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(001) GaAs interface
Journal of Crystal Growth 127 (1993) 339—342 North-Holland
jo~~ o~
CRYSTAL GROWT H
Formation of the ZnTe/(OO1) GaAs interface S. Tatarenko a J• Cibert a K. Saminadayar M. Sauvage-Simkin C and R. Pinchaux b
b
PH. Jouneau b V.H. Etgens
C
Laboratoire de Spectrométrie Physique, CNRS, Université J. Fourier, BP 87, F-38042 Saint-Martin-d’Hères, France Laboratoire de Physique des Semiconducteurs, DRFMC/SP2M, BP 85X, F-38041 Grenoble, France CNRS — LURE, Bat. 209D, F-91405 Orsay, France
We report on the formation of the ZnTe/(O01) GaAs interface by molecular beam epitaxy. Techniques include X-ray photoelectron spectroscopy (XPS), high energy electron diffraction, grazing incidence X-ray diffraction and high resolution transmission electron microscopy. For film thicknesses smaller than 5 monolayers (ML), a two-dimensional (2D) growth of ZnTe on the As-rich c(4 x 4) reconstructed (001) GaAs surface is demonstrated. Analysis of the XPS Te 3d and Zn2~, signals from 1—4 ML thick ZnTe films reveals the existence of an interfacial As—Zn bonding state. Thus the 2D ZnTe growth on (001) GaAs is initiated by a Ga—As—Zn—Te sequence, in contrast with the GaAs—Te—Cd sequence in CdTe/(001) GaAs growth.
1. Introduction ZnTe can be used as intermediate buffer layer in the molecular beam epitaxy (MBE) growth of (001) CdTe on (001) GaAs substrate since it has an intermediate lattice parameter [11:in the case of CdTe, the interface structure is GaAs—Te—Cd, with Te bound to Ga or As [2]. On (001) GaAs substrate, CdTe grows (111) or, like ZnTe, (001), thus is often assumed that ZnTe growth starts with Te. We have already reported a study of ZnTe growth on the Te—GaAs (001) precursor surface: In this case, islanding and three-dimensional (3D) growth occur, yielding difficulties in the interpretation of the experimental results since an uncovered Te—GaAs surface remains between ZnTe islands [31.The present results concern the growth by direct exposure of an As-rich c(4 x 4) reconstructed GaAs (001) surface. It will be shown that a two-dimensional (2D) growth is observed which makes the interpretation of the results much easier. 2. Experimental procedure Experimental techniques include X-ray photoelectron spectroscopy (XPS), high energy electron 0022-0248/93/$06.00 © 1993
—
diffraction (RHEED), grazing incidence X-ray diffraction (GIXD) and 400 kV high resolution transmission electron microscopy (HRTEM). Growth is performed in two different systems. The first is a 2300 Riber MBE system equipped with RHEED and XPS: 50 keV electron gun for RHEED and MgKa line (1253.6 eV) as X-ray source. The second system, dedicated to GIXD measurements, is located at the LURE synchrotron facility: X-ray measurements are performed in an ultrahigh vacuum compatible 4-circle diffractometer coupled to a MBE chamber. In the two systems, ZnTe layers are grown using a ZnTe effusion cell, typically at a growth rate of 0.1 to 0.3 monolayer (ML) per second. Samples for HRTEM study are protected by an amorphous GaAs layer of about 100 A (fig. 1).
3. Results and discussion When growth of ZnTe is initiated on the Asrich c(4 x 4) reconstructed (001) GaAs surface [41,a 2D growth is evidenced by the observation of a streaky (2 x 1) reconstructed RHEED pattern along with RHEED oscillations [5]. This result is confirmed by HRTEM. Figs. la and lb present micrographs of a 5 ML thick ZnTe film
Elsevier Science Publishers B.V. All rights reserved
340
S. Tatarenko ~ al.
/ Formation of ZnTe/(OOI)
GaAs interface
which is obtained under this condition: A continuous strained layer is observed. Such a 2D growth is observed for thicknesses lower than 5 ML; otherwise relaxation of the layer and islanding occur, inducing a 3D growth as illustrated in fig. ic for a 30 ML layer. Precise measurement of the
This 2D growth of ZnTe is not observed for other (001) GaAs or Te—GaAs reconstructed sur-
thickness is made possible using RHEED oscillations and HRTEM. The misfit at the interface is accommodated through the introduction in the layer of stacking faults and different types of dislocations [6]. Further confirmation of the relaxation process comes from sequence GIXD cxperiments on ZnTe epilayers with increasing thickness: evolution of the (200) reflection profile of the heterostructure demonstrates a perfect elastically accommodated layer for film thicknesses below 5 ML, but a plastic relaxation of the misfit strain takes place for thicknesses higher than 6 ML [7].
1019.6, 1019.2 and 1020.1 eV, and two for Te: Teb and Te~at 572.6 and 573.0 eV; the subscripts b, i and s refer to bulk, interface or surface atoms, as will be shown below. Figs. 2a and 2b show the Te3d~2 and the Zn2~~2XPS line recorded for a ~ ML layer. From deconvolution of fig. 2a, we can extract peaks Teb and Te~.For fig. 2b the same treatment leads to peaks Znh and Zn1. Attributions of peaks Teb and Zflb rely on the fact that they dominate the spectrum of bulk ZnTe [3]. A thin strained ZnTe layer in a ZnTe/GaAs heteroepitaxy should induce bonding states,
a
.~
faces [8]. Conditions inducing such a 2D growth are studied by XPS of continuous layers. We shall show that XPS spectra (figs. 2 and 3) involve three components for Zn: Zflb, Zn1 and Zn~ at
b
Fig. 1. HRTEM micrographs of (001) ZnTe layers grown on an As-rich c(4X 4) GaAs (001) reconstructed surface. (a), (b) Coherent 5 ML thick ZnTe layer viewed along [1101and [1001; a 2D growth is observed without islanding. (c) Relaxed 30 ML thick ZnTe layer viewed along [110]: strain relaxation induces formation of islands and nucleation of misfit dislocations in the layer.
S. Tatarenko ci a!.
/ Formation
of ZnTe / (001) GaAs interface
Zn
namely interface or surface states, different from that of Zn and Te atoms in bulk ZnTe: in fig. 2, peaks Zn1, Zn~and Te~are such specific bonding states. We checked this assumption by an XPS study of increasingly thick layers (figs. 2b—2d): a progressive decrease of peak Zn1 is observed. The point is then to determine the localization of the atoms (surface or interface). This is done by modifying the surface of the sample. While the growing surface is usually a (2 x 1) reconstructed surface (which is attributed to excess Te and formation of Te dimers [9]), we can change it to a c(2 x 2) (i.e. Zn stabilized [9])just by exposing the sample to a Zn beam, at 340°C(figs. 2e and 3a). Then: peak Te5 disappears (fig. 2e), showing that it has to be attributed to surface Te atoms which were present on the Te-rich growing surface, while peak Zn5 appears (fig. 3a), which we
Z ‘~ 2
__________
b
b2ML
~ 1020
1018
BindingEnergy (eV)
Fig. 3. Zn213312 XPS line recorded on a coherent ZnTe layer grown on an As-rich c(4X4) GaAs (001) and exposed to a Zn flux at 340°C(a) or on a (* x 3) Te—GaAs (001) surface (b). Peaks Znh (1019.6 eV), Zn, (1020,1 eV) and Zn1 (1019.2 eV) are attributed to bulk, surface and interface Zn atoms, respectively.
ML
Te __________________
~
assign to the new Zn surface atoms. Note that peak Zn1 (fig. 3a) can be due to interface Zn there is no evidence of interface Te atoms, while atoms, first because a second Zn configuration (different from the configuration giving peak Zn5) is improbable on the Te-rich surface and more convincingly because this Zn1 does not appear if the 2 ML layer is grown on a saturated (* X 3) Te—GaAs (001) surface [10],bonds i.e., a(fig. surface presents essentially 3b) which To sum up (figs. Te—Ga 2 and 3), on the Te3d52 XPS line, we have identified one bulk peak Teb and ~
a
one surface peak Te5 while on the Zn2 P3/2 XPS line we peak Zn5have and one one interface bulk peak peak Znb, Zn1.one surface
b2~
~
573
341
571
1020
018
Binding Energy (eV) Fig. 2. Te3d572 and Zn21,372 XPS lines recorded on a coherent ZnTe layer grown on an As-rich c(4X4) GaAs (001) (a to d) and exposed to a Zn flux at 340°C(e). Peaks Teb (572.6 eV) and Te, (573.0 eV) are attributed to bulk and surface Te and peaks. Peaks Znb (1019.6 eV), Zn~(1019.2 eV) and Zn5 (1020,1 eV) are attributed to bulk, interface and surface Zn atoms, respectively,
4. Summary and conclusion We demonstrate that a 2D growth of ZnTe on As-rich c(4 x 4) GaAs (001) substrate is observed #1
Here, * refers to an incommensurate phase in the 1110] direction.
342
S. Tatarenko ci al.
/ Formation of ZnTe / (00])
if the layer thickness stays below 5 ML; otherwise, plastic relaxation occurs inducing islanding and 3D growth. The 2D growth is due to the formation of a Zn/GaAs interface which is dif ferent from that obtained for growth on either Te—GaAs precursor surfaces or Ga-rich GaAs surfaces. Comparing the (2 x 1) ZnTe and the (2 x 4) GaAs reconstructions, we suggest that the relationship between ZnTe and GaAs consists of a Zn—As interface: the half order in the (2 X 1) ZnTe/GaAs surface (Te dimers) is in the same direction as the half order in the well-known (2 x 4) GaAs (001) surface (As dimers). We note that a (1 x 2) surface structure is observed when Zn atoms are deposited on an As-rich c(4 X 4) GaAs surface [11] while no Cd sticks on GaAs. This means that the stabilized Zn/GaAs (001) presents As—Zn bonds. Thus, Zn atoms occupy the Ga sites while Te atoms are on the As sites: as a consequence, the 2D growth is initiated by a Ga—As—Zn—Te sequence, different from the well-known GaAs—Te—Cd sequence assumed in the case of CdTe/GaAs (001) growth [12]. This excludes, for this case of ZnTe/GaAs, the formation of a Ill—VI interface which has been demonstrated for other systems [13]. The nature of the interface on the other GaAs surfaces is still to be elucidated and it should be interesting to apply these techniques also to the growth of ZnTe on GaSb and ZnSe on GaAs.
GaAs interface
References [1] See for instance, RD. Feldman, R.F. Austin, A.H. Dayem and E.H. Westerwick, AppI. Phys. Letters 49 (1986) 797. [2] HA. Mar, N. Salansky and K.T. Chee, Appi. Phys. Letters 44 (1984) 898; J.P. Faurie, C. Hsu, S. Sivanathan and X. Chu, Surface Sci. 168 (1986) 473. [3] S. Tatarenko, K. Saminadayar and J. Cibert, AppI. Phys. Letters 51(1987)1690. [4] P.K. Larsen, J.N. Neave, iF. van der Veen, P.J. Dobson and B.A. Joyce, Phys. Rev. B 27 (1983) 4966. [5] J. Cibert, R. André, C. Deshayes, G. Feuillet, P.11. Jouneau, Le Si Dang, R. Mallard, A. Nahmani, K. Saminadayar and S. Tatarenko, Superlattices Microstruct. 9 (1991) 271. [6] G. Feuillet, J. Cibert, Y. Gobil, PH. Jouneau, S. Tatarenko, K. Saminadayar, AC. Chami and E. Ligeon, Phys. Scripta T 35 (1991) 268. [71yE. Etgens, R. Pinchaux, M. Sauvage-Simkin, J. Massies, N. Jedrecy, N. Greiser, A. Waldhauer and S. Tatarenko, Appi. Surface. Sci. 56/57 (1992) 597. [8] Y. Gobil, J. Cibert, K. Saminadayar and S. Tatarenko, Surface Sci. 211/212 (1989) 969. [9] T. Yao and T. Takeda, Appi. Phys. Letters 48 (1986) 160. [10] J. Cibert, K. Saminadayar, S. Tatarenko and Y. Gobil, Phys. Rev. B 39 (1989) 12047. [11] The As-rich c(4x4) GaAs (001) surface is terminated by a double arsenic layer:
M. Sauvage-Simkin, R. Pinchaux, J. Massies, P. Claverie, N. Jedreny, I. Bonnet and 1K. Robinson, Phys. Rev. Letters 62 (1989) 563. [12] G. Cohen-Solal, F. Bailly and M. Barbe, AppI. Phys. Letters 49 (1986) 1519. [13] M. Kolodziejczyk, T. Filz, A. Krost, W. Richter and D.R.T. Zahn, J. Crystal Growth 117 (1992) 549.
jo~~ o~
CRYSTAL GROWT H
Formation of the ZnTe/(OO1) GaAs interface S. Tatarenko a J• Cibert a K. Saminadayar M. Sauvage-Simkin C and R. Pinchaux b
b
PH. Jouneau b V.H. Etgens
C
Laboratoire de Spectrométrie Physique, CNRS, Université J. Fourier, BP 87, F-38042 Saint-Martin-d’Hères, France Laboratoire de Physique des Semiconducteurs, DRFMC/SP2M, BP 85X, F-38041 Grenoble, France CNRS — LURE, Bat. 209D, F-91405 Orsay, France
We report on the formation of the ZnTe/(O01) GaAs interface by molecular beam epitaxy. Techniques include X-ray photoelectron spectroscopy (XPS), high energy electron diffraction, grazing incidence X-ray diffraction and high resolution transmission electron microscopy. For film thicknesses smaller than 5 monolayers (ML), a two-dimensional (2D) growth of ZnTe on the As-rich c(4 x 4) reconstructed (001) GaAs surface is demonstrated. Analysis of the XPS Te 3d and Zn2~, signals from 1—4 ML thick ZnTe films reveals the existence of an interfacial As—Zn bonding state. Thus the 2D ZnTe growth on (001) GaAs is initiated by a Ga—As—Zn—Te sequence, in contrast with the GaAs—Te—Cd sequence in CdTe/(001) GaAs growth.
1. Introduction ZnTe can be used as intermediate buffer layer in the molecular beam epitaxy (MBE) growth of (001) CdTe on (001) GaAs substrate since it has an intermediate lattice parameter [11:in the case of CdTe, the interface structure is GaAs—Te—Cd, with Te bound to Ga or As [2]. On (001) GaAs substrate, CdTe grows (111) or, like ZnTe, (001), thus is often assumed that ZnTe growth starts with Te. We have already reported a study of ZnTe growth on the Te—GaAs (001) precursor surface: In this case, islanding and three-dimensional (3D) growth occur, yielding difficulties in the interpretation of the experimental results since an uncovered Te—GaAs surface remains between ZnTe islands [31.The present results concern the growth by direct exposure of an As-rich c(4 x 4) reconstructed GaAs (001) surface. It will be shown that a two-dimensional (2D) growth is observed which makes the interpretation of the results much easier. 2. Experimental procedure Experimental techniques include X-ray photoelectron spectroscopy (XPS), high energy electron 0022-0248/93/$06.00 © 1993
—
diffraction (RHEED), grazing incidence X-ray diffraction (GIXD) and 400 kV high resolution transmission electron microscopy (HRTEM). Growth is performed in two different systems. The first is a 2300 Riber MBE system equipped with RHEED and XPS: 50 keV electron gun for RHEED and MgKa line (1253.6 eV) as X-ray source. The second system, dedicated to GIXD measurements, is located at the LURE synchrotron facility: X-ray measurements are performed in an ultrahigh vacuum compatible 4-circle diffractometer coupled to a MBE chamber. In the two systems, ZnTe layers are grown using a ZnTe effusion cell, typically at a growth rate of 0.1 to 0.3 monolayer (ML) per second. Samples for HRTEM study are protected by an amorphous GaAs layer of about 100 A (fig. 1).
3. Results and discussion When growth of ZnTe is initiated on the Asrich c(4 x 4) reconstructed (001) GaAs surface [41,a 2D growth is evidenced by the observation of a streaky (2 x 1) reconstructed RHEED pattern along with RHEED oscillations [5]. This result is confirmed by HRTEM. Figs. la and lb present micrographs of a 5 ML thick ZnTe film
Elsevier Science Publishers B.V. All rights reserved
340
S. Tatarenko ~ al.
/ Formation of ZnTe/(OOI)
GaAs interface
which is obtained under this condition: A continuous strained layer is observed. Such a 2D growth is observed for thicknesses lower than 5 ML; otherwise relaxation of the layer and islanding occur, inducing a 3D growth as illustrated in fig. ic for a 30 ML layer. Precise measurement of the
This 2D growth of ZnTe is not observed for other (001) GaAs or Te—GaAs reconstructed sur-
thickness is made possible using RHEED oscillations and HRTEM. The misfit at the interface is accommodated through the introduction in the layer of stacking faults and different types of dislocations [6]. Further confirmation of the relaxation process comes from sequence GIXD cxperiments on ZnTe epilayers with increasing thickness: evolution of the (200) reflection profile of the heterostructure demonstrates a perfect elastically accommodated layer for film thicknesses below 5 ML, but a plastic relaxation of the misfit strain takes place for thicknesses higher than 6 ML [7].
1019.6, 1019.2 and 1020.1 eV, and two for Te: Teb and Te~at 572.6 and 573.0 eV; the subscripts b, i and s refer to bulk, interface or surface atoms, as will be shown below. Figs. 2a and 2b show the Te3d~2 and the Zn2~~2XPS line recorded for a ~ ML layer. From deconvolution of fig. 2a, we can extract peaks Teb and Te~.For fig. 2b the same treatment leads to peaks Znh and Zn1. Attributions of peaks Teb and Zflb rely on the fact that they dominate the spectrum of bulk ZnTe [3]. A thin strained ZnTe layer in a ZnTe/GaAs heteroepitaxy should induce bonding states,
a
.~
faces [8]. Conditions inducing such a 2D growth are studied by XPS of continuous layers. We shall show that XPS spectra (figs. 2 and 3) involve three components for Zn: Zflb, Zn1 and Zn~ at
b
Fig. 1. HRTEM micrographs of (001) ZnTe layers grown on an As-rich c(4X 4) GaAs (001) reconstructed surface. (a), (b) Coherent 5 ML thick ZnTe layer viewed along [1101and [1001; a 2D growth is observed without islanding. (c) Relaxed 30 ML thick ZnTe layer viewed along [110]: strain relaxation induces formation of islands and nucleation of misfit dislocations in the layer.
S. Tatarenko ci a!.
/ Formation
of ZnTe / (001) GaAs interface
Zn
namely interface or surface states, different from that of Zn and Te atoms in bulk ZnTe: in fig. 2, peaks Zn1, Zn~and Te~are such specific bonding states. We checked this assumption by an XPS study of increasingly thick layers (figs. 2b—2d): a progressive decrease of peak Zn1 is observed. The point is then to determine the localization of the atoms (surface or interface). This is done by modifying the surface of the sample. While the growing surface is usually a (2 x 1) reconstructed surface (which is attributed to excess Te and formation of Te dimers [9]), we can change it to a c(2 x 2) (i.e. Zn stabilized [9])just by exposing the sample to a Zn beam, at 340°C(figs. 2e and 3a). Then: peak Te5 disappears (fig. 2e), showing that it has to be attributed to surface Te atoms which were present on the Te-rich growing surface, while peak Zn5 appears (fig. 3a), which we
Z ‘~ 2
__________
b
b2ML
~ 1020
1018
BindingEnergy (eV)
Fig. 3. Zn213312 XPS line recorded on a coherent ZnTe layer grown on an As-rich c(4X4) GaAs (001) and exposed to a Zn flux at 340°C(a) or on a (* x 3) Te—GaAs (001) surface (b). Peaks Znh (1019.6 eV), Zn, (1020,1 eV) and Zn1 (1019.2 eV) are attributed to bulk, surface and interface Zn atoms, respectively.
ML
Te __________________
~
assign to the new Zn surface atoms. Note that peak Zn1 (fig. 3a) can be due to interface Zn there is no evidence of interface Te atoms, while atoms, first because a second Zn configuration (different from the configuration giving peak Zn5) is improbable on the Te-rich surface and more convincingly because this Zn1 does not appear if the 2 ML layer is grown on a saturated (* X 3) Te—GaAs (001) surface [10],bonds i.e., a(fig. surface presents essentially 3b) which To sum up (figs. Te—Ga 2 and 3), on the Te3d52 XPS line, we have identified one bulk peak Teb and ~
a
one surface peak Te5 while on the Zn2 P3/2 XPS line we peak Zn5have and one one interface bulk peak peak Znb, Zn1.one surface
b2~
~
573
341
571
1020
018
Binding Energy (eV) Fig. 2. Te3d572 and Zn21,372 XPS lines recorded on a coherent ZnTe layer grown on an As-rich c(4X4) GaAs (001) (a to d) and exposed to a Zn flux at 340°C(e). Peaks Teb (572.6 eV) and Te, (573.0 eV) are attributed to bulk and surface Te and peaks. Peaks Znb (1019.6 eV), Zn~(1019.2 eV) and Zn5 (1020,1 eV) are attributed to bulk, interface and surface Zn atoms, respectively,
4. Summary and conclusion We demonstrate that a 2D growth of ZnTe on As-rich c(4 x 4) GaAs (001) substrate is observed #1
Here, * refers to an incommensurate phase in the 1110] direction.
342
S. Tatarenko ci al.
/ Formation of ZnTe / (00])
if the layer thickness stays below 5 ML; otherwise, plastic relaxation occurs inducing islanding and 3D growth. The 2D growth is due to the formation of a Zn/GaAs interface which is dif ferent from that obtained for growth on either Te—GaAs precursor surfaces or Ga-rich GaAs surfaces. Comparing the (2 x 1) ZnTe and the (2 x 4) GaAs reconstructions, we suggest that the relationship between ZnTe and GaAs consists of a Zn—As interface: the half order in the (2 X 1) ZnTe/GaAs surface (Te dimers) is in the same direction as the half order in the well-known (2 x 4) GaAs (001) surface (As dimers). We note that a (1 x 2) surface structure is observed when Zn atoms are deposited on an As-rich c(4 X 4) GaAs surface [11] while no Cd sticks on GaAs. This means that the stabilized Zn/GaAs (001) presents As—Zn bonds. Thus, Zn atoms occupy the Ga sites while Te atoms are on the As sites: as a consequence, the 2D growth is initiated by a Ga—As—Zn—Te sequence, different from the well-known GaAs—Te—Cd sequence assumed in the case of CdTe/GaAs (001) growth [12]. This excludes, for this case of ZnTe/GaAs, the formation of a Ill—VI interface which has been demonstrated for other systems [13]. The nature of the interface on the other GaAs surfaces is still to be elucidated and it should be interesting to apply these techniques also to the growth of ZnTe on GaSb and ZnSe on GaAs.
GaAs interface
References [1] See for instance, RD. Feldman, R.F. Austin, A.H. Dayem and E.H. Westerwick, AppI. Phys. Letters 49 (1986) 797. [2] HA. Mar, N. Salansky and K.T. Chee, Appi. Phys. Letters 44 (1984) 898; J.P. Faurie, C. Hsu, S. Sivanathan and X. Chu, Surface Sci. 168 (1986) 473. [3] S. Tatarenko, K. Saminadayar and J. Cibert, AppI. Phys. Letters 51(1987)1690. [4] P.K. Larsen, J.N. Neave, iF. van der Veen, P.J. Dobson and B.A. Joyce, Phys. Rev. B 27 (1983) 4966. [5] J. Cibert, R. André, C. Deshayes, G. Feuillet, P.11. Jouneau, Le Si Dang, R. Mallard, A. Nahmani, K. Saminadayar and S. Tatarenko, Superlattices Microstruct. 9 (1991) 271. [6] G. Feuillet, J. Cibert, Y. Gobil, PH. Jouneau, S. Tatarenko, K. Saminadayar, AC. Chami and E. Ligeon, Phys. Scripta T 35 (1991) 268. [71yE. Etgens, R. Pinchaux, M. Sauvage-Simkin, J. Massies, N. Jedrecy, N. Greiser, A. Waldhauer and S. Tatarenko, Appi. Surface. Sci. 56/57 (1992) 597. [8] Y. Gobil, J. Cibert, K. Saminadayar and S. Tatarenko, Surface Sci. 211/212 (1989) 969. [9] T. Yao and T. Takeda, Appi. Phys. Letters 48 (1986) 160. [10] J. Cibert, K. Saminadayar, S. Tatarenko and Y. Gobil, Phys. Rev. B 39 (1989) 12047. [11] The As-rich c(4x4) GaAs (001) surface is terminated by a double arsenic layer:
M. Sauvage-Simkin, R. Pinchaux, J. Massies, P. Claverie, N. Jedreny, I. Bonnet and 1K. Robinson, Phys. Rev. Letters 62 (1989) 563. [12] G. Cohen-Solal, F. Bailly and M. Barbe, AppI. Phys. Letters 49 (1986) 1519. [13] M. Kolodziejczyk, T. Filz, A. Krost, W. Richter and D.R.T. Zahn, J. Crystal Growth 117 (1992) 549.