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GaSe valence band structure from angle-resolved photoemission spectroscopy
Volume 58A, number 6
PHYSICS LETTERS
4 October 1976
GaSe VALENCE BAND STRUCFURE FROM ANGLE-RESOLVED PHOTOEMISSION SPECTROSCOPY P.K. LARSEN*, G. MARGARITONDO**, J.E. ROWE, M. SCHLUTER and N.y. SMITH Bell Laboratories, Murray Hill, New Jersey 07974, USA Received 2 August 1976 Energy-wavevector dispersion curves for the GaSe valence bands obtained from angle-resolved photoemission spectra are compared with pseudopotential band calculations. It is found that the third density-of-states feature below the top of the valence bands (peak C) is related to the Ga-Se bond rather than the Ga-Ga bond. A peak not appearing in the angle-integrated spectra appears as the most intense feature at normal emission, and helps to identify completely the nature of the valence bands down to 7—8 eV below the Fermi energy.
A direct relationship has recently been found in
A
various layer compounds [1—3] between experimental polar-angle resolved photoelectron spectra and calculated energy band dispersion curves. Although anticipated theoretically [4], this result is of some importance since it shows that a specular refraction approximation holds in spite of the complexity of the photoelectron emission process. We have extended this method to a layered semiconductor, GaSe, with the intention of resolving some ambiguities in the interpretation of recent angle-integrated photoemission measurements on this material [51.In particular we find that the third main density of states feature below the top of the valence bands (peak C) is related to the GaSe rather than to the Ga-Ga bond. Similar work has been reported by lloyd et al. [2]. The experiments were performed using synchrotron radiation emitted by the Tantalus I storage ring at the University of Wisconsin Physical Sciences Laboratory, monochromatized by a bakable vertical Seya-Namioka monochromator. The photon beam was incident at 450 with respect to the sample normal and the plane of incidence was horizontal (p-polarization). The photoelectron energy distribution was measured with a plane-mirror analyzer whose position could be varied so as to sample electron propagating in a vertical plane. The energy resolution was “‘0.3 eV and the angular -
*
**
Permanent address: Philips Research Laboratories,
Eindhoven, The Netherlands. Fellow of the Italian National Research Council — GNSM.
eo~cr
C B
E
c~
~ e~o°
A.B hw°l9eV I
12
I
84
I
I I
-~ ~
Fig. 1. Photoelectron spectra of GaSe taken at three different polar angles 0 = 0°(normal emission), 200, and 40°respectively. The electron collector movesin a plane intersecting the sample surface along the FM direction in the first Brillouin zone.
resolution “±2°. The data were taken on a freshly cleaved sample under ultrahigh vacuum conditions. Fig. 1 shows spectra taken at three different polar angles with the electron collector moving in a plane corresponding to the I’MLAI’ plane of the Brillouin zone. The labeling of observed structures is in accordance with previous angle-integrated photoemission measurements [5]. Fig. 2 shows the energy position of these features as a function of K0, the parallel wave423
Volume 58A, number 6
PHYSICS LETTERS
o
~ -2
assigned to be similar to peak B [8] or to be due to slike Ga orbitals (with some p~component) belonging
~
M -
~
-
~
•
to an antibonding state of the Ga-Ga bond [7, 9]. Both interpretations are compatible with band theory calculations which predict the Ga-Se bonding states somewhat above the Ga-Ga bonding and anti-bonding states of mostly s-character [6] (lowest two bands in fig.2). The present angular-resolved results (fig. 1) show that an intense feature (C’) is present in the C-peak
-
-
-
~
—
0
I
I
04
08
I
12 K,,(A~)
4 October 1976
I
1.6
Position of peaks A (x), B (.), C (+), C’ (o) and E (~) as a function of the wave vector component K11, along the FM Fig. 2.
direction. Pseudopotential bandstructure results arc shown for compailson. Each band corresponds to a dashed region rather than to a line because of non-zero interlayer interaction
(kj dispersion). The experimental points have been shifted by 0.9 eV to higher energy. 1/21r’ sinO vector component, as given by Ki1 = (2mE) (E photoelectron kinetic energy). The results of a pseudopotential band structure calculation [6] are su perimposed in fig. 2 for comparison, This calculation has been performed for a particular
region at normal emission, corresponding to the F point of the Brillouin zone. A detailed inspection of the dispersion relations in both theory and experiment in fig. 2 indicates that peak C’ (open circles) and peak C (crosses) correspond to two different bands. No band-crossing is expected to occur due to strong hybrid.
-
ization effects. The band corresponding to peak C gives rise to a peak in the calculated density of states at about —4 eV [8] . This band is fairly flat in this energy range andzone extends over a large of k-space close to the boundary. Peak volume C observed in angle-
integrated spectra [5, 7] can therefore be unambiguously assigned to this band which is based primarily on the p~,p~,orbitals of the Ga-Se bond, similar to peak B. Peak C’ occurs at lower energy for increasing
stacking sequence (fl-type) resulting in an inter-layer splitting of the bands given by the widths of the shaded regions. Since the experimental data have been obtained on c-type (mixed with 7-type structure)
Ku and is identified with the aforementioned Ga-Ga anti-bonding band. This band approaches its bonding partner (see fig. 2) close to the zone boundary, and the two bands give rise to the structures in the density
GaSe, some energy differences between theory and
of states which correspond to the structures labeled D and E in the angle-integrated spectra discussed in ref. [8] In conclusion we find that for GaSe the results of
experiment of the order of the aforementioned band widths (~O.5eV) are expected to occur for some features. Nevertheless the behavior of the upper two
bands of fig. 2 is in very good agreement with theory. The nature of peaks A and B as previously identified from angle-integrated spectra [5, 8] and photon polarization effects [7] is thus confirmed to be due to p~like orbitals in a Ga-Ga bonding state mixed with some nonbonding pr-like çhalcogen orbitals (i.e. peak A is due to n-like bonding) and to p~,p~,orbitals in the Ga-Se bond (i.e. peak B is due to a bonds). The main emission in the angle-integrated spectra results from regions in k-space close to the zone boundaries and a bond-like interpretation is possible for experimental peaks which are due to an average over energy bands of a well-defined symmetry. The nature of peak C at about—4 eV is more controversial. On the basis of angle-integrated spectra its origin has either been 424
angle-resolved photoemission spectra with varying polar angle are in good agreement with pseudopotential band structure calculations. This allows one to settle a residual controversy of the identification of the valence band symmetry in GaSe. This combination of pseudopotential theory and angle-resolved photoemission spectroscopy appears to be quite powerful and should prove useful for other compounds. We are grateful to M.M. Traum for helpful discussions and to E.M. Rowe and all the Synchrotron Radiation Center Staff for hospitality and assistance. The Synchrotron Radiation Center was supported by the National Science Foundation under Grant No. DMR-74-1 5089.
Volume 58A, number 6
PHYSICS LETTERS
4 October 1976
References
[4] E.O. Kane, Phys. Rev. Lett. 12 (1964) 97; H.P. Hughes and W.J. Liang, J. Phys. C6 (1973) 1684.
[11 N.y. Smith, M.M. Traum and F.J. DiSalvo, Solid State Commun. 15 (1974) 211; N.y. Smith and M.M. Traum, Phys. Rev. Bli (1975)
[5] R.H. Williams et al., J. Phys. C7 (1974) L29; F.R. Shepherd and P.M. Williams, Phys. Rev. B12 (1975) 5705;
2087. [2] D.R. Lloyd, C.M. Quinn, N.y. Richardson and P.M. Williams, Communications onPhysics 1(1976) 11.We are grateful to P.M. Williams for private communication of further details of this work. [3] R.Z. Bachrach, M. Skibowski and F.C. Brown, Phys. Rev. Lett. 37 (1976) 40.
S.P. Kowalczyk et al., Solid State Commun. 17 (1975)
463. [6] M. Schiuter, Nuovo Cimento B13 (1973) 313; M. Schluter et al., Phys. Rev. BI 3 (1976) 3534. [7] J.E. Rowe, G. Margaritondo, H. Kasper and A. Baldereschi, to be published. [8] M. Schluter and M.L. Cohen, Phys. Rev. B, in press. [9] A. Baldereschi, K. Maschke and M. Schiuter, Helvetica Phys. Acta 47 (1974) 434.
425
PHYSICS LETTERS
4 October 1976
GaSe VALENCE BAND STRUCFURE FROM ANGLE-RESOLVED PHOTOEMISSION SPECTROSCOPY P.K. LARSEN*, G. MARGARITONDO**, J.E. ROWE, M. SCHLUTER and N.y. SMITH Bell Laboratories, Murray Hill, New Jersey 07974, USA Received 2 August 1976 Energy-wavevector dispersion curves for the GaSe valence bands obtained from angle-resolved photoemission spectra are compared with pseudopotential band calculations. It is found that the third density-of-states feature below the top of the valence bands (peak C) is related to the Ga-Se bond rather than the Ga-Ga bond. A peak not appearing in the angle-integrated spectra appears as the most intense feature at normal emission, and helps to identify completely the nature of the valence bands down to 7—8 eV below the Fermi energy.
A direct relationship has recently been found in
A
various layer compounds [1—3] between experimental polar-angle resolved photoelectron spectra and calculated energy band dispersion curves. Although anticipated theoretically [4], this result is of some importance since it shows that a specular refraction approximation holds in spite of the complexity of the photoelectron emission process. We have extended this method to a layered semiconductor, GaSe, with the intention of resolving some ambiguities in the interpretation of recent angle-integrated photoemission measurements on this material [51.In particular we find that the third main density of states feature below the top of the valence bands (peak C) is related to the GaSe rather than to the Ga-Ga bond. Similar work has been reported by lloyd et al. [2]. The experiments were performed using synchrotron radiation emitted by the Tantalus I storage ring at the University of Wisconsin Physical Sciences Laboratory, monochromatized by a bakable vertical Seya-Namioka monochromator. The photon beam was incident at 450 with respect to the sample normal and the plane of incidence was horizontal (p-polarization). The photoelectron energy distribution was measured with a plane-mirror analyzer whose position could be varied so as to sample electron propagating in a vertical plane. The energy resolution was “‘0.3 eV and the angular -
*
**
Permanent address: Philips Research Laboratories,
Eindhoven, The Netherlands. Fellow of the Italian National Research Council — GNSM.
eo~cr
C B
E
c~
~ e~o°
A.B hw°l9eV I
12
I
84
I
I I
-~ ~
Fig. 1. Photoelectron spectra of GaSe taken at three different polar angles 0 = 0°(normal emission), 200, and 40°respectively. The electron collector movesin a plane intersecting the sample surface along the FM direction in the first Brillouin zone.
resolution “±2°. The data were taken on a freshly cleaved sample under ultrahigh vacuum conditions. Fig. 1 shows spectra taken at three different polar angles with the electron collector moving in a plane corresponding to the I’MLAI’ plane of the Brillouin zone. The labeling of observed structures is in accordance with previous angle-integrated photoemission measurements [5]. Fig. 2 shows the energy position of these features as a function of K0, the parallel wave423
Volume 58A, number 6
PHYSICS LETTERS
o
~ -2
assigned to be similar to peak B [8] or to be due to slike Ga orbitals (with some p~component) belonging
~
M -
~
-
~
•
to an antibonding state of the Ga-Ga bond [7, 9]. Both interpretations are compatible with band theory calculations which predict the Ga-Se bonding states somewhat above the Ga-Ga bonding and anti-bonding states of mostly s-character [6] (lowest two bands in fig.2). The present angular-resolved results (fig. 1) show that an intense feature (C’) is present in the C-peak
-
-
-
~
—
0
I
I
04
08
I
12 K,,(A~)
4 October 1976
I
1.6
Position of peaks A (x), B (.), C (+), C’ (o) and E (~) as a function of the wave vector component K11, along the FM Fig. 2.
direction. Pseudopotential bandstructure results arc shown for compailson. Each band corresponds to a dashed region rather than to a line because of non-zero interlayer interaction
(kj dispersion). The experimental points have been shifted by 0.9 eV to higher energy. 1/21r’ sinO vector component, as given by Ki1 = (2mE) (E photoelectron kinetic energy). The results of a pseudopotential band structure calculation [6] are su perimposed in fig. 2 for comparison, This calculation has been performed for a particular
region at normal emission, corresponding to the F point of the Brillouin zone. A detailed inspection of the dispersion relations in both theory and experiment in fig. 2 indicates that peak C’ (open circles) and peak C (crosses) correspond to two different bands. No band-crossing is expected to occur due to strong hybrid.
-
ization effects. The band corresponding to peak C gives rise to a peak in the calculated density of states at about —4 eV [8] . This band is fairly flat in this energy range andzone extends over a large of k-space close to the boundary. Peak volume C observed in angle-
integrated spectra [5, 7] can therefore be unambiguously assigned to this band which is based primarily on the p~,p~,orbitals of the Ga-Se bond, similar to peak B. Peak C’ occurs at lower energy for increasing
stacking sequence (fl-type) resulting in an inter-layer splitting of the bands given by the widths of the shaded regions. Since the experimental data have been obtained on c-type (mixed with 7-type structure)
Ku and is identified with the aforementioned Ga-Ga anti-bonding band. This band approaches its bonding partner (see fig. 2) close to the zone boundary, and the two bands give rise to the structures in the density
GaSe, some energy differences between theory and
of states which correspond to the structures labeled D and E in the angle-integrated spectra discussed in ref. [8] In conclusion we find that for GaSe the results of
experiment of the order of the aforementioned band widths (~O.5eV) are expected to occur for some features. Nevertheless the behavior of the upper two
bands of fig. 2 is in very good agreement with theory. The nature of peaks A and B as previously identified from angle-integrated spectra [5, 8] and photon polarization effects [7] is thus confirmed to be due to p~like orbitals in a Ga-Ga bonding state mixed with some nonbonding pr-like çhalcogen orbitals (i.e. peak A is due to n-like bonding) and to p~,p~,orbitals in the Ga-Se bond (i.e. peak B is due to a bonds). The main emission in the angle-integrated spectra results from regions in k-space close to the zone boundaries and a bond-like interpretation is possible for experimental peaks which are due to an average over energy bands of a well-defined symmetry. The nature of peak C at about—4 eV is more controversial. On the basis of angle-integrated spectra its origin has either been 424
angle-resolved photoemission spectra with varying polar angle are in good agreement with pseudopotential band structure calculations. This allows one to settle a residual controversy of the identification of the valence band symmetry in GaSe. This combination of pseudopotential theory and angle-resolved photoemission spectroscopy appears to be quite powerful and should prove useful for other compounds. We are grateful to M.M. Traum for helpful discussions and to E.M. Rowe and all the Synchrotron Radiation Center Staff for hospitality and assistance. The Synchrotron Radiation Center was supported by the National Science Foundation under Grant No. DMR-74-1 5089.
Volume 58A, number 6
PHYSICS LETTERS
4 October 1976
References
[4] E.O. Kane, Phys. Rev. Lett. 12 (1964) 97; H.P. Hughes and W.J. Liang, J. Phys. C6 (1973) 1684.
[11 N.y. Smith, M.M. Traum and F.J. DiSalvo, Solid State Commun. 15 (1974) 211; N.y. Smith and M.M. Traum, Phys. Rev. Bli (1975)
[5] R.H. Williams et al., J. Phys. C7 (1974) L29; F.R. Shepherd and P.M. Williams, Phys. Rev. B12 (1975) 5705;
2087. [2] D.R. Lloyd, C.M. Quinn, N.y. Richardson and P.M. Williams, Communications onPhysics 1(1976) 11.We are grateful to P.M. Williams for private communication of further details of this work. [3] R.Z. Bachrach, M. Skibowski and F.C. Brown, Phys. Rev. Lett. 37 (1976) 40.
S.P. Kowalczyk et al., Solid State Commun. 17 (1975)
463. [6] M. Schiuter, Nuovo Cimento B13 (1973) 313; M. Schluter et al., Phys. Rev. BI 3 (1976) 3534. [7] J.E. Rowe, G. Margaritondo, H. Kasper and A. Baldereschi, to be published. [8] M. Schluter and M.L. Cohen, Phys. Rev. B, in press. [9] A. Baldereschi, K. Maschke and M. Schiuter, Helvetica Phys. Acta 47 (1974) 434.
425