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Influence of anodisation time, current density and electrolyte concentration on the photoconductivity spectra of porous silicon
Thin Solid Films 315 Ž1998. 281–285
Influence of anodisation time, current density and electrolyte concentration on the photoconductivity spectra of porous silicon R.M. Mehra a
a,)
, Vivechana Agarwal a , V.K. Jain b, P.C. Mathur
a
Department of Electronic Science, UniÕersity of Delhi South Campus, New Delhi-110021, India b Solid State Physics Laboratory, Lucknow Road, Delhi-110007, India Received 15 July 1997; accepted 3 September 1997
Abstract Porous silicon layers emitting red photoluminescence ŽPL. have been prepared by the anodisation of p-type ²100: monocrystalline Si substrate in different HF concentrations. The steady state photoconductivity of porous silicon ŽPS. layers as a function of electrolyte concentration, anodisation time and current density has been studied. The photoconductivity ŽPC. peak was observed to shift towards the shorter wavelength with the decrease in the crystallite size and it was interpreted to be the result of band gap widening. The recombination is found to have contribution from both the monomolecular and the bimolecular processes. q 1998 Elsevier Science S.A. Keywords: Photoconductance; Porous silicon; Photoluminescence and recombination
1. Introduction The discovery of room temperature ŽRT. visible photoluminescence ŽPL. from porous silicon ŽPS. prepared by wet electrochemical anodisation of c-Si, has stimulated a lot of interest among the scientists in the recent years w1–5x and therefore its characteristics are being thoroughly investigated. The main stimulation in this area is due to the possible application of PS in solar cells Žbecause of its low surface refractivity and band gap variation due to quantum size effect. w6–9x, LED’s w10–16x, moisture detector w17x, SOI structures w18–20x, etc. Most of the work in this area has been done on the study of PL in the visible part of the spectrum w21x. Thus significant progress has been achieved in the understanding of the formation mechanism of PS w22x. However, in spite of systematic investigations performed by many authors very little data exists in the area of electrical transport properties as well as on their relationship with the preparation conditions. In most of the published literature on the transport measurements, the PS layer has been treated like a Schottky diode formed between the metal and the PS hetrostruc-
)
Corresponding author.
0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 0 7 5 6 - 6
ture w23x, and the current–voltage characteristics ŽI–V. were interpreted in terms of a diode equation w24,25x. In such samples only the barrier effect could be seen and no information about the material could be obtained. It has been observed by M. Ben-Chorin et al. w26x that when the thickness of the porous layer is ) 1 m m, the I–V characteristics are nearly symmetrical for forward and reverse bias. Photoconductivity ŽPC. is an important tool for the study of transport of photogenerated charge carriers and also gives an insight into recombination mechanism. In PS, since a strong PL is observed even at room temperature, the PC which is controlled mainly by nonradiative recombination could give us complimentary information for the understanding of the transport and microscopic origin of PL and EL. One of the major problem faced in PC measurements on PSrc-Si hetrostructures in the visible light region is the possible photogeneration of free carriers not only in thin PS layer but also in the thick c-Si substrate. We have minimized this problem by taking PC measurements on planar contacts and only those samples were used whose I–V characteristics show nonrectifying behavior. In this work an attempt is made to study the relationship between PS microstructure and its anodisation parameters through steady state PC measurements.
R.M. Mehra et al.r Thin Solid Films 315 (1998) 281–285
282
2. Sample preparation The samples were prepared from ²100: orientation, 350 m m thick, p-type polished silicon wafers with 1 V cm resistivity. The wafer was cleaned in trichloroethylene for 2 min at 608C before anodisation. It was subsequently rinsed in the deionised water and then the native SiO 2 layer was removed with dilute HF acid. An aluminum contact was deposited by evaporation on the back side of the wafer and covered with an acid proof wax for protection. Etching was performed in dark using standard galvano static etching process. Porous layers were grown under different experimental conditions given in the Table 1. Immediately after anodisation, the samples were taken out from the cell, rinsed in deionised water for half an hour Žto have a smooth PS layer w27x. and left to dry in ambient conditions. For electrical measurements rectangular aluminum contacts 2 = 1 mm2 were evaporated Žseparated by 1 mm gap. on the top of the porous layer.
3. Experimental results The PS samples were characterized by scanning electron microscopy ŽSEM.. It can be seen ŽFig. 1. from SEM micrographs that the crystallite size decreases with the increase in the etching time, keeping concentration constant. PL spectrum was taken by Arq Laser and as expected, PL peak was found to shift towards the shorter wavelength with the increase in the anodisation time Žin terms of energy 1.8 to 1.92 eV. Fig. 2. The spectral photoresponse to continuous wave Žcw. illumination was determined by applying an electric field of 45 Vrcm between the two planer contacts. Since the contacts in the present case were nonrectifying, no effect on the change of bias voltage was observed. The spectral response of the PC is measured using quartz halogen lamp,
Table 1 Conditions for the preparation of PS samples at 300 K, prepared on p-type Si with orientation ²100: and resistivity 1 ohm cm Sample no.
Solution concentration wHFŽ48 wt.%.:C 2 H 5 OH:H 2 Ox in volume
Etching time Žmin.
Current density ŽmArcm2 .
1 2 3 4 5 6 7 8 9 10 11
1:1 1:1:1 1:1:1 1:1:1 1:1:1 1:2:2 1:2:2 1:2:2 1:2:2 1:2:2 1:2:2
30 15 30 45 60 15 30 45 60 30 30
10 10 10 10 10 10 10 10 10 2 30
Fig. 1. The anodisation time dependent morphologies of PS samples observed by SEM. Fabricated in the diluted electrolyte HF:C 2 H 5 OH:H 2 O Ž1:2:2., at 10 mArcm2 , with anodisation times as Ža. 15 min Žb. 30 min Žc. 45 min.
which was calibrated with the help of a standard solar cell ŽSpectro Laboratory, USA.. The illumination intensity was varied by changing the distance between the sample and the source. Carl Zeisis IF filters, in the range of 350 to 1100 nm wavelength, with a step of 25 nm difference, have been used. The bandwidth of the filters ranged from 6.5–11 nm. The transmittance Ž%. ranged from 25–48%. Data has been normalised for the percentage transmittance of the filters and also corrected for the lamp response.
R.M. Mehra et al.r Thin Solid Films 315 (1998) 281–285
283
Fig. 2. The dependence of PL spectra of PS on anodisation time.
The variation of photoconductance Ž Gph . with intensity of illumination ŽF. is shown in Fig. 3 for a typical microporous sample ŽSample no. 4 in Table 1.. The dependence of Gph on intensity is found to obey the power law: Gph A F g . The value of exponent g is an indicator of a particular dominant recombination process and lies between 0.5 Žbimolecular. to 1 Žmonomolecular.. In the present case g is found to be ; 0.84 indicating that the recombination is taking place by both the processes monomolecular and bimolecular. The correlation of the steady state PC spectra and structure of PS is shown in Fig. 4. The most notable characteristic feature of the spectral response is the shift of the PC peak, on changing the preparation conditions. A peak independent of preparation parameters was observed at 1082 nm.
4. Discussion PS is, judging by the published work, a conglomerate of Si filaments or columns, channel shaped pores and cavities w1x and bulk silicon microcrystals w28x, which remain during growth of pores or are formed as a result of the reactions of disproportionation of Siq2 ions generated initially during anodisation w29x.
Fig. 4. The spectral dependence of PC as a function of Ža. current density for samples 7, 10, 11; inset shows peak wavelengths of PC spectra as a function of current density. Žb. Electrolyte concentration Žin volume. for samples 1, 3, 7; inset shows peak wavelengths of PC spectra as a function of electrolyte concentration. Žc. Anodisation time for samples 2, 3, 4, 5; inset shows peak wavelengths of PC spectra as a function of anodisation time.
2Siq2 ™ Siq4 q Si It was therefore natural to find that the spectral response of the PS have some characteristics of the substrates on which it was grown, as we have observed a peak at 1082 nm independent of the formation parameter of the PS layer. Preparation conditions of the samples strongly affect the properties of PS, as can be seen from Fig. 4. 4.1. Effect of current density and etching time
Fig. 3. The variation of photoconductance with intensity of illumination Žlog Gph vs. log F .. ŽThe dots and the solid line represent respectively, the experimental data and the curve fit to the equation..
PC peak has a blue shift from 721 to 523 nm increasing the current density from 2 mArcm2 to mArcm 2 , keeping the electrolyte concentration HFŽ48%.:ethanolŽ98%.:H 2 O as 1:2:2 Žin volume. and
on 40 of the
284
R.M. Mehra et al.r Thin Solid Films 315 (1998) 281–285
etching time as 30 min. The PC peak wavelength positions blue shifts from 673 to 546 nm with increasing anodisation time from 15–60 min, keeping the electrolyte concentration of HFŽ48%.:ethanolŽ98%.:H 2 O as 1:1:1 Žin volume. and current density as 10 mArcm2 . Because the sizes of the residual silicon skeleton decreases with increasing total charge density flowing across the sample surface, the peak of the PC spectra blue shifts with increasing current densityretching time. This is consistent with the model of the Q.C. effects w1x, which in turn is supported by the SEM and PL results. The thinner the Si wires are, the stronger the Q.C. effects are. The samples in which there exist thinner Si wires will show more band gap widening. It is noticed that the intensity of the PC peak energy increases from 15 to 45 min then decreases for the sample prepared at anodisation time of 60 min. This can be similarly explained due to the initial increase of surface area but if the anodisation time is further increased, it results in the etching of the upper surface of the PS layer which in turn decrease the surface area. The larger the illuminated surface area, the more will be the photogenerated charge carriers and hence, the PC. 4.2. Effect of electrolyte concentration PC peak has a blue shift from 655 nm to 580 nm by decreasing the concentration of HF from 1.5 M to 8 M, keeping the current density at 10 mArcm2 and etching time as 30 min. This too can be explained according to the model of quantum confinement ŽQ.C.. effects w1x. It is reasonable that the dimension of Si wires is larger in the samples prepared with the high HF concentration than the sample with lower HF concentration w27x. It is noticed that the intensity of the PC peak energy also increases with the increasing dilution of the solution. This can be explained due to the increased porosity which in turn results in the enhanced surface area. Thus, shift of the PC peak towards the shorter wavelength can be accounted for in terms of the crystallite size. By increasing the current densityrdilution of the electrolyteranodisation time, the crystallite size is found to decrease. The overall increase in the PC can be explained qualitatively in terms of an increase in the absorption of light by a highly developed surface of PS and by a reduction in the surface recombination velocity w30x. These peaks, we suppose, do not represent strictly the band gap of the material. The observed shift can be interpreted to be the result of the increased band gap widening with the decreasing crystallite size. The smaller the crystallite size, the more is the band gap widening. The appearance of wings in the PC spectrum is obviously related to the surface states. A significant drop of the photoresponse for the shorter wavelengths, shown in Fig. 4a–c, is presumably due to surface recombination losses of carriers generated in the region very close to the Al contact. The difference between
the peak wavelengths of the photoconduction and the PL spectra could be due to a potential fluctuation along the depth direction in PS.
5. Conclusion The study of PC and transport properties in PS is still in its infancy. Some fundamental points regarding the photoelectric response of PS with the change in anodisation timersolution concentrationrcurrent density have been clarified. The observed photoconduction characteristics can be interpreted to be the result of the band-gap widening in PS with increase of anodisation timercurrent density and decrease of solution concentration. Such understanding is a necessary first step for the understanding of the transport in the PS layer. Although further analysis of the microscopic mechanisms of the optical excitation and the subsequent carrier transport in PS are required, the results presented here support the hypothesis that the optical activity of PS is based on quantum confinement in nanocrystallites with disordered electronic boundaries along with the surface states Žthe existence of the surface states has already been proved by Koch et al. w4x.. The recombination in PS is found to have contribution from both the monomolecular and the bimolecular processes.
References w1x L.T. Canham, Appl. Phys. Lett. 57 Ž1990. 1046. w2x J.C. Vial, A. Bsiesy, F. Gaspard, R. Herino, M. Ligeon, F. Muller, R. Romestain, R.M. Macfarlane, Phys. Rev. B 45 Ž1992. 14171. w3x M.S. Brandt, H.D. Fuchs, M. Stutzmann, J. Weber, M. Cardona, Solid State Commun. 81 Ž1992. 307. w4x F. Koch, V. Petrova-Koch, T. Muschik, J. Lumin. 57 Ž1993. 271. w5x W. Lang, P. Steiner, F. Kozlowski, J. Lumin. 57 Ž1993. 341. w6x G. Smestad, M. Kunst, C. Vial, Solar Energy Mater Solar Cells 26 Ž1992. 277. w7x H. Zimnsermann, F.H. Cocks, U. Gosele, Mater. Chem. Phys. 32 Ž1992. 310. w8x Y.S. Tsuo, M.J. Heben, X. Wu, Y. Xiao, C.A. Moore, P. Verlinden, S.K. Deb, Mat. Res. Soc. Symp. Proc. 283 Ž1993. 405. w9x Y.S. Tsuo, Y. Xiao, M.J. Heben, X. Wu, F.J. Pern, S.K. Deb, Proc. 23rd IEEE Photovoltaic Specialists Conf., 1993, p. 287. w10x A. Richter, P. Steiner, F. Kozlowski, W. Lang, IEEE Electron Devices Lett. 12 Ž1991. 691. w11x N. Koshida, H. Koyama, Appl. Phys. Lett. 60 Ž1992. 347. w12x F. Namavar, H.P. Maruska, N.M. Kalkhoran, Appl. Phys. Lett. 60 Ž1992. 2514. w13x T. Futagi, T. Mastsunoto, M. Katsuno, Y. Ohta, H. Mimura, K. Kitamura, Appl. Phys. Lett. 63 Ž1993. 1209. w14x Z. Chen, G. Bosman, R. Ochoa, Appl. Phys. Lett. 62 Ž1993. 708. w15x P. Steiner, F. Kozlowski, W. Lang, Appl. Phys. Lett. 62 Ž1993. 2700. w16x C. Peng, K.D. Hirschman, P.M. Fauchet, J. Appl. Phys. 80 Ž1. Ž1996. 295. w17x M.J. Kelly, T.R. Gullinger, D.W. Peterson, M.R. Tuck, J.N. Sweet, Mat. Res. Soc. Symp. Proc. 225 Ž1991. 313. w18x K. Imai, H. Unno, H. Takaoka, J. Cryst. Growth 63 Ž1983. 547. w19x R.C. Frye, Mat. Res. Soc. Symp. Proc. 33 Ž1984. 53.
R.M. Mehra et al.r Thin Solid Films 315 (1998) 281–285 w20x G. Bomchil, A. Halimaoui, R. Herino, Appl. Surf. Sci. 41r42 Ž1989. 602. w21x T. van Buuren, S. Eisebitt, S. Patitsas, S. Richtie, T. Tiedje, J.F. Young, Y. Gao, Mater. Res. Soc. Symp. Proc. 358 Ž1995. 441. w22x D.W. Zheng, Y.P. Huang, Z.H. He, A.Z. Li, T.A. Tang, R. Kwor, Q. Cui, X.J. Zhang, J. Appl. Phys. 81 Ž1. Ž1997. 492. w23x H. Koyama, N. Khoshida, J. Lumin. 57 Ž1993. 293. w24x V. Paubutas, A. Krotkus, I. Simkiene, R. Viselga, J. Appl. Phys. 77 Ž6. Ž1995. 2501.
285
w25x L.V. Belyakov, D.N. Goryachev, O.M. Sreseli, I.D. Yaroshetskii, Semiconductors 27 Ž8. Ž1993. 758. w26x M. Ben-Chorin, F. Moller, F. Koch, Phys. Rev. B 49 Ž1994. 2981. w27x C.-H. Lin, S.C. Lee, Y.-F. Chen, J. Appl. Phys. 75 Ž12. Ž1994. 7728. w28x M.W. Cole, J.F. Harvey, R.A. Lux, D.W. Eckart, R. Tsu, Appl. Phys. Lett. 60 Ž1992. 2800. w29x R.L. Smith, S.D. Collins, J. Appl. Phys. 71 Ž1992. R1. w30x J.P. Zheng, K.L. Jiao, W.P. Shen, W.A. Anderson, H.S. Kwok, Appl. Phys. Lett. 60 Ž1992. 459.
Influence of anodisation time, current density and electrolyte concentration on the photoconductivity spectra of porous silicon R.M. Mehra a
a,)
, Vivechana Agarwal a , V.K. Jain b, P.C. Mathur
a
Department of Electronic Science, UniÕersity of Delhi South Campus, New Delhi-110021, India b Solid State Physics Laboratory, Lucknow Road, Delhi-110007, India Received 15 July 1997; accepted 3 September 1997
Abstract Porous silicon layers emitting red photoluminescence ŽPL. have been prepared by the anodisation of p-type ²100: monocrystalline Si substrate in different HF concentrations. The steady state photoconductivity of porous silicon ŽPS. layers as a function of electrolyte concentration, anodisation time and current density has been studied. The photoconductivity ŽPC. peak was observed to shift towards the shorter wavelength with the decrease in the crystallite size and it was interpreted to be the result of band gap widening. The recombination is found to have contribution from both the monomolecular and the bimolecular processes. q 1998 Elsevier Science S.A. Keywords: Photoconductance; Porous silicon; Photoluminescence and recombination
1. Introduction The discovery of room temperature ŽRT. visible photoluminescence ŽPL. from porous silicon ŽPS. prepared by wet electrochemical anodisation of c-Si, has stimulated a lot of interest among the scientists in the recent years w1–5x and therefore its characteristics are being thoroughly investigated. The main stimulation in this area is due to the possible application of PS in solar cells Žbecause of its low surface refractivity and band gap variation due to quantum size effect. w6–9x, LED’s w10–16x, moisture detector w17x, SOI structures w18–20x, etc. Most of the work in this area has been done on the study of PL in the visible part of the spectrum w21x. Thus significant progress has been achieved in the understanding of the formation mechanism of PS w22x. However, in spite of systematic investigations performed by many authors very little data exists in the area of electrical transport properties as well as on their relationship with the preparation conditions. In most of the published literature on the transport measurements, the PS layer has been treated like a Schottky diode formed between the metal and the PS hetrostruc-
)
Corresponding author.
0040-6090r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 4 0 - 6 0 9 0 Ž 9 7 . 0 0 7 5 6 - 6
ture w23x, and the current–voltage characteristics ŽI–V. were interpreted in terms of a diode equation w24,25x. In such samples only the barrier effect could be seen and no information about the material could be obtained. It has been observed by M. Ben-Chorin et al. w26x that when the thickness of the porous layer is ) 1 m m, the I–V characteristics are nearly symmetrical for forward and reverse bias. Photoconductivity ŽPC. is an important tool for the study of transport of photogenerated charge carriers and also gives an insight into recombination mechanism. In PS, since a strong PL is observed even at room temperature, the PC which is controlled mainly by nonradiative recombination could give us complimentary information for the understanding of the transport and microscopic origin of PL and EL. One of the major problem faced in PC measurements on PSrc-Si hetrostructures in the visible light region is the possible photogeneration of free carriers not only in thin PS layer but also in the thick c-Si substrate. We have minimized this problem by taking PC measurements on planar contacts and only those samples were used whose I–V characteristics show nonrectifying behavior. In this work an attempt is made to study the relationship between PS microstructure and its anodisation parameters through steady state PC measurements.
R.M. Mehra et al.r Thin Solid Films 315 (1998) 281–285
282
2. Sample preparation The samples were prepared from ²100: orientation, 350 m m thick, p-type polished silicon wafers with 1 V cm resistivity. The wafer was cleaned in trichloroethylene for 2 min at 608C before anodisation. It was subsequently rinsed in the deionised water and then the native SiO 2 layer was removed with dilute HF acid. An aluminum contact was deposited by evaporation on the back side of the wafer and covered with an acid proof wax for protection. Etching was performed in dark using standard galvano static etching process. Porous layers were grown under different experimental conditions given in the Table 1. Immediately after anodisation, the samples were taken out from the cell, rinsed in deionised water for half an hour Žto have a smooth PS layer w27x. and left to dry in ambient conditions. For electrical measurements rectangular aluminum contacts 2 = 1 mm2 were evaporated Žseparated by 1 mm gap. on the top of the porous layer.
3. Experimental results The PS samples were characterized by scanning electron microscopy ŽSEM.. It can be seen ŽFig. 1. from SEM micrographs that the crystallite size decreases with the increase in the etching time, keeping concentration constant. PL spectrum was taken by Arq Laser and as expected, PL peak was found to shift towards the shorter wavelength with the increase in the anodisation time Žin terms of energy 1.8 to 1.92 eV. Fig. 2. The spectral photoresponse to continuous wave Žcw. illumination was determined by applying an electric field of 45 Vrcm between the two planer contacts. Since the contacts in the present case were nonrectifying, no effect on the change of bias voltage was observed. The spectral response of the PC is measured using quartz halogen lamp,
Table 1 Conditions for the preparation of PS samples at 300 K, prepared on p-type Si with orientation ²100: and resistivity 1 ohm cm Sample no.
Solution concentration wHFŽ48 wt.%.:C 2 H 5 OH:H 2 Ox in volume
Etching time Žmin.
Current density ŽmArcm2 .
1 2 3 4 5 6 7 8 9 10 11
1:1 1:1:1 1:1:1 1:1:1 1:1:1 1:2:2 1:2:2 1:2:2 1:2:2 1:2:2 1:2:2
30 15 30 45 60 15 30 45 60 30 30
10 10 10 10 10 10 10 10 10 2 30
Fig. 1. The anodisation time dependent morphologies of PS samples observed by SEM. Fabricated in the diluted electrolyte HF:C 2 H 5 OH:H 2 O Ž1:2:2., at 10 mArcm2 , with anodisation times as Ža. 15 min Žb. 30 min Žc. 45 min.
which was calibrated with the help of a standard solar cell ŽSpectro Laboratory, USA.. The illumination intensity was varied by changing the distance between the sample and the source. Carl Zeisis IF filters, in the range of 350 to 1100 nm wavelength, with a step of 25 nm difference, have been used. The bandwidth of the filters ranged from 6.5–11 nm. The transmittance Ž%. ranged from 25–48%. Data has been normalised for the percentage transmittance of the filters and also corrected for the lamp response.
R.M. Mehra et al.r Thin Solid Films 315 (1998) 281–285
283
Fig. 2. The dependence of PL spectra of PS on anodisation time.
The variation of photoconductance Ž Gph . with intensity of illumination ŽF. is shown in Fig. 3 for a typical microporous sample ŽSample no. 4 in Table 1.. The dependence of Gph on intensity is found to obey the power law: Gph A F g . The value of exponent g is an indicator of a particular dominant recombination process and lies between 0.5 Žbimolecular. to 1 Žmonomolecular.. In the present case g is found to be ; 0.84 indicating that the recombination is taking place by both the processes monomolecular and bimolecular. The correlation of the steady state PC spectra and structure of PS is shown in Fig. 4. The most notable characteristic feature of the spectral response is the shift of the PC peak, on changing the preparation conditions. A peak independent of preparation parameters was observed at 1082 nm.
4. Discussion PS is, judging by the published work, a conglomerate of Si filaments or columns, channel shaped pores and cavities w1x and bulk silicon microcrystals w28x, which remain during growth of pores or are formed as a result of the reactions of disproportionation of Siq2 ions generated initially during anodisation w29x.
Fig. 4. The spectral dependence of PC as a function of Ža. current density for samples 7, 10, 11; inset shows peak wavelengths of PC spectra as a function of current density. Žb. Electrolyte concentration Žin volume. for samples 1, 3, 7; inset shows peak wavelengths of PC spectra as a function of electrolyte concentration. Žc. Anodisation time for samples 2, 3, 4, 5; inset shows peak wavelengths of PC spectra as a function of anodisation time.
2Siq2 ™ Siq4 q Si It was therefore natural to find that the spectral response of the PS have some characteristics of the substrates on which it was grown, as we have observed a peak at 1082 nm independent of the formation parameter of the PS layer. Preparation conditions of the samples strongly affect the properties of PS, as can be seen from Fig. 4. 4.1. Effect of current density and etching time
Fig. 3. The variation of photoconductance with intensity of illumination Žlog Gph vs. log F .. ŽThe dots and the solid line represent respectively, the experimental data and the curve fit to the equation..
PC peak has a blue shift from 721 to 523 nm increasing the current density from 2 mArcm2 to mArcm 2 , keeping the electrolyte concentration HFŽ48%.:ethanolŽ98%.:H 2 O as 1:2:2 Žin volume. and
on 40 of the
284
R.M. Mehra et al.r Thin Solid Films 315 (1998) 281–285
etching time as 30 min. The PC peak wavelength positions blue shifts from 673 to 546 nm with increasing anodisation time from 15–60 min, keeping the electrolyte concentration of HFŽ48%.:ethanolŽ98%.:H 2 O as 1:1:1 Žin volume. and current density as 10 mArcm2 . Because the sizes of the residual silicon skeleton decreases with increasing total charge density flowing across the sample surface, the peak of the PC spectra blue shifts with increasing current densityretching time. This is consistent with the model of the Q.C. effects w1x, which in turn is supported by the SEM and PL results. The thinner the Si wires are, the stronger the Q.C. effects are. The samples in which there exist thinner Si wires will show more band gap widening. It is noticed that the intensity of the PC peak energy increases from 15 to 45 min then decreases for the sample prepared at anodisation time of 60 min. This can be similarly explained due to the initial increase of surface area but if the anodisation time is further increased, it results in the etching of the upper surface of the PS layer which in turn decrease the surface area. The larger the illuminated surface area, the more will be the photogenerated charge carriers and hence, the PC. 4.2. Effect of electrolyte concentration PC peak has a blue shift from 655 nm to 580 nm by decreasing the concentration of HF from 1.5 M to 8 M, keeping the current density at 10 mArcm2 and etching time as 30 min. This too can be explained according to the model of quantum confinement ŽQ.C.. effects w1x. It is reasonable that the dimension of Si wires is larger in the samples prepared with the high HF concentration than the sample with lower HF concentration w27x. It is noticed that the intensity of the PC peak energy also increases with the increasing dilution of the solution. This can be explained due to the increased porosity which in turn results in the enhanced surface area. Thus, shift of the PC peak towards the shorter wavelength can be accounted for in terms of the crystallite size. By increasing the current densityrdilution of the electrolyteranodisation time, the crystallite size is found to decrease. The overall increase in the PC can be explained qualitatively in terms of an increase in the absorption of light by a highly developed surface of PS and by a reduction in the surface recombination velocity w30x. These peaks, we suppose, do not represent strictly the band gap of the material. The observed shift can be interpreted to be the result of the increased band gap widening with the decreasing crystallite size. The smaller the crystallite size, the more is the band gap widening. The appearance of wings in the PC spectrum is obviously related to the surface states. A significant drop of the photoresponse for the shorter wavelengths, shown in Fig. 4a–c, is presumably due to surface recombination losses of carriers generated in the region very close to the Al contact. The difference between
the peak wavelengths of the photoconduction and the PL spectra could be due to a potential fluctuation along the depth direction in PS.
5. Conclusion The study of PC and transport properties in PS is still in its infancy. Some fundamental points regarding the photoelectric response of PS with the change in anodisation timersolution concentrationrcurrent density have been clarified. The observed photoconduction characteristics can be interpreted to be the result of the band-gap widening in PS with increase of anodisation timercurrent density and decrease of solution concentration. Such understanding is a necessary first step for the understanding of the transport in the PS layer. Although further analysis of the microscopic mechanisms of the optical excitation and the subsequent carrier transport in PS are required, the results presented here support the hypothesis that the optical activity of PS is based on quantum confinement in nanocrystallites with disordered electronic boundaries along with the surface states Žthe existence of the surface states has already been proved by Koch et al. w4x.. The recombination in PS is found to have contribution from both the monomolecular and the bimolecular processes.
References w1x L.T. Canham, Appl. Phys. Lett. 57 Ž1990. 1046. w2x J.C. Vial, A. Bsiesy, F. Gaspard, R. Herino, M. Ligeon, F. Muller, R. Romestain, R.M. Macfarlane, Phys. Rev. B 45 Ž1992. 14171. w3x M.S. Brandt, H.D. Fuchs, M. Stutzmann, J. Weber, M. Cardona, Solid State Commun. 81 Ž1992. 307. w4x F. Koch, V. Petrova-Koch, T. Muschik, J. Lumin. 57 Ž1993. 271. w5x W. Lang, P. Steiner, F. Kozlowski, J. Lumin. 57 Ž1993. 341. w6x G. Smestad, M. Kunst, C. Vial, Solar Energy Mater Solar Cells 26 Ž1992. 277. w7x H. Zimnsermann, F.H. Cocks, U. Gosele, Mater. Chem. Phys. 32 Ž1992. 310. w8x Y.S. Tsuo, M.J. Heben, X. Wu, Y. Xiao, C.A. Moore, P. Verlinden, S.K. Deb, Mat. Res. Soc. Symp. Proc. 283 Ž1993. 405. w9x Y.S. Tsuo, Y. Xiao, M.J. Heben, X. Wu, F.J. Pern, S.K. Deb, Proc. 23rd IEEE Photovoltaic Specialists Conf., 1993, p. 287. w10x A. Richter, P. Steiner, F. Kozlowski, W. Lang, IEEE Electron Devices Lett. 12 Ž1991. 691. w11x N. Koshida, H. Koyama, Appl. Phys. Lett. 60 Ž1992. 347. w12x F. Namavar, H.P. Maruska, N.M. Kalkhoran, Appl. Phys. Lett. 60 Ž1992. 2514. w13x T. Futagi, T. Mastsunoto, M. Katsuno, Y. Ohta, H. Mimura, K. Kitamura, Appl. Phys. Lett. 63 Ž1993. 1209. w14x Z. Chen, G. Bosman, R. Ochoa, Appl. Phys. Lett. 62 Ž1993. 708. w15x P. Steiner, F. Kozlowski, W. Lang, Appl. Phys. Lett. 62 Ž1993. 2700. w16x C. Peng, K.D. Hirschman, P.M. Fauchet, J. Appl. Phys. 80 Ž1. Ž1996. 295. w17x M.J. Kelly, T.R. Gullinger, D.W. Peterson, M.R. Tuck, J.N. Sweet, Mat. Res. Soc. Symp. Proc. 225 Ž1991. 313. w18x K. Imai, H. Unno, H. Takaoka, J. Cryst. Growth 63 Ž1983. 547. w19x R.C. Frye, Mat. Res. Soc. Symp. Proc. 33 Ž1984. 53.
R.M. Mehra et al.r Thin Solid Films 315 (1998) 281–285 w20x G. Bomchil, A. Halimaoui, R. Herino, Appl. Surf. Sci. 41r42 Ž1989. 602. w21x T. van Buuren, S. Eisebitt, S. Patitsas, S. Richtie, T. Tiedje, J.F. Young, Y. Gao, Mater. Res. Soc. Symp. Proc. 358 Ž1995. 441. w22x D.W. Zheng, Y.P. Huang, Z.H. He, A.Z. Li, T.A. Tang, R. Kwor, Q. Cui, X.J. Zhang, J. Appl. Phys. 81 Ž1. Ž1997. 492. w23x H. Koyama, N. Khoshida, J. Lumin. 57 Ž1993. 293. w24x V. Paubutas, A. Krotkus, I. Simkiene, R. Viselga, J. Appl. Phys. 77 Ž6. Ž1995. 2501.
285
w25x L.V. Belyakov, D.N. Goryachev, O.M. Sreseli, I.D. Yaroshetskii, Semiconductors 27 Ž8. Ž1993. 758. w26x M. Ben-Chorin, F. Moller, F. Koch, Phys. Rev. B 49 Ž1994. 2981. w27x C.-H. Lin, S.C. Lee, Y.-F. Chen, J. Appl. Phys. 75 Ž12. Ž1994. 7728. w28x M.W. Cole, J.F. Harvey, R.A. Lux, D.W. Eckart, R. Tsu, Appl. Phys. Lett. 60 Ž1992. 2800. w29x R.L. Smith, S.D. Collins, J. Appl. Phys. 71 Ž1992. R1. w30x J.P. Zheng, K.L. Jiao, W.P. Shen, W.A. Anderson, H.S. Kwok, Appl. Phys. Lett. 60 Ž1992. 459.