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A photoemission and inverse-photoemission study of trigonal Se
Journal of Electron Spectroscopy and Related Phenomena 79 (1996) 1-4
A P h o t o e m i s s i o n and I n v e r s e - P h o t o e m i s s i o n Study of T r i g o n a l Se I. Ono, P. C. Grekos, T. Kouchi, M. Nakatake, M. Tamura, S. Hosokawa, H. Namatame and M. Taniguchi Department of Materials Science, Faculty of Science, Hiroshima University, Kagamiyama 1-3, Higashi-Hiroshima 739, Japan
We present valence-band and conduction-band spectra of trigonal Se measured by means of photoemission and inverse-photoemission spectroscopies (UPS and IPES). From comparison between the theoretical density of states (DOS) and the experimental whole DOS spectrum, it has been shown that the coupling strength between the hybrid p-like orbitals on different atoms should be increased, while the interaction between the s orbitals decreased. In addition, from comparison between IPES and core-absorption spectra, the main peak in the 0-4 eV region and the structure above 4 eV in conduction band are assigned to the p-like antibonding and 4d and/or 5s states, respectively.
1. I N T R O D U C T I O N A trigonal Se consists of helical chains arranged in a hexagonal array oriented along the c axis and each atom is covalently bonded to two neighbors with a bonding angle of 105.5 °. The bonding between individual chains is much weaker and is often believed to be of the weak van der Waals type. Valence orbital configuration of Se is (4s) e (4p) 4. Two electrons per atom fill the deepest-lying s-like levels. The remaining four electrons fill the p orbitals: two of them form covalent bonds with electrons on the nearest neighbors, while the remaining two form lone-pair (LP) states. The density of states (DOS) of trigonal Se in the valenceband region has been investigated by means of ultraviolet and X-ray photoemission spectroscopies [1-3]. The electronic band-structure calculations of trigonal Se have been performed by various approaches over the years. However, little comparison has been made between theory and experiment on the conduction-band DOS, because the direct experimental information on the unoccupied electronic states was fairly limited, so far. In this study, we have investigated valence-band 0368-2048/96/$15.00© 1996 ElsevierScienceB.V.All rights reserved PH S0368- 2048 (96) 0278%9
and conduction-band spectra of trigonal Se by means of photoemission and inverse-photoemission spectroscopies (UPS and IPES, respectively). The IPES spectra reveal, for the first time, the structural features of the conduction-band DOS. We discuss the relative energy positions of the experimental DOS peaks and compare these with the theoretical DOS for trigonal Se. We also compare the IPES spectrum with Se 2p and 3d core-absorption spectra to evaluate the contributions of the 4p, 4d and 5s states to conduction-band DOS.
2. EXPERIMENTAL UPS spectrometer used for the present experiment is composed of a He discharge lamp (He I 21.2 eV, He II 40.8 eV) and a double-stage cylindrical-mirror analyzer. The energy resolution was set to be 0.2 eV. IPES spectrometer [4] connected with the UPS spectrometer consists of an electron gun of Erdman-Zipf type and a bandpass photon detector centered at 9.43 eV. Overall energy resolution was 0.56 eV. Energy calibration of the UPS and IPES spectra [5] were experimentally made using the spectra for a fresh film of polycrystalline Au. The UPS and IPES
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ES
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,'%
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a
0
-15
0 Energy (eV)
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10
15
Figure I. Valence-band photoemission and conduction-band inverse-photoemission spectra (dotted lines) and the theoretical DOS (solid lines) calculated by Joannopoulos et al. using EPM [7] for trigonal Se. Energy is referred to the VBM. Vertical bars represent energy positions of structures. spectra have been in situ measured for the same sample-surface under 3x10 -9 and 8x10 -10 Torr, respectively, and connected at the Fermi level (EF). All measurements were carried out at room temperature. Because of high resistivity of bulk trigonal Se, IPES measurements suffer from an electrostatic charging effect. To overcome the difficulty, thin film of trigonal Se was specially prepared on Au substrate. The substrate was heated at 105 °C for 13 min to transform amorphous into trigonal form, after the evaporation of Se on the substrate at room temperature with a rate of 9 A/s until ~ 1 ktm in thickness. During the heating, re-evaporation of deposited Se atoms was observed. Deposition rate and thickness of film were measured by means of a quartz thickness monitor placed near the sample. The thin film of the trigonal phase was checked by X-ray diffraction, Raman s c a t t e r i n g and UPS measurements. The film prepared in the crystal growth chamber was in situ transferred into the UPS or IPES spectrometer.
3. R E S U L T S
AND D I S C U S S I O N
The dotted lines in Fig. 1 show the valenceband UPS and conduction-band IPES spectra of trigonal Se. Energy of the spectra is drawn with respect to the valence-band maximum (VBM), which is determined by extrapolating the leading edge of valence bands to the base line, after the connection of the UPS and IPES spectra at the EF. Threshold energy of the IPES spectrum evaluated by extrapolating the leading edge is in good agreement with the band-gap energy of 1.6 eV [6]. Excitation photon-energies for UPS spectra are 21.2 eV (He I) and 40.8 eV (He II). The He II spectrum is shown to demonstrate the structures in the energy region from -10 to -16 eV. The UPS spectra agree with earlier results [1, 2] with respect to their spectral shape. Solid curves in Fig. 1 exhibit the theoretical DOS of trigonal Se calculated by Joannopoulos et al. using empirical pseudopotential method (EPM) [7]. Energy of the theoretical curves is also referred to the VBM. The valence-band UPS spectra exhibit
structures at -1.0, -2.1, --4.4, -5.4, -12.0 and -15.2 eV, while the conduction-band IPES spectrum shows prominent structures at 3.4 eV, and weak structures at 7.6 and 8.4 eV. Calculated valence-bands in the energy regions from 0 to -2.5 eV, from -2.5 to -5.5 eV, and from -10 to -16 eV are derived from the LP, p-like bonding and s-like states, respectively. Peaks around -3.2 a n d - 4 . 5 eV have been related to interchain and intrachain bonding states, respectively [8]. The conduction-bands in the energy region from 2 to 5 eV are due to the p-like antibonding states. One can recognize that the theory describes well the features of valence-band spectra. In particular, with respect to the relative energy positions of DOS peaks, there appears an good agreement between the theory and experiment. In the present results, however, all peaks of the theoretical DOS are reproduced shallower in energy than those of the experimental one. Theoretical widths of the LP and p-like bonding bands are narrower than experimental one, while the width of 4s bands is wider than that one. Relative intensities of theoretical DOS peaks in the p-like bonding bands are also in good agreement with those in the UPS spectrum. Although the calculation has been performed with no direct information on the experimental DOS of conduction bands, the theoretical curve reproduces fairly well the features of conduction-band spectrum in energy position of the p-like antibonding bands. However, the amount of the p-like bondingantibonding splitting energy is smaller than that in the present experiments. From a comparison between experimental and theoretical results, we point out that the center-ofmass energy of the LP, p-like bonding and lower s bands in the theoretical DOS should be placed at energies deeper by about 1.0, 1.2 and 0.5 eV, respectively. Then, the bonding-antibonding splitting energy of the p-like bands should be also increased by about 1.2 eV. An increase of the coupling strength between the hybrid p-like orbitals on different atoms would increase the splitting energy, and provide, thus, an increase of widths of the p-like bonding and antibonding bands. The width of the s bands should be decreased by about 1.1 eV.
|
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Figure 2. The 2p3/2 core-absorption spectrum [9] (solid curve) and the IPES spectrum (dotted curve) of trigonal Se.
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Figure 3. The 3d core-absorption spectrum [10] (solid curve) and the IPES spectrum (dotted curves) of trigonal Se. The dotted curve (a+b) represents a spectrum constructed by a superposition of two experimental DOS curves (a) and (b) weighted in the ratio of 6:4 and shifted by 0.9 eV.
This means that the 4s band is more isolated than that in theoretical calculation. Next, we compare the IPES spectrum with the 2p3/2 and 3d core-absorption spectra of trigonal Se measured by Belin et al. [9] and Cardona et al. [10] in Figs. 2 and 3, respectively (by solid curves). The IPES spectrum shown by dotted curve in Fig. 2 is arranged as the threshold coincides with that of the core-absorption spectrum, and horizontal axis is referred to these thresholds. The 3d core-absorption
spectrum in Fig. 3 consists of 3d5/2 and 3d3t2 components. The threshold of the IPES spectrum (a) is adjusted to the 3d5/2 core-absorption threshold (56.2 eV), estimated from the sum of the band-gap energy (1.6 eV) and the Se 3d5/2 core-level energy with respect to the VBM (54.6 eV). Curve (a+b) represents a spectrum constructed by a superposition of two curves (a) and (b) weighted in the ratio of degeneracies of 3d5/2 and 3d3t2 states, 6:4, shifted by spin-orbit splitting energy of 0.9 eV. Horizontal axis is referred to the threshold of the IPES spectrum (a). The core-absorption spectra provide information on the conduction-band DOS including the selection rule of angular momentum on the optical transition. Therefore, we can derive the contributions of each orbital components to the conduction-band DOS, from comparison between the IPES and coreabsorption spectra. One notices that the shape of the Se 2p and 3d core-absorption spectra are quite different to each other due to the selection rule for the excitation. The Se 2p core spectrum in Fig. 2 exhibits a weak and broad peak in the energy region from 0 to 4 eV, and a structure with a steep threshold near 4 eV in agreement with the feature of IPES spectrum in the energy region from 4 to 10 eV. On the other hand, the Se 3d core spectrum in Fig. 3 exhibits doublet structures separated by about 0.9 eV in the energy region from -2 to 1 eV, and also a weak structure above 5 eV. The prominent structure at lower energy region would be due to the Se 3d core exciton originated from an interaction between the excited electron and core hole. The feature above 5 eV corresponds well to that in the IPES (a+b) spectrum. From such remarkable contrast between the Se p and d core-absorption spectra, we find that the prominent structure in 0-4 eV region in the IPES spectrum consists mainly of the 4p orbitals, and weak structures above 4 eV region is mainly made up of the 4d and/or 5s states. ACKNOWLEDGEMENTS The authors are grateful to Dr.
Osamu
MATSUDA of Osaka University for the Raman Scattering experiments and aid in measurements. We also thank to Ms. Chizuru UEDA and Mr. Takashi OHBA for their assistance in measurements. One of us (I. O.) was supported by Fellowships of the Japan Society for the Promotion of Science for Japanese Junior Scientists, and P. C. G. would like to express his sincere acknowledgements for the financial support from the Japanese-German Center in Berlin [JGCB] and the Japanese Ministry of Foreign Affairs. This work is partly supported by the Grant-in-Aid for Science Research from the Ministry of Education, Science and Culture, Japan, Iketani Science and Technology Foundation, The Ogasawara Foundation for the Promotion of Science and Technology, The Murata Science Foundation and Shimazu Science Foundation, Tokuyama Science Foundation.
REFERENCES 1.
N . J . Shevchik, M. Cardona and J. Tejeda, Phys. Rev., B 8 (1973) 2833. 2. N.J. Shevchik, J. Tejeda, M. Cardona and D. W. Langer, Solid State Commun., 12 (1973) 1285 3. T. Takahashi, K. Murano, K. Nagata and Y. Miyamoto, Phys. Rev., B 28 (1983) 4893. 4. K. Yokoyama, K. Nishihara, K. Mimura, Y. Hari, M. Taniguchi, Y. Ueda and M. Fujisawa, Rev. Sci. Instrum., 64 (1993) 87. 5. Y. Ueda, K. Nishihara, K. Mimura, Y. Hari, M. Taniguchi and M. Fujisawa, Nucl. Instrum. Methods, A 330 (1993) 140. 6. J. Stuke, J. Nos-Cryst. Sokids, 4 (1970) 1. 7. J. D. Joannopoulos, M. Scliiter and M. L. Cohen, Phys. Rev., B 11 (1975) 2186. 8. M. SchliJter, J. D. Joannopoulos and M. L. Cohen, Phys. Rev. Lett., 33 (1974) 89. 9. E. Belin, C. Senemaud and C. Bonnelle, J. Non-Cryst. Solids, 27 (1978) 119. 10. M. Cardona, W. Gudat, B. Sonntag and P. Yu, Proc. of the 10th Int. Conf. on the Physics of semiconductors, 1970.
A P h o t o e m i s s i o n and I n v e r s e - P h o t o e m i s s i o n Study of T r i g o n a l Se I. Ono, P. C. Grekos, T. Kouchi, M. Nakatake, M. Tamura, S. Hosokawa, H. Namatame and M. Taniguchi Department of Materials Science, Faculty of Science, Hiroshima University, Kagamiyama 1-3, Higashi-Hiroshima 739, Japan
We present valence-band and conduction-band spectra of trigonal Se measured by means of photoemission and inverse-photoemission spectroscopies (UPS and IPES). From comparison between the theoretical density of states (DOS) and the experimental whole DOS spectrum, it has been shown that the coupling strength between the hybrid p-like orbitals on different atoms should be increased, while the interaction between the s orbitals decreased. In addition, from comparison between IPES and core-absorption spectra, the main peak in the 0-4 eV region and the structure above 4 eV in conduction band are assigned to the p-like antibonding and 4d and/or 5s states, respectively.
1. I N T R O D U C T I O N A trigonal Se consists of helical chains arranged in a hexagonal array oriented along the c axis and each atom is covalently bonded to two neighbors with a bonding angle of 105.5 °. The bonding between individual chains is much weaker and is often believed to be of the weak van der Waals type. Valence orbital configuration of Se is (4s) e (4p) 4. Two electrons per atom fill the deepest-lying s-like levels. The remaining four electrons fill the p orbitals: two of them form covalent bonds with electrons on the nearest neighbors, while the remaining two form lone-pair (LP) states. The density of states (DOS) of trigonal Se in the valenceband region has been investigated by means of ultraviolet and X-ray photoemission spectroscopies [1-3]. The electronic band-structure calculations of trigonal Se have been performed by various approaches over the years. However, little comparison has been made between theory and experiment on the conduction-band DOS, because the direct experimental information on the unoccupied electronic states was fairly limited, so far. In this study, we have investigated valence-band 0368-2048/96/$15.00© 1996 ElsevierScienceB.V.All rights reserved PH S0368- 2048 (96) 0278%9
and conduction-band spectra of trigonal Se by means of photoemission and inverse-photoemission spectroscopies (UPS and IPES, respectively). The IPES spectra reveal, for the first time, the structural features of the conduction-band DOS. We discuss the relative energy positions of the experimental DOS peaks and compare these with the theoretical DOS for trigonal Se. We also compare the IPES spectrum with Se 2p and 3d core-absorption spectra to evaluate the contributions of the 4p, 4d and 5s states to conduction-band DOS.
2. EXPERIMENTAL UPS spectrometer used for the present experiment is composed of a He discharge lamp (He I 21.2 eV, He II 40.8 eV) and a double-stage cylindrical-mirror analyzer. The energy resolution was set to be 0.2 eV. IPES spectrometer [4] connected with the UPS spectrometer consists of an electron gun of Erdman-Zipf type and a bandpass photon detector centered at 9.43 eV. Overall energy resolution was 0.56 eV. Energy calibration of the UPS and IPES spectra [5] were experimentally made using the spectra for a fresh film of polycrystalline Au. The UPS and IPES
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1.0
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I
UPS (He I)
t~
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ES
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,,.
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:
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,,,
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0
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,'%
-10
-5
..-== O . n
-->'.6 0.5 t-- ....~ Q
a
0
-15
0 Energy (eV)
5
10
15
Figure I. Valence-band photoemission and conduction-band inverse-photoemission spectra (dotted lines) and the theoretical DOS (solid lines) calculated by Joannopoulos et al. using EPM [7] for trigonal Se. Energy is referred to the VBM. Vertical bars represent energy positions of structures. spectra have been in situ measured for the same sample-surface under 3x10 -9 and 8x10 -10 Torr, respectively, and connected at the Fermi level (EF). All measurements were carried out at room temperature. Because of high resistivity of bulk trigonal Se, IPES measurements suffer from an electrostatic charging effect. To overcome the difficulty, thin film of trigonal Se was specially prepared on Au substrate. The substrate was heated at 105 °C for 13 min to transform amorphous into trigonal form, after the evaporation of Se on the substrate at room temperature with a rate of 9 A/s until ~ 1 ktm in thickness. During the heating, re-evaporation of deposited Se atoms was observed. Deposition rate and thickness of film were measured by means of a quartz thickness monitor placed near the sample. The thin film of the trigonal phase was checked by X-ray diffraction, Raman s c a t t e r i n g and UPS measurements. The film prepared in the crystal growth chamber was in situ transferred into the UPS or IPES spectrometer.
3. R E S U L T S
AND D I S C U S S I O N
The dotted lines in Fig. 1 show the valenceband UPS and conduction-band IPES spectra of trigonal Se. Energy of the spectra is drawn with respect to the valence-band maximum (VBM), which is determined by extrapolating the leading edge of valence bands to the base line, after the connection of the UPS and IPES spectra at the EF. Threshold energy of the IPES spectrum evaluated by extrapolating the leading edge is in good agreement with the band-gap energy of 1.6 eV [6]. Excitation photon-energies for UPS spectra are 21.2 eV (He I) and 40.8 eV (He II). The He II spectrum is shown to demonstrate the structures in the energy region from -10 to -16 eV. The UPS spectra agree with earlier results [1, 2] with respect to their spectral shape. Solid curves in Fig. 1 exhibit the theoretical DOS of trigonal Se calculated by Joannopoulos et al. using empirical pseudopotential method (EPM) [7]. Energy of the theoretical curves is also referred to the VBM. The valence-band UPS spectra exhibit
structures at -1.0, -2.1, --4.4, -5.4, -12.0 and -15.2 eV, while the conduction-band IPES spectrum shows prominent structures at 3.4 eV, and weak structures at 7.6 and 8.4 eV. Calculated valence-bands in the energy regions from 0 to -2.5 eV, from -2.5 to -5.5 eV, and from -10 to -16 eV are derived from the LP, p-like bonding and s-like states, respectively. Peaks around -3.2 a n d - 4 . 5 eV have been related to interchain and intrachain bonding states, respectively [8]. The conduction-bands in the energy region from 2 to 5 eV are due to the p-like antibonding states. One can recognize that the theory describes well the features of valence-band spectra. In particular, with respect to the relative energy positions of DOS peaks, there appears an good agreement between the theory and experiment. In the present results, however, all peaks of the theoretical DOS are reproduced shallower in energy than those of the experimental one. Theoretical widths of the LP and p-like bonding bands are narrower than experimental one, while the width of 4s bands is wider than that one. Relative intensities of theoretical DOS peaks in the p-like bonding bands are also in good agreement with those in the UPS spectrum. Although the calculation has been performed with no direct information on the experimental DOS of conduction bands, the theoretical curve reproduces fairly well the features of conduction-band spectrum in energy position of the p-like antibonding bands. However, the amount of the p-like bondingantibonding splitting energy is smaller than that in the present experiments. From a comparison between experimental and theoretical results, we point out that the center-ofmass energy of the LP, p-like bonding and lower s bands in the theoretical DOS should be placed at energies deeper by about 1.0, 1.2 and 0.5 eV, respectively. Then, the bonding-antibonding splitting energy of the p-like bands should be also increased by about 1.2 eV. An increase of the coupling strength between the hybrid p-like orbitals on different atoms would increase the splitting energy, and provide, thus, an increase of widths of the p-like bonding and antibonding bands. The width of the s bands should be decreased by about 1.1 eV.
|
|
|
1.0
|
i
f ~ IPES
"
•
•~
:
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• "~
~,
,
:
.
:¢t / 2p3/2 . .. / core absorption
~¢
0
1'0 Energy (eV)
Figure 2. The 2p3/2 core-absorption spectrum [9] (solid curve) and the IPES spectrum (dotted curve) of trigonal Se.
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sorption
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' 15
Figure 3. The 3d core-absorption spectrum [10] (solid curve) and the IPES spectrum (dotted curves) of trigonal Se. The dotted curve (a+b) represents a spectrum constructed by a superposition of two experimental DOS curves (a) and (b) weighted in the ratio of 6:4 and shifted by 0.9 eV.
This means that the 4s band is more isolated than that in theoretical calculation. Next, we compare the IPES spectrum with the 2p3/2 and 3d core-absorption spectra of trigonal Se measured by Belin et al. [9] and Cardona et al. [10] in Figs. 2 and 3, respectively (by solid curves). The IPES spectrum shown by dotted curve in Fig. 2 is arranged as the threshold coincides with that of the core-absorption spectrum, and horizontal axis is referred to these thresholds. The 3d core-absorption
spectrum in Fig. 3 consists of 3d5/2 and 3d3t2 components. The threshold of the IPES spectrum (a) is adjusted to the 3d5/2 core-absorption threshold (56.2 eV), estimated from the sum of the band-gap energy (1.6 eV) and the Se 3d5/2 core-level energy with respect to the VBM (54.6 eV). Curve (a+b) represents a spectrum constructed by a superposition of two curves (a) and (b) weighted in the ratio of degeneracies of 3d5/2 and 3d3t2 states, 6:4, shifted by spin-orbit splitting energy of 0.9 eV. Horizontal axis is referred to the threshold of the IPES spectrum (a). The core-absorption spectra provide information on the conduction-band DOS including the selection rule of angular momentum on the optical transition. Therefore, we can derive the contributions of each orbital components to the conduction-band DOS, from comparison between the IPES and coreabsorption spectra. One notices that the shape of the Se 2p and 3d core-absorption spectra are quite different to each other due to the selection rule for the excitation. The Se 2p core spectrum in Fig. 2 exhibits a weak and broad peak in the energy region from 0 to 4 eV, and a structure with a steep threshold near 4 eV in agreement with the feature of IPES spectrum in the energy region from 4 to 10 eV. On the other hand, the Se 3d core spectrum in Fig. 3 exhibits doublet structures separated by about 0.9 eV in the energy region from -2 to 1 eV, and also a weak structure above 5 eV. The prominent structure at lower energy region would be due to the Se 3d core exciton originated from an interaction between the excited electron and core hole. The feature above 5 eV corresponds well to that in the IPES (a+b) spectrum. From such remarkable contrast between the Se p and d core-absorption spectra, we find that the prominent structure in 0-4 eV region in the IPES spectrum consists mainly of the 4p orbitals, and weak structures above 4 eV region is mainly made up of the 4d and/or 5s states. ACKNOWLEDGEMENTS The authors are grateful to Dr.
Osamu
MATSUDA of Osaka University for the Raman Scattering experiments and aid in measurements. We also thank to Ms. Chizuru UEDA and Mr. Takashi OHBA for their assistance in measurements. One of us (I. O.) was supported by Fellowships of the Japan Society for the Promotion of Science for Japanese Junior Scientists, and P. C. G. would like to express his sincere acknowledgements for the financial support from the Japanese-German Center in Berlin [JGCB] and the Japanese Ministry of Foreign Affairs. This work is partly supported by the Grant-in-Aid for Science Research from the Ministry of Education, Science and Culture, Japan, Iketani Science and Technology Foundation, The Ogasawara Foundation for the Promotion of Science and Technology, The Murata Science Foundation and Shimazu Science Foundation, Tokuyama Science Foundation.
REFERENCES 1.
N . J . Shevchik, M. Cardona and J. Tejeda, Phys. Rev., B 8 (1973) 2833. 2. N.J. Shevchik, J. Tejeda, M. Cardona and D. W. Langer, Solid State Commun., 12 (1973) 1285 3. T. Takahashi, K. Murano, K. Nagata and Y. Miyamoto, Phys. Rev., B 28 (1983) 4893. 4. K. Yokoyama, K. Nishihara, K. Mimura, Y. Hari, M. Taniguchi, Y. Ueda and M. Fujisawa, Rev. Sci. Instrum., 64 (1993) 87. 5. Y. Ueda, K. Nishihara, K. Mimura, Y. Hari, M. Taniguchi and M. Fujisawa, Nucl. Instrum. Methods, A 330 (1993) 140. 6. J. Stuke, J. Nos-Cryst. Sokids, 4 (1970) 1. 7. J. D. Joannopoulos, M. Scliiter and M. L. Cohen, Phys. Rev., B 11 (1975) 2186. 8. M. SchliJter, J. D. Joannopoulos and M. L. Cohen, Phys. Rev. Lett., 33 (1974) 89. 9. E. Belin, C. Senemaud and C. Bonnelle, J. Non-Cryst. Solids, 27 (1978) 119. 10. M. Cardona, W. Gudat, B. Sonntag and P. Yu, Proc. of the 10th Int. Conf. on the Physics of semiconductors, 1970.