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Porous silicon obtained by anodization in the transition regime
Thin Solid Films 253 ( 1995) 152- 154
Porous silicon obtained by anodization M. Bertolotti,
F. Carassiti,
in the transition regime
E. Fazio, A. Ferrari, S. La Monica, S. Lazarouk*, G. Maiello, E. Proverbio, L. Schirone
G. Liakhou**,
_ Abstract A process is presented based on anodization of silicon in the transition region between the porous silicon formation regime and the electropolishing regime. Bright and stable photoluminescence was obtained on p-, n and p+, n+ (even degenerate) silicon, We report the photoluminescence, IR absorbance and thermal diffusivity of porous silicon formed by anodizing in the transition regime the four types of silicon. Fourier transform IR and gravimetric investigations showed that anodic silicon suboxide is formed on the surface. A model is proposed for the porous structure. which is suggested to consist of silicon crystallites built inside an anodic oxide. Keywords:
Luminescence;
Optical spectroscopy; Silicon
1. Introduction Since Canham reported luminescence phenomena observed under UV exposure of anodized crystalline silicon [ 11, porous silicon (PS) has attracted much attention because it offers the possibility to develop silicon-based optoelectronic components. The strong visible photoluminescence (PL) was mainly explained in terms of quantum effects occurring in columnar nanostructures formed on the surface of the material [ I-31. The size of pores and silicon needles in the porous structure is strictly dependent on the type and amount of doping and on the electrochemical etching regime [ 1, 4-71. Most recent investigations deal with PS formed on p- silicon owing to the relative ease of obtaining quantum-sized structures with respect to substrates with different doping [ 1, 51. As a matter of fact, in order to obtain quantum-sized structures on p+, n+ and nsilicon substrates, an additional pore enlargement step generally is performed [ 11. Unfortunately, on increasing the pore size by common methods such as chemical dissolution, the Si skeleton is damaged and the PL disappears. * On leave from: Bielorussian State University of lnformatics and Electronics, P. Brovki 6, 220600 Minsk, Belarus. **On leave from: Technical University of Moldova, Stephen cei Mare, 277012 Kishinev, Moldova. Elsevier Science S.A. SSDI 0040-6090( 94)05680-3
Zhang et al. [8] showed that the four types of silicon can be anodized in HF aqueous solutions in three different regimes: (i) porous silicon formation, (ii) electropolishing (no pores are formed on the silicon surface, which is covered by a homogenous silicon oxide) and (iii) a transition regime from one to the other. In the transition regime both pores and silicon oxide are formed [8, 91. By anodization in the transition regime we obtained bright and stable PL from porous silicon (thickness l-2 urn) formed on either p-, n or p+, n+ (even degenerate) substrates without a second step for the enlargement of the pores. We report the IR absorbance, thermal diffusivity, gravimetric investigation and photoluminescence of porous silicon formed by anodizing the different types of silicon in the transition regime.
2. Fabrication The silicon substrates were (100) oriented: p-type substrates were boron doped with 1220.005 Q cm resistivity; n-type substrates were phosphorous and arsenic doped with resistivities of 4.5 and 0.001 Q cm respectively. Before anodization a contact was formed on the back side with silver paste. Anodization of all types of silicon substrates in the transition regime was per-
formed in 1% (by weight) HF aqueous solution. A 2-5 mA cm-’ current density was applied in an electrochemical cell with a platinum reference electrode. During anodization the n-type substrates were illuminated by a tungsten lamp from the front side. The PS layer was I - 2 pm thick. Strong PL was observed at room temperature when the samples were irradiated by the UV light of an Hg Wood glass lamp or by a 488 nm Ar laser. The emission peak was shifted depending on the process and the full width at half-maximum (FWHM) was about 100 nm. Fourier transform IR (FTIR) investigations were performed with a Mattson 5000 spectrometer whose resolution was I cm ‘.
3. Results and discussion FTTR investigations confirm that silicon oxide is present on the surface: the spectrum exhibits the expected vibrational mode for SiO, (with x varying in the range 0.5-.2) and the Si-O-Si stretching frequency from 967 to I 105 cm-’ [10,II]. On approaching the electropolishing regime, oxide absorption is increased. Fig. I shows the FTTR reflection spectra of PS samples formed under various anodization conditions: (a) porous silicon formation, (b) boundary of porous silicon formation and (c) transition regime. The samples prepared in the porous silicon formation regime (Fig. I (a)) show small peaks in the range corresponding to native oxide vibrational modes. At the boundary of this regime we can see (Fig. l(b)) an increase in the oxide peaks between 965 and 1000 cm ‘, suggesting that oxygen was incorporated as a suboxide SiO,,j and SiO [IO].
In the transition regime (Fig. l(c)) the oxide peaks are further increased and the main peak shifts from 1000 to 1030cm-‘. This can be explained by an increase in the amount of SiO,., suboxide. The amount of SiOl is very small. In the case of PS formed by anodization in the transition regime, gravimetric measurements [ 61 provide an indirect estimate of the film porosity. as the formation of anodic oxide must be taken into account in the computation of the dissolved silicon mass. For this reason the result of gravimetric measurements is more properly referred to as the relative change in mats. After anodization in 1% HF solution at 2 mA cm ’ for 10 min the specific relative change in mass of n samples was about 20%. The sample was then placed in 40% HF solution for a few seconds: the relative change in mass reached 70%80%. This confirms that PS formed in the transition regime is mostly composed of anodic silicon oxide: the estimated mass of the silicon needles is 20’!&30%. Further information on the nature of the obtained PS can be gained from its thermal conductivity and diffusivity. The measurement has been performed with the photodeflection technique [121and comparison of the obtained results has been made with the silicon substrate. Preliminary measurements show that PS has a lower thermal conductivity than the original Si substrate. The thermal diffusivity of a sample formed on an n. substrate (/I = 0.01 R cm, thickness 20 pm) after dissolution of anodic oxide has been measured as II = 0.16 cm’ s ‘, to be contrasted with D = 0.6 cm’ s ’ for Si. This large difference can be put into relation with the porosity and allows us to give an estimate of this characteristic. If a model D = D,,( I - P) of linear dependence of diffusivity on porosity is accepted [ 13. 141, where /I),, is the substrate diffusivity, one has P 2 73’%). which is in satisfactory agreement with the value estimated by other methods. The photoluminescence of PS films formed in the transition regime on all types of doped silicon (p , p ’ , n and n+) was stable and noticeable with the naked eye in normal daylight. Fig. 2 shows Ihe PL intensity dependence on the forming current density for samples
5
P- ij P+ A n- ! l n+
l
1188
IEEE
Wavenumber
980
(cm-‘)
~_ 2
Fig. 1. FTIR reflection (0 = 1R cm) in various ’ m 40% HF (5mAcm silicon formation (:! mA transition regime (5 mA
spectra of PS formed on n-type substrates regimes: (a) porous silicon formation solution for 4 min); (b) boundary of porous cm-’ in I% HF solution for IO min); (c) cm 7 in 1% HF solution for 4min).
3 I
4
5
7
(mA/cm’)
Fig. 2. PL intensity of PS formed m 1% HF solution as a function of anodizing current density for the four types 01 silicon substrate. The PS thickness varied in the range 1.0 I.5 ltrn.
M. Bertolocri et al. 1 Thin Solid Films 255 (1995) 152- 154
154
tb)(oIoI( HO10
L--_l OHOH
HO10
n Silicon
q Holes
0
Oxide
Fig. 3. Idealized plan view of the surface of porous silicon formed (a) in the porous formation regime (from Ref. [I]) and (b) in the transition regime.
prepared in 1% HF aqueous solution. An increase in current density generally results in an increase in PL intensity. However, when the forming current exceeds 5 mA cm-*, electropolishing takes place [4] and no PL is observed. On increasing the current density, a blue shift in the PL spectra was observed. This blue shift suggests that a reduction in size of silicon crystallites is a consequence of approaching the electropolishing regime [S, 91. The PL obtained from p+, n- and n+ substrates can be related to silicon suboxide being present on the surface of porous silicon anodized in the transition regime. The porous structure is suggested to be described according to the model depicted in Fig. 3(b). The suboxide grown in the transition regime might add mechanical stability to silicon crystallites, allowing us to reduce their size without destroying the porous film skeleton. In this way it was also possible to reach quantum sizes for p+, nand n+ porous silicon, where the interpore spacing is relatively large [4-71. For comparison, a model of the surface after anodization in the porous silicon formation regime is shown in Fig. 3(a) [ 11. In this case, when 80% porosity is approached, the destruction of the silicon skeleton sets a limit to the reduction in size of silicon needles by means of pore enlargement. PS formed by anodization in the transition regime allowed us to obtain bright and stable electroluminescence [ 151.
4. Conclusions
Anodization in the transition regime allows us to obtain PS built in the anodic oxide through a single electrochemical step. In this way it is possible to form quantum-sized structures over n- and p+, n+ (even degenerate) substrates, where the interpore spacing is larger than 50 8, [4-71, without destroying the silicon structure. PS formed in this regime on the four types of silicon showed bright and stable PL.
References [I] L. T. Canham.
Appl. PIzys. Lett., 57 ( 1990) 1046. [2] V. Lehmann and U. Gosele, Appl. Phys. Letr., 58 (1991) 856. [3] K. H. Jung, S. Shih and D. L. Kwong, J. Elecfrochem. Sac.. 140 ( 1993) 3046. [4] V. Lehmann and H. Foil, J. Elecfrochem. Sot., 137 (1990) 653. [5] R. L. Smith and S. D. Collins, J. Appl. Phys, 71 ( 1992) RI. [6] R. Herino, G. Bomchil, K. Barla and C. Bertrand, J. Eiectrochem. Sot.. 134 (1987) 1994. [7] 1. Berbezier and A. Halimaoui, J. Appl. Phys., 74 (1993) 5421. [8] X. G. Zhang, S. D. Collins and R. L. Smith, J. Electrochem. Sot., 136 (1989) 1561. [9] P. C. Searson and X. G. Zhang, J. Electroche,w. Sot., 137 (1990) 2539. [IO] P. G. Pai, S. S. Chao and Y. Takagi, J. Vat. Sci. Tech&. A. 4 ( 1986) 689. [I I] Y. Xiao. M. J. Heben, J. M. McCullough, Y. S. Tsuo, J. I. Pankove and S. K. Deb. Appl. Phys. Left., 62 ( 1993) 1152. [12] M. Bertolotti, R. Li Voti, G. Leakhou and C. Sibilia, Rev. Sri. Instrum., 64 (1993) 1576. M. Bertolotti, L. Fabbri. C. Sibilia, A. Ferrari, F. Genel-Ricciardiello, C. Alvani and G. Suber. Marer Sci. Eng. A, 109 (1989) 153. [13] J. Franel and W. D. Kingery, J. Am. Ceram. Sot.. 37 (1954) 49. [I41 D. M. Liu, C. J. Chen and L. J. Lin. J. Appl. Phys.. 75 ( 1994) 3765. [ 151 In preparation.
Porous silicon obtained by anodization M. Bertolotti,
F. Carassiti,
in the transition regime
E. Fazio, A. Ferrari, S. La Monica, S. Lazarouk*, G. Maiello, E. Proverbio, L. Schirone
G. Liakhou**,
_ Abstract A process is presented based on anodization of silicon in the transition region between the porous silicon formation regime and the electropolishing regime. Bright and stable photoluminescence was obtained on p-, n and p+, n+ (even degenerate) silicon, We report the photoluminescence, IR absorbance and thermal diffusivity of porous silicon formed by anodizing in the transition regime the four types of silicon. Fourier transform IR and gravimetric investigations showed that anodic silicon suboxide is formed on the surface. A model is proposed for the porous structure. which is suggested to consist of silicon crystallites built inside an anodic oxide. Keywords:
Luminescence;
Optical spectroscopy; Silicon
1. Introduction Since Canham reported luminescence phenomena observed under UV exposure of anodized crystalline silicon [ 11, porous silicon (PS) has attracted much attention because it offers the possibility to develop silicon-based optoelectronic components. The strong visible photoluminescence (PL) was mainly explained in terms of quantum effects occurring in columnar nanostructures formed on the surface of the material [ I-31. The size of pores and silicon needles in the porous structure is strictly dependent on the type and amount of doping and on the electrochemical etching regime [ 1, 4-71. Most recent investigations deal with PS formed on p- silicon owing to the relative ease of obtaining quantum-sized structures with respect to substrates with different doping [ 1, 51. As a matter of fact, in order to obtain quantum-sized structures on p+, n+ and nsilicon substrates, an additional pore enlargement step generally is performed [ 11. Unfortunately, on increasing the pore size by common methods such as chemical dissolution, the Si skeleton is damaged and the PL disappears. * On leave from: Bielorussian State University of lnformatics and Electronics, P. Brovki 6, 220600 Minsk, Belarus. **On leave from: Technical University of Moldova, Stephen cei Mare, 277012 Kishinev, Moldova. Elsevier Science S.A. SSDI 0040-6090( 94)05680-3
Zhang et al. [8] showed that the four types of silicon can be anodized in HF aqueous solutions in three different regimes: (i) porous silicon formation, (ii) electropolishing (no pores are formed on the silicon surface, which is covered by a homogenous silicon oxide) and (iii) a transition regime from one to the other. In the transition regime both pores and silicon oxide are formed [8, 91. By anodization in the transition regime we obtained bright and stable PL from porous silicon (thickness l-2 urn) formed on either p-, n or p+, n+ (even degenerate) substrates without a second step for the enlargement of the pores. We report the IR absorbance, thermal diffusivity, gravimetric investigation and photoluminescence of porous silicon formed by anodizing the different types of silicon in the transition regime.
2. Fabrication The silicon substrates were (100) oriented: p-type substrates were boron doped with 1220.005 Q cm resistivity; n-type substrates were phosphorous and arsenic doped with resistivities of 4.5 and 0.001 Q cm respectively. Before anodization a contact was formed on the back side with silver paste. Anodization of all types of silicon substrates in the transition regime was per-
formed in 1% (by weight) HF aqueous solution. A 2-5 mA cm-’ current density was applied in an electrochemical cell with a platinum reference electrode. During anodization the n-type substrates were illuminated by a tungsten lamp from the front side. The PS layer was I - 2 pm thick. Strong PL was observed at room temperature when the samples were irradiated by the UV light of an Hg Wood glass lamp or by a 488 nm Ar laser. The emission peak was shifted depending on the process and the full width at half-maximum (FWHM) was about 100 nm. Fourier transform IR (FTIR) investigations were performed with a Mattson 5000 spectrometer whose resolution was I cm ‘.
3. Results and discussion FTTR investigations confirm that silicon oxide is present on the surface: the spectrum exhibits the expected vibrational mode for SiO, (with x varying in the range 0.5-.2) and the Si-O-Si stretching frequency from 967 to I 105 cm-’ [10,II]. On approaching the electropolishing regime, oxide absorption is increased. Fig. I shows the FTTR reflection spectra of PS samples formed under various anodization conditions: (a) porous silicon formation, (b) boundary of porous silicon formation and (c) transition regime. The samples prepared in the porous silicon formation regime (Fig. I (a)) show small peaks in the range corresponding to native oxide vibrational modes. At the boundary of this regime we can see (Fig. l(b)) an increase in the oxide peaks between 965 and 1000 cm ‘, suggesting that oxygen was incorporated as a suboxide SiO,,j and SiO [IO].
In the transition regime (Fig. l(c)) the oxide peaks are further increased and the main peak shifts from 1000 to 1030cm-‘. This can be explained by an increase in the amount of SiO,., suboxide. The amount of SiOl is very small. In the case of PS formed by anodization in the transition regime, gravimetric measurements [ 61 provide an indirect estimate of the film porosity. as the formation of anodic oxide must be taken into account in the computation of the dissolved silicon mass. For this reason the result of gravimetric measurements is more properly referred to as the relative change in mats. After anodization in 1% HF solution at 2 mA cm ’ for 10 min the specific relative change in mass of n samples was about 20%. The sample was then placed in 40% HF solution for a few seconds: the relative change in mass reached 70%80%. This confirms that PS formed in the transition regime is mostly composed of anodic silicon oxide: the estimated mass of the silicon needles is 20’!&30%. Further information on the nature of the obtained PS can be gained from its thermal conductivity and diffusivity. The measurement has been performed with the photodeflection technique [121and comparison of the obtained results has been made with the silicon substrate. Preliminary measurements show that PS has a lower thermal conductivity than the original Si substrate. The thermal diffusivity of a sample formed on an n. substrate (/I = 0.01 R cm, thickness 20 pm) after dissolution of anodic oxide has been measured as II = 0.16 cm’ s ‘, to be contrasted with D = 0.6 cm’ s ’ for Si. This large difference can be put into relation with the porosity and allows us to give an estimate of this characteristic. If a model D = D,,( I - P) of linear dependence of diffusivity on porosity is accepted [ 13. 141, where /I),, is the substrate diffusivity, one has P 2 73’%). which is in satisfactory agreement with the value estimated by other methods. The photoluminescence of PS films formed in the transition regime on all types of doped silicon (p , p ’ , n and n+) was stable and noticeable with the naked eye in normal daylight. Fig. 2 shows Ihe PL intensity dependence on the forming current density for samples
5
P- ij P+ A n- ! l n+
l
1188
IEEE
Wavenumber
980
(cm-‘)
~_ 2
Fig. 1. FTIR reflection (0 = 1R cm) in various ’ m 40% HF (5mAcm silicon formation (:! mA transition regime (5 mA
spectra of PS formed on n-type substrates regimes: (a) porous silicon formation solution for 4 min); (b) boundary of porous cm-’ in I% HF solution for IO min); (c) cm 7 in 1% HF solution for 4min).
3 I
4
5
7
(mA/cm’)
Fig. 2. PL intensity of PS formed m 1% HF solution as a function of anodizing current density for the four types 01 silicon substrate. The PS thickness varied in the range 1.0 I.5 ltrn.
M. Bertolocri et al. 1 Thin Solid Films 255 (1995) 152- 154
154
tb)(oIoI( HO10
L--_l OHOH
HO10
n Silicon
q Holes
0
Oxide
Fig. 3. Idealized plan view of the surface of porous silicon formed (a) in the porous formation regime (from Ref. [I]) and (b) in the transition regime.
prepared in 1% HF aqueous solution. An increase in current density generally results in an increase in PL intensity. However, when the forming current exceeds 5 mA cm-*, electropolishing takes place [4] and no PL is observed. On increasing the current density, a blue shift in the PL spectra was observed. This blue shift suggests that a reduction in size of silicon crystallites is a consequence of approaching the electropolishing regime [S, 91. The PL obtained from p+, n- and n+ substrates can be related to silicon suboxide being present on the surface of porous silicon anodized in the transition regime. The porous structure is suggested to be described according to the model depicted in Fig. 3(b). The suboxide grown in the transition regime might add mechanical stability to silicon crystallites, allowing us to reduce their size without destroying the porous film skeleton. In this way it was also possible to reach quantum sizes for p+, nand n+ porous silicon, where the interpore spacing is relatively large [4-71. For comparison, a model of the surface after anodization in the porous silicon formation regime is shown in Fig. 3(a) [ 11. In this case, when 80% porosity is approached, the destruction of the silicon skeleton sets a limit to the reduction in size of silicon needles by means of pore enlargement. PS formed by anodization in the transition regime allowed us to obtain bright and stable electroluminescence [ 151.
4. Conclusions
Anodization in the transition regime allows us to obtain PS built in the anodic oxide through a single electrochemical step. In this way it is possible to form quantum-sized structures over n- and p+, n+ (even degenerate) substrates, where the interpore spacing is larger than 50 8, [4-71, without destroying the silicon structure. PS formed in this regime on the four types of silicon showed bright and stable PL.
References [I] L. T. Canham.
Appl. PIzys. Lett., 57 ( 1990) 1046. [2] V. Lehmann and U. Gosele, Appl. Phys. Letr., 58 (1991) 856. [3] K. H. Jung, S. Shih and D. L. Kwong, J. Elecfrochem. Sac.. 140 ( 1993) 3046. [4] V. Lehmann and H. Foil, J. Elecfrochem. Sot., 137 (1990) 653. [5] R. L. Smith and S. D. Collins, J. Appl. Phys, 71 ( 1992) RI. [6] R. Herino, G. Bomchil, K. Barla and C. Bertrand, J. Eiectrochem. Sot.. 134 (1987) 1994. [7] 1. Berbezier and A. Halimaoui, J. Appl. Phys., 74 (1993) 5421. [8] X. G. Zhang, S. D. Collins and R. L. Smith, J. Electrochem. Sot., 136 (1989) 1561. [9] P. C. Searson and X. G. Zhang, J. Electroche,w. Sot., 137 (1990) 2539. [IO] P. G. Pai, S. S. Chao and Y. Takagi, J. Vat. Sci. Tech&. A. 4 ( 1986) 689. [I I] Y. Xiao. M. J. Heben, J. M. McCullough, Y. S. Tsuo, J. I. Pankove and S. K. Deb. Appl. Phys. Left., 62 ( 1993) 1152. [12] M. Bertolotti, R. Li Voti, G. Leakhou and C. Sibilia, Rev. Sri. Instrum., 64 (1993) 1576. M. Bertolotti, L. Fabbri. C. Sibilia, A. Ferrari, F. Genel-Ricciardiello, C. Alvani and G. Suber. Marer Sci. Eng. A, 109 (1989) 153. [13] J. Franel and W. D. Kingery, J. Am. Ceram. Sot.. 37 (1954) 49. [I41 D. M. Liu, C. J. Chen and L. J. Lin. J. Appl. Phys.. 75 ( 1994) 3765. [ 151 In preparation.