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Photoemission and inverse-photoemission studies of glassy GexSe1−x
Journal of Electron Spectroscopy and Related Phenomena 78 ( 1996)507-5 10
PHOTOEMISSION AND INVEFtSE-PHOTOEMISSION OF GLASSY Ge,Sel,,
STUDIES
M.Taniguchia, T.Kouchia, I.Onoa, S.Hosokawaa, M.Nakatakea, H.Namatamea, and K.Muraseb aDepartment of Materials Science, Faculty of Science, Hiroshima University, Kagamiyama l-3, Higashi-Hiroshima 739, Japan bDepartment of Physics, Faculty of Science, Osaka University, Machikaneyama l-l, Toyonaka, Osaka 560, Japan Valence-band UPS and conduction-band IPES spectra of glassy Ge,Se,_, (01x10.33) exhibit a distinct change in their spectral features near x=0.2. This observation is consistent with an occurrence of percolation threshold in non-crystalline covalent networks predicted by Phillips and Thorpe. The threshold is characterized by the percolation of a specific Ge(Se&4 molecular unit over the material.
1. INTRODUCTION For non-crystalline covalent networks constrained by bond-stretching and bond-bending forces, Phillips proposed that the covalent bonding may be optimized when the number of constraints equals the dimensionality or number of degrees of freedom in the system[l]. Thorpe predicted that a rigidity percolation takes place as the average coordination number (r) of the system passes through the threshold value of 2.4[2]. The character of the covalent network undergoes a change from being easily deformable ((r)c2.4) to being rigid ((r)>2.4). The prediction of the rigidity percolation threshold in a glassy network at (r)=2.4 has stimulated a number of experimental works over the years. The binary glass of Ge,Ser_x(g-GexSe1_x), in which Ge and Se atoms are known to be four- and two-fold-coordinated, respectively, provides an ideal test system. One can prepare the glasses over a wide Ge-composition (x) range of 01x10.4[3]. The average coordination number of (r)=2.4 is realized for x=0.2. Raman A, mode frequencies of Ge(Se&4 units[4], rlgSn LambMossbauer f-factors of Sn(Set,~), sites[5], 12gI Miissbauer site intensity ratios[6] and molar volumes[7], all investigated as a function of x, were reported to exhibit some anomalous features near x=0.2. With an interest in testing our idea that the 0368-2048/%/$15.00 6 19% Elsevier Science B.V. All rights reserved PII SO368-2048 (W) 02785-5
percolation phenomenon in non-crystalline covalent networks might be sensitively reflected in the electronic states, we have investigated the density of states of both valence and conduction bands of gGe,Ser_, by means of ultraviolet photoemission and inverse-photoemission spectroscopies (UPS and IPES), taking into account the pronounced photoabsorption cross-sections of the Ge 4p and Se 4p states in the ultraviolet region[8]. We report in this paper a dramatical change in features of the UPS and IPES spectra of g-GexSer _xnear x=0.2 in detail and discuss the critical behaviour in terms of the PhillipsThorpe rigidity percolation theory. 2. EXPERIMENTAL Figure 1 shows schematically the apparatus used in the present experiments. The UPS spectrometer connected with the IPES apparatus is composed of a He discharge lamp (hv=21.2eV) and a double-stage cylindrical-mirror analyzer (DCMA) to obtain angleintegrated spectra. The energy resolution was set to be 0.2 eV. Working pressure of the UPS chamber was 3~10~~Torr under the operation of the discharge lamp, though the base pressure was 4~10-‘~ Torr. The IPES spectrometer[9] consists of a low energy electron gun of Erdman-Zipf type with an energy spread of 0.25 eV, an Al reflection mirror coated with a MgF, film, and the bandpass photon
508
Band pass Photon Detector
Transfer Rod
w
Channeltron
Al Reflection Mirror
I UPS
Figure 1 : Schematic illustration of the experimental apparatus composed of the UPS and IPES spectrometers, and sample preparation chamber. Energy positions of the Fermi level in the UPS and IPES spectra are experimentally determined using spectra for a fresh film of polycrystalline Au. In situ measurements of the UPS and IPES spectra realize a connection of both spectra at the Fermi level. detector with a full width at half maximum of 0.47 eV and a maximum response at 9.43 eV. All components are mounted in an ultra high vacuum chamber under the base pressure below 7x10-l 1 Ton. Overall energy resolution of the spectrometer was 0.56 eV with a maximumresponse at 9.37 eV. The electron beam was injected normal to the sample surfaces. Energy calibration of the UPS and IPES spectra were experimentally made using the spectra for a fresh film of polycrystalline Au[9]. The UPS and IPES spectra measured in siru for the same sample-surface were connected at the Fermi level. All measurements were carried out at room temperature. The energy was referred to the Fermi level. The g-GqSer_, films were obtained by thermal evaporation from GexSer_x bulk glasses with x varying from 0 to 0.33. The bulk glasses were prepared by a standard melt-quenching method, and were in situ deposited on Au substrates within a sample preparation chamber using specially designed quartz furnace. During evaporation, the substrates were kept constant at room temperature and the pressure was of the order of 10sgTom The composition of films with thickness of -3l~m, evaluated carefully by electronprobe microanalysis, was in all cases close to the bulk
composition within 2-3 % [9]. For in situ measurements of the UPS and IPES spectra, the thickness of films was reduced to 50-lOO A to avoid an electrostatic charging effect in the IPES measurements. Then, the UPS spectra for these thin films, which exhibited no changing effect in the IPES measurements, were checked to be fully consistent with those for thick films. The deposition rate was controlled by means of a quartz thickness monitor placed near the sample. Typical value was 0.2-0.3 &sec. 3. RESULTS AND DISCUSSION Figure 2 shows a series of valence-band UPS and conduction-band IPES spectra of g-Ge,Ser_x with x from 0 to 0.33. The UPS and IPES spectra for x=0 (g-Se) exhibit structures at -6.4 and -5.4, -2.7,3.1 and 7.4 eV due to the Se 4p bonding, 4p lone pair (LPI, 4p antibonding and 4d and/or 5s states, respectively[ 101, while those for x=0.33 (g-GeSe2) represent peaks at -6.Oand-4.3,-2.7,2.4and4.9,and8.4eVoriginating mainly from the Ge sp3-Se p bonding, Se p LP, Ge sp3-Se p antibonding states, and 4d and/or 5s states of Ge and Se atoms, respectively[l 11,as summalized in Table 1. With increasing x from 0 to 0.18, the UPS
g -
Ge,Sel_,
IPES hv = 9.37 eV
UPS hv=21.2eV
‘10
-5
0 5 Energy (eV)
10
15
Figure 2: A series of the valence-band UPS and conduction-band IPES spectra of g-Ge,Sel_x with x from 0 to 0.33. Intensities of the UPS and IPES spectra are tentatively normalized at -2.7eV, and 3.1 (x=0,0.10,0.15 and 0.18) and 2.4eV (x=0.20,0.25 and 0.33), respectively. Vertical bars indicate the positions of structures. Energies are referred to the Fermi level. and IPES spectra do not show any noticeable change with respect to their spectral shape and energy positions of structures. At x=0.2, however, one can recognize a slight blurring of the structures at -6.4 and -5.4 eV and increasing components around -6.0 and -4.3 eV for the UPS spectrum. A remarkable change is also observed for the shape of the IPES spectrum as well as for the number of peaks. For further increase of x, features of the UPS and IPES spectra reach those of g-GeSez. We find that the UPS spectra for x=0.20 and 0.33 in Fig.2 are in qualitative agreement with spectra obtained by X-ray photoemission spectroscopy (XPS)[12,13]. From modifications of the Ge 4s and Se 4s bands with x, these XPS studies emphasize that
the 4(Ge):2(Se) coordination is manifested [12,13] and chemically ordered structural units are favoured[ 121. The composition dependence of the UPS and IPES spectra in Fig.2 can be understood in terms of the Phillips-Thorpe rigidity percolation theory[ 1,2]. In g-Ge,Se,_, with ~~0.2, there exist Se chains and Ge(Sel,& tetrahedral units, where Ge atoms act as cross-links between Se chains. Near x=0.2, the rigid Ge(Se,,*), molecular units, which nucleate in the glass and gradually coalesce with the increase of x, may contact each other and percolate over the material. For x above 0.2, the Ge(Sel,&, units grow to form fragments of the two-dimensional form of
510
Table 1. Energy positions of peak structures in tbe UPS and IPES spectra of g-Se and g-GeSe2, and electronic states which contribute predominantly to the peak structures. Energy(eV) 8.4 4.9 g-GeSe2 2.4 1 -2.7 -4.3 -6.0 I 7.4 3.1 g-Se -2.7 -5.4 -6.4 1
Electronic states 4d and/or 5s states of Ge and Se atoms Ge sp3-Se 4p antibonding states
Japan, The Ogasawara Foundation for the Promotion of Science and Technology, The Murata Science Foundation, Shimazu Science Foundation, Nissan Science Foundation and Tokuyama Science Foundation. REFERENCES 1.
Se 4p lone pair states Ge sp3-Se 4p bonding states
2.
Se 4d and/or 5s states Se 4p antibonding states Se 4p lone pair states
3.
Se 4p bonding states
4.
GeSe2. Such a transition from one kind of network to another shows up clearly in the UPS and IPES spectra near x=0.2. One notices also that the present results correlate well with those of Raman scattering[4], Mijssbauer experiments[5,6] and molar volumes[7]. We believe that the present UPS and IPES studies of g-Ge,Sel_, provide an experimental support for the rigidity percolation threshold to take place near x=0.2, in much more direct form in comparison with those reported so far. The distinct change of the UPS and IPES spectra is reasonably assumed to reflect the threshold between the nucleation and subsequent growth of clusters just after the percolation. Finally, we point out that there is an interest to establish whether the threshold in the 4:2 coordinated networks takes place just at (r)=2.4 or at (r) greater than 2.4, depending on the presence of broken angularconstraints. For this kind of subject, further experimental works are required.
5. 6.
7. 8. 9.
10. 11.
ACKNOWLEDGMENTS 12. The authors are grateful to Dr. 0. Matsuda for fruitful discussions and to A. Minami for an electronprobe microanalysis, respectively. This work is partly supported by the Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture,
13.
J.C.Phillips, J. Non-Cryst. Solids 34 (1979) 153. M.F.Thorpe, J. Non-Cryst. Solids 57 (1983) 355; also see J.C.Phillips and M.F.Thorpe, Solid State Commun. 53 (1985) 699. P.Tronc, M.Bensoussan, A.Brenac, G.Errandonea, and C.Sebenne, J. Phys. 38 (1977) 1493. K.Murase, T.Fukunaga, K.Yakusiji, T.Yoshimi, and IYunoki, J. Non-Cryst. Solids 59&60 (1983) 883. M.Stevens, J.Grothaus, P.Boolchand, and J.G.Hernandez, Phys. Rev. B31 (1985) 981. W.Bresser, P.Boolchand, and P.Suranyi, Phys. Rev. Lett. 56 (1986) 2493; also see P.Boolchand, W.Bresser, M.Zhang, Y.Wu, J.Wells, and R.N.Enzweiler, J. Non-Cryst. Solids 182 (1995) 143. A.Feltz, H.Aust, and A.Bleyer, J. Non-Cryst. Solids 55 (1983) 179. J.J.Yeh, and I.Lindau, At Data Nucl. Tables 32 (1985) 52; 32 (1985) 54. K.Yokoyama, K.Nishihara, K.Mimura, Y .Hari, M.Taniguchi, Y.&da, and M.Fujisawa, Rev. Sci. Instrum. 64 (1993) 87; also see Y.Ueda, K.Nishihara, K.Mimura, Y .Hari, M.Taniguchi, and M.Fujisawa, Nucl. Instrum. Methods A330 (1993) 140. J.D.Joanopoulos, M.Schluter, and M.L.Cohen, Phys. Rev. Bll (1975) 2186. SHosokawa, K.Nishihara, Y.Hari, M.Taniguchi, O.Matsuda and K.Murase, Phys. Rev. B47 (1993) 15509. M.L.Theye, A.Gheorghiu, C.Senemaud, M.F.Kotkata, and K.M.Kandil, Phil. Mag. B69 (1994) 209. E.Bergignat, G.Hollinger, H.Chermette, P.Pertosa, DLohez, M.Larmoo, and M.Bensoussan, Phys. Rev. B37 (1988) 4506.
PHOTOEMISSION AND INVEFtSE-PHOTOEMISSION OF GLASSY Ge,Sel,,
STUDIES
M.Taniguchia, T.Kouchia, I.Onoa, S.Hosokawaa, M.Nakatakea, H.Namatamea, and K.Muraseb aDepartment of Materials Science, Faculty of Science, Hiroshima University, Kagamiyama l-3, Higashi-Hiroshima 739, Japan bDepartment of Physics, Faculty of Science, Osaka University, Machikaneyama l-l, Toyonaka, Osaka 560, Japan Valence-band UPS and conduction-band IPES spectra of glassy Ge,Se,_, (01x10.33) exhibit a distinct change in their spectral features near x=0.2. This observation is consistent with an occurrence of percolation threshold in non-crystalline covalent networks predicted by Phillips and Thorpe. The threshold is characterized by the percolation of a specific Ge(Se&4 molecular unit over the material.
1. INTRODUCTION For non-crystalline covalent networks constrained by bond-stretching and bond-bending forces, Phillips proposed that the covalent bonding may be optimized when the number of constraints equals the dimensionality or number of degrees of freedom in the system[l]. Thorpe predicted that a rigidity percolation takes place as the average coordination number (r) of the system passes through the threshold value of 2.4[2]. The character of the covalent network undergoes a change from being easily deformable ((r)c2.4) to being rigid ((r)>2.4). The prediction of the rigidity percolation threshold in a glassy network at (r)=2.4 has stimulated a number of experimental works over the years. The binary glass of Ge,Ser_x(g-GexSe1_x), in which Ge and Se atoms are known to be four- and two-fold-coordinated, respectively, provides an ideal test system. One can prepare the glasses over a wide Ge-composition (x) range of 01x10.4[3]. The average coordination number of (r)=2.4 is realized for x=0.2. Raman A, mode frequencies of Ge(Se&4 units[4], rlgSn LambMossbauer f-factors of Sn(Set,~), sites[5], 12gI Miissbauer site intensity ratios[6] and molar volumes[7], all investigated as a function of x, were reported to exhibit some anomalous features near x=0.2. With an interest in testing our idea that the 0368-2048/%/$15.00 6 19% Elsevier Science B.V. All rights reserved PII SO368-2048 (W) 02785-5
percolation phenomenon in non-crystalline covalent networks might be sensitively reflected in the electronic states, we have investigated the density of states of both valence and conduction bands of gGe,Ser_, by means of ultraviolet photoemission and inverse-photoemission spectroscopies (UPS and IPES), taking into account the pronounced photoabsorption cross-sections of the Ge 4p and Se 4p states in the ultraviolet region[8]. We report in this paper a dramatical change in features of the UPS and IPES spectra of g-GexSer _xnear x=0.2 in detail and discuss the critical behaviour in terms of the PhillipsThorpe rigidity percolation theory. 2. EXPERIMENTAL Figure 1 shows schematically the apparatus used in the present experiments. The UPS spectrometer connected with the IPES apparatus is composed of a He discharge lamp (hv=21.2eV) and a double-stage cylindrical-mirror analyzer (DCMA) to obtain angleintegrated spectra. The energy resolution was set to be 0.2 eV. Working pressure of the UPS chamber was 3~10~~Torr under the operation of the discharge lamp, though the base pressure was 4~10-‘~ Torr. The IPES spectrometer[9] consists of a low energy electron gun of Erdman-Zipf type with an energy spread of 0.25 eV, an Al reflection mirror coated with a MgF, film, and the bandpass photon
508
Band pass Photon Detector
Transfer Rod
w
Channeltron
Al Reflection Mirror
I UPS
Figure 1 : Schematic illustration of the experimental apparatus composed of the UPS and IPES spectrometers, and sample preparation chamber. Energy positions of the Fermi level in the UPS and IPES spectra are experimentally determined using spectra for a fresh film of polycrystalline Au. In situ measurements of the UPS and IPES spectra realize a connection of both spectra at the Fermi level. detector with a full width at half maximum of 0.47 eV and a maximum response at 9.43 eV. All components are mounted in an ultra high vacuum chamber under the base pressure below 7x10-l 1 Ton. Overall energy resolution of the spectrometer was 0.56 eV with a maximumresponse at 9.37 eV. The electron beam was injected normal to the sample surfaces. Energy calibration of the UPS and IPES spectra were experimentally made using the spectra for a fresh film of polycrystalline Au[9]. The UPS and IPES spectra measured in siru for the same sample-surface were connected at the Fermi level. All measurements were carried out at room temperature. The energy was referred to the Fermi level. The g-GqSer_, films were obtained by thermal evaporation from GexSer_x bulk glasses with x varying from 0 to 0.33. The bulk glasses were prepared by a standard melt-quenching method, and were in situ deposited on Au substrates within a sample preparation chamber using specially designed quartz furnace. During evaporation, the substrates were kept constant at room temperature and the pressure was of the order of 10sgTom The composition of films with thickness of -3l~m, evaluated carefully by electronprobe microanalysis, was in all cases close to the bulk
composition within 2-3 % [9]. For in situ measurements of the UPS and IPES spectra, the thickness of films was reduced to 50-lOO A to avoid an electrostatic charging effect in the IPES measurements. Then, the UPS spectra for these thin films, which exhibited no changing effect in the IPES measurements, were checked to be fully consistent with those for thick films. The deposition rate was controlled by means of a quartz thickness monitor placed near the sample. Typical value was 0.2-0.3 &sec. 3. RESULTS AND DISCUSSION Figure 2 shows a series of valence-band UPS and conduction-band IPES spectra of g-Ge,Ser_x with x from 0 to 0.33. The UPS and IPES spectra for x=0 (g-Se) exhibit structures at -6.4 and -5.4, -2.7,3.1 and 7.4 eV due to the Se 4p bonding, 4p lone pair (LPI, 4p antibonding and 4d and/or 5s states, respectively[ 101, while those for x=0.33 (g-GeSe2) represent peaks at -6.Oand-4.3,-2.7,2.4and4.9,and8.4eVoriginating mainly from the Ge sp3-Se p bonding, Se p LP, Ge sp3-Se p antibonding states, and 4d and/or 5s states of Ge and Se atoms, respectively[l 11,as summalized in Table 1. With increasing x from 0 to 0.18, the UPS
g -
Ge,Sel_,
IPES hv = 9.37 eV
UPS hv=21.2eV
‘10
-5
0 5 Energy (eV)
10
15
Figure 2: A series of the valence-band UPS and conduction-band IPES spectra of g-Ge,Sel_x with x from 0 to 0.33. Intensities of the UPS and IPES spectra are tentatively normalized at -2.7eV, and 3.1 (x=0,0.10,0.15 and 0.18) and 2.4eV (x=0.20,0.25 and 0.33), respectively. Vertical bars indicate the positions of structures. Energies are referred to the Fermi level. and IPES spectra do not show any noticeable change with respect to their spectral shape and energy positions of structures. At x=0.2, however, one can recognize a slight blurring of the structures at -6.4 and -5.4 eV and increasing components around -6.0 and -4.3 eV for the UPS spectrum. A remarkable change is also observed for the shape of the IPES spectrum as well as for the number of peaks. For further increase of x, features of the UPS and IPES spectra reach those of g-GeSez. We find that the UPS spectra for x=0.20 and 0.33 in Fig.2 are in qualitative agreement with spectra obtained by X-ray photoemission spectroscopy (XPS)[12,13]. From modifications of the Ge 4s and Se 4s bands with x, these XPS studies emphasize that
the 4(Ge):2(Se) coordination is manifested [12,13] and chemically ordered structural units are favoured[ 121. The composition dependence of the UPS and IPES spectra in Fig.2 can be understood in terms of the Phillips-Thorpe rigidity percolation theory[ 1,2]. In g-Ge,Se,_, with ~~0.2, there exist Se chains and Ge(Sel,& tetrahedral units, where Ge atoms act as cross-links between Se chains. Near x=0.2, the rigid Ge(Se,,*), molecular units, which nucleate in the glass and gradually coalesce with the increase of x, may contact each other and percolate over the material. For x above 0.2, the Ge(Sel,&, units grow to form fragments of the two-dimensional form of
510
Table 1. Energy positions of peak structures in tbe UPS and IPES spectra of g-Se and g-GeSe2, and electronic states which contribute predominantly to the peak structures. Energy(eV) 8.4 4.9 g-GeSe2 2.4 1 -2.7 -4.3 -6.0 I 7.4 3.1 g-Se -2.7 -5.4 -6.4 1
Electronic states 4d and/or 5s states of Ge and Se atoms Ge sp3-Se 4p antibonding states
Japan, The Ogasawara Foundation for the Promotion of Science and Technology, The Murata Science Foundation, Shimazu Science Foundation, Nissan Science Foundation and Tokuyama Science Foundation. REFERENCES 1.
Se 4p lone pair states Ge sp3-Se 4p bonding states
2.
Se 4d and/or 5s states Se 4p antibonding states Se 4p lone pair states
3.
Se 4p bonding states
4.
GeSe2. Such a transition from one kind of network to another shows up clearly in the UPS and IPES spectra near x=0.2. One notices also that the present results correlate well with those of Raman scattering[4], Mijssbauer experiments[5,6] and molar volumes[7]. We believe that the present UPS and IPES studies of g-Ge,Sel_, provide an experimental support for the rigidity percolation threshold to take place near x=0.2, in much more direct form in comparison with those reported so far. The distinct change of the UPS and IPES spectra is reasonably assumed to reflect the threshold between the nucleation and subsequent growth of clusters just after the percolation. Finally, we point out that there is an interest to establish whether the threshold in the 4:2 coordinated networks takes place just at (r)=2.4 or at (r) greater than 2.4, depending on the presence of broken angularconstraints. For this kind of subject, further experimental works are required.
5. 6.
7. 8. 9.
10. 11.
ACKNOWLEDGMENTS 12. The authors are grateful to Dr. 0. Matsuda for fruitful discussions and to A. Minami for an electronprobe microanalysis, respectively. This work is partly supported by the Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture,
13.
J.C.Phillips, J. Non-Cryst. Solids 34 (1979) 153. M.F.Thorpe, J. Non-Cryst. Solids 57 (1983) 355; also see J.C.Phillips and M.F.Thorpe, Solid State Commun. 53 (1985) 699. P.Tronc, M.Bensoussan, A.Brenac, G.Errandonea, and C.Sebenne, J. Phys. 38 (1977) 1493. K.Murase, T.Fukunaga, K.Yakusiji, T.Yoshimi, and IYunoki, J. Non-Cryst. Solids 59&60 (1983) 883. M.Stevens, J.Grothaus, P.Boolchand, and J.G.Hernandez, Phys. Rev. B31 (1985) 981. W.Bresser, P.Boolchand, and P.Suranyi, Phys. Rev. Lett. 56 (1986) 2493; also see P.Boolchand, W.Bresser, M.Zhang, Y.Wu, J.Wells, and R.N.Enzweiler, J. Non-Cryst. Solids 182 (1995) 143. A.Feltz, H.Aust, and A.Bleyer, J. Non-Cryst. Solids 55 (1983) 179. J.J.Yeh, and I.Lindau, At Data Nucl. Tables 32 (1985) 52; 32 (1985) 54. K.Yokoyama, K.Nishihara, K.Mimura, Y .Hari, M.Taniguchi, Y.&da, and M.Fujisawa, Rev. Sci. Instrum. 64 (1993) 87; also see Y.Ueda, K.Nishihara, K.Mimura, Y .Hari, M.Taniguchi, and M.Fujisawa, Nucl. Instrum. Methods A330 (1993) 140. J.D.Joanopoulos, M.Schluter, and M.L.Cohen, Phys. Rev. Bll (1975) 2186. SHosokawa, K.Nishihara, Y.Hari, M.Taniguchi, O.Matsuda and K.Murase, Phys. Rev. B47 (1993) 15509. M.L.Theye, A.Gheorghiu, C.Senemaud, M.F.Kotkata, and K.M.Kandil, Phil. Mag. B69 (1994) 209. E.Bergignat, G.Hollinger, H.Chermette, P.Pertosa, DLohez, M.Larmoo, and M.Bensoussan, Phys. Rev. B37 (1988) 4506.