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Homogeneous luminescent stain etched porous silicon elaborated by a new multi-step stain etching method
Applied Surface Science 284 (2013) 324–330
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Homogeneous luminescent stain etched porous silicon elaborated by a new multi-step stain etching method M. Hajji a,b,∗ , M. Khalifa a , S. Ben Slama a , H. Ezzaouia a a b
Laboratoire de Photovoltaïque, Centre de Recherche et des Technologies de l’Energie, Technopôle de Borj-Cédria BP 95, Hammam-Lif 2050, Tunisia Institut Supérieur d’Electronique et de Communication de Sfax, route Menzel Chaker Km 0.5, BP 868, Sfax 3018, Tunisia
a r t i c l e
i n f o
Article history: Received 17 November 2012 Received in revised form 17 July 2013 Accepted 20 July 2013 Available online 27 July 2013 Keywords: Stain etching Porous silicon Photoluminescence Optical reflectivity Antireflection coating
a b s t r a c t This paper presents a new method to produce porous silicon which derived from the conventional stain etching (SE) method. But instead of one etching step that leads to formation of porous layer, the substrate is subjected to an initial etching step with a duration t0 followed by a number of supplementary short steps that differs from a layer to another. The duration of the initial step is just the necessary time to have a homogenous porous layer on the whole surface of the substrate. It was found that this duration is largely dependent of the doping type and level of the silicon substrate. The duration of supplementary steps was kept as short as possible to prevent the formation of bubbles on the silicon surface during silicon dissolution which leads generally to inhomogeneous porous layers. It is found from surface investigation by atomic force microscopy (AFM) that multistep stain etching (MS-SE) method allows to produce homogeneous porous silicon nanostructures compared to the conventional SE method. The chemical composition of the obtained porous layers has been evaluated using Fourier transform infrared spectroscopy (FTIR). Photoluminescence (PL) measurement shows that porous layers produced by SE and MS-SE methods have comparable spectra indicating that those layers are composed of nanocrystallites with comparable sizes. But the intensity of photoluminescence of layer elaborated by MS-SE method is higher than that elaborated by the SE method. Total reflectance characteristics show that the presented method allows the production of porous silicon layers with controllable thicknesses and optical properties. Results for porous silicon layers elaborated on heavily doped n-type silicon show that the reflectance can be reduced to values less than 3% in the major part of the spectrum. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Porous silicon (PSi) is a sponge-like materiel having a large specific surface and diversified structural and optical properties. It can be produced by electrochemical etching in an anodization cell in which the silicon wafer is taken as anode [1] or by simple immersion in HF/HNO3 called stain etching method [2–5]. The main advantages of stain etching method, if compared with electrochemical one, are its simplicity and its capability to produce large area porous silicon layers. Among the attractive applications of stain etched porous silicon its exploitation in the field of silicon solar cells treatment and fabrication. It can be used like a sacrificial layer for the gettering of the undesirable impurities present in solar grade silicon [6,7]. Its use as antireflection layer was also reported by several groups [8–12]. But major problems of the stain
∗ Corresponding author at: Laboratoire de Photovoltaïque, Centre de Recherche et des Technologies de l’Energie, Technopôle de Borj-Cédria BP 95, Hammam-Lif 2050, Tunisia. Tel.: +216 97 516880; fax: +216 71 430934. E-mail address: [email protected] (M. Hajji). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.07.101
etching method, carried out using a HF/HNO3 mixture, are the inhomogeneous etching and limited thicknesses of porous layers. To overcome those problems a modified stain etching (SE) method called multi-step stain etching (MS-SE) was developed. In this method silicon wafers are subjected to a succession of etching steps in a mixture HF/HNO3 instead of the one step etching used in the conventional stain etching method. In this paper we report the elaboration of PSi on different silicon wafers using MS-SE method, as well as the study of its structural and optical properties. The surface morphology and chemical composition of PSi layers were investigated by atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FTIR), while optical properties were studied by photoluminescence (PL) and UV–vis reflectance measurements. 2. Experimental setup The stain films were produced by immersion of Si substrates in a HF:HNO3 :H2 O solution with ratios of 1:3:5 by volume. Reagents used were standard electronic-grade 40% HF and 64% HNO3 , and the water was deionized. The elaboration of porous silicon by the MS-SE method is done in two stages. In the first stage all samples
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Table 1 Initiation etching times for different silicon wafers. Substrate type
p-type silicon
p+ -type silicon
n+ -type silicon
Resistivity ( cm) t0 (s)
1–10 300–360
<0.02 180–240
0.005–0.02 30–40
were etched at the same time during an initiation time (t0 ). 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. It was found that this initiation time strongly depends on doping level and type of silicon wafers. Table 1 recapitulates the values of initiation time (t0 ) for all silicon wafers used in this work. The homogeneity of the layer obtained in this stage is crucial for next steps of the process. In the second stage samples are subjected to a number of supplementary etching steps that differs from a sample to another. All etching steps are performed using the same etching solution. Between two successive steps all samples are rinsed in deionized water. Porous silicon elaboration by MS-SE for a fixed composition of the etching solution is controlled by two important parameters; (1) the initiation time (t0 ), (2) the supplementary steps duration (t). Different silicon substrates with different doping types (n or p) and different surface states (polished or rough surface) for the same doping level and type were used in this study. Table 2 summarizes the experimental conditions used to elaborate all series of samples prepared for this study. Samples are abundantly rinsed in deionized water and dried at ambient temperature under nitrogen flow before characterizations. The images of surface topography of fabricated porous layers were taken by a Nanoscope III atomic force microscope (AFM) from Digital Instruments Inc. which was working in tapping mode. Information about the different species present at the surface has been obtained from FTIR measurements. PL measurements were performed at room temperature under 488 nm line of an Argon ion laser as excitation source. The signal was detected through a Jobin-Yvon monochromator and by GaAs photomultiplier associated with a standard lock-in technique. Reflectance measurements were taken by means of Perkin–Elmer Lambda 950 UV–vis spectrometer.
Fig. 1. AFM images of a PSi layer prepared by SE method during 720 s (a) and a second layer prepared by MS-SE method during an initialization time of 360 s followed by 6 steps of 60 s (b).
3. Results and discussion Morphological analysis is carried out using AFM analysis in order to compare the structural quality of the PSi layers prepared by conventional SE and MS-SE methods and to study the evolution of surface morphology of PSi layers after each step of MS-SE method. PSi layers for this analysis were elaborated on p-type electronic grade silicon substrates. A series of samples (Series p-Si360-60) contains 8 samples elaborated under different conditions was prepared. The initiation time for this series was taken equal to 360 s and the duration of each supplementary step was fixed to 60 s. The obtained images were treated by means of the WSxM software [13]. Fig. 1a shows an AFM image of a PSi thin film prepared by conventional SE during 720 s. Fig. 1b is taken for porous layer prepared by MS-SE method during the same etching time (720 s), but in this case instead of one step etching the sample was subjected to 360 s
of etching (initiation time) and 6 etching steps of one minute each one. These images show that the sample prepared by SE method is composed with large crystallites compared to the one elaborated by MS-SE method. Fig. 2 displays the profiles of the sample prepared in one step (fine line) and the sample prepared by MS-SE method (thick line). Results show that the surface of the PSi layer elaborated in single step (720 s) reveals an enormous roughness of the sample with abrupt structures of some tens of nm. On the other hand the layer elaborated with MS-SE presents a much smoother surface. The etching reaction yielding the formation of chemically etched porous silicon has been already satisfactorily explained as 100 80
Table 2 Experimental conditions used for the preparation of porous silicon by MS-SE.
Z[nm]
60 40 20
n+ -Type silicon
Substrate type
p-Type silicon
Series Initialization time, t0 (s) Step duration, t (s)
p-Si360-60 360
p-Si300-30 300
n+ -Si40-10 40
n+ -Si40-05 40
60
30
10
05
0 0
0.5
1
1.5
2
2.5
3
3.5
4
X[µm]
Fig. 2. Profiles of AFM images of a PSi layer prepared by SE method during 720 s (fine line) and a second layer prepared by MS-SE method during an initialization time of 360 s followed by 6 steps of 60 s (thick line).
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Fig. 3. AFM images of a PSi layer prepared by MS-SE method during the same initialization time of 360 s (a) followed by a step (b), 2 steps (c), 3 steps (d), 4 steps (e) and 5 steps (f).
a localized electrochemical process in which anode and cathode randomly switch from one site to another of the silicon surface [14,15]. The pore formation starts by a preferential etching at some anodic sites, which are randomly distributed, on the silicon surface. During etching bubbles were continuously formed on the surface and for long etching duration large bubbles cover some regions and stop the etch in those regions which results in an inhomogeneous porous layer with a large gains size distribution. This behavior was observed for the sample prepared by SE method for 720 s (Fig. 1a). This problem does not occur in the MS-SE
method since the sample is subjected to a series of short etching steps. During a short etching step only very small bubbles were observed on the surface. Those bubbles were removed by rinsing in deionized water between two successive etching steps. This suggestion is confirmed by the low surface roughness (2.15 nm) of the sample prepared by MS-SE method compared to that of the PSi layer prepared by the conventional SE method (11.4 nm). Fig. 3 presents the evolution of PSi morphology with the etching steps for six samples prepared by MS-SE method. Images from Fig. 3a–f correspond respectively to samples that were subjected
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0,07
(c)
0,06
632 cm-1 864 cm-1 1064 cm-1 2100
0,06
intensity at the peak
Absorbance
0,05 0,04
(b)
0,02
(a)
0,04
0,03
0,02
0,00
0,01 400
600
800
1000
1200
1400
1600
1800
2000
2200
0,00
-1
wavenumber (cm )
-1
0
1
2
3
4
5
6
7
8
the number of steps
to a number of etching steps of 60 s going from 0 to 5. With each image is indicated the corresponding rms of surface roughness. It is clear from results that the roughness of samples prepared by MSSE varies from a sample to another depending on the number of etching steps but it remains even smoother than the sample prepared by conventional SE method (rms = 11.95 nm) (Fig. 1a). This smooth surface is very attractive for silicon solar cells application where a metallic contact is required on the top surface of the layer [10,11]. Homogeneity and smooth surface is also a great requirement for impurities gettering since it allows a uniform diffusion of impurities from silicon bulk to the porous surface. In order to study the effect of MS-SE conditions on the chemical composition of silicon surface, FTIR spectra were recorded. Typical FTIR spectra are shown in Fig. 4 for three different layers, grown on samples cut from the same wafer, the first layer (Fig. 4a) etched for 360 s (initiation time), the second layer (Fig. 4b) after the 360 s etching was subjected to additional etching steps (6 steps) of 60 s each one and the last one (Fig. 4c) prepared by conventional SE method during the same total etching time as the second layer. Observed FTIR bands are located at around 2050–2200 cm−1 (stretching modes), 800–1000 cm−1 (bending modes) and 600–750 cm−l (wagging modes) associated with Si Hn (n ≥ 1) bondings and finally the band 1000–1300 cm−l corresponds to the stretching modes of the Si O Si bonds in the SiOx . The peak at 1065 cm−1 corresponds to the stretching modes of the Si O Si bonds in the SiOx and the peak at 1140 cm−1 is generated by the asymmetrical stretching of Si O Si bonds in stoichiometric SiO2 [3,11,16]. Fig. 4 shows a clear increase in the intensities of all bands by increasing the number of etching steps. The figure shows also a comparison between samples prepared by SE (Fig. 4c) and MS-SE (Fig. 4b) methods. These two spectra present the same absorption bands with a slight increase in the intensities (the peak height) at the peak of the bands for the sample elaborated by MS-SE method. This result can be probably attributed to an improvement of the thickness of the layer prepared by MS-SE method compared to that prepared by SE method. In Fig. 5 is presented the evolution of the intensity at the peak of different observed bands versus the number of etching steps used to prepare MS-SE porous layers. Results show that the intensities of the peaks increase progressively with the number of etching steps. This increase of intensities indicates a continuous enhancement of the specific surface of PSi layer during successive etching steps. This result is an indication of the continuity of chemical etching during the successive etching steps to which samples were subjected.
Fig. 5. Evolution of the intensity at the peak of different FTIR bands versus the number of etching steps.
The photoluminescence (PL) measurements were carried out in ambient air. Degradation in the PL was observed under laser irradiation which results in a decrease of the PL intensity without a shift of the peak, suggesting probably that the intrinsic properties are not disturbed due to local heating. Moreover 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 [17]. Fig. 6 shows the PL spectra from samples prepared by SE and MS-SE methods. The spectrum of the PSi prepared by single step SE method during 720 s is plotted by a dashed line. The spectrum of the sample subjected to multistep chemical etching process during the same total duration, consisting of 360 s initiation time followed by six etching steps of 60 s, is plotted by continues line. This figure shows that the sample elaborated by conventional SE presents a weaker PL intensity and a less broad PL band without a significant shift of the peak position of the PL emission band if compared to the layer produced using MS-SE method. This result suggests that those two samples are composed of luminescent silicon nanocrystallites with comparable average size but with different size distribution [18]. In order to study the effect of MS-SE method parameters (t0 and t), on the luminescence properties of chemical etched PSi, two 0,030
(b)
0,025
PL intensity (a. u.)
Fig. 4. Typical FTIR spectra of a PSi layers prepared by MS-SE method (a) during 360 s without additional steps and (b) with 6 additional steps of 60 s and (c) prepared by SE method during the same total etching time than the second sample.
SE 720 s MS-SE 360 s + 6*60s
0,020
(a) 0,015
0,010
0,005
0,000 1,6
1,8
2,0
2,2
2,4
E(eV) Fig. 6. PL spectra of PSi layers prepared by SE method during 720 s (a) and MS-SE method during the same duration (b).
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Table 3 Summary of the photoluminescence measurements in PSi layers grown by MS-SE for the series p-Si360-60. Sample
Etching time (s)
peak (nm)
FWHM (meV)
Imax (a.u.)
Eg (eV)
p-Si360-60-0 p-Si360-60-2 p-Si360-60-4 p-Si360-60-6 p-Si720 (SE)
360 480 600 720 720
627.2 624.7 625.0 633.1 627.1
277 299 295 288 299
0.048 0.024 0.033 0.026 0.021
1.97 1.98 1.98 1.95 1.97
70
60
50
c-Si R(%)
40
30
6
3
0 20
0 step (Δ t0 = 300s) 1 step of 30s 2 steps of 30s 3 steps of 30s
1
0.05
2
PL intensity (au)
0.04
10
0 400
500
600
700
800
900
1000
Wavelength (nm)
0.03
3 0.02
Fig. 8. Reflectance spectra of PSi layers prepared by MS-SE during 360 s (0) followed by three steps (3) and six steps (6) of 60 s.
0
0.01
0.00 1.6
1.8
2.0
2.2
2.4
E (eV) Fig. 7. Evolution of the PL band with the number of etching steps for the four first samples of the series p-Si300-30.
series of samples were prepared. The most important results of the PL measurements for the first series (p-Si360-60) are summarized in Table 3. As it is shown in this table, the emission band is located around 630 nm (1.97 eV) with approximately 300 meV full width at height middle (FWHM). For this series a slight red or blue shift of the peak position occurs depending on the number of etching steps. In this case the successive etching steps did not strongly affect the energy at the peak and FWHM of the PL band, but a clear variation of the PL intensity was observed. This result indicates that all samples are composed with nanocrystallites with comparable sizes but their densities vary from a sample to another. This behavior is due to continuous formation and dissolution of silicon nanocrystallites during chemical etching [19]. This result is in agreement with which was observed by AFM analyses in Fig. 3. With an aim of better understanding the dissolution mechanism, a second series of samples was prepared in which the two parameters t0 and t are reduced respectively from 360 to 300 s and from 60 to 30 s. Fig. 7 shows the PL bands for the first four samples of this series and Table 4 summarize the main results for this series of samples. Fig. 7 shows the evolution of the PL band with increasing etching steps. In this figure each curve is indicated by a number that gives the number of etching steps to which the sample was subjected. The curve indicated by 0 corresponds to the PL Table 4 Summary of the photoluminescence measurements in porous silicon layers grown by MS-SE for the series p-Si300-30. Sample
Etching time (s)
peak (nm)
FWHM (meV)
Imax (a.u.)
Eg (eV)
p-Si300-30-0 p-Si300-30-2 p-Si300-30-4 p-Si300-30-6 p-Si300-30-8
300 360 420 480 540
654.3 625.8 625.3 635.4 630.2
218 286 338 302 316
0.018 0.041 0.019 0.026 0.022
1.89 1.98 1.98 1.95 1.97
band of a sample etched during 300 s (initiation time for this series) which is located at an energy of 1.89 eV (654 nm) with a FWHM of 218 meV. After the first supplementary etching step of 30 s (curve 1) the PL intensity is increased by about three times. This increase of intensity is accompanied by a significant blueshift from 1.89 to 1.95 eV and a broadening of the PL band (from 218 meV of FWHM to 262 meV) indicating a reduction in the size and an enlargement of the size distribution of luminescent silicon nanocrystallites. The second etching step (curve 2) leads to a slight additional blueshift from 1.95 to 1.98 eV and a reduction of about 20% of the PL intensity. Since for a given wavelength the PL intensity is proportional to the crystallites number [20] so the crystallites density is reduced during this step due to the continuous formation and dissolution of nanocrystallites during chemical etching of silicon. After the third step (curve 3) the PL intensity is reduced by 50% compared to the sample prepared by one step (curve 1) indicating a supplementary reduction of the crystallites density due to the lateral dissolution of silicon crystallites. On the other hand the PL peak is still located at the same energy (1.98 eV) and it does not exceeds this limiting value after further steps (Tables 3 and 4) indicating that in stain etching process silicon nanocrystallites reach a limiting size. This result is in agreement with that found by Di Francia [19]. Fig. 8 shows the reflectance spectra of the porous silicon samples prepared by MS-SE method with different steps number on p-type silicon substrate (series p-Si360-60). Each spectrum is indicated by a number which gives the number of supplementary steps to which the substrate was subjected. For the clarity of the figure only four spectra were presented. The intensity of the reflectance was significantly lowered after porous layer formation. The reflectance spectra for samples with porous layers present interference oscillations reflecting the homogeneity of porous layers. The presence of those oscillations allows the determination of the optical thickness of the layer. Extracted optical thicknesses for all samples of this series are presented in Fig. 9 as a function of the wavelength. This figure shows that the optical thickness continuously increases with the number of etching steps. In Fig. 10 are presented the evolution of the optical thickness at a wavelength of 450 nm and the SiOx absorption band intensity versus the number of etching steps. From this figure it is clear that both the optical thickness and SiOx band intensity increase when increasing the number of etching steps. This behavior indicates the continue dissolution of silicon and the improvement of the thickness of the porous silicon layer during different etching steps used in MS-SE method. This property makes
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70
450 nm
600
60
50
7 6
500
reflectance (%)
Optical thickness (nm)
550
5 450
4 3
400
0 step (a)
30
20
2
350
c-Si 40
2 steps (b) 4 steps (c)
10
1 300
6 steps (d)
0 0
300
400
500
600
700
800
900
400
1000
500
600
700
800
900
1000
wavelenght (nm)
wavelength (nm) Fig. 9. Optical thickness as a function of the wavelength and the number of etching steps for PSi layers of the series p-Si360-60.
the MS-SE a very interesting method for the elaboration of porous silicon layers with controllable thicknesses and optical properties. One of the important applications of porous silicon is its use as an antireflection coating (ARC) for silicon solar cells. The porous silicon layer is generally elaborated on the n+ emitter of a p–n+ junction. In order to optimize the conditions which give a PSi layer with appropriate properties for efficient ARC, a set of samples was prepared on n+ -type silicon substrates. In this optimization both the number and the duration (t) of the supplementary etching steps were varied (Table 2). The effect of surface roughness was also studied using two substrates of different surface roughnesses. It was found that the reflectance of silicon substrates after porous silicon formation depends strongly on the elaboration conditions and the surface roughness of the starting material. In Fig. 11 are presented the total reflectance spectra of PSi samples prepared on the polished face of a silicon substrate with a surface roughness of 1 nm (series n+ -Si40-10). Results show that the surface reflectivity can be continuously reduced during successive etching steps and it reaches its minimum after the sixth step. This reflectance reduction is due to the continuous dissolution of silicon during successive etching steps which transform the substrate surface from a flat surface to a porous one that enhance the light absorption. As the number of etching steps increases the formed pores propagate laterally and in-depth leading to an enhancement of the specific
Fig. 11. Reflectance spectra of PSi layers prepared by MS-SE on a polished heavily doped n-type silicon substrate during 40 s followed by different numbers of supplementary etching steps of 10 s.
surface and then what results in a reduction in the reflectivity. But this phenomenon is limited by the lateral propagation of pores that produce the dissolution of the formed PSi nanocrystallites resulting in a reduction of the specific surface what explains the increase of the reflectivity after the sixth step. On the other hand, for PSi layers prepared, under same conditions, on a substrate having the same doping type and level but with a surface roughness of 12 nm the reflectivity reaches minimum values since the first step (Fig. 12a). The values reached, after this first step, are much weaker than those reached after the sixth one with the smoother substrate (Fig. 11). This behavior is due to a difference in the etching process related to the contact surface between the substrate and the etching solution. It is well known that rough substrates have generally a greater specific surface than the smooth ones, thus a greater contact surface with the solution leading to amplification in the density of nucleation centers, from which starts the silicon dissolution, it results an increase in the etching rate and an improvement in the nanostructure of the produced layer. For better control of the surface reflectance evolution of PSi layers, produced on the rough substrates, the duration of the
45
(a) 1 step of 10s (RS) (b) 2 steps of 10s (RS)
35
(c) 1 step of 5s (RS)
0,07
550
(d) 3 steps of 5s (RS)
500
450 0,04 400
0,03 0,02
350
SiOx peak intensity
0,05
Reflectance (%)
0,06
Optical thickness (nm)
40
30
0
1
2
20
(d)
15
(e)
3
4
5
6
7
5
0,00
0
8
Fig. 10. Evolution of the optical thickness at 450 nm and the intensity at the peak of the SiOx band with the number of etching steps for the PSi layers of the series p-Si360-60.
(c)
10
0,01
steps number
(f) 4 steps of 5s (SS) (f)
300 -1
(e) 4 steps of 5s (RS)
25
(b) (a) 400
600
800
1000
1200
wavelength (nm) Fig. 12. Reflectance spectra of PSi layers prepared by MS-SE on heavily doped n-type silicon substrates with different surface roughness during 40 s followed by different number and durations of supplementary etching steps.
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supplementary steps was reduced from 10 to 5 s (series n+ -Si4005 Table 2). Fig. 12 shows the most important results obtained for series elaborated on the rough substrate (RS). For the first series, with supplementary etching steps of 10 s, the reflectance is highly reduced since the first step but increases after the further steps. However for the second series, for which the time of supplementary etching steps is reduced to 5 s, it was found that the reflectance of PSi layers can be reduced step by step and it reaches values less than 3% in the major part of the spectrum after four steps (Fig. 12). To clearly show the effect of the roughness in the same figure is also presented the reflectance spectrum for a porous layer obtained after four etching steps of 5 seconds on the smooth substrate (SS). Finally, it is clearly seen from the reflectance characteristic of the PS layers formed by MS-SE method that such layers can be used as an effective ARC for silicon solar cells. 4. Conclusion A new method to produce porous silicon was presented. In this method instead of one etching step that leads to formation of porous layer, the substrate is subjected to an initial etching step with a duration t0 followed by a number of supplementary short steps that differs from a layer to another. It is found that this duration is largely dependent of the doping type and level of the silicon substrate. Surface investigation by AFM shows that MS-SE method allows the production of very homogeneous porous silicon nanostructures compared to the conventional SE method. This homogeneity is very interesting for many applications such as gettering in solar grade silicon or antireflection coating in silicon solar cells. The chemical composition of the obtained porous layers has been evaluated using Fourier transform infrared spectroscopy (FTIR). It is found that the intensities of different absorption bands increase as the number of supplementary etching steps increase. Photoluminescence (PL) measurement shows that porous layers produced by SE and MS-SE methods have comparable spectra indicating that those layers are composed of nanocrystallites with comparable sizes. But the intensity of photoluminescence of layer elaborated by MS-SE method is higher than that elaborated by the SE method. Total reflectance characteristics show that the presented method allows the production of porous silicon layers with controllable thicknesses and optical properties. Results for porous silicon layers elaborated on heavily doped n-type silicon show that the reflectance can be reduced to values less than 3% in the major part of the spectrum.
References [1] R.L. Smith, S.F. Chuang, S.D. Collins, Porous silicon morphologies and formation mechanism, Sensors and Actuators A 23 (1990) 825–829. [2] S. Shih, K.H. Jung, T.Y. Hsieh, J. Sarathy, J.C. Campbell, D.L. Kwong, Photoluminescence and formation mechanism of chemically etched silicon, Applied Physics Letters 60 (15) (1992) 1863–1865. [3] S¸. Kalem, D. Göbelek, R. Kurtar, Z. Misirli, A. Aydinli, R. Ellialtio˘glu, The effects of surface treatment on optical and vibrational properties of stain-etched silicon, Nanostructured Materials 6 (5–8) (1995) 847–850. [4] E.V. Astrova, S.V. Belov, A.A. Lebedev, A.D. Remenjuk, Yu Remenjuk, V. Rud, Optical and electrical properties of porous silicon and stain-etched films, Thin Solid Films 255 (1995) 196–199. [5] É. Vázsonyi, E. Szilágyi, P. Petrik, Z.E. Horváth, T. Lohner, M. Fried, G. Jalsovszky, Porous silicon formation by stain etching, Thin Solid Films 388 (2001) 295–302. [6] M. Hajji, M. Hassen, H. Ezzaouia, A. Selmi, H. Bouchriha, Electrical properties of purified solar grade silicon substrates using a combination of porous silicon and SiCl4 , Applied Surface Science 253 (2007) 5341–5344. [7] N. Khedher, M. Hajji, M. Bouaïcha, M.F. Boujmil, H. Ezzaouia, B. Bessaïs, R. Bennaceur, Improvement of transport parameters in solar grade monocrystalline silicon by application of a sacrificial porous silicon layer, Solid State Communications 123 (2002) 7–10. [8] M. Rajabi, R.S. Dariani, Current improvement of porous silicon photovoltaic devices by using double layer porous silicon structure: applicable in porous silicon solar cells, Journal of Porous Materials 16 (2009) 513–519. [9] R.J. Mart´n-Palma, L. Vázquez, P. Herroro, J.M. Mart´nez-Duart, M. Schnell, S. Schaefer, Morphological, optical and electrical characterization of antireflective porous silicon coatings for solar cells, Optical Materials 17 (2001) 75–78. [10] B. González-D´az, R. Guerrero-Lemus, D. Borchert, C. Hernandez-Rodrıguez, J.M. Mart´nez-Duart, Low-porosity porous silicon nanostructures on monocrystalline silicon solar cells, Physica E 38 (2007) 215–218. [11] B. González-D´az, R. Guerrero-Lemus, J. Méndez-Ramos, B. D´az-Herrera, V.D. Rodr´guez, Gradual oxidation of stain etched porous silicon nanostructures applied to silicon-based solar cells, Sensors and Actuators A 150 (2009) 97–101. [12] N. Marrero, R. Guerrero-Lemus, B. González-Díaz, D. Borchert, Effect of porous silicon stain etched on large area alkaline textured crystalline silicon solar cells, Thin Solid Films 517 (2009) 2648–2650. [13] I. Horcas, R. Fernandez, J.M. Gomez-Rodriguez, J. Colchero, J. Gomez-Herrero, A.M. Baro, W.S.X.M: a software for scanning probe microscopy and a tool for nanotechnology, Review of Scientific Instruments 78 (2007) 13705–13712. [14] D.R. Turner, On the mechanism of chemically etching germanium and silicon, Journal of The Electrochemical Society 107 (1960) 810–816. [15] M.I.J. Beale, J.D. Benjamin, M.J. Uren, N.G. Chew, A.G. Cullis, The formation of porous silicon by chemical stain etches, Journal of Crystal Growth 75 (1986) 408–414. [16] P. Lange, U. Schnakenberg, S. Ullerich, H.-J. Schliwinski, Disorder in vitreous SiO2 : the effect of thermal annealing on structural properties, Journal of Applied Physics 68 (1990) 3532–3538. [17] M.V. Wolkin, J. Jorne, P.M. Fauchet, G. Allan, C. Delerue, Electronic states and luminescence in porous silicon quantum dots: the role of oxygen, Physical Review Letters 82 (1999) 197–200. [18] B. Huy, P.H. Binh, B.Q. Diep, P.V. Luong, Effect of ageing on the luminescence intensity and lifetime of porous silicon: roles of recombination centers, Physica E 17 (2003) 134–136. [19] G. Di Francia, P. Maddalena, D. Ninno, Formation of luminescent chemically etched porous silicon, Solid State Communications 96 (8) (1995) 579–581. [20] H. Yorikawa, S. Muramatsu, Photoluminescence and particle size distribution in porous silicon, Journal of Luminescence 87-89 (2000) 423–425.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Homogeneous luminescent stain etched porous silicon elaborated by a new multi-step stain etching method M. Hajji a,b,∗ , M. Khalifa a , S. Ben Slama a , H. Ezzaouia a a b
Laboratoire de Photovoltaïque, Centre de Recherche et des Technologies de l’Energie, Technopôle de Borj-Cédria BP 95, Hammam-Lif 2050, Tunisia Institut Supérieur d’Electronique et de Communication de Sfax, route Menzel Chaker Km 0.5, BP 868, Sfax 3018, Tunisia
a r t i c l e
i n f o
Article history: Received 17 November 2012 Received in revised form 17 July 2013 Accepted 20 July 2013 Available online 27 July 2013 Keywords: Stain etching Porous silicon Photoluminescence Optical reflectivity Antireflection coating
a b s t r a c t This paper presents a new method to produce porous silicon which derived from the conventional stain etching (SE) method. But instead of one etching step that leads to formation of porous layer, the substrate is subjected to an initial etching step with a duration t0 followed by a number of supplementary short steps that differs from a layer to another. The duration of the initial step is just the necessary time to have a homogenous porous layer on the whole surface of the substrate. It was found that this duration is largely dependent of the doping type and level of the silicon substrate. The duration of supplementary steps was kept as short as possible to prevent the formation of bubbles on the silicon surface during silicon dissolution which leads generally to inhomogeneous porous layers. It is found from surface investigation by atomic force microscopy (AFM) that multistep stain etching (MS-SE) method allows to produce homogeneous porous silicon nanostructures compared to the conventional SE method. The chemical composition of the obtained porous layers has been evaluated using Fourier transform infrared spectroscopy (FTIR). Photoluminescence (PL) measurement shows that porous layers produced by SE and MS-SE methods have comparable spectra indicating that those layers are composed of nanocrystallites with comparable sizes. But the intensity of photoluminescence of layer elaborated by MS-SE method is higher than that elaborated by the SE method. Total reflectance characteristics show that the presented method allows the production of porous silicon layers with controllable thicknesses and optical properties. Results for porous silicon layers elaborated on heavily doped n-type silicon show that the reflectance can be reduced to values less than 3% in the major part of the spectrum. © 2013 Elsevier B.V. All rights reserved.
1. Introduction Porous silicon (PSi) is a sponge-like materiel having a large specific surface and diversified structural and optical properties. It can be produced by electrochemical etching in an anodization cell in which the silicon wafer is taken as anode [1] or by simple immersion in HF/HNO3 called stain etching method [2–5]. The main advantages of stain etching method, if compared with electrochemical one, are its simplicity and its capability to produce large area porous silicon layers. Among the attractive applications of stain etched porous silicon its exploitation in the field of silicon solar cells treatment and fabrication. It can be used like a sacrificial layer for the gettering of the undesirable impurities present in solar grade silicon [6,7]. Its use as antireflection layer was also reported by several groups [8–12]. But major problems of the stain
∗ Corresponding author at: Laboratoire de Photovoltaïque, Centre de Recherche et des Technologies de l’Energie, Technopôle de Borj-Cédria BP 95, Hammam-Lif 2050, Tunisia. Tel.: +216 97 516880; fax: +216 71 430934. E-mail address: [email protected] (M. Hajji). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.07.101
etching method, carried out using a HF/HNO3 mixture, are the inhomogeneous etching and limited thicknesses of porous layers. To overcome those problems a modified stain etching (SE) method called multi-step stain etching (MS-SE) was developed. In this method silicon wafers are subjected to a succession of etching steps in a mixture HF/HNO3 instead of the one step etching used in the conventional stain etching method. In this paper we report the elaboration of PSi on different silicon wafers using MS-SE method, as well as the study of its structural and optical properties. The surface morphology and chemical composition of PSi layers were investigated by atomic force microscopy (AFM) and Fourier transform infrared spectroscopy (FTIR), while optical properties were studied by photoluminescence (PL) and UV–vis reflectance measurements. 2. Experimental setup The stain films were produced by immersion of Si substrates in a HF:HNO3 :H2 O solution with ratios of 1:3:5 by volume. Reagents used were standard electronic-grade 40% HF and 64% HNO3 , and the water was deionized. The elaboration of porous silicon by the MS-SE method is done in two stages. In the first stage all samples
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Table 1 Initiation etching times for different silicon wafers. Substrate type
p-type silicon
p+ -type silicon
n+ -type silicon
Resistivity ( cm) t0 (s)
1–10 300–360
<0.02 180–240
0.005–0.02 30–40
were etched at the same time during an initiation time (t0 ). 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. It was found that this initiation time strongly depends on doping level and type of silicon wafers. Table 1 recapitulates the values of initiation time (t0 ) for all silicon wafers used in this work. The homogeneity of the layer obtained in this stage is crucial for next steps of the process. In the second stage samples are subjected to a number of supplementary etching steps that differs from a sample to another. All etching steps are performed using the same etching solution. Between two successive steps all samples are rinsed in deionized water. Porous silicon elaboration by MS-SE for a fixed composition of the etching solution is controlled by two important parameters; (1) the initiation time (t0 ), (2) the supplementary steps duration (t). Different silicon substrates with different doping types (n or p) and different surface states (polished or rough surface) for the same doping level and type were used in this study. Table 2 summarizes the experimental conditions used to elaborate all series of samples prepared for this study. Samples are abundantly rinsed in deionized water and dried at ambient temperature under nitrogen flow before characterizations. The images of surface topography of fabricated porous layers were taken by a Nanoscope III atomic force microscope (AFM) from Digital Instruments Inc. which was working in tapping mode. Information about the different species present at the surface has been obtained from FTIR measurements. PL measurements were performed at room temperature under 488 nm line of an Argon ion laser as excitation source. The signal was detected through a Jobin-Yvon monochromator and by GaAs photomultiplier associated with a standard lock-in technique. Reflectance measurements were taken by means of Perkin–Elmer Lambda 950 UV–vis spectrometer.
Fig. 1. AFM images of a PSi layer prepared by SE method during 720 s (a) and a second layer prepared by MS-SE method during an initialization time of 360 s followed by 6 steps of 60 s (b).
3. Results and discussion Morphological analysis is carried out using AFM analysis in order to compare the structural quality of the PSi layers prepared by conventional SE and MS-SE methods and to study the evolution of surface morphology of PSi layers after each step of MS-SE method. PSi layers for this analysis were elaborated on p-type electronic grade silicon substrates. A series of samples (Series p-Si360-60) contains 8 samples elaborated under different conditions was prepared. The initiation time for this series was taken equal to 360 s and the duration of each supplementary step was fixed to 60 s. The obtained images were treated by means of the WSxM software [13]. Fig. 1a shows an AFM image of a PSi thin film prepared by conventional SE during 720 s. Fig. 1b is taken for porous layer prepared by MS-SE method during the same etching time (720 s), but in this case instead of one step etching the sample was subjected to 360 s
of etching (initiation time) and 6 etching steps of one minute each one. These images show that the sample prepared by SE method is composed with large crystallites compared to the one elaborated by MS-SE method. Fig. 2 displays the profiles of the sample prepared in one step (fine line) and the sample prepared by MS-SE method (thick line). Results show that the surface of the PSi layer elaborated in single step (720 s) reveals an enormous roughness of the sample with abrupt structures of some tens of nm. On the other hand the layer elaborated with MS-SE presents a much smoother surface. The etching reaction yielding the formation of chemically etched porous silicon has been already satisfactorily explained as 100 80
Table 2 Experimental conditions used for the preparation of porous silicon by MS-SE.
Z[nm]
60 40 20
n+ -Type silicon
Substrate type
p-Type silicon
Series Initialization time, t0 (s) Step duration, t (s)
p-Si360-60 360
p-Si300-30 300
n+ -Si40-10 40
n+ -Si40-05 40
60
30
10
05
0 0
0.5
1
1.5
2
2.5
3
3.5
4
X[µm]
Fig. 2. Profiles of AFM images of a PSi layer prepared by SE method during 720 s (fine line) and a second layer prepared by MS-SE method during an initialization time of 360 s followed by 6 steps of 60 s (thick line).
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Fig. 3. AFM images of a PSi layer prepared by MS-SE method during the same initialization time of 360 s (a) followed by a step (b), 2 steps (c), 3 steps (d), 4 steps (e) and 5 steps (f).
a localized electrochemical process in which anode and cathode randomly switch from one site to another of the silicon surface [14,15]. The pore formation starts by a preferential etching at some anodic sites, which are randomly distributed, on the silicon surface. During etching bubbles were continuously formed on the surface and for long etching duration large bubbles cover some regions and stop the etch in those regions which results in an inhomogeneous porous layer with a large gains size distribution. This behavior was observed for the sample prepared by SE method for 720 s (Fig. 1a). This problem does not occur in the MS-SE
method since the sample is subjected to a series of short etching steps. During a short etching step only very small bubbles were observed on the surface. Those bubbles were removed by rinsing in deionized water between two successive etching steps. This suggestion is confirmed by the low surface roughness (2.15 nm) of the sample prepared by MS-SE method compared to that of the PSi layer prepared by the conventional SE method (11.4 nm). Fig. 3 presents the evolution of PSi morphology with the etching steps for six samples prepared by MS-SE method. Images from Fig. 3a–f correspond respectively to samples that were subjected
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0,07
(c)
0,06
632 cm-1 864 cm-1 1064 cm-1 2100
0,06
intensity at the peak
Absorbance
0,05 0,04
(b)
0,02
(a)
0,04
0,03
0,02
0,00
0,01 400
600
800
1000
1200
1400
1600
1800
2000
2200
0,00
-1
wavenumber (cm )
-1
0
1
2
3
4
5
6
7
8
the number of steps
to a number of etching steps of 60 s going from 0 to 5. With each image is indicated the corresponding rms of surface roughness. It is clear from results that the roughness of samples prepared by MSSE varies from a sample to another depending on the number of etching steps but it remains even smoother than the sample prepared by conventional SE method (rms = 11.95 nm) (Fig. 1a). This smooth surface is very attractive for silicon solar cells application where a metallic contact is required on the top surface of the layer [10,11]. Homogeneity and smooth surface is also a great requirement for impurities gettering since it allows a uniform diffusion of impurities from silicon bulk to the porous surface. In order to study the effect of MS-SE conditions on the chemical composition of silicon surface, FTIR spectra were recorded. Typical FTIR spectra are shown in Fig. 4 for three different layers, grown on samples cut from the same wafer, the first layer (Fig. 4a) etched for 360 s (initiation time), the second layer (Fig. 4b) after the 360 s etching was subjected to additional etching steps (6 steps) of 60 s each one and the last one (Fig. 4c) prepared by conventional SE method during the same total etching time as the second layer. Observed FTIR bands are located at around 2050–2200 cm−1 (stretching modes), 800–1000 cm−1 (bending modes) and 600–750 cm−l (wagging modes) associated with Si Hn (n ≥ 1) bondings and finally the band 1000–1300 cm−l corresponds to the stretching modes of the Si O Si bonds in the SiOx . The peak at 1065 cm−1 corresponds to the stretching modes of the Si O Si bonds in the SiOx and the peak at 1140 cm−1 is generated by the asymmetrical stretching of Si O Si bonds in stoichiometric SiO2 [3,11,16]. Fig. 4 shows a clear increase in the intensities of all bands by increasing the number of etching steps. The figure shows also a comparison between samples prepared by SE (Fig. 4c) and MS-SE (Fig. 4b) methods. These two spectra present the same absorption bands with a slight increase in the intensities (the peak height) at the peak of the bands for the sample elaborated by MS-SE method. This result can be probably attributed to an improvement of the thickness of the layer prepared by MS-SE method compared to that prepared by SE method. In Fig. 5 is presented the evolution of the intensity at the peak of different observed bands versus the number of etching steps used to prepare MS-SE porous layers. Results show that the intensities of the peaks increase progressively with the number of etching steps. This increase of intensities indicates a continuous enhancement of the specific surface of PSi layer during successive etching steps. This result is an indication of the continuity of chemical etching during the successive etching steps to which samples were subjected.
Fig. 5. Evolution of the intensity at the peak of different FTIR bands versus the number of etching steps.
The photoluminescence (PL) measurements were carried out in ambient air. Degradation in the PL was observed under laser irradiation which results in a decrease of the PL intensity without a shift of the peak, suggesting probably that the intrinsic properties are not disturbed due to local heating. Moreover 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 [17]. Fig. 6 shows the PL spectra from samples prepared by SE and MS-SE methods. The spectrum of the PSi prepared by single step SE method during 720 s is plotted by a dashed line. The spectrum of the sample subjected to multistep chemical etching process during the same total duration, consisting of 360 s initiation time followed by six etching steps of 60 s, is plotted by continues line. This figure shows that the sample elaborated by conventional SE presents a weaker PL intensity and a less broad PL band without a significant shift of the peak position of the PL emission band if compared to the layer produced using MS-SE method. This result suggests that those two samples are composed of luminescent silicon nanocrystallites with comparable average size but with different size distribution [18]. In order to study the effect of MS-SE method parameters (t0 and t), on the luminescence properties of chemical etched PSi, two 0,030
(b)
0,025
PL intensity (a. u.)
Fig. 4. Typical FTIR spectra of a PSi layers prepared by MS-SE method (a) during 360 s without additional steps and (b) with 6 additional steps of 60 s and (c) prepared by SE method during the same total etching time than the second sample.
SE 720 s MS-SE 360 s + 6*60s
0,020
(a) 0,015
0,010
0,005
0,000 1,6
1,8
2,0
2,2
2,4
E(eV) Fig. 6. PL spectra of PSi layers prepared by SE method during 720 s (a) and MS-SE method during the same duration (b).
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Table 3 Summary of the photoluminescence measurements in PSi layers grown by MS-SE for the series p-Si360-60. Sample
Etching time (s)
peak (nm)
FWHM (meV)
Imax (a.u.)
Eg (eV)
p-Si360-60-0 p-Si360-60-2 p-Si360-60-4 p-Si360-60-6 p-Si720 (SE)
360 480 600 720 720
627.2 624.7 625.0 633.1 627.1
277 299 295 288 299
0.048 0.024 0.033 0.026 0.021
1.97 1.98 1.98 1.95 1.97
70
60
50
c-Si R(%)
40
30
6
3
0 20
0 step (Δ t0 = 300s) 1 step of 30s 2 steps of 30s 3 steps of 30s
1
0.05
2
PL intensity (au)
0.04
10
0 400
500
600
700
800
900
1000
Wavelength (nm)
0.03
3 0.02
Fig. 8. Reflectance spectra of PSi layers prepared by MS-SE during 360 s (0) followed by three steps (3) and six steps (6) of 60 s.
0
0.01
0.00 1.6
1.8
2.0
2.2
2.4
E (eV) Fig. 7. Evolution of the PL band with the number of etching steps for the four first samples of the series p-Si300-30.
series of samples were prepared. The most important results of the PL measurements for the first series (p-Si360-60) are summarized in Table 3. As it is shown in this table, the emission band is located around 630 nm (1.97 eV) with approximately 300 meV full width at height middle (FWHM). For this series a slight red or blue shift of the peak position occurs depending on the number of etching steps. In this case the successive etching steps did not strongly affect the energy at the peak and FWHM of the PL band, but a clear variation of the PL intensity was observed. This result indicates that all samples are composed with nanocrystallites with comparable sizes but their densities vary from a sample to another. This behavior is due to continuous formation and dissolution of silicon nanocrystallites during chemical etching [19]. This result is in agreement with which was observed by AFM analyses in Fig. 3. With an aim of better understanding the dissolution mechanism, a second series of samples was prepared in which the two parameters t0 and t are reduced respectively from 360 to 300 s and from 60 to 30 s. Fig. 7 shows the PL bands for the first four samples of this series and Table 4 summarize the main results for this series of samples. Fig. 7 shows the evolution of the PL band with increasing etching steps. In this figure each curve is indicated by a number that gives the number of etching steps to which the sample was subjected. The curve indicated by 0 corresponds to the PL Table 4 Summary of the photoluminescence measurements in porous silicon layers grown by MS-SE for the series p-Si300-30. Sample
Etching time (s)
peak (nm)
FWHM (meV)
Imax (a.u.)
Eg (eV)
p-Si300-30-0 p-Si300-30-2 p-Si300-30-4 p-Si300-30-6 p-Si300-30-8
300 360 420 480 540
654.3 625.8 625.3 635.4 630.2
218 286 338 302 316
0.018 0.041 0.019 0.026 0.022
1.89 1.98 1.98 1.95 1.97
band of a sample etched during 300 s (initiation time for this series) which is located at an energy of 1.89 eV (654 nm) with a FWHM of 218 meV. After the first supplementary etching step of 30 s (curve 1) the PL intensity is increased by about three times. This increase of intensity is accompanied by a significant blueshift from 1.89 to 1.95 eV and a broadening of the PL band (from 218 meV of FWHM to 262 meV) indicating a reduction in the size and an enlargement of the size distribution of luminescent silicon nanocrystallites. The second etching step (curve 2) leads to a slight additional blueshift from 1.95 to 1.98 eV and a reduction of about 20% of the PL intensity. Since for a given wavelength the PL intensity is proportional to the crystallites number [20] so the crystallites density is reduced during this step due to the continuous formation and dissolution of nanocrystallites during chemical etching of silicon. After the third step (curve 3) the PL intensity is reduced by 50% compared to the sample prepared by one step (curve 1) indicating a supplementary reduction of the crystallites density due to the lateral dissolution of silicon crystallites. On the other hand the PL peak is still located at the same energy (1.98 eV) and it does not exceeds this limiting value after further steps (Tables 3 and 4) indicating that in stain etching process silicon nanocrystallites reach a limiting size. This result is in agreement with that found by Di Francia [19]. Fig. 8 shows the reflectance spectra of the porous silicon samples prepared by MS-SE method with different steps number on p-type silicon substrate (series p-Si360-60). Each spectrum is indicated by a number which gives the number of supplementary steps to which the substrate was subjected. For the clarity of the figure only four spectra were presented. The intensity of the reflectance was significantly lowered after porous layer formation. The reflectance spectra for samples with porous layers present interference oscillations reflecting the homogeneity of porous layers. The presence of those oscillations allows the determination of the optical thickness of the layer. Extracted optical thicknesses for all samples of this series are presented in Fig. 9 as a function of the wavelength. This figure shows that the optical thickness continuously increases with the number of etching steps. In Fig. 10 are presented the evolution of the optical thickness at a wavelength of 450 nm and the SiOx absorption band intensity versus the number of etching steps. From this figure it is clear that both the optical thickness and SiOx band intensity increase when increasing the number of etching steps. This behavior indicates the continue dissolution of silicon and the improvement of the thickness of the porous silicon layer during different etching steps used in MS-SE method. This property makes
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70
450 nm
600
60
50
7 6
500
reflectance (%)
Optical thickness (nm)
550
5 450
4 3
400
0 step (a)
30
20
2
350
c-Si 40
2 steps (b) 4 steps (c)
10
1 300
6 steps (d)
0 0
300
400
500
600
700
800
900
400
1000
500
600
700
800
900
1000
wavelenght (nm)
wavelength (nm) Fig. 9. Optical thickness as a function of the wavelength and the number of etching steps for PSi layers of the series p-Si360-60.
the MS-SE a very interesting method for the elaboration of porous silicon layers with controllable thicknesses and optical properties. One of the important applications of porous silicon is its use as an antireflection coating (ARC) for silicon solar cells. The porous silicon layer is generally elaborated on the n+ emitter of a p–n+ junction. In order to optimize the conditions which give a PSi layer with appropriate properties for efficient ARC, a set of samples was prepared on n+ -type silicon substrates. In this optimization both the number and the duration (t) of the supplementary etching steps were varied (Table 2). The effect of surface roughness was also studied using two substrates of different surface roughnesses. It was found that the reflectance of silicon substrates after porous silicon formation depends strongly on the elaboration conditions and the surface roughness of the starting material. In Fig. 11 are presented the total reflectance spectra of PSi samples prepared on the polished face of a silicon substrate with a surface roughness of 1 nm (series n+ -Si40-10). Results show that the surface reflectivity can be continuously reduced during successive etching steps and it reaches its minimum after the sixth step. This reflectance reduction is due to the continuous dissolution of silicon during successive etching steps which transform the substrate surface from a flat surface to a porous one that enhance the light absorption. As the number of etching steps increases the formed pores propagate laterally and in-depth leading to an enhancement of the specific
Fig. 11. Reflectance spectra of PSi layers prepared by MS-SE on a polished heavily doped n-type silicon substrate during 40 s followed by different numbers of supplementary etching steps of 10 s.
surface and then what results in a reduction in the reflectivity. But this phenomenon is limited by the lateral propagation of pores that produce the dissolution of the formed PSi nanocrystallites resulting in a reduction of the specific surface what explains the increase of the reflectivity after the sixth step. On the other hand, for PSi layers prepared, under same conditions, on a substrate having the same doping type and level but with a surface roughness of 12 nm the reflectivity reaches minimum values since the first step (Fig. 12a). The values reached, after this first step, are much weaker than those reached after the sixth one with the smoother substrate (Fig. 11). This behavior is due to a difference in the etching process related to the contact surface between the substrate and the etching solution. It is well known that rough substrates have generally a greater specific surface than the smooth ones, thus a greater contact surface with the solution leading to amplification in the density of nucleation centers, from which starts the silicon dissolution, it results an increase in the etching rate and an improvement in the nanostructure of the produced layer. For better control of the surface reflectance evolution of PSi layers, produced on the rough substrates, the duration of the
45
(a) 1 step of 10s (RS) (b) 2 steps of 10s (RS)
35
(c) 1 step of 5s (RS)
0,07
550
(d) 3 steps of 5s (RS)
500
450 0,04 400
0,03 0,02
350
SiOx peak intensity
0,05
Reflectance (%)
0,06
Optical thickness (nm)
40
30
0
1
2
20
(d)
15
(e)
3
4
5
6
7
5
0,00
0
8
Fig. 10. Evolution of the optical thickness at 450 nm and the intensity at the peak of the SiOx band with the number of etching steps for the PSi layers of the series p-Si360-60.
(c)
10
0,01
steps number
(f) 4 steps of 5s (SS) (f)
300 -1
(e) 4 steps of 5s (RS)
25
(b) (a) 400
600
800
1000
1200
wavelength (nm) Fig. 12. Reflectance spectra of PSi layers prepared by MS-SE on heavily doped n-type silicon substrates with different surface roughness during 40 s followed by different number and durations of supplementary etching steps.
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supplementary steps was reduced from 10 to 5 s (series n+ -Si4005 Table 2). Fig. 12 shows the most important results obtained for series elaborated on the rough substrate (RS). For the first series, with supplementary etching steps of 10 s, the reflectance is highly reduced since the first step but increases after the further steps. However for the second series, for which the time of supplementary etching steps is reduced to 5 s, it was found that the reflectance of PSi layers can be reduced step by step and it reaches values less than 3% in the major part of the spectrum after four steps (Fig. 12). To clearly show the effect of the roughness in the same figure is also presented the reflectance spectrum for a porous layer obtained after four etching steps of 5 seconds on the smooth substrate (SS). Finally, it is clearly seen from the reflectance characteristic of the PS layers formed by MS-SE method that such layers can be used as an effective ARC for silicon solar cells. 4. Conclusion A new method to produce porous silicon was presented. In this method instead of one etching step that leads to formation of porous layer, the substrate is subjected to an initial etching step with a duration t0 followed by a number of supplementary short steps that differs from a layer to another. It is found that this duration is largely dependent of the doping type and level of the silicon substrate. Surface investigation by AFM shows that MS-SE method allows the production of very homogeneous porous silicon nanostructures compared to the conventional SE method. This homogeneity is very interesting for many applications such as gettering in solar grade silicon or antireflection coating in silicon solar cells. The chemical composition of the obtained porous layers has been evaluated using Fourier transform infrared spectroscopy (FTIR). It is found that the intensities of different absorption bands increase as the number of supplementary etching steps increase. Photoluminescence (PL) measurement shows that porous layers produced by SE and MS-SE methods have comparable spectra indicating that those layers are composed of nanocrystallites with comparable sizes. But the intensity of photoluminescence of layer elaborated by MS-SE method is higher than that elaborated by the SE method. Total reflectance characteristics show that the presented method allows the production of porous silicon layers with controllable thicknesses and optical properties. Results for porous silicon layers elaborated on heavily doped n-type silicon show that the reflectance can be reduced to values less than 3% in the major part of the spectrum.
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