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Photoelectric comparison of electron states of amorphous and crystalline antimony
PHYSICS LETTERS
Volume 48A, number 1
PHOTOELECTRIC
COMPARISON
OF ELECTRON
AND CRYSTALLINE I. ABBATI*, L. BRAICOVICH*,
20 May 1974
STATES
OF AMORPHOUS
ANTIMONY
B. De MICHELIS* and A. FASANA
Institute of Physics of the Politecnico, Milan, Italy
Received 29 March 1974 The photoelectron energy distributions of amorphous Sb (aSb) with 6.22 d hv Q 9.22 eV are given and compared with those of crystalline Sb. The barycenter of p-like valence states of aSb is,95-110 meV upward shifted; no final state effects are seen in aSb.
In this letter we give ultraviolet photoelectron spectroscopy results on amorphous Sb (aSb), whose energy distribution curves (EDCs) were not known till now [ 11. The results are discussed briefly in connection with those [2] on crystalline Sb (cSb) and with X-ray photoelectron spectroscopy (XPS) results of ref. [3] (ref. [3] is supposed to be known to the reader). Amorphous Sb was prepared by evaporation as in [l] ; the pressure during the measurements was 5 X lo- l1 torr and in the lo- lo torr range during the evaporation. The EDCs with 6.22 Q hv < 9.22 eV are collected [4] in fig. 1A, as a function of the initial energy in the solid measured [5] from the extrapolated upper edge E,. In fig. 1B the EDCs of aSb are compared at equal values of (hv - Q), so that the same binding energy interval (measured from Ev) is explored. In the present experiment the thresholds @ of aSb and cSb agree with those given in ref. [ I]. The EDCs of aSb show a broad structure which is completed on the left at increasing hv, consistently with the existence, at these binding energies, of a single valence structure seen in XPS [3] ; the present results cover this “p-like” structure almost completely. The EDCs of fig. 1 show the effects on electron states of the structural properties of Sb and, generally speaking, the present results are relevant in connection with the physics of amorphous 3-coordinated semiconductors. The main new results are the following. i) No final state effects are seen in the EDCs; the conduction density of states has not structures * Gruppo Nazionale di Struttura della Materia de1 C.N.R.
1
-5.
I
-4.
-3.
I
-2.
I
- 1. (eV)Ev
Fig. 1. A: The EDCs of aSb with 6.22 4 h < 9.22 eV plotted against initial energy. B: comparison between the EDCs of aSb and cSb (dotted line) at the same (hv - a) values; the photon energies of the EDCs of aSb are given. C: the barycenters (measured from EV) of the EDCs of aSb (line (a)) and of cSb (line (c)) versus (hv - m). between 5 and 9 eV above Ev. An analogous property holds for 4-coordinated amorphous semiconductors and its relevance is discussed in [6]. ii) Fig. 1B shows a shift of p-valence states of aSb 33
Volume 48A, number 1
PHYSICS LETTERS
towards E,, which is connected (as discussed by Joannoupoulos and Cohen [7] for Ge and Si and in ref. [3] for As, Sb, Bi) with the variation of Coulomb and kinetic energies due to bond angle modifications, passing from cSb to aSb. The measurement of this shift is important because it is an index of “local disorder” (in the sense of ref. [7]) and, in connection with future theoretical progress, will greatly help in discriminating between the various random networks which can be assumed for 3coordinated amorphous systems [8]. The present results allow an accurate measurement of this shift which was already seen with XPS by Ley et al. [3] who were able to estimate it no better than “a few tenths of eV”. We define here this shift as the difference A between the first momenta (barycenters) of the EDCs of aSb and cSb taken, as in fig. 1B, at the same (hv - Cp).The success of the procedure is shown by the linear dependence of these momenta (with constant A = 110 meV) on hv as is shown by the lines of tig. 1C; the measurement of A thus does not depend on hv and is not affected by final state effects [2] present in the EDCs of cSb. By discarding the part of the EDCs between the onset and IZ for all C =G1.2 eV. A values are between 95 and
110 meV; thus escape effects do not influence A significantly and 15 meV is the accuracy in the measured A, which is an intrinsic property of the p-like valence states of Sb.
References [ 1] The thresholds @ of aSb and cSb are given in T.W. Hall, R.M. Eastment and C.H.B. Mee, Phys. St. Sol. A2, (1970) 327; only the EDC of aSb at hu = 5.80 eV is given in E. Taft and L. Apker, Phys. Rev. 96 (1954) 1496. [2] I. Abbati, L. Braicovich, B. De Michelis and A. Fasana, J. Phys. C. Solid St. Phys. 7 April 1974 pp. 1412; the comparison between aSb and cSb is very accurate since both phases were studied with the same apparatus. [ 31 L. Ley, R.A. Pollak, S.P. Kowalczyk, R. McFeely and D.A. Schirley, Phys. Rev. B8 (1973) 641. [4] The upper limit to hv is caused by the use of a McPherson 218 monochromator owing to economic reasons. [ 51 EV is a convenient reference point and is easily defined in theoretical results. [6] M.F. Thorpe, D. Weare and R. Alben, Phys. Rev. 7B, (1973) 3777 and ref. quoted therein. [ 71 J. D. Joannoupoulos and M.L. Cohen, Phys. Rev. 7B (1973) 2644. [8] D. Turnbull, and.D.E. Polk, J. NonCrystalline
(1972) 19.
34
20 May 1974
Solids 8-10,
Volume 48A, number 1
PHOTOELECTRIC
COMPARISON
OF ELECTRON
AND CRYSTALLINE I. ABBATI*, L. BRAICOVICH*,
20 May 1974
STATES
OF AMORPHOUS
ANTIMONY
B. De MICHELIS* and A. FASANA
Institute of Physics of the Politecnico, Milan, Italy
Received 29 March 1974 The photoelectron energy distributions of amorphous Sb (aSb) with 6.22 d hv Q 9.22 eV are given and compared with those of crystalline Sb. The barycenter of p-like valence states of aSb is,95-110 meV upward shifted; no final state effects are seen in aSb.
In this letter we give ultraviolet photoelectron spectroscopy results on amorphous Sb (aSb), whose energy distribution curves (EDCs) were not known till now [ 11. The results are discussed briefly in connection with those [2] on crystalline Sb (cSb) and with X-ray photoelectron spectroscopy (XPS) results of ref. [3] (ref. [3] is supposed to be known to the reader). Amorphous Sb was prepared by evaporation as in [l] ; the pressure during the measurements was 5 X lo- l1 torr and in the lo- lo torr range during the evaporation. The EDCs with 6.22 Q hv < 9.22 eV are collected [4] in fig. 1A, as a function of the initial energy in the solid measured [5] from the extrapolated upper edge E,. In fig. 1B the EDCs of aSb are compared at equal values of (hv - Q), so that the same binding energy interval (measured from Ev) is explored. In the present experiment the thresholds @ of aSb and cSb agree with those given in ref. [ I]. The EDCs of aSb show a broad structure which is completed on the left at increasing hv, consistently with the existence, at these binding energies, of a single valence structure seen in XPS [3] ; the present results cover this “p-like” structure almost completely. The EDCs of fig. 1 show the effects on electron states of the structural properties of Sb and, generally speaking, the present results are relevant in connection with the physics of amorphous 3-coordinated semiconductors. The main new results are the following. i) No final state effects are seen in the EDCs; the conduction density of states has not structures * Gruppo Nazionale di Struttura della Materia de1 C.N.R.
1
-5.
I
-4.
-3.
I
-2.
I
- 1. (eV)Ev
Fig. 1. A: The EDCs of aSb with 6.22 4 h < 9.22 eV plotted against initial energy. B: comparison between the EDCs of aSb and cSb (dotted line) at the same (hv - a) values; the photon energies of the EDCs of aSb are given. C: the barycenters (measured from EV) of the EDCs of aSb (line (a)) and of cSb (line (c)) versus (hv - m). between 5 and 9 eV above Ev. An analogous property holds for 4-coordinated amorphous semiconductors and its relevance is discussed in [6]. ii) Fig. 1B shows a shift of p-valence states of aSb 33
Volume 48A, number 1
PHYSICS LETTERS
towards E,, which is connected (as discussed by Joannoupoulos and Cohen [7] for Ge and Si and in ref. [3] for As, Sb, Bi) with the variation of Coulomb and kinetic energies due to bond angle modifications, passing from cSb to aSb. The measurement of this shift is important because it is an index of “local disorder” (in the sense of ref. [7]) and, in connection with future theoretical progress, will greatly help in discriminating between the various random networks which can be assumed for 3coordinated amorphous systems [8]. The present results allow an accurate measurement of this shift which was already seen with XPS by Ley et al. [3] who were able to estimate it no better than “a few tenths of eV”. We define here this shift as the difference A between the first momenta (barycenters) of the EDCs of aSb and cSb taken, as in fig. 1B, at the same (hv - Cp).The success of the procedure is shown by the linear dependence of these momenta (with constant A = 110 meV) on hv as is shown by the lines of tig. 1C; the measurement of A thus does not depend on hv and is not affected by final state effects [2] present in the EDCs of cSb. By discarding the part of the EDCs between the onset and IZ for all C =G1.2 eV. A values are between 95 and
110 meV; thus escape effects do not influence A significantly and 15 meV is the accuracy in the measured A, which is an intrinsic property of the p-like valence states of Sb.
References [ 1] The thresholds @ of aSb and cSb are given in T.W. Hall, R.M. Eastment and C.H.B. Mee, Phys. St. Sol. A2, (1970) 327; only the EDC of aSb at hu = 5.80 eV is given in E. Taft and L. Apker, Phys. Rev. 96 (1954) 1496. [2] I. Abbati, L. Braicovich, B. De Michelis and A. Fasana, J. Phys. C. Solid St. Phys. 7 April 1974 pp. 1412; the comparison between aSb and cSb is very accurate since both phases were studied with the same apparatus. [ 31 L. Ley, R.A. Pollak, S.P. Kowalczyk, R. McFeely and D.A. Schirley, Phys. Rev. B8 (1973) 641. [4] The upper limit to hv is caused by the use of a McPherson 218 monochromator owing to economic reasons. [ 51 EV is a convenient reference point and is easily defined in theoretical results. [6] M.F. Thorpe, D. Weare and R. Alben, Phys. Rev. 7B, (1973) 3777 and ref. quoted therein. [ 71 J. D. Joannoupoulos and M.L. Cohen, Phys. Rev. 7B (1973) 2644. [8] D. Turnbull, and.D.E. Polk, J. NonCrystalline
(1972) 19.
34
20 May 1974
Solids 8-10,