Chemical deposition of bismuth selenide thin films using N,N-dimethylselenourea
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Semicond. Sci. Technol. 12 (1997) 645–653. Printed in the UK
Chemical deposition of bismuth selenide thin films using N ,N -dimethylselenourea V M Garc´ıa†, M T S Nair†, P K Nair† and R A Zingaro‡ ´ † Laboratorio de Energ´ıa Solar, IIM, Universidad Nacional Autonoma de Mexico, Temixco-62580, Morelos, Mexico ‡ Department of Chemistry, Texas A&M University, College Station, TX 77843-3255, USA Received 23 July 1996, in final form 11 November 1996, accepted for publication 17 January 1997 Abstract. Good quality thin films of bismuth selenide of thickness up to 0.28 µm were deposited from solutions containing bismuth nitrate, triethanolamine and N ,N -dimethylselenourea maintained at temperatures ranging from room temperature to 40 ◦ C. X-ray diffraction patterns of the samples annealed at 200 ◦ C in air match the standard pattern of hexagonal Bi2 Se3 (paraguanajuatite, JCPDS 33-0214). The films exhibit strong optical absorption corresponding to a bandgap of about 1.7–1.41 eV in the as-prepared films. These values decrease to about 1.57–1.06 eV upon annealing the films at 200 ◦ C for 1 h in nitrogen. As-deposited, the films show high sheet resistance (∼1012 −1 ) in the dark. Annealing the films in air or in nitrogen enhances the dark current by about seven orders of magnitude; the resulting dark conductivity is about 10 −1 cm−1 . This enhancement in conductivity results from improved crystallinity as well as from partial loss of selenium.
1. Introduction Solid solutions of bismuth selenide with bismuth telluride are well known thermoelectric cooling materials. This prompted investigations on preparation of crystals of solid solutions of Bi2 Te3 –Bi2 Se3 and their alloys. Their structure, composition, mechanical and electrical properties [1, 2] and the influence of gravity on the crystallization process and electrical properties  have been previously reported. Thin films of bismuth selenide have been prepared by chemical bath deposition [4, 5] and the molecular jet  methods. In chemical bath deposition, the authors [4, 5] used sodium selenosulphate, Na2 SeSO3 , as the source of selenide in a bath containing bismuth nitrate and triethanolamine. From the electronic absorption spectra of these films, two absorption edges, one at 3500 nm corresponding to a bandgap of 0.354 eV and another at 1200 nm corresponding to a bandgap of 1.03 eV, have been reported for the films . Based on its bandgap value of 0.354 eV, the authors have suggested application of Bi2 Se3 thin films as photographic films in infrared photography . The chemical bath deposition technique is known to yield good quality thin films with easily reproducible properties in the case of thin films of metal chalcogenides. We have already reported the chemical deposition of thin films of CdSe [7, 8], ZnSe , CuSe  and PbSe  from solutions containing soluble complexes of c 1997 IOP Publishing Ltd 0268-1242/97/050645+09$19.50
the corresponding metal ions and N ,N -dimethylselenourea. The present work describes a similar procedure for depositing thin films of bismuth selenide on glass substrates using a bath containing solutions of bismuth(III), triethanolamine and N ,N-dimethylselenourea. The effect of annealing the films in air and nitrogen on the structural, optical and electrical properties of the films is presented. The issue of the optical bandgap is discussed and possible applications of the films are considered.
2. Experimental details 2.1. Materials Baker-analysed reagent-grade bismuth nitrate pentahydrate (Bi(NO3 )3 ·5H2 O) and triethanolamine (TEA, N(CH2 CH2 OH)3 ), anhydrous sodium sulphite (Na2 SO3 ) of analytical reagent quality from Productos Qu´ımicos Monterrey, and N ,N -dimethylselenourea ((CH3 )2 NCSeNH2 ) prepared in our laboratory following the method reported earlier  were used in the deposition of the bismuth selenide thin films. Corning glass microscope slides of 26 mm × 76 mm × 1 mm were used as substrates. The substrates were cleaned well using detergent and water and dried prior to film deposition. 645
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2.2. Deposition of the thin films
3. Results and discussion
A solution of approximately 0.5 M bismuth nitrate was prepared by dissolving 24.25 g of Bi(NO3 )3 ·5H2 O in 65 ml of 3.7 M TEA and making the volume to 100 ml with deionized water in a standard volumetric flask. The deposition bath was prepared in a 100 ml beaker by the sequential addition (under stirring) of 7 ml of the bismuth solution, 7 ml of 3.7 M TEA, 20 ml of 0.066 M solution of N ,N-dimethylselenourea prepared freshly in 0.01 M Na2 SO3 and 56 ml of deionized water. Thus the total concentration of the constituents of the freshly constituted bath may be described approximately as: Bi(III), 0.035 mol l−1 ; triethanolamine, 0.42 mol l−1 ; N,N-dimethylselenourea, 0.013 mol l−1 ; and sodium sulphite, 0.002 mol l−1 . The cleaned glass substrates were placed in the bath against the wall of the beaker. This was covered with a larger beaker kept inverted. The depositions were allowed to proceed without stirring for different durations at room temperature (22–26 ◦ C) as well as at 30 ◦ C, 34 ◦ C and 40 ◦ C. A temperature-controlled oven was used for this purpose. At the end of the deposition, the substrates were removed from the bath and washed well with deionized water and dried. Both sides of the glass substrates were coated with thin films. The substrate surface facing the wall of the beaker during the deposition was coated with a specularly reflective uniform thin film. This film was retained for all the measurements. The coating on the surface of the substrate facing the interior of the bath exhibited a mosaic appearance due to precipitate settling on the growing thin film surface. This film was removed by a cotton swab moistened with dilute HCl. An Alfa Step 100 unit (Tencor Inc., CA, USA) was used to measure the film thickness.
3.1. Film growth
2.3. Characterization of the films The optical transmittance and the near-normal specular reflectance spectra of the films were recorded on a Shimadzu UV-3101PC UV–VIS–NIR scanning spectrophotometer. The light beams were incident from the film side. The reference was air while recording the transmittance spectra and a front aluminized mirror in the case of reflectance spectra. X-ray diffraction (XRD) patterns of the films were recorded on a Siemens D 500 machine with CuKα radiation. X-ray fluorescence spectra were recorded on selected samples in a Siemens SRS 303 spectrometer. Electrical conductivity measurements were done using a Keithley 230 programmable voltage source coupled with a 619 electrometer/multimeter and an XT personal computer. The electrical contacts to the film surface were made through a pair of silver paint electrodes of 5 mm length printed at a separation of 5 mm. For the photocurrent response measurements the samples were illuminated with a tungsten–halogen lamp producing an intensity of 2 kW m−2 over the plane of the sample. The applied bias was 10 V. The samples were allowed to reach the steady-state dark current and subsequently the photocurrent response curves were recorded for 20 s in the dark, 20 s under illumination and for 20 s after the illumination was shut off. 646
The basic principles involved in the chemical bath deposition of metal chalcogenides have been discussed in earlier papers [12–15]. In the present case, the formation of Bi2 Se3 in the bath involves the hydrolysis of dimethylselenourea which releases selenide ions into the bath: [(CH3 )2 NCSeNH2 ] + OH− → (CH3 )2 NCN+H2 O + HSe− HSe− + OH− → H2 O + Se2− and the dissociation of triethanolaminebismuth(III) complex ions which releases bismuth ions: [Bi(TEA)n ]3+ → Bi3+ + nTEA. These ions can now condense on an ion-by-ion basis to produce thin film on the glass substrate and over the wall of the beaker. The ions also condense and settle down as precipitate in the bath. The overall chemical equation for the formation of bismuth selenide in the present bath can be stated as: 2[Bi(TEA)]3+ + 3[(CH3 )2 NCSeNH2 ] + 6OH− → 3[(CH3 )2 NCONH2 ] + Bi2 Se3 + 3H2 O + 2TEA. In chemical bath deposition, the formation of a thin film on a substrate takes place through a nucleation process followed by a growth phase until a terminal thickness is reached [15, 16]. The choice of N ,N -dimethylselenourea as a source of selenide ions in an aqueous medium was made after a careful study of various substituted selenoureas, selenazolones and selenazoles . N ,N -dimethylselenourea was found to be the most stable against decomposition into elemental selenium. The use of sodium sulphite in the chemical bath containing dimethylselenourea further inhibits the oxidation of selenide to selenium, as discussed in . Thus we expect the films deposited by the present technique to be devoid of elemental selenium. Figure 1 shows the increase in thickness of the bismuth selenide thin films as a function of duration of deposition at room temperature (22–26 ◦ C), 30 ◦ C, 34 ◦ C and 40 ◦ C. The rate of deposition increases with the bath temperature. This is expected because of the increase in concentrations of Bi(III) and Se2− ions in the bath due to enhanced dissociations of the metal complex ion as well as of N ,N-dimethylselenourea in solution at higher temperatures. Such behaviour is very common in the chemical bath deposition of metal chalcogenide thin films [15, 16]. 3.2. Structure and composition of the films Figure 2 shows the XRD patterns of the films, as-prepared and after they were annealed in air for 1 h at 200 ◦ C. The XRD profile of the glass substrate, which is typical of amorphous glass, is also given. The as-prepared film shows little crystallinity—the XRD profile is not distinct from that of the glass substrate. Well defined peaks corresponding to
Chemical deposition of Bi2 Se3 thin films
Figure 1. Variation of Bi2 Se3 thin film thickness as a function of the duration of deposition at different bath temperatures: room temperature (22–26 ◦ C), 30 ◦ C, 34 ◦ C and 40 ◦ C.
the thin film are absent in the XRD pattern. However, the crystallinity of the film improves upon annealing and XRD peaks appear superimposed on the XRD profile of the glass substrate. This XRD pattern matches well the standard pattern of Bi2 Se3 mineral paraguanajuatite (JCPDS 33-0214) which possesses hexagonal layer structure with alternating layers of Bi and Se. The crystallographic data for this structure of bismuth selenide have been cited as a = 0.41396 nm and c = 2.8636 nm. The XRD results therefore establish that the films deposited by the present technique possess this structure and hence a composition Bi2 Se3 . The grain size of the crystallites (mean crystallite diameter, D) in the annealed film was calculated using Scherrer’s equation : D = 0.9λ/β cos θ , where λ is the wavelength of the x-ray (0.154 06 nm in the present case), β is the full width in radians at half maximum of the peak and θ is the Bragg angle of the x-ray diffraction peak. Calculation made on the (015) peak at 2θ = 29.48◦ (θ = 14.74◦ ) in figure 2 gave a value of 12 nm for the crystallite diameter. A close value, 11 nm, was from the (006) peak at 2θ = 18.44◦ . The conversion of the as-deposited amorphous thin films to crystalline films with well defined crystal structure and grain size in the range of 10–20 nm is a behaviour shared by chemically deposited bismuth sulphide thin films as well [18, 19]. In the latter case, annealing the as-deposited amorphous bismuth sulphide thin films at 200 ◦ C leads to crystallization into a structure corresponding to that of the mineral sample bismuthinite with composition Bi2 S3 .
Figure 2. CuKα XRD patterns of bismuth selenide thin films, as-prepared and air annealed for 1 h at 200 ◦ C along with the XRD profile of the glass substrate and the standard XRD powder pattern (JCPDS 33-0214) of Bi2 Se3 (paraguanajuatite).
3.3. Optical transmittance and reflectance spectra The optical transmittance and reflectance spectra of the bismuth selenide films of three different thicknesses, recorded in the wavelength range of 200 to 2500 nm, are given in figure 3. These curves exhibit superimposed optical interference patterns in the case of thicker films. The quality of the films is attested to by the fact that the sum of the percentage reflectance and transmittance in the long-wavelength region, away from the absorption edge, adds up to almost 100. In the figure we have also included transmittance curves corrected for the reflection losses. The reflectance of the films vary depending on the film thickness. It is more than 40% at certain wavelength ranges. The transmittance corrected for the reflectance losses (Tcorr ) may be expressed to a first approximation as Tcorr (%) = 100 T (%)/(100 − R(%)) for any wavelength. This correction modifies the transmittance curves considerably and the characteristic optical absorption of the semiconductor material emerges. The corrected transmittance curves will be used for the calculation of the optical bandgap of the material of the film. Figure 4 shows the effect of annealing the films in air or nitrogen on the optical transmittance and reflectance spectra. Here again, the corrected transmittance spectra is given. Comparison of the corrected transmittance curves of the as-prepared and annealed films of thickness 0.15 µm or higher, suggests a shift in the absorption edge towards the longer wavelength upon annealing. 647
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Figure 3. Optical transmittance (T %) and reflectance (R %) spectra of the as-prepared Bi2 Se3 thin films of three different thicknesses: 0.09 µm, 0.15 µm and 0.25 µm. The optical transmittance data corrected for reflection losses (Tcorr ) and T + R data for one of the samples are also given.
3.4. Evaluation of optical bandgap Figure 5 shows the plots of α 2 versus hν of representative films, where α is the absorption coefficient and hν the photon energy. These values were computed from the corrected optical transmittance curves illustrated in figures 3 and 4. The absorption coefficient is calculated at any wavelength (and hence at the corresponding photon energy) using the relation α = (1/d) ln(100/Tcorr (%)), where d is the film thickness. Extrapolation of the straight line part in each of the plots to the abscissa gives a value for the direct bandgap. Optical bandgap values distinct from that of the bulk crystalline material are known to exist in polycrystalline thin films. The variation is ascribed to very small crystallites constituting a thin film which results in the quantum confinement of charge carriers in the crystallites. The resultant effect is an increase in the bandgap in thin films, as compared with its value in 648
bulk crystalline material, when crystallite size is typically less than 10 nm . In a thin film, the crystallite size depends on the specific technique of deposition and the annealing conditions. For example, in the case of chemically deposited CdSe thin films, optical bandgaps in the range of 1.7 eV to 2.4 eV have been reported, depending on the deposition conditions . In the case of the bismuth selenide thin film of 0.09 µm thickness, the as-prepared film in figure 5 indicates a direct bandgap of 1.70 eV. When annealed in nitrogen at 200 ◦ C for 1 h, this value drops to 1.57 eV. In the case of thicker samples (0.15 µm, 0.18 µm and 0.25 µm), the as-prepared films indicate a direct bandgap of about 1.41 eV. When annealed at 200 ◦ C for 1 h in air or nitrogen the value drops to 1.08–1.06 eV. For the sake of clarity, only the plot for the film of 0.15 µm thickness is given to represent the thicker films. The shift in the absorption edge toward longer
Chemical deposition of Bi2 Se3 thin films
Figure 4. Optical transmittance (T %), reflectance (R %) and Tcorr (%) spectra of the films of figure 3 recorded after annealing in air or nitrogen at 200 ◦ C for 1 h each.
wavelength upon annealing the chemically deposited thin films is associated with an improvement in the crystallinity of the films [18, 19]. In the present case the improvement in the crystallinity of the films upon annealing is evidenced in the XRD pattern in figure 2. In the literature, Bi2 Se3 is reported as a semiconductor material with a direct bandgap. Two different values for the minimum energy gap are reported for bulk Bi2 Se3 : 0.35 eV  and 0.16 eV . In the case of chemically deposited thin films of Bi2 Se3 , the presence of two absorption edges has been reported [4, 5]: one at 3500 nm corresponding to 0.354 eV and the other at 1200 nm corresponding to 1.03 eV. The latter value is close to what is obtained for the annealed films (of thickness >0.15 µm) reported here. Figure 5 establishes that this value is that of a direct bandgap associated with the strong optical absorption in the film. Figure 3 shows that very little optical absorption takes place at wavelengths above 1500 nm (or a photon energy of
less than 0.83 eV) in any of the films. The optical absorption is not significant at such wavelengths in the case of annealed films (figure 4). Particularly notable is the near-zero optical absorption at wavelengths above 850 nm (photon energy less than about 1.5 eV) in the case of as-prepared or annealed films of thickness 0.09 µm. Figure 6 shows the plots of α 2 and α 1/2 versus hν, evaluated from the corrected optical transmittance curves represented in figure 4 for the lowenergy region (0.5–0.8 eV). Data for thicker films which are not given in figure 4 (for the sake of clarity) are included in figure 6. A straight line fit is possible over the entire low-energy spectral region in the case of the α 1/2 versus hν plots for the films of different thicknesses. This indicates that the very low optical absorption may be related to an indirect bandgap with energy in the 0.2–0.35 eV range. The presence of direct bandgaps at energies of 0.16 eV or 0.34 eV reported by other researchers [5, 22, 23] is not evident in the case of the present films. 649
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3.5. Electrical properties
Figure 5. Plots of α 2 versus h ν in the region of high optical absorption of two of the samples of figures 3 and 4 computed from Tcorr (%) values.
We also consider that the weak optical absorption in the low-energy region of the transmittance spectra may be related to diffuse scattering losses from the film surface. Such scattering will depend on the roughness of the surface . Figure 7 shows the plot of log A versus log λ, where A is the extinction of the transmitted beam (due to scattering losses from the surfaces and absorption in the medium), A = 1 − (Tcorr /100). It is seen that the data fit a straight line in the log–log plot, i.e. they obey a relation of the type, A = constant × λ−n . This is typical of many scattering mechanisms. Rayleigh scattering, which applies to particles of size less than the incident wavelength, is characterized by n = 4, and for intermediate cases, n = 2 . In figure 7, n has values 1.9 to 4.3 illustrating that the extinction data in the low-energy region may also fit into specific scattering mechanisms. Further work is required to establish the extinction processes and confirm whether the weak extinction observed in the low-energy region of the films should be associated with an indirect optical bandgap or whether it results from scattering losses, or both. We wish to point out that most of the published results on the optical properties of bismuth selenide [25–27] are based on optical reflectance on single crystals of Bi2 Se3 with relatively high electrical conductivities (103 −1 cm−1 ). These results cannot be readily correlated with the present work. In the case of thin films prepared by vacuum techniques of the same range of electrical conductivities, the optical properties have not been discussed [28, 29]. 650
Figure 8 shows the photocurrent response curves of the thin films as a function of thickness of the bismuth selenide thin films, as-prepared and after having been annealed for 1 h each in air or nitrogen at 200 ◦ C. The as-prepared samples are seen to be very resistive—showing sheet resistances of 8.8 × 1012 –2.1 × 1011 −1 in the dark. Under illumination, the corresponding values are 1.3 × 1011 – 1.2 × 1010 −1 ; the resistance being lower at higher film thicknesses. The highest photocurrent-to-dark current ratio (70) is obtained for the thinnest of the samples (0.09 µm)— a result similar to what was reported for bismuth sulphide thin films . After annealing in air, the films show a systematic increase in the dark current with film thickness, even though such an effect was not obvious in the case of the as-prepared films. Annealing in a nitrogen atmosphere increases the dark current of the samples by about an order of magnitude compared with the samples annealed in air. The increase in the current is due to the improvement in the crystallinity of the films, which would increase the charge carrier mobility. Annealing in a nitrogen atmosphere inhibits incorporation of oxygen in the grain boundaries and hence further enhances the mobility . The photosensitivity is hardly noticed in the case of the thicker samples, which acquire high electrical conductivities, of about 10 −1 cm−1 . The carrier type in the annealed films was tested by the hot probe method and the material was found to be n-type. Again, similar behaviour has been reported in the case of chemically deposited bismuth sulphide thin films . The XRF spectra of the as-prepared and annealed films shown in figure 9 suggests that the drastic increase in the electrical conductivity of the annealed films may also be due to a loss of selenium—a process which would make the films bismuth rich on an atomic scale and hence make them n-type. The ratio of the area under the XRF peak of bismuth Lα1 (λ = 0.11439 nm, 2θ = 47.36◦ ) and of the combined XRF peaks of selenium Kα1 (λ = 0.11048 nm, 2θ = 45.65◦ ) and Kα2 (λ = 0.11088 nm, 2θ = 45.83◦ ) in the as-prepared film (full curve) was considered to correspond to the atomic ratio of a stochiometric Bi2 Se3 material, i.e. 2/3 = 0.67. This indicated a ratio of the sensitivity factors for the bismuth peak and the combined selenium peaks of 0.5. The ratio of the area under the corresponding peaks of the annealed film was multiplied by the factor 0.5 to estimate the atomic ratio of bismuth and selenium in the annealed film. This indicated a value 0.98, suggesting a notably higher atomic percentage of bismuth in the annealed film. The annealing at a higher temperature, 300 ◦ C, for 1 h was done in order to produce a more notable effect than is possible under annealing at 200 ◦ C. 3.6. Applications of the films The above results show that the chemically deposited bismuth sulphide thin films reported here possess strong optical absorption corresponding to a direct bandgap in the 1.7–1.06 eV region. Annealed thin films of thickness >0.15 µm indicate a bandgap of nearly 1.06 eV, which in solar cells should result in conversion efficiencies up
Chemical deposition of Bi2 Se3 thin films
Figure 6. Plots of α 2 and α 1/2 versus h ν for the region of low optical absorption of various samples annealed in air or nitrogen at 200 ◦ C for 1 h each computed from Tcorr (%) values represented in figure 4.
to 21% according to Prince-Loferski analyses . The electrical conductivity (n-type) of the chemically deposited bismuth selenide thin films may be modified using postdeposition treatments such as nitrogen annealing for making them suitable for integration into solar cell structures. Recent work on chemically deposited CuS films on Bi2 S3 films has indicated the formation of a new compound, Cu3 BiS3 , with crystal structure corresponding to the mineral sample wittichenite [19, 33]. Wittichenite is a mineral sample obtained from Wittichen mine in Germany . The formation of the thin film material of this composition was achieved by depositing a copper sulphide thin film on a bismuth sulphide thin film and subjecting the stack to annealing at 280 ◦ C for about 1 h. This leads to interfacial diffusion of the atoms—a process also previously reported in chemically deposited PbS–CuS and ZnS–CuS stacks . The solid state reaction has been suggested as: 6CuS + Bi2 S3 → 2Cu3 BiS3 + 3S↑. We consider that the same reaction may be extended to Bi2 Se3 – CuSe stacks as well, in which the Bi2 Se3 film is deposited by the present method and the CuSe film is deposited as reported in . The formation of the compound Cu3 BiSe3 in the bulk form has been reported previously , which indicates the feasibility of this approach. This material
is expected to be of p-type conductivity, due to copper deficiency. It may also be electrochemically stable in device configuration because of the inhibition of copper (Cu+ ) diffusion through the material by Bi3+ —in a similar manner as achieved in the case of CuInSe2 —a proven absorber material for thin film solar cells . 4. Conclusions In this paper we have reported a method for producing good quality bismuth selenide thin films from a chemical bath containing N,N-dimethylselenourea as the source of selenide ions. The as-prepared samples have a structure which is mainly amorphous, having a high sheet resistance which decreases with the increase in thickness. Asprepared, the films are nearly intrinsic. We report that the air and nitrogen annealing process at 200 ◦ C increases the dark conductivity by many orders of magnitude, which arises from an enhancement of the crystallinity of the material indicated by the XRD pattern matching JCPDS 33-0214 of paraguanajuatite, and loss of Se from the film. Both these processes transform the as-prepared material into a conductive state, having an n-type conductivity of 651
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Figure 7. Plots of log of the extinction coefficient (A) versus log of wavelength (λ) of the representative annealed thin films.
Figure 9. X-ray fluorescence spectra of the as-prepared (full curves) and air annealed (300 ◦ C, 1 h) Bi2 Se3 thin films. The plots show only the Kα1 (λ = 0.110 48 nm, 2θ = 45.65◦ ) and the Kα2 (λ = 0.110 88 nm, 2θ = 45.83◦ ) peaks for Se and the Lα1 (λ = 0.114 39 nm, 2θ = 47.36◦ ) peak for Bi, which are the strongest peaks for these elements.
Acknowledgments The authors are grateful to Leticia Ba˜nos of IIM, UNAM for recording the XRD and XRF patterns and to CONACYT (Mexico) and DGAPA (UNAM-Mexico) for financial support. We also acknowledge the financial support given to one of us (VMG) by the Universidad Autonoma de Zacatecas, Mexico. References
Figure 8. Photocurrent response curves of Bi2 Se3 thin films of different thicknesses recorded for samples: as-prepared (A, 0.09 µm; B, 0.15 µm; C, 0.18 µm; D, 0.25 µm); annealed in air (A1–D1); and annealed in nitrogen (A2–D2) with an applied bias of 10 V across 5 mm.
about 10 −1 cm−1 . This may open up various possibilities for the application of these films prepared by the chemical deposition technique. 652
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