- Email: [email protected]
Photo-induced crystallization in amorphous GeSe2 studied by Raman scattering
Journal of Non-Crystalline Solids 227–230 Ž1998. 728–731
Photo-induced crystallization in amorphous GeSe 2 studied by Raman scattering Yong Wang a
a,)
, Osamu Matsuda a , Koichi Inoue b, Kazuo Murase
a
Department of Physics, Graduate School of Science, Osaka UniÕersity, 1-1 Machikaneyama, Toyonaka 560, Japan b The Institute of Scientific and Industrial Research, Osaka UniÕersity, 8-1 Mihogaoka, Ibaraki 567, Japan
Abstract The structural difference, due to different preparation methods, in glassy GeSe 2 samples is investigated by photo-induced crystallization phenomena using time-resolved Raman measurement. Crystallization temperatures of two samples, which were cooled from melts at different cooling rates, were almost the same in thermal crystallization processes. However, in photo-induced crystallization processes, threshold temperatures of the crystallization directly depended on their cooling rates, the smaller cooling rate the lower the threshold temperature. In the sample cooled at a smaller cooling rate, its medium-range structure is more similar to crystalline nuclei, or easier to transform into crystalline nuclei. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Raman scattering; Amorphous GeSe 2 ; Crystallization
1. Introduction Photo-induced crystallization ŽPIC. in GeSe 2 w1– 8x is one of the typical photo-induced structural changes which has attracted interest. Although complete knowledge of the mechanism of the process has not been obtained yet, a schematic model of electronic and thermal effects has been proposed recently w7,8x. Before the onset of the PIC, local microscopic excitations of what excited by near band-gap light, changed the metastable amorphous state towards the crystalline state with a cooperative promotion of phonons. The study of the phenomena during the PIC process is expected to bring important clues to an understanding of the structure, particularly for the medium-range structure. )
Corresponding author: Tel.: q81 6 850 5375; fax: q81 6 850 5376; e-mail: [email protected].
In this paper, the PIC processes of glassy GeSe 2 films are studied through the time-resolved Raman measurements w4–8x. Based on measurements of the dependence of the crystallization tendency on cooling rates, structural differences in micro- andror mesoscopic scales in the glasses are discussed.
2. Experimental Thin films of GeSe 2 glasses Žthickness of ; 1 m m. were prepared by quenching GeSe 2 liquids held in evacuated fused-silica cells with a small distance between two parallel plates. The liquids in the cells were held at 9408C for 24 h to completely fill the space between the parallel plates. We cooled the cells Ž1. in an ice water bath or Ž2. in air. In this paper, glassy GeSe 2 films obtained by quenching the liquids Žg-GeSe 2 . are labeled as M and S corre-
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 9 3 - 8
Y. Wang et al.r Journal of Non-Crystalline Solids 227–230 (1998) 728–731
729
sponding to the two cooling ways Ž1. and Ž2., respectively. Also, amorphous GeSe 2 films Ža-GeSe 2 . with a thickness of ; 600 nm were prepared by vacuum evaporation of glassy GeSe 2 onto glass substrate ŽCorning 7059. at room temperature. The 2.54 eV Ž488 nm. light of an Ar ion laser, with the light power in the range of 5–50 mW, was focused onto the sample with a typical illumination region of about 50 m m in diameter. The glassy samples M and S were measured in the cells and the amorphous samples were measured in air at room temperature. Using the excitation light for the PIC as a Raman probe, the transformation of the Raman spectra during the PIC process was recorded every 1 min.
3. Results Typical time-resolved Raman spectra of the gGeSe 2 films in the PIC process are shown in Fig. 1. After an appropriate period, named as the latent period, the initial amorphous spectra changed into the crystalline spectra with a characteristic 2D Žlayered structure. band, A, at ; 210 cmy1 w5x. The latent period becomes extremely long Ž) 60 min. at a threshold excitation power. We estimate the local temperature of the illuminated portion by the shift of the crystalline band, A, whose temperature coefficient is about y0.01 cmy1 rK w6x. An error of estimated temperatures is "20.
Fig. 2. Latent period vs. estimated temperature in g-GeSe 2 samples S, M, and in a-GeSe 2 film. The vertical arrows point the crystallization temperature in the thermal processes at 3208C for a-GeSe 2 films and 4408C for g-GeSe 2 films.
Fig. 2 displays the estimated temperature dependence of the latent period in the g- and a-GeSe 2 films. In the sample S, we find that: Ž1. the crystallization occurs at lower temperature than that in sample M; Ž2. its threshold temperature Tthre under which the latent period becomes extremely long is approximately 1308C which is lower than that of the a-GeSe 2 film Ž; 2008C.; Ž3. the Tthre is much lower than the thermal crystallization ŽTC. temperature Tc Ž; 4508C. w9x. On the other hand, in sample M, the Tthre is about 4308C which is near Tc Ž; 4508C. for the TC process. In spite of the fact that the threshold temperatures in the sample S and M differ, the Tc for their TC processes are almost the same. For the a-GeSe 2 film, its Tc which was obtained by annealing the a-GeSe 2 film in the dark condition is about 3208C. The Tc of a-GeSe 2 film is almost 1008C less than those of the g-GeSe 2 films. In the a-GeSe 2 films, the Tc is usually ; 1008C higher than its Tthre .
4. Discussion
Fig. 1. Time-resolved Raman spectra during the PIC process in g-GeSe 2 film with an excitation power of 20 MW. The characteristic crystalline peaks begin to appear after a latent period of 4 min.
Matsuda et al. w8x have proposed a model of electronic and thermal process in the latent period of the PIC process, where the evolution towards the formation of crystalline nuclei takes place. After photo-excitations in the initial glassy states Ža., as
730
Y. Wang et al.r Journal of Non-Crystalline Solids 227–230 (1998) 728–731
Fig. 3. The diagram of the electronic and thermal processes during the latent period of the PIC process. The detail of each state and processes are described in the text.
shown in Fig. 3, excited electrons relax to intermediate states Žb. by electron–phonon interactions. From the states Žb., the system transforms toward the crystalline nuclei overcoming the energy barrier, UB , by thermal excitation. Thus in the PIC process, the threshold temperature, which is defined as the lowest crystallizable temperature in the 60 min of our experimental time period, is related to the height of the energy barrier, UB . In the pure thermal process in which the glass transforms from the initial states Ža. to the crystalline nuclei Žc., the system overcomes the barrier UA ) UB by thermal excitation. In the a-GeSe 2 film, which was vapor deposited, the cooling rate is expected to be larger than in the case of the quenched samples. The glassy state is assumed to a metastable state Ža. with an initial energy, Uini w8x. This Uini metastable state Ža. is accompanied by a barrier UA corresponding to the crystallization temperature Tc s 3208C for the TC process. For the g-GeSe 2 films cooled in silica cells, the cooling rates are not so large as that of the a-GeSe 2 film. These smaller cooling rates lead the g-GeSe 2 films to be in a metastable state Že. with a smaller initial energy, UiniX . In the g-GeSe 2 films, as the Tc s for their TC processes are almost the same Ž; 4508., we suppose that the barrier UAX of sample S and M are almost in the same value. Thus, in two different preparation methods of the samples, vacuum evaporation and quenching liquids, we obtain two kinds of samples whose metastable states are
accompanied by two different barrier UA and UAX , respectively, where the barrier UA in the a-GeSe 2 film is less than UAX in the g-GeSe 2 films. The threshold temperature in a-GeSe 2 films is ; 2008C, while in g-GeSe 2 films they depend on the cooling rates. In the sample S, its intermediate glassy state Žd. may overcome a smaller barrier, corresponding to the Tthre s 1308C, by thermal excitation. In sample M, the magnitude of barrier UBX corresponds to Tthre s 4308C. Although the Tthre of aGeSe 2 films is between those of the g-GeSe 2 films M and S, its medium-range structure ŽMRS. should not be assumed to be the same series as of those of g-GeSe 2 films based on the obtained crystalline phases in the PIC process. In the vapor deposited films, a-GeSe 2 , the PIC process is classified into type A and B, where in type A case only the 2D form grows, while in the type B case both the 2D and 3D forms grow w5x. Only type A has been observed in the PIC process in the films produced by cooling in the silica cells. The appearance of 3D form in the PIC suggests that it is easier to construct a network structure of corner-sharing GeSe 4r2 tetrahedra w7x starting from the vapor deposited films, a-GeSe 2 . In the g-GeSe 2 films cooled in the silica cells, although the Tc s are similar, the different Tthre s indicate that a smaller cooling rate makes crystallization easier. These different Tthre s may indicate differing structures due to the different cooling rates. During the preparation of the films, a super-cooled liquid is frozen in a low energy state by a small cooling rate. But for an additionally small cooling rate, the liquid can be transformed into the crystal. Though it has not been examined what the critical cooling rate is, it is reasonable to suppose that in the glass prepared by a smaller cooling rate, the MRS is topologically more similar to the crystalline nuclei, or easier to transform into crystalline nuclei. Here, this MRS may be named as a near-nuclei glass structure. According to the results, a good near-nuclei glass structure is expected in sample S. Thus, we can experimentally clarify the different MRS by investigating the behaviours in the PIC. It should be noted that in films cooled in silica cells, the glass-transition temperature, Tg , above which the structural change can be observed by Raman spectroscopy in an experimental time period,
Y. Wang et al.r Journal of Non-Crystalline Solids 227–230 (1998) 728–731
is about 3708C w9x. Because the Tthre of sample S, 1308C, is less than the Tg , its PIC process is considered as a typical mixed electronic and thermal effect. While for the sample M whose Tthre , 4308C, is higher than the Tg , we contribute its PIC process to the thermal one. Thus, the type of the PIC processes depends on the cooling rates.
5. Conclusion The initial metastable energy of a-GeSe 2 films are larger than those of g-GeSe 2 films and hence the thermal crystallization temperature is decreased. In the g-GeSe 2 films, a small cooling rate decreases the threshold temperature for the PIC though it does not affect crystallization in thermal crystallization process. We assume that in the glass prepared by a smaller cooling rate, the medium-range structure is more similar to crystalline nuclei, or easier to transform into crystalline nuclei.
Acknowledgements This work is partially supported by a Grant-in-Aid for Scientific Research ŽB. and a grant for Scientific
731
Research in the Priority Area ‘Cooperative Phenomena in Complex Liquids’, both from the Ministry of Education, Science, and Culture ŽJapan..
References w1x J.E. Griffiths, G.P. Espinosa, J.P. Remeika, J.C. Phillips, Solid State Commun. 40 Ž1981. 1077. w2x E. Haro, Z.S. Xu, J.F. Morhange, M. Balkanski, G.P. Espinosa, J.C. Phillips, Phys. Rev. B 32 Ž1985. 969. w3x S. Sugai, Phys. Rev. Lett. 57 Ž1986. 456. w4x K. Murase, K. Inoue, in: M.A. Kastner, G.A. Thomas, S.R. Ovshinsky ŽEds.., Disordered Semiconductors, Plenum, New York, 1987, p. 297. w5x K. Inoue, K. Kawamoto, K. Murase, J. Non-Cryst. Solids 95–96 Ž1987. 517. w6x K. Inoue, O. Matsuda, K. Murase, in: W. Zawadzki ŽEd.., Proc. 19th Int. Conf. on Physics of Semiconductors, Warsaw, Poland, Institute of Physics, Polish Academy of Sciences, Poland, 1988, p. 1665. w7x K. Murase, K. Inoue, O. Matsuda, in: Y. Sakurai, Y. Hamakawa, T. Masumoto. K. Shirae, K. Suzuki ŽEds.., Current Topics in Amorphous Materials: Physics and Technology, Elsevier, Amsterdam, 1993, p. 47. w8x O. Matsuda, H. Oe, K. Inoue, K. Murase, J. Non-Cryst. Solids 192–193 Ž1995. 524. w9x Y. Wang, M. Nakamura, O. Matsuda, K. Inoue, K. Murase, J. Non-Cryst. Solids 198–200 Ž1996. 753.
Photo-induced crystallization in amorphous GeSe 2 studied by Raman scattering Yong Wang a
a,)
, Osamu Matsuda a , Koichi Inoue b, Kazuo Murase
a
Department of Physics, Graduate School of Science, Osaka UniÕersity, 1-1 Machikaneyama, Toyonaka 560, Japan b The Institute of Scientific and Industrial Research, Osaka UniÕersity, 8-1 Mihogaoka, Ibaraki 567, Japan
Abstract The structural difference, due to different preparation methods, in glassy GeSe 2 samples is investigated by photo-induced crystallization phenomena using time-resolved Raman measurement. Crystallization temperatures of two samples, which were cooled from melts at different cooling rates, were almost the same in thermal crystallization processes. However, in photo-induced crystallization processes, threshold temperatures of the crystallization directly depended on their cooling rates, the smaller cooling rate the lower the threshold temperature. In the sample cooled at a smaller cooling rate, its medium-range structure is more similar to crystalline nuclei, or easier to transform into crystalline nuclei. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Raman scattering; Amorphous GeSe 2 ; Crystallization
1. Introduction Photo-induced crystallization ŽPIC. in GeSe 2 w1– 8x is one of the typical photo-induced structural changes which has attracted interest. Although complete knowledge of the mechanism of the process has not been obtained yet, a schematic model of electronic and thermal effects has been proposed recently w7,8x. Before the onset of the PIC, local microscopic excitations of what excited by near band-gap light, changed the metastable amorphous state towards the crystalline state with a cooperative promotion of phonons. The study of the phenomena during the PIC process is expected to bring important clues to an understanding of the structure, particularly for the medium-range structure. )
Corresponding author: Tel.: q81 6 850 5375; fax: q81 6 850 5376; e-mail: [email protected].
In this paper, the PIC processes of glassy GeSe 2 films are studied through the time-resolved Raman measurements w4–8x. Based on measurements of the dependence of the crystallization tendency on cooling rates, structural differences in micro- andror mesoscopic scales in the glasses are discussed.
2. Experimental Thin films of GeSe 2 glasses Žthickness of ; 1 m m. were prepared by quenching GeSe 2 liquids held in evacuated fused-silica cells with a small distance between two parallel plates. The liquids in the cells were held at 9408C for 24 h to completely fill the space between the parallel plates. We cooled the cells Ž1. in an ice water bath or Ž2. in air. In this paper, glassy GeSe 2 films obtained by quenching the liquids Žg-GeSe 2 . are labeled as M and S corre-
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 9 3 - 8
Y. Wang et al.r Journal of Non-Crystalline Solids 227–230 (1998) 728–731
729
sponding to the two cooling ways Ž1. and Ž2., respectively. Also, amorphous GeSe 2 films Ža-GeSe 2 . with a thickness of ; 600 nm were prepared by vacuum evaporation of glassy GeSe 2 onto glass substrate ŽCorning 7059. at room temperature. The 2.54 eV Ž488 nm. light of an Ar ion laser, with the light power in the range of 5–50 mW, was focused onto the sample with a typical illumination region of about 50 m m in diameter. The glassy samples M and S were measured in the cells and the amorphous samples were measured in air at room temperature. Using the excitation light for the PIC as a Raman probe, the transformation of the Raman spectra during the PIC process was recorded every 1 min.
3. Results Typical time-resolved Raman spectra of the gGeSe 2 films in the PIC process are shown in Fig. 1. After an appropriate period, named as the latent period, the initial amorphous spectra changed into the crystalline spectra with a characteristic 2D Žlayered structure. band, A, at ; 210 cmy1 w5x. The latent period becomes extremely long Ž) 60 min. at a threshold excitation power. We estimate the local temperature of the illuminated portion by the shift of the crystalline band, A, whose temperature coefficient is about y0.01 cmy1 rK w6x. An error of estimated temperatures is "20.
Fig. 2. Latent period vs. estimated temperature in g-GeSe 2 samples S, M, and in a-GeSe 2 film. The vertical arrows point the crystallization temperature in the thermal processes at 3208C for a-GeSe 2 films and 4408C for g-GeSe 2 films.
Fig. 2 displays the estimated temperature dependence of the latent period in the g- and a-GeSe 2 films. In the sample S, we find that: Ž1. the crystallization occurs at lower temperature than that in sample M; Ž2. its threshold temperature Tthre under which the latent period becomes extremely long is approximately 1308C which is lower than that of the a-GeSe 2 film Ž; 2008C.; Ž3. the Tthre is much lower than the thermal crystallization ŽTC. temperature Tc Ž; 4508C. w9x. On the other hand, in sample M, the Tthre is about 4308C which is near Tc Ž; 4508C. for the TC process. In spite of the fact that the threshold temperatures in the sample S and M differ, the Tc for their TC processes are almost the same. For the a-GeSe 2 film, its Tc which was obtained by annealing the a-GeSe 2 film in the dark condition is about 3208C. The Tc of a-GeSe 2 film is almost 1008C less than those of the g-GeSe 2 films. In the a-GeSe 2 films, the Tc is usually ; 1008C higher than its Tthre .
4. Discussion
Fig. 1. Time-resolved Raman spectra during the PIC process in g-GeSe 2 film with an excitation power of 20 MW. The characteristic crystalline peaks begin to appear after a latent period of 4 min.
Matsuda et al. w8x have proposed a model of electronic and thermal process in the latent period of the PIC process, where the evolution towards the formation of crystalline nuclei takes place. After photo-excitations in the initial glassy states Ža., as
730
Y. Wang et al.r Journal of Non-Crystalline Solids 227–230 (1998) 728–731
Fig. 3. The diagram of the electronic and thermal processes during the latent period of the PIC process. The detail of each state and processes are described in the text.
shown in Fig. 3, excited electrons relax to intermediate states Žb. by electron–phonon interactions. From the states Žb., the system transforms toward the crystalline nuclei overcoming the energy barrier, UB , by thermal excitation. Thus in the PIC process, the threshold temperature, which is defined as the lowest crystallizable temperature in the 60 min of our experimental time period, is related to the height of the energy barrier, UB . In the pure thermal process in which the glass transforms from the initial states Ža. to the crystalline nuclei Žc., the system overcomes the barrier UA ) UB by thermal excitation. In the a-GeSe 2 film, which was vapor deposited, the cooling rate is expected to be larger than in the case of the quenched samples. The glassy state is assumed to a metastable state Ža. with an initial energy, Uini w8x. This Uini metastable state Ža. is accompanied by a barrier UA corresponding to the crystallization temperature Tc s 3208C for the TC process. For the g-GeSe 2 films cooled in silica cells, the cooling rates are not so large as that of the a-GeSe 2 film. These smaller cooling rates lead the g-GeSe 2 films to be in a metastable state Že. with a smaller initial energy, UiniX . In the g-GeSe 2 films, as the Tc s for their TC processes are almost the same Ž; 4508., we suppose that the barrier UAX of sample S and M are almost in the same value. Thus, in two different preparation methods of the samples, vacuum evaporation and quenching liquids, we obtain two kinds of samples whose metastable states are
accompanied by two different barrier UA and UAX , respectively, where the barrier UA in the a-GeSe 2 film is less than UAX in the g-GeSe 2 films. The threshold temperature in a-GeSe 2 films is ; 2008C, while in g-GeSe 2 films they depend on the cooling rates. In the sample S, its intermediate glassy state Žd. may overcome a smaller barrier, corresponding to the Tthre s 1308C, by thermal excitation. In sample M, the magnitude of barrier UBX corresponds to Tthre s 4308C. Although the Tthre of aGeSe 2 films is between those of the g-GeSe 2 films M and S, its medium-range structure ŽMRS. should not be assumed to be the same series as of those of g-GeSe 2 films based on the obtained crystalline phases in the PIC process. In the vapor deposited films, a-GeSe 2 , the PIC process is classified into type A and B, where in type A case only the 2D form grows, while in the type B case both the 2D and 3D forms grow w5x. Only type A has been observed in the PIC process in the films produced by cooling in the silica cells. The appearance of 3D form in the PIC suggests that it is easier to construct a network structure of corner-sharing GeSe 4r2 tetrahedra w7x starting from the vapor deposited films, a-GeSe 2 . In the g-GeSe 2 films cooled in the silica cells, although the Tc s are similar, the different Tthre s indicate that a smaller cooling rate makes crystallization easier. These different Tthre s may indicate differing structures due to the different cooling rates. During the preparation of the films, a super-cooled liquid is frozen in a low energy state by a small cooling rate. But for an additionally small cooling rate, the liquid can be transformed into the crystal. Though it has not been examined what the critical cooling rate is, it is reasonable to suppose that in the glass prepared by a smaller cooling rate, the MRS is topologically more similar to the crystalline nuclei, or easier to transform into crystalline nuclei. Here, this MRS may be named as a near-nuclei glass structure. According to the results, a good near-nuclei glass structure is expected in sample S. Thus, we can experimentally clarify the different MRS by investigating the behaviours in the PIC. It should be noted that in films cooled in silica cells, the glass-transition temperature, Tg , above which the structural change can be observed by Raman spectroscopy in an experimental time period,
Y. Wang et al.r Journal of Non-Crystalline Solids 227–230 (1998) 728–731
is about 3708C w9x. Because the Tthre of sample S, 1308C, is less than the Tg , its PIC process is considered as a typical mixed electronic and thermal effect. While for the sample M whose Tthre , 4308C, is higher than the Tg , we contribute its PIC process to the thermal one. Thus, the type of the PIC processes depends on the cooling rates.
5. Conclusion The initial metastable energy of a-GeSe 2 films are larger than those of g-GeSe 2 films and hence the thermal crystallization temperature is decreased. In the g-GeSe 2 films, a small cooling rate decreases the threshold temperature for the PIC though it does not affect crystallization in thermal crystallization process. We assume that in the glass prepared by a smaller cooling rate, the medium-range structure is more similar to crystalline nuclei, or easier to transform into crystalline nuclei.
Acknowledgements This work is partially supported by a Grant-in-Aid for Scientific Research ŽB. and a grant for Scientific
731
Research in the Priority Area ‘Cooperative Phenomena in Complex Liquids’, both from the Ministry of Education, Science, and Culture ŽJapan..
References w1x J.E. Griffiths, G.P. Espinosa, J.P. Remeika, J.C. Phillips, Solid State Commun. 40 Ž1981. 1077. w2x E. Haro, Z.S. Xu, J.F. Morhange, M. Balkanski, G.P. Espinosa, J.C. Phillips, Phys. Rev. B 32 Ž1985. 969. w3x S. Sugai, Phys. Rev. Lett. 57 Ž1986. 456. w4x K. Murase, K. Inoue, in: M.A. Kastner, G.A. Thomas, S.R. Ovshinsky ŽEds.., Disordered Semiconductors, Plenum, New York, 1987, p. 297. w5x K. Inoue, K. Kawamoto, K. Murase, J. Non-Cryst. Solids 95–96 Ž1987. 517. w6x K. Inoue, O. Matsuda, K. Murase, in: W. Zawadzki ŽEd.., Proc. 19th Int. Conf. on Physics of Semiconductors, Warsaw, Poland, Institute of Physics, Polish Academy of Sciences, Poland, 1988, p. 1665. w7x K. Murase, K. Inoue, O. Matsuda, in: Y. Sakurai, Y. Hamakawa, T. Masumoto. K. Shirae, K. Suzuki ŽEds.., Current Topics in Amorphous Materials: Physics and Technology, Elsevier, Amsterdam, 1993, p. 47. w8x O. Matsuda, H. Oe, K. Inoue, K. Murase, J. Non-Cryst. Solids 192–193 Ž1995. 524. w9x Y. Wang, M. Nakamura, O. Matsuda, K. Inoue, K. Murase, J. Non-Cryst. Solids 198–200 Ž1996. 753.