I. Experimental I. Experimental 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.

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

M. Hajji*, M. Ben M. Hajji*, M. Ben RabhaRabha, B. , B. BessaisBessais, H. , H. EzzaouiaEzzaouia

Laboratoire de Photovoltaïque, Centre de Recherches et des Technologies de l’Energie, Technopole de Laboratoire de Photovoltaïque, Centre de Recherches et des Technologies de l’Energie, Technopole de BorjBorj--CédriaCédria, BP 95, 2050 Hammam, BP 95, 2050 Hammam--Lif, Lif, TunisiaTunisia..

* Corresponding author: e* Corresponding author: e--mail mail mhajji2001@yahoo.frmhajji2001@yahoo.fr , Phone: +216 79325160, Fax: +216 79325825, Phone: +216 79325160, Fax: +216 79325825

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 mc-

silicon 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.

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

AbstractAbstract

Correlation between structural, photoluminescence properties and gettering Correlation between structural, photoluminescence properties and gettering

effect of stain etched porous silicon in multieffect of stain etched porous silicon in multi--crystalline siliconcrystalline silicon

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

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.

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).

500 1000 1500 2000 2500 3000 3500 4000

0,0

0,2

0,4

0,6

0,8

Ab

so

rban

ce

(a,u

)

Wavenumber (cm-1

)

SiHx (x=1,2,3)

SiO2

SiH2

SiH

500 550 600 650 700

0,000

0,002

0,004

0,006

0,008

0,010

0,012

0,014

0,016

0,018

0,020

PL

in

ten

cit

y (

a. u

.)

Wavelength (nm)

(a)

(b)

(c)

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.

IIII.. ResultsResults andand discussionsdiscussions

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.

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.

III. ConclusionIII. Conclusion

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.

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

Sample S1 S2 S3

Initiation time (min) 11 11 11

Step duration (s) 60 60 60

Number of etching steps 1 2 3

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

StructuralStructural andand opticaloptical propertiesproperties ofof porousporous siliconsilicon

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

a b c

0 10 20 30 40 50 60

0

4000

8000

Nu

mb

er

of

ev

en

ts

Topography [nm]

(a)

(b)

(c)

400 500 600 700 800 900 1000 1100

0

8

16

24

32

40

48

R (

%)

Wavelength (nm)

mc-Si

(a)

(b) (c)

Figure 2: Roughness histograms of PSi layers

obtained for samples S1 (a), S2 (b) and S3 (c).

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.

Sample Bare mc-Si S1 S2 S3

Lifetime (µs) 3 47 9 11

Table 2: Effective lifetime evolution of mc-Si after Stain

Etching and thermal treatment in a N2 atmosphere.

GetteringGettering effecteffect ofof porousporous siliconsilicon

1 2 3

0,01

0,02

0,03

number of etching steps

PL

in

tesit

y (

a.

u.)

0

5

10

15

20

25

30

35

40

45

50

Lif

eti

me( µµ µµ

s)

Figure 6: PL intensity after PS and effective bulk carrier

lifetime measured for mc-Si wafers after PS and photo-

thermal annealing in a N2 atmosphere 800°C.

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.