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Photoluminescence of inhomogeneous porous silicon at gas adsorption
Applied Surface Science 157 Ž2000. 145–150 www.elsevier.nlrlocaterapsusc
Photoluminescence of inhomogeneous porous silicon at gas adsorption V.A. Skryshevsky ) Laboratory of Semiconductor DeÕices, Radiophysics Department, National KieÕ SheÕchenko UniÕersity, 64 Vladimirskaya, 01033, KieÕ, Ukraine Received 2 October 1999; accepted 22 November 1999
Abstract An impact of inhomogeneities on photoluminescence ŽPL. of porous silicon ŽPS. is analysed using numerical simulation and supported experiment under gas adsorption. Depending on the excitation wavelength and the condition of measurement Žsteady-state or transient mode., the gas adsorption can result in the quenching or increase of PL. The emission efficiency of the as-prepared porous layers is shown to decrease at the adsorption of acetone molecules for each excitation wavelength in the 405–546-nm range. However, if the 546-nm excitation causes small change of the photoluminescence spectrum during exposition, the 405-nm excitation quenches the PL in the ambient air and increases emission efficiency in acetone vapours. Effects are discussed with the model approach of different recombination properties, and the contribution to the PL incoming of the upper photooxidised and bottom nanocrystalline layers. We describe the method of the PL spectra fitting in order to determine the gradient profile of porous layers having depth irregularity of PL. q 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Porous silicon; Photoluminescence; Inhomogeneity; Gas adsorption
1. Introduction Recently, the luminescent porous silicon ŽPS. has been regarded as attractive material for various sensor applications w1–4x. Chemical sensors based on PS photoluminescence ŽPL. can bring advantages due to a strong sensing of PL efficiency to adsorbates. In fact, the adsorption of some molecules results in the reduction or even disappearance of the
)
Tel.: q380-44-266-4009; fax: q380-44-265-6744. E-mail address: [email protected] ŽV.A. Skryshevsky..
strong visible PL that recovers entirely after the removing of solvents w5,6x, because no net chemical reaction occurs during the adsorptionrdesorption of molecules. The mechanisms of PL quenching by adsorbates are summarised in Refs. w1,7–9x and include the following models: Ži. the increase of the non-radiative recombination rate in the nanoparticles due to the alteration of the dielectric medium outside the Si nanocrystallites; Žii. the enhancement of the non-radiative vibronic coupling to the surface vibrational modes; Žiii. the change of the nanoparticle surface electronic structure; Živ. the capture increase on the non-radiative traps at the forming of the
0169-4332r00r$ - see front matter q 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 5 6 0 - 7
146
V.A. SkrysheÕskyr Applied Surface Science 157 (2000) 145–150
strain-induced defects when molecules are adsorbed, etc. The correlation between PL quenching and solvent dipole moment, established by authors of Ref. w6x, means that the solvent induces the capture of electrons or holes by the surface traps. The molecules with the large dipole moment, as methanol or ethanol, quench the PL stronger in comparison with the weak quenching by benzene or toluene having a small dipole moment. The reversible quenching phenomenon has been interpreted as the stabilisation of the surface traps by the alignment of molecular dipoles on the PS surface. Usually, the PL quenching is accompanied by the asymmetrical shape of the PL spectrum. The blue shifting or the decrease of the emission in the long wavelength region was observed after the treatment in the boiling CCl 4 w10x, organoamine molecules w11x, C 2 H 5 OH w12x, and methanol w13x. However, there are the experimental evidences of anomalous behaviour of PL quantum efficiency at the adsorption of some molecules w14,15x. Taking into consideration that the PL of PS is correlated closely with the nanostructure of porous materials and their surface reactivity, the nature of the observed asymmetrical shape of the PL is correlated with the inhomogeneity of the luminescent layer. Thus, the study of the alteration of PL properties in various environments can be a useful tool for the analysis of the PS inhomogeneity. To get an understanding of the effects of the structural–chemical inhomogeneity on PL, that can take place in the electrochemically etched PS, the spectra and kinetics of PS emission in the ambient atmosphere and saturated acetone vapours were investigated in the present work.
2. Experimental The porous layers were prepared on phosphorus doped, Ž100. oriented silicon substrates of 4.5-V cm resistivity and 350-mm thickness. The growing of the porous layers was provided by the anodization in the solution of 48 wt.% HF: C 2 H 6 O Ž1:2. with two regimes: at the constant current density of 20 mArcm2 for 15 min and at discrete subsequence
steps of the current, namely, 40, 32, 24, 16 and 8 mArcm2 for 3 min of each current value. The selected technological regimes allow the formation of the material with the porosity within 60–70% w16x. The rate of PS layer growth is about 6–10 nmrs. In this case, the thickness of each layers formed by the second regime is estimated to be equal 1.1–1.8 mm. The PL of PS has been excited by the DRSh-250-2 mercury lamp through the system of the optical filters with a singular selected emission at 365, 405, 436, 546 or 578 nm. The PL spectra of the as-prepared PS films were measured in transient and steady-state modes in two atmospheres — ambient air and saturated acetone vapours. The steady state mode is achieved by the stabilization of the PL intensity when the patterns are excited at 405 nm for 30 min, then by 436 nm for 5 min and 546 nm for 5 min. Such a treatment evokes the 1.8-time reduction of the PL yield for 10 min and then PL exhibits no change during measurement neither in ambient atmosphere nor in acetone vapours.
3. PL of multilayer PS film: simulation Let us consider a multi-layer film where each thin sub-layer thickness of d is characterised by the proper absorption and emission spectra. Hence, the depth profile of the PL incoming is tightly coupled with the depth profile of nanocrystallite sizes Žand the porosity. and the light absorption by the upper PS layers. Owing to the absorption, whole upper sub-layers act as the selective optic filters for the bulk PL. In general, the number of the absorbed photons that excites PL per unit of time, unit of area and unit of energy interval for i sub-layer is given by: G s G 0 Ž 1 y R . exp Ž ya 1 x . 1 y exp Ž ya 1 d . ,
Ž 1. where G 0 is the density of the photon flux from the excitation source, R is the coefficient of the surface reflectivity, a 1 is the PS absorption coefficient at
V.A. SkrysheÕskyr Applied Surface Science 157 (2000) 145–150
Fig. 1. The calculated PL spectra of PS consisting of two layers: 0.25 mm top layer with 1.9-eV emission and 0.75-mm bottom layer with 1.7-eV emission. Here x indicates the depth Žin mm. of full PL quenching. Excitation energy is 3.4 eV.
the wavelength of the excited illumination, x is the depth of the sub-layer from the PS surface. The number of photons emitted by every sub-layer i in the approximation of Gauss-type spectrum is written as: Ie i Ž hn , x . s G 0 Ž 1 y R . exp Ž ya 1 x . = 1 y exp Ž ya 1 d . =exp y
hi Di'2p
Ž hn y hnm i .
2
2 Di2
,
Ž 2.
where hi is the internal quantum efficiency for isotropic emission; hnm i , Di are parameters of Gauss-type PL spectrum. Summarising the emission from each sub-layer and taking into consideration the optical losses at the absorption of emission by upper PS sub-layers, the intensity of the measured PL is expressed as: N hi Ie Ž hn . s G 0 Ž 1 y R . 1 y exp Ž ya 1 d . Ý ' is1 D i 2 p
147
Fig. 1 shows the calculated PL spectra for film, consisting of 0.25-mm top layer having a 1.9-eV emission and a 0.75-mm bottom layer with 1.7-eV emission, considering that h1 s h 2 and a Ž hn . taken from Ref. w17x. The resulting PL spectrum is the sum of the strong short-wavelength emission from the surface region and slow long-wavelength emission from the deeper part of PS Žcurve 1.. If the gas exposure results in the PL quenching, the emission spectrum changes asymmetrically. The deeper the penetration of adsorbed molecules into PS layer, the stronger quenching of the short wavelength PL. The reduction of the PL efficiency is accompanied by the red shifting of the PL spectra Žcurves 2–5.. Curve 6 indicates the case of full suppression of 1.9-eV emission. There are few evidences that the anodically formed PS films exhibit the depth irregularity of the luminescence properties. The full width at half maximum ŽFWHM. of the PS emission lies within of 150–500 meV w18x. Large FWHM implies that the emission is coming from the ensemble of nanocrystallites having a size dispersion. Besides, the short wavelength component of PL decays faster than the long one w18x. Usually, the PL and the electroluminescence ŽEL. exhibit non-coincident spectra because the different depth regions of PS are responsible for these emissions. It was shown w19x that the PL yield is maximum from a few micrometers below the surface, whereas only a thin top layer shows EL. The shorter excitation wavelength Žfrom 578 to 365 nm., the stronger the shift of PL maximum in the short-wavelength region Žfrom 730 to 660 nm.
=exp y Ž a 1 q a . Ž i y 1 . d =exp y
Ž hn y hnm i . 2 Di2
2
,
Ž 3.
where a s a Ž hn . is the PS absorption coefficient in the spectral region of emission, N is the number of luminescent sub-layers.
Fig. 2. The normalised steady-state PL spectra of PS formed at the constant current at the following excitation: 365 Ž1., 405 Ž2., 436 Ž3., 546 Ž4., 578 nm Ž5..
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V.A. SkrysheÕskyr Applied Surface Science 157 (2000) 145–150
ŽFig. 2.. This phenomenon can be challenged by the non-uniformity of incoming PL, because at the increase of the excitation energy the depth of absorption in PS decreases w20x and consequently the depth of PL incoming decreases as well.
4. PL of the inhomogeneous PS film: influence of the acetone adsorption Fig. 3 shows the PL spectra of two PS patterns prepared at the constant and step-like constant current when the PL was excited at lex s 405, 436 and 546 nm and registered in steady-state conditions. At these excited illuminations the full absorption occurs at 1.0-, 1.7- and 8-mm depths, respectivelly w17x. Taking into account the average thickness of PS sub-layers formed at the step-like constant current, the 405-nm illumination is absorbed by the top layer exceptionally, the 436 nm one is absorbed by the upper and underneath layers, the 546 nm one — by
Fig. 3. The normalised steady-state PL spectra of PS formed at 20 mArcm2 Ža–c. and the step-like constant current Žd–f. in the ambient atmosphere Ž1. and in the saturated acetone vapours Ž2. at the excitation of 405 nm Ža,d., 436 nm Žb,e., 546 nm Žc,f..
the whole PS film. For the first type of PS, the ratio of PL yield for 405-, 436- and 546-nm excitation is 0.44:0.78:0.20 in the ambient atmosphere and 0.8:1.0:0.20 in the saturated acetone vapours. For the second type of PS film the ratio is 0.36:0.87:0.44 in the ambient atmosphere and 0.54:1.0:0.30 in the saturated acetone vapours. The main experimental results are summarised below. Ži. For both type of PS samples the increase of lex evokes the red shifting of the PL spectra from 680 nm at lex s 405 nm up to 730 nm at lex s 546 nm. Simultaneously, the FWHM decreases from 360 to 250 meV. Žii. The effect of the acetone adsorption depends on lex . At 405-nm illumination the acetone adsorption results in PL yield increasing and the red shifting of the PL maximum, at the 436-nm illumination it results in the increase of PL yield and blue shifting of the peak, at the 546-nm illumination it reduces the PL intensity and leads to the blue shifting of spectra. Žiii. The PS layers formed at the step-like constant current Žthat means the enlargement of PS inhomogeneity. qualitatively reveals the same peculiarities of PL, however, the acetone adsorption leads to more significant suppressing of PL. Kinetics of PL yield of PS films formed at the constant current strongly depends on lex . At the beginning of measurement of the as-prepared PS layer the intensity of PL in the acetone vapours is less than in the ambient atmosphere for every lex . However, at the 405-nm excitation, the PL intensity decreases in the ambient atmosphere and increases in acetone vapours up to the saturation ŽFig. 4a.. After a 6-min excitation the PL yield is higher when the acetone molecules are adsorbed. On the other hand, the excitation at 546 nm causes no change of PL intensity ŽFig. 4b.. For the as-prepared PS layer formed at the step-like constant current, the stronger quenching is observed in the acetone atmosphere ŽFig. 4a,b.. However, the trend of the PL increase during measurement in acetone atmosphere rests for a 405-nm excitation. The difference in PL kinetics among short- and long-wavelength excitation can be explained by the photooxidation of the silicon nanocrystallites of the uppermost sub-layers, which is caused by the shortwavelength excitation w21–23x. The chemical or electrochemical derivatization reactions can modify the
V.A. SkrysheÕskyr Applied Surface Science 157 (2000) 145–150
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the principal contribution to the PL incoming is given by the inner sub-layers of PS that was predicted in item 3 of this article ŽFig. 2.. For a more inhomogeneous PS film formed at the step-like constant current, the PL of the oxidised upper layers excited at 546 nm is noticeable.
5. Conclusion
Fig. 4. The normalised PL intensity at 650 nm versus time of the measurement at the excitation of 405 nm Ža. and 546 nm Žb. in the ambient atmosphere and in the saturated acetone vapours of PS formed at the 20 mArcm2 Ž1. and the step-like constant current Ž2..
ability of PS to gas sensing w6,24,25x. For example, reactions that impart a degree of hydrophilicity to the PS surface change the ability of H 2 O and ethanol to quench the PL. Figs. 3 and 4 show that the acetone adsorption affects differently on the as-prepared nonoxidised and oxidised layers owing to 405-nm illumination. The intensity of the as-prepared PS is strongly quenched by acetone vapours ŽFig. 4.. Vice-versa, the material that was made more hydrophilic by the surface oxidation is quenched to a lesser degree by the acetone molecules ŽFig. 3. with good according to the hydrophobic–hydrophilic PS modification. The acetone adsorption increases the non-radiative recombination canal for the as-prepared layers, whereas the adsorbed acetone molecules act as a passivation coating for the oxidised PS that reduces the non-radiative recombination. For the 546-nm excitation, as the PL is not changed neither in the ambient atmosphere nor in acetone vapours,
The inhomogeneity of PS layers transforms significantly the PL behaviour under the gas adsorption. For PL excited by the long-wavelength illumination, the quenching of the intensity is observed under adsorption of acetone molecules. At the excitation of the PL by the short-wavelength illumination, the photooxidation of surface region of PS occurs in the ambient atmosphere. The acetone adsorption on this top layer increases the PL yield, whereas the PL of the inner layers is quenched. Thus, the total PL incoming will be defined by the absorption depth of the excitation and ratio of nonoxidisedroxidised thickness of a porous film. This circumstance should be considered in elaboration of PL gas sensors with PS layers. On the other hand, the specific sensing of the different PS sub-layers to the adsorbates can be used to improve the selectivity of PL gas sensors.
References w1x M.J. Sailor, C.L. Curtis, K.P. Dancil, E.J. Lee, J.H. Song, in: Proc. Int. Conf. ‘‘Porous Semiconductors, Science and Technology’’, Mallorca, 1998, p. 114. w2x G. Cullis, L.T. Canham, P.D.G. Calcott, J. Appl. Phys. 82 Ž1997. 909. w3x Y.S. Tsuo, Y. Xiao, C.A. Moore, in: Z.C. Fengs, R. Tsu ŽEds.., Porous silicon, World Scientific, Singapore, 1994, p. 347. w4x V.A. Skryshevsky, A. Laugier, V.I. Strikha, V.A. Vikulov, Mat. Sci. Eng. B 40 Ž1996. 54. w5x M.J. Sailor, E. Lee, J. Adv. Mater. 9 Ž1997. 783. w6x J.M. Lauerhaas, G.M. Credo, J.L. Heinrich, M.J. Sailor, J. Am. Chem. Soc. 114 Ž1992. 1911. w7x G. Dolino, D. Bellet, Thin Solid Films 255 Ž1995. 132. w8x J.M. Rehm, G.L. Mclendon, L. Tsybeskov, P.M. Fauchet, Appl. Phys. Lett. 66 Ž1995. 3669. w9x V.M. Dubin, F. Ozanam, J.N. Chazalviel, Phys. Rev. B 50 Ž1994. 14867. w10x M.A. Hory, R. Herino, M. Ligeon, F. Muller, F. Gaspard, I. Mihalcescu, J.C. Vial, Thin Solid Films 255 Ž1995. 200.
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w11x J.L. Coffer, S.C. Lilley, R.A. Martin, L.A. Files-Sesler, J. Appl. Phys. 74 Ž1993. 2094. w12x T. Dittrich, E.A. Konstantinova, V.Ya. Timoshenko, Thin Solid Films 255 Ž1995. 238. w13x M. Ben-Chorin, A. Kux, I. Schechter, Appl. Phys. Lett. 64 Ž1994. 481. w14x K. Li, D.C. Diaz, J.C. Campbell, C. Tsai, in: Z.C. Fengs, R. Tsu ŽEds.., Porous Silicon, World Scientific, Singapore, 1994, p. 261. w15x G.D. Francia, V.L. Ferrara, L. Quercia, in: Proc. Int. Conf. ‘‘Porous Semiconductors, Science and Technology’’, Mallorca, 1998, p. 173. w16x St. Frohnhoff, M.G. Berger, M. Thonissen, C. Dieker, L. Vescan, H. Munder, H. Luth, Thin Solid Films 255 Ž1995. 59. w17x N. Koshida, H. Koyama, Y. Suda, Y. Yamamoto, M. Araki, T. Saito, K. Sato, N. Sata, S. Shin, Appl. Phys. Lett. 63 Ž1993. 774. w18x P.M. Fauchet, in: Z.C. Feng, R. Tsu ŽEds.., Porous Silicon, World Scientific, Singapore, 1994, p. 449.
w19x F. Kozlowski, P. Steiner, W. Lang, in: Z.C. Feng, R. Tsu ŽEds.., Porous Silicon, World Scientific, Singapore, 1994, p. 149. w20x S. Guha, P. Steiner, F. Kozlowski, W. Lang, J. Porous Mater. 4 Ž1997. 227. w21x V.A. Skryshevsky, V.A. Vikulov, V.I. Strikha, L. Tristani, in: M. Balkanski ŽEd.., Fabrication, Properties and Applications of Low-Dimensional Semiconductor Structures, NATO ASI Series, Kluwer Academic Publishers, 1995, p. 173. w22x H. Elhouichet, M. Oueslati, B. Bessais, H. Ezzaouia, in: Proc. Int. Conf. ‘‘Porous Semiconductors, Science and Technology’’, Mallorca, 1998, p. 194. w23x V.A. Skryshevsky, V.I. Strikha, V.A. Vikulov, A.V. Kozinetc, A.V. Mamikin, A. Laugier, in: Proc. First Polish– Ukrainian Symposium ‘‘New Photovoltaic Materials for Solar Cells’’, Cracow, E-MRS, 1996, p. 202. w24x T.F. Harper, M.J. Sailor, J. Am. Chem. Soc. 119 Ž1997. 6943. w25x E.J. Lee, J.S. Ha, M.J. Sailor, J. Am. Chem. Soc. 117 Ž1995. 8295.
Photoluminescence of inhomogeneous porous silicon at gas adsorption V.A. Skryshevsky ) Laboratory of Semiconductor DeÕices, Radiophysics Department, National KieÕ SheÕchenko UniÕersity, 64 Vladimirskaya, 01033, KieÕ, Ukraine Received 2 October 1999; accepted 22 November 1999
Abstract An impact of inhomogeneities on photoluminescence ŽPL. of porous silicon ŽPS. is analysed using numerical simulation and supported experiment under gas adsorption. Depending on the excitation wavelength and the condition of measurement Žsteady-state or transient mode., the gas adsorption can result in the quenching or increase of PL. The emission efficiency of the as-prepared porous layers is shown to decrease at the adsorption of acetone molecules for each excitation wavelength in the 405–546-nm range. However, if the 546-nm excitation causes small change of the photoluminescence spectrum during exposition, the 405-nm excitation quenches the PL in the ambient air and increases emission efficiency in acetone vapours. Effects are discussed with the model approach of different recombination properties, and the contribution to the PL incoming of the upper photooxidised and bottom nanocrystalline layers. We describe the method of the PL spectra fitting in order to determine the gradient profile of porous layers having depth irregularity of PL. q 2000 Published by Elsevier Science B.V. All rights reserved. Keywords: Porous silicon; Photoluminescence; Inhomogeneity; Gas adsorption
1. Introduction Recently, the luminescent porous silicon ŽPS. has been regarded as attractive material for various sensor applications w1–4x. Chemical sensors based on PS photoluminescence ŽPL. can bring advantages due to a strong sensing of PL efficiency to adsorbates. In fact, the adsorption of some molecules results in the reduction or even disappearance of the
)
Tel.: q380-44-266-4009; fax: q380-44-265-6744. E-mail address: [email protected] ŽV.A. Skryshevsky..
strong visible PL that recovers entirely after the removing of solvents w5,6x, because no net chemical reaction occurs during the adsorptionrdesorption of molecules. The mechanisms of PL quenching by adsorbates are summarised in Refs. w1,7–9x and include the following models: Ži. the increase of the non-radiative recombination rate in the nanoparticles due to the alteration of the dielectric medium outside the Si nanocrystallites; Žii. the enhancement of the non-radiative vibronic coupling to the surface vibrational modes; Žiii. the change of the nanoparticle surface electronic structure; Živ. the capture increase on the non-radiative traps at the forming of the
0169-4332r00r$ - see front matter q 2000 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 9 . 0 0 5 6 0 - 7
146
V.A. SkrysheÕskyr Applied Surface Science 157 (2000) 145–150
strain-induced defects when molecules are adsorbed, etc. The correlation between PL quenching and solvent dipole moment, established by authors of Ref. w6x, means that the solvent induces the capture of electrons or holes by the surface traps. The molecules with the large dipole moment, as methanol or ethanol, quench the PL stronger in comparison with the weak quenching by benzene or toluene having a small dipole moment. The reversible quenching phenomenon has been interpreted as the stabilisation of the surface traps by the alignment of molecular dipoles on the PS surface. Usually, the PL quenching is accompanied by the asymmetrical shape of the PL spectrum. The blue shifting or the decrease of the emission in the long wavelength region was observed after the treatment in the boiling CCl 4 w10x, organoamine molecules w11x, C 2 H 5 OH w12x, and methanol w13x. However, there are the experimental evidences of anomalous behaviour of PL quantum efficiency at the adsorption of some molecules w14,15x. Taking into consideration that the PL of PS is correlated closely with the nanostructure of porous materials and their surface reactivity, the nature of the observed asymmetrical shape of the PL is correlated with the inhomogeneity of the luminescent layer. Thus, the study of the alteration of PL properties in various environments can be a useful tool for the analysis of the PS inhomogeneity. To get an understanding of the effects of the structural–chemical inhomogeneity on PL, that can take place in the electrochemically etched PS, the spectra and kinetics of PS emission in the ambient atmosphere and saturated acetone vapours were investigated in the present work.
2. Experimental The porous layers were prepared on phosphorus doped, Ž100. oriented silicon substrates of 4.5-V cm resistivity and 350-mm thickness. The growing of the porous layers was provided by the anodization in the solution of 48 wt.% HF: C 2 H 6 O Ž1:2. with two regimes: at the constant current density of 20 mArcm2 for 15 min and at discrete subsequence
steps of the current, namely, 40, 32, 24, 16 and 8 mArcm2 for 3 min of each current value. The selected technological regimes allow the formation of the material with the porosity within 60–70% w16x. The rate of PS layer growth is about 6–10 nmrs. In this case, the thickness of each layers formed by the second regime is estimated to be equal 1.1–1.8 mm. The PL of PS has been excited by the DRSh-250-2 mercury lamp through the system of the optical filters with a singular selected emission at 365, 405, 436, 546 or 578 nm. The PL spectra of the as-prepared PS films were measured in transient and steady-state modes in two atmospheres — ambient air and saturated acetone vapours. The steady state mode is achieved by the stabilization of the PL intensity when the patterns are excited at 405 nm for 30 min, then by 436 nm for 5 min and 546 nm for 5 min. Such a treatment evokes the 1.8-time reduction of the PL yield for 10 min and then PL exhibits no change during measurement neither in ambient atmosphere nor in acetone vapours.
3. PL of multilayer PS film: simulation Let us consider a multi-layer film where each thin sub-layer thickness of d is characterised by the proper absorption and emission spectra. Hence, the depth profile of the PL incoming is tightly coupled with the depth profile of nanocrystallite sizes Žand the porosity. and the light absorption by the upper PS layers. Owing to the absorption, whole upper sub-layers act as the selective optic filters for the bulk PL. In general, the number of the absorbed photons that excites PL per unit of time, unit of area and unit of energy interval for i sub-layer is given by: G s G 0 Ž 1 y R . exp Ž ya 1 x . 1 y exp Ž ya 1 d . ,
Ž 1. where G 0 is the density of the photon flux from the excitation source, R is the coefficient of the surface reflectivity, a 1 is the PS absorption coefficient at
V.A. SkrysheÕskyr Applied Surface Science 157 (2000) 145–150
Fig. 1. The calculated PL spectra of PS consisting of two layers: 0.25 mm top layer with 1.9-eV emission and 0.75-mm bottom layer with 1.7-eV emission. Here x indicates the depth Žin mm. of full PL quenching. Excitation energy is 3.4 eV.
the wavelength of the excited illumination, x is the depth of the sub-layer from the PS surface. The number of photons emitted by every sub-layer i in the approximation of Gauss-type spectrum is written as: Ie i Ž hn , x . s G 0 Ž 1 y R . exp Ž ya 1 x . = 1 y exp Ž ya 1 d . =exp y
hi Di'2p
Ž hn y hnm i .
2
2 Di2
,
Ž 2.
where hi is the internal quantum efficiency for isotropic emission; hnm i , Di are parameters of Gauss-type PL spectrum. Summarising the emission from each sub-layer and taking into consideration the optical losses at the absorption of emission by upper PS sub-layers, the intensity of the measured PL is expressed as: N hi Ie Ž hn . s G 0 Ž 1 y R . 1 y exp Ž ya 1 d . Ý ' is1 D i 2 p
147
Fig. 1 shows the calculated PL spectra for film, consisting of 0.25-mm top layer having a 1.9-eV emission and a 0.75-mm bottom layer with 1.7-eV emission, considering that h1 s h 2 and a Ž hn . taken from Ref. w17x. The resulting PL spectrum is the sum of the strong short-wavelength emission from the surface region and slow long-wavelength emission from the deeper part of PS Žcurve 1.. If the gas exposure results in the PL quenching, the emission spectrum changes asymmetrically. The deeper the penetration of adsorbed molecules into PS layer, the stronger quenching of the short wavelength PL. The reduction of the PL efficiency is accompanied by the red shifting of the PL spectra Žcurves 2–5.. Curve 6 indicates the case of full suppression of 1.9-eV emission. There are few evidences that the anodically formed PS films exhibit the depth irregularity of the luminescence properties. The full width at half maximum ŽFWHM. of the PS emission lies within of 150–500 meV w18x. Large FWHM implies that the emission is coming from the ensemble of nanocrystallites having a size dispersion. Besides, the short wavelength component of PL decays faster than the long one w18x. Usually, the PL and the electroluminescence ŽEL. exhibit non-coincident spectra because the different depth regions of PS are responsible for these emissions. It was shown w19x that the PL yield is maximum from a few micrometers below the surface, whereas only a thin top layer shows EL. The shorter excitation wavelength Žfrom 578 to 365 nm., the stronger the shift of PL maximum in the short-wavelength region Žfrom 730 to 660 nm.
=exp y Ž a 1 q a . Ž i y 1 . d =exp y
Ž hn y hnm i . 2 Di2
2
,
Ž 3.
where a s a Ž hn . is the PS absorption coefficient in the spectral region of emission, N is the number of luminescent sub-layers.
Fig. 2. The normalised steady-state PL spectra of PS formed at the constant current at the following excitation: 365 Ž1., 405 Ž2., 436 Ž3., 546 Ž4., 578 nm Ž5..
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V.A. SkrysheÕskyr Applied Surface Science 157 (2000) 145–150
ŽFig. 2.. This phenomenon can be challenged by the non-uniformity of incoming PL, because at the increase of the excitation energy the depth of absorption in PS decreases w20x and consequently the depth of PL incoming decreases as well.
4. PL of the inhomogeneous PS film: influence of the acetone adsorption Fig. 3 shows the PL spectra of two PS patterns prepared at the constant and step-like constant current when the PL was excited at lex s 405, 436 and 546 nm and registered in steady-state conditions. At these excited illuminations the full absorption occurs at 1.0-, 1.7- and 8-mm depths, respectivelly w17x. Taking into account the average thickness of PS sub-layers formed at the step-like constant current, the 405-nm illumination is absorbed by the top layer exceptionally, the 436 nm one is absorbed by the upper and underneath layers, the 546 nm one — by
Fig. 3. The normalised steady-state PL spectra of PS formed at 20 mArcm2 Ža–c. and the step-like constant current Žd–f. in the ambient atmosphere Ž1. and in the saturated acetone vapours Ž2. at the excitation of 405 nm Ža,d., 436 nm Žb,e., 546 nm Žc,f..
the whole PS film. For the first type of PS, the ratio of PL yield for 405-, 436- and 546-nm excitation is 0.44:0.78:0.20 in the ambient atmosphere and 0.8:1.0:0.20 in the saturated acetone vapours. For the second type of PS film the ratio is 0.36:0.87:0.44 in the ambient atmosphere and 0.54:1.0:0.30 in the saturated acetone vapours. The main experimental results are summarised below. Ži. For both type of PS samples the increase of lex evokes the red shifting of the PL spectra from 680 nm at lex s 405 nm up to 730 nm at lex s 546 nm. Simultaneously, the FWHM decreases from 360 to 250 meV. Žii. The effect of the acetone adsorption depends on lex . At 405-nm illumination the acetone adsorption results in PL yield increasing and the red shifting of the PL maximum, at the 436-nm illumination it results in the increase of PL yield and blue shifting of the peak, at the 546-nm illumination it reduces the PL intensity and leads to the blue shifting of spectra. Žiii. The PS layers formed at the step-like constant current Žthat means the enlargement of PS inhomogeneity. qualitatively reveals the same peculiarities of PL, however, the acetone adsorption leads to more significant suppressing of PL. Kinetics of PL yield of PS films formed at the constant current strongly depends on lex . At the beginning of measurement of the as-prepared PS layer the intensity of PL in the acetone vapours is less than in the ambient atmosphere for every lex . However, at the 405-nm excitation, the PL intensity decreases in the ambient atmosphere and increases in acetone vapours up to the saturation ŽFig. 4a.. After a 6-min excitation the PL yield is higher when the acetone molecules are adsorbed. On the other hand, the excitation at 546 nm causes no change of PL intensity ŽFig. 4b.. For the as-prepared PS layer formed at the step-like constant current, the stronger quenching is observed in the acetone atmosphere ŽFig. 4a,b.. However, the trend of the PL increase during measurement in acetone atmosphere rests for a 405-nm excitation. The difference in PL kinetics among short- and long-wavelength excitation can be explained by the photooxidation of the silicon nanocrystallites of the uppermost sub-layers, which is caused by the shortwavelength excitation w21–23x. The chemical or electrochemical derivatization reactions can modify the
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the principal contribution to the PL incoming is given by the inner sub-layers of PS that was predicted in item 3 of this article ŽFig. 2.. For a more inhomogeneous PS film formed at the step-like constant current, the PL of the oxidised upper layers excited at 546 nm is noticeable.
5. Conclusion
Fig. 4. The normalised PL intensity at 650 nm versus time of the measurement at the excitation of 405 nm Ža. and 546 nm Žb. in the ambient atmosphere and in the saturated acetone vapours of PS formed at the 20 mArcm2 Ž1. and the step-like constant current Ž2..
ability of PS to gas sensing w6,24,25x. For example, reactions that impart a degree of hydrophilicity to the PS surface change the ability of H 2 O and ethanol to quench the PL. Figs. 3 and 4 show that the acetone adsorption affects differently on the as-prepared nonoxidised and oxidised layers owing to 405-nm illumination. The intensity of the as-prepared PS is strongly quenched by acetone vapours ŽFig. 4.. Vice-versa, the material that was made more hydrophilic by the surface oxidation is quenched to a lesser degree by the acetone molecules ŽFig. 3. with good according to the hydrophobic–hydrophilic PS modification. The acetone adsorption increases the non-radiative recombination canal for the as-prepared layers, whereas the adsorbed acetone molecules act as a passivation coating for the oxidised PS that reduces the non-radiative recombination. For the 546-nm excitation, as the PL is not changed neither in the ambient atmosphere nor in acetone vapours,
The inhomogeneity of PS layers transforms significantly the PL behaviour under the gas adsorption. For PL excited by the long-wavelength illumination, the quenching of the intensity is observed under adsorption of acetone molecules. At the excitation of the PL by the short-wavelength illumination, the photooxidation of surface region of PS occurs in the ambient atmosphere. The acetone adsorption on this top layer increases the PL yield, whereas the PL of the inner layers is quenched. Thus, the total PL incoming will be defined by the absorption depth of the excitation and ratio of nonoxidisedroxidised thickness of a porous film. This circumstance should be considered in elaboration of PL gas sensors with PS layers. On the other hand, the specific sensing of the different PS sub-layers to the adsorbates can be used to improve the selectivity of PL gas sensors.
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