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Large pressure effect on photoluminescence lines in 6H SiC:Ti crystal
Solid State Communications, Printed in Great Britain.
LARGE PRESSURE
Vol. 88, No. 7, pp. 537-540, 1993.
EFFECT ON PHOTOLUMINESCENCE
0038- 1098/93 $6.00 + .OO Pergamon Press Ltd
LINES IN 6H SiC:Ti CRYSTAL
A. Niilisk and A. Laisaar Institute of Physics, Estonian Academy of Sciences, Riia St. 142, EE2400 Tartu, Estonia and I.S. Gorban and A.V. Slobodyanyuk Taras Shevchenko Kiev University, Prospekt Glushkova 6, 252127 Kiev, Ukraine (Received
5 July 1993 by P. Wachter)
The effect of hydrostatic pressure up to 7.5 kbar on the so-called ABC lines in the photoluminescence spectrum of hexagonal 6H Sic crystals with traces of Ti atoms has been studied at 77 K. Linear pressure shifts of these lines were found to be rather large (1.08-1.25 meV kbar-‘) considering low compressibility of the crystal and the expected very small pressure coefficient for the band gap in 6H Sic. The recorded pressure shifts exceed by more than 10 times those of the well-known R luminescence lines in ruby, A1203 : Cr3+., .which has roughly the same compressibility as Sic. Fairly high sensitivity of ABC lines to the applied pressure is interpreted in terms of a ligand field model for a quasimolecular impurity centre where Ti atom substitutes an about 20% smaller Si atom in SIC crystal.
WHEN excited by ultraviolet radiation at low silicon carbide crystals of various temperature, polytypes, including the most common hexagonal 6H Sic, often exhibit the so-called ABC lines in the luminescence spectrum at wavelengths between 430 and 450 nm [l-4]. Due to high Debye temperature of Sic these narrow emission zero-phonon lines (ZPLs) and also their phonon replicas are clearly seen at temperatures as high as 77K and even more. The lines are assigned to the radiative transitions in impurity Ti atoms substituting Si atoms of the host lattice [5-81. For this kind of impurity centre an overlap of the wave functions of Ti atom and its ligands [7, 81 is rather sensitive to small variations of interatomic distances in the centre because the atomic radius of the substitutional Ti atom is by about 20% larger than that of Si atom (1.47 and 1.17 A, respectively). Therefore, the electronic states of this quasimolecular impurity centre can considerably be changed by various external factors, especially by hydrostatic compression of the crystal. As a continuation of piezospectroscopic investigations of doped silicon carbide crystals [9, lo], we have studied the effect of hydrostatic pressure up to 7.5 kbar on the ABC luminescence lines in 6H Sic : Ti crystal at 77 K. Compensated 6H Sic single crystals were grown by using a titanium getter. For
experiments a special high-pressure setup [ll] was used, where a sample in the form of a small platelet was subjected to helium gas pressure in an optical cell provided with two sapphire windows. The cell was immersed in liquid nitrogen inside a simple vacuumless cryostat. Luminescence of the sample was excited by near-ultraviolet radiation from a 1000-W xenon lamp through proper optical filters. The ZPLs under study were recorded by the system consisting of a 80 cm focal-length double grating monochromator and a photomultiplier with photon counting electronics. The results of the study are illustrated in Fig. 1, where the luminescence spectrum of 6H SIC : Ti at normal pressure (lower curve) and at 7.5 kbar (upper curve) is presented. At 77 K there can be seen the ZPLs A,, B, and C,, that are caused by the electronic transitions from the lowest level of the excited state of Ti centres occupying three different inequivalent sites in the crystal lattice, and also the ZPLs A*, A3, B2 and C, that are due to transitions from higher, thermally populated levels in the excited state of the same centres [3]. The peak A-90 is supposed to be a vibrational replica of Al and/or A2 line(s) (with the generation of a local gap mode of the energy of about 90meV), while the peak A-30 is a phonon replica of the same line(s) (with the excitation of TA phonon having an energy of about 30meV) [3]. Pressure shifts of all the lines studied are given in
537
PRESSURE 2.78
282
I
I
EFFECT
I
I
ON PHOTOLUMINESCENCE
2.86
hJ (eV)
I
I
A 21
A-90
-_:
I
LLO
L45
L35 Wovelength
L3 inmi
Fig. 1. ABC luminescence spectrum of 6H SIC : Ti crystal at atmospheric pressure (curve a) and at 7.5 kbar (curve b); T = 77 K. Table 1. The sole effect observed is a linear shift of the lines towards higher energies without any detectable change in their relative intensities. The pressure coefficients for individual lines range from 1.08 to 1.25 meV kbar-‘. These coefficients seem to be unexpectedly large, bearing in mind the following three points. Firstly, the observed pressure shifts of ABC luminescence lines in 6H Sic are surprisingly large in contrast with very small temperature shifts of the same lines. Indeed, the overall shift of ABC lines towards lower energies in the temperature range of 4.2-77 K does not exceed the average width of these lines at 4.2 K (about 0.3 meV): the shift equals to only -0.26 meV for Bi line and even less, merely -0.16 and -0.18 meV, for the other lines [12]. Table 1. Peak positions at atmospheric pressure and pressure coe#icients for ABC lines in 6H Sic : Ti photoluminescence spectrum at T = 77 K Line
Al A2 -43
A - 90 A - 30 BI B2
Cl c2
Position (eV)”
Pressure shift (meV kbar-‘)b
2.862 2.863 2.868 2.772 2.834 2.821 2.822 2.787 2.789
1.23 f 0.03 1.22 f 0.03 1.23 f 0.03 1.20 f 0.03 1.25 f 0.06 1.13 f 0.03 1.15f0.06 1.14 f 0.03 1.08 f 0.03
“, Obtained from the values of l/X in vacuum. Calculated as a slope of the least-squares straight line through the data points.
LINES
Vol. 88. No. 7
Secondly, the pressure shifts of ABC lines seem to be rather large, taking into account low compressibility of 6H SIC crystal, 0.48 x 10m6 bar-’ as estimated by us from elastic constants at 300 K [13]. It is notewq$hy that for instance in a ruby crystal, A1203:Cr having approximately the same compressibility ‘as Sic crystals do, the pressure shifts of the well-known R luminescence lines are more than 10 times smaller, amounting to only -0.76 cm-’ kbar-’ or -0.094 meV kbar-‘, irrespective of the temperature down to 4.2K [14, 151. Thirdly, the shifts of ABC lines are also large (and opposite in sign) as compared to the anticipated minute pressure shift of the absorption edge of the hexagonal 6H SIC crystal. The fundamental absorption edge of this polytype of SIC is formed by indirect band-to-band transitions from the top of the valence band at l? point to the minima of a complicated conduction band located most likely near M points of the Brillouin zone of the hexagonal wurtzite lattice [16]. These conduction band minima are similar in symmetry to X minima of the cubic 3C SIC crystal with zinc-blende lattice [16], where absorption edge is associated with indirect r - X transitions. Recently, the pressure dependence of the absorption edge in cubic SIC crystals was measured up to about 150 kbar at room temperature [ 17, IS]. Very small (for an absorption edge) sublinear pressure shift towards lower energies was detected, from which the pressure coefficient of the indirect band gap at the low pressure limit was estimated to be -0.40 meV kbar-’ [ 181, which is in a good agreement with the theoretical value of -0.36meVkbaY’ [19]. By analogy an approximately the same pressure shift can be expected for the absorption edge of the hexagonal Ti
Ligands
Combined a? I
\
Fig. 2. Schematic diagram showing the combination of Ti and ligand states in Sic to form bonding (a! and ti), nonbonding (e) and antibonding (a; and t;) orbitals. Degenerate levels are shown separated for clarity [7].
Vol. 88, No. 7
PRESSURE
EFFECT
ON PHOTOLUMINESCENCE
SIC crystal and thus the indirect band gap of 6H Sic probably decrease at a rate of should -0.4 meV kbar-’ or so. Rather large pressure shifts towards higher energies for ABC lines in 6H Sic : Ti can be understood on the basis of the energy level scheme of Ti impurity centre in SIC crystal [7], reproduced in Fig. 2. According to the ligand-field theory a system consisting of Ti atom and four tetrahedrally coordinated C atoms in Sic gives rise to bonding (ti, a!), antibonding (t;, a;) and nonbonding (e) orbitals. ABC luminescence is assigned to the separation AE transitions e -+ ti. The energy between the bonding ti and the antibonding t; states depends strongly on the overlap of the wavefunctions of an impurity atom and its ligands, and hence on Ti-C distance, while the energy of nonbonding e state depends on Ti-C spacing much less because the wavefunctions of host lattice atoms do not contribute to the e state. The decrease of Ti-C distance under pressure leads to a strong increase in the bondingantibonding splitting AE and to much weaker increase in the energy of e level as an upper component, 3d(e), of a crystal-field split 3d level of a free Ti atom. In consequence, the energy of radiative transition e -+ ti must considerably increase with pressure. It is noteworthy that the pressure shift of the main Al line is somewhat (about 10%) larger than that for the lines B, and C,, reflecting the dependence of electronic transition energy in the luminescence centre upon slightly different environment of the impurity atom occupying various crystallographically inequivalent sites. Taking into account a small deviation from tetrahedral symmetry, arising from the effect of the second and the third nearest neighbours on electronic energy states of the impurity atom, the inequivalent sites in 6H SIC lattice may be separated into a hexagonal-like (one site) and a cubic-like (two sites) one [8, 201. It seems reasonable to assign B1 and Ci lines, showing practically the same pressure shifts, to two cubiclike sites, whereas A, line with about 10% larger pressure shift should belong to the single hexagonallike site. However, this assignment seems to be invalid, considering an optically detected magnetic resonance (ODMR) study of these spectral lines [8], where the hexagonal-like site was ascertained to correspond to the line B. In the light of such a contradiction the question about small differences in the pressure shifts of A,, B, and Ci lines remains open. For the vibronic line A-90 associated with the creation of a local gap mode, somewhat smaller
LINES
539
pressure shift than that for the main Al line is observed (Table 1). It is reasonable to suppose that this small difference in pressure shifts, about is caused by the increase of the 0.03 meV kbar-‘, local mode frequency under pressure, although the pressure coefficient for that mode thus obtained does not exceed the standard deviation of the linear least-squares fit for the shifts of both A, and A-90 lines. For comparison, it should be noted that in cubic 3C Sic crystals the energy of TO and LO phonons, as recorded in first-order Raman spectra, was found to increase under pressure at the rate of 0.048 f 0.001 and 0.059 & 0.001 meV kbar-‘, respectively [21].
REFERENCES
5.
10.
11.
12. 13.
14.
15.
L. Patrick, D.R. Hamilton & W.J. Choyke, Phys. Rev. 132, 2023 (1963). W.J. Choyke, D.R. Hamilton & L. Patrick, Phys. Rev. 139, Al262 (1965). D.R. Hamilton, W.J. Choyke & L. Patrick, Phys. Rev. 131, 127 (1963). I.S. Gorban & V.A. Gubanov, Fiz. Tverd. Tela 13, 2076 (1971) [English transl.: Sov. Phys.Solid State 13, 1741 (1971)]. W.J. Choyke & L. Patrick, in Silicon Carbide1973 (Edited by R.C. Marshall, J.W. Faust & C.E. Ryan), p. 261, University of South Carolina Press, Columbia, South Carolina (1974). A.W.C. van Kemenade & S.H. Hagen, Solid State Commun. 14, 1331 (1974). L. Patrick & W.J. Choyke, Phys. Rev. BlO, 5091 (1974). K.M. Lee, Le Si Dang, G.D. Watkins & W.J. Choyke, Phys. Rev. B32, 2273 (1985). I.S. Gorban, V.A. Gubanov, V.D. Kulakovskii, A.S. Skirda & B.N. Shepel’, Fiz. Tverd. Tela 30, 1605 (1988) [English transl.: Sov. Phys.-Solid State 30, 928 (1988)]. I.S. Gorban, A.P. Krokhmal’ & I.A. Rozhko, Fiz. Tverd. Tela 31, No. 12, 126 (1989) [English transl.: Sov. Phys.-Solid State 31, 2095 (1989)]. W. Bujnowski, S. Porowski & A.I. Laisaar, Prib. Tekh. Eksp. No. 1, 224 (1973) [English transl.: Znstrum. Exp. Tech. 16, 274 (1973)]. V.A. Gubanov, Thesis, Kiev State University (1975). V.M. Lyubimskii, Fiz. Tverd. Tela 18, 3111 (1976) [English transl.: Sov. Phys.-Solid State 18, 1814 (1976)]. R.A. Noack & W.B. Holzapfel, in High Pressure Science and Technology (Edited by K.D. Timmerhaus & M.S. Barber), Vol. 1, p. 748, Plenum, New York (1979). A. Jayaraman, Rev. Sci. Znstrum. 57, 1013 (1986).
PRESSURE
540 16. 17. 18.
EFFECT ON PHOTOLUMINESCENCE
L. Patrick, Phys. Rev. B5, 2198 (1972). M. Kobayashi, M. Yamanaka & M. Shinohara, J. Phys. Sot. Jpn 58, 2673 (1989). M. Kobayashi, R. Akimoto, S. Endo, M. Yamanaka, M. Shinohara & K. Ikoma, in
20.
Amorphous
21.
and Crystalline
Silicon Carbide III
(Edited by G.L. Harris, M.G. Spencer & C.Y.
19.
LINES
Vol. 88, No. 7
Yang), Springer Proc. in Physics, Vol. 56, p. 263, Springer, Berlin (1992). K.J. Chang & M.L. Cohen, Phys. Rev. B35, 8196 (1987). L. Patrick, Phys. Rev. 127, 1878 (1962). D. Olego, M. Cardona & P. Vogl, Phys. Rev. B25, 3878 (1982).
LARGE PRESSURE
Vol. 88, No. 7, pp. 537-540, 1993.
EFFECT ON PHOTOLUMINESCENCE
0038- 1098/93 $6.00 + .OO Pergamon Press Ltd
LINES IN 6H SiC:Ti CRYSTAL
A. Niilisk and A. Laisaar Institute of Physics, Estonian Academy of Sciences, Riia St. 142, EE2400 Tartu, Estonia and I.S. Gorban and A.V. Slobodyanyuk Taras Shevchenko Kiev University, Prospekt Glushkova 6, 252127 Kiev, Ukraine (Received
5 July 1993 by P. Wachter)
The effect of hydrostatic pressure up to 7.5 kbar on the so-called ABC lines in the photoluminescence spectrum of hexagonal 6H Sic crystals with traces of Ti atoms has been studied at 77 K. Linear pressure shifts of these lines were found to be rather large (1.08-1.25 meV kbar-‘) considering low compressibility of the crystal and the expected very small pressure coefficient for the band gap in 6H Sic. The recorded pressure shifts exceed by more than 10 times those of the well-known R luminescence lines in ruby, A1203 : Cr3+., .which has roughly the same compressibility as Sic. Fairly high sensitivity of ABC lines to the applied pressure is interpreted in terms of a ligand field model for a quasimolecular impurity centre where Ti atom substitutes an about 20% smaller Si atom in SIC crystal.
WHEN excited by ultraviolet radiation at low silicon carbide crystals of various temperature, polytypes, including the most common hexagonal 6H Sic, often exhibit the so-called ABC lines in the luminescence spectrum at wavelengths between 430 and 450 nm [l-4]. Due to high Debye temperature of Sic these narrow emission zero-phonon lines (ZPLs) and also their phonon replicas are clearly seen at temperatures as high as 77K and even more. The lines are assigned to the radiative transitions in impurity Ti atoms substituting Si atoms of the host lattice [5-81. For this kind of impurity centre an overlap of the wave functions of Ti atom and its ligands [7, 81 is rather sensitive to small variations of interatomic distances in the centre because the atomic radius of the substitutional Ti atom is by about 20% larger than that of Si atom (1.47 and 1.17 A, respectively). Therefore, the electronic states of this quasimolecular impurity centre can considerably be changed by various external factors, especially by hydrostatic compression of the crystal. As a continuation of piezospectroscopic investigations of doped silicon carbide crystals [9, lo], we have studied the effect of hydrostatic pressure up to 7.5 kbar on the ABC luminescence lines in 6H Sic : Ti crystal at 77 K. Compensated 6H Sic single crystals were grown by using a titanium getter. For
experiments a special high-pressure setup [ll] was used, where a sample in the form of a small platelet was subjected to helium gas pressure in an optical cell provided with two sapphire windows. The cell was immersed in liquid nitrogen inside a simple vacuumless cryostat. Luminescence of the sample was excited by near-ultraviolet radiation from a 1000-W xenon lamp through proper optical filters. The ZPLs under study were recorded by the system consisting of a 80 cm focal-length double grating monochromator and a photomultiplier with photon counting electronics. The results of the study are illustrated in Fig. 1, where the luminescence spectrum of 6H SIC : Ti at normal pressure (lower curve) and at 7.5 kbar (upper curve) is presented. At 77 K there can be seen the ZPLs A,, B, and C,, that are caused by the electronic transitions from the lowest level of the excited state of Ti centres occupying three different inequivalent sites in the crystal lattice, and also the ZPLs A*, A3, B2 and C, that are due to transitions from higher, thermally populated levels in the excited state of the same centres [3]. The peak A-90 is supposed to be a vibrational replica of Al and/or A2 line(s) (with the generation of a local gap mode of the energy of about 90meV), while the peak A-30 is a phonon replica of the same line(s) (with the excitation of TA phonon having an energy of about 30meV) [3]. Pressure shifts of all the lines studied are given in
537
PRESSURE 2.78
282
I
I
EFFECT
I
I
ON PHOTOLUMINESCENCE
2.86
hJ (eV)
I
I
A 21
A-90
-_:
I
LLO
L45
L35 Wovelength
L3 inmi
Fig. 1. ABC luminescence spectrum of 6H SIC : Ti crystal at atmospheric pressure (curve a) and at 7.5 kbar (curve b); T = 77 K. Table 1. The sole effect observed is a linear shift of the lines towards higher energies without any detectable change in their relative intensities. The pressure coefficients for individual lines range from 1.08 to 1.25 meV kbar-‘. These coefficients seem to be unexpectedly large, bearing in mind the following three points. Firstly, the observed pressure shifts of ABC luminescence lines in 6H Sic are surprisingly large in contrast with very small temperature shifts of the same lines. Indeed, the overall shift of ABC lines towards lower energies in the temperature range of 4.2-77 K does not exceed the average width of these lines at 4.2 K (about 0.3 meV): the shift equals to only -0.26 meV for Bi line and even less, merely -0.16 and -0.18 meV, for the other lines [12]. Table 1. Peak positions at atmospheric pressure and pressure coe#icients for ABC lines in 6H Sic : Ti photoluminescence spectrum at T = 77 K Line
Al A2 -43
A - 90 A - 30 BI B2
Cl c2
Position (eV)”
Pressure shift (meV kbar-‘)b
2.862 2.863 2.868 2.772 2.834 2.821 2.822 2.787 2.789
1.23 f 0.03 1.22 f 0.03 1.23 f 0.03 1.20 f 0.03 1.25 f 0.06 1.13 f 0.03 1.15f0.06 1.14 f 0.03 1.08 f 0.03
“, Obtained from the values of l/X in vacuum. Calculated as a slope of the least-squares straight line through the data points.
LINES
Vol. 88. No. 7
Secondly, the pressure shifts of ABC lines seem to be rather large, taking into account low compressibility of 6H SIC crystal, 0.48 x 10m6 bar-’ as estimated by us from elastic constants at 300 K [13]. It is notewq$hy that for instance in a ruby crystal, A1203:Cr having approximately the same compressibility ‘as Sic crystals do, the pressure shifts of the well-known R luminescence lines are more than 10 times smaller, amounting to only -0.76 cm-’ kbar-’ or -0.094 meV kbar-‘, irrespective of the temperature down to 4.2K [14, 151. Thirdly, the shifts of ABC lines are also large (and opposite in sign) as compared to the anticipated minute pressure shift of the absorption edge of the hexagonal 6H SIC crystal. The fundamental absorption edge of this polytype of SIC is formed by indirect band-to-band transitions from the top of the valence band at l? point to the minima of a complicated conduction band located most likely near M points of the Brillouin zone of the hexagonal wurtzite lattice [16]. These conduction band minima are similar in symmetry to X minima of the cubic 3C SIC crystal with zinc-blende lattice [16], where absorption edge is associated with indirect r - X transitions. Recently, the pressure dependence of the absorption edge in cubic SIC crystals was measured up to about 150 kbar at room temperature [ 17, IS]. Very small (for an absorption edge) sublinear pressure shift towards lower energies was detected, from which the pressure coefficient of the indirect band gap at the low pressure limit was estimated to be -0.40 meV kbar-’ [ 181, which is in a good agreement with the theoretical value of -0.36meVkbaY’ [19]. By analogy an approximately the same pressure shift can be expected for the absorption edge of the hexagonal Ti
Ligands
Combined a? I
\
Fig. 2. Schematic diagram showing the combination of Ti and ligand states in Sic to form bonding (a! and ti), nonbonding (e) and antibonding (a; and t;) orbitals. Degenerate levels are shown separated for clarity [7].
Vol. 88, No. 7
PRESSURE
EFFECT
ON PHOTOLUMINESCENCE
SIC crystal and thus the indirect band gap of 6H Sic probably decrease at a rate of should -0.4 meV kbar-’ or so. Rather large pressure shifts towards higher energies for ABC lines in 6H Sic : Ti can be understood on the basis of the energy level scheme of Ti impurity centre in SIC crystal [7], reproduced in Fig. 2. According to the ligand-field theory a system consisting of Ti atom and four tetrahedrally coordinated C atoms in Sic gives rise to bonding (ti, a!), antibonding (t;, a;) and nonbonding (e) orbitals. ABC luminescence is assigned to the separation AE transitions e -+ ti. The energy between the bonding ti and the antibonding t; states depends strongly on the overlap of the wavefunctions of an impurity atom and its ligands, and hence on Ti-C distance, while the energy of nonbonding e state depends on Ti-C spacing much less because the wavefunctions of host lattice atoms do not contribute to the e state. The decrease of Ti-C distance under pressure leads to a strong increase in the bondingantibonding splitting AE and to much weaker increase in the energy of e level as an upper component, 3d(e), of a crystal-field split 3d level of a free Ti atom. In consequence, the energy of radiative transition e -+ ti must considerably increase with pressure. It is noteworthy that the pressure shift of the main Al line is somewhat (about 10%) larger than that for the lines B, and C,, reflecting the dependence of electronic transition energy in the luminescence centre upon slightly different environment of the impurity atom occupying various crystallographically inequivalent sites. Taking into account a small deviation from tetrahedral symmetry, arising from the effect of the second and the third nearest neighbours on electronic energy states of the impurity atom, the inequivalent sites in 6H SIC lattice may be separated into a hexagonal-like (one site) and a cubic-like (two sites) one [8, 201. It seems reasonable to assign B1 and Ci lines, showing practically the same pressure shifts, to two cubiclike sites, whereas A, line with about 10% larger pressure shift should belong to the single hexagonallike site. However, this assignment seems to be invalid, considering an optically detected magnetic resonance (ODMR) study of these spectral lines [8], where the hexagonal-like site was ascertained to correspond to the line B. In the light of such a contradiction the question about small differences in the pressure shifts of A,, B, and Ci lines remains open. For the vibronic line A-90 associated with the creation of a local gap mode, somewhat smaller
LINES
539
pressure shift than that for the main Al line is observed (Table 1). It is reasonable to suppose that this small difference in pressure shifts, about is caused by the increase of the 0.03 meV kbar-‘, local mode frequency under pressure, although the pressure coefficient for that mode thus obtained does not exceed the standard deviation of the linear least-squares fit for the shifts of both A, and A-90 lines. For comparison, it should be noted that in cubic 3C Sic crystals the energy of TO and LO phonons, as recorded in first-order Raman spectra, was found to increase under pressure at the rate of 0.048 f 0.001 and 0.059 & 0.001 meV kbar-‘, respectively [21].
REFERENCES
5.
10.
11.
12. 13.
14.
15.
L. Patrick, D.R. Hamilton & W.J. Choyke, Phys. Rev. 132, 2023 (1963). W.J. Choyke, D.R. Hamilton & L. Patrick, Phys. Rev. 139, Al262 (1965). D.R. Hamilton, W.J. Choyke & L. Patrick, Phys. Rev. 131, 127 (1963). I.S. Gorban & V.A. Gubanov, Fiz. Tverd. Tela 13, 2076 (1971) [English transl.: Sov. Phys.Solid State 13, 1741 (1971)]. W.J. Choyke & L. Patrick, in Silicon Carbide1973 (Edited by R.C. Marshall, J.W. Faust & C.E. Ryan), p. 261, University of South Carolina Press, Columbia, South Carolina (1974). A.W.C. van Kemenade & S.H. Hagen, Solid State Commun. 14, 1331 (1974). L. Patrick & W.J. Choyke, Phys. Rev. BlO, 5091 (1974). K.M. Lee, Le Si Dang, G.D. Watkins & W.J. Choyke, Phys. Rev. B32, 2273 (1985). I.S. Gorban, V.A. Gubanov, V.D. Kulakovskii, A.S. Skirda & B.N. Shepel’, Fiz. Tverd. Tela 30, 1605 (1988) [English transl.: Sov. Phys.-Solid State 30, 928 (1988)]. I.S. Gorban, A.P. Krokhmal’ & I.A. Rozhko, Fiz. Tverd. Tela 31, No. 12, 126 (1989) [English transl.: Sov. Phys.-Solid State 31, 2095 (1989)]. W. Bujnowski, S. Porowski & A.I. Laisaar, Prib. Tekh. Eksp. No. 1, 224 (1973) [English transl.: Znstrum. Exp. Tech. 16, 274 (1973)]. V.A. Gubanov, Thesis, Kiev State University (1975). V.M. Lyubimskii, Fiz. Tverd. Tela 18, 3111 (1976) [English transl.: Sov. Phys.-Solid State 18, 1814 (1976)]. R.A. Noack & W.B. Holzapfel, in High Pressure Science and Technology (Edited by K.D. Timmerhaus & M.S. Barber), Vol. 1, p. 748, Plenum, New York (1979). A. Jayaraman, Rev. Sci. Znstrum. 57, 1013 (1986).
PRESSURE
540 16. 17. 18.
EFFECT ON PHOTOLUMINESCENCE
L. Patrick, Phys. Rev. B5, 2198 (1972). M. Kobayashi, M. Yamanaka & M. Shinohara, J. Phys. Sot. Jpn 58, 2673 (1989). M. Kobayashi, R. Akimoto, S. Endo, M. Yamanaka, M. Shinohara & K. Ikoma, in
20.
Amorphous
21.
and Crystalline
Silicon Carbide III
(Edited by G.L. Harris, M.G. Spencer & C.Y.
19.
LINES
Vol. 88, No. 7
Yang), Springer Proc. in Physics, Vol. 56, p. 263, Springer, Berlin (1992). K.J. Chang & M.L. Cohen, Phys. Rev. B35, 8196 (1987). L. Patrick, Phys. Rev. 127, 1878 (1962). D. Olego, M. Cardona & P. Vogl, Phys. Rev. B25, 3878 (1982).