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Effect of electron-induced dichroism in vitreous As2S3
Journal of Non-Crystalline Solids 227–230 Ž1998. 837–841
Effect of electron-induced dichroism in vitreous As 2 S 3 O.I. Shpotyuk b
a,b,)
, V.O. Balitska a , M.M. Vakiv
a
a LÕiÕ Scientific Research Institute of Materials, Stryjska str. 202, LÕiÕ UA-290031, Ukraine Physics Institute, Pedagogical UniÕersity of Czestochowa, Al. Armii Krajowej 13 r 15, Czestochowa PL-42201, Poland
Abstract Spectral and temperature dependencies of the effect of electron-induced dichroism, its stability at room temperature, as well as microstructural mechanism are investigated in cubic samples of vitreous As 2 S 3. It is shown that this effect is observed in the region of optical absorption edge. The corresponding spectral dependence has an exponential form with an edge in the short-wave region and an extended ‘tail’ in the long-wave direction. The dichroism disappears completely at room temperature after ) 10 days or at the annealing to ; 425 K. The dichroism mechanism is connected with electron-induced formation of new oriented Žrelatively to electron flow. defects in the form of undercoordinated atomic pairs. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Electron-induced dichroism; Vitreous As 2 S 3 ; Optical absorption edge
1. Introduction The effect of electron-induced dichroism ŽEID. was first observed in vitreous v-As 2 S 3 cubic samples using a specific geometry of electron irradiation and experimental measurements w1x. It belongs to a wider group of radiation phenomena in amorphous chalcogenides caused by energetic Ž E ) 1 MeV. ionizing irradiation w2x. In this article we shall consider spectral and temperature dependence of the EID in v-As 2 S 3 , its stability at room temperature and mechanism to compare this effect with well-known linear photoinduced dichroisms studied during the last several years w3,4x.
)
Corresponding author. Tel.: q7-380-322-638303; fax: q7380-322-632228; e-mail: [email protected].
2. Experimental The v-As 2 S 3 samples were synthesized from high-purity constituents Ž99.999%. in evacuated quartz ampoules as described previously w1x. All investigated samples had cubic forms with 6–8-mm rib lengths and high-quality polished plane borders. The irradiation was carried out by a beam of electrons with 2.8 MeV energy and F ) 10 15 cmy2 fluences up to F s 5 = 10 17 cmy2 perpendicularly to the BB1C 1C plane of the v-As 2 S 3 cube ŽFig. 1.. We denoted this plane by 5 sign as its normal vector was parallel to the electron-beam direction. The light beam had a diameter ; 3 mm and passed through the sample at ; 2 mm distance from the irradiated BB1C 1C plane. In similar way, the ABCD plane having the normal vector with perpendicular orientation to the electron beam direction was denoted by H sign. The complete penetration of the accelerated
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 5 9 - 8
838
O.I. Shpotyuk et al.r Journal of Non-Crystalline Solids 227–230 (1998) 837–841
Fig. 1. Scheme illustrating the experimental procedure for EID observation in v-As 2 S 3 .
electrons through the investigated zone was achieved in these conditions w5x. Hence we could distinguish two mutually orthogonal directions for light with parallel aa Ž5. and perpendicular bb ŽH. polarizations with respect to the direction of accelerated electrons ŽFig. 1.. All measurements were carried out one day after electron irradiation using a spectrophotometer ŽSpecord M-40; 200 to 900 nm.. Effects produced by inhomogeneity of scalar electron-induced darkening were excluded by changing the direction of the light beam by 1808 with respect to the sample surface. The EID magnitude was described by k in accordance with the expression
ksD aPds Ž a 5 ya H. Pds
2 Ž t H yt 5 .
Ž t H qt 5 .
Ž 1.
where a 5 Žt 5 . and a H Žt H . are the absorption coefficients Žtransmission coefficients. for light with parallel Žaa. and perpendicular Žbb. orientations of polarization plane, and d is the sample thickness w3,4x. The far IR Fourier spectroscopy Žin reflection geometry. was used to study the microstructural mechanism of EID. All measurements were carried out with a device ŽIFS-113V ‘Bruker’. in the region of v-As 2 S 3 main vibrational bands Ž400 to 100 cmy1 .. Reflection spectra obtained before and after electron irradiation were multiply Ž256-fold. accumulated and then subtracted. By this way we ob-
tained the spectra of additional reflectivity, D R, produced by electron irradiation. Positive D R corresponded to complexes created by irradiation and negative D R corresponded to destroyed complexes. The v-As 2 S 3 was taken as a model material for our investigation as it has well-resolved vibrational bands, corresponding to its main structural fragments, in particular the pyramidal AsS 3 and the bridge As–S–As units based on heteropolar As–S bonds Ž335 to 285 cmy1 ., as well as the structural units with ‘wrong’ homopolar As–As Ž379, 340, 231, 210, 168 and 140 cmy1 . and S–S bonds Ž243 and 188 cmy1 . w6–9x. The far IR Fourier spectra of electron-irradiated v-As 2 S 3 were measured after thermal annealing at the temperatures of 300 to 423 K, i.e., below and above the thermal bleaching threshold of the scalar radiation induced darkening Ž390 to 400 K. w10x. All measurements were made on planes of the cube oriented perpendicularly ŽABCD or H. and parallel ŽBB1C 1C or 5. to the electron beam direction Žsee Fig. 1..
3. Results The spectral dependence of k for v-As 2 S 3 cubic samples after electron irradiation with F s 5 = 10 16 cmy2 fluence and following annealing during 2 h at various temperatures are shown in Fig. 2 Žcurves 1–5.. It can be noted that EID is observed in the spectral region of optical absorption coefficient edge Žsee insert to Fig. 2.. This effect appears after electron irradiation with F ) 5 = 10 15 cmy2 and increases without saturation up to F s 5 = 10 17 cmy2 . It is important to consider the main features of EID after irradiation with F s 5 = 10 16 cmy2 fluence. The exponential edge of EID is detected at the photon energies hn ) 1.9 eV ŽFig. 2, curve 1.. The k reaches ; 0.7, which is more than that in the case of photoinduced dichroism w3x. The slope of the k Ž hn . curve is approximately equal to s ; 7 eVy1 . Further thermal annealing leads to a short-wave shift of this curve as in the case of thermally induced optical properties bleaching in g-irradiated glasses w10x. But, contrary to the latter process, the EID thermal bleaching begins at temperatures greater than 300 K and saturates near 425 K ŽFig. 2, curves 2–5.
O.I. Shpotyuk et al.r Journal of Non-Crystalline Solids 227–230 (1998) 837–841
Fig. 2. Spectral dependencies of EID in v-As 2 S 3 after irradiation by accelerated electrons at F s 5=10 16 cmy2 Ž1. and following thermal annealing at 343 Ž2., 373 Ž3., 398 Ž4. and 423 K Ž5.. Insert: spectral dependence of v-As 2 S 3 optical absorption coefficient. Lines are drawn as guides for the eyes.
without any thresholds in this temperature region. The slope, s , increases at annealing up to ; 18 eVy1 . The extended ‘tail’ of EID is observed in more long-wave region Ž k - 0.15.. This EID ‘tail’ decreases after annealing, but linear approximations of these k Ž hn . curves cross through one point Žsee Fig. 2.. We established that EID disappeared completely at T s 300 K during ) 10 days, while the photoinduced dichroism disappeared partially w3,4x. Comparing R H and R 5 spectra of the v-As 2 S 3 just after irradiation ŽF s 5 = 10 16 cmy2 ., we note that background R H is more than R H . The vibrational band at 420 cmy1 due to As–O complexes w11x is more intense in the R H spectrum, while vibrational bands at between 285 and 335 cmy1 associated with As–S bonds w7–9x are of smaller intensities. The D R H spectrum for electron-irradiated vAs 2 S 3 ŽF s 5 = 10 16 cmy2 . induced by thermal annealing at the T s 333 K is shown in Fig. 3. We can resolve more than ten additional reflectivity bands Ž D R H ) 0. corresponding to various structural units of v-As 2 S 3 based on the heteropolar As–S, as well as homopolar As–As and S–S bonds. Hence in accordance with Fig. 3, the fraction of major structural units in the samples increases after thermal annealing. The most essential increase is proper to
839
ŽAs–S.-based complexes associated with D R H bands in the 335 to 285 cmy1 region w9x. This effect is not observed in nonirradiated samples annealed at temperatures below T - 450 K w12x. With an annealing temperature T ; 395 K, the intensities of the v-As 2 S 3 vibrational modes increased, especially those with homopolar bonds Ž379, 340, 243, 231, 188, 168 and 140 cmy1 .. Based on these increases we conclude that electron-induced defects on the basis of destroyed As–S bonds are the most unstable and sensitive to thermal annealing. At higher temperatures Ž) 390 K. the signal of additional reflectivity has a more complicated shape. We can resolve two components in the D R H spectrum: Ž1. a component similar to that shown in Fig. 3 with more intense vibrational bands in the region corresponding to homopolar bonds; Ž2. component of bonds switching process connected with thermal annihilation of coordination defects at the reversible stage of scalar radiation induced structural transformations which have been described for girradiated samples previously w2,13,14x. The second component appears only after thermal annealing at the temperatures over bleaching threshold Ž) 390 K. w10x. The rheological electron-induced damages in the perpendicular BB1C 1C plane of vAs 2 S 3 cube decrease in amplitude under these conditions and, as a result, the background reflectivity, R 5 , of this plane increases. However, the quantitative estimation of the both components of additional reflectivity is difficult because of overlap. The EID is fully bleached at the thermal annealing near T ; 425 K when both D R H and D R 5 spectra are saturated.
Fig. 3. Spectrum of the additional reflectivity D R H of electronirradiated v-As 2 S 3 induced by thermal annealing at 333 K.
840
O.I. Shpotyuk et al.r Journal of Non-Crystalline Solids 227–230 (1998) 837–841
4. Discussion Physical features of the investigated EID in vAs 2 S 3 testify that microstructural mechanism of this effect differs relatively to the photoinduced one. Comparing R H and R 5 spectra, it is shown that two radiation-structural transformations take place at the surface of the investigated samples: Ž1. the process of surface damages Žrheological changes. due to direct bombardment of high-energetic accelerated electrons Žthe analogous process was recently studied in Ref. w15x.; Ž2. secondly, the process of electron-induced surface oxidation due to chemical interaction of destroyed complexes and absorbed oxygen atoms as in the case of surface oxidation of the amorphous chalcogenide semiconductors after g-irradiation w16x. We found the first process is larger on the parallel ŽBB1C 1C. plane of v-As 2 S 3 cube, whereas the second one is larger on the perpendicular ŽABCD. plane. The first process leads to a decrease of background reflectivity Ž R 5 . and, consequently, prevents correct identification of the structural transformations on this ŽBB1C 1C. plane Žat least at temperatures - 390 K.. The second process Žoxidation. does not change the vibrational R H spectrum of v-As 2 S 3 in the range of the main structural units Ž400 to 100 cmy1 . as stretch and bend modes corresponding to As–O complexes are located ) 400 cmy1 w11,17x. Hence we can use only the D R H spectrum to study the microstructural mechanism of EID. One of the possible reasons to explain our results of far IR Fourier spectroscopy ŽFig. 3. is the existence of broken chemical bonds in the structural network of electron-irradiated samples. In other words, the 8-N rule is not satisfied in these samples because some atoms do not have their normal valency. In accordance with our calculations, the fraction of such atoms based on the spectrum ŽFig. 3. equals ; 9% Žcomparing the 335 to 285 cmy1 vibrational bands intensities before and after electron irradiation.. These atoms with unsaturated valencies may be specific structural defects with smaller coordination Žundercoordinated atoms. w18x. Whereas the process of bond breaking in AChS has a homolytical character, i.e., electrons forming the covalent bond are localized after bond breaking only at one atom w19x, there are no unpaired spins in this process and the
defects are really pairs of undercoordinated atoms with opposite electrical charges. These defects appear in the glassy matrix owing to displacements of covalently-linked atoms at the bombardment by electrons in accordance with a threshold mechanism w20x. The coincidence of the calculated concentration of such displaced atoms w18x and above-mentioned defects, estimated using our experimental results ŽFig. 3., supports such explanation. These defects are preferentially oriented ones along the direction of electron-beam propagation and, as a result, they can be considered as oriented electrical dipoles producing the observed EID. The final concentration of undercoordinated atoms Žwith respect to the total concentration of atoms., annihilated at the temperature ; 425 K, is equal to ; 11%. Using the model of v-As 2 S 3 random covalent network w21x, we conclude that defects indicated as y. Ž y q. undercoordinated atoms are ŽAsq 2 , S 1 , As 2 , As 2 y q and ŽS1 , S1 .. The charge of a defect centre is defined by the superscript and the quantity of nearest neighbouring atoms by the subscript. They appear in the structural matrix by pairs, keeping change neutrality. Such defects are stable just after irradiation at room temperature and unstable at higher temperatures in accordance with obtained temperature dependence of EID. The process of defect annihilation has a long-time component with duration of ) 10 days, explaining the time dependence of EID. These defects annihilate when the broken chemical bonds are restored: As–S heteropolar bonds are reclaimed at the ŽAsq 2, . Sy defect pair annihilation; 1 As–As homopolar bonds are reclaimed at the ŽAsy 2, . Asq 2 defect pair annihilation, and S–S homopolar bonds are reclaimed at the ŽSy 1, . defect pair annihilation. Sq 1 From the general point of view, the first process of annihilation is predominant over the two others in accordance with our experimental results. It rises from both concentration and energetic preference of the heteropolar covalent bonds in v-As 2 S 3 w19,21x. Thus, we conclude that the effect of EID in v-As 2 S 3 originate from electron-induced formation of oriented Žrelatively to electron beam direction. defects in the form of pairs of undercoordinated atoms with opposite charges. The bond breaking is the main feature of this process contrary to linear
O.I. Shpotyuk et al.r Journal of Non-Crystalline Solids 227–230 (1998) 837–841
photoinduced dichroism connected due to existing suppositions w3,4x rather with oriented effect of polarized light on native defects than with creation of new ones.
5. Conclusion The nature of EID is connected with oriented defects formation due to displacements of atoms and chemical bond breaking, induced by accelerated electrons. These defects are pairs of undercoordinated atoms with opposite electrical charges.
References w1x O.I. Shpotyuk, Ukr. Fiz. Zh. 8 Ž1993. 1484. w2x O.I. Shpotyuk, A.O. Matkovski, J. Non-Cryst. Solids 176 Ž1994. 45. w3x V.M. Lyubin, V.K. Tichomirov, Fiz. Tverd. Tela ŽLeningrad. 33 Ž1991. 2063. w4x V.M. Lyubin, V.K. Tichomirov, Fiz. Tverd. Tela ŽLeningrad. 32 Ž1991. 1838. w5x A.K. Pikaev, Modern Radiation Chemistry. Basic Principles,
w6x w7x w8x w9x w10x w11x w12x w13x w14x w15x
w16x w17x w18x w19x w20x w21x
841
Experimental Technique and Methods, Nauka, Moscow, 1985, p. 374. D.W. Scott, J.P. McCullough, F.H. Kruse, J. Mol. Spectrosc. 13 Ž1964. 313. S.A. Solin, G.N. Papatheodorou, Phys. Rev. B. 15 Ž1977. 2084. U. Strom, T.P. Martin, Solid State Commun. 29 Ž1979. 527. T. Mori, K. Matsuishi, T. Arai, J. Non-Cryst. Solids 65 Ž1984. 269. O.I. Shpotyuk, Zh. Prikl. Spektrosk. 46 Ž1. Ž1987. 22. I.I. Rosola, P.P. Puga, V.V. Chiminies, D.V. Chepur, Ukr. Fiz. Zh. 26 Ž1981. 1875. V.N. Kornelyuk, Visnyk Lviv. Univer., Ser. Fiz. 22 Ž1989. 92. O.I. Shpotyuk, Phys. Status Solidi A 145 Ž1994. 69. O.I. Shpotyuk, Phys. Status Solidi B 183 Ž1994. 365. K. Tanaka, in: A. Andriesh, M. Bertolotti, Physics and Applications of Non-Crystalline Semiconductors in Optoelectronics, Kluwer, 1997, p. 31. O.I. Shpotyuk, Ukr. Fiz. Zh. 23 Ž1987. 509. J.A. Savage, J. Non-Cryst. Solids 47 Ž1982. 101. A.O. Matkovsky, S.B. Ubizsky, O.I. Shpotyuk, Fiz. Tverd. Tela ŽLeningrad. 6 Ž1990. 1790. A. Feltz, Amorphous and Vitreous Inorganic Solids, Mir, Moscow, 1986, p. 556. M.I. Klinger, Izv. Akad. Nauk Latv. SSR, Ser. Fiz. Tekh. Nauk 4 Ž1987. 58. M.H. Brodsky, Amorphous Semiconductors, Mir, Moscow, 1982, p. 419.
Effect of electron-induced dichroism in vitreous As 2 S 3 O.I. Shpotyuk b
a,b,)
, V.O. Balitska a , M.M. Vakiv
a
a LÕiÕ Scientific Research Institute of Materials, Stryjska str. 202, LÕiÕ UA-290031, Ukraine Physics Institute, Pedagogical UniÕersity of Czestochowa, Al. Armii Krajowej 13 r 15, Czestochowa PL-42201, Poland
Abstract Spectral and temperature dependencies of the effect of electron-induced dichroism, its stability at room temperature, as well as microstructural mechanism are investigated in cubic samples of vitreous As 2 S 3. It is shown that this effect is observed in the region of optical absorption edge. The corresponding spectral dependence has an exponential form with an edge in the short-wave region and an extended ‘tail’ in the long-wave direction. The dichroism disappears completely at room temperature after ) 10 days or at the annealing to ; 425 K. The dichroism mechanism is connected with electron-induced formation of new oriented Žrelatively to electron flow. defects in the form of undercoordinated atomic pairs. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Electron-induced dichroism; Vitreous As 2 S 3 ; Optical absorption edge
1. Introduction The effect of electron-induced dichroism ŽEID. was first observed in vitreous v-As 2 S 3 cubic samples using a specific geometry of electron irradiation and experimental measurements w1x. It belongs to a wider group of radiation phenomena in amorphous chalcogenides caused by energetic Ž E ) 1 MeV. ionizing irradiation w2x. In this article we shall consider spectral and temperature dependence of the EID in v-As 2 S 3 , its stability at room temperature and mechanism to compare this effect with well-known linear photoinduced dichroisms studied during the last several years w3,4x.
)
Corresponding author. Tel.: q7-380-322-638303; fax: q7380-322-632228; e-mail: [email protected].
2. Experimental The v-As 2 S 3 samples were synthesized from high-purity constituents Ž99.999%. in evacuated quartz ampoules as described previously w1x. All investigated samples had cubic forms with 6–8-mm rib lengths and high-quality polished plane borders. The irradiation was carried out by a beam of electrons with 2.8 MeV energy and F ) 10 15 cmy2 fluences up to F s 5 = 10 17 cmy2 perpendicularly to the BB1C 1C plane of the v-As 2 S 3 cube ŽFig. 1.. We denoted this plane by 5 sign as its normal vector was parallel to the electron-beam direction. The light beam had a diameter ; 3 mm and passed through the sample at ; 2 mm distance from the irradiated BB1C 1C plane. In similar way, the ABCD plane having the normal vector with perpendicular orientation to the electron beam direction was denoted by H sign. The complete penetration of the accelerated
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 5 9 - 8
838
O.I. Shpotyuk et al.r Journal of Non-Crystalline Solids 227–230 (1998) 837–841
Fig. 1. Scheme illustrating the experimental procedure for EID observation in v-As 2 S 3 .
electrons through the investigated zone was achieved in these conditions w5x. Hence we could distinguish two mutually orthogonal directions for light with parallel aa Ž5. and perpendicular bb ŽH. polarizations with respect to the direction of accelerated electrons ŽFig. 1.. All measurements were carried out one day after electron irradiation using a spectrophotometer ŽSpecord M-40; 200 to 900 nm.. Effects produced by inhomogeneity of scalar electron-induced darkening were excluded by changing the direction of the light beam by 1808 with respect to the sample surface. The EID magnitude was described by k in accordance with the expression
ksD aPds Ž a 5 ya H. Pds
2 Ž t H yt 5 .
Ž t H qt 5 .
Ž 1.
where a 5 Žt 5 . and a H Žt H . are the absorption coefficients Žtransmission coefficients. for light with parallel Žaa. and perpendicular Žbb. orientations of polarization plane, and d is the sample thickness w3,4x. The far IR Fourier spectroscopy Žin reflection geometry. was used to study the microstructural mechanism of EID. All measurements were carried out with a device ŽIFS-113V ‘Bruker’. in the region of v-As 2 S 3 main vibrational bands Ž400 to 100 cmy1 .. Reflection spectra obtained before and after electron irradiation were multiply Ž256-fold. accumulated and then subtracted. By this way we ob-
tained the spectra of additional reflectivity, D R, produced by electron irradiation. Positive D R corresponded to complexes created by irradiation and negative D R corresponded to destroyed complexes. The v-As 2 S 3 was taken as a model material for our investigation as it has well-resolved vibrational bands, corresponding to its main structural fragments, in particular the pyramidal AsS 3 and the bridge As–S–As units based on heteropolar As–S bonds Ž335 to 285 cmy1 ., as well as the structural units with ‘wrong’ homopolar As–As Ž379, 340, 231, 210, 168 and 140 cmy1 . and S–S bonds Ž243 and 188 cmy1 . w6–9x. The far IR Fourier spectra of electron-irradiated v-As 2 S 3 were measured after thermal annealing at the temperatures of 300 to 423 K, i.e., below and above the thermal bleaching threshold of the scalar radiation induced darkening Ž390 to 400 K. w10x. All measurements were made on planes of the cube oriented perpendicularly ŽABCD or H. and parallel ŽBB1C 1C or 5. to the electron beam direction Žsee Fig. 1..
3. Results The spectral dependence of k for v-As 2 S 3 cubic samples after electron irradiation with F s 5 = 10 16 cmy2 fluence and following annealing during 2 h at various temperatures are shown in Fig. 2 Žcurves 1–5.. It can be noted that EID is observed in the spectral region of optical absorption coefficient edge Žsee insert to Fig. 2.. This effect appears after electron irradiation with F ) 5 = 10 15 cmy2 and increases without saturation up to F s 5 = 10 17 cmy2 . It is important to consider the main features of EID after irradiation with F s 5 = 10 16 cmy2 fluence. The exponential edge of EID is detected at the photon energies hn ) 1.9 eV ŽFig. 2, curve 1.. The k reaches ; 0.7, which is more than that in the case of photoinduced dichroism w3x. The slope of the k Ž hn . curve is approximately equal to s ; 7 eVy1 . Further thermal annealing leads to a short-wave shift of this curve as in the case of thermally induced optical properties bleaching in g-irradiated glasses w10x. But, contrary to the latter process, the EID thermal bleaching begins at temperatures greater than 300 K and saturates near 425 K ŽFig. 2, curves 2–5.
O.I. Shpotyuk et al.r Journal of Non-Crystalline Solids 227–230 (1998) 837–841
Fig. 2. Spectral dependencies of EID in v-As 2 S 3 after irradiation by accelerated electrons at F s 5=10 16 cmy2 Ž1. and following thermal annealing at 343 Ž2., 373 Ž3., 398 Ž4. and 423 K Ž5.. Insert: spectral dependence of v-As 2 S 3 optical absorption coefficient. Lines are drawn as guides for the eyes.
without any thresholds in this temperature region. The slope, s , increases at annealing up to ; 18 eVy1 . The extended ‘tail’ of EID is observed in more long-wave region Ž k - 0.15.. This EID ‘tail’ decreases after annealing, but linear approximations of these k Ž hn . curves cross through one point Žsee Fig. 2.. We established that EID disappeared completely at T s 300 K during ) 10 days, while the photoinduced dichroism disappeared partially w3,4x. Comparing R H and R 5 spectra of the v-As 2 S 3 just after irradiation ŽF s 5 = 10 16 cmy2 ., we note that background R H is more than R H . The vibrational band at 420 cmy1 due to As–O complexes w11x is more intense in the R H spectrum, while vibrational bands at between 285 and 335 cmy1 associated with As–S bonds w7–9x are of smaller intensities. The D R H spectrum for electron-irradiated vAs 2 S 3 ŽF s 5 = 10 16 cmy2 . induced by thermal annealing at the T s 333 K is shown in Fig. 3. We can resolve more than ten additional reflectivity bands Ž D R H ) 0. corresponding to various structural units of v-As 2 S 3 based on the heteropolar As–S, as well as homopolar As–As and S–S bonds. Hence in accordance with Fig. 3, the fraction of major structural units in the samples increases after thermal annealing. The most essential increase is proper to
839
ŽAs–S.-based complexes associated with D R H bands in the 335 to 285 cmy1 region w9x. This effect is not observed in nonirradiated samples annealed at temperatures below T - 450 K w12x. With an annealing temperature T ; 395 K, the intensities of the v-As 2 S 3 vibrational modes increased, especially those with homopolar bonds Ž379, 340, 243, 231, 188, 168 and 140 cmy1 .. Based on these increases we conclude that electron-induced defects on the basis of destroyed As–S bonds are the most unstable and sensitive to thermal annealing. At higher temperatures Ž) 390 K. the signal of additional reflectivity has a more complicated shape. We can resolve two components in the D R H spectrum: Ž1. a component similar to that shown in Fig. 3 with more intense vibrational bands in the region corresponding to homopolar bonds; Ž2. component of bonds switching process connected with thermal annihilation of coordination defects at the reversible stage of scalar radiation induced structural transformations which have been described for girradiated samples previously w2,13,14x. The second component appears only after thermal annealing at the temperatures over bleaching threshold Ž) 390 K. w10x. The rheological electron-induced damages in the perpendicular BB1C 1C plane of vAs 2 S 3 cube decrease in amplitude under these conditions and, as a result, the background reflectivity, R 5 , of this plane increases. However, the quantitative estimation of the both components of additional reflectivity is difficult because of overlap. The EID is fully bleached at the thermal annealing near T ; 425 K when both D R H and D R 5 spectra are saturated.
Fig. 3. Spectrum of the additional reflectivity D R H of electronirradiated v-As 2 S 3 induced by thermal annealing at 333 K.
840
O.I. Shpotyuk et al.r Journal of Non-Crystalline Solids 227–230 (1998) 837–841
4. Discussion Physical features of the investigated EID in vAs 2 S 3 testify that microstructural mechanism of this effect differs relatively to the photoinduced one. Comparing R H and R 5 spectra, it is shown that two radiation-structural transformations take place at the surface of the investigated samples: Ž1. the process of surface damages Žrheological changes. due to direct bombardment of high-energetic accelerated electrons Žthe analogous process was recently studied in Ref. w15x.; Ž2. secondly, the process of electron-induced surface oxidation due to chemical interaction of destroyed complexes and absorbed oxygen atoms as in the case of surface oxidation of the amorphous chalcogenide semiconductors after g-irradiation w16x. We found the first process is larger on the parallel ŽBB1C 1C. plane of v-As 2 S 3 cube, whereas the second one is larger on the perpendicular ŽABCD. plane. The first process leads to a decrease of background reflectivity Ž R 5 . and, consequently, prevents correct identification of the structural transformations on this ŽBB1C 1C. plane Žat least at temperatures - 390 K.. The second process Žoxidation. does not change the vibrational R H spectrum of v-As 2 S 3 in the range of the main structural units Ž400 to 100 cmy1 . as stretch and bend modes corresponding to As–O complexes are located ) 400 cmy1 w11,17x. Hence we can use only the D R H spectrum to study the microstructural mechanism of EID. One of the possible reasons to explain our results of far IR Fourier spectroscopy ŽFig. 3. is the existence of broken chemical bonds in the structural network of electron-irradiated samples. In other words, the 8-N rule is not satisfied in these samples because some atoms do not have their normal valency. In accordance with our calculations, the fraction of such atoms based on the spectrum ŽFig. 3. equals ; 9% Žcomparing the 335 to 285 cmy1 vibrational bands intensities before and after electron irradiation.. These atoms with unsaturated valencies may be specific structural defects with smaller coordination Žundercoordinated atoms. w18x. Whereas the process of bond breaking in AChS has a homolytical character, i.e., electrons forming the covalent bond are localized after bond breaking only at one atom w19x, there are no unpaired spins in this process and the
defects are really pairs of undercoordinated atoms with opposite electrical charges. These defects appear in the glassy matrix owing to displacements of covalently-linked atoms at the bombardment by electrons in accordance with a threshold mechanism w20x. The coincidence of the calculated concentration of such displaced atoms w18x and above-mentioned defects, estimated using our experimental results ŽFig. 3., supports such explanation. These defects are preferentially oriented ones along the direction of electron-beam propagation and, as a result, they can be considered as oriented electrical dipoles producing the observed EID. The final concentration of undercoordinated atoms Žwith respect to the total concentration of atoms., annihilated at the temperature ; 425 K, is equal to ; 11%. Using the model of v-As 2 S 3 random covalent network w21x, we conclude that defects indicated as y. Ž y q. undercoordinated atoms are ŽAsq 2 , S 1 , As 2 , As 2 y q and ŽS1 , S1 .. The charge of a defect centre is defined by the superscript and the quantity of nearest neighbouring atoms by the subscript. They appear in the structural matrix by pairs, keeping change neutrality. Such defects are stable just after irradiation at room temperature and unstable at higher temperatures in accordance with obtained temperature dependence of EID. The process of defect annihilation has a long-time component with duration of ) 10 days, explaining the time dependence of EID. These defects annihilate when the broken chemical bonds are restored: As–S heteropolar bonds are reclaimed at the ŽAsq 2, . Sy defect pair annihilation; 1 As–As homopolar bonds are reclaimed at the ŽAsy 2, . Asq 2 defect pair annihilation, and S–S homopolar bonds are reclaimed at the ŽSy 1, . defect pair annihilation. Sq 1 From the general point of view, the first process of annihilation is predominant over the two others in accordance with our experimental results. It rises from both concentration and energetic preference of the heteropolar covalent bonds in v-As 2 S 3 w19,21x. Thus, we conclude that the effect of EID in v-As 2 S 3 originate from electron-induced formation of oriented Žrelatively to electron beam direction. defects in the form of pairs of undercoordinated atoms with opposite charges. The bond breaking is the main feature of this process contrary to linear
O.I. Shpotyuk et al.r Journal of Non-Crystalline Solids 227–230 (1998) 837–841
photoinduced dichroism connected due to existing suppositions w3,4x rather with oriented effect of polarized light on native defects than with creation of new ones.
5. Conclusion The nature of EID is connected with oriented defects formation due to displacements of atoms and chemical bond breaking, induced by accelerated electrons. These defects are pairs of undercoordinated atoms with opposite electrical charges.
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