- Email: [email protected]
Reversible photodarkening in As2S3 nanolayers
Journal of Non-Crystalline Solids 227–230 Ž1998. 700–704
Reversible photodarkening in As 2 S 3 nanolayers I.Z. Indutnyi ) , P.E. Shepeljavi Institute of Semiconductor Physics, National Academy of Sciences, prosp. Nauki 45, KyiÕ-28, 252028, Ukraine
Abstract The dependence of reversible photodarkening on film thickness has been measured for island type As 2 S 3 layers embedded in a dielectric SiO matrix, when the layer effective thicknesses are comparable with the scale of the intermediate-range order. The photostimulated shift of the optical absorption edge to smaller energies increases when the As 2 S 3 layer thickness decreases Žfrom 0.08 eV for continuous films to 0.23 eV for composite multilayer structures with As 2 S 3 layers of 0.7 nm effective thickness.. This feature is related to confinement of the photoexcited carrier diffusion length with decreasing As 2 S 3 cluster sizes. A gradual decrease of the absorption coefficient with decreasing As 2 S 3 cluster sizes is caused by As–O and S–S bond formation at the As 2 S 3 –SiO interface. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Photodarkening; As 2 S 3 ; SiO
1. Introduction The photostimulated changes in the optical properties of chalcogenide glasses ŽChG. have been studied for more than two decades. The most investigated model system is that of As 2 S 3 vacuumevaporated layers which exhibit both irreversible Žphotopolymerization. and reversible photostimulated transformations. The reversible changes are displayed in both layers and glasses. They are fundamentally interesting and have attracted much attention in practical aspects, especially the red shift of the optical absorption edge Ži.e., photodarkening. and photostimulated optical anisotropy w1–3x. To exclude irreversible processes, As 2 S 3 films are usually annealed after deposition and then exposed to
)
Corresponding author. Tel.: q7-380-44-265-57-77; fax: q7380-44-265-83-42; e-mail: [email protected].
interband light. It has been revealed that the photodarkening efficiency depends on temperature; it increases with decreasing temperature. At low temperatures, an absorption edge shift of up to 0.15 eV was obtained w4x. These changes may be reversed by heat treatment near the glass-transition temperature. Such recording–erasing cycles may be repeated many times, illustrating the possibilities for reversible information recording. Numerous investigations involving different experimental methods have led to the suggestion that reversible photostimulated changes of ChG optical properties are connected with structural changes w5– 10x. A number of mechanisms associated with photostructural changes have been proposed w8,9x, but as yet there is no complete explanation of all experimental results. Models based on the latest experimental data suppose that photostructural relaxation results in changes in the medium-range order on a scale of 0.4 to 0.8 nm w10x. It is of interest therefore
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 1 4 9 - 5
I.Z. Indutnyi, P.E. ShepeljaÕir Journal of Non-Crystalline Solids 227–230 (1998) 700–704
to study the dependence of photostructural transformations and the corresponding optical changes on layer thickness. Such investigations were performed on As 2 S 3 at film thicknesses of 10 to 800 nm and no thickness dependence was revealed w11x. But this interval of thicknesses is rather far from the medium-range order scale to account for the results. In the present paper, we have studied the reversible photodarkening of island-type As 2 S 3 layers with effective thicknesses comparable to intermediate-range order scale. The results obtained are discussed in the framework of the latest models for reversible photostimulated processes in ChG.
2. Experimental procedures The samples were prepared by thermal evaporation at a pressure of 10y5 Torr and deposition onto polished silica substrates at room temperature. Optical studies of superthin films are difficult because of their small optical density w12x. To enhance the accuracy of the optical measurements we prepared multilayer samples by sequential deposition of As 2 S 3 and SiO layers. A large number of layer couples was used to obtain a total As 2 S 3 thickness in multilayer samples from 90 to 130 nm. The effective thickness of each SiO layer was equal to 5 nm for all samples, but the effective thicknesses of the As 2 S 3 layers were changed for different samples from 2.5 to 0.7 nm Žthe number of layers were also changed to obtain the required total As 2 S 3 thickness.. The layers were deposited at a rate of 0.1 nmrs, the thickness being monitored ‘in situ’ by a quartz-crystaloscillator monitoring system ŽKIT-1.. During the SiO deposition, part of the sample was screened to obtain a control film of As 2 S 3 composition. It is possible to measure the total As 2 S 3 thickness using a microinterferometer ŽMII-4. and then to obtain Žwith ; 10% accuracy. the effective thickness of each As 2 S 3 layer in the composite samples by dividing by the number of layers. The deposited films were annealed at 450 K in a N2 atmosphere for 2 h. Photodarkening was induced at 77 K by exposure to a high-pressure Hg lamp Žlight intensity 50 mWrcm2 . for 1 h. The composite samples obtained form an effective SiO–As 2 S 3 media with a smaller fraction of As 2 S 3 .
701
SiO layers are transparent in the spectral range of As 2 S 3 interband transitions which enables the observation of the photodarkening of As 2 S 3 particles in the SiO matrix. The transmittance T, air-incident R, and substrate-incident RX reflectances of the samples were measured at normal incidence using a spectrophotometer ŽKSVU-23.. The optical constants of the investigated films may be calculated, taking into account interference effects, by using these measured values, the sample thickness and the optical constants of the silica substrate Žfor details, see Ref. w12x.. This technique was used to measure the continuous As 2 S 3 films as control samples. Composite SiO–As 2 S 3 layers and silica have rather similar indexes of refraction, which results in reflectances, ; 3%, at a layer–substrate interface. This is why for absorption coefficient Ž a . calculations of superthin As 2 S 3 layers, it is possible to use, with an accuracy ; 5%, the approximate formula neglecting interference: 1 1yR a s ln Ž 1 y a q aRX . q Ta d T
Ž 1.
where a s Ž n 3 y 1. 2rŽ4 n 3 ., where n 3 is the silica index of refraction. This expression was applied to the SiO–As 2 S 3 films, where d is the total As 2 S 3 thickness in the multilayer samples. The measured a values are due to the As 2 S 3 absorption in composite SiO–As 2 S 3 samples.
3. Results Fig. 1 shows absorption spectra of Ža. an As 2 S 3 layer and Žb. a composite ŽSiO–As 2 S 3 .N films Ž N s number of SiO–As 2 S 3 layer-couples in the sample. with an effective thickness of each As 2 S 3 layer of 1.4 nm, N s 90 and Žc. 0.7 nm, N s 130. The open circles are the absorption after annealing of the films, while the solid circles are the same ones after illumination. All samples exhibit photodarkening, i.e., a red shift of the optical absorption edge. The absorption edge of all samples can be expressed by w13x:
a hn A Ž hn y Eg .
2
Ž 2.
702
I.Z. Indutnyi, P.E. ShepeljaÕir Journal of Non-Crystalline Solids 227–230 (1998) 700–704
where hn is the photon energy. Eg is often used as a measure of the optical band gap. However, the span of the square dependence is not large enough in the multilayer sample spectra to obtain an accurate value of Eg by this method. The values of the photostimulated optical-absorption edge shift, D Eg , are shown in Table 1, where the first row displays data for continuous 120 nm thickness As 2 S 3 films, the next four rows for multilayers ŽSiO–As 2 S 3 .N composite structures. The optical gap changes, D Eg , were obtained by two methods: D Eg1 , the absorption edge shift, at a s 2.5 = 10 4 cmy1 , D Eg2 , using Eq. Ž2.. In Table 1 are shown also the D Eg data for the second cycle of annealing-exposure. The second annealing was performed in air for a continuous ‘thick’ As 2 S 3 film and two composite films with N s 90 and 130. The following differences were found in absorption spectra and photodarkening of composite sam-
Table 1 Photostimulated optical gap change D Eg1 ŽeV.
D Eg2 ŽeV.
First cycle 1 120 48 2.5 60 1.6 90 1.4 130 0.7
0.08"0.01 0.12"0.01 0.14"0.01 0.19"0.01 0.23"0.01
0.09"0.01 0.12"0.02 0.14"0.02 0.17"0.02 0.22"0.02
Second cycle 1 120 90 1.4 130 0.7
0.08"0.01 0.29"0.02 0.35"0.02
N
Layer thickness Žnm.
ples with ultrathin As 2 S 3 layers in comparison with continuous As 2 S 3 films: 1. The photostimulated optical-absorption edge shift, D Eg , increases when the As 2 S 3 layer thickness decreases. 2. The absorption coefficient of the layers is smaller than that of continuous control films. 3. At the second cycle annealing in air exposure, D Eg for the continuous film does not change, but the layers D Eg increases. 4. At the second annealing, the absorptivities of samples with N s 90 and 130 decreases by ; 10%.
4. Discussion
Fig. 1. Spectral dependence of Ž a hn .1r 2 of As 2 S 3 layers with thickness Ža. 120 nm, and multilayers samples ŽSiO–As 2 S 3 .N with effective As 2 S 3 layer thicknesses Žb. 1.4 nm, N s90 and Žc. 0.7 nm, N s130. Ž ± . Annealed, Žv . exposed samples. Solid lines are linear approximations.
We compare these results with existing models of reversible As 2 S 3 photodarkening. Many authors use a phenomenological cofigurational-coordinate model w14x which illustrates metastable state formation in ChG as a result of excitation of the electron system and subsequent relaxation of the amorphous structure. However, many microscopic mechanisms of this process have been proposed: for example, breaking of inter- or intra-molecular bonds w8x, atomic relaxation due to non-radiative recombination of photoexcited carriers through transient self-trapped excitons ŽSTE. which results in a change in the intermediate-range structure w9x, and excitation of lone-pair electrons of the helical S atoms with subsequent relaxation of the structure w10x. Most of the
I.Z. Indutnyi, P.E. ShepeljaÕir Journal of Non-Crystalline Solids 227–230 (1998) 700–704
more recent investigations have led to the conclusion that photodarkening is related to photostimulated changes in the medium-range structure of As 2 S 3 w7,9,11x; previous models which considered photogenerated defects Žhomopolar bonds, valence-alternation pairs. to be the cause of photodarkening are inconsistent with the experimental results. Photoexcited electron-hole pairs may recombine via a number of non-radiative and radiative paths. The non-radiative recombination path was suggested by Street w14x —the excited carriers form a localized exciton-like state with a bonding rearrangement. Such a self-trapped exciton can relax to other bonding configuration which differs from the original state. As suggested by Fritzsche w9x, these recombinationinduced bond rearrangements accumulate during exposure to produce a more disordered structure Žon the medium-range scale. that results in photodarkening. Recombination through localized exciton-like state is also proposed in a model w10x based on X-ray absorption measurements. When a photoexcited electron diffuses over a long distance, the probability of STE creation diminishes and the effectiveness of photodarkening becomes less. The models which include a recombination stage through STE consistently explain the temperature dependence of photodarkening: with decreasing temperature the electrons are localized near the sites of excitation, the probability of STE formation increases, and the effect increases. By using such models, we can explain the observed dependence of D Eg on effective layer thickness Žwhich is related to the As 2 S 3 particle dimensions.. When the size of the As 2 S 3 particles embedded in the dielectric SiO matrix is decreased, the diffusion length of the photoexcited electrons is confined, resulting in an increase of the rate for STE formation, photostructural transformations and photodarkening. The increase of the photostimulated change, D Eg , in As 2 S 3 layers embedded in a SiO matrix may also be related to the effects of strain. It is established that under hydrostatic compression the energy gap of As 2 S 3 decreases w15x. On the other hand, As 2 S 3 photodarkening is accompanied by a reversible increase in volume w2x. Photostimulated expansion of particles, embedded in a non-expanding SiO matrix could produce mechanical strain and decreasing Eg
703
Žincreasing D Eg .. After Ref. w16x, for light-soaked As 2 S 3 films, the total photostimulated increase of volume fraction is DVrV s 6 = 10y3 . Using this value along with the dependencies of Eg on pressure w15x and the compressibility of As 2 S 3 w17x, it is possible to estimate the corresponding value of D Eg . For As 2 S 3 , we obtained D Eg - 0.02 eV, which is smaller than the experimental values. Beside, by using the strain hypothesis, it is difficult to explain the dependence of D Eg on effective layer thickness. We therefore conclude that our results are more consistent with a confinement of the photoexcited electron diffusion length which results in an increased probability of transient STE formation and photodarkening. We believe that the decrease of a in thin-layers structures in comparison with continuous As 2 S 3 layers is connected with the creation of As–O and S–S bonds at the As 2 S 3 –SiO interface. We assume that these local regions are microinclusions of phases with different absorptivity Žthe optical gap of sulfur is approximately 3 eV, and that of As 2 O 3 is 5 eV w18x.. These differing absorptions results in decreased sample absorptivity in the spectral region of As 2 S 3 interband absorption edge. The second cycle of annealing-illumination leads to an oxidation of the particle surfaces and the observed decrease of a . Such decreasing of As 2 S 3 particle sizes increase the confinement of the photoexcited electron diffusion length and leads to an increased D Eg . For continuous films, the relative number of surface atoms is smaller by two to three orders of magnitude and surface oxidation is not detected; hence a reversible change of Eg was obtained.
5. Conclusions In this work, we have determined the thickness dependence of the photostimulated optical absorption edge shift in island-type As 2 S 3 layers embedded in a SiO matrix. Based on our results, we propose the effect is related to confinement of the photoexcited electron diffusion length when the As 2 S 3 particle dimensions decrease. This results in an increased probability of STE formation and subsequent recombination-induced bond rearrangements.
704
I.Z. Indutnyi, P.E. ShepeljaÕir Journal of Non-Crystalline Solids 227–230 (1998) 700–704
References w1x V.M. Lyubin, Avtometrija ŽRussia. 14 Ž1988. 18. w2x Ke. Tanaka, Avtometrija ŽRussia. 14 Ž1988. 12. w3x A.E. Owen, A.P. Firth, P.J.S. Ewen, Philos. Mag. B 52 Ž1985. 347. w4x Ke. Tanaka, J. Non-Cryst. Solids 59–60 Ž1983. 925. w5x C.Y. Yang, M.A. Paesler, D.E. Sayers, Phys. Rev. B 36 Ž1987. 9160. w6x Ke. Tanaka, Thin Solid Films 157 Ž1988. 35. w7x W. Zhou, J.M. Lee, D.E. Sayers, M.A. Paesler, J. Non-Cryst. Solids 114 Ž1989. 43. w8x S.R. Elliott, J. Non-Cryst. Solids 81 Ž1986. 71. w9x H. Fritzsche, Philos. Mag. B 68 Ž1993. 561. w10x J.M. Lee, M.A. Paesler, D.E. Sayers, J. Non-Cryst. Solids 123 Ž1990. 295.
w11x H. Eguchi, Y. Suzuki, M. Hirai, J. Non-Cryst. Solids 95–96 Ž1987. 757. w12x I.Z. Indutnyi, A.I. Stetsun, Proc. SPIE 2113 Ž1993. 55. w13x N.F. Mott, F.A. Davis, Electron Processes in Non-Crystalline Materials, Claredon Press, Oxford, 1979. w14x R.A. Street, Solid State Commun. 24 Ž1977. 363. w15x Ke. Tanaka, J. Non-Cryst. Solids 90 Ž1987. 363. w16x C.Y. Yang, M.A. Paesler, D.E. Sayers, Phys. Rev. B 36 Ž1987. 9160. w17x S.V. Svechnikov, V.V. Himinets, N.J. Dovgoshey, Complex Non-Crystalline Chalcogenide and Chalcohalide and Their Applications in Optoelectronics, Naukova Dumka, Kyiv, 1992. w18x W.M. Pontuschka, P.C. Taylor, Solid State Commun. 38 Ž1981. 573.
Reversible photodarkening in As 2 S 3 nanolayers I.Z. Indutnyi ) , P.E. Shepeljavi Institute of Semiconductor Physics, National Academy of Sciences, prosp. Nauki 45, KyiÕ-28, 252028, Ukraine
Abstract The dependence of reversible photodarkening on film thickness has been measured for island type As 2 S 3 layers embedded in a dielectric SiO matrix, when the layer effective thicknesses are comparable with the scale of the intermediate-range order. The photostimulated shift of the optical absorption edge to smaller energies increases when the As 2 S 3 layer thickness decreases Žfrom 0.08 eV for continuous films to 0.23 eV for composite multilayer structures with As 2 S 3 layers of 0.7 nm effective thickness.. This feature is related to confinement of the photoexcited carrier diffusion length with decreasing As 2 S 3 cluster sizes. A gradual decrease of the absorption coefficient with decreasing As 2 S 3 cluster sizes is caused by As–O and S–S bond formation at the As 2 S 3 –SiO interface. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Photodarkening; As 2 S 3 ; SiO
1. Introduction The photostimulated changes in the optical properties of chalcogenide glasses ŽChG. have been studied for more than two decades. The most investigated model system is that of As 2 S 3 vacuumevaporated layers which exhibit both irreversible Žphotopolymerization. and reversible photostimulated transformations. The reversible changes are displayed in both layers and glasses. They are fundamentally interesting and have attracted much attention in practical aspects, especially the red shift of the optical absorption edge Ži.e., photodarkening. and photostimulated optical anisotropy w1–3x. To exclude irreversible processes, As 2 S 3 films are usually annealed after deposition and then exposed to
)
Corresponding author. Tel.: q7-380-44-265-57-77; fax: q7380-44-265-83-42; e-mail: [email protected].
interband light. It has been revealed that the photodarkening efficiency depends on temperature; it increases with decreasing temperature. At low temperatures, an absorption edge shift of up to 0.15 eV was obtained w4x. These changes may be reversed by heat treatment near the glass-transition temperature. Such recording–erasing cycles may be repeated many times, illustrating the possibilities for reversible information recording. Numerous investigations involving different experimental methods have led to the suggestion that reversible photostimulated changes of ChG optical properties are connected with structural changes w5– 10x. A number of mechanisms associated with photostructural changes have been proposed w8,9x, but as yet there is no complete explanation of all experimental results. Models based on the latest experimental data suppose that photostructural relaxation results in changes in the medium-range order on a scale of 0.4 to 0.8 nm w10x. It is of interest therefore
0022-3093r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 Ž 9 8 . 0 0 1 4 9 - 5
I.Z. Indutnyi, P.E. ShepeljaÕir Journal of Non-Crystalline Solids 227–230 (1998) 700–704
to study the dependence of photostructural transformations and the corresponding optical changes on layer thickness. Such investigations were performed on As 2 S 3 at film thicknesses of 10 to 800 nm and no thickness dependence was revealed w11x. But this interval of thicknesses is rather far from the medium-range order scale to account for the results. In the present paper, we have studied the reversible photodarkening of island-type As 2 S 3 layers with effective thicknesses comparable to intermediate-range order scale. The results obtained are discussed in the framework of the latest models for reversible photostimulated processes in ChG.
2. Experimental procedures The samples were prepared by thermal evaporation at a pressure of 10y5 Torr and deposition onto polished silica substrates at room temperature. Optical studies of superthin films are difficult because of their small optical density w12x. To enhance the accuracy of the optical measurements we prepared multilayer samples by sequential deposition of As 2 S 3 and SiO layers. A large number of layer couples was used to obtain a total As 2 S 3 thickness in multilayer samples from 90 to 130 nm. The effective thickness of each SiO layer was equal to 5 nm for all samples, but the effective thicknesses of the As 2 S 3 layers were changed for different samples from 2.5 to 0.7 nm Žthe number of layers were also changed to obtain the required total As 2 S 3 thickness.. The layers were deposited at a rate of 0.1 nmrs, the thickness being monitored ‘in situ’ by a quartz-crystaloscillator monitoring system ŽKIT-1.. During the SiO deposition, part of the sample was screened to obtain a control film of As 2 S 3 composition. It is possible to measure the total As 2 S 3 thickness using a microinterferometer ŽMII-4. and then to obtain Žwith ; 10% accuracy. the effective thickness of each As 2 S 3 layer in the composite samples by dividing by the number of layers. The deposited films were annealed at 450 K in a N2 atmosphere for 2 h. Photodarkening was induced at 77 K by exposure to a high-pressure Hg lamp Žlight intensity 50 mWrcm2 . for 1 h. The composite samples obtained form an effective SiO–As 2 S 3 media with a smaller fraction of As 2 S 3 .
701
SiO layers are transparent in the spectral range of As 2 S 3 interband transitions which enables the observation of the photodarkening of As 2 S 3 particles in the SiO matrix. The transmittance T, air-incident R, and substrate-incident RX reflectances of the samples were measured at normal incidence using a spectrophotometer ŽKSVU-23.. The optical constants of the investigated films may be calculated, taking into account interference effects, by using these measured values, the sample thickness and the optical constants of the silica substrate Žfor details, see Ref. w12x.. This technique was used to measure the continuous As 2 S 3 films as control samples. Composite SiO–As 2 S 3 layers and silica have rather similar indexes of refraction, which results in reflectances, ; 3%, at a layer–substrate interface. This is why for absorption coefficient Ž a . calculations of superthin As 2 S 3 layers, it is possible to use, with an accuracy ; 5%, the approximate formula neglecting interference: 1 1yR a s ln Ž 1 y a q aRX . q Ta d T
Ž 1.
where a s Ž n 3 y 1. 2rŽ4 n 3 ., where n 3 is the silica index of refraction. This expression was applied to the SiO–As 2 S 3 films, where d is the total As 2 S 3 thickness in the multilayer samples. The measured a values are due to the As 2 S 3 absorption in composite SiO–As 2 S 3 samples.
3. Results Fig. 1 shows absorption spectra of Ža. an As 2 S 3 layer and Žb. a composite ŽSiO–As 2 S 3 .N films Ž N s number of SiO–As 2 S 3 layer-couples in the sample. with an effective thickness of each As 2 S 3 layer of 1.4 nm, N s 90 and Žc. 0.7 nm, N s 130. The open circles are the absorption after annealing of the films, while the solid circles are the same ones after illumination. All samples exhibit photodarkening, i.e., a red shift of the optical absorption edge. The absorption edge of all samples can be expressed by w13x:
a hn A Ž hn y Eg .
2
Ž 2.
702
I.Z. Indutnyi, P.E. ShepeljaÕir Journal of Non-Crystalline Solids 227–230 (1998) 700–704
where hn is the photon energy. Eg is often used as a measure of the optical band gap. However, the span of the square dependence is not large enough in the multilayer sample spectra to obtain an accurate value of Eg by this method. The values of the photostimulated optical-absorption edge shift, D Eg , are shown in Table 1, where the first row displays data for continuous 120 nm thickness As 2 S 3 films, the next four rows for multilayers ŽSiO–As 2 S 3 .N composite structures. The optical gap changes, D Eg , were obtained by two methods: D Eg1 , the absorption edge shift, at a s 2.5 = 10 4 cmy1 , D Eg2 , using Eq. Ž2.. In Table 1 are shown also the D Eg data for the second cycle of annealing-exposure. The second annealing was performed in air for a continuous ‘thick’ As 2 S 3 film and two composite films with N s 90 and 130. The following differences were found in absorption spectra and photodarkening of composite sam-
Table 1 Photostimulated optical gap change D Eg1 ŽeV.
D Eg2 ŽeV.
First cycle 1 120 48 2.5 60 1.6 90 1.4 130 0.7
0.08"0.01 0.12"0.01 0.14"0.01 0.19"0.01 0.23"0.01
0.09"0.01 0.12"0.02 0.14"0.02 0.17"0.02 0.22"0.02
Second cycle 1 120 90 1.4 130 0.7
0.08"0.01 0.29"0.02 0.35"0.02
N
Layer thickness Žnm.
ples with ultrathin As 2 S 3 layers in comparison with continuous As 2 S 3 films: 1. The photostimulated optical-absorption edge shift, D Eg , increases when the As 2 S 3 layer thickness decreases. 2. The absorption coefficient of the layers is smaller than that of continuous control films. 3. At the second cycle annealing in air exposure, D Eg for the continuous film does not change, but the layers D Eg increases. 4. At the second annealing, the absorptivities of samples with N s 90 and 130 decreases by ; 10%.
4. Discussion
Fig. 1. Spectral dependence of Ž a hn .1r 2 of As 2 S 3 layers with thickness Ža. 120 nm, and multilayers samples ŽSiO–As 2 S 3 .N with effective As 2 S 3 layer thicknesses Žb. 1.4 nm, N s90 and Žc. 0.7 nm, N s130. Ž ± . Annealed, Žv . exposed samples. Solid lines are linear approximations.
We compare these results with existing models of reversible As 2 S 3 photodarkening. Many authors use a phenomenological cofigurational-coordinate model w14x which illustrates metastable state formation in ChG as a result of excitation of the electron system and subsequent relaxation of the amorphous structure. However, many microscopic mechanisms of this process have been proposed: for example, breaking of inter- or intra-molecular bonds w8x, atomic relaxation due to non-radiative recombination of photoexcited carriers through transient self-trapped excitons ŽSTE. which results in a change in the intermediate-range structure w9x, and excitation of lone-pair electrons of the helical S atoms with subsequent relaxation of the structure w10x. Most of the
I.Z. Indutnyi, P.E. ShepeljaÕir Journal of Non-Crystalline Solids 227–230 (1998) 700–704
more recent investigations have led to the conclusion that photodarkening is related to photostimulated changes in the medium-range structure of As 2 S 3 w7,9,11x; previous models which considered photogenerated defects Žhomopolar bonds, valence-alternation pairs. to be the cause of photodarkening are inconsistent with the experimental results. Photoexcited electron-hole pairs may recombine via a number of non-radiative and radiative paths. The non-radiative recombination path was suggested by Street w14x —the excited carriers form a localized exciton-like state with a bonding rearrangement. Such a self-trapped exciton can relax to other bonding configuration which differs from the original state. As suggested by Fritzsche w9x, these recombinationinduced bond rearrangements accumulate during exposure to produce a more disordered structure Žon the medium-range scale. that results in photodarkening. Recombination through localized exciton-like state is also proposed in a model w10x based on X-ray absorption measurements. When a photoexcited electron diffuses over a long distance, the probability of STE creation diminishes and the effectiveness of photodarkening becomes less. The models which include a recombination stage through STE consistently explain the temperature dependence of photodarkening: with decreasing temperature the electrons are localized near the sites of excitation, the probability of STE formation increases, and the effect increases. By using such models, we can explain the observed dependence of D Eg on effective layer thickness Žwhich is related to the As 2 S 3 particle dimensions.. When the size of the As 2 S 3 particles embedded in the dielectric SiO matrix is decreased, the diffusion length of the photoexcited electrons is confined, resulting in an increase of the rate for STE formation, photostructural transformations and photodarkening. The increase of the photostimulated change, D Eg , in As 2 S 3 layers embedded in a SiO matrix may also be related to the effects of strain. It is established that under hydrostatic compression the energy gap of As 2 S 3 decreases w15x. On the other hand, As 2 S 3 photodarkening is accompanied by a reversible increase in volume w2x. Photostimulated expansion of particles, embedded in a non-expanding SiO matrix could produce mechanical strain and decreasing Eg
703
Žincreasing D Eg .. After Ref. w16x, for light-soaked As 2 S 3 films, the total photostimulated increase of volume fraction is DVrV s 6 = 10y3 . Using this value along with the dependencies of Eg on pressure w15x and the compressibility of As 2 S 3 w17x, it is possible to estimate the corresponding value of D Eg . For As 2 S 3 , we obtained D Eg - 0.02 eV, which is smaller than the experimental values. Beside, by using the strain hypothesis, it is difficult to explain the dependence of D Eg on effective layer thickness. We therefore conclude that our results are more consistent with a confinement of the photoexcited electron diffusion length which results in an increased probability of transient STE formation and photodarkening. We believe that the decrease of a in thin-layers structures in comparison with continuous As 2 S 3 layers is connected with the creation of As–O and S–S bonds at the As 2 S 3 –SiO interface. We assume that these local regions are microinclusions of phases with different absorptivity Žthe optical gap of sulfur is approximately 3 eV, and that of As 2 O 3 is 5 eV w18x.. These differing absorptions results in decreased sample absorptivity in the spectral region of As 2 S 3 interband absorption edge. The second cycle of annealing-illumination leads to an oxidation of the particle surfaces and the observed decrease of a . Such decreasing of As 2 S 3 particle sizes increase the confinement of the photoexcited electron diffusion length and leads to an increased D Eg . For continuous films, the relative number of surface atoms is smaller by two to three orders of magnitude and surface oxidation is not detected; hence a reversible change of Eg was obtained.
5. Conclusions In this work, we have determined the thickness dependence of the photostimulated optical absorption edge shift in island-type As 2 S 3 layers embedded in a SiO matrix. Based on our results, we propose the effect is related to confinement of the photoexcited electron diffusion length when the As 2 S 3 particle dimensions decrease. This results in an increased probability of STE formation and subsequent recombination-induced bond rearrangements.
704
I.Z. Indutnyi, P.E. ShepeljaÕir Journal of Non-Crystalline Solids 227–230 (1998) 700–704
References w1x V.M. Lyubin, Avtometrija ŽRussia. 14 Ž1988. 18. w2x Ke. Tanaka, Avtometrija ŽRussia. 14 Ž1988. 12. w3x A.E. Owen, A.P. Firth, P.J.S. Ewen, Philos. Mag. B 52 Ž1985. 347. w4x Ke. Tanaka, J. Non-Cryst. Solids 59–60 Ž1983. 925. w5x C.Y. Yang, M.A. Paesler, D.E. Sayers, Phys. Rev. B 36 Ž1987. 9160. w6x Ke. Tanaka, Thin Solid Films 157 Ž1988. 35. w7x W. Zhou, J.M. Lee, D.E. Sayers, M.A. Paesler, J. Non-Cryst. Solids 114 Ž1989. 43. w8x S.R. Elliott, J. Non-Cryst. Solids 81 Ž1986. 71. w9x H. Fritzsche, Philos. Mag. B 68 Ž1993. 561. w10x J.M. Lee, M.A. Paesler, D.E. Sayers, J. Non-Cryst. Solids 123 Ž1990. 295.
w11x H. Eguchi, Y. Suzuki, M. Hirai, J. Non-Cryst. Solids 95–96 Ž1987. 757. w12x I.Z. Indutnyi, A.I. Stetsun, Proc. SPIE 2113 Ž1993. 55. w13x N.F. Mott, F.A. Davis, Electron Processes in Non-Crystalline Materials, Claredon Press, Oxford, 1979. w14x R.A. Street, Solid State Commun. 24 Ž1977. 363. w15x Ke. Tanaka, J. Non-Cryst. Solids 90 Ž1987. 363. w16x C.Y. Yang, M.A. Paesler, D.E. Sayers, Phys. Rev. B 36 Ž1987. 9160. w17x S.V. Svechnikov, V.V. Himinets, N.J. Dovgoshey, Complex Non-Crystalline Chalcogenide and Chalcohalide and Their Applications in Optoelectronics, Naukova Dumka, Kyiv, 1992. w18x W.M. Pontuschka, P.C. Taylor, Solid State Commun. 38 Ž1981. 573.