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Effect of pressure on near-infrared abc photoluminescence spectrum of 6H SiC crystal
~pergamon
Solid State Communications, Vol. 94, No.1, pp. 71-74, 1995 Elsevier Science Ltd Printed in Great Britain. 0038-1098/95 $9.50 + .00
0038-1098(94)00843-4
EFFECT OF PRESSURE ON NEAR-INFRARED abc PHOTOLUMINESCENCE SPECTRUM OF 6H SiC CRYSTAL A. Niilisk and A. Laisaar Institute of Physics, Estonian Academy of Sciences, Riia Street 142, EE2400 Tartu, Estonia and A.V. Slobodyanyuk Taras Shevchenko Kiev University, Prospekt Glushkova 6, 252127 Kiev, Ukraine
(Received 21 September 1994; accepted in revised form 11 November 1994 by M. Cardona) The effect of hydrostatic pressure up to 10.5 kbar on the abc zerophonon lines (ZPLs) in the near-infrared photoluminescence spectrum of n-type hexagonal 6H SiC single crystals has been studied at 77 K. For the ZPLs examined linear pressure shifts to higher energies, amounting from 1.68 to 1.80 meV kbar- I , were found. The recorded shifts exceed by about 1.5 times those obtained earlier for the ZPLs in ABC spectrum of 6H SiC: Ti crystal [Solid State Commun. 88, 537 (1993)], pointing to a stronger lattice distortion around a quasimolecular centre responsible for the ZPLs in the abc spectrum as compared to the case of the Ti impurity centre. It is conjectured that the abc lines in n-type SiC crystals are caused by an impurity atom from the scandium subgroup, most likely yttrium. Keywords: A. semiconductors; C. impurities in semiconductors; D. electronic states (localized); E. high pressure; E. luminescence.
splittings in the abc spectrum are, to some extent, polytype-dependent. The ZPLs a, b, c in 6H polytype [3, 6, 7] and a, b, c, d in 15R polytype [4, 6, 7] are caused by radiative transitions in some kind of impurity centres occupying a number of different, crystallographically inequivalent lattice sites: three in 6H polytype and four of the five possible in 15R polytype. In the case of 33R SiC [5], where 11 inequivalent lattice sites exist, only three ZPLs a, b and c are seen, the lines band c being split into three well-resolved components, whereas the line a does not show any splitting. Instead, line a is wider and has a higher intensity than the components of the b and c lines. The triplets band c are assumed to belong to two "cubic-like" centres each occupying three different lattice sites; however, the line a belongs to a "hexagonal-like" centre for which the splitting between four possible sites may be much smaller and therefore is not seen in the spectrum [5]. From the thermal quenching of the luminescence lines in the case of the 6H polytype it was deduced [7]
IN OUR previous paper [1] the effect of hydrostatic pressure on the visible ABC photoluminescence in a SiC: Ti crystal of 6H polytype was examined. Rather large pressure shifts of zero-phonon lines (ZPLs) in the spectrum were found and interpreted proceeding from the quasimolecular nature [2] of the substitutional Ti centre involved. As an extension of our efforts to elucidate the origin of some of the strongly localized luminescence centres in silicon carbide we have studied the effect of hydrostatic pressure up to 10.5 kbar on ZPLs in the near-infrared abc photoluminescence spectrum of a hexagonal 6H SiC crystal at 77 K. As is known, the so-called abc spectrum can be observed at the wavelengths between 860 and 960 nm only in n-type SiC crystals of various polytypes 6H, 15R and 33R [3-7]. At liquid He and N 2 temperatures the spectrum consists of narrow ZPLs and their phonon replicas with the generation of an LA phonon having an energy of about 76meV. The number of lines as well as their spectral positions and 71
72
abc PHOTOLUMINESCENCE SPECTRUM OF 6H SiC CRYSTAL
that the lowest energy level in the excited electronic state of all the three luminescence centres responsible for the a, band c lines is located at about 80meV below the bottom of the conduction band. Accordingly, the ground state level of each centre must lie at an energy about 1.4eV lower (hv ~ 1.4eV for a, b, c lines), i.e. it is placed roughly in the middle of the band gap of the crystal (Eg ~ 3 eV for 6H SiC [2]). The chemical nature of the centres producing the near-infrared abc luminescence in SiC crystals is not yet clear. It has been found [6] that an abc luminescence of remarkable intensity appears even in the purest samples of 6H and 15R SiC (with accidental impurities at a level below 0.5 ppm). This circumstance makes the identification of the centres rather troublesome since very low concentrations of unintentional dopants may be involved. Based on some similarity between the spectral and polarization properties of the visible ABC luminescence and the near-infrared abc luminescence, which is present, as mentioned above, only in n-type crystals, it has been suggested [7] that in both cases the impurity atom responsible for the luminescence centre may have the same configuration of valence electrons, d 2s2, as is the case for a neutral Ti atom. In an n-type SiC crystal such a configuration may be realized by capturing an additional electron from the host lattice into a d shell of an impurity with d I i electron configuration, peculiar to the elements of scandium subgroup [7]. To our knowledge, there have been no additional proposals to these earlier speculations about the origin of the centres responsible for the abc emission, although in the last years notable progress has been achieved in the identification of various defects in SiC crystals (see, for instance, [8]). In this situation, any new information about these centres, including their behaviour under pressure, should be useful. To study the near-infrared abc photoluminescence of silicon carbide, reported in this paper, we used an unintentionally doped 6H SiC crystal with ntype conductivity. The crystal was grown in argon atmosphere with a low content of nitrogen (less than 0.003%). The concentration of uncompensated nitrogen donors amounted to 3 x 1017 cm- 3 . No acceptor impurities were specially added into the starting material. The high-pressure setup was the same as the one used in our previous work [1]. The sample was subjected to helium gas pressure in an optical cell made of a maraging steel. The cell was provided with two opposite sapphire windows. It was immersed in liquid nitrogen inside a simple vacuumless cryostat surrounded by a foam plastic envelope for thermal insulation. Hence, the sample temperature was very
Vol. 94, No.
close to 77 K in all our experiments. Pressure wa: measured with a calibrated manganin resistance gauge placed inside the final stage of a 15kbar gas compresso and thus maintained at room temperature. The abc luminescence of 6H SiC crystal wa: excited by near-ultraviolet radiation from a 1000" mercury lamp by using proper optical filters. The emission from the sample was analysed with an 80 err focal-length double grating monochromator (dispersion 10 Amm -I in the Ist order) by using a spectral slit width of lOA (1.4-1.7meV). A cooled photomultiplier sensitive to near-infrared radiation was used along with photon counting electronics. The spectra were recorded without polarization analysis of the emitted light. The results of the study are illustrated in Fig. I where the abc luminescence spectrum of a 6H SiC crystal at normal pressure (lower curve) and at a pressure of 1O.5kbar (upper curve) is presented. At 77 K three ZPLs a, band c as well as two phonon replicas a-76 and b-76 can be seen in the spectrum. The peak energies versus pressure for these ZPLs and their phonon replicas are given in Fig. 2. In general, the spectrum is quite similar to that published earlier [3], except for the lack of splitting of the a line. In fact, at 77 K this line was found [3J to consist of two components, labelled as a and a' and being caused by radiative transitions originating, respectively, from the lowest and from a higher, thermally populated level of the excited electronic state of the centre. The energy separation between these components is about 1.2meV. In our study they Photon Energy (eV)
1.34
1.38
1.42
'b 1nm -it-
,a
1.7meV
940
900
860
Wavelength (nm)
Fig. 1. Near-infrared abc photoluminescence spectrum of 6H SiC crystal at atmospheric pressure (lower curve) and at 10.5kbar (upper curve); T = 77 K. The spectrum measured at 10.5kbar is shifted somewhat upwards for clarity.
Table 1. Peak positions at atmospheric pressure and pressure coefficients for abc lines in 6H SiC nearinfrared photoluminescence spectrum at T = 77K
1.44
Line
Position* (eV)
Pressure shift t (mev kbar")
._____0
a
1.433 1.397 1.368 1.359 1.323
1.76 ± 0.04 1.74 ± 0.03 1.80 ± 0.03 1.73 ± 0.04 1.68 ± 0.04
o~
* Obtained from the values of 1/>. in vacuum . t Calculated as a slope of the least-squares straight
______0
>. Ol
~
C W
.--0 1.40 .... ~o
0
00
C
.9 o
.!:
73
abc PHOTOLUMINESCENCE SPECTRUM OF 6H SiC CRYSTAL
Vol. 94, No.1
....
Q.
c <$>
0 _0----__ 0---"
0
----
o~
b c a-76 b-76
line through the data points.
__0
_ _0
1.36
attributed to the radiative annihilation of bound excitons with emission of LA phonons. The pressure shifts of the lines in the abc _____ 0 luminescence spectrum may be compared with the shifts of various lines in the ABC spectrum of 6H SiC: Ti. The latter shifts, also toward s higher energies, ~Owere found to run between 1.08 and 1.25meVkbar- 1 I I 1.32 If' 1 [I]. It can be seen that the pressure shifts of the lines 10 8 6 o 2 4 in abc spectrum are, on the average, 50% larger than Pressure (kbar) those in the ABC spectrum. Fig. 2. Pressure dependence of the peak energy for As in the case of the ABC luminescence [1], various lines in the abc photoluminescence spectrum the large pressure shifts of the abc lines are in contrast of 6H SiC crystal at T = 77 K. to their very small temperature shifts, which in the range from 4.2 to 77 K do not exceed the average were not resolved (see Fig. I) because of insufficient width of these lines at 4.2 K [3] (about 0.3-0.4meV). spectral resolution (1.7 meV for the line a) due to The other two point s from our discussion on the rather a low intensity of abc luminescence in our behaviour of ABC lines [I] are also valid for abc lines: sample and light losses in the high pressure cell. (i) the pressure shifts of abc lines are surprisingly Pressure shifts of the lines along with their large considering the low compressibility of the hard spectral positions at atm ospheric pressure are given SiC crystals; (ii) these shifts are also large in in Table 1. Linear blue shifts of the lines occur , comparison with the expected marginal pressure the pressure coefficients ranging from 1.68 to shift of the absorption edge of a hexagonal 6H SiC 1.80meV kbar I for the individual lines. crystal in the opposite direction , towards lower Note that the phon on replicas a-76 and b-76 have energies (presumably only about - 0.4 meV kbar- J somewhat smaller pressure coefficients than the [1 D. respective ZPLs a and b. These small differences in The fact that the lines in both ABC and abc pressure shifts probably refer to the increase of the spectra are shifted under hydrostatic pressure in the energy of LA phonon with pressure . The pressure same direction and at a comparable average rate coefficient for that phonon thus obtained (0.03 or lends further support to the idea of a similar origin of 0.06meV kbar", see Table I) is in moderately good the centres under discussion, as mentioned earlier agreement with the pressure increase of the LA [7]. From the above it is plausible that the abc phonon energy in SiC crystals of 3C and 15R luminescence can be assigned to some impurit y atom polytypes obtained from the first-order Raman belonging to the scandium subgroup (Sc, Y, La, or spectra at 300 K (0.02-0.03 meV kbar"! as estimated some lanthanide atom) and having an electron from Fig. 2 in [9D and also with the pressure configuration d Ii [7]. Such an atom in n-type coefficient of 0.023 meV kbar- 1 for LA(X) phonon crystal can acquire the same configuration as a in cubic 3C SiC crystal estimated in [IO] from the neutral titanium atom, i.e. d 2i , by capturing an pressure shift of the photoluminescence line at 1.8K extra electron into its d shell. ." a-76
0-----
74
abc PHOTOLUMINESCENCE SPECTRUM OF 6H SiC CRYSTAL
All the elements of Sc subgroup have markedly larger atomic radii than the Ti atom (see, e.g. (11)) and, therefore, when such an atom substitutes Si atom in SiC crystal, the induced lattice distortion certainly exceeds that caused by the substitutional Ti atom. In this case a centre responsible for abc luminescence may have a quasi molecular character analogous to that of Ti centre [2] with the only quantitative difference that the greater lattice distortion leads to a stronger crystal field splitting of combined electronic levels of the centre and, as a consequence, also to larger pressure shifts of the lines in abc spectrum as compared to those in ABC spectrum. Such a possibility is in line with the earlier findings [6] that a very low impurity concentration is involved in the abc luminescence because it is evident that only a very limited quantity of impurity atoms with a large radius may be dissolved in a compact SiC crystal matrix. The most probable candidate for such impurity of scandium subgroup seems to be yttrium (Y) which was present in the crystal growing process of the samples that revealed a relatively intense abc luminescence (yttrium is an accompanying impurity in the zirconium ceramics which was used in a crystal growing furnace). The atomic radius of Y is 1.81 A against the value of 1.47 A for Ti [11] and therefore a larger lattice relaxation can be expected by the formation of such a point defect in silicon carbide. Unfortunately, to our knowledge there has been no elaborate trace impurity analysis of SiC crystals exhibiting abc luminescence, and convincing arguments for or against yttrium as an impurity giving rise to a quasimolecular titanium-like centre in SiC crystal are absent up to now. Also, other possibilities to explain the occurrence of abc luminescence in SiC (for instance, as caused by a more complex local centre) should not be discarded at present. One of the ways to tackle this problem might be to examine the possible isotope shifts and also the hyperfine structure of abc lines in doped SiC crystals at liquid helium temperatures by using highresolution spectroscopy methods. Note that the
Vol. 94, No. I
isotope structure of ABC luminescence lines [2, 12] was a crucial argument for attributing these lines to Ti impurity atoms in SiC lattice. Acknowledgement - The authors are indebted to Prof. M. Cardona for critically reading the manuscript and for valuable remarks.
REFERENCES A Niilisk, A Laisaar, I.S. Gorban' & A.V. Slobodyanyuk, Solid State Commun. 88, 537 (1993). 2. L. Patrick & W.J. Choyke, Phys. Rev. BI0, 5091 (1974). 3. I.S. Gorban' & AV. Slobodyanyuk, Fiz. Tverd. Tela 15, 789 (1973) [English trans!': Sov. Phys.Solid State 15, 548 (1973»). 4. I.S. Gorban' & AV. Slobodyanyuk, Fiz. Tverd. Tela 15, 2877 (1973) [English transl.: Sov. Phys.i-Solid State 15, 1925 (1974)]. 5. LS. Gorban' & A.V. Slobodyanyuk, Fiz. Tverd. Tela 16, 1789 (1974) [English transl.: Sov. Phys.-Solid State 16, 1163 (1974)]. 6. S.H. Hagen & AW.C. van Kemenade, J. Luminesc. 9,9 (1974). 7. I.S. Gorban' & A.V. Slobodyanyuk, Fiz. Tekh. Poluprovodn. 10, 1125 (1976) [English transl.: Sov. Phys.-Semicond. 10, 668 (1976)]. 8. J. Schneider & K. Maier, Physica B185, 199 (1993). 9. A.F. Goncharov, E.V. Yakovenko & S.M. Stishov, Pis'ma Zh. Eksp. Teor. Fiz. 52, 1092 (1990) [English transl.: JETP Lett. 52, 491 (1990)]. 10. M. Kobayashi, R. Akimoto, S. Endo, M. Yamanaka, M. Shinohara & K. Ikoma, in Amorphous and Crystalline Silicon Carbide III (Edited by G.L. Harris, M.G. Spencer & c.Y. Yang), Springer Proc. in Physics, Vol. 56, p. 263. Springer, Berlin (1992). II. B.K. Vainshtein, V.M. Fridkin & V.L. lndenborn, Modern Crystallography II. Structure of Crystals, Springer Series in Solid-State Sciences, Vol. 21, Chapter 1, Table 7. Springer-Verlag, Berlin (1982). 12. AW.C. van Kemenade & S.H. Hagen, Solid State Commun. 14, 1331 (1974). 1.
Solid State Communications, Vol. 94, No.1, pp. 71-74, 1995 Elsevier Science Ltd Printed in Great Britain. 0038-1098/95 $9.50 + .00
0038-1098(94)00843-4
EFFECT OF PRESSURE ON NEAR-INFRARED abc PHOTOLUMINESCENCE SPECTRUM OF 6H SiC CRYSTAL A. Niilisk and A. Laisaar Institute of Physics, Estonian Academy of Sciences, Riia Street 142, EE2400 Tartu, Estonia and A.V. Slobodyanyuk Taras Shevchenko Kiev University, Prospekt Glushkova 6, 252127 Kiev, Ukraine
(Received 21 September 1994; accepted in revised form 11 November 1994 by M. Cardona) The effect of hydrostatic pressure up to 10.5 kbar on the abc zerophonon lines (ZPLs) in the near-infrared photoluminescence spectrum of n-type hexagonal 6H SiC single crystals has been studied at 77 K. For the ZPLs examined linear pressure shifts to higher energies, amounting from 1.68 to 1.80 meV kbar- I , were found. The recorded shifts exceed by about 1.5 times those obtained earlier for the ZPLs in ABC spectrum of 6H SiC: Ti crystal [Solid State Commun. 88, 537 (1993)], pointing to a stronger lattice distortion around a quasimolecular centre responsible for the ZPLs in the abc spectrum as compared to the case of the Ti impurity centre. It is conjectured that the abc lines in n-type SiC crystals are caused by an impurity atom from the scandium subgroup, most likely yttrium. Keywords: A. semiconductors; C. impurities in semiconductors; D. electronic states (localized); E. high pressure; E. luminescence.
splittings in the abc spectrum are, to some extent, polytype-dependent. The ZPLs a, b, c in 6H polytype [3, 6, 7] and a, b, c, d in 15R polytype [4, 6, 7] are caused by radiative transitions in some kind of impurity centres occupying a number of different, crystallographically inequivalent lattice sites: three in 6H polytype and four of the five possible in 15R polytype. In the case of 33R SiC [5], where 11 inequivalent lattice sites exist, only three ZPLs a, b and c are seen, the lines band c being split into three well-resolved components, whereas the line a does not show any splitting. Instead, line a is wider and has a higher intensity than the components of the b and c lines. The triplets band c are assumed to belong to two "cubic-like" centres each occupying three different lattice sites; however, the line a belongs to a "hexagonal-like" centre for which the splitting between four possible sites may be much smaller and therefore is not seen in the spectrum [5]. From the thermal quenching of the luminescence lines in the case of the 6H polytype it was deduced [7]
IN OUR previous paper [1] the effect of hydrostatic pressure on the visible ABC photoluminescence in a SiC: Ti crystal of 6H polytype was examined. Rather large pressure shifts of zero-phonon lines (ZPLs) in the spectrum were found and interpreted proceeding from the quasimolecular nature [2] of the substitutional Ti centre involved. As an extension of our efforts to elucidate the origin of some of the strongly localized luminescence centres in silicon carbide we have studied the effect of hydrostatic pressure up to 10.5 kbar on ZPLs in the near-infrared abc photoluminescence spectrum of a hexagonal 6H SiC crystal at 77 K. As is known, the so-called abc spectrum can be observed at the wavelengths between 860 and 960 nm only in n-type SiC crystals of various polytypes 6H, 15R and 33R [3-7]. At liquid He and N 2 temperatures the spectrum consists of narrow ZPLs and their phonon replicas with the generation of an LA phonon having an energy of about 76meV. The number of lines as well as their spectral positions and 71
72
abc PHOTOLUMINESCENCE SPECTRUM OF 6H SiC CRYSTAL
that the lowest energy level in the excited electronic state of all the three luminescence centres responsible for the a, band c lines is located at about 80meV below the bottom of the conduction band. Accordingly, the ground state level of each centre must lie at an energy about 1.4eV lower (hv ~ 1.4eV for a, b, c lines), i.e. it is placed roughly in the middle of the band gap of the crystal (Eg ~ 3 eV for 6H SiC [2]). The chemical nature of the centres producing the near-infrared abc luminescence in SiC crystals is not yet clear. It has been found [6] that an abc luminescence of remarkable intensity appears even in the purest samples of 6H and 15R SiC (with accidental impurities at a level below 0.5 ppm). This circumstance makes the identification of the centres rather troublesome since very low concentrations of unintentional dopants may be involved. Based on some similarity between the spectral and polarization properties of the visible ABC luminescence and the near-infrared abc luminescence, which is present, as mentioned above, only in n-type crystals, it has been suggested [7] that in both cases the impurity atom responsible for the luminescence centre may have the same configuration of valence electrons, d 2s2, as is the case for a neutral Ti atom. In an n-type SiC crystal such a configuration may be realized by capturing an additional electron from the host lattice into a d shell of an impurity with d I i electron configuration, peculiar to the elements of scandium subgroup [7]. To our knowledge, there have been no additional proposals to these earlier speculations about the origin of the centres responsible for the abc emission, although in the last years notable progress has been achieved in the identification of various defects in SiC crystals (see, for instance, [8]). In this situation, any new information about these centres, including their behaviour under pressure, should be useful. To study the near-infrared abc photoluminescence of silicon carbide, reported in this paper, we used an unintentionally doped 6H SiC crystal with ntype conductivity. The crystal was grown in argon atmosphere with a low content of nitrogen (less than 0.003%). The concentration of uncompensated nitrogen donors amounted to 3 x 1017 cm- 3 . No acceptor impurities were specially added into the starting material. The high-pressure setup was the same as the one used in our previous work [1]. The sample was subjected to helium gas pressure in an optical cell made of a maraging steel. The cell was provided with two opposite sapphire windows. It was immersed in liquid nitrogen inside a simple vacuumless cryostat surrounded by a foam plastic envelope for thermal insulation. Hence, the sample temperature was very
Vol. 94, No.
close to 77 K in all our experiments. Pressure wa: measured with a calibrated manganin resistance gauge placed inside the final stage of a 15kbar gas compresso and thus maintained at room temperature. The abc luminescence of 6H SiC crystal wa: excited by near-ultraviolet radiation from a 1000" mercury lamp by using proper optical filters. The emission from the sample was analysed with an 80 err focal-length double grating monochromator (dispersion 10 Amm -I in the Ist order) by using a spectral slit width of lOA (1.4-1.7meV). A cooled photomultiplier sensitive to near-infrared radiation was used along with photon counting electronics. The spectra were recorded without polarization analysis of the emitted light. The results of the study are illustrated in Fig. I where the abc luminescence spectrum of a 6H SiC crystal at normal pressure (lower curve) and at a pressure of 1O.5kbar (upper curve) is presented. At 77 K three ZPLs a, band c as well as two phonon replicas a-76 and b-76 can be seen in the spectrum. The peak energies versus pressure for these ZPLs and their phonon replicas are given in Fig. 2. In general, the spectrum is quite similar to that published earlier [3], except for the lack of splitting of the a line. In fact, at 77 K this line was found [3J to consist of two components, labelled as a and a' and being caused by radiative transitions originating, respectively, from the lowest and from a higher, thermally populated level of the excited electronic state of the centre. The energy separation between these components is about 1.2meV. In our study they Photon Energy (eV)
1.34
1.38
1.42
'b 1nm -it-
,a
1.7meV
940
900
860
Wavelength (nm)
Fig. 1. Near-infrared abc photoluminescence spectrum of 6H SiC crystal at atmospheric pressure (lower curve) and at 10.5kbar (upper curve); T = 77 K. The spectrum measured at 10.5kbar is shifted somewhat upwards for clarity.
Table 1. Peak positions at atmospheric pressure and pressure coefficients for abc lines in 6H SiC nearinfrared photoluminescence spectrum at T = 77K
1.44
Line
Position* (eV)
Pressure shift t (mev kbar")
._____0
a
1.433 1.397 1.368 1.359 1.323
1.76 ± 0.04 1.74 ± 0.03 1.80 ± 0.03 1.73 ± 0.04 1.68 ± 0.04
o~
* Obtained from the values of 1/>. in vacuum . t Calculated as a slope of the least-squares straight
______0
>. Ol
~
C W
.--0 1.40 .... ~o
0
00
C
.9 o
.!:
73
abc PHOTOLUMINESCENCE SPECTRUM OF 6H SiC CRYSTAL
Vol. 94, No.1
....
Q.
c <$>
0 _0----__ 0---"
0
----
o~
b c a-76 b-76
line through the data points.
__0
_ _0
1.36
attributed to the radiative annihilation of bound excitons with emission of LA phonons. The pressure shifts of the lines in the abc _____ 0 luminescence spectrum may be compared with the shifts of various lines in the ABC spectrum of 6H SiC: Ti. The latter shifts, also toward s higher energies, ~Owere found to run between 1.08 and 1.25meVkbar- 1 I I 1.32 If' 1 [I]. It can be seen that the pressure shifts of the lines 10 8 6 o 2 4 in abc spectrum are, on the average, 50% larger than Pressure (kbar) those in the ABC spectrum. Fig. 2. Pressure dependence of the peak energy for As in the case of the ABC luminescence [1], various lines in the abc photoluminescence spectrum the large pressure shifts of the abc lines are in contrast of 6H SiC crystal at T = 77 K. to their very small temperature shifts, which in the range from 4.2 to 77 K do not exceed the average were not resolved (see Fig. I) because of insufficient width of these lines at 4.2 K [3] (about 0.3-0.4meV). spectral resolution (1.7 meV for the line a) due to The other two point s from our discussion on the rather a low intensity of abc luminescence in our behaviour of ABC lines [I] are also valid for abc lines: sample and light losses in the high pressure cell. (i) the pressure shifts of abc lines are surprisingly Pressure shifts of the lines along with their large considering the low compressibility of the hard spectral positions at atm ospheric pressure are given SiC crystals; (ii) these shifts are also large in in Table 1. Linear blue shifts of the lines occur , comparison with the expected marginal pressure the pressure coefficients ranging from 1.68 to shift of the absorption edge of a hexagonal 6H SiC 1.80meV kbar I for the individual lines. crystal in the opposite direction , towards lower Note that the phon on replicas a-76 and b-76 have energies (presumably only about - 0.4 meV kbar- J somewhat smaller pressure coefficients than the [1 D. respective ZPLs a and b. These small differences in The fact that the lines in both ABC and abc pressure shifts probably refer to the increase of the spectra are shifted under hydrostatic pressure in the energy of LA phonon with pressure . The pressure same direction and at a comparable average rate coefficient for that phonon thus obtained (0.03 or lends further support to the idea of a similar origin of 0.06meV kbar", see Table I) is in moderately good the centres under discussion, as mentioned earlier agreement with the pressure increase of the LA [7]. From the above it is plausible that the abc phonon energy in SiC crystals of 3C and 15R luminescence can be assigned to some impurit y atom polytypes obtained from the first-order Raman belonging to the scandium subgroup (Sc, Y, La, or spectra at 300 K (0.02-0.03 meV kbar"! as estimated some lanthanide atom) and having an electron from Fig. 2 in [9D and also with the pressure configuration d Ii [7]. Such an atom in n-type coefficient of 0.023 meV kbar- 1 for LA(X) phonon crystal can acquire the same configuration as a in cubic 3C SiC crystal estimated in [IO] from the neutral titanium atom, i.e. d 2i , by capturing an pressure shift of the photoluminescence line at 1.8K extra electron into its d shell. ." a-76
0-----
74
abc PHOTOLUMINESCENCE SPECTRUM OF 6H SiC CRYSTAL
All the elements of Sc subgroup have markedly larger atomic radii than the Ti atom (see, e.g. (11)) and, therefore, when such an atom substitutes Si atom in SiC crystal, the induced lattice distortion certainly exceeds that caused by the substitutional Ti atom. In this case a centre responsible for abc luminescence may have a quasi molecular character analogous to that of Ti centre [2] with the only quantitative difference that the greater lattice distortion leads to a stronger crystal field splitting of combined electronic levels of the centre and, as a consequence, also to larger pressure shifts of the lines in abc spectrum as compared to those in ABC spectrum. Such a possibility is in line with the earlier findings [6] that a very low impurity concentration is involved in the abc luminescence because it is evident that only a very limited quantity of impurity atoms with a large radius may be dissolved in a compact SiC crystal matrix. The most probable candidate for such impurity of scandium subgroup seems to be yttrium (Y) which was present in the crystal growing process of the samples that revealed a relatively intense abc luminescence (yttrium is an accompanying impurity in the zirconium ceramics which was used in a crystal growing furnace). The atomic radius of Y is 1.81 A against the value of 1.47 A for Ti [11] and therefore a larger lattice relaxation can be expected by the formation of such a point defect in silicon carbide. Unfortunately, to our knowledge there has been no elaborate trace impurity analysis of SiC crystals exhibiting abc luminescence, and convincing arguments for or against yttrium as an impurity giving rise to a quasimolecular titanium-like centre in SiC crystal are absent up to now. Also, other possibilities to explain the occurrence of abc luminescence in SiC (for instance, as caused by a more complex local centre) should not be discarded at present. One of the ways to tackle this problem might be to examine the possible isotope shifts and also the hyperfine structure of abc lines in doped SiC crystals at liquid helium temperatures by using highresolution spectroscopy methods. Note that the
Vol. 94, No. I
isotope structure of ABC luminescence lines [2, 12] was a crucial argument for attributing these lines to Ti impurity atoms in SiC lattice. Acknowledgement - The authors are indebted to Prof. M. Cardona for critically reading the manuscript and for valuable remarks.
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