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Band structure of hexagonal CdTe
Volume
ZEA, number
7
PHYSICS
BAND
STRUCTURE
Yu. P. GNATENKO,
Institute
LETTERS
of Physics.
Academy
OF
13 January
HEXAGONAL
CdTe
M. V. KURIK and V. V. MATLAK of Sciences
Received
of the Ukrainian
27 November
1969
SSR. Kiev.
USSR
1968
The reflection spectrum of hexagonal CdTe is measured in the region of exciton bands. The magnitudes of spin-orbit splitting, the splitting by the crystal field as well as the electron and hole effective masses are calculated.
Some semiconductor compounds of AIIBVI group can exist in two modifications: cubic (T2) and hexagonal (C&). Thus, two types of crys-d tals ZnS,HgS are known. As for the crystals of CdTe, they are, as a rule, of a cubic modification, whereas the films of CdTe may be of a hexagonal modification. It is shown in ref. 1 that hexagonal CdTe is obtained at 1000°K which turns into a stable cubic modification upon slow cooling down to room temperature. We could obtain crystals of hexagonal structure by the Bridgeman method as the melt was rapidly cooled from 1050’K down to room temperature in 5-6 minutes. It is possible that during this time the metastable phase does not turn to the stable cubic phase completely. According to the X-ray data, the hexagonal phase is indeed present in the crystal. Reflection spectra of CdTe were measured at ‘77OK on the natural cleavages of the crystal with normal incident light (fig. 1). Three reflection bands are pronounced in the spectrum. These bands, like those for CdS [2], are due to the three exciton bands. From the position of the reflection bands, the magnitudes of the spin-orbit splitting (A,,) as well as the splitting by the crystal field (A,.) were calculated. The band structure of hexagonal CdTe was assumed to be of the same form Parameters Crystal
Eg(eV)
symmetry
77OK
T$
1.592
0.95
'6~4
1.598
0.926
522
Aso (eV)
A,
6 5-9
f63
bCh I?52
2.5r
hwv)
2.56
--+
Fig. 1.
as that proposed for the wurtzite semiconductors of CdS type [3]. The values obtained are given in table 1 in comparison with those for the cubic CdTe [4]. Note a good agreement between A,, and AC and values obtained earlier for the hexagonal films of CdTe [5]. From the knowledge of E (forbidden gap) and Aso, the effective masses o B electrons and holes could be calculated. In the calculation of tensor components of effective masses, the expressions of [6] taking account of the interaction between bands of a corresponding wurtzite structure were used. The other necessary parameters, e.g. the values of matrix elements were taken from their extrapolation for the series CdS-CdSe-CdTe of wurtzite type. me and mh are also given in the table.
Table 1 of CdTe band structure. Ref.
mh/mo
me/m0
(eV)
0.046
L6/
_?hh
mln
msh
0.11
2.15
0.13
0.28
0.069 0.078
0.36 0.37
0.66 0.19
0.48 0.2s
141
Volume
28A, number
7
PHYSICS
LETTERS
13 January
1969
4. M.S. Brodin, M. V. Kurik, V. V. Matlak, B. I. Okiiabirskii, Fizika i Teknika Poluprovodnikov 2 (1968) 727. 5. Yu. E. Maronchuk, Author’s abstract of the candidates dissertation, Novosibirsk, 1966. 6. H.Hirosi, Bussei 6 (1965) 246.
References 1. J. Von Appel, 2. Naturforsch. 9a (1953) 265. 2. A. Lempicki, Proc. Phys. Sot. 74 (1959) 138. 3. J. J.Hopfield, J. Phys. Chem. Sol. 15 (1960) 97.
*****
NUCLEAR MAGNETIC RESONANCE OF 27A1 THE GARNET GROSSULARITE Ca3A12(Si04)3
IN
B. DERIGHETTI and S. GHOSE * Physik-Institut
der Received
UniversitEt 5 December
Ziirich, 1968
The quadrupole coupling constant e%Q/h for 27Al in the octahedral Ca3A12(SiG4)3 has been determined to be (3.609 l 0.006) MHz.
Grossularite Ca3A12(Si04)3 is a rather commun natural garnet, whose structure has been refined by Prandl [l] (1966). The Al-atoms, which occupy the 16(a) position in the space group Ia3d have the point symmetry 3, the symmetry axis being parallel to the (111) directions. The nuclear magnetic resonance of 27Al in a rose coloured natural grossularite from Canada has been investigated at room temperature. The crystal was rotated with one of its [llO] axes normal to the magnetic field. The angular dependence of the measured NMR lines revealed the presence of 3 sets of Al-spectra (fig. 1). Two of these sets have similar angular behaviour but are displaced 110’ from one another. At the crystal orientations (fig. 2) showing the maximum splitting the satellites are equidistant (488 gauss). These spectra have been assigned to Al-nuclei in axially symmetric field gradients, and have been correlated to the Al-sites whose symmetry axes (3) are parallel to the two [ill] directions, which
Switzerland
position
in the garnet grossularite
lie on the (110) plane, the magnetic field direction being always in this plane.
* Present address: Flight Center,
Planetology Branch, Goddard Space NASA, Greenbelt. Maryland, USA.
Fig. 1. Angular dependence of measured (points) and calculated (solid and dotted lines) NMR lines of 27Al at 9.2992 MHz for a rotation with the magnetic field perpendicular to the [IlO] direction (e2qQ /h = 3.609 MHz). The linewidths are approximately indicated. The origin of the angle is chosen for the magnetic field parallel to the [ilO] direction.
0’
w
60-
900
PO-
150'
1800
523
ZEA, number
7
PHYSICS
BAND
STRUCTURE
Yu. P. GNATENKO,
Institute
LETTERS
of Physics.
Academy
OF
13 January
HEXAGONAL
CdTe
M. V. KURIK and V. V. MATLAK of Sciences
Received
of the Ukrainian
27 November
1969
SSR. Kiev.
USSR
1968
The reflection spectrum of hexagonal CdTe is measured in the region of exciton bands. The magnitudes of spin-orbit splitting, the splitting by the crystal field as well as the electron and hole effective masses are calculated.
Some semiconductor compounds of AIIBVI group can exist in two modifications: cubic (T2) and hexagonal (C&). Thus, two types of crys-d tals ZnS,HgS are known. As for the crystals of CdTe, they are, as a rule, of a cubic modification, whereas the films of CdTe may be of a hexagonal modification. It is shown in ref. 1 that hexagonal CdTe is obtained at 1000°K which turns into a stable cubic modification upon slow cooling down to room temperature. We could obtain crystals of hexagonal structure by the Bridgeman method as the melt was rapidly cooled from 1050’K down to room temperature in 5-6 minutes. It is possible that during this time the metastable phase does not turn to the stable cubic phase completely. According to the X-ray data, the hexagonal phase is indeed present in the crystal. Reflection spectra of CdTe were measured at ‘77OK on the natural cleavages of the crystal with normal incident light (fig. 1). Three reflection bands are pronounced in the spectrum. These bands, like those for CdS [2], are due to the three exciton bands. From the position of the reflection bands, the magnitudes of the spin-orbit splitting (A,,) as well as the splitting by the crystal field (A,.) were calculated. The band structure of hexagonal CdTe was assumed to be of the same form Parameters Crystal
Eg(eV)
symmetry
77OK
T$
1.592
0.95
'6~4
1.598
0.926
522
Aso (eV)
A,
6 5-9
f63
bCh I?52
2.5r
hwv)
2.56
--+
Fig. 1.
as that proposed for the wurtzite semiconductors of CdS type [3]. The values obtained are given in table 1 in comparison with those for the cubic CdTe [4]. Note a good agreement between A,, and AC and values obtained earlier for the hexagonal films of CdTe [5]. From the knowledge of E (forbidden gap) and Aso, the effective masses o B electrons and holes could be calculated. In the calculation of tensor components of effective masses, the expressions of [6] taking account of the interaction between bands of a corresponding wurtzite structure were used. The other necessary parameters, e.g. the values of matrix elements were taken from their extrapolation for the series CdS-CdSe-CdTe of wurtzite type. me and mh are also given in the table.
Table 1 of CdTe band structure. Ref.
mh/mo
me/m0
(eV)
0.046
L6/
_?hh
mln
msh
0.11
2.15
0.13
0.28
0.069 0.078
0.36 0.37
0.66 0.19
0.48 0.2s
141
Volume
28A, number
7
PHYSICS
LETTERS
13 January
1969
4. M.S. Brodin, M. V. Kurik, V. V. Matlak, B. I. Okiiabirskii, Fizika i Teknika Poluprovodnikov 2 (1968) 727. 5. Yu. E. Maronchuk, Author’s abstract of the candidates dissertation, Novosibirsk, 1966. 6. H.Hirosi, Bussei 6 (1965) 246.
References 1. J. Von Appel, 2. Naturforsch. 9a (1953) 265. 2. A. Lempicki, Proc. Phys. Sot. 74 (1959) 138. 3. J. J.Hopfield, J. Phys. Chem. Sol. 15 (1960) 97.
*****
NUCLEAR MAGNETIC RESONANCE OF 27A1 THE GARNET GROSSULARITE Ca3A12(Si04)3
IN
B. DERIGHETTI and S. GHOSE * Physik-Institut
der Received
UniversitEt 5 December
Ziirich, 1968
The quadrupole coupling constant e%Q/h for 27Al in the octahedral Ca3A12(SiG4)3 has been determined to be (3.609 l 0.006) MHz.
Grossularite Ca3A12(Si04)3 is a rather commun natural garnet, whose structure has been refined by Prandl [l] (1966). The Al-atoms, which occupy the 16(a) position in the space group Ia3d have the point symmetry 3, the symmetry axis being parallel to the (111) directions. The nuclear magnetic resonance of 27Al in a rose coloured natural grossularite from Canada has been investigated at room temperature. The crystal was rotated with one of its [llO] axes normal to the magnetic field. The angular dependence of the measured NMR lines revealed the presence of 3 sets of Al-spectra (fig. 1). Two of these sets have similar angular behaviour but are displaced 110’ from one another. At the crystal orientations (fig. 2) showing the maximum splitting the satellites are equidistant (488 gauss). These spectra have been assigned to Al-nuclei in axially symmetric field gradients, and have been correlated to the Al-sites whose symmetry axes (3) are parallel to the two [ill] directions, which
Switzerland
position
in the garnet grossularite
lie on the (110) plane, the magnetic field direction being always in this plane.
* Present address: Flight Center,
Planetology Branch, Goddard Space NASA, Greenbelt. Maryland, USA.
Fig. 1. Angular dependence of measured (points) and calculated (solid and dotted lines) NMR lines of 27Al at 9.2992 MHz for a rotation with the magnetic field perpendicular to the [IlO] direction (e2qQ /h = 3.609 MHz). The linewidths are approximately indicated. The origin of the angle is chosen for the magnetic field parallel to the [ilO] direction.
0’
w
60-
900
PO-
150'
1800
523