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Early stage of growth for (001) ZnTe and (111) CdTe on (001) GaAs: A structural study of the interface using conventional and grazing-incidence X-ray diffraction
Solid State Communications, Vol. 74, No. 6, pp. 433-437, 1990. Printed in Great Britain.
0038-1098/9053.00+.00 Pergamon Press plc
EARLY STAGE OF GROWTH FOR (001) ZnTe AND (111) CdTe ON (001) GaAs : A STRUCTURAL STUDY OF THE INTERFACE USING CONVENTIONAL AND GRAZING-INCIDENCE X-RAY DIFFRACTION. G. Patrat, E. Soyez, M. Brunel CNRS, Laboratoire de Cristallographie,166X, 38042 Grenoble Cedex, France J. Cibert, S. Tatarenko Laboratoire de Spectromttrie Physique, 87 X, 38402 St Martin d'H~res Cedex, France K. Saminadayar Centre d'Etudes Nucl6aires, DRF/SPh-PSC, 85X, 38041 Grenoble Cedex, France (Received 23 January 1990 by P. Burlet)
Strain evolution and lattice relaxation near the interface in (001) ZnTe and (111) CdTe thin layers grown by molecular beam epitaxy on (001) GaAs have been studied using conventional and grazing incidence X-ray techniques. Thinner ZnTe layers exhibit inhomogeneous lattice distortions that consist in weakly (e//=-1%) and strongly (e//=4.5%) strained regions; the thickness of the last one has been estimated to be about 10 to 13 .~ close to the interface. On (111) CdTe epilayers, the strain along [110] is e//< 2.5%, compared to the 14.6% lattice mismatch : CdTe is very early relaxed along this direction. On the contrary, along CdTe [ 11?-] the 0.65% lattice mismatch is accomodated by elastic strain.
strains parallel and perpendicular to the interface are measured (3). In the GIRD technique, the incident and diffracted beams are all at glancing angles, near the critical angle etc = (-2n') 1/2, where n' is the real part of the refractive index of solids for the X-ray wavelength (n = 1 + n' + in", with n', n"< 0 and of the order of 10-6). In this case, the penetration depth of the evanescent wave may be reduced to a few lattice constants (4). The good reflectivity conditions of X-ray near the critical angle provide with a suitable tool for probing the surface and interface effects. Epitaxial films were characterized on a set of special "four axes" goniometer and a 18 KW rotating anode. Monochromatic CuKct radiation was selected by a vertically bent pyrolytic graphite monochromator in front of a ponctual X-ray focus of 0.8 x 0.8 mm2. The two squared slits in front of the crystal and the soller slits in front of the Si(Li) detector limit the horizontal and vertical divergences to 0.15 °. Small monocrystalline slabs of = 8 x 8 mm2 were mounted on a goniometric head; an optical autocollimation system allows to adjust the value of incident ~. The incident and grazing diffracted beams in the GIRD technique were kept at incident ot = emergent ot = 0.28 ° with respect to the surface ; the critical angle for ZnTe, CdTe and GaAs is about 0.32 ° for the CuKet wavelength. By rotating the sample by an angle ~ around the normal to the surface, we observed Bragg diffraction by planes perpendicular to the interface (4). At grazing incidence, the angle within the sample plane between the projection of the incident beam and the projection of the diffracted beam on the crystal surface plane is practically equal to 20, where 0 is the Bragg angle.
INTRODUCTION How to accomodate the lattice mismatch of two semiconductors near the interface is a main and sophisticated problem in heteroepitaxy. Generally, it is assumed that the lattices are uniformly strained to the substrate up to a critical thickness where the strain is gradually relaxed by the generation of misfit dislocations. In this paper, we report on the comparative observations on two systems : (001) ZnTe and (111) CdTe on (001) GaAs. The ZnTe lattice is 7.9% and the CdTe lattice 14.6% larger than the GaAs one. On well-prepared (001) GaAs, CdTe naturally grows with the (111) orientation while ZnTe keeps the (001) orientation. Samples have been grown by Molecular Beam Epitaxy (MBE) at the joint CNRS- CEA Group for MBE of II-VI Compounds in Grenoble. Substrate preparation and growth procedure have been described elsewhere (1). The substrates were deoxydized either in vacuum or in an As flux. Nominal layer thickness is simply the growth time by the growth rate as measured on thick layers (typically 1 monolayer / second). Growth temperature was 320°C for ZnTe and 280 °C for CdTe. As usual (2), broad spots are obtained in RHEED ( Reflection High Energy Electron Diffraction) for ZnTe, which indicates a 3D growth with small islands. For CdTe streaky patterns are observed; however electron microscopy revealed that the growth is not completly 2D, but occurs through large flat islands (1), The samples were removed from the sample holder under dry nitrogen in a glove box. All measurements were carried out in the air, just after growth, specially for the thinnest layers. The samples were characterized by X-ray Grazing Incidence and Reflection Diffraction (GIRD) and by conventional Bragg Scattering Diffraction (BSD). So lattice 433
434
EARLY STAGE OF GROWTH FOR (001) ZnTe AND (III) CdTe ON (001) GaAs
Conventional BSD technique, where incident and diffracted beams make 0 with respect to the surface of the sample, completes our study by detecting the lattice distortions perpendicular to the interface plane. In this case, for (001) GaAs and (001) ZnTe, the (00~ Bragg planes were examined; for (111) CdTe we used the (hhh) planes.
Vol. 74, No. 6
TABLE I. Strains of epitaxial (001) ZnTe on (001) GaAs. thickness
E//(%)
(A)
~3X(%)
1000
T H E (001) Z n T e ON (001) G a A s S Y S T E M
0
250
-- 0
-- 0
75
- 0,6
+0.7
- 1
+2
50
GaAs and ZnTe have the zinc blende structure; the lattice parameters are a (GaAs) = 5.65A and a (ZnTe) = 6.10A. The evolution of the lattice parameter in the ZnTe layers was observed on films with thicknesses ranging from 1000/~ to 20 ]k.
-
4.9
-
-1 4.5
20
Using GIRD the strain parallel to the interface, ~3//, was measured by recording the position of the (220) ZnTe Bragg reflection with respect to the (220) GaAs peak used as an angular reference. As the thickness of the epitaxial layer increases, the (220) ZnTe Bragg peaks are shifted toward their position for bulk ZnTe (Fig. 1). U s i n g c o n v e n t i o n a l d i f f r a c t i o n , the strain perpendicular to the surface was deduced from the shift of the (002) and (004) ZnTe peaks with respect to the corresponding GaAs peaks. The main result is that for any ZnTe contraction parallel to the interface there is a corresponding lattice elongation normal to the surface.
_
GaAs one (Fig.l)..This residual peak is still slightly detectable on the 75A thick film. These observations seem to be correlated to an inhomogeneous lattice distortion with a non uniformly strained lattice. Similar observations have been made on InAs/GaAs (5) and interpreted as being due to the coexistence of two zones : a strongly strained zone close to the interface, and an upper one weakly strained. We estimated the respective parts of. the two zones by fitting the spectra on the 20A and 50A ZnTe layers with two peaks (Fig. 2). The integrated peaks where corrected for the absorption of the upper zone of the layer in the GIRD case (4). From the peak intensity, the thickness of the strongly strained zone amounts to ~10 to 13.~ (if uniformly distributed close to the interface). For the 50/~
The results of parallel and perpendicular strains (5//, ~31 ) are shown in Table I. The lattice cell of epitaxial ZnTe is not commensurate with the GaAs cell even for the thinnest layer. However from data on the 50 and 20A ZnTe layers, we observed a small residual peak between the ZnTe main peak and the
thick film, the two peak positions give £//= -1% for the weakly strained part and £ / / = -4.9 % for the strongly strained one. The 20/~ thick film gives the same results with a lower accuracy.
5 0 0 0
75A
>
ZnTe a
o
tO t-" mC
GaAs
4000 II
/
bulk ZnTe position 3000' n
m
•
1~050A
\
•
•
I
•
%
2000 m
~
a
*
•
o,
mmmmmm
~
o~ °
1000
• ••*e°°'•**eoo•*° • ,•ee ee•
eeee
" 0 19,5
m
oo~ ula
(220) reflections
~,~ " ' , _ _
4oeeeeee • • •
20,0
20,5
21,0
21',5
22,0
22,5 Theta
Fig. l. GIRD on (001) ZnTe / (001) GaAs. Shifts of the (220) ZnTe Bragg peak from the indicated bulk position give the strain parallel to the interface. The weak peak between the main ZnTe peak and the GaAs one is still slightly detectable on the 75A thick film. Spectra are not to scale.
23,0
23,5
Voi. 74, No. 6
EARLY STAGE OF GROWTH FOR (001) ZnTe AND (III) CdTe ON (001) GaAs
435
O~ r"
!
~,÷ ~,+
¢
k
'., + ~'~ "~~+++ ++
f
I
~"
--<-~---. ~-~./
I 19.70
I 20.90
I
l
22.10
Theta
23.30
Fig. 2. Fit of the experimental data (Fig. 1) on the 50/~ thick film, using two lines corresponding to two zones with different strains. Input parameters are the position, maximum and width of each individual line. 8000 bulk
CdTe .
100A
E
__
__
5ooA
\
GaAs
p ~, / \ • \ (~) [_1 1 1] CdTe o" I'=" "." -an~f "1[-" " ~ - Q--,~ 4 - - . , ~ e- -- IO01]GaAs IL~ directio~ T b ! ol ~ • [1 1 ~ CdTe 1~_!.]_ CdTe clirections direction • G__aAs lat31ce " 4 - CdTe lattice
0~ ¢..
6000
,
~.-[A~. " - e - - ~ I ' - " ~ ' - ~ T e and ~,,,r \ / \ /. GaAs directions
position
|ll
t,
50A
4000-
2000
•
•
" "
..//
'
A a
20A / A
(2 2 0) reflections
Ii II iiiinii .
19,0
19,5
20,0
20,5
ma R I a
21,0
21,5
22,0
22,5
23,0
"lheta
Fig. 3. GIRD on (11 l) CdTe / (001) GaAs. Shifts of the (220) CdTe Bragg peak from the indicated bulk position give the strain parallel to the interface. Spectra are normalized. The projections of the (001) GaAs and (11 l) CdTe lattices are also given. Taking the GaAs linewidth as the instrumental width, the broadening of the ZnTe line (Fig,2) may be interpreted in terms of coherent domain size parallel to the interface (although microstrains inside coherent domains may induce some line-broadening; for the ZnTe system, we did not try to measure other order such as the (440) one, because we did not get single ZnTe Bragg peak). From the linewidth of
the two ZnTe Bragg peaks on the 50/~ thick film, we found that the strongly strained part nearest to the interface have a domain size (=90/~) larger than the domain size in the upper zone (= 60A). Since we neglected the microstrains effects, the quoted values are a lower limit for the actual domain size.
436
EARLY STAGE OF GROWTH FOR (001) ZnTe AND (111) CdTe ON (001) GaAs
[1-1~
On the 250.~ thick film, the linewidth is only sligthy larger than the GaAs one and the parallel and perpendicular lattice parameters are about the same as for bulk. T H E (111) CdTe ON (001) GaAs S Y S T E M
bulk CdTe
The lattice parameter is a(CdTe) = 6.48/~ compared to 5.65,~ for GaAs. With such a great mismatch, 14.6%, it is easier to grow (111) than (001) CdTe on (001) GaAs. Here we have the epitaxial relationships (Fig. 3) : [112] CdTe // [110] GaAs (tensile mismatch 0.65%), [1 i0] CdTe//[1 i0] GaAs (compressive mismatch 14.6%). Studies of CdTe epilayers from 500/~ to 20,~ have been performed in order to measure the strains parallel and perpendicular to the interface in an anisotropic mismatch case. Coherent domain size, microstrains, misorientations have also been examined mainly on the 50/~ thick layer. Before this, the crystalline quality of the layers has been assessed using specular reflection on the 100A and 50/~ layers. Intense specular reflected beam accompagnied by interference oscillations have been seen on the two samples; moreover the (11 I) Bragg reflection for the 100/~ layer exhibits Bragg interference satellites characteristic of a well-defined layer. Strain
parallel to the interface
Unlike the ZnTe case, we did not observe any strongly strained domain near the interface. e// a l o n g C d T e
[ l l 0 ] . Along this direction, the shift
of the (220) CdTe Bragg peak is compared with the (27.0) GaAs peak, used as internal angular reference (Fig. 3). In spite of the great mismatch along this direction (14.6%), the lattice contraction, e//, is always less than 2.5% (Table II). Thus the lattice is relaxed very early .This is confirmed in situ by a close examination of RHEED patterns. e / / a l o n g C d T e [ll~t]. Along this direction, the (224t) CdTe Bragg peak positions have been compared to the (220) GaAs one. In Table I1, we see that, for the thinnest layers, the lattice mismatch is practically accomodated by elastic strain, since the CdTe lattice tension e//[ 1 12] amounts to 0.6%. e//along
CdTe
[10i]. Specially on the 50A thick
layer, linear and angular deformation for the [10i] direction have been measured in order to complete our information about the CdTe lattice strain parallel to the interface.The shift of the (202) CdTe peaks was measured (Table II). By pointing out the maximum of the (224) and (202) Bragg reflections we obtained an angle of 29 ° +0.3 ° between the [112] and [10i] directions, instead of 30 ° for bulk material (Fig. 4). This difference ~ =1 ° is quite small, but is in full agreement with the measured strains. TABLE II. Strains of epitaxial (111) CdTe on (001) GaAs. thickness (A) 500 100 50 35
13//[1i0] (%) 0 -0.7 -2.3 -2.2
~/[101] (%)
e//[ 11~] (%)
-0.2 -0.45
=0 +0.34
E±[lll] (%) 0 =0 -1 -2
Vol. 74, No.
CdTe 50A epitaxy
expansion
[10-1] bulk
direction
r [11-21
[10-1]
Fig. 4. CdTe strains parallel to the interface. On the 50A thick film 13 = 0.6 - 0.7 o, e//[1 i0] CdTe = - 2.3%, e//[112] = +0.34%, e//[10i] -~ -0.43%.
Strain perpendicular to the interface. 0-0 diffraction has been used to evaluate the strains perpendicular to the interface ; in this case (11 l) and (333) CdTe Bragg peak shifts have been compared to (002) and (004) GaAs reflection positions. For the 500,~ and 100A thick layers, there is no detectable perpendicular strain. For thinner layers, a lattice contraction is measured : = 1% for the 50A thick layer and = 2% for the 35A one. This observation of a contraction along [111] is not expected : since we observe in average a compression within the (111) plane ( i.e. E//[1 i0] + I~//[11 2] < 0,) we would expect an elongation along [11 l] so as to preserve the bond length (and in this case (El[111] / (13//[1 i0] + 13// [112]) = -1) or to match the elastic coefficients for CdTe (then e ± [ l l l ] / (E//[ 1 ].0] + E// [ 1 12] ) = -0.4). This "abnormal" result indicates an apparent decrease of the unit cell volume. It cannot be interpreted in terms of a stacking fault effect (6,7) since the (hhh) reflections are not affected by stacking faults. Such an abnormal strain has not been observed in the ZnTe (001) case. The easiest way to explain this observation is to assume that the layer is not pure CdTe, but a solid solution with some compound with a smaller lattice. This hypothetical solid solution may involve GaAs (with a 12.7% smaller lattice in accordance with the 14.6% misfit): this would imply an interdiffusion across the interface. However it can also involve the formation of an oxide: it was pointed out in ref. 9, that TeO2 exhibits a Te sublattice fairly similar to the Te sublattice of CdTe, whith the (2i 1) TeO2 plane replacing the (111) CdTe plane. A closer examination of the TeO2 structure (10) shows that the TeTe atomic distances are in fact almost identical to the As-As distances in GaAs. It is then straightforward to check that if we write: e (x)=[(1-x) a(CdTe)+x a(GaAs)-a(experimental)]/a(exp.) for each measured parameter, then the condition 13_1_[111]/(El/[1 [0] +E/l[l12] )=-0.4 to -1 imply x~10 to 12% for the 35/~ thick layer, x=7 to 8% for the 50,~ one and x=l to 2% for the 100/~ layer. These are average compositions : clearly the concentration can be nonhomogeneous. Corresponding residual strains are shown in Table III. These values are not unreasonnable, but the nature of the diffused species (GaAs from the interface, or TeO2 from the surface) cannot be precised without in-situ experiments.
Vol. 74, No. 6
EARLY STAGE OF GROWTH FOR (001) ZnTe AND (Iii) CdTe ON (001) GaAs
Table III. Residual strains for the hypothetical solid
Misorientation parallel to the interface
solution x GaAs-(1-x) CdTe or x TeO2 - (l-x) CdTe
The domain misorientations may be obtained from a"
thickness ix of the solid i E//[1]0] layer (/~) solution (%)
(%)
E//[l121
E±[lll]
(%)
(%)
0.2
0.2
¢ profile", obtained with the detector fixed on the (220) maximum by rotating the sample by an angle ¢ around the normal to its surface. This profile has been corrected for the domain size broadening as : (At~)2misorientation= (A0)2total- (A~)2domain
0
500
0
0
100
1- 2
-0.5
50
7- 8
-1
+1
0
35
10 - 12
-1
2
-1
This method, applied around the [1 i0] direction, gives a misorientation of the domains + 0.5 ° parallel to the surface, for the 50A thick layer. CONCLUSION Parallel and perpendicular strains( £//and E_L)have been measured on thin layers of (001) ZnTe and (111.) CdTe ~rown on (001) GaAs. Thinner ZnTe layers, 50 A and 20A, exhibit inhomogeneous lattice strains that consist
Size of coherent domains and microstrains These informations may be obtained from the corrected linewitdth A0 measured along the scattering vector. This linewitdth has two main origins: the domain size and microstrains; these two effects are convoluted with the instrumental linewitdth. They can be obtained separately if one measures two different orders of one reflection, in our case (22.0) and (440). We estimated the instrumental resolution from the GaAs (220) linewitdth, assumed to be of good quality. On all reflections, the ratio of the full width at half maximum to the integral width is 0.90, which indicates a rather gaussian profile (9). Thus we made a gaussian correction to the lineshapes. For the 50/~ CdTe layer we obtained an average domain size of ~150/~ and Ad microstrains-~- = + 0.8%. Along the [112] direction we could not obtain the second order reflection.
437
in weakly (E//ZnTe=-1%) and strongly (E//ZnTe~-4.5%) strained regions. The thickness of the latter one has been estimated around 10 to 13/~ close to the interface. Parallel to the interface we found fairly larger domain size near the interface than in the upper zone. On the (111) CdTe epilayers, the maximum contraction value along [110] CdTe is far from the 14.6% lattice mismatch : the CdTe lattice is early relaxed along this direction. Along [ 112] the 0.65% lattice mismatch is accomodated by elastic strain in CdTe. However an "abnormal" strain along the [111] growth direction has been observed which may either be due to a weak oxidation of the layer or reveal an interdiffusion across the CdTe/GaAs interface. We acknowledge fruitful discussions with F. de Bergevin, G. Feuillet and E. Ligeon.
REFERENCES 1 2 3 4 5
S. Tatarenko,J. Cibert,Y. Gobil,G. Feuillet, K. Saminadayar,A.C. Chami and E. Ligeon, Appl.Surface Sci., 41/42, 470 (1989) R.D. Felman,R.F. Austin,D.W. Kisker, K.S. Jeffers and P.N. Bridenbaugh, Appl.Phys.Lett. 48, 248 (1986) W.C. Marra, P. Einsenberger and A.Y. Cho, J. Appl. Phys., 5 0 , 6 9 2 7 (1979) H. Dosh, B.W. Batterman and D.C. Wack, Phys. Rev. Letter, 56, 1144 (1986) H. Munekata, A. Segmiiller and L.L. Chang, Appl. Phys. Lett. 51,587 (1987)
A. Guinier, Th6orie et technique de la radiocristallographie Dunod (1964). 7 M.S. Paterson J. Appl. Phys., 2 , 805 (1952). 8 F.A. Ponce, R. Sinclair and R.H. Bube, Appl. Phys. Lett., 39, 951 (1981). 9 R.W.G. Wyckoff, Crystal structures, 2nd edition Interscience, New York (1963), Vol 1, p 254. 10 G.K.Williamson and W.H.HalI, Acta Metall., 1, 22 (1952).
6
0038-1098/9053.00+.00 Pergamon Press plc
EARLY STAGE OF GROWTH FOR (001) ZnTe AND (111) CdTe ON (001) GaAs : A STRUCTURAL STUDY OF THE INTERFACE USING CONVENTIONAL AND GRAZING-INCIDENCE X-RAY DIFFRACTION. G. Patrat, E. Soyez, M. Brunel CNRS, Laboratoire de Cristallographie,166X, 38042 Grenoble Cedex, France J. Cibert, S. Tatarenko Laboratoire de Spectromttrie Physique, 87 X, 38402 St Martin d'H~res Cedex, France K. Saminadayar Centre d'Etudes Nucl6aires, DRF/SPh-PSC, 85X, 38041 Grenoble Cedex, France (Received 23 January 1990 by P. Burlet)
Strain evolution and lattice relaxation near the interface in (001) ZnTe and (111) CdTe thin layers grown by molecular beam epitaxy on (001) GaAs have been studied using conventional and grazing incidence X-ray techniques. Thinner ZnTe layers exhibit inhomogeneous lattice distortions that consist in weakly (e//=-1%) and strongly (e//=4.5%) strained regions; the thickness of the last one has been estimated to be about 10 to 13 .~ close to the interface. On (111) CdTe epilayers, the strain along [110] is e//< 2.5%, compared to the 14.6% lattice mismatch : CdTe is very early relaxed along this direction. On the contrary, along CdTe [ 11?-] the 0.65% lattice mismatch is accomodated by elastic strain.
strains parallel and perpendicular to the interface are measured (3). In the GIRD technique, the incident and diffracted beams are all at glancing angles, near the critical angle etc = (-2n') 1/2, where n' is the real part of the refractive index of solids for the X-ray wavelength (n = 1 + n' + in", with n', n"< 0 and of the order of 10-6). In this case, the penetration depth of the evanescent wave may be reduced to a few lattice constants (4). The good reflectivity conditions of X-ray near the critical angle provide with a suitable tool for probing the surface and interface effects. Epitaxial films were characterized on a set of special "four axes" goniometer and a 18 KW rotating anode. Monochromatic CuKct radiation was selected by a vertically bent pyrolytic graphite monochromator in front of a ponctual X-ray focus of 0.8 x 0.8 mm2. The two squared slits in front of the crystal and the soller slits in front of the Si(Li) detector limit the horizontal and vertical divergences to 0.15 °. Small monocrystalline slabs of = 8 x 8 mm2 were mounted on a goniometric head; an optical autocollimation system allows to adjust the value of incident ~. The incident and grazing diffracted beams in the GIRD technique were kept at incident ot = emergent ot = 0.28 ° with respect to the surface ; the critical angle for ZnTe, CdTe and GaAs is about 0.32 ° for the CuKet wavelength. By rotating the sample by an angle ~ around the normal to the surface, we observed Bragg diffraction by planes perpendicular to the interface (4). At grazing incidence, the angle within the sample plane between the projection of the incident beam and the projection of the diffracted beam on the crystal surface plane is practically equal to 20, where 0 is the Bragg angle.
INTRODUCTION How to accomodate the lattice mismatch of two semiconductors near the interface is a main and sophisticated problem in heteroepitaxy. Generally, it is assumed that the lattices are uniformly strained to the substrate up to a critical thickness where the strain is gradually relaxed by the generation of misfit dislocations. In this paper, we report on the comparative observations on two systems : (001) ZnTe and (111) CdTe on (001) GaAs. The ZnTe lattice is 7.9% and the CdTe lattice 14.6% larger than the GaAs one. On well-prepared (001) GaAs, CdTe naturally grows with the (111) orientation while ZnTe keeps the (001) orientation. Samples have been grown by Molecular Beam Epitaxy (MBE) at the joint CNRS- CEA Group for MBE of II-VI Compounds in Grenoble. Substrate preparation and growth procedure have been described elsewhere (1). The substrates were deoxydized either in vacuum or in an As flux. Nominal layer thickness is simply the growth time by the growth rate as measured on thick layers (typically 1 monolayer / second). Growth temperature was 320°C for ZnTe and 280 °C for CdTe. As usual (2), broad spots are obtained in RHEED ( Reflection High Energy Electron Diffraction) for ZnTe, which indicates a 3D growth with small islands. For CdTe streaky patterns are observed; however electron microscopy revealed that the growth is not completly 2D, but occurs through large flat islands (1), The samples were removed from the sample holder under dry nitrogen in a glove box. All measurements were carried out in the air, just after growth, specially for the thinnest layers. The samples were characterized by X-ray Grazing Incidence and Reflection Diffraction (GIRD) and by conventional Bragg Scattering Diffraction (BSD). So lattice 433
434
EARLY STAGE OF GROWTH FOR (001) ZnTe AND (III) CdTe ON (001) GaAs
Conventional BSD technique, where incident and diffracted beams make 0 with respect to the surface of the sample, completes our study by detecting the lattice distortions perpendicular to the interface plane. In this case, for (001) GaAs and (001) ZnTe, the (00~ Bragg planes were examined; for (111) CdTe we used the (hhh) planes.
Vol. 74, No. 6
TABLE I. Strains of epitaxial (001) ZnTe on (001) GaAs. thickness
E//(%)
(A)
~3X(%)
1000
T H E (001) Z n T e ON (001) G a A s S Y S T E M
0
250
-- 0
-- 0
75
- 0,6
+0.7
- 1
+2
50
GaAs and ZnTe have the zinc blende structure; the lattice parameters are a (GaAs) = 5.65A and a (ZnTe) = 6.10A. The evolution of the lattice parameter in the ZnTe layers was observed on films with thicknesses ranging from 1000/~ to 20 ]k.
-
4.9
-
-1 4.5
20
Using GIRD the strain parallel to the interface, ~3//, was measured by recording the position of the (220) ZnTe Bragg reflection with respect to the (220) GaAs peak used as an angular reference. As the thickness of the epitaxial layer increases, the (220) ZnTe Bragg peaks are shifted toward their position for bulk ZnTe (Fig. 1). U s i n g c o n v e n t i o n a l d i f f r a c t i o n , the strain perpendicular to the surface was deduced from the shift of the (002) and (004) ZnTe peaks with respect to the corresponding GaAs peaks. The main result is that for any ZnTe contraction parallel to the interface there is a corresponding lattice elongation normal to the surface.
_
GaAs one (Fig.l)..This residual peak is still slightly detectable on the 75A thick film. These observations seem to be correlated to an inhomogeneous lattice distortion with a non uniformly strained lattice. Similar observations have been made on InAs/GaAs (5) and interpreted as being due to the coexistence of two zones : a strongly strained zone close to the interface, and an upper one weakly strained. We estimated the respective parts of. the two zones by fitting the spectra on the 20A and 50A ZnTe layers with two peaks (Fig. 2). The integrated peaks where corrected for the absorption of the upper zone of the layer in the GIRD case (4). From the peak intensity, the thickness of the strongly strained zone amounts to ~10 to 13.~ (if uniformly distributed close to the interface). For the 50/~
The results of parallel and perpendicular strains (5//, ~31 ) are shown in Table I. The lattice cell of epitaxial ZnTe is not commensurate with the GaAs cell even for the thinnest layer. However from data on the 50 and 20A ZnTe layers, we observed a small residual peak between the ZnTe main peak and the
thick film, the two peak positions give £//= -1% for the weakly strained part and £ / / = -4.9 % for the strongly strained one. The 20/~ thick film gives the same results with a lower accuracy.
5 0 0 0
75A
>
ZnTe a
o
tO t-" mC
GaAs
4000 II
/
bulk ZnTe position 3000' n
m
•
1~050A
\
•
•
I
•
%
2000 m
~
a
*
•
o,
mmmmmm
~
o~ °
1000
• ••*e°°'•**eoo•*° • ,•ee ee•
eeee
" 0 19,5
m
oo~ ula
(220) reflections
~,~ " ' , _ _
4oeeeeee • • •
20,0
20,5
21,0
21',5
22,0
22,5 Theta
Fig. l. GIRD on (001) ZnTe / (001) GaAs. Shifts of the (220) ZnTe Bragg peak from the indicated bulk position give the strain parallel to the interface. The weak peak between the main ZnTe peak and the GaAs one is still slightly detectable on the 75A thick film. Spectra are not to scale.
23,0
23,5
Voi. 74, No. 6
EARLY STAGE OF GROWTH FOR (001) ZnTe AND (III) CdTe ON (001) GaAs
435
O~ r"
!
~,÷ ~,+
¢
k
'., + ~'~ "~~+++ ++
f
I
~"
--<-~---. ~-~./
I 19.70
I 20.90
I
l
22.10
Theta
23.30
Fig. 2. Fit of the experimental data (Fig. 1) on the 50/~ thick film, using two lines corresponding to two zones with different strains. Input parameters are the position, maximum and width of each individual line. 8000 bulk
CdTe .
100A
E
__
__
5ooA
\
GaAs
p ~, / \ • \ (~) [_1 1 1] CdTe o" I'=" "." -an~f "1[-" " ~ - Q--,~ 4 - - . , ~ e- -- IO01]GaAs IL~ directio~ T b ! ol ~ • [1 1 ~ CdTe 1~_!.]_ CdTe clirections direction • G__aAs lat31ce " 4 - CdTe lattice
0~ ¢..
6000
,
~.-[A~. " - e - - ~ I ' - " ~ ' - ~ T e and ~,,,r \ / \ /. GaAs directions
position
|ll
t,
50A
4000-
2000
•
•
" "
..//
'
A a
20A / A
(2 2 0) reflections
Ii II iiiinii .
19,0
19,5
20,0
20,5
ma R I a
21,0
21,5
22,0
22,5
23,0
"lheta
Fig. 3. GIRD on (11 l) CdTe / (001) GaAs. Shifts of the (220) CdTe Bragg peak from the indicated bulk position give the strain parallel to the interface. Spectra are normalized. The projections of the (001) GaAs and (11 l) CdTe lattices are also given. Taking the GaAs linewidth as the instrumental width, the broadening of the ZnTe line (Fig,2) may be interpreted in terms of coherent domain size parallel to the interface (although microstrains inside coherent domains may induce some line-broadening; for the ZnTe system, we did not try to measure other order such as the (440) one, because we did not get single ZnTe Bragg peak). From the linewidth of
the two ZnTe Bragg peaks on the 50/~ thick film, we found that the strongly strained part nearest to the interface have a domain size (=90/~) larger than the domain size in the upper zone (= 60A). Since we neglected the microstrains effects, the quoted values are a lower limit for the actual domain size.
436
EARLY STAGE OF GROWTH FOR (001) ZnTe AND (111) CdTe ON (001) GaAs
[1-1~
On the 250.~ thick film, the linewidth is only sligthy larger than the GaAs one and the parallel and perpendicular lattice parameters are about the same as for bulk. T H E (111) CdTe ON (001) GaAs S Y S T E M
bulk CdTe
The lattice parameter is a(CdTe) = 6.48/~ compared to 5.65,~ for GaAs. With such a great mismatch, 14.6%, it is easier to grow (111) than (001) CdTe on (001) GaAs. Here we have the epitaxial relationships (Fig. 3) : [112] CdTe // [110] GaAs (tensile mismatch 0.65%), [1 i0] CdTe//[1 i0] GaAs (compressive mismatch 14.6%). Studies of CdTe epilayers from 500/~ to 20,~ have been performed in order to measure the strains parallel and perpendicular to the interface in an anisotropic mismatch case. Coherent domain size, microstrains, misorientations have also been examined mainly on the 50/~ thick layer. Before this, the crystalline quality of the layers has been assessed using specular reflection on the 100A and 50/~ layers. Intense specular reflected beam accompagnied by interference oscillations have been seen on the two samples; moreover the (11 I) Bragg reflection for the 100/~ layer exhibits Bragg interference satellites characteristic of a well-defined layer. Strain
parallel to the interface
Unlike the ZnTe case, we did not observe any strongly strained domain near the interface. e// a l o n g C d T e
[ l l 0 ] . Along this direction, the shift
of the (220) CdTe Bragg peak is compared with the (27.0) GaAs peak, used as internal angular reference (Fig. 3). In spite of the great mismatch along this direction (14.6%), the lattice contraction, e//, is always less than 2.5% (Table II). Thus the lattice is relaxed very early .This is confirmed in situ by a close examination of RHEED patterns. e / / a l o n g C d T e [ll~t]. Along this direction, the (224t) CdTe Bragg peak positions have been compared to the (220) GaAs one. In Table I1, we see that, for the thinnest layers, the lattice mismatch is practically accomodated by elastic strain, since the CdTe lattice tension e//[ 1 12] amounts to 0.6%. e//along
CdTe
[10i]. Specially on the 50A thick
layer, linear and angular deformation for the [10i] direction have been measured in order to complete our information about the CdTe lattice strain parallel to the interface.The shift of the (202) CdTe peaks was measured (Table II). By pointing out the maximum of the (224) and (202) Bragg reflections we obtained an angle of 29 ° +0.3 ° between the [112] and [10i] directions, instead of 30 ° for bulk material (Fig. 4). This difference ~ =1 ° is quite small, but is in full agreement with the measured strains. TABLE II. Strains of epitaxial (111) CdTe on (001) GaAs. thickness (A) 500 100 50 35
13//[1i0] (%) 0 -0.7 -2.3 -2.2
~/[101] (%)
e//[ 11~] (%)
-0.2 -0.45
=0 +0.34
E±[lll] (%) 0 =0 -1 -2
Vol. 74, No.
CdTe 50A epitaxy
expansion
[10-1] bulk
direction
r [11-21
[10-1]
Fig. 4. CdTe strains parallel to the interface. On the 50A thick film 13 = 0.6 - 0.7 o, e//[1 i0] CdTe = - 2.3%, e//[112] = +0.34%, e//[10i] -~ -0.43%.
Strain perpendicular to the interface. 0-0 diffraction has been used to evaluate the strains perpendicular to the interface ; in this case (11 l) and (333) CdTe Bragg peak shifts have been compared to (002) and (004) GaAs reflection positions. For the 500,~ and 100A thick layers, there is no detectable perpendicular strain. For thinner layers, a lattice contraction is measured : = 1% for the 50A thick layer and = 2% for the 35A one. This observation of a contraction along [111] is not expected : since we observe in average a compression within the (111) plane ( i.e. E//[1 i0] + I~//[11 2] < 0,) we would expect an elongation along [11 l] so as to preserve the bond length (and in this case (El[111] / (13//[1 i0] + 13// [112]) = -1) or to match the elastic coefficients for CdTe (then e ± [ l l l ] / (E//[ 1 ].0] + E// [ 1 12] ) = -0.4). This "abnormal" result indicates an apparent decrease of the unit cell volume. It cannot be interpreted in terms of a stacking fault effect (6,7) since the (hhh) reflections are not affected by stacking faults. Such an abnormal strain has not been observed in the ZnTe (001) case. The easiest way to explain this observation is to assume that the layer is not pure CdTe, but a solid solution with some compound with a smaller lattice. This hypothetical solid solution may involve GaAs (with a 12.7% smaller lattice in accordance with the 14.6% misfit): this would imply an interdiffusion across the interface. However it can also involve the formation of an oxide: it was pointed out in ref. 9, that TeO2 exhibits a Te sublattice fairly similar to the Te sublattice of CdTe, whith the (2i 1) TeO2 plane replacing the (111) CdTe plane. A closer examination of the TeO2 structure (10) shows that the TeTe atomic distances are in fact almost identical to the As-As distances in GaAs. It is then straightforward to check that if we write: e (x)=[(1-x) a(CdTe)+x a(GaAs)-a(experimental)]/a(exp.) for each measured parameter, then the condition 13_1_[111]/(El/[1 [0] +E/l[l12] )=-0.4 to -1 imply x~10 to 12% for the 35/~ thick layer, x=7 to 8% for the 50,~ one and x=l to 2% for the 100/~ layer. These are average compositions : clearly the concentration can be nonhomogeneous. Corresponding residual strains are shown in Table III. These values are not unreasonnable, but the nature of the diffused species (GaAs from the interface, or TeO2 from the surface) cannot be precised without in-situ experiments.
Vol. 74, No. 6
EARLY STAGE OF GROWTH FOR (001) ZnTe AND (Iii) CdTe ON (001) GaAs
Table III. Residual strains for the hypothetical solid
Misorientation parallel to the interface
solution x GaAs-(1-x) CdTe or x TeO2 - (l-x) CdTe
The domain misorientations may be obtained from a"
thickness ix of the solid i E//[1]0] layer (/~) solution (%)
(%)
E//[l121
E±[lll]
(%)
(%)
0.2
0.2
¢ profile", obtained with the detector fixed on the (220) maximum by rotating the sample by an angle ¢ around the normal to its surface. This profile has been corrected for the domain size broadening as : (At~)2misorientation= (A0)2total- (A~)2domain
0
500
0
0
100
1- 2
-0.5
50
7- 8
-1
+1
0
35
10 - 12
-1
2
-1
This method, applied around the [1 i0] direction, gives a misorientation of the domains + 0.5 ° parallel to the surface, for the 50A thick layer. CONCLUSION Parallel and perpendicular strains( £//and E_L)have been measured on thin layers of (001) ZnTe and (111.) CdTe ~rown on (001) GaAs. Thinner ZnTe layers, 50 A and 20A, exhibit inhomogeneous lattice strains that consist
Size of coherent domains and microstrains These informations may be obtained from the corrected linewitdth A0 measured along the scattering vector. This linewitdth has two main origins: the domain size and microstrains; these two effects are convoluted with the instrumental linewitdth. They can be obtained separately if one measures two different orders of one reflection, in our case (22.0) and (440). We estimated the instrumental resolution from the GaAs (220) linewitdth, assumed to be of good quality. On all reflections, the ratio of the full width at half maximum to the integral width is 0.90, which indicates a rather gaussian profile (9). Thus we made a gaussian correction to the lineshapes. For the 50/~ CdTe layer we obtained an average domain size of ~150/~ and Ad microstrains-~- = + 0.8%. Along the [112] direction we could not obtain the second order reflection.
437
in weakly (E//ZnTe=-1%) and strongly (E//ZnTe~-4.5%) strained regions. The thickness of the latter one has been estimated around 10 to 13/~ close to the interface. Parallel to the interface we found fairly larger domain size near the interface than in the upper zone. On the (111) CdTe epilayers, the maximum contraction value along [110] CdTe is far from the 14.6% lattice mismatch : the CdTe lattice is early relaxed along this direction. Along [ 112] the 0.65% lattice mismatch is accomodated by elastic strain in CdTe. However an "abnormal" strain along the [111] growth direction has been observed which may either be due to a weak oxidation of the layer or reveal an interdiffusion across the CdTe/GaAs interface. We acknowledge fruitful discussions with F. de Bergevin, G. Feuillet and E. Ligeon.
REFERENCES 1 2 3 4 5
S. Tatarenko,J. Cibert,Y. Gobil,G. Feuillet, K. Saminadayar,A.C. Chami and E. Ligeon, Appl.Surface Sci., 41/42, 470 (1989) R.D. Felman,R.F. Austin,D.W. Kisker, K.S. Jeffers and P.N. Bridenbaugh, Appl.Phys.Lett. 48, 248 (1986) W.C. Marra, P. Einsenberger and A.Y. Cho, J. Appl. Phys., 5 0 , 6 9 2 7 (1979) H. Dosh, B.W. Batterman and D.C. Wack, Phys. Rev. Letter, 56, 1144 (1986) H. Munekata, A. Segmiiller and L.L. Chang, Appl. Phys. Lett. 51,587 (1987)
A. Guinier, Th6orie et technique de la radiocristallographie Dunod (1964). 7 M.S. Paterson J. Appl. Phys., 2 , 805 (1952). 8 F.A. Ponce, R. Sinclair and R.H. Bube, Appl. Phys. Lett., 39, 951 (1981). 9 R.W.G. Wyckoff, Crystal structures, 2nd edition Interscience, New York (1963), Vol 1, p 254. 10 G.K.Williamson and W.H.HalI, Acta Metall., 1, 22 (1952).
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