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Fe schottky barrier to Cd1−xFexTe semimagnetic semiconductor
applied surface s c i e n c e ELSEVIER
Applied Surface Science 123/124 (1998) 631-635
From CdTe/Fe Schottky barrier to _xFexTesemimagnetic semiconductor B.A. Orlowski a.*, E. G u z i e w i c z , B.J. Kowalski " N. Barrett b,c R. Belkhou b,c, D. Radosavkic b,c, D. Martinotti b,c, C. Guillot b.c, j._p. Lacharme d, C.A. Sdbenne d '~ Institute o/'Physics, Polish Academy (~['Sciences, Warsaw, Poland b LURE, Orsay, France c DRECAM-SRSIM, Saclay, France d Laboratoire de Min~';ralogie-C(vstallographie, URA9-CNRS, Unil,ersitg Pierre et Marie Curie, Paris, France
Abstract Synchrotron radiation tuned to the Fe 3p-3d transition ( h u = 56 eV) was used to study Fano type resonant photoemission spectra lbr a clean CdTe(110) surface sequentially covered in the monolayer (ML) range by Fe atoms and annealed. The results showed that, in the first stage of the deposition, the Fe atoms are mainly involved in the creation of a Cd~ .~Fe,Te ternary crystal (0.2-0.6 ML). At higher coverages (1.2 ML), the contribution of Fe metallic islands becomes visible. CdTe dissociation in the surface region leads to the appearance of C d - F e interaction at higher Fe deposition (3.6 ML). After deposition of 20 ML, sample annealing in the 300°C range leads to Fe diffusion into the crystal and the measured spectra correspond well to the Cd~ ,Fe,Te calculated spectra. © 1998 Elsevier Science B.V. PACS: 73.20.
r: 79.60.Jv
Kevwords: Resonant photoemission: Semimagnetic semiconductors: Schottky barrier formation
1. Introduction
The present study is a new approach to investigate the C d T e / F e interface formation and to observe the changes in the valence band electronic structure which accompany the appearance of a superficial Cd l_xFe,Te semimagnetic semiconductor (SMSC) [1]. The study is strongly stimulated by the recent research of the electronic properties of interfaces and superstructures grown with SMSC [2]. The present
* Corresponding author.
results concern the actual Schottky barrier and apply to the case when the deposited metal reacts with the semiconductor compound surface. In this paper results obtained for the Fe atoms deposited on clean CdTe(1 10) surface are presented. The procedure of the experiment is similar to that applied to Fe atoms deposited on a Cd I .,Fe,Se(110) surface [3]. For Cd~_ ,Fe ,Se crystal, iron atoms were already introduced into the bulk of the crystal by a modified Bridgman method; additional Fe atoms were deposited on the freshly cleaved surface of a ternary crystal Cd0 86Feo.14Se. The Fe atoms can be involved in two stages: as a bulk component of the SMSC
0169-4332/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0169-4332(97)00560-6
632
B.A. Orlowski et al./ Applied Surface Science 123/124 (1998) 631-635
crystal, and then as metallic islands of iron created along the crystal surface. For C d j _ , F e , S e crystal the limit of Fe solubility is about x = 0.16 while for Cdl_,Fe~Te it is x = 0.03 [4]. Photoemission EDC data for Cdj_xFexTe bulk crystal are not yet available in the literature mainly because of this low x limit. However, the contribution of Fe 3d electrons has been calculated for C d l _ , FexTe crystal by Masek [5] using the coherent potential approximation method. The results of this DOS calculation are in good agreement with the EDC's measured by us for a Cd I ~FexTe layer prepared and presented in the paper. The Fe impurity level is located 0.16 eV above the valence band edge of CdTe and the valence band edge of the Cdl_,Fe~Te SMSC alloy, once formed, will be pinned about there.
Fe CdTe(110)/Fe
coverage i
4oo hv=56 eV J
1
3oo
V e"
Jr:" !
•
I
i%
•
"
,--
."
oo.
2. Experimental conditions and results The samples of CdTe were grown by the modified Bridgman method [6] in the Institute of Physics, Polish Academy of Sciences. The clean (110) surface of the CdTe crystal was obtained in the preparation chamber by Ar + sputtering (600 eV, 10 /xA, 30 min) and then by vacuum annealing at about 270°C. The experimental conditions were similar to those applied to Fe atoms deposited on a Cd 1 xFexSe(110) surface [3]. In Fig. 1 the set of EDC's measured for both the valence band and the Cd 4d core level electrons after the sequential Fe deposition on the clean CdTe(110) surface is presented. The Fe coverage starts at 0.2 ML and increases by steps up to 20 ML. The lowest curve (0 ML) corresponds to the clean surface of CdTe(110)(1 × 1). For the first step of the deposition (0.2 ML) the change of the valence band EDC is slightly visible. The shift of the Cd 4d level, equal to 0.2 eV, can be explained as caused by the shift of the Fermi level coming from the Fe doping of the CdTe surface a n d / o r from the band bending down at the very first stages of the M - S barrier formation. In the next curve (0.6 ML) the Fe atoms contribute mainly to the Cd l_XFexTe crystal creation while for 1.2 ML the Fermi edge appears due to the growth of the metallic iron islands. These effects are illustrated by the appearance of a sharpening of the Fermi edge and an increase of the Fe 3d electrons contribution to
o.
~ I
I
I
I
-12
-8
-4
0
Binding Energy (eV) Fig. 1. The set of the EDC's of the valence band and Cd 4d core electrons measured for hv = 56 eV (Fe 3p-3d resonant energy) for a C d T e ( l l 0 ) surface; first clean, then after increasing Fe coverages.
the Cd~_,FexTe valence band (see Fig. 4b and c) and, in the Cd 4d band, by the appearance of a right hand side shoulder corresponding to the raise of a C d - F e interaction. Upon further Fe deposition (3.6, 10 and 20 ML) metal islands dominate the spectra with a sharp metallic Fermi edge. In this range of coverages, the Cd 4d peak decreases due to the coverage of the surface by Fe islands and, the Cd-Fe interaction leads to the increase of the additional Cd 4d peak located at the lower binding energy than it is in CdTe. It has to be mentioned that in the case of CdTe the contribution of a metallic Fe Fermi edge is already visible at 1.2 ML coverage while in the case of Cdo.s6Fe0.j4Se it is not yet visible [3]. This illustrates the effect of a higher stability of the CdTe surface as compared to the CdSe one. In Fig. 2 the set of EDC's presents the changes of the density of states obtained after successive 300°C annealings of the CdTe(110) surface covered by 20
B.A. Orlowski et al. /Applied Surface Science 123/124 (1998) 631-635 ML of iron. After such annealings, the shapes of the EDC's change as if they were going backward compared to Fig. 1, where the effect of Fe coverage increase was shown. In particular, the metallic edge and the Fe 3d top peak are progressively decreasing. This is related to the Fe diffusion into the CdTe crystal. Upon the third heating, the contribution of the Fe 3d electrons coming from a ternary crystal Cd~ xFe,Te highly dominates the contribution of metallic Fe islands. After further heating the Fe atoms diffuse deep into the crystal and their contribution to the Cd I .,Fe.,Te crystal within the probed surface region decreases. Fig. 3 presents the set of EDC's measured at photon energy h v = 90 eV after the sequential Fe deposition. There, the electrons excited from the upper valence band states have the lowest escape depth. In this case, the contribution of the metallic Fe islands is amplified in the EDC's in comparison to the other contributions. As expected, the upper
300
f
I,, /'
250
./I
CdTe/Fe L
20 ML Fe
=
hv=56 eV
Ix
time (15 mill)
200'
=. ,t~ 150"
._~
/,J
loo.
50
l
J/\/ J i
i
i
i
-12
-8
-4
0
Binding Energy (eV) Fig. 2. The set of the EDC's of the valence band density of states obtained after successive 300°C annealings of a CdTe(110) surface first covered by 20 ML of Fe.
300
633
t. g° "~ ".
CdTe(110)/Fe ;
250
hv=90 eV
..,:,':
j" ~t • off
, - , 200
"725
/ A"
: Fe coverage
EDC
¢:.
]
P""
B
,
20 ML
,,,.,,,
..@
at:
..
,,:-
=
-:
10 ,,..,"¢'" ML ,,o<,..~
loo
•
~:.:.-'~"
~
50
".. -
~
1.2 ML
o~ ~ - " - . 2 I
I
-8
-6
ME
I
I
I
I
-4
-2
0
2
Binding Energy (eV) Fig. 3. The set of EDC's measured for hv = 9 0 eV (minimum electron escape depth) on a CdTe(110) surface upon increasing Fe coverages. Arrows A and B correspond to the peak positions of the Fe crystal [7].
curve (20 ML) reproduces the EDC measured for an Fe crystal [7]. In Fig. 4 the set of E D C ' s corresponding to the resonant photon energy, h v = 56 eV, and to the antiresonant one, h v = 54 eV, are presented together with their differences. The EDC differences can be treated as representing the contribution of the Fe 3d electrons to the valence band energy region. For 0.6 ML Fe coverage (Fig. 4b) the difference curve corresponds well to the calculated [5] contribution of the Fe 3d electrons to the valence band of a Cd~_ ,Fe~Te crystal. The peak at - 5 . 5 eV is built mainly of Fe 3d e electrons with a small contribution of Fe 3d t, while the peak at - 2 eV is built of hybridised Fe 3d t electrons [5]. The contribution of metallic Fe increases upon further Fe coverage and for 20 ML, it dominates the spectra (Fig. 4d). Similar curves can be drawn in the case of a 20 ML Fe-covered CdTe surface upon successive annealings: they show again
B.A. Orlowski et al. / Applied Surface Science 123 / 124 (1998) 631-635
634
a)
b)
C4Ta(IIWI', o~ F,M~
•
~o-
;, ga
.Z..
orr, omtF,
: " I
0.6 Fo ML
1500,
•
..
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.,
,~ ,~
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•,
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.;,
re~
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=56eV
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)
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!
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Binding Energy (eV)
BindingEnergy (eV)
2000
C)
d)
CdTe(110)/Fe 1.2 Fe ML
..
/.
4500
1500 -
• .~ . ;.
: .~-
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:.
/
h,= 5 6 , v " ' - " - /
hv= 54eV "~.
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:/
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~ 3o00.
a"
;,
¸
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•
i
104)0.
CdTe(110)/Fe 20 Fe M L
v
•
/
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,:',.
/
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!
!
!
!
!
-8
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Binding E n © r ~ (©V)
;
n=
-EDC
m:z
!
-8
!
-6
-4
-2
0
2
Binding En©qD' (eV)
Fig. 4. The set of E D C ' s at Fe resonant photon energy hu = 56 eV and at the antiresonant one h u = 54 eV together with their difference for a CdTe(l | 0 ) surface upon increasing Fe coverages.
B.A. Orlowski et al. / Applied SurJace Science 123/124 (1998) 631-635
the reversibility of the metallic Fe contribution, as in Fig. 2.
3. Summary Resonant photoemission was applied to determine the contribution of Fe 3d electrons after Fe deposition on the crystal CdTe(110) surface and then after the diffusion of Fe into the crystal. The results show that in the first stage of the deposition the atoms of Fe are mainly involved in creation of Cd 1 ~ F e J e ternary crystal (0.2 and 0.6 ML). Upon increasing coverage, the Fe Fermi edge appears (1.2 ML). CdTe dissociation in the surface region leads to the appearance of C d - F e interaction at higher deposition (3.6 ML). The 300°C heating of the 20 ML covered sample leads to Fe diffusion into the crystal, and the measured spectra correspond well to the calculated ones [5] for the Cdl_xFe~Te crystal.
635
Acknowledgements This paper was partly supported by KBN PB 2P03B 089 10.
References [1] J.K. Furdyna, J. Appl. Phys. 64 (1988) R29. [2] J. Kossut, Thin Solid Films 267 (1995) 22. [3] B.A. Orlowski, B.J. Kowalski, N. Barrett, D. Martinotti, C. Guillot, J.-P. Lacharme, C.A. S6benne, Appl. Surf. Sci. 104/105 (1996) 282. [4] A. Mycielski, J, Appl. Phys. 63 (1988) 3279. [5] J. Masek, Acta Phys. Polon., in print. [6] R. Galazka, in: Proc. XIV Int. Conf. on the Physics of Semiconductors, Inst. Phys. Conf., Edinburg, vol. 43, 1979, p. 133. [7] L. Ley, O.B. Dabbousi, S.P. Kowalczyk, F.R. McFeely, D.A. Shirley, Phys. Rev. B 16 (1977) 5372.
Applied Surface Science 123/124 (1998) 631-635
From CdTe/Fe Schottky barrier to _xFexTesemimagnetic semiconductor B.A. Orlowski a.*, E. G u z i e w i c z , B.J. Kowalski " N. Barrett b,c R. Belkhou b,c, D. Radosavkic b,c, D. Martinotti b,c, C. Guillot b.c, j._p. Lacharme d, C.A. Sdbenne d '~ Institute o/'Physics, Polish Academy (~['Sciences, Warsaw, Poland b LURE, Orsay, France c DRECAM-SRSIM, Saclay, France d Laboratoire de Min~';ralogie-C(vstallographie, URA9-CNRS, Unil,ersitg Pierre et Marie Curie, Paris, France
Abstract Synchrotron radiation tuned to the Fe 3p-3d transition ( h u = 56 eV) was used to study Fano type resonant photoemission spectra lbr a clean CdTe(110) surface sequentially covered in the monolayer (ML) range by Fe atoms and annealed. The results showed that, in the first stage of the deposition, the Fe atoms are mainly involved in the creation of a Cd~ .~Fe,Te ternary crystal (0.2-0.6 ML). At higher coverages (1.2 ML), the contribution of Fe metallic islands becomes visible. CdTe dissociation in the surface region leads to the appearance of C d - F e interaction at higher Fe deposition (3.6 ML). After deposition of 20 ML, sample annealing in the 300°C range leads to Fe diffusion into the crystal and the measured spectra correspond well to the Cd~ ,Fe,Te calculated spectra. © 1998 Elsevier Science B.V. PACS: 73.20.
r: 79.60.Jv
Kevwords: Resonant photoemission: Semimagnetic semiconductors: Schottky barrier formation
1. Introduction
The present study is a new approach to investigate the C d T e / F e interface formation and to observe the changes in the valence band electronic structure which accompany the appearance of a superficial Cd l_xFe,Te semimagnetic semiconductor (SMSC) [1]. The study is strongly stimulated by the recent research of the electronic properties of interfaces and superstructures grown with SMSC [2]. The present
* Corresponding author.
results concern the actual Schottky barrier and apply to the case when the deposited metal reacts with the semiconductor compound surface. In this paper results obtained for the Fe atoms deposited on clean CdTe(1 10) surface are presented. The procedure of the experiment is similar to that applied to Fe atoms deposited on a Cd I .,Fe,Se(110) surface [3]. For Cd~_ ,Fe ,Se crystal, iron atoms were already introduced into the bulk of the crystal by a modified Bridgman method; additional Fe atoms were deposited on the freshly cleaved surface of a ternary crystal Cd0 86Feo.14Se. The Fe atoms can be involved in two stages: as a bulk component of the SMSC
0169-4332/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0169-4332(97)00560-6
632
B.A. Orlowski et al./ Applied Surface Science 123/124 (1998) 631-635
crystal, and then as metallic islands of iron created along the crystal surface. For C d j _ , F e , S e crystal the limit of Fe solubility is about x = 0.16 while for Cdl_,Fe~Te it is x = 0.03 [4]. Photoemission EDC data for Cdj_xFexTe bulk crystal are not yet available in the literature mainly because of this low x limit. However, the contribution of Fe 3d electrons has been calculated for C d l _ , FexTe crystal by Masek [5] using the coherent potential approximation method. The results of this DOS calculation are in good agreement with the EDC's measured by us for a Cd I ~FexTe layer prepared and presented in the paper. The Fe impurity level is located 0.16 eV above the valence band edge of CdTe and the valence band edge of the Cdl_,Fe~Te SMSC alloy, once formed, will be pinned about there.
Fe CdTe(110)/Fe
coverage i
4oo hv=56 eV J
1
3oo
V e"
Jr:" !
•
I
i%
•
"
,--
."
oo.
2. Experimental conditions and results The samples of CdTe were grown by the modified Bridgman method [6] in the Institute of Physics, Polish Academy of Sciences. The clean (110) surface of the CdTe crystal was obtained in the preparation chamber by Ar + sputtering (600 eV, 10 /xA, 30 min) and then by vacuum annealing at about 270°C. The experimental conditions were similar to those applied to Fe atoms deposited on a Cd 1 xFexSe(110) surface [3]. In Fig. 1 the set of EDC's measured for both the valence band and the Cd 4d core level electrons after the sequential Fe deposition on the clean CdTe(110) surface is presented. The Fe coverage starts at 0.2 ML and increases by steps up to 20 ML. The lowest curve (0 ML) corresponds to the clean surface of CdTe(110)(1 × 1). For the first step of the deposition (0.2 ML) the change of the valence band EDC is slightly visible. The shift of the Cd 4d level, equal to 0.2 eV, can be explained as caused by the shift of the Fermi level coming from the Fe doping of the CdTe surface a n d / o r from the band bending down at the very first stages of the M - S barrier formation. In the next curve (0.6 ML) the Fe atoms contribute mainly to the Cd l_XFexTe crystal creation while for 1.2 ML the Fermi edge appears due to the growth of the metallic iron islands. These effects are illustrated by the appearance of a sharpening of the Fermi edge and an increase of the Fe 3d electrons contribution to
o.
~ I
I
I
I
-12
-8
-4
0
Binding Energy (eV) Fig. 1. The set of the EDC's of the valence band and Cd 4d core electrons measured for hv = 56 eV (Fe 3p-3d resonant energy) for a C d T e ( l l 0 ) surface; first clean, then after increasing Fe coverages.
the Cd~_,FexTe valence band (see Fig. 4b and c) and, in the Cd 4d band, by the appearance of a right hand side shoulder corresponding to the raise of a C d - F e interaction. Upon further Fe deposition (3.6, 10 and 20 ML) metal islands dominate the spectra with a sharp metallic Fermi edge. In this range of coverages, the Cd 4d peak decreases due to the coverage of the surface by Fe islands and, the Cd-Fe interaction leads to the increase of the additional Cd 4d peak located at the lower binding energy than it is in CdTe. It has to be mentioned that in the case of CdTe the contribution of a metallic Fe Fermi edge is already visible at 1.2 ML coverage while in the case of Cdo.s6Fe0.j4Se it is not yet visible [3]. This illustrates the effect of a higher stability of the CdTe surface as compared to the CdSe one. In Fig. 2 the set of EDC's presents the changes of the density of states obtained after successive 300°C annealings of the CdTe(110) surface covered by 20
B.A. Orlowski et al. /Applied Surface Science 123/124 (1998) 631-635 ML of iron. After such annealings, the shapes of the EDC's change as if they were going backward compared to Fig. 1, where the effect of Fe coverage increase was shown. In particular, the metallic edge and the Fe 3d top peak are progressively decreasing. This is related to the Fe diffusion into the CdTe crystal. Upon the third heating, the contribution of the Fe 3d electrons coming from a ternary crystal Cd~ xFe,Te highly dominates the contribution of metallic Fe islands. After further heating the Fe atoms diffuse deep into the crystal and their contribution to the Cd I .,Fe.,Te crystal within the probed surface region decreases. Fig. 3 presents the set of EDC's measured at photon energy h v = 90 eV after the sequential Fe deposition. There, the electrons excited from the upper valence band states have the lowest escape depth. In this case, the contribution of the metallic Fe islands is amplified in the EDC's in comparison to the other contributions. As expected, the upper
300
f
I,, /'
250
./I
CdTe/Fe L
20 ML Fe
=
hv=56 eV
Ix
time (15 mill)
200'
=. ,t~ 150"
._~
/,J
loo.
50
l
J/\/ J i
i
i
i
-12
-8
-4
0
Binding Energy (eV) Fig. 2. The set of the EDC's of the valence band density of states obtained after successive 300°C annealings of a CdTe(110) surface first covered by 20 ML of Fe.
300
633
t. g° "~ ".
CdTe(110)/Fe ;
250
hv=90 eV
..,:,':
j" ~t • off
, - , 200
"725
/ A"
: Fe coverage
EDC
¢:.
]
P""
B
,
20 ML
,,,.,,,
..@
at:
..
,,:-
=
-:
10 ,,..,"¢'" ML ,,o<,..~
loo
•
~:.:.-'~"
~
50
".. -
~
1.2 ML
o~ ~ - " - . 2 I
I
-8
-6
ME
I
I
I
I
-4
-2
0
2
Binding Energy (eV) Fig. 3. The set of EDC's measured for hv = 9 0 eV (minimum electron escape depth) on a CdTe(110) surface upon increasing Fe coverages. Arrows A and B correspond to the peak positions of the Fe crystal [7].
curve (20 ML) reproduces the EDC measured for an Fe crystal [7]. In Fig. 4 the set of E D C ' s corresponding to the resonant photon energy, h v = 56 eV, and to the antiresonant one, h v = 54 eV, are presented together with their differences. The EDC differences can be treated as representing the contribution of the Fe 3d electrons to the valence band energy region. For 0.6 ML Fe coverage (Fig. 4b) the difference curve corresponds well to the calculated [5] contribution of the Fe 3d electrons to the valence band of a Cd~_ ,Fe~Te crystal. The peak at - 5 . 5 eV is built mainly of Fe 3d e electrons with a small contribution of Fe 3d t, while the peak at - 2 eV is built of hybridised Fe 3d t electrons [5]. The contribution of metallic Fe increases upon further Fe coverage and for 20 ML, it dominates the spectra (Fig. 4d). Similar curves can be drawn in the case of a 20 ML Fe-covered CdTe surface upon successive annealings: they show again
B.A. Orlowski et al. / Applied Surface Science 123 / 124 (1998) 631-635
634
a)
b)
C4Ta(IIWI', o~ F,M~
•
~o-
;, ga
.Z..
orr, omtF,
: " I
0.6 Fo ML
1500,
•
..
•
.,
,~ ,~
$ ",
•°
". • ~ ' . -
•,
I;
e~
tO00
EDC
.;,
re~
• ?/;:
i %
"~
500.
]
hv= 5
;
= 5~v~: \
,oo ~ , ~ ' : E~c
"..\
..',"
,
,
~
•
~
-6
•
~
-4
•
)
•
0
-2
~o. "..
.~-¢.
~EDC=EDC-EDC
Z'~
-8
".~._...',~
~" -'
::.~:~. "
1
"
:: -Z~"
~.',,"
0-
-500
hv~ 56eV
10OO-
..
#a
-..
0 e.
=56eV
•
)
!
I
!
!
!
2
-g
-6
.4
-2
0
Binding Energy (eV)
BindingEnergy (eV)
2000
C)
d)
CdTe(110)/Fe 1.2 Fe ML
..
/.
4500
1500 -
• .~ . ;.
: .~-
,.,.-,:
:.
/
h,= 5 6 , v " ' - " - /
hv= 54eV "~.
h,,=54eV :', -%: ..;:"•
:/
4 J
""
A
~ 3o00.
a"
;,
¸
hv = 56eV
•
i
104)0.
CdTe(110)/Fe 20 Fe M L
v
•
/
5oo.
,:',.
/
•
N
,b.
",." 0-
M~DC= EDC -EDC -J00
0.
M~DC= EDC
!
!
!
!
!
-8
-6
..4
-2
0
Binding E n © r ~ (©V)
;
n=
-EDC
m:z
!
-8
!
-6
-4
-2
0
2
Binding En©qD' (eV)
Fig. 4. The set of E D C ' s at Fe resonant photon energy hu = 56 eV and at the antiresonant one h u = 54 eV together with their difference for a CdTe(l | 0 ) surface upon increasing Fe coverages.
B.A. Orlowski et al. / Applied SurJace Science 123/124 (1998) 631-635
the reversibility of the metallic Fe contribution, as in Fig. 2.
3. Summary Resonant photoemission was applied to determine the contribution of Fe 3d electrons after Fe deposition on the crystal CdTe(110) surface and then after the diffusion of Fe into the crystal. The results show that in the first stage of the deposition the atoms of Fe are mainly involved in creation of Cd 1 ~ F e J e ternary crystal (0.2 and 0.6 ML). Upon increasing coverage, the Fe Fermi edge appears (1.2 ML). CdTe dissociation in the surface region leads to the appearance of C d - F e interaction at higher deposition (3.6 ML). The 300°C heating of the 20 ML covered sample leads to Fe diffusion into the crystal, and the measured spectra correspond well to the calculated ones [5] for the Cdl_xFe~Te crystal.
635
Acknowledgements This paper was partly supported by KBN PB 2P03B 089 10.
References [1] J.K. Furdyna, J. Appl. Phys. 64 (1988) R29. [2] J. Kossut, Thin Solid Films 267 (1995) 22. [3] B.A. Orlowski, B.J. Kowalski, N. Barrett, D. Martinotti, C. Guillot, J.-P. Lacharme, C.A. S6benne, Appl. Surf. Sci. 104/105 (1996) 282. [4] A. Mycielski, J, Appl. Phys. 63 (1988) 3279. [5] J. Masek, Acta Phys. Polon., in print. [6] R. Galazka, in: Proc. XIV Int. Conf. on the Physics of Semiconductors, Inst. Phys. Conf., Edinburg, vol. 43, 1979, p. 133. [7] L. Ley, O.B. Dabbousi, S.P. Kowalczyk, F.R. McFeely, D.A. Shirley, Phys. Rev. B 16 (1977) 5372.