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Correlation between structural, photoluminescence properties and gettering effect of stain etched porous silicon in multimulti-crystalline silicon M. Hajji*, M. Ben Rabha Rabha,, B. Bessais Bessais,, H. Ezzaouia Laboratoire de Photovoltaïque, Centre de Recherches et des Technologies de l’Energie, Technopole de Borj Borj--Cédria Cédria,, BP 95, 2050 Hammam Hammam--Lif, Tunisia Tunisia.. * Corresponding author: ee-mail mhajji2001@yahoo.fr , Phone: +216 79325160, Fax: +216 79325825

Abstract Porous silicon layers have been obtained by stain etching of multi-crystalline silicon substrates in a HF/HNO3 solution for different etching times. Atomic Force Microscopy (AFM) was used to analyze the morphology of the surface nanostructures. The photoluminescence and total reflectance of obtained porous silicon layers were measured. Obtained results show that the PSi nanostructure is largely affected by the conditions of elaboration. For short etching times the porous surface reaches reflectance minimum values and the photoluminescence intensity attain maximum values. The gettering effect of porous silicon was also studied and related to its structural and optical properties. It is found that after thermal treatment of mcsilicon substrates with a thin porous layer on both sides in a N2 atmosphere the effective minority carrier lifetime increases from 3 to about 47 µs.

This behavior is due to continuous formation and destruction of silicon nanocrystallites during chemical etching of silicon. The roughness histograms (Figure 2) show that the surface diversification increases by increasing the etching time. The observed shift to the high grains height signifies an increase of the PSi layer thickness during successive etching steps. This increase in the thickness is also indicated by the evolution of interference fringes in the reflection spectra in figure 3. It is also clear from this figure that the surface reflectance is largely reduced after PSi formation indicating that the PSi layer can acts as antireflection coating in mc-Si solar cells. 0,020

Keywords: Porous silicon, stain etching, photoluminescence, reflectance, gettering, carrier lifetime.

0,8

0,018

SiHx (x=1,2,3)

(a)

Absorbance (a,u)

The substrates used are multi-crystalline, p-type boron doped silicon wafers, with a resistivity ranging from 0.5 to 2 Ω cm and a thickness of 450 mm.

PL intencity (a. u.)

0,016

I. Experimental

0,6

SiO2

0,4

SiH2

0,014 0,012 0,010

(c)

0,008 0,006

The stain films were produced by immersion of mc-Si substrates in a HF: HN03:H20 solution with ratios of 1:3:5 by volume. The elaboration of porous silicon was done in two stages. In the first stage all samples were etched at the same time during an initiation time of 11 minutes. This initiation time is the time elapsed from the moment that the sample is immersed in the solution until the etching begins and the porous layer is propagated on the whole surface of the sample. In the second stage samples are subjected to a number of supplementary etching steps, of 60 s, that differs from a sample to another. The surface morphology and chemical composition of PSi layers were investigated by Atomic Force Microscopy (AFM) and Fourier Transform Infrared Spectroscopy (FTIR). Optical properties were studied by photoluminescence (PL) and UV-Vis reflectance measurements. Thermal annealing of mc-Si substrates with porous layers on both sides was carried out under nitrogen atmosphere at a temperature of 800°C for an annealing duration of 2 hours. The gettering efficiency of stain etched porous silicon was monitored by life time measurements and obtained results are correlated with structural and optical properties of porous silicon layers. S1 11 60 1

S2 11 60 2

S3 11 60 3

0,004 0,002

0,0 500

1000

1500

2000

2500

3000

3500

0,000 500

4000

550

-1

600

650

700

Wavelength (nm)

Wavenumber (cm )

Figure 4: FTIR spectra of PSi prepared by Stain Etching method.

Figure 5: PL spectra of PSi layers obtained for samples S1 (a), S2 (b) and S3 (c).

Figure 5 shows the PL bands for all studied samples. Spectrum (a) corresponds to the PL band of a sample etched during 11 minutes (initiation time) followed one supplementary etching step which is located at an energy of 2 eV (619 nm) with a FWHM of 302 meV. The second etching step (b) leads to a reduction of about 76 % of the PL intensity and a reduction of the FWHM from 302 to 292 meV but no shift was observed. The reduction of the PL intensity during the second step is due to the continuous formation and dissolution of nanocrystallites during chemical etching of silicon. After the third step (c) the PL intensity is twofold increased compared to the last sample indicating a supplementary reduction in the size crystallites formed during the second step. No correlation between the chemical composition of PSi layers, extracted from FTIR measurements, and the photoluminescence properties was observed. Thus the most accepted mechanism of PL in chemically etched PSi is the quantum confinement effect.

Gettering effect of porous silicon 50

0,03

Sample Lifetime (µs)

45

Table 1: Experimental conditions used for stain etched porous silicon elaboration.

40

Bare mc-Si 3

S1 47

S2 9

S3 11

II. II. Results and discussions Structural and optical properties of porous silicon Morphological analysis is carried out using Atomic Force Microscope (AFM) (Digital Instruments Nanoscope) analysis in order to study the structural quality of the stained porous layers obtained under different conditions.

PL intesity (a. u.)

35 0,02

30 25 20

Lifetime(µ s)

Sample Initiation time (min) Step duration (s) Number of etching steps

(b)

SiH

0,2

Table 2: Effective lifetime evolution of mc-Si after Stain Etching and thermal treatment in a N2 atmosphere.

15 0,01 10 5

Figure 6: PL intensity after PS and effective bulk carrier lifetime measured for mc-Si wafers after PS and photothermal annealing in a N2 atmosphere 800°C.

0 1

2

3

number of etching steps

a

b

The minority carriers lifetime is largely increased after substrates thermal annealing (Table 2). This increase is essentially due to the diffusion, during annealing, of undesirable impurities, that act as traps for carriers, from silicon bulk to porous layer where they will be localized and then removed with the porous layer. Figure 6 shows a comparison between the evolution of the PL intensity at the peak before annealing and minority carrier lifetime after annealing. It is clear that the increase of the lifetime is strongly correlated to structural and related optical properties of PSi layers. The gettering effect of porous silicon is as efficient as the porous layer is composed of efficiently luminescent nanocrystallites with their density as high as possible and homogeneously distributed on substrate surface.

c

Figure 1: AFM images of PSi layers obtained for samples S1 (a), S2 (b) and S3 (c). 48

(b) (a)

III. Conclusion

40

32

mc-Si R (%)

Number of events

8000

24

4000

(c)

(b)

16

(a) 8

(c)

0

0 0

10

20

30

40

50

Topography [nm]

Figure 2: Roughness histograms of PSi layers obtained for samples S1 (a), S2 (b) and S3 (c).

60

400

500

600

700

800

900

1000

1100

Wavelength (nm)

Figure 3: Variation of the surface reflectivity before and after PS formation for samples S1 (a), S2 (b) and S3 (c).

AFM results show an important change of the PSi nanostructure after successive etching steps. The sample prepared by a single step presents a rough surface with an rms of about 7.8 nm, after the second step the substrate surface becomes slightly smoother (7 nm) but after the third step this roughness increases another time to reach 9.6 nm.

Homogenous porous silicon layers were obtained using a stain etching method. AFM, FTIR, PL and UV-vis reflectance were used to study the structural and optical properties of obtained PSi layers. It found that PL in PSi layer is strongly affected by the layer nanostructure and the PL intensity is as high as PSi layer is composed of small crystallites homogeneously distributed. On the other hand, PSi layer acts as a perfect light diffusor and provides an appropriate reflectance which is quite comparable to the reflectance of a textured Si surface covered by conventional ARC. The gettering effect of PSi layers was also studied. This study shows that after gettering the minority carrier life time is largely increased due to a reduction of undesirable impurities in the substrate after the thermal annealing. It is also found that the gettering effect efficiency of PSi layer is strongly correlated to its nanostructure and related optical properties.

Acknowledgment : This work was supported by the Ministry of High Education and Scientific Research.

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nanotechnology

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