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Relaxation process of band-edge exciton in layered crystalline GeSe2
Journal of Luminescence 87}89 (2000) 617}619
Relaxation process of band-edge exciton in layered crystalline GeSe Toshihiro Nakaoka *, Yong Wang , Osamu Matsuda, Koichi Inoue, Kazuo Murase
Department of Physics, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka 560-0043, Japan Division of Applied Physics, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan The institute of Scientixc and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
Abstract Relaxation processes of the photoexcited states in layered crystalline GeSe are studied by time-resolved photo luminescence measurement. Two photoluminescence bands, P1 and P2, are clearly resolved from their decay kinetics. It is found that one of the relaxation pathways to the P2 band arises from a band-edge exciton state. The P2 band shows no polarization dependence, in spite of the fact that the photoexcitation of the exciton shows a strongly anisotropic optical character. We discuss the relaxation process on the basis of the luminescence characteristics and the structural model of the band-edge exciton quasi-localized at edge-sharing tetrahedra. 2000 Elsevier Science B.V. All rights reserved. Keywords: Chalcogenides; Photoluminescence; Relaxation process
Due to strong lattice relaxation following optical excitation, photoluminescence (PL) spectra of chalcogenide semiconductors exhibits a large Stokes shift and a fairly broad line width. Such interesting PL properties have been the subject of extensive studies [1}8]. The PL spectra and time decays are similar in both crystalline and amorphous forms of several chalcogenide semiconductors, including As Se and GeSe [1,2]. This sug gests a common relaxation mechanism that arises from the intrinsic chemical bonds of the chalcogenide alloys rather than native defects or impurities. Toward a general understanding of the relaxation process of the photoexcited state in chalcogenide semiconductors, we study the PL properties of a typical chalcogenide semiconductor, layered crystalline GeSe (c-GeSe ) which is `a key stonea to investigate those of the Ge}Se glasses [4]. The basic structural units of c-GeSe are GeSe tetrahedra. One layer of c-GeSe consists of parallel chains of cor * Corresponding author. Fax: #81-6-6850-5376. E-mail address: [email protected] (T. Nakaoka)
ner-sharing tetrahedra, interconnected by pairs of edgesharing tetrahedra. Ristein et al. have studied the relaxation processes of crystalline As Se by the optically detected magnetic resonance (ODMR) measurement [5]. They have argued that the luminescent center is a self-trapped triplet exciton, and the relaxation of the photoexcited states is initiated by trapping of a hole at a center of inversion symmetry. Around the inversion center, the lone-pair electrons of two Se atoms strongly interact with each other. The Se atoms are located on neighboring layers across the inversion center. The local con"guration of the initial state of the relaxation in c-As Se is similar to that of a band-edge exciton in c-GeSe , proposed on the basis of resonant Raman study [9,10]. The exciton is quasilocalized to the edge-sharing bi-tetrahedra around the inversion center of c-GeSe . The exciton peak has been observed around 2.84 eV at 15 K in E""a polarization, while no clear peak has been observed in E""b polarization [11]. The fairly broad line width of the exciton peak has been attributed to strong electron}phonon interactions. As we shall show later, this exciton relaxes to a luminescent state. In this work, we discuss the electronic
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 3 3 0 - 0
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T. Nakaoka et al. / Journal of Luminescence 87}89 (2000) 617}619
and structural relaxation processes of the band edge exciton, leading to a PL band. Single-GeSe crystals were prepared by a vapor-phase growth method from melt-quenched GeSe bulk glasses [10]. The cw laser light in an energy range of 2.41} 2.81 eV chopped by an acousto-optic modulator or an electro-optic modulator was used for the time-resolved PL measurement. The pulse width was 3 ms and the frequency was 166.6 Hz. The PL spectra were taken by using a monochromator, a photo-multiplier (HAMAMATSU R5509-71A), and a gated photon counter (Stanford SR400). The excitation by near band-gap light (&2.7 eV) causes a Gaussian-shaped PL band with &0.3 eV FWHM around 1 eV. The PL peak energy increases from 0.99 to 1.14 eV with increasing excitation energies from 2.54 to 2.81 eV [7]. This shift of the peak energy will be explained by the fact that the PL spectra consist of two PL bands. The two PL bands of c-GeSe could be clearly separated by the time-resolved measurement. Fig. 1 shows the excitation energy dependence of the decay kinetics at various luminescent photon energies. The decay curves do not much depend on the luminescent photon energy with excitation photon energies of 2.54 and 2.81 eV. We denote the PL band excited at 2.54 eV as P1, and the band excited at 2.81 eV as P2. At excitation energy of 2.62 eV, the decay curve at the lower-energy side of the PL resembles that of the P1 band. With increasing detected luminescent photon energies, the
decay curve approaches that of the P2 band. This fact re#ects that the PL spectrum is composed of the P1 and the P2 bands. Details of the decay kinetics in the P1 and the P2 bands will be described elsewhere. In this paper, we focus on the relaxation process to the P2 band. The band-edge exciton transition in c-GeSe is ob served only in the E""a absorption spectrum around 2.8 eV. On the other hand, the PL spectra and the decay kinetics, excited at 2.81 eV, do not depend on the polarization of the incident and detected lights, as shown in Fig. 2. Moreover, the quantum e$ciency of the PL excited at 2.81 eV is also the same for both polarizations, in spite of the fact that the exciton absorption spectra is strongly anisotropic around 2.8 eV, where practically all the incident photons are absorbed. Non-polarization dependence of the PL properties suggests that the photoexcited carriers created by the light of E""a and b polarizations relax to the same PL band. It follows that both excited electron}hole pairs and the quasi-localized band-edge excitons relax to the P2 band. In addition, we investigated the PL spectra and the decay curves for various orientations of the detected polarization in the a}b, and the a}c, planes, and found that they are independent of the orientation of the polarization. The possibility that the PL has a circular polarization is excluded from the experimental results using a j/4 waveplate. In the following we discuss the relaxation process to the P2 band starting from the band-edge exciton. It is well known that the top of the valence band of c-GeSe consists of the Se lone-pair orbitals. There are exactly parallel lone-pair orbitals of the Se atoms displaced on neighboring layers across the center of the inversion symmetry, as shown in Fig. 3. A large overlap integral of those lone-pair orbitals leads to a strong p-type interaction between the Se atoms, denoted by the dashed line in Fig. 3. The interaction may cause large p}pH splittings. The pH state will be pushed up and form the top of the
Fig. 1. Decay kinetics of the PL of c-GeSe at various detected luminescent photon energies for the excitation energies of 2.54, 2.62, and 2.81 eV at 15 K. The incident light polarization is parallel to the a-axis, and the detected polarization is also parallel to the a-axis. We use the symbol (a,a) to refer to this con"guration.
Fig. 2. No polarization dependence of (a) the PL spectra and (b) the decay kinetics of the P2 band for the excitation energy of 2.81 eV at 15 K. The PL spectra are normalized at the respective peak. The decay curves are measured at the PL peak energy of 1.14 eV. The spectra and the decay curves are displaced vertically for clarity.
T. Nakaoka et al. / Journal of Luminescence 87}89 (2000) 617}619
619
concluded that the luminescent center of the P2 band of c-GeSe should be strongly localized around the particu lar Se atoms on neighboring layers.
Acknowledgements
Fig. 3. Projection of the layered structure of c-GeSe . The inversion center is indicated by a cross symbol. Thick dashed lines indicate the strong p-type interlayer interaction between the Se atoms whose lone-pair orbitals are parallel to each other.
This work was supported by Grants-in-Aid for Scienti"c Research (B) (No. 09440117), for Encouragement of Young Scientists (No. 11740173), and for Scienti"c Research on Priority Area &Cooperative Phenomena in Complex Liquids', from the Ministry of Education, Science and Culture (Japan).
References valence band. On the other hand, the bottom of the conduction band consists of the antibonding states of Ge}Se covalent bonds [12,13]. From these facts, the exciton is assigned to the transition from the pH state of the lone-pair electrons to the antibonding state of Ge}Se bonds. Because the hole state arising from the pH state is localized around the Se atoms, the creation of the hole state will reduce the electron repulsive force between the Se atoms. It makes the Se atoms approach each other, and causes the enhancement of the p interaction, leading to further increase of the p}pH splitting. On the other hand, the photoexcitation of electron to the antibonding states will weaken the local covalent Ge}Se bonds, which causes further relaxation. Thus, both electron and hole states are moved deeper into the optical gap to form the luminescent center of the P2 band. It is noteworthy that this relaxation process is similar to a formation of a selftrapped exciton in c-As Se [5]. As mentioned before, there is no polarization dependence in the P2 bands for the a}b and the a}c planes. If this luminescent state is relatively extended along the layer plane, the PL would show an anisotropic optical property. The band-edge exciton quasi-localized at the edge-sharing tetrahedra is further localized to form the luminescent center as a consequence of the relaxation with distortion of local bonds, especially around the Se atoms located on neighboring layers across the center of the inversion symmetry. It is
[1] V.A. Vassilyev, M. KooH s, I. KoH sa Somogyi, Sov. Phys. Solid State 31 (1989) 782. [2] L.H. Robins, M.A. Kastner, in: P.C. Taylor, S.G. Bishop (Eds.), Optical E!ects in Amorphous Semiconductors, AIP Conference Proceedings No. 120, AIP, New York, 1984, p. 183. [3] V.A. Vassilyev, M. KooH s, I. KoH sa Somogyi, Philos. Mag. B 39 (1979) 333. [4] K. Murase, in: P. Boolchand (Ed.), Insulating and Semiconducting Glasses, World Scienti"c, Singapore, 1999 (Chapter 6c). [5] J. Ristein, P.C. Taylor, W.D. Ohlsen, G. Weiser, Phys. Rev. B 42 (1990) 11 845. [6] L.H. Robins, M.A. Kastner, Phys. Rev. B 35 (1987) 2867. [7] O. Matsuda, T. Nakane, K. Inoue, K. Murase, in: P. Jiang, H. Zhi Zheng (Eds.), Proceedings of the 21st International Conference on the Physics of Semiconductors, Beijing, China, World Scienti"c, Singapore, 1992, p. 125. [8] O. Matsuda, Y. Saitoh, K. Yamagata, Y. Wada, Y. Wang, K. Inoue, K. Murase, J. Non-Cryst. Solids 227}330 (1998) 829. [9] O. Matsuda, K. Inoue, K. Murase, Solid State Commun. 75 (1990) 303. [10] T. Nakaoka, Y. Wang, O. Matsuda, K. Inoue, K. Murase, to be published. [11] S.A. Boiko, D.I. Bletskan, S.F. Terekhova, Phys. Stat. Sol. B 90 (1978) K49. [12] Steven G. Louie, Phys. Rev. B 26 (1982) 5993. [13] S. Hosokawa, K. Nishihara, Y. Hari, M. Taniguchi, O. Matsuda, K. Murase, Phys. Rev. B 47 (1993) 15 509.
Relaxation process of band-edge exciton in layered crystalline GeSe Toshihiro Nakaoka *, Yong Wang , Osamu Matsuda, Koichi Inoue, Kazuo Murase
Department of Physics, Graduate School of Science, Osaka University, 1-1 Machikaneyama, Toyonaka 560-0043, Japan Division of Applied Physics, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan The institute of Scientixc and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan
Abstract Relaxation processes of the photoexcited states in layered crystalline GeSe are studied by time-resolved photo luminescence measurement. Two photoluminescence bands, P1 and P2, are clearly resolved from their decay kinetics. It is found that one of the relaxation pathways to the P2 band arises from a band-edge exciton state. The P2 band shows no polarization dependence, in spite of the fact that the photoexcitation of the exciton shows a strongly anisotropic optical character. We discuss the relaxation process on the basis of the luminescence characteristics and the structural model of the band-edge exciton quasi-localized at edge-sharing tetrahedra. 2000 Elsevier Science B.V. All rights reserved. Keywords: Chalcogenides; Photoluminescence; Relaxation process
Due to strong lattice relaxation following optical excitation, photoluminescence (PL) spectra of chalcogenide semiconductors exhibits a large Stokes shift and a fairly broad line width. Such interesting PL properties have been the subject of extensive studies [1}8]. The PL spectra and time decays are similar in both crystalline and amorphous forms of several chalcogenide semiconductors, including As Se and GeSe [1,2]. This sug gests a common relaxation mechanism that arises from the intrinsic chemical bonds of the chalcogenide alloys rather than native defects or impurities. Toward a general understanding of the relaxation process of the photoexcited state in chalcogenide semiconductors, we study the PL properties of a typical chalcogenide semiconductor, layered crystalline GeSe (c-GeSe ) which is `a key stonea to investigate those of the Ge}Se glasses [4]. The basic structural units of c-GeSe are GeSe tetrahedra. One layer of c-GeSe consists of parallel chains of cor * Corresponding author. Fax: #81-6-6850-5376. E-mail address: [email protected] (T. Nakaoka)
ner-sharing tetrahedra, interconnected by pairs of edgesharing tetrahedra. Ristein et al. have studied the relaxation processes of crystalline As Se by the optically detected magnetic resonance (ODMR) measurement [5]. They have argued that the luminescent center is a self-trapped triplet exciton, and the relaxation of the photoexcited states is initiated by trapping of a hole at a center of inversion symmetry. Around the inversion center, the lone-pair electrons of two Se atoms strongly interact with each other. The Se atoms are located on neighboring layers across the inversion center. The local con"guration of the initial state of the relaxation in c-As Se is similar to that of a band-edge exciton in c-GeSe , proposed on the basis of resonant Raman study [9,10]. The exciton is quasilocalized to the edge-sharing bi-tetrahedra around the inversion center of c-GeSe . The exciton peak has been observed around 2.84 eV at 15 K in E""a polarization, while no clear peak has been observed in E""b polarization [11]. The fairly broad line width of the exciton peak has been attributed to strong electron}phonon interactions. As we shall show later, this exciton relaxes to a luminescent state. In this work, we discuss the electronic
0022-2313/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 3 3 0 - 0
618
T. Nakaoka et al. / Journal of Luminescence 87}89 (2000) 617}619
and structural relaxation processes of the band edge exciton, leading to a PL band. Single-GeSe crystals were prepared by a vapor-phase growth method from melt-quenched GeSe bulk glasses [10]. The cw laser light in an energy range of 2.41} 2.81 eV chopped by an acousto-optic modulator or an electro-optic modulator was used for the time-resolved PL measurement. The pulse width was 3 ms and the frequency was 166.6 Hz. The PL spectra were taken by using a monochromator, a photo-multiplier (HAMAMATSU R5509-71A), and a gated photon counter (Stanford SR400). The excitation by near band-gap light (&2.7 eV) causes a Gaussian-shaped PL band with &0.3 eV FWHM around 1 eV. The PL peak energy increases from 0.99 to 1.14 eV with increasing excitation energies from 2.54 to 2.81 eV [7]. This shift of the peak energy will be explained by the fact that the PL spectra consist of two PL bands. The two PL bands of c-GeSe could be clearly separated by the time-resolved measurement. Fig. 1 shows the excitation energy dependence of the decay kinetics at various luminescent photon energies. The decay curves do not much depend on the luminescent photon energy with excitation photon energies of 2.54 and 2.81 eV. We denote the PL band excited at 2.54 eV as P1, and the band excited at 2.81 eV as P2. At excitation energy of 2.62 eV, the decay curve at the lower-energy side of the PL resembles that of the P1 band. With increasing detected luminescent photon energies, the
decay curve approaches that of the P2 band. This fact re#ects that the PL spectrum is composed of the P1 and the P2 bands. Details of the decay kinetics in the P1 and the P2 bands will be described elsewhere. In this paper, we focus on the relaxation process to the P2 band. The band-edge exciton transition in c-GeSe is ob served only in the E""a absorption spectrum around 2.8 eV. On the other hand, the PL spectra and the decay kinetics, excited at 2.81 eV, do not depend on the polarization of the incident and detected lights, as shown in Fig. 2. Moreover, the quantum e$ciency of the PL excited at 2.81 eV is also the same for both polarizations, in spite of the fact that the exciton absorption spectra is strongly anisotropic around 2.8 eV, where practically all the incident photons are absorbed. Non-polarization dependence of the PL properties suggests that the photoexcited carriers created by the light of E""a and b polarizations relax to the same PL band. It follows that both excited electron}hole pairs and the quasi-localized band-edge excitons relax to the P2 band. In addition, we investigated the PL spectra and the decay curves for various orientations of the detected polarization in the a}b, and the a}c, planes, and found that they are independent of the orientation of the polarization. The possibility that the PL has a circular polarization is excluded from the experimental results using a j/4 waveplate. In the following we discuss the relaxation process to the P2 band starting from the band-edge exciton. It is well known that the top of the valence band of c-GeSe consists of the Se lone-pair orbitals. There are exactly parallel lone-pair orbitals of the Se atoms displaced on neighboring layers across the center of the inversion symmetry, as shown in Fig. 3. A large overlap integral of those lone-pair orbitals leads to a strong p-type interaction between the Se atoms, denoted by the dashed line in Fig. 3. The interaction may cause large p}pH splittings. The pH state will be pushed up and form the top of the
Fig. 1. Decay kinetics of the PL of c-GeSe at various detected luminescent photon energies for the excitation energies of 2.54, 2.62, and 2.81 eV at 15 K. The incident light polarization is parallel to the a-axis, and the detected polarization is also parallel to the a-axis. We use the symbol (a,a) to refer to this con"guration.
Fig. 2. No polarization dependence of (a) the PL spectra and (b) the decay kinetics of the P2 band for the excitation energy of 2.81 eV at 15 K. The PL spectra are normalized at the respective peak. The decay curves are measured at the PL peak energy of 1.14 eV. The spectra and the decay curves are displaced vertically for clarity.
T. Nakaoka et al. / Journal of Luminescence 87}89 (2000) 617}619
619
concluded that the luminescent center of the P2 band of c-GeSe should be strongly localized around the particu lar Se atoms on neighboring layers.
Acknowledgements
Fig. 3. Projection of the layered structure of c-GeSe . The inversion center is indicated by a cross symbol. Thick dashed lines indicate the strong p-type interlayer interaction between the Se atoms whose lone-pair orbitals are parallel to each other.
This work was supported by Grants-in-Aid for Scienti"c Research (B) (No. 09440117), for Encouragement of Young Scientists (No. 11740173), and for Scienti"c Research on Priority Area &Cooperative Phenomena in Complex Liquids', from the Ministry of Education, Science and Culture (Japan).
References valence band. On the other hand, the bottom of the conduction band consists of the antibonding states of Ge}Se covalent bonds [12,13]. From these facts, the exciton is assigned to the transition from the pH state of the lone-pair electrons to the antibonding state of Ge}Se bonds. Because the hole state arising from the pH state is localized around the Se atoms, the creation of the hole state will reduce the electron repulsive force between the Se atoms. It makes the Se atoms approach each other, and causes the enhancement of the p interaction, leading to further increase of the p}pH splitting. On the other hand, the photoexcitation of electron to the antibonding states will weaken the local covalent Ge}Se bonds, which causes further relaxation. Thus, both electron and hole states are moved deeper into the optical gap to form the luminescent center of the P2 band. It is noteworthy that this relaxation process is similar to a formation of a selftrapped exciton in c-As Se [5]. As mentioned before, there is no polarization dependence in the P2 bands for the a}b and the a}c planes. If this luminescent state is relatively extended along the layer plane, the PL would show an anisotropic optical property. The band-edge exciton quasi-localized at the edge-sharing tetrahedra is further localized to form the luminescent center as a consequence of the relaxation with distortion of local bonds, especially around the Se atoms located on neighboring layers across the center of the inversion symmetry. It is
[1] V.A. Vassilyev, M. KooH s, I. KoH sa Somogyi, Sov. Phys. Solid State 31 (1989) 782. [2] L.H. Robins, M.A. Kastner, in: P.C. Taylor, S.G. Bishop (Eds.), Optical E!ects in Amorphous Semiconductors, AIP Conference Proceedings No. 120, AIP, New York, 1984, p. 183. [3] V.A. Vassilyev, M. KooH s, I. KoH sa Somogyi, Philos. Mag. B 39 (1979) 333. [4] K. Murase, in: P. Boolchand (Ed.), Insulating and Semiconducting Glasses, World Scienti"c, Singapore, 1999 (Chapter 6c). [5] J. Ristein, P.C. Taylor, W.D. Ohlsen, G. Weiser, Phys. Rev. B 42 (1990) 11 845. [6] L.H. Robins, M.A. Kastner, Phys. Rev. B 35 (1987) 2867. [7] O. Matsuda, T. Nakane, K. Inoue, K. Murase, in: P. Jiang, H. Zhi Zheng (Eds.), Proceedings of the 21st International Conference on the Physics of Semiconductors, Beijing, China, World Scienti"c, Singapore, 1992, p. 125. [8] O. Matsuda, Y. Saitoh, K. Yamagata, Y. Wada, Y. Wang, K. Inoue, K. Murase, J. Non-Cryst. Solids 227}330 (1998) 829. [9] O. Matsuda, K. Inoue, K. Murase, Solid State Commun. 75 (1990) 303. [10] T. Nakaoka, Y. Wang, O. Matsuda, K. Inoue, K. Murase, to be published. [11] S.A. Boiko, D.I. Bletskan, S.F. Terekhova, Phys. Stat. Sol. B 90 (1978) K49. [12] Steven G. Louie, Phys. Rev. B 26 (1982) 5993. [13] S. Hosokawa, K. Nishihara, Y. Hari, M. Taniguchi, O. Matsuda, K. Murase, Phys. Rev. B 47 (1993) 15 509.