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Bremsstrahlung isochromat studies of conduction band states in GaSe
~i.~ Solid State Communications, Vol.65,No.l, pp.ll-13, 1988. ,,_~ Printed in Great Britain.
0038-1098/88 $3.00 + .00 Pergamon Journals Ltd.
BREMSSTRAHLUNG ISOCHROMAT STUDIES OF CONDUCTION BAND STATES IN GaSe Y. Gao, B. Smandek, M. Nikaido, and J.H. Weaver Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455 F. Ldvy Laboratoire de Physique Appliqude, Ecole Polytechnique Fdddrale, Lausanne, Switzerland and G. Margaritondo Department of Physics and Synchrotron Radiation Center, University of Wisconsin, Madison, WI 53706
(Received
12 June 1987 by E. Tosatti)
Using bremsstrahlung isochromat spectroscopy, we have directly studied of the conduction band density of states of the prototypical layer compound GaSe. We find two main features in the lower conduction band density of states, in qualitative agreement with core-level ,effectivity data. However, there is a large discrepancy between the energy positions of the optical and BIS peaks. This discrepancy is discussed in terms of excitonic effects and of self-energy corrections.
Gallium selenide is a prototypical non-transition-metal
For these measurements, the electron beam size was 15 x 5
layer compound. As such, it has been studied extensively
ram, and the typical electron current to the sample of 150
with a variety of experimental probes and theoretical
ttA. A fraction of those electrons of energy E i injected into high-lying states of the solid decay radiatively and emitted
techniques[l]. In particular, it has been used as a test case for band structure mapping of the occupied electronic states with angle resolved photoemission[2] and it has provided a
photons of energy hv = E i - El. With our monochromator, we collected only those photons with hv = 1486.6 eV. To
test for calculations of these electronic states[3-8]. There is a detailed picture of the valence band states and a reasonable amount of information on upper conduction band
obtain these spectra, we measured the photon yield as a function of incident electron energy for 1482 < E i < 1512 eV where E i is referenced to the Fermi level of the sample. The peaks in the BIS spectra reflect features in the density
states[l,9,10]. In contrast, the information on the conduction band states near the conduction band minimum, Ec, is limited.
of unoccupied states, modulated by transition probabilities and by many-body effects. The overall resolution (electrons
In this paper, we present bremsstrahlung isochromat spectroscopy (BIS) results which add new insight into the electronic structure of this fundamental material[11]. We
plus photons) was 0.8 eV as determined by the width of the leading edge of the BIS spectrum of Au (10-90% points). More details of the spectrometer will be published elsewhere[ 14]. Clean surfaces were prepared by cleaving in the measurement chamber (base pressure -5 x 10"11 Torr)
have detected two sharp peaks in the region 0-5 eV above Ec at 1.9 and 3.9 eV (3.8 and 5.8 eV relative to El=). These features are explained qualitatively by several theoretical
and the measurements were performed at 2 x 10"1° Tort
approaches[3-6], but none of the calculations is able to
x-ray photoemission. The incident electron beam was
because of electron stimulated desorption[14]. For these measurements, the count rate ranged from 3 sec-1 below the onset (dark current) to 20 sec -1 a few eV above E c. The typical data acquisition time per spectrum was -8 hours. The spectral features could be reproduced from cleave to cleave and exhibited no symptoms of contamination, consistent with the inert nature of the layer compound observed previously.
provided by a custom designed Pierce electron gun[14].
In Fig. l(b),(c) we show BIS spectra taken on two
correctly predict the positions in energy of the peaks. Likewise, previous estimates derived from optical data give peak positions with large deviations with respect to the above values [5,12,13]. The experiments were performed with a Vacuum Generators 0.5 m x-ray monochromator ordinarily used for
11
12
STUDIES OF CONDUCTION BAND STATES IN GaSe
Vol. 65
No.
In Fig. 2 we compare our BIS results to optical and
GQSe
%
theoretical estimates of the GaSe conduction band density of states. Curves (b) and (c) correspond to reflecdvity spectra
B,s i
which measured optical transitions from the Ga 3d spin-orbit-split core level to the conduction band states, reported by Mamy et aL[12] and by Thiry[13]. As can be seen, there is a qualitative agreement between BIS and optical curves, since they both exhibit two main peaks in the
,
first 5 eV above E c. However, it is also evident that the optical peaks are systematically shifted to lower energies. The discrepancy is approximately 1.2 eV for peak A and 0.8 eV for peak B. Such a discrepancy should be explained, at least in part, by core excitonic shifts affecting the reflectivity data. In fact, large core excitonic shifts were observed in other layered materials[17].
I
-5
I
I
I
!
I
0 5 I0 15 20 ENERGY RELATIVE TO E F (eV)
In this case, however, several
II
25 e
Fig. 1. Bremsstrahlung isochromat spectroscopy curves for (a) a gold sample and (b), (c) two different GaSe samples cleaved GaSe samples taken with hv = 1486.6 eV. Tic Fermi level is determined from the gold BIS spectra. different samples of cleaved GaSe. The position of the
i
I ,,, oO l--
= ,II;, ""
li I I I I i ~ I
ii Jl
d /',~
GaSe ~ I
j] i t
I
'
b
Fermi level, EF, was determined by measuring the BIS spectra of a gold sample in electrical contact with the GaSe
1
crystal (Fig. l(a)). On clean cleaved GaSe, El= is known to ~I/"\
be -0.2 eV above the valence band maximum, Ev[15], and
<
the gap is 2.05 eV. This gives E c - El= = 1.85 eV. Using a linear extrapolation of the leading spectral edge of the BIS spectra, we find E c - El= = 1.9 eV, in good agreement with the above value[15]. The sharpness of the features in Fig.
l(b),(c)
result of the quasi-molecular character of the energy levels states. The two peaks A and B are 3.8 and is a
5.8 eV above El=. The rising photon yield at higher energies is a consequence of inelastic scattering of incident electrons followed by radiative decay. These secondaries are the counterparts to secondaries of direct photoemission and have been discussed in detail by Dose[11]. The empty states of GaSe have been previously investigated experimentally with optical spectroscopy[5,12,13,16] and with synchrotron radiation photoemission spectroscopy in the constant-initial-state (CIS) mode[9]. Unfortunately, the CIS technique cannot explore states at energies between E c and the vacuum level.
-5
I
I
I
I
I
0
5
10
15
20
25
ENERGY RELATIVE TO EF (eV) Fig. 2. Comparison between GaSc BIS results (a) and those of optical and theoretical investigations. Curves (b) and (c) are sections of reflectivity curves for photon energies corresponding to Ga 3d ---> conduction band excitations from Rcfs, 12 and 13. Curves (d) and (e) arc calculated conduction band densities of states with pseudopotenfials3 and tight-binding methods[7].
Vol, 65, No.
1
STUDIES OF CONDUCTION BAND STATES IN GaSe
considerations argue against identifying the above discrepancies with excitonic shifts. Piacentini et al.[5], for
above discrepancy is a combination of core excitonic shifts for the reflectivity data, and of self-energy corrections for
example, found that the Ga 3d core excitonic shift for the GaSe edge position was smaller, 0.4 eV. Furthermore, both pseudopotential[3] and tight-binding[7] calculations agree in predicting the first density-of-states peak above E c in a position close to the first reflectivity peak. This point is evident by comparing the reflectivity data of Fig. 2, curves (b) and (c), and the theoretical results of Refs. 3 and 7, curves (d) and (e). The sharp peak immediately above E c is derived from the sp and Pz states of both Ga and Se, and originates from a group of nearly flat bands. The same peak is predicted by other theoretical approaches[5,6]. The closeness in energy of the first theoretical peak and of the fast*effectivity peak suggests that the latter is not affected by large core-excitonic shifts. Therefore, excitonic effects cannot entirely explain the large shift between peak A and its reflectivity counterpart. The situation is not equally clear for peak B, since there is no straightforward correspondence between optical and theoretical data. The magnitude of the discrepancy, however, again suggests contributions from factors other than ,effectivity. In principle, the BIS data could be affected by the symmetry dependence of the transition probability. Effects of this kind appear important in some cases[ 18]. However, they are not expected to play a major role at these large transition energies. The most likely explanation for the
the BIS energies. Self-energy terms appear to play a major role in other BIS experiments and cause upward shifts in energy of the BIS features whose magnitude is comparable
13
to the above discrepancies[ 18]. The qualitative explanation in terms of self-energy and excitonic effects does not remove all the questions about the observed shifts between optical and BIS peaks. In particular, self-energy corrections appear much different for different classes of layer compounds. In a recent experiment[19], we demonstrated that the discrepancies between reflectivity and BIS features for Sn 2 and SnSe 2 was primarily due to core excitonic shifts -- with little or no contribution from self- energy corrections. We find no simple explanation for the different magnitude of the self-energy corrections in GaSe and in the tin dichalcogenides. The solution of this open question will greatly contribute to our understanding of the BIS mechanisms and to the interpretation of the BIS data.
Acknowledgments -- This work was supported by the National Science Foundation through DMR-86-10837 (Minnesota) and DMR-84-21292 (Wisconsin). We gratefully acknowledge the critical efforts of M. Grioni, K. Riggs, and S. Krahn in the initial stages of setting up the BIS spectrometer. We are grateful to O. Bisi, C. Calandra, and F. Baidereschi for illuminating discussions.
REFERENCES 1.
For an extensive review of GaSe and other layer compounds, see V. Grasso, Electronic Structure and Electronic Transitions in Layered Materials, (Reidel, Dordrecht 1986). 2. P.K. Larsen, G. Margaritondo, J.E. Rowe, M. Schliiter, and N.V. Smith, Phys, Lett. 58A, 423 (1976). 3. M. Schltiter and M.L. Cohen, Phys. Rev. BI4, 430 (1976). 4. E. Doni, R. Girlanda, V. Grasso, A. Balzarotti, and M. Piacentini, Nuovo Cimento BS1, 254 (1979). 5. M. Piacentini, C.G. Olson, A. Balzarotti, R. Girlanda, V. Grasso, and E. Doni, Nuovo Cimento B54, 248 (1979). 6. S. Nagel, A. Baldereschi, and K. Maschke, J. Phys. C12, 1625 (1979). 7. J. Robertson, J. Phys. C12, 4777 (1979). 8. Y. Depeursinge, Nuovo Cimento B64, 111 (1981). 9. G. Margaritondo, J.E. Rowe, and S.B. Christman, Phys. Rev. BI5, 3844 (1977). 10. G. Leveque, J. Phys. C 10, 4645 (1977). 11. For a discussion of the BIS techniques, see V. Dose, Progr. Surf. Sci. 13, 225 (1983).
12. 13. 14.
15. 16. 17. 18. 19.
R. Mamy, L. Martin, G. Leveque, and C. Raisin, Phys. Star. Sol. (b) 62, 201 (1974). P. Thiry, thesis, University of Paris VI (1975) (unpublished). M. Grioni, Y. Gao, B. Smandek, J.H. Weaver, and T. Tyrie, to be discussed in detail elsewhere. The electron gun was manufactured by Kimball Physics, Inc. R.R. Daniels, G. Margaritondo, C. Quaresima, M. Capozi, P. Perfetti, and F. L6vy, Solid State Commun. 51, 495 (1984). F. Antonangeli, A.L. Apicella, A. Balzarotti, L. Incoccia, and M. Piacentini, Physica 10$B, 25 (1981). G. Margaritondo, J.E. Rowe, M. Schliiter, F. l.~vy, and E. Mooser, Phys. Rev. BI6, 2938 (1977). O. Bisi and C. Caiandra, private communication. Y. Gao, B. Smandek, T.J. Wagener, J.H. Weaver, F. L~vy, and G. Margaritondo, Phys. Rev.- Rapid Commun. B35, xxx (1987) (in press).
0038-1098/88 $3.00 + .00 Pergamon Journals Ltd.
BREMSSTRAHLUNG ISOCHROMAT STUDIES OF CONDUCTION BAND STATES IN GaSe Y. Gao, B. Smandek, M. Nikaido, and J.H. Weaver Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, MN 55455 F. Ldvy Laboratoire de Physique Appliqude, Ecole Polytechnique Fdddrale, Lausanne, Switzerland and G. Margaritondo Department of Physics and Synchrotron Radiation Center, University of Wisconsin, Madison, WI 53706
(Received
12 June 1987 by E. Tosatti)
Using bremsstrahlung isochromat spectroscopy, we have directly studied of the conduction band density of states of the prototypical layer compound GaSe. We find two main features in the lower conduction band density of states, in qualitative agreement with core-level ,effectivity data. However, there is a large discrepancy between the energy positions of the optical and BIS peaks. This discrepancy is discussed in terms of excitonic effects and of self-energy corrections.
Gallium selenide is a prototypical non-transition-metal
For these measurements, the electron beam size was 15 x 5
layer compound. As such, it has been studied extensively
ram, and the typical electron current to the sample of 150
with a variety of experimental probes and theoretical
ttA. A fraction of those electrons of energy E i injected into high-lying states of the solid decay radiatively and emitted
techniques[l]. In particular, it has been used as a test case for band structure mapping of the occupied electronic states with angle resolved photoemission[2] and it has provided a
photons of energy hv = E i - El. With our monochromator, we collected only those photons with hv = 1486.6 eV. To
test for calculations of these electronic states[3-8]. There is a detailed picture of the valence band states and a reasonable amount of information on upper conduction band
obtain these spectra, we measured the photon yield as a function of incident electron energy for 1482 < E i < 1512 eV where E i is referenced to the Fermi level of the sample. The peaks in the BIS spectra reflect features in the density
states[l,9,10]. In contrast, the information on the conduction band states near the conduction band minimum, Ec, is limited.
of unoccupied states, modulated by transition probabilities and by many-body effects. The overall resolution (electrons
In this paper, we present bremsstrahlung isochromat spectroscopy (BIS) results which add new insight into the electronic structure of this fundamental material[11]. We
plus photons) was 0.8 eV as determined by the width of the leading edge of the BIS spectrum of Au (10-90% points). More details of the spectrometer will be published elsewhere[ 14]. Clean surfaces were prepared by cleaving in the measurement chamber (base pressure -5 x 10"11 Torr)
have detected two sharp peaks in the region 0-5 eV above Ec at 1.9 and 3.9 eV (3.8 and 5.8 eV relative to El=). These features are explained qualitatively by several theoretical
and the measurements were performed at 2 x 10"1° Tort
approaches[3-6], but none of the calculations is able to
x-ray photoemission. The incident electron beam was
because of electron stimulated desorption[14]. For these measurements, the count rate ranged from 3 sec-1 below the onset (dark current) to 20 sec -1 a few eV above E c. The typical data acquisition time per spectrum was -8 hours. The spectral features could be reproduced from cleave to cleave and exhibited no symptoms of contamination, consistent with the inert nature of the layer compound observed previously.
provided by a custom designed Pierce electron gun[14].
In Fig. l(b),(c) we show BIS spectra taken on two
correctly predict the positions in energy of the peaks. Likewise, previous estimates derived from optical data give peak positions with large deviations with respect to the above values [5,12,13]. The experiments were performed with a Vacuum Generators 0.5 m x-ray monochromator ordinarily used for
11
12
STUDIES OF CONDUCTION BAND STATES IN GaSe
Vol. 65
No.
In Fig. 2 we compare our BIS results to optical and
GQSe
%
theoretical estimates of the GaSe conduction band density of states. Curves (b) and (c) correspond to reflecdvity spectra
B,s i
which measured optical transitions from the Ga 3d spin-orbit-split core level to the conduction band states, reported by Mamy et aL[12] and by Thiry[13]. As can be seen, there is a qualitative agreement between BIS and optical curves, since they both exhibit two main peaks in the
,
first 5 eV above E c. However, it is also evident that the optical peaks are systematically shifted to lower energies. The discrepancy is approximately 1.2 eV for peak A and 0.8 eV for peak B. Such a discrepancy should be explained, at least in part, by core excitonic shifts affecting the reflectivity data. In fact, large core excitonic shifts were observed in other layered materials[17].
I
-5
I
I
I
!
I
0 5 I0 15 20 ENERGY RELATIVE TO E F (eV)
In this case, however, several
II
25 e
Fig. 1. Bremsstrahlung isochromat spectroscopy curves for (a) a gold sample and (b), (c) two different GaSe samples cleaved GaSe samples taken with hv = 1486.6 eV. Tic Fermi level is determined from the gold BIS spectra. different samples of cleaved GaSe. The position of the
i
I ,,, oO l--
= ,II;, ""
li I I I I i ~ I
ii Jl
d /',~
GaSe ~ I
j] i t
I
'
b
Fermi level, EF, was determined by measuring the BIS spectra of a gold sample in electrical contact with the GaSe
1
crystal (Fig. l(a)). On clean cleaved GaSe, El= is known to ~I/"\
be -0.2 eV above the valence band maximum, Ev[15], and
<
the gap is 2.05 eV. This gives E c - El= = 1.85 eV. Using a linear extrapolation of the leading spectral edge of the BIS spectra, we find E c - El= = 1.9 eV, in good agreement with the above value[15]. The sharpness of the features in Fig.
l(b),(c)
result of the quasi-molecular character of the energy levels states. The two peaks A and B are 3.8 and is a
5.8 eV above El=. The rising photon yield at higher energies is a consequence of inelastic scattering of incident electrons followed by radiative decay. These secondaries are the counterparts to secondaries of direct photoemission and have been discussed in detail by Dose[11]. The empty states of GaSe have been previously investigated experimentally with optical spectroscopy[5,12,13,16] and with synchrotron radiation photoemission spectroscopy in the constant-initial-state (CIS) mode[9]. Unfortunately, the CIS technique cannot explore states at energies between E c and the vacuum level.
-5
I
I
I
I
I
0
5
10
15
20
25
ENERGY RELATIVE TO EF (eV) Fig. 2. Comparison between GaSc BIS results (a) and those of optical and theoretical investigations. Curves (b) and (c) are sections of reflectivity curves for photon energies corresponding to Ga 3d ---> conduction band excitations from Rcfs, 12 and 13. Curves (d) and (e) arc calculated conduction band densities of states with pseudopotenfials3 and tight-binding methods[7].
Vol, 65, No.
1
STUDIES OF CONDUCTION BAND STATES IN GaSe
considerations argue against identifying the above discrepancies with excitonic shifts. Piacentini et al.[5], for
above discrepancy is a combination of core excitonic shifts for the reflectivity data, and of self-energy corrections for
example, found that the Ga 3d core excitonic shift for the GaSe edge position was smaller, 0.4 eV. Furthermore, both pseudopotential[3] and tight-binding[7] calculations agree in predicting the first density-of-states peak above E c in a position close to the first reflectivity peak. This point is evident by comparing the reflectivity data of Fig. 2, curves (b) and (c), and the theoretical results of Refs. 3 and 7, curves (d) and (e). The sharp peak immediately above E c is derived from the sp and Pz states of both Ga and Se, and originates from a group of nearly flat bands. The same peak is predicted by other theoretical approaches[5,6]. The closeness in energy of the first theoretical peak and of the fast*effectivity peak suggests that the latter is not affected by large core-excitonic shifts. Therefore, excitonic effects cannot entirely explain the large shift between peak A and its reflectivity counterpart. The situation is not equally clear for peak B, since there is no straightforward correspondence between optical and theoretical data. The magnitude of the discrepancy, however, again suggests contributions from factors other than ,effectivity. In principle, the BIS data could be affected by the symmetry dependence of the transition probability. Effects of this kind appear important in some cases[ 18]. However, they are not expected to play a major role at these large transition energies. The most likely explanation for the
the BIS energies. Self-energy terms appear to play a major role in other BIS experiments and cause upward shifts in energy of the BIS features whose magnitude is comparable
13
to the above discrepancies[ 18]. The qualitative explanation in terms of self-energy and excitonic effects does not remove all the questions about the observed shifts between optical and BIS peaks. In particular, self-energy corrections appear much different for different classes of layer compounds. In a recent experiment[19], we demonstrated that the discrepancies between reflectivity and BIS features for Sn 2 and SnSe 2 was primarily due to core excitonic shifts -- with little or no contribution from self- energy corrections. We find no simple explanation for the different magnitude of the self-energy corrections in GaSe and in the tin dichalcogenides. The solution of this open question will greatly contribute to our understanding of the BIS mechanisms and to the interpretation of the BIS data.
Acknowledgments -- This work was supported by the National Science Foundation through DMR-86-10837 (Minnesota) and DMR-84-21292 (Wisconsin). We gratefully acknowledge the critical efforts of M. Grioni, K. Riggs, and S. Krahn in the initial stages of setting up the BIS spectrometer. We are grateful to O. Bisi, C. Calandra, and F. Baidereschi for illuminating discussions.
REFERENCES 1.
For an extensive review of GaSe and other layer compounds, see V. Grasso, Electronic Structure and Electronic Transitions in Layered Materials, (Reidel, Dordrecht 1986). 2. P.K. Larsen, G. Margaritondo, J.E. Rowe, M. Schliiter, and N.V. Smith, Phys, Lett. 58A, 423 (1976). 3. M. Schltiter and M.L. Cohen, Phys. Rev. BI4, 430 (1976). 4. E. Doni, R. Girlanda, V. Grasso, A. Balzarotti, and M. Piacentini, Nuovo Cimento BS1, 254 (1979). 5. M. Piacentini, C.G. Olson, A. Balzarotti, R. Girlanda, V. Grasso, and E. Doni, Nuovo Cimento B54, 248 (1979). 6. S. Nagel, A. Baldereschi, and K. Maschke, J. Phys. C12, 1625 (1979). 7. J. Robertson, J. Phys. C12, 4777 (1979). 8. Y. Depeursinge, Nuovo Cimento B64, 111 (1981). 9. G. Margaritondo, J.E. Rowe, and S.B. Christman, Phys. Rev. BI5, 3844 (1977). 10. G. Leveque, J. Phys. C 10, 4645 (1977). 11. For a discussion of the BIS techniques, see V. Dose, Progr. Surf. Sci. 13, 225 (1983).
12. 13. 14.
15. 16. 17. 18. 19.
R. Mamy, L. Martin, G. Leveque, and C. Raisin, Phys. Star. Sol. (b) 62, 201 (1974). P. Thiry, thesis, University of Paris VI (1975) (unpublished). M. Grioni, Y. Gao, B. Smandek, J.H. Weaver, and T. Tyrie, to be discussed in detail elsewhere. The electron gun was manufactured by Kimball Physics, Inc. R.R. Daniels, G. Margaritondo, C. Quaresima, M. Capozi, P. Perfetti, and F. L6vy, Solid State Commun. 51, 495 (1984). F. Antonangeli, A.L. Apicella, A. Balzarotti, L. Incoccia, and M. Piacentini, Physica 10$B, 25 (1981). G. Margaritondo, J.E. Rowe, M. Schliiter, F. l.~vy, and E. Mooser, Phys. Rev. BI6, 2938 (1977). O. Bisi and C. Caiandra, private communication. Y. Gao, B. Smandek, T.J. Wagener, J.H. Weaver, F. L~vy, and G. Margaritondo, Phys. Rev.- Rapid Commun. B35, xxx (1987) (in press).