SYNTHESISED GALLIUM NITRIDE POWDERS. .................................................... 64

FIGURE 3-5: EXAFS DATA FOR GALLIUM NITRIDE TAKEN UP TO 41GPA. ............... 66 FIGURE 3-6: NEAREST NEIGHBOURS PRESSURE DEPENDENCE CHART, USING EXAFS DATA, FOR FLOWING AMMONIA SYNTHESISED GALLIUM NITRIDE TAKEN TO ≈

450KBAR. ....................................................................................................... 67

3 FIGURE 3-7: PHOTOLUMINESCENCE SPECTRA FOR FLOWING AMMONIA SYNTHESISED GALLIUM NITRIDE POWDER TAKEN UP TO HIGH PRESSURES USING A DAC. ..... 68

FIGURE 3-8: X-RAY DIFFRACTION IMAGE FOR GAN TAKEN INSIDE A DIMANOD ANVIL CELL (THE ARROW INDICATES A GASKET LINE). ................................... 69

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GROUP-III

Nitride Semiconductors

1.1 Applications of GaN 1.1.1

GaN within society as LEDs and LDs.

When comparing the changes of the efficiency of light that are knitted within the structure of society and the modern world, one can easily see that it is gradually being maximised. The gradual integration of the light emitting diode (LED) into every facet of society is just one extension of the drive towards a less wasteful and more efficient culture. The LED can be considered the ultimate general source of continuous light due its high luminous efficiency, quick response time, and long lifetime. A standard light bulb has its electrical efficiency almost halved when one chooses to changes its colour because a filter is necessary to do so. Yet, the LED has no such problems when colour is concerned.

Practically, if one

examines the number of light bulbs presently utilised across the world in, for example, traffic lights, it can be instantly realised LED’s would be a far superior replacement. This would not just make sense in terms of electrical efficiency, but would also be noticeable economically due to the maintenance needed and lack of repetitive replacements (present light bulbs must be replaced every 6 months (approximately)). The only hindrance (until recently) towards the wide spread use of LED’s for traffic lights, was the lack of high intensity blue and green LED’s.

As blue is one of the primary colours then it is essential if intentions are aimed towards producing the full spectrum (or even white light). The possibilities open within society for the use of high intensity LED’s (which are highly reliable coupled with a low energy consumption) encompassing a wide range of available colours is limitless. Such a range of products would have the possibility of replacing the conventional light bulb, and fluorescent lamps along with the present technology behind big screen televisions etc.

Over the 1980’s the evolution of high brightness LED’s produced high brightness red LED’s (the first by Nishizawa et al1) which were eventually mass produced, replacing the standard red lights and neon tubes. The wider ranges of uses were soon recognised including multicoloured displays (utilising (green and red) GaP LED’s), which are now seen world-wide as information boards etc. in a variety of

5 locations.

Full colour displays could not be created purely because of the

relatively low luminescent properties of the LED’s utilised. SiC LED’s were the main source of relatively reliable blue LED’s but relied on a material with an indirect bandgap and therefore had very low luminescence brightness; hence they were less than perfect for practical applications.

Several years ago the maximum amount of information that could be carried outside of a computer (i.e. via floppy disc) was minimal compared to the presently used CD’s. CD’s at the moment are read by near infrared solid-state lasers (780nm). Replacing these infrared lasers with blue or ultra-violet lasers, (i.e. lasers of a much shorter wavelength) then a four-fold increase in the information that could be recorded on the CD would be facilitated.

The need for an efficient and bright blue LED is in keeping with the rapid movement of an efficient society. The necessity for such an LED lead to research

by Shuji Nakamura (of the Nichia Chemical Industries ltd.) into the GaN semiconductor system.

By 1993 high brightness blue LED’s were being

developed2. The first group III-V nitride based violet LED’s (around 400nm) were also being developed 3.

Obviously, the applications of GaN, for use within blue LED’s and laser diode’s, is in keeping with modern society, not only from an the efficiency point of view, but also from an economical, and environmental stance1. The ongoing research into the mechanisms of GaN, as a highly efficient blue semiconducting material is both essential and worthy as a valid area of research. This is especially so when one considers the effect it will eventually have (and already has had2) on society as a whole.

1.1.2

Overview of GaN growth methods on substrate

During 1971, Pankove (RCA Princeton Laboratory) showed a metal-insulatorsemiconductor light emitting diode (LED) based upon gallium nitride. Although 1

The energy consumption from traffic lights alone is within the gigawatt range, hence energy savings and cost savings due to decreased replacement and service costs will be substantial3.

2 GaN LED’s have already been used in big screen televisions in Japan. In addition, traffic lights utilising the technology have already started to be located in various countries and cities phasing out the present versions.

6 this was successful to a point, a p-type form of GaN had not been produced. This is very important when considering highly efficient LED’s that use a p-n junction.

In order to produce GaN based LED’s several problems needed to be overcome.

The first was to produce high quality crystalline layers and the second to achieve p-type doping3 (which was achieved in 1988 using low energy electron beam irradiation (LEEBI) 4.

The problems of finding a suitable substrate material on which to grow high quality gallium nitride is the main hindrance to the process of making relatively non-defected gallium nitride in LED’s. It was shown in 1986

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that a method

known as metal organic vapour phase epitaxy (MOVPE) could be used to grow relatively high quality gallium nitride layers onto sapphire, using a sequence of buffer layers4 grown at different substrate temperatures. The lattice constants of the substrate (sapphire) surface and of the wurtzite (hexagonal) gallium nitride differ by 15%, along with a difference in thermal expansion3. This usually leads to films of gallium nitride that are very poor and open to extreme cracking of any gallium nitride layer which has no buffer layers between it and the sapphire substrate. The use of the buffer layers on the sapphire substrate help to obtain acceptable crystal quality GaN films despite the large lattice mismatch.

3

Although p-type doping is an important concern overall when considering GaN LED’s it is not dealt with in this report.

4 Buffer layers are grown in the same way as the intended layer (in this case of GaN) but act to reduce the mismatch between the layer and the substrate.

7 1.1.3

Basic structure and substrate related defects

Gallium nitride can crystallise in two different structure types, the more common wurtzite (hexagonal structure) or the zincblende (cubic) structure. Although the properties of both wurtzite and zincblende are related, they can show significant differences.

In terms of crystallography, both structure types are very closely related. The neighbour bonding is tetrahedral. The Bravais lattice of the wurtzite structure is hexagonal. Along the c-axis (the axis perpendicular to the hexagons) the structure can be described as a sequence of layers of atoms of the same element (e.g. Ga or N), which is built up from regular hexagons. This is described in Figure 1-1. For the wurtzite structure the lattice constants a and b are equal (a=b).

Ga or N N or Ga C

A

B

Figure 1-1: The Wurtzite structure of GaN

The diagram below (figure 11) shows a schematic of a GaN film grown onto a sapphire substrate, obviously showing the large lattice mismatch of the two materials.

8 1.1.4

Use of GaN powders for analysis

Due to the aforementioned structural defects inherent with substrate grown GaN, another method needs to be utilised in order to ascertain whether the structural defects are hindering or aiding the luminescent properties of the material. By looking at gallium nitride powders rather than the substrate grown versions, it is possible to analyse the material in an unstressed environment. Further to this, the material can be looked at under pressure, hence giving more insight into the effects of pressure (and hence stresses) upon the material without changing its immediate environment. Often, materials grown on substrates must be removed from the substrate in order to apply pressure to them, therefore relaxing the stresses and hence the material looked at will be slightly different from that on the substrate.

As most substrate grown materials (in this case GaN) is not the only material upon the substrate (or at least their counterparts will be grown in a slightly different manner) then the analysis of the material may be affected. With the use of powders, this kind of problem disappears because the whole sample will have been produced in a manner that is both recorded and uniform. In order to fulfil the latter statement, the powders must be synthesised ‘in house’. This is for two reasons; 1. The method of synthesis of the GaN, which is commercially created, is not known, therefore the process can only be assumed. Hence, if their process changes from batch to batch the results may change yet no record of the processes causing the change (during synthesis) will be known.

2. The GaN powders presently available do not appear to be totally pure, therefore allowing room for improvement. Attempting to synthesise much purer GaN will improve on the accuracy of the results gained through further analysis

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2 Experimental Theory Several techniques were used in characterising the synthesised materials.

Facilities were available at Daresbury laboratories and De Montfort for both online and off-line measurements. The techniques overviewed here include EXAFS, x-ray powder diffraction (on-line and off-line), photoluminescence and various high-pressure techniques.

2.1 EXAFS The main process for X-ray absorption is that the incident photon is completely absorbed and kicks out a core photoelectron from the absorbing atom leaving behind a core hole. This photoelectron will be ejected with an energy equal to energy of the incoming photon less its binding energy when in the core. This photoelectron will interact with the surrounding atoms.

By considering the wave nature of the ejected photoelectron and regarding the atoms as point scatterers, a simplified picture can be imagined whereby the waves backscattered from surrounding atoms (i.e. the incoming) interfere with the outgoing wave to produce either peaks or troughs in what?

10 This is an interference effect on the final state, and affects the absorption coefficient which is the property determined from an XAS experiment.

The interference only needs to be calculated at the centre of the absorbing atom??

The photoelectron wavelength is inversely proportional to its energy, therefore the phase of the incoming back scattered wave at the central absorbing atom will change with the energy of the incoming photon. This leads to the oscillatory interference effect. The back-scattering amplitude and phase are dependent on the type of atom doing the back-scattering and the distance it is from the central atom, and information regarding the local environment of the absorbing atom can be obtained by analysing the EXAFS signal.

The EXAFS technique employed used the standard transmission method, which uses a tuneable monochromatic x-ray beam. The 2 GeV Synchrotron radiation source at CCLRC Daresbury Laboratories was used as a source of continuous xray energies. Most of the data collected were taken on station 9.3 with the aid of the Hi-Prexx facility.

The transmission EXAFS method requires a scanned monochromatic beam. This is done on station 9.3 at DL via a Si water-cooled channel cut harmonic-rejecting monochromator. Stepper motors with set increments drive the monochromator, which vary the energy of the incident beam. The sample is placed between two ion chambers which record the intensity of the incident beam before (Io) and after (It) transmission through the sample. This information is then used to produce the EXAFS function (It/Io), the initial EXAFS spectrum, which can then be analysed further.

The sample preparation for good EXAFS data collection is extremely important, especially in the case of high-pressure techniques. An overview of sample preparation is given in the high pressure chapter.

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2.2 X-Ray Powder Diffraction.

X-ray powder diffraction is a versatile, non-destructive analytical tool for the identification and quantitative determination of the amount of the various crystalline forms (phases) of compounds present in powdered and solid samples.

X-rays are used because they have wavelengths of just a few Angstroms (Å) which is similar to the interatomic distances in crystalline solids. Materials all have their own unique ‘fingerprint’ diffraction pattern, hence the pattern can be used to ascertain the constituents present within a sample.

Diffraction will occur when an incident beam interacts with a regular structure whose repeat distances are about the wavelength of the beam. X-ray diffraction is an elastic scattering process in which a large number of atoms are involved. Due to the periodic arrangement of the atoms in the lattice, the waves scattered by the atoms have definite phase relationships among them. As known from diffraction theory, constructive interference occurs if the path difference between successive wave fronts is nλ, where n is an integer and λ is the wavelength of the incident xrays.

W.L.Bragg (1912) described the relationship between the wavelength, the interatomic distances and the diffracted angles, whereby: nλ = 2dsinθ where n is an integer (1, 2 …..) d is the distance of adjacent parallel planes in the crystal θ is the angle between the incident wave and the reflected wave plane λ is the wavelength.

2.2.1

Off-line Powder Diffraction.

To obtain the powder diffraction pattern of the synthesised materials, X-ray powder diffraction of the samples produced was performed using a Siemens

12 Kristalloflex 710/710H Tube X-ray generator, with a Cu Kι target. The diffractometer measures the intensity of the diffracted beams as a function of the angle 2θ. The angle that is measured is a multiple of 2 because the incoming beam is placed horizontally???. The detector moves over a given angle interval and measures the intensity at a given frequency.

The resulting information from the diffractometer is given in the form of a diffractogram which consists of a number of peaks at various angles and with various intensities. The angles at which the peaks are present can be used in combination with the Bragg equation to calculate the lattice spacing. The widths of the peaks provide an indication of the average crystallite size. Large crystallites give rise to very sharp peaks. This is all standard stuff. You should state what is important for fitting data

Sample preparation is very important when recording powder diffraction measurements. The sample is firstly ground into a fine powder and then uniformly placed onto a small glass sample holder. Amyl acetate is used to fix the powder to the glass, and this is then gently evaporated using a furnace set to above room temperature.

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2.3 Powder Diffraction Techniques (on-line) Using the Hi-Prexx facility (outlined in chapter XX: Hi-Prexx), in-situ powder diffraction measurements were made inbetween recording the EXAFS data.

The on-line method implemented was the angle-dispersive powder diffraction technique. This method utilises a monochromatic beam of a particular energy, which passes through the sample, and the x-ray diffraction pattern is are recorded on a two dimensional flat image plate detector. The image plate and plate scanner used were made by Molecular Dynamics.

The detector (i.e. the image plate) is a phosphorous-covered film which is read using a He-Ne laser (is it?). This works by inducing photoluminescence in the phosphor which in turn emits a characteristic light, the intensity of which is proportional to the number of absorbed incident x-ray photons. The luminescence is detected by a photomultiplier and converted to a digital signal.

Image plates are used rather than photographic films due to their wider dynamic range, higher detector efficiency, convenience in digital representation. To obtain the intensity versus angle dependence, integration of the image plate read out is undertaken.

During the high-pressure (where energy dispersive EXAFS were used), in-situ measurements the monochromatic beam was obtained by cutting the narrow (~1 eV) band from the energy range used for the EXAFS experiment. To do this, slits were placed in front of the sample, the sample being mounted inside the diamond anvil cell.

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2.4 Overview of experimental High Pressure techniques.

To perform the high-pressure measurements, the Hi-Prexx facility was used, the description of which is given in chapter XX ‘Hi-Prexx’. To take the materials up to high pressures, whilst allowing EXAFS, powder diffraction and photoluminescence measurements to be taken, a Diamond Anvil Cell (DAC) is utilised. The transparency of diamonds over the ultraviolet, visible, infrared, and X-ray spectral range allows the freedom for a wide range of experimental techniques up to high pressures The operation of a diamond anvil cell (DAC) relies on two diamonds, which exert uniform pressure, upon a metal gasket with a small sample filled hole. Rewrite.

Refer to diagram.

The diamond anvil cells used in the Hi-Prexx experiments were supplied by Dr. David Adams of Diacell Ltd, and use a pressure transmitting membrane filled by an inert gas to apply pressure to the diamond anvils. The diamond anvils are produced from gem-quality single crystals and cut in the simple form of a brilliant (8 or 16 faces cut), with the working plane as a culet. The alignment of the anvils is crucial to reduce the risk of damage. The usual alignment method is through observation of the interference fringes between the diamond anvils. The anvils are slowly rotated until the fringes disappear. This method gives a high degree of parallelism: to better than 200nm across the culet. The relative lateral rotation of the anvils is then marked so that the diamonds can always be simply aligned laterally with respect to each other.

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To measure the high pressures inside the cell, a ruby-fluorescence technique is utilised. This method, as outlined in chapter XX ‘Hi-Prexx’, allows the pressure inside the gasket hole to be determined in-situ, without any change of the cell’s environment.

The sample preparation is crucial to allow good results to be measured why?? and to allow high pressures to be attained. To achieve this, each step in filling the gasket hole must be carefully observed.

The pressure-transmitting medium used inside the gasket is silicon oil. Gaskets are generally made of Inconel, some other work-hardened or spring stainless steel, rhenium or tungsten. The typical diameter of the hole in the gaskets is approximately 150 – 300µm. Why?? Before loading the sample, the gasket hole must be drilled. The drilling can only commence after the gasket has been indented. Pre-indenting the gasket is intended to avoid large deformation and instability of the hole (during the experiment). The position of the gasket relative to the anvils must be marked so that it may be replaced in the same position later.

This is also to allow higher pressures to be reached how?? and to avoid any possibility of side movements (which could significantly distort the EXAFS signal).

16 Once pre-indentation is completed, a hole must be drilled in the centre of the indent. Miniature holes can be drilled using specially designed miniature drill bits, or more accurately by using a (make/model etc.) spark eroder. The holes drilled for the experiments described are usually between 100-300Âľm.

Great pic but the relative sizes of the gasket hole and culet diameter is not very typical.

To be filled, the gasket must be placed upon one of the anvils and in the same orientation as when indented. The order of loading the gasket is determined by the arrangement of the DAC with regards to the experimental instrumentation: the ruby needs to be positioned in front of the sample for photoluminescence (which includes the pressure determination) measurements.

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2.5 Photoluminescence For several years semiconductor photoluminescence within the visible wavelength region has been intensively studied. The results of such experiments have been very important for both industry (mainly electronics) and for theoretical physicists.

An overview of the principles of photoluminescence is given here. Initially optical absorption in semiconductors must be considered.

Within a crystal, the crystal momentum of an electron with a wave vector k is

hk

2π

.

The momentum of a photon (p) is given by:

p=ε p=h

c

λ

Consider light of energy ℇi incident upon a semiconductor, where ℇi is greater than the difference between the valence and conduction band energies. Then the photon will have the ability to excite an electron from the valence to the conduction band; leaving behind a hole.

Due to the laws of conservation of momentum the crystal momentum must be conserved, therefore the k value of the electron in both bands must be the same to within ±10-4Å-1. This form of transition is called a vertical transition. The change in the momentum of the electron is approximately hk i 2π , where ki is the wave vector of the incident photon and is approximately zero on the scale of the Brillouin zone.

18 With direct band gap materials, the electrons are excited vertically in k space, from the top of the valence band to the bottom of the conduction band, i.e. across ℇg .

Many ambient photoluminescence measurements were taken using a set up in De Montfort University. The initial arrangement (as shown below) consisted of a HeCd UV/Visible (325/442nm) laser, directed at a sample perpendicular to the spectrometer, and used a simple lens system to focus the excited photons into the spectrometer entrance slits.

The arrangement shown below utilised UV-transmitting optical fibres, which guided the laser light to the sample and the photoluminescent light to the spectrometer entrance slits.

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3 High Pressure Research 3.1 Use of Combined High Pressure Techniques.

3.1.1

Overview of Combined high pressure studies

The need for combined structural studies of materials has often been identified as an important step in the growth of information within material sciences. Due to the significant advantages in the structural information data achievable when using combined studies, their use has become more become more popular over recent years7-13.

Combined extended x-ray absorption of fine structure (EXAFS) and x-ray diffraction (XRD) studies are able to provide the comprehensive information on materials structure due to their intrinsic sensitivity to short and long range order structure, respectively. The widespread use of in-situ combined structural studies has been significantly held back due to the lack of appropriate facilities at synchrotron sources (although some combined studies facilities area available they are not necessarily where we wanted in terms of research capabilities). Only very recently has there been a growth in development of suitable facilities for such studies (for example; DW11A at LURE and ID24 at ESRF, France, etc).

Although these stations create research environments suitable for combined studies, they are still limited in some areas and as such have disadvantages. For example, at LURE experimental EXAFS and XRD data are collected at different stations, which involve moving the high-pressure cell from one station to another.

At the ESRF, where there are significant advances in the quality of the focusing optics, there are still hindrances due to the limited EXAFS range available.

The techniques, which are currently used, for combined high-pressure structural experiments are energy dispersive EXAFS (EDE) and powder diffraction. One of the advantages to this approach (i.e. EDE) is the fast spectra collection times. The use of fast collection times allows truly in-situ measurements to be taken whereby the environment and material can be measured with little alteration during the

21 experimental period, which can be very useful for less stable materials. Although this makes it a very favourable technique, several disadvantages can act to reduce its effectiveness. Especially when used in combination with x-ray diffraction collection. These hindrances include the strict requirements on stability and size of focus, which are crucial for high quality EXAFS data. The use of focusing also degrades the experimental resolution. However, the most significant disadvantage is with the restriction of choice of wavelength for diffraction, if XRD and EXAFS experiments are to be done in-situ. Restrictions are created in the choice of energy for XRD by the range used by the EDE, since the EDE set up is a time consuming procedure and change of region is quite impossible with the time scale of a standard experiment. In addition, a general restriction in the EXAFS preedge energies is imposed, since resonance and absorption effects can affect an XRD spectrum in the absorption edge region of an EXAFS spectrum. Thus, the flexibility in the choice of wavelength is lost, which is one of the main advantages of synchrotron radiation for such studies.

Various experiments previously attempted for combined high-pressure studies (14,15) highlighted the opportunities of using QEXAFS (16) as a preferred technique for combined XRD/EXAFS experiments. The advantages of short spectra collection times and that it is a standard transmission technique, allow high quality EXAFS data to be collected under high pressures without the loss of resolution associated with the EDE detection. This also minimises the distortions in the EXAFS spectrum due to Bragg reflections from diamonds in a reasonable amount of time.

3.1.2

The Hi-Prexx Facility

Knowing that high quality EXAFS could be taken in-situ, with XRD data under high pressures, led to the idea behind Hi-Prexx. Hi Prexx is a facility, which has been developed for the in-situ measurements of EXAFS, x-ray diffraction and optical measurements under high-pressure conditions. The facility was developed at the Daresbury Laboratory Synchrotron Radiation Source both on-line on station 9.3 and off line within the Material Sciences Laboratory.

22 The facility can be used at any other suitable beamline at a synchrotron source.

The main feature of the Hi-Prexx facility is the ability to fully control all of the functions from outside of the experimental enclosure (i.e. the beamline hutch).

This allows the simultaneous collection of optical and structural data whilst varying the pressure.

The arrangement is extremely flexible and therefore allows the equipment to be tailored for particular experiments, such as time or temperature dependant measurements.

During the development of the facility used the relatively new EDE approach, which allowed significantly shorter experimental times to be used and an increase in the data collected. This opens up the possibility for EXAFS collection at any pressure obtainable by a diamond anvil cell.

Figure 3-1: Optical alignment schematic of Hi-Prexx facility used for high pressure combined studies.

The X-ray diffraction data are collected using an image plate, placed behind the high-pressure cell (diamond anvil cell, as described in chapter 2.4). The EXAFS data can be collected in the energy-dispersive mode, (as described earlier in this chapter) or the standard transmission mode.

23 The optical paths of the system are diverted from the path of the x-rays via the use of a mirror. The mirror is designed with a central hole of dimensions (x,x,x,) such that the x-rays can pass through to be focused in the high pressure cell. The mirror is positioned at 45oC to the direction of the x-ray path. The x-ray and optical alignment procedures are decoupled using fibre optics. In this way the alignment of the diamond anvil cell (DAC) for x-rays does not interfere with the optical alignment for the laser radiation and photoluminescence (PL).

For the PL spectroscopy, a He-Cd (325nm and 442nm) laser from Kimmon is utilised to excite the sample, with a TRIAX 320 spectrometer by ISA inc. used for spectral analysis. The motor control and data acquisition is provided via software upon a personal computer placed outside the experimental area.

The pressure calibration of the cell is carried out using the above optical system (using the 442nm filter setting on the laser). The system measures the ruby fluorescence from the ruby chips mounted with the sample with the DAC (see chapter 2.4 for details).

Successful data collection of the PL data involves scanning over a wide range of wavelength (often ultra violet (UV) through to the infrared (IR), in the case of GaN the range was usually set from 330nm (with filter) to 800nm). The dispersion of refractive index within such a wide range is a very important consideration and hence UV achromatic lenses are used. The confocal geometry of the optical arrangement makes optical alignment particularly simple to achieve.

The whole system is fixed to a breadboard, which is mounted upon translational and rotational stages to allow the positioning of the diamond anvil cell with the sample mounted inside. The translational movements are necessary to align the sample in the x-ray beam, and the rotational freedom allows the diamond glitches to be minimised within the collected EXAFS spectra.

Diacell Products Ltd provides the cells used with low fluorescence 0.3mm culet diamonds mounted to apply pressure to the sample. The sample is loaded, as previously described (chapter 2.4) in a pre-indented gasket with 0.1mm hole for the sample, ruby chip and pressure-transmitting medium (silica oil). The imaging

24 plate behind the DAC is used as the x-ray detector for the XRD studies. The working functions of the facility (including the pressure changes) can all be remotely controlled from outside the experimental hutch, hence allowing very sensitive samples to be free from any environmental changes.

25

3.2 High-pressure research into GaN It is widely believed that the ionicity of a material dictates, largely, the structural forms of high-pressure phases of III-V and II-VI compounds. Thus, the more covalent materials, i.e. most of the III-V semiconductors, transform predominately to a tetragonal β-Sn structure under pressures. Whereas, the more ionic materials, i.e., the II-VI semiconductors, transform to a rocksalt 17). Although this is the case in general, GaN can be characterised by its high ionicity of approximately 0.5, which is the highest among the group III-V compounds. This unexpected difference highlighted that the phase change of gallium nitride at high pressures was not necessarily a straightforward conclusion and hence experimental investigations were needed to parallel theoretical research. Ultrahigh pressure is a very efficient way to tune interatomic distances in condensed matter and research pre 1992 expected interesting results from the high-pressure study of structural properties of GaN. Until the early 1990’s, the experimental research into the high-pressure phase of gallium nitride was undetermined and the early theoretical work by theorists such as Van Vechten still expected the structure to change to β-Sn at extremely high pressures. Although investigations showed GaN approaches a phase transition as the material is put under hydrostatic pressures advancing towards 50GPa, the nature of the phase change was undetermined 18. By 1992, Perlin et al 12used xray-absorption spectroscopy to determine the phase change in GaN. Their studies took GaN up to 50 Gpa using a diamond anvil cell (DAC), the experiment was performed at the Ga K edge (10367 eV), with an energy range of 400eV above the edge. Their results showed the beginning of a phase transition occurring around 50 Gpa and expected a full transformation at approximately 54 GPa. Their results identified that GaN was similar to the II-VI ZnS, ZnSe or ZnTe. These results gave a clear indication that the gallium nitride structure would be expected to have a phase transition towards the rocksalt structure when the pressure reached the mid 50GPa region. This information was very useful for theoretical calculations at the time, as it was able to give a clearer indication of the outcome of transitions in GaN as higher pressures, e.g. Camp et al 19.

26 At ambient conditions GaN has a wurtzite structure with space group P63mmc and a fourfold co-ordination, with lattice parameters of a=3.190Ă…, c=5.189Ă…, u=0.377, c/a = 1.627 (these values are approximate due to the varied values given in most reports. The successful investigation into higher pressures was furthered

by Ueno et al 20; the literature described the study of the effects of pressure upon the axial ratio of the wurtzite structure to further the study of the phase transition of gallium nitride. The high-pressure methods utilised a diamond anvil cell to obtain the high pressures needed for the transition and used x-ray diffraction to examine the changes in the materials structure. They found the wurtzite phase continued up to 50GPa. Above 52.2GPa, all of the XRD peaks from the wurtzite phase had weakened and low peaks had begun to appear. By 60.6GPa data was published showing that all of the peaks for the wurtzite structure had disappeared and three new peaks had appeared, which were assigned to the (111), (200) and (220) reflections of the rocksalt type structure. This evidence gave experimental proof that the gallium nitride wurtzite structure has a phase change to the rocksalt structure at high pressures. The group continued to take measurements when releasing pressure and found that the pressure-induced transition is reversible.

One of the points Ueno et al 20 highlighted during the study was the difference between the reported transition pressure for the Perlin 12report, where x-ray absorption spectroscopy had been utilised, and their own work using x-ray diffraction. The difference was accepted at 5GPa, and concluded that the use of synchrotron radiation to study the transition of GaN would be more effective due to a higher sensitivity shown (whereby the onset of the transition was observed at 37GPa).

During 1993 the interest into gallium nitride grew rapidly with the fabrication of first high brightness blue LED by Nakamura et al 2,3,21. The importance grew concerning the optical properties of gallium nitride and the effects of high pressure upon GaN became a great source of interest for research groups. In particular, the pressure dependence of the photoluminescence (PL) of GaN was seen to be useful in the understanding of the electronic energy band structure and structural properties 22. There had been little experimental determination of the pressure dependence of the band gap for gallium nitride up to 1995. From the literature available, the most informative work had shown the pressure dependence of the band gap for wurtzite GaN at room temperature up to 55kbar

27 using transmission spectroscopy 12. Further research by groups such as Shan et al, Tuschman et al and Hwang et al 6,23-25 gave new insight into the electronic properties of GaN at higher pressures (although at this point the studies were on substrate grown GaN and not single crystals or powders as studied later). The high-pressure studies were all taken using diamond anvil cells, using the 325nm laser line (from He-Cd lasers). The research by Shan et al 6and Tuchman et al both used MOCVD grown GaN grown upon a sapphire substrate. The measurements were taken at low temperatures (≈10K) as well as at room temperature using the ruby calibration system to ascertain the pressure variations.

The data showed the MOCVD GaN samples were exhibiting strong, narrow, near band edge emission structures corresponding to the radiative decay of bound and free excitons with a broad emission band in the yellow spectral region (yellow band). The exciton emission lines were found to shift almost linearly towards higher energies with increasing pressure.

The intrinsic difficulties with the measurements at high pressures of the optical and structural properties of materials have meant that the growth of knowledge within this research field has grown slowly. Through to the late 1990’s PL data for gallium nitride at ultrahigh pressures (such as those earlier investigated for EXAFS and XRD by Perlin 12and Xia17, respectively) was not available. Studies

by Reimann et al 26took GaN up to pressures of approximately 12Gpa but found that (as previously reported) the spectral lines shapes stay similar although they shift to higher energies.

The information corresponding to experimental research into gallium nitride is given in the table above. Although the research into the properties of gallium nitride has been quite extensive, the overall body of research has been undertaken using different techniques using gallium nitride grown through different methods.

Thus far, there has been no research looking at the material using such techniques in-situ and with a single source of gallium nitride. By using the Hi-Prexx facility (described in Chapter 3.1.2), investigations into gallium nitride synthesised using the techniques (described in chapter 4.1.3.2) were able to be undertaken to give a characterisation of the material up to high pressures.

28

4 Investigations into Group III-N synthesis 4.1 Synthesis of Gallium Nitride 4.1.1

Recent research into GaN powder/crystals

Various techniques have been investigated for the synthesis of Gallium Nitride powders. The major problems, which affect gallium nitride during the present moment, are due to the inability’s to find a suitable substrate material for GaN to grow upon. Even the most popular substrate materials used in GaN investigations have lattice mismatches and differing thermal expansion coefficients. Hence, this leads to stresses and dislocations within the GaN structure grown upon the substrate, due to the incompatibility. Various attempts have been made to reduce the effect of these mismatches; the most popular reduction exercises have involved the growth of ‘buffer’ layers between the substrate and the initial GaN layer. Although this does help reduce the mismatches, it doesn’t eradicate them.

With such problems hindering the growth of defect free growth of GaN upon foreign substrate materials, research aimed at eradicating these problems would focus upon growing the GaN layers upon GaN itself. The majority of crystal process routes applicable to GaN, such as sublimation and high pressure solution methods, need the availability of high purity, single phase, well characterised powder sources

27.

Due to the limited research in this area, the purity, quantity

and cost of commercially available GaN powders have generally been poor, limited and quite excessive, respectively.

Research into growing GaN substrates, relies upon the production of GaN crystals large enough to be suitable for future commercial use, the largest single crystal recently reported was 10mm2, prepared by Aoki et al

28.

The technique used

required both high pressures and temperatures and an ongoing process of adding crystals to the process to act as seeds for the creation of larger crystals.

Although various techniques of epitaxial GaN growth have been researched by numerous groups world-wide, few studies on bulk GaN growth can be found. {kurai jpn j app phys 1996, vodakov et al 183 1998 –need to add to endnote!1} 29-

29 31.

This lack of literature includes a deficiency of information about the synthesis

of GaN as a raw material for crystal growth.

The high stability of nitrides results in very high melting points, e.g. 2800K for GaN {Vechten Phys Rev B 1973 1479- add to endnote} and have very high decomposition pressures. Therefore, the growth of bulk GaN can be very difficult.

A further hindrance to the process of producing Gallium Nitride powders/crystals are encountered due to Gallium metal (Ga) and Nitrogen gas (N2) being unreactive to each other under atmospheric pressure (with ammonia gas NH3 often utilised as a nitrogen source). Until recently the thermodynamic properties of nitrides had seemingly eliminated any standard method of melt growth such as Czochralski or Bridgeman 30}.

30 4.1.2

The Solid State Metathesis technique.

4.1.2.1 Overview of the technique

The initial research undertaken for the production of bulk GaN powders followed an initial development by Wallace et al

32,33.

Using a method known as a ‘solid-

state metathesis reaction’, under pressure.

In solid-state synthesis, reactions are heated, such that the precursor materials thermally decompose to produce the required fragments for direct combination.

In addition to this, the higher temperature allows some movement diffusion through the solid at a sufficient rate such that the desired product can eventually be obtained. A more beneficial system is created if one of the components melts, hence overcoming the solid-state diffusion layer. This barrier arises because reactions can only occur between neighbouring atoms; in a solid these atoms have to migrate into contact through the rigid solid lattice. This process is slow unless the temperature is raised significantly to allow rapid migration.

The solid-state metathesis reaction is a synthetic technique, which relies upon an exothermic chemical reaction. The solid state metathesis reaction (SSM) involves an exchange reaction between two compounds in which the atoms are swapped between the groups, such that in general (see Reaction 1).

Reaction 1:

Where;

MI n + LinQ → MQ + nLiI

M=transition (main group) metal or (lanthanide, actinide) Q=N (or P, As, Sb, O, S, Se, Te, Si, B)

The driving force for reaction is the high lattice energy of the co-produced salt (in the above example this is LiN), which can account for 90% of reaction enthalpy 34.

The formation of the co-produced salt has important consequences for the

overall maximum reaction temperature. With the ever-increasing interest into the production of GaN powders, SSM reactions acted as an alternative strategy to their synthesis.

31 By using the sensible precursor materials in the above reaction system one can produce GaN powders. The reaction between Gallium Halides and Li3N, although favourable (especially thermodynamically (>500 kJ/mol)), when studied above, are not favourable when considered under ambient conditions. Therefore, these reactions were considered for high-pressure synthesis 32,33. When considering the Gallium Halides, consideration has been given to the knowledge that halides such as GaBr3, GaCl3 or GaF3 spontaneously detonate on mixing with lithium nitride.

The method reported by Wallace used different equipment and hence relatively different methods for the production of Gallium Nitride powders, although the principal and experimental nature was very similar.

The method previously

described used Bridgeman anvils within a hydraulic ram press.

32 4.1.2.2 GaN Synthesis using SSM method.

The attempted synthesis comprised of a high temperature reaction between GaI3 and Li3N contained by a Walker Cell.

The Walker cell is named after Professor David Walker and is a design progression from the first generation multi-anvil cells used in the 1950’s for diamond synthesis studies.

The design has several features, which create

advantages over other types of high-pressure cells. These include; •

Almost homogenous pressure.

•

Almost homogenous temperature.

•

Pressure control with repeatability (due to pre-programmed pressure systems)

•

Large sample sizes - which are useful for synthesis techniques.

The cell is taken up to pressures of 5 GPa in the high pressure press where high temperatures are achieved by passing a high current through a set of anvils containing a 12mm TEL‡ octahedra which holds the precursor materials. The temperature is regulated through a Eurotherm BPC900 temperature controller, which is attached to the internal thermocouple held within the high-pressure cell.

Figure 4-1: Diagrams of Gold cups during fabrication

The finely ground precursor materials GaI3 and Li3N, which are finely ground and mixed together inside an Argon filled glove box are compacted into a gold capsule. The gold capsule (as shown above (Figure 4-1) with in the three main states) has an initial length of 13mm which then reduces to 10mm once the ends are sealed, the radius of the gold tubing stays at 3mm until compression occurs.

‡

The size of octahedra is identified by its length along the truncated edge, hence it is known as a 12mm truncated edge length octahedra.

33 The gold capsule is then compressed further by hand using a manually controlled bench clamp and dye, is further reduced to a pellet of length of approx. 6mm.

Figure 4-2: Diagram of fired octohedra with hole drilled through faces.

The compressed capsule is placed into a prefabricated octahedra. The 12mm truncated edge length (TEL) octahedra has a 6mm hole drilled from face to face, centred. Within the drilled hole, a pre-manufactured graphite furnace is gently fitted (see Figure 4-2). The furnace must be placed inside the hole, such that, the furnace is neither loose nor too tight.

A further hole is drilled from fin to fin perpendicular to the furnace. A hand crafted S-type thermocouple is then threaded via the drilled hole and through the furnace. The thermocouple is fabricated with ceramic tubing and stainless steel tubing to protect it from the furnace and the pressure of octahedra, respectively, during the experiment. The ceramic (Al2O3) tubing acts as an insulator between the thermocouple and the furnace along with the gold pellet, during compression.

The stainless steel tubing acts to protect the thermocouple from undue pressure when the compression is initiated.

When the octahedra are further constructed as below, the gold capsule containing the precursor materials is placed inside the fitted octahedra furnace. The capsule is subsequently insulated from the furnace and thermocouple via extra tubing and powder.

The production of working octahedra involves the careful construction of the constituent parts (as shown below).

If any component is faulty, then the

experiment may fail during either compression or during the increase of

34 temperature. If this occurs during a sample run, then several days of experimental time may be lost; hence, the construction must be carefully considered at each point of assembly and fabrication.

Figure 4-3: Diagram of constituent parts of octahedra and synthesis components

Once the majority of the octahedra manufacture is completed (Figure 4-3), the stainless steel end-caps are connected either side of the graphite furnace. The whole assembly (as seen in Figure 4-4) is carefully placed within an 8 tungsten carbide cube set. The cubes allow even pressure to be applied to the octahedra when inside the press.

35

Figure 4-4: Inside view of fully fabricated octahedra with gold cup and sample.

The cubes are held together by sheets of thin fibreglass (using Araldite super-glue as a fixative), which are attached to the outside of the whole construct. The newly completed cube set is then sprayed with a non-adhesive spray, which allows the cube set (and hence the final sample) to be removed easily after the experiment is completed. This is important for safety purposes as the tungsten carbide cubes can explode when the pressure is released; therefore, any reduction in the time needed in handling the cubes to obtain the pressurised octahedra reduces the risks involved. The press can be seen in Figure 4-5.

Figure 4-5: Photograph of the high pressure press used to take cell to required pressures.

36 When the octahedra are placed within the cube set, there are two cubes, which correspond to the stainless steel end caps touching the ends of the furnace inside the octahedra. The two opposing cube faces have copper discs attached to allow the flow of current through to the furnace.

This arrangement is carefully positioned and connected in the high-pressure press, where the thermocouple is attached to the Eurotherm EPC900 temperature controller.

The cell is taken up to a pressure of ≈ 5 GPa. To initiate the reaction between the precursor materials a high current is passed through the anvils, this acts to heats the graphite furnace. The capsule is taken to a temperature of approximately 1450K and is contained for several hours before the temperature is reduced and the pressure released.

The pressure is applied due to the decomposition of GaN at 1170K at ambient pressures. Wallace et al [1] previously calculated the reaction temperature, where a temperature ≈ 1443K is needed with this applied pressure.

To remove the sample from the pressurised cell, the octahedra is broken apart and the pellet is opened. Care is taken to limit any unwanted powders from mixing with the sample inside the pellet.

Once removed, the powder is washed and centrifuged in de-ionised water to remove the LiI and then the sample is scanned using the off-line x-ray diffractrometer.

The overall reaction is ideally a full reaction following the principle formulae (Reaction 1). Whereby (Reaction 2);

Reaction 2:

GaI 3 + Li3 N → GaN + 3LiI

The reaction is driven by a salt formation with ΔHRXN=-515 kJ/mol) which accounts for 79% of the heat released during the reaction.

37 The assuming a complete reaction and adiabatic conditions, then a calculated maximum reaction temperature, Tad, of 1443 K was calculated by Wallace et al.

Differential scanning calorimetry was reported to indicate the reaction of Gallium Iodide and Lithium Nitride initiates at 503.7 K, where a large irreversible exotherm is found. The maximum reaction temperature is almost twice that of the melting point of lithium iodide and as mentioned earlier (when one component melts a more beneficial system is produced) hence the reaction is expected to propagate more rapidly.

The Gallium Nitride produced from the synthesis technique was very low yield and included several unidentified by products.

38

4.1.2.2.1 SSM GaN results.

Methods for separating the GaN from the unwanted by-products were generally unsuccessful. Although within the ideal reaction the by-product should be lithium iodide which could be removed using the de-ionised water, there appeared to be several other compounds which were unable to be identified using x-ray diffraction powder diffraction data available. The x-ray diffractogram below (Figure 4-6) identify the presence of gallium nitride but also further peaks within the graph indicate that further impurities are present within the powder even after cleaning via centrifuging with de-ionised water.

Figure 4-6: X-ray diffractograms for gallium nitride synthesised using SSM method.

Another problem identified with the technique was the low yield, which was characteristic of the experimental procedure. Due to the small quantity produced, further characterisation meant the sample needed to be carefully looked after. The use of off-line x-ray diffraction methods to test the sample meant the sample had to be mixed with Ethyl Acetate (which makes the powder have the constitution of a paste). The sample is then evenly pasted onto the front off a glass slide (and then dried), which can then be used in the x-ray diffractometer. The small yield meant that the entire sample extracted from the gold cell was used and so further characterisation needed the powder from the slide to be used.

39 Using the powder diffraction files to identify the synthesised material indicated the gallium nitride present within the powders had a hexagonal structure with space group P63mc (186). The cell parameters were compared to Balkas et al27 and showed them to be similar with, a=3.19Ă…, c=5.18Ă… with cell volume of V=45.65Ă….

40 4.1.3

GaN synthesis using Flowing Ammonia Technique.

4.1.3.1 Overview of technique and recent research Following the relatively poor yield from the SSM method (inc. the complexity of the procedure), another route for the synthesis of high quality GaN powders was investigated.

During the late 1990’s, several research groups had followed

investigations whereby GaN had been synthesised via a reaction between Gallium compounds and ammonia gas (NH3) using various techniques. Many of the techniques relied upon specialised equipment The most notable study was an investigation by Balkas et al

27.

Their studies

explored earlier research by Johnson et al, from 1930. Johnson reported attempts of GaN synthesis through a direct reaction between gallium and flowing nitrogen gas at temperatures ranging from 500-1000oC, for several days. The results gave no indication of a reaction and hence no evidence of any combination of the two constituents were observed.

When the constituents are considered for the

reaction, the very low reactivity between gallium and nitrogen gas must be taken into account whereby; ΔGf = 31 kJ/mol and Kp = 0.004 at 900oC, and ΔGf = -18 kJ/mol and Kp = 17 at 500oC. Hence concentrating on the precursor materials proved to be important and the research was focussed on reacting gallium metal with flowing ammonia. The temperatures for synthesis were more favourable when held at approximately 900-1000oC.

Although the reports indicated

successful GaN synthesis had been achieved the methods of characterising the materials was limited and hence no evidence could be given for the purity of the powders produced. Balkas et al further investigated the flowing ammonia method 35-39

and various Ga compounds (halides and oxides) as precursors. Apart from

the use of Ga or Ga2O as precursors, the results were unfavourable as methods for practical GaN synthesis. This can be seen by the positive free energy calculations reproduced from the Balkas report (Table 4-1); Table 4-1: Reaction overviews for production of GaN (taken from Balkas et al)

Reaction

Temperature

Free

energy

reaction ΔGr(kJ/mol)

GaBr( g ) + NH 3 ( g ) ⇔ GaN (s) + HBr ( g ) + H 2 ( g ) 300-1000

-17 - +25

of

41

GaI ( g ) + NH 3 ( g ) ⇔ GaN (s) + HI ( g ) + H 2 ( g )

300-1000

-8 - +33

GaCl3 ( g ) + NH 3 ( g ) ⇔ GaN (s) + 3HCl ( g )

300-1000

+54 - +4

GaF3 (s) + NH 3 ( g ) ⇔ GaN (s) + 3HF ( g )

300-1000

+138 - -25

GaCl( g ) + NH 3 ( g ) ⇔ GaN (s) + HCl ( g ) + H 2 ( g ) 300-1000

-30 - +17

Considerations to the synthesis technique adopted include the financial viability of the method. In many of the cases initially investigated, the production of the GaN would be far higher than the commercially available GaN and would therefore only be a worthy route if the purity proved to be exceptionally higher than the commercially available GaN powders.

The conclusions of the Balkas research highlighted the use of gallium metal as the most successful precursor for the powder production.

42 4.1.3.2 GaN synthesis using Flowing Ammonia method

Following the reports from Balkas et al

27,

a further simplified technique was

attempted whereby the production of GaN powder could be realised without the need for expensive equipment and extensive preparation.

The overall arrangement (Figure 4-7) utilised an ammonia source connected via a stainless steel pipe to the stainless steel vessel containing a silica boat containing the precursor material. The vessel (containing the boat) is enclosed within the MTF 12/38 Carbolite tube furnace.

The precursor material, gallium metal, is stored in cool conditions until placed in the boat prior to the start of the experiment. This is due to the low melting temperature of 30oC. The purity of the gallium metal was given as 99.9999 %.

The vessel, which contains the boat, has inner proportions of 150mm length with a 35mm-core radius. The boat has approximate proportions 75mm by 12mm by 20mm (although the whole boat wasn’t always utilised when the gallium was placed within). The boat was always placed centrally within the vessel, although the gallium was usually placed nearer the exit end of the boat (as seen in other diagram).

Figure 4-7: Diagram of equipment used for flowing ammonia technique used for gallium nitride synthesis.

43

The simple arrangement passes ammonia (given as 99.9995% purity gas from BOC Edwards) through the vessel (containing the silica boat and precursor) inside the furnace. A gradual increase in temperature is maintained from RT up to the desired 1050oC over the period of 1 hour to sustain a regular and steady flow of ammonia throughout. The temperature inside the vessel was held at a temperature of 1050oC for a period of approximately 3 hours (this temperature was always more favourable to the synthesis of the powder, lower temperatures generally led to un-reacted free gallium metal to be found in x-ray diffractograms of synthesised materials).

Once the excess ammonia passed out through the vessel, a further stainless steel pipe acts to direct the ammonia (at high temperature) and constituent gases into a large beaker of water. This cools the ammonia whilst converting the NH3 into the less toxic ammonium hydroxide on reacting with the water as shown below (Reaction 3);

Reaction 3:

NH 3 ( g ) + H 2O(l ) → NH 4OH (l )

The ammonia was set to flow at a rate of ≈ 450 standard cubic centimetres per minute (sccm) through the vessel during the whole period of heating. After three hours the vessel (and its contents) were allowed to cool, the flow of NH3 was continued until room temperature at which point the system was flushed with nitrogen gas, to remove any trace of ammonia within the vessel. Once the N2 current had been halted the silica boat and its contents were removed for examination.

44 The experimental set up differs from the Balkas technique in several ways. The reported technique used quartz boats to hold the precursor material, which were contained within a vessel 90cm in length. The technique seemingly relied heavily upon the boat position within the vessel (with the optimised position at 50cm from the NH3 inlet) whereas the technique presented here, the boat position was not as strict. This may be due to the use of a shorter vessel, which is contained inside the tube furnace, whereas the reported vessel protrudes outside the furnace to create a cooling zone. Hence, the gas temperature within the vessel would be lower nearer the back end of the vessel, possibly causing strong convection currents, unlike the arrangement given here, whereby the whole vessel acts to promote a consistent temperature inside the vessel. This is a possibility for the consistency of the sample quality with the boat position.

Other differences

between the experimental details are the flow rate and temperature; these were reported to be optimal at 400 standard cubic centimetres per minute (sccm) and 975oC, respectively. The findings here give optimal results with a flow rate of 450 sccm and a temperature setting of 1050oC.

45 4.1.3.3 Results from the Flowing Ammonia method

The synthesis produced high quality GaN, with total conversion from Ga to GaN through the following reaction (Reaction 4):

Reaction 4:

2Ga(l) + 2NH3 ( g ) ⇔ 2GaN (s) + 3H 2 ( g )

The free energy of reaction was previously calculated [balkas] as ∆G = -54 – -50 (kJ/mol).

The powder produced is light grey in colour and examined after synthesis with xray diffractometry using a Siemens Kristalloflex 710/71 OH Tube X-ray generator, with a Cu Kα target. The powder contained no contaminants or free Ga as shown below (Figure 4-8), energy dispersive x-ray analysis also showed no other materials present other than Ga or N. The diffractograms were compared to the PCPDF data. The powder can be seen to be highly-crystalline wurtzite GaN, with space group P63mc (186). The cell parameters are (from the xrd data) a=3.186Å and c=5.178Å, giving a cell volume of V=45.518Å5. Further results and analysis for the flowing ammonia synthesised powders are given in the results sections. The XRD below also contains a silicon marker (Figure 4-8).

Figure 4-8: Diffractogram of gallium nitride produced using the flowing ammonia method

5

Where the volume is calculated for the hexagonal cell structure using V=a2csin60

46 Since the research undertaken here, further techniques have been attempted by other research to reproduce the GaN synthesis using slightly different methods. A further variation of this technique has been reported by Shibata et al where the ammonia flows into a gallium melt at temperatures of 900-980oC, the GaN powder is retrieved from the Ga melt where it is cleaned with HCl and H2O2 and then filtered. The powder was high quality but the effectiveness of the method is hindered due to contamination with free gallium. Unlike the technique applied in this report, the process is a longer and more complicated process whereby contamination is a risk if post processing is not applied properly. To date, no other methods of synthesis have shown any major improvements to the synthesis technique, nor have they produced GaN of higher quality.

47

4.2 Indium Nitride Synthesis 4.2.1

Overview of recent research into indium nitride synthesis

Indium Nitride is known to have a direct bandgap of around 1.9 eV with a wurtzite crystalline structure 20,40-46, and has seen a lot of interest as part of projects involving gallium nitride and its alloys.

Since GaN can be fabricated to incorporate InN (and AlN) within its structure, opportunities opened for the possibility of creating optoelectronic devices that can have band-gaps, which vary from 1.9 to 6.2 eV (i.e. from orange through to ultra violet). Even though III-N materials have seen an increased volume of interest of the recent years the bulk of the research has been focused upon GaN, with further research upon its alloys e.g. InGaN, AlGaN etc. In spite of this, experimental data on the properties of InN are surprisingly scarce, and the understandings of its properties have not really grown with proportion with the wider III-N research.

Further to this, reports involving InN research, had either used indium nitride which had been grown using MOCVD techniques, or sputtering methods 45,47 or used commercially synthesised InN which was created using an unknown method and shown to include impurities such as free Indium or In2O3 20. Further to this, the availability of any InN powder commercially is very low (as commented by O’Donnel45); hence, a need was highlighted for highly pure InN samples, which were synthesised using a known and repeatable method. This would create the opportunity for accurate characterisation and allow further synthesis of pure alloyed III-N powders.

No reports could be found for the synthesis of indium nitride powders via the usual methods of research; therefore, the synthesis investigations of InN powders were looked at using previous knowledge gained via the synthesis of gallium nitride. Unpublished work undertaken by a student within the material science laboratory at Daresbury Laboratories had highlighted methods previously attempted for the creation of indium nitride powders and the results gave a very brief overview of the techniques that had been attempted and failed. This information was observed, such that, mistakes were not repeated.

48 4.2.2

Synthesis of Indium Nitride using Flowing Ammonia Technique

Due to the simplicity of the flowing ammonia technique used for the GaN synthesis, a continuation of the sample preparation technique was attempted for the InN manufacture. Most literature identifies that InN breaks down at temperatures close to 600oC 47 report degradation of InN above 500oC) and therefore attempting to use the use temperatures higher than this would be futile when considering a successful experiment.

After considering the precursor materials possible for use within the synthesis (including economical factors), indium fluoride (InF3) was identified as a possible successful precursor, following the reaction shown below (Reaction 5).

InF3 + NH 3 → InN + 3HF

Reaction 5:

To identify a safe level of InF3 to prepare for the technique (with the knowledge that HF would be produced if a successful reaction would take place), the reaction was considered carefully.

By taking the approximate volume of the vessel as 240cm3, and the highest temperature used as 600oC (as explained above) then it was found that the ideal weight of InF3 to be used would be 0.144g. This changes the original reaction outline (Reaction 5) to incorporate the excess ammonia to produce excess NH4F, which is soluble in water (H2O), which would be a safer by product for the reaction (Reaction 6), especially when considering the temperatures involved.

Reaction 6:

4 NH 3 (excess) + InF3 → InN + 3NH 3 F

A schematic of the equipment used is shown in figure above gives the approximate volumes involved. The furnace, a MTF 12/38 Carbolite tube furnace, held a ‘home-made’ stainless steel tube acting as the vessel to hold the boat and precursor powder. The pipes were all made of stainless steel to reduce any possible risks from the production of Hydrofluoric acid (which may occur if

49 the reaction is not ideal). The reaction boat was the same as highlighted in the gallium nitride synthesis and was fabricated for the experiment from silica glass.

The equipment, set up as shown previously (Figure 4-7: Diagram of equipment used for flowing ammonia technique used for gallium nitride synthesis., where InF3 has replaces the Ga) has the ammonia set to flow at a rate of 450 standard cubic centimetres per minute (cm3s-1). The tube furnace temperature is initially set to 100ยบ C for the initial 30 minutes gradually increasing to 590ยบ C over the following 48 minutes. The furnace is held at 590 ยบC for a period of 3 hours. Once this period has passed, the furnace is turned off and allowed to return to room temperature over the subsequent period, with the ammonia continuing to flow through the system.

Whilst attempting the synthesis temperatures below and above 590oC were used, these were all generally unsuccessful, whereby un-reacted InF3 was the main contaminant or other unidentified by-products were created, respectively. This may be due to the InN (formed during the experiment), being broken down by the temperatures used and forming further, more complicated, compounds which were unavailable in the powder diffraction files available for comparison.

The ammonia continues to flow throughout the process. The ammonia is set to flow through the pipes until it is safe to remove the synthesised material (approx.

8 hours later, this is due to the natural cooling time of the vessel inside the furnace). When the vessel approaches room temperature the ammonia flow is halted and N2 flows through the vessel for a following hour, this is to remove any excess ammonia from the system before manually opening the vessel.

50 4.2.2.1 Indium nitride results from Flowing Ammonia Method.

This reaction, although heavily reliant upon the flow rate and temperature used, is a very simple system to use and produced matte black fine powder which was then analysed using x-ray diffraction, energy dispersive x-rays and scanning electron microscopy to ascertain the purity.

The powder was found to be highly pure indium nitride with no contaminants identified within the tests attempted on the material. The x-ray diffractograms (as seen below) show no other noticeable peaks other than InN identified by the powder diffraction files available for comparison. The diffraction data identifies the powder to be highly pure wurtzite indium nitride.

The powders were then taken to De Montfort University and analysed using scanning electron microscopy, the microscope used was a Leica S430 scanning electron microscope. This is a 30 KeV instrument, which is fully controlled from within a computer controlled windows environment. The SEM images show hexagonal platelets, which average in cross section at approximately 1Âľm.

Using the same equipment as outlined above, the powders were examined using energy dispersive x-rays (EDX) see Figure 4-9, which indicated that only indium and nitrogen was present within crystals examined whilst in the scanning electron microscope. The EDX graphs and maps data can be seen below, along with a xray diffractogram.

51

Figure 4-9: Energy dispersive x-ray analysis of indium nitride produced using flowing ammonia method.

+ xrd from msl (need to contact ray) Photoluminescence from InN hasn’t been extensively examined (in fact, even O’Donnel et al 45,48 highlighted during their research into InN that only one report of InN PL has been reported. Reports have been published into the properties of InN under high pressure 20 and investigations have used techniques such as EXAFS 45 and x-ray diffraction to determine the characteristics of the material.

Therefore, this would be an ideal material to study in depth using the Hi-Prexx facility.

Ueno et al 20investigated InN under pressure using a diamond anvil cell. Their findings were that InN has a phase transition from the wurtzite phase to the rocksalt structure. The material contained In2O3 as an impurity from the beginning and the peaks assigned to this needed to be accounted for. The data showed that 4 new diffraction peaks appear at 12.1GPa indicating the phase transition, these peaks grew stronger when the material was placed under pressures of 18.2 GPa. They found the transition was reversible with the xrd pattern exhibiting the re-appearance of the wurtzite phase after the material was removed from the DAC. EXAFS work on InN was done at Daresbury Laboratories on station 9.2 looking at the Indium K-edge absorption near 27,800 eV by O’Donnel et al. The materials looked at were grown at differing

52 temperatures (100, 300 and 500oC) using sputtering techniques.

53 Overview of III-N alloys.

4.2.3

Reasons for Investigating III-N alloys

Al(or In)GaN/GaN heterostructures are the basis of optoelectronic devices tunable in the whole spectral region from ultraviolet (6.2 eV for AlN, 3.4 eV for GaN) to infrared (1.9 eV for InN) by varying the composition. Highly efficient light emitting diodes from ultraviolet to amber and laser diodes in the ultraviolet have been already fabricated by Nakamura et al 2-4,21 yet a full understanding of the properties of such materials hasn’t been fully comprehended.

The alloys are generally grown and hence examined when created as substrate grown materials. Hence, the techniques for investigation have needed to be fine tuned to measure the properties of the alloy itself and not the substrate or neighbouring layers. To allow a true examination of the alloys without these inherent problems incurred by the usual growth routes (i.e. grown upon a substrate using Two Flow Metal Organic Chemical Vapour Deposition) an investigation was begun to attempt the synthesis of alloy powders using high-pressure techniques.

Previous reports, whereby the association of GaN is always favoured over InGaN when attempts had been make to synthesise InGaN using similar techniques to the flowing ammonia methods reported in chapters GaN Synthesis using SSM method.4.1.2.2{ref: I know I have some references somewhere}, hence, this method was an unlikely candidate for III-N powder synthesis. The route of synthesis chosen was a high-pressure, high temperature route using a gold cup within a Walker cell as the precursor. The attempts for AlGaN used high temperatures of approx. 950oC and a pressure of approximately 5GPa, and the InGaN used the same pressures but temperatures ranging from 550oC to 950oC. Although the results indicated no alloys were produced, an overview of the methods and research surrounding the materials are highlighted, as further work may be possible to create successful conditions for such a process to be successful. The AlGaN attempts showed no effects upon either precursor. The

54 InGaN attempts indicate that the InN breaks down quite early in the heating process but again the GaN stayed unaffected.

Growth of these crystals on substrates was mainly limited to GaN/AlGaN systems because there was only a small lattice mismatch between GaN and AlGaN. For InGaN a much greater mismatch was expected between the GaN/InGaN interface hence making the growth of InGaN much more difficult. Due to the unknown potential (and later realised potential) of InGaN as an emission layer for GaN based LED’s further research was undertaken to determine the properties of the alloy.

Figure 4-10: Graph identifying the varying band-gaps of III-N materials.

The reason for the importance of the materials within the structure of the GaN based LED’s was due to the wide range of band gaps between the three main IIIN materials (along with the structural similarities (i.e. wurtzite (hexagonal) and direct energy band gaps which they all possess). As seen above (Figure 4-10), the band gaps of the materials AlN, InN and GaN vary from 1.92eV (InN), 3.45eV (GaN) to 6.2eV (AlN). In terms of wavelength, these energy values correlate to values of approximately 636nm (red region) for InN, 400nm (green region) for GaN and 200nm (non-visible part of spectrum) for AlN. The values were calculated approximately using h=6.6x10-34, c=3x108 and ec=1.602x10-19. The wide range of band gap energies indicates that a material such as AlGaInN (i.e. incorporating all the group III materials investigated) could be produced with a

55 full range of visible and near visible wavelengths, depending on the stoichiometry of the group III products within the III-N alloy. Beyond the importance of the properties, in practice, the GaN related compounds are grown above (or around) 1000oC, and can therefore withstand the processes involved in annealing and high temperature processing.

In the production of LED’s and LD’s the growth of the InGaN and AlGaN layers generally uses a method known as two-flow MOCVD. The two-flow Metal Organic Chemical Vapour Deposition method is similar to the single flow technique but uses both a main flow across the surface of the substrate and a second ‘sub-flow’, which acts to transport the inactive gas perpendicular to the substrate. One reason for the complexity of the flow system is due to the heat needed to produce a reaction (reports vary but approx. temperatures of 1050oC are often quoted). The need for the film to be of high quality using a single flow system needed a very high flow rate to reduce the effects of the convection mechanisms occurring near the surface of the substrate. Hence, the high temperatures resulted in the flow of the gas to be non-uniform across the surface of the substrate. By introducing a relatively strong second flow perpendicular to the surface of the substrate, enable the reactive gases (usually Triethylgallium (TMG) and Ammonia (NH3) and Hydrogen (H2) to flow across the surface in a more even and regular flow. This method utilizes very expensive equipment, which needs to be within extreme clean environments to reduce the possibility of contaminants to a negligible minimum.

Without access to the equipment described the synthesis of substrate-based alloys is very difficult, although the synthesis away from the substrate is still a very important topic for investigation. Production of the alloy materials without growth upon a substrate allows the materials to be investigated in a purer form, whereby the effects of the materials underlying the alloys and the various mechanisms involved with the layer growth (both electronically and structurally) would be removed. This therefore allows an accurate representation of the properties of the alloys to be created.

56 4.2.4

Overview of III-N Material Properties

The properties of the various III-N materials investigated (generally theoretically more than practically) have resulted in various values for the general properties of the materials. Lattice constants and unit cell parameters, which have been calculated previously, are given below (Table 4-2), although they do not agree 100% with all literature (there are many variations, mainly because of the effects of the substrate upon the materials when found through experimentation).

Table 4-2: Basic lattice parameters of III-N materials

AlN

GaN

InN

a

3.091

3.174

3.538

c

4.954

5.169

5.707

The main problems that need to be overcome for the production of the alloys are the known differences between the III-N materials. These are summarised below (Table 4-3).

Table 4-3: Table identifying the crystal system, approximate band-gap and melting points of III-N materials.

Material

AlN

GaN

InN

Crystal system

Wurtzite

Wurtzite

Wurtzite

Band Gap (eV)

6.2eV

3.45eV

1.92eV

Melting point

22000C

Sublimes at

Stable to 300oC

800oC

Breaks down at 600oC

High-pressure techniques for the synthesis of the alloys allows the materials to be contained during the reaction process.

The temperature differences for the

melting (/sublimation) points of the materials shows that there are problems incurred in the process and the use of high pressure would seem to be a route to overcome these difficulties.

57 4.2.5

Overview SSM method to produce Alloy Powders.

The synthesis technique investigated involves initiating a high temperature reaction between GaN and AlN or InN contained by a Walker Cell within a highpressure press. The precursor materials are finely ground and mixed; this is done using a simple pestle and mortar method. The ground powders are placed and compacted into a gold capsule of radius 3mm and length 10mm, which is then compressed again. The capsule is placed into prefabricated octahedra.

The 12mm TEL octahedra has a 6mm hole drilled through the centre; within this, a graphite furnace is fitted. A further hole is made perpendicular to the furnace where an S-type thermocouple is threaded. The furnace is insulated from the thermocouple by stainless steel and Al2O3 tubing. The gold capsule is placed within the octahedra and is further insulated from the furnace and thermocouple.

Stainless steel end-caps are connected either side of the graphite furnace and the whole assembly placed within an 8 tungsten carbide cube set to allow even pressure to be applied. The cubes are held together by sheets of fibreglass attached to the outside of the whole construct. The two opposing cube faces, which correspond to the stainless steel end-caps, have copper discs attached to allow the flow of current through to the furnace. This arrangement is carefully positioned and connected in the high-pressure press, where the thermocouple is attached to a Eurotherm EPC900 temperature controller (the overall set up is the similar to that described for the GaN SSM synthesis given in chapter 4.1.2).

Using the high-pressure press the cell has a force exerted upon it of 3000kN, which is equivalent to approximately 5GPa. The reaction is initiated by passing a high current through the anvils, which acts to heats the graphite furnace. The capsule is taken to the required temperature and contained for 6 hours before the temperature is reduced and the pressure released.

Ideally, the reaction between the GaN and AlN or InN would produce an AlGaN or InGaN alloy, whereby impurities would be more likely be free In, Al or Ga or un-reacted InN, AlN or GaN.

58 4.2.5.1 Results from SSM method for Alloy Powders.

As can be seen in the XRD patterns, there are no indications of alloy production (using the approximated ICDD files).

The AlGaN results suggest that the AlN and GaN were unaffected; there were several spurious peaks, which have been identified as a form of Al2O3, which is most probably due to the ceramic materials used within the cell.

The InGaN results show that the temperatures used (above 650oC) break down the InN within the cell leaving In. The GaN is unaffected in all the runs. None of the XRD scans show any peaks that could be matched to any of the calculated ICDD peaks for the alloy materials.

The InGaN runs gave varied results, none of these showed any effect upon the GaN present. The temperature changes showed there are various modifications to the InN within the cell, but all of these (apart from the 550oC result) broke down the InN leaving In as a contaminant. Further peaks in some scans could be identified as Aluminium Oxide, which can be determined as the same Al2O3 used within the cell manufacture.

The AlGaN results all indicated that both the AlN and the GaN were unaffected by the synthesis technique. This is most likely to be due to the high melting temperature of the AlN, coupled with the GaN subliming at temperatures above 800oC without breaking down.

Further work would need to look at the effects of AlGaN synthesis at higher temperatures, possibly without the gold cup as a precursor container. This could be done with the materials packed directly into the inner ceramic tubing for trial runs. This would allow higher temperatures but would need a new cell calibration to be made.

The indium gallium nitride (InGaN) work indicated that the InN breaks down, leaving In. No evidence of a reaction with the gallium nitride could be seen, even

59 at relatively high temperatures. Increasing the pressure to contain the Indium Nitride would mean the temperature of the reaction would need to be higher and therefore this would not produce conditions that are more favourable.

Overall, further attempts using the high-pressure press for the synthesis of III-N alloys should concentrate on AlGaN rather than InGaN as the results so far indicate the InN breaks down too early in the heating process. Although if this is continued the temperatures needed to take AlN near to any melting point would need a cell capable of going to over 2200oC, due to the raised melting point under pressure.

Work on synthesizing AlInN alloys has not been attempted and perhaps could be a possible first route for the continuation of the project, although this again may have problems associated due to the low temperatures at which InN breaks down.

60

4.3 Overview of etching of gallium nitride powders 4.3.1

Previous research

Etching techniques were carried out using gallium nitride powders bought commercially and produced using the flowing ammonia technique. The system was a simple acid etching process using hydrochloric acid and hydrofluoric acid to promote etching on the surface of the crystals.

The powders were first washed in de-ionised water and then placed in hydrochloric acid for a period of 8 hours. During this time, a magnetic stirrer was used to allow all of the powders to be affected by the acid. Once this process had completed the powders were carefully washed in de-ionised water once again.

The powder was then covered with hydrofluoric acid for 5 hours (with the magnetic stirrer in place again). The powder was then carefully cleaned with deionised water again.

The powders were then examined using scanning electron microscopy. The micrographs produced show a successful level of etching on the surface of the hexagonal crystallites. The surfaces of the crystals exhibit hexagonal rods of proportions on the nano-scale. The commercially bought etched samples were further examined using photoluminescence spectroscopy shown in chapter 5.2.

61

5 Results 5.1 Initial data - An Overview.

During the course of the research several powders were investigated, these were Commercially available GaN, and two ‘in-house’ synthesised powders using solid-state metathesis and flowing ammonia. These were all investigated at the point of purchase or synthesis using techniques such as photoluminescence and xray powder diffraction. Data from these initial ambient x-ray powder diffraction6 investigations are given in the table below (Table 5-1), with corresponding x-ray diffractograms given underneath (Figure 5-1, Figure 5-2, Figure 5-3).

Table 5-1: Basic structural data for III-N materials.

GaN type

Quality

Structure a (Å)

c (Å)

Volume (V)

Alfa GaN

Impurities present

Wurtzite

3.199 5.213 46.243

SSM GaN

poor yield, impurities present Wurtzite

3.186 5.178 45.581

Flow NH3

good yield, no impurities

3.186 5.178 45.581

GaN

found.

Wurtzite

Figure 5-1: X-ray Diffractogram for commercial gallium nitride powder obtained from Alfa chemicals.

62

Figure 5-2: X-ray Diffractogram for gallium nitride produced using the solid state metathesis method

Figure 5-3: X-ray Diffractogram for gallium nitride produced using the flowing ammonia method.

6

The data was taken on a Siemens Kristalloflex 710/71 OH Tube X-ray generator with a Cu KÎą target and matched with GaN PCPDF files and used X-Plot software available for use at Daresbury Laboratories to determine the lattice parameters.

63 As identified in the table the purity of the powders is highlighted from the diffractograms, whereby the Alfa gallium nitride has an unidentified peak at the start of the spectra, which cannot be identified as gallium nitride. The SSM synthesised gallium nitride is clearly hindered by unidentified impurities and the structure, although identified as hexagonal, isn’t highly crystalline. The gallium nitride synthesised by the flowing ammonia method shows no impurities within the graphical data. The GaN can be identified as being highly crystalline, this is noted by the width of the peaks that are all identified as being associated with gallium nitride (apart from those identified for silicon which is used as a standard).

The gallium nitride powders have been examined using high-pressure techniques previously outlined (3.1.2 and experiment chapters). The investigations were undertaken using the Hi-Prexx facility at Daresbury Laboratories and the Advanced Photon Source in Argonne Illinois. The samples were prepared as described in chapter 2.4, whereby, the samples were placed in the gasket hole with a ruby chip (to allow the pressure to be determined) with silica oil used to create an environment such that quasi-hydrostatic conditions could be reached. The samples were ground using a pestle and mortar to make them as homogenous as possible. These preparations are essential to create the best environment for successful high-pressure results.

Early investigations looked at the commercially available powders (along with powders after etching techniques had been applied). Whereas later experiments looked at the gallium nitride produced using the flowing ammonia method and the commercial powders.

64

5.2 Initial data investigating commercial powders.

Initial measurements of the gallium nitride powders looked at the photoluminescence (PL) properties of the gallium nitride powders7 (pre-flowing ammonia synthesised GaN - chapter 4.1.3.2). The powders were initially studied using the simple photoluminescence set-up at De Montfort University and later using the Hi-Prexx facility.

Examples of the PL data can be seen below (Figure 5-4):

PL from GaN Powders (exc. Wavelength 325nm)

Intensity /a.u.

140000 120000

A lfa  G aN

100000 80000 60000 40000

SSM GaN

20000 0 350

400

450

500

550

600

650

700

750

800

Wavelength / nm

Figure 5-4: Early photoluminescence spectra for commercial and SSM synthesised gallium nitride powders.

The initial PL spectra of the commercial and SSM powders showed that although the PL was intense (could easily be seen with lights on in the room), the spectra was dominated by the broad peak at approximately 560nm (~2.21eV) which differs from reported values whereby a ‘yellow band’ emission is dominant around 365nm (~3.4eV){29,32,33,49}, where, although the broader peak is identified, it has a much lower intensity, which would inference towards this peak being due to either impurities or defects within the material. When studied more

7

All photoluminescence measurements were undertaken using a He-Cd laser with a 325/442nm filter. The measurements were undertaken using the 325nm filter unless otherwise stated.

65 closely a very small peak can be seen in the spectra around this region, but is swamped by the start of the broader peak8.

Investigations into etching gallium nitride powders (commercial) following papers identifying methods for the surface cleaning of gallium nitride using wet chemical techniques 50 attempts were made to ‘clean up’ the powders used for these investigations. The commercial powders were etched using HCl and HF, and then examined using photoluminescence techniques (and scanning electron microscopy - these images are shown in chapter 4.3).

The results identified that the ‘yellow band’ peak increases in intensity after etching has occurred (the same system as above was used to take PL measurements), these can be seen below.

The initial measurement using the Hi-Prexx facility (outlined in chapter 3.1.2) to investigate the Alfa GaN and etched (Alfa) GaN were taken at the Advanced Photon Source in Argonne, Illinois. Using the beamline {SRI-CAT) the Hi-Prexx facility was configured to work with the system requirements for the stations configuration. The beamtime was primarily used for testing the Hi-Prexx experimental set-up and secondly for the investigation of gallium nitride powders under pressure.

The results obtained were achieved from GaN powders taken up to pressures of 125kbar using a diamond anvil cell (as described in chapter 2.4). Both commercial and etched commercial powders were investigated. Plots of the main two peak positions of the GaN PL spectra were made such that a trend could be identified between the pressure increase and a change in peak position. The plots indicate that there is a linear shift with pressure for both peaks. These can be identified as the same shift parameters that Ueno et al 20,51and Teisseyre et al 49 also reported when investigating similar shifts.

Due to the nature of the beamtime, successful in-situ XRD and EXAFS data was unable to be taken at the time.

8

This reduction in size for the initial yellow band luminescence was also further reduced due to the absorption of the lenses initially used for ranges below 410nm.

66

5.3 Hi-Prexx investigations into GaN synthesised using the Flowing Ammonia technique.

Using the EXAFS procedure highlighted in chapter 3.1.1 - whereby the EXAFS data are taken twice with the DAC slightly rotated to enable glitches to be removed, successful investigations into the effects of pressure upon gallium nitride were enabled. The method allowed the EXAFS range to be increased to k=12-14Ă….

The quality of the high-pressure measurements collected for EXAFS were such that the data even up to 41GPa were of comparable quality to ambient measurements taken using the standard transmission technique {52,53}.

Typical EXAFS spectra collected at different pressures are shown below (Figure 5-5);

Figure 5-5: EXAFS data for gallium nitride taken up to 41Gpa.

67 The pressure dependence of the Ga-N and Ga-N-Ga ditsances are shown below in Figure 5-6);

Figure 5-6: Nearest neighbours pressure dependence chart, using EXAFS data, for flowing ammonia synthesised gallium nitride taken to ≈ 450kBar.

The nearest neighbours distances-pressure dependence of GaN powder extracted from EXAFS data. The open circles represent the Ga-N-Ga distance and the solid squares correspond to the Ga-N distance.

The Ga-N-Ga distance extracted from the EXAFS spectra corresponds to the alattice parameter. The observed dependence is similar to that previously reported

by Ueno et al 20, where GaN powders were studied using x-ray diffraction (and PL).

The high-pressure dependence of GaN powder PL is displayed in Figure 5-79. It can be observed that the luminescence peaks (A and B) shift towards shorter wavelengths with a corresponding pressure increase. This effect has been reported for different types of GaN.

9

The dip in the spectra around 475nm can be accounted for by the harmonic from a notch filter used to cut the 325nm laser line

68 The data shows that the pressure dependence of the ‘yellow band’ luminescence (peak B) was found to be ≈37 meV/GPa, which is close to reported values given

by Perlin et al for a monocrystal12. The pressure dependence of the position of peak A was found to be approximately 30 meV/GPa.

Figure 5-7: Photoluminescence spectra for flowing ammonia synthesised gallium nitride powder taken up to high pressures using a DAC.

Using an image plate to collect the diffraction data allowed a simple and effective method to be used for the collection of XRD data10. The diffraction data indicated a further similarity to reports by Ueno et al 20for the pressure dependency of the GaN powder.

Figure 5-8 shows an example of the x-ray diffraction data taken during the investigation. The data shows a triplet, which is typical to the hexagonal phase of GaN.

10

The method was similar to that previously reported by Nelmes et al 1992

69

Figure 5-8: X-ray diffraction image for GaN taken inside a diamond anvil cell (the arrow indicates a gasket line).

70

6 Conclusions and Recommendations The aim of the project was to synthesise and characterise gallium nitride powders.

The synthesis produced highly pure gallium nitride powders and indium nitride powders which have not been found in any literature to have been improved upon, in terms of quality, yield, cost effectiveness and simplicity of replication.

Several techniques were used identify a synthesis route for the production of gallium nitride powders. These systems; solid-state metathesis and flowing ammonia technique both produced gallium nitride powders, which were later examined using x-ray diffraction techniques. The results concluded the lattice parameters of the materials were similar to those reported 24,44,54-62) and identified the powders to be wurtzite P63mc structure.

The methods highlighted the flowing ammonia technique as a highly successful and easily reproducible method of synthesis. The reaction was identified by Reaction 4 given below;

2Ga(l) + 2NH3 ( g ) ⇔ 2GaN (s) + 3H 2 ( g ) The reaction was a total conversion to gallium nitride, under conditions whereby ammonia was flowed at a rate of 450 standard cubic centimetres per second and maintained at a temperature of 11500C for 3 hours. The reaction took place inside a stainless steel vessel (with the gallium placed inside a silica boat).

The technique has improvements over the technique identified by Balkas et al whereby the procedure is further simplified, such that, the boat position is less important and no rapid cooling is needed for high quality, high yield GaN to be produced.

The XRD, SEM and EDX analysis showed only gallium and nitrogen to be present within the synthesised GaN powders examined and that the powders consisted of crystals ≈1Âľ in diameter.

71 The synthesised powder was examined using combined high-pressure x-ray techniques along with ambient x-ray diffraction, scanning electron microscopy (SEM) and energy dispersive x-ray analysis (EDX). Using EXAFS, x-ray diffraction, photoluminescence to examine the properties of pure GaN powders under high pressures, the powder was taken up to 41GPa inside a diamond anvil cell. The results were possible in-situ using the Hi-Prexx facility. The high pressure data indicates the pressure dependence of the powders is similar to previously reported measurements of mono-crystals by Perlin et al12, with the identified ‘yellow band’ peak having a higher correlation to the mono-crystals than the broader defect peak.

The pressures reached (41GPa) approach the expected phase transition to cubic gallium nitride at ≈55GPa and indicate the pressure dependency of the first and second shell nearest neighbours (Ga-N and Ga-Ga) to be ≈0.05Å/100kbar {ref. to my paper}.

Further to the gallium nitride investigation, indium nitride powder has been successfully synthesised which has not been reported previously (according to the research undertaken). The synthesis technique followed the success of the gallium nitride flowing ammonia synthesis and produced highly pure indium nitride powder which consisted of fine matte black crystals The technique followed the outline for flowing ammonia synthesis outlined for GaN. The reaction took place inside a stainless steel vessel at a temperature of 590oC for 3 hours. The reaction was identified as (Reaction 6);

4 NH 3 (excess) + InF3 → InN + 3NH 3 F The possible production of hydrofluoric acid identified in Reaction 5, highlighted the importance of safety needed for a fully successful reaction to take place.

Hence, this led to the need for the excess ammonia needed to produce a relatively harmless ammonium fluoride.

72 Following synthesis, the InN powder was examined under ambient conditions using SEM, EDX and XRD investigative techniques. The initial XRD data indicated that there were no impurities in the powder and highlighted that the powder was wurtzite indium nitride with lattice parameters a=x, c=y, V=z. The powders were then examined using EDX and SEM, which identified that, no other impurities were present and that the powder consisted of extremely thin hexagonal platelets with a diameter of xx. The tests resulted in conclusive proof that the first highly pure indium nitride powder had been synthesised using the flowing ammonia technique. The method is a simple, easily reproducible, cost-effective method of producing highly pure indium nitride powders.

Further to the III-N research, the Hi-Prexx facility was developed allowing high pressure, in-situ, EXAFS, XRD and PL measurements to be taken. The experimental rig was modified over the course of the project and is now a fully functional experimental facility. The initial developments used a geometrical array of lenses and beam splitters to act as a collection and delivery system for photoluminescence (and for high pressure ruby calibration), and allow the x-ray based measurements to be taken. The evolution of Hi-PREXX 52,53,63 has led to the optical based arrangement to compliment the x-ray developments and now uses optical fibres in conjunction with lenses to make the system as efficient as possible. The can be fully controlled from outside of the experimental hutch via the use of camera equipment, motors and computer software/hardware.

Overall, the conclusions are that the initial project aims concerning gallium nitride were achieved and advanced. The further investigation of InN has allowed the development of a cheap and efficient synthesis technique, which is simple to reproduce in any laboratory. The development of Hi-Prexx has helped further the expansion of combined studies for high-pressure measurements.

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