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Structural investigations of CuInSe2 epitaxial layers on {110}- and {100}-oriented GaAs Substrates
534
Journal of Crystal Growth 54 (1981) 534—540 North-Holland Publishing Company
STRUCTURAL INVESTIGATIONS OF CuInSe2 EPITAXIAL LAYERS ON ~l 1O}- AND 1100}-ORIENTED GaAs SUBSTRATES A. TEMPEL, B. SCHUMANN, K. KOLB and G. KUHN Sektion Chemie, Fachbereich Kristallographie, Karl-Marx-Universitöt, Scharnhorststrasse 20, DDR- 7030 Leipzig, GDR Received 7 January 1981
Epitaxial layers of CuInSe2 on {uo}- and {100}-oriented GaAs substrates were deposited by flash evaporation and investigated using the RHEED technique. The temperature range of epitaxial growth was found to be 720—920 K. The epitaxial relationships expected from geometrical considerations are realized in the growth process, but the kinds of epitaxial relation having the smallest misfit are preferred. In layers grown on {100}-oriented substrates a superlattice structure occurs.
1. Introduction
studied using reflection high energy electron diffraction (RHEED) and the C—Pt-replica technique.
Currently there is considerable interest in the Cu— III—V12 compounds because of their potential application m photovoltaic devices [1—3].For this reason, thin films of these compounds have been grown and investigated by different methods [4] In this connection it is also of interest to investigate thin epitaxial layers with different orientations because their properties are, in general, anisotropic. So far, the epitaxial relationships for CuInSe2 epitaxial layers on [11 1}A and ~11 1} B-oriented GaAs substrates have been studied in detail [5,6]. It is the purpose of this paper to investigate the structural properties of CuInSe2 thin epitaxial ifims grown on ~1l0}- and {100}-oriented substrates and to clear the influence of the substrate orientations on the layer structure. -
3. Basic considerations CuInSe2 isa chalcopyrite-type semiconductor with the space group I 42d. The structure is very similar to that of sphalerite but the cation sublattice is occupied alternately by copper and indium, respectively. So the structure is not cubic but tetragonal with the lattice parameters a0 = 5.782 A and c0 = 11.62 A and the lattice constant ratio co/ao = 2.01 [7]. The lattice parameter a0 = 5.6533 A of GaAs differs only slightly from the value a~1l~Se2and, therefore, the lattice misfit is small. In principle, one can expect three possibilities of epitaxial overgrowth for the deposition of CulnSe2 onto {110} and {iOO} -oriented GaAs substrates, respectively, illustrated schematically in figs. la and 2a. Geometrical considerations allow us to write down immediately the epitaxial relationships expected: -
2. Sample preparation and experimental The growth and the structural investigations of the layers were carried out with the same methods described previously [5]. CuInSe2 thin films were prepared by flash evaporation of prereacted material onto ~1l0}- and ~100}-orientedGaAs. The substrate temperature was varied in the range from 620 to 920 Km 25 K. Theproperties depositionofrate aboutwere 5 ~ 1. steps The of structural thewas layers s~
0022-0248/81 /0000—0000/S02 .50 © 1981 North-Holland
[110}GaAs II {102}cuinsez 100> 11)0> CuInSe2,
(la)
dlujGa~s 1~’”-’fCuiiiSe2, ~l°°>GaAs (001>CuInSe 2,
(ib)
GaAs ,-~
ri
1n~
——
A. Tempel eta!.
~
/ Structural investigations of CuInSe2 epitaxial layers
535
Fig. 1. The three possibilities for epitaxial overgrowth of CuInSe2 on {i 10}-oriented GaAs. Schematical representation (a), calculated RHEED diagrams for CuInSe2 in the azimuths (100> (b), (100> (c), (001) (d), and RHEED pattern expected if all the three possibilities of overgrowth are realized (e). The arrows indicate the direction of the c-axis ofthe deposit and the diameter of the spots characterizes the intensity expected.
~strite
Fig. 2. The three possibilities for cpitaxial overgro~~thofCulnSe2 on ~100}-oriented GaAs. Schematical representation ~a), calculated RI-TEED diagrams for CuInSe2 in the azimuths (100> (b), (100> (c), (001> (d), and RHEED pattern expected if all the three possibilities of overgrowth are realized (e). The arrows indicate the direction of the c-axis of the deposit and the diameter ofthe spots characterizes the intensity expected.
A. Tempel et a!. / Structural investigations of CuInSe
536
2 epitaxial layers
~Il00}GaMII ~00l}~uin~e2 00>CuinSe K l°°>GaAsIlK 1 2
Table 2
(2a)
~iOO}GaAs II {100}CuInSe2 1>CuInSe Ki°°)GaAs (°° 2.
hkl
(2b) Figs. lb—id and 2b—2d give calculated RHEED diagrams for all cases of overgrowth on ~1l0}- and {l00}oriented GaAs substrates, respectively, expected for azimuths (100) with reference to the substrate. The arrows mark the directions of the c-axis of the deposit. If all orientations of the deposit corresponding to figs. la and 2a, are realized in the growth process, an experimental investigation of the layers by RHEED should give characteristic diffraction patterns according to figs. le and 2e consisting ofa superposition of the diagrams lb—id and 2b—2d, respectively. For instance, such types of electron diffraction diagram were obtained by Manolikas et al. [8]. These authors have investigated CuIn 5Se8 bulk material in transmission electron diffraction and have explained these diffraction patterns by domains with different (perpendicular) directions of the c-axes. For an exact analysis ofthe diffraction pattern obtamed experimentally we have calculated the structure factors for (hkl) reflections in CuInSe2. The calculation of F~kl
~
[f~ exp((u0h +v~k+ w~l)2iri)]
n Table 1 The five types of structure factor of CulnSe2
_______________________________________________
F~kl
Additional hkl conditions
6(fCu ~fin
Intensity order of magnitude
220 400 204 440 008
F~k1
hkl
5 3.35 x 10~ 3.30x10 3.38 X 10~
240 200 402 600 208
2.70x103 1.80 x l0~ 2.46 X lO~ 3.74 x l0~ 1.80 x l0~
202 130 134 402 242
5.80 x 10° 2.30 10~ 2.30 xx 10~ 9.l2xlOi 5.80 x 100
3.19 3.41
xx l0~ 10~
112 132 116 332 512 101
1.71 x 10~ 1.68 l0~ 1.71 xx 10~ 1.66x105 1.63 x 10~ 4.81 x l0~
103 301
1.32 x l0~ 3.47 X 102
211 213
2.57 x 103 5.46x103
-
-
-
-
-
gives five types of reflection listed in table 1. Here, j;, is the atomic scattering factor of the ,ith atom and u, v, w are the atomic coordinates. In table 1, A is the free parameter of the chalcopyrite structure and means here the deviation of the Se atoms from the position (n/4, n/4, z), ii = 1, 3 in x- and y-direction, respectively [9]. Table 2 gives some values ofF~kjfor CuInSe2. The values of table 2 were calculated with A = 0.015 [7] and putting the atomic numbers of Cu, In, Se forfcu,fin,fse. If the F~kl values and, therefore, the reflection intensities expected are known it is possible to decide whether the epita.xial relationships according to figs. la and 2a will be realized in the same strength or not by analysing the RHEED diagrams experimentally obtamed.
h + k +1/2 = 4n, h, k even
1O~
4. Experimental results
‘6~fCu+f~~)2 + 64f~e
h 1k +1/2 odd, h, k odd
1O~
4.].
8(fcU
h + k +1 even, 1 odd
10~
h + k + 1/2 = 4n + 2,h, k even
1i11
h, k, 1/2 mixed
100 1o2
CuInSe 2 epitaxial layers were obtained in the whole temperature range of 720—920 K. Below 720 K the layers are partly polycrystalline. The structure of all layers investigated was the chalcopyrite type, layers with a cubic structure were not found. Twins
+
2f~e)2
Calculated F~k1values for sonic chosen reflections
‘
—
fin)2
6(fCu ~fIn — 2f~e)2 ‘ ~ 0 due to X ~ 0
~1]O}-Oriented
substrates
/ Structural investigations of CuInSe2 epitaxial layers
A. Tempel et a!.
~
537
up to 795 K as it follows from spikes of the spots in [221] (fIg. 3a). At growth temperatures higher than 870 K stacking faults do not occur. Since the specific spots caused by crystallites according to fig. lb are very weak in the RHEED diagrams it follows that the epitaxial relationships (1 a) {l 10}GaM II
{102}cuin~e2, 00)GaAs UK l°°>CuInSe
K1
2,
are preferred in the layer growth process(fig. 3b). Furthermore, because of the occurrence of spikes the faces {OOi}, ~i00} and ~102}, ~1lO}, respectively, should be well developed in the layer morphology. At high growth temperatures the layers become very smooth. Table 3 gives a summary of the experimental results obtained for CuInSe2 deposition onto {110}oriented GaAs.
~
4.2. ~1OO}-Orientedsubstrates -
,
.
.
-
hg. .,. RHELD diagrams ot a C ulnSe2 epitaxlal 1dm on [110]. -oriented GaAs substrate, T = 795 K: (a) azimuth (110) with regard to the substrate; (b) azimuth (100) with regard to the substrate.
of the type T221 and T22~were observed in the RHEED diagrams. Twins around [221] appear only in layers grown at temperatures of 695 K. The tendency to stacking fault formation seems to increase
In the case of {iOO}-oriented substrates the sub. strate temperature was varied from 620 to 920 K. The epitaxial temperature was found to be 720 K. Below 720K the layers were polycrystalline but in all cases the films had the chalcopyrite structure. With regard to twin formation we found similar results as in the case of ~1l0} .oriented substrates. The tendency to twin formation increases up to temperatures of about 795 K, at higher temperatures the twin con-
Table 3 Results of epitaxy of CulnSe2 on ~1I0}-oriented GaAs T(K)
Epitaxial overgrowth
Twins T221
620 695 720 745 770 795 820 845 870 895 920
Polycrystalline Epitaxial and polycrystalline Epitaxial Epitaxial Epitaxial Epitaxial Epitaxial Epitaxial Epitaxial Epitaxial Polycrystalline
Stacking faults
Preference of (la)
T221
—
—
—
—
++
++
—
—
—
+
+
—
+
+
—
+
+ +
÷
++ ++ ++ ++
—
—
++
—
+ ++
— —
++ +
++
—
++
++
—
—
+
—
—
—
—
538
A. Tempel et at
/ Structural investigations of CuIuSe2
centration decreases. Twins of the type T2~1were observed only at 820 K. It is necessary to remark that stacking faults occur only in layer regions having a twin position with regard to the matrix. In the films investigated the epitaxial relationships (2a) {100}GaAs
K
II {00l}cuinse2,
l°°>GaAsIlK
100>cuinSe 2
are clearly favoured as can be concluded from the
_ _ Fig. 4. RHEED diagrasns of a CulnSe2 epitaxial film on Øoo} -oriented GaAs substrate, T5 = 870 K: (a) azimuth (100) with regard to the substrate; (b) azhnuth (110) with regard to the substrate; (c) azimuth (110) with regard to the substrate (see text).
epitaxial layers
absence of specific spots caused by crystallites grown with prismatic faces parallel to the substrate surface (fig. 4a). A discussion of this factwifi be given in section 5. It should be noted that the RHEED diagrams of CuInSe2 epitaxial layers grown on {lOO} -oriented GaAs show a special feature. The RHEED patterns obtained in the azimuth K 110) with respect to the substrate show additional spots (fig. 4b) which can be understood as due to a periodic structure with the fourfold interplanar distances. The reflections vanish, if the sample is rotated around the surface normal into a neighbouring azimuth of KllO) type (fig. 4c). This result is in accordance with the symmetry of the [100]. faces. In principle, there are two possibilities to explain these additional reflections: (i) Superlattice structures due to a defmed array of point defects (vacancies, interstitials etc.) (see for instance refs. [10,11]). (ii) Superlattice structures due to surface reconstruction (see for instance refs. [12—14]). In this case, the observed patterns can be interpreted as due to a 4 X 1 reconstructed surface structure. Additional spots of the same type were observed in CuGa07In0~Se2thin films and Cu—In—Se layers obtained with source material having a composition near CuIn2 0Se3 For an exact understanding of this effect further investigations including a systematic variation of the deposition parameters, of the composition of the source materials, and of annealing paramaters are needed. In table 4 the experimental results obtained for CuInSe2 deposition onto [100].-oriented GaAs are summarized. ~.
5. Discussion The epitaxial relationships experimentally found are in good agreement with those expected from pure geometrical considerations. However, it is not clear, why the relationships (la) and (2a) are preferred compared to (lb) and (2b). It may be that the difference in the lattice misfit of CuInSe2 on GaAs is responsible for this effect. In table 5 the lattice misfit =
(ddePosit
—
d5ub5t~~te)/d5r~b5tr5te, .
.
.
.
is given for the different epita.xial relationships. As
A. Tempel et al.
/ Structural investigations ofCuinSe2
539
epitaxial layers
Table 4 Results of epitaxy of CulnSe2 on {i 00)-oriented GaAs T (K)
Epitaxial overgrowth
Twins
Stacking faults
T22i 620 670 695 720 745 770 795 820 845 870 895 920
Polycrystalline Epitaxial and polycrystalline Epitaxial and polycrystalline Epitaxial Epitaxial Epitaxial Epitaxial Epitaxial Epitaxial Epitaxial Epitaxial Polycrystalline
T22r —
—
—
—
—
+
—
—
+
+
+ ++
+ ++
—
++
++ ++
+ +
—
—
—
—
++
+
+
+
+
++
+
++
++
—
++
+
+
+
+
++
++
+
+
+
—
+
++
+
—
++
++
+
+
++
+
—
—
—
+
+ —
Misfit for epitaxial growth of CulnSe2 on fi io}- and {ioo)oriented GaAs substrates; the upper indices are related to deposit directions, the lower to substrate directions, respectively Misfit
Average
(~/~)
misfit
(la) (ib) 2 a1 (2b)
l3ioo
= =
2.28, 2.77,
201 13i 10
j31 ~
=
=
2.52 2.28
13 13
ioo
—
i~i00
—
2.28, ~
2.28
j3
=
2.28, 13?~~ = 2.77
13
100
— —
T211
+
Table S
100
Superlattice structure
—
can be seen the misfit in the cases (1 a) and (2a) is smaller than in the cases (ib) and (2b), respectively, Particularly large is the average misfit difference in the case of )100}-oriented substrates. Thus, it can be suggested that the preference of the relationships (2a) compared to (2b) should be more pronounced than the preference of(la) to (ib) for {l 10)-oriented substrates which is in accord~tncewith the trends observed in our experiments. Thus, we conclude that the lattice misfit between deposit and substrate plays an important role hi establishing the epitaxial relationships in those cases
Epitaxial relationships
Preference of (2a)
= =
2.40 2.53
— —
2.28
=
2.53
—
where different relationships with different lattice misfits are possible from geometrical considerations. In order to get further in evidence for this supposition it could be very interesting to investigate epitaxial layers of CuInSe2 or related compounds on substrate materials with different lattice parameters. Acknowledgements The authors would like to thank Dr. H. Neumann for critical reading of the manuscript. This work was performed in the Arbeitsgemeinschaft A111Bv -Halbleiter of the Karl-Marx-University, Leipzig. References [1] S. Wagner, Inst. Phys. Conf. Ser. 35 (1977) 205. [2] L.L. Kazmerski Inst. Phys. Conf. Ser. 35 (1977) 217. [3] E. Bucher, Appi. Phys. 17 (1978) 1. [4] H. Neumann, G. Kuhn and B. Schumann, Progr. Crystal Growth Characterization 3 (1981) 157. [5] B. Schumann, C. Georgi, A. Tempel, G. Kuhn, Nguyen Van Nam, H. Neumann and W. Hong, Thin Solid Films 52 (1978) 45. [6] B. Schumann, H. Neumann, E. Nowak and G. Kuhn, Knistal Tech., to be published. [7] J. Parkes, R.D. Tomlinson and M.J. Hampshire, J. Appi. Cryst. 6 (1973) 414.
540
A. Tempel et a!.
/ Stnictural investigations of CuInSe2 epita.xial layers
[8] L. Manolikas, J. Van Landuyt, R. Dc Ridder and S. Amelinckx, Phys. Status Solidi (a) 55 (1979) 709. [9]A. Tempel and B. Schumann, Kristal Tech. 13 (1978)
389. [10]L. Däweritz, Kristal Tech. 11(1976) 745. [11]D. Van Dyck, C. Conde-Amiano and S. Amehinckx, Phys. Status Solidi (a) 58 (1980) 451.
[12] A.Y. Cho and J.R. Arthur, Progr. Soiid-State Chem. 10 (1975) 157. [13]J. Massies and P. Etienne, Rev. Tech. Thomson CS1’ 8 (1976) 5. [14]K. Ploog, in: Crystals, Vol. 3, Ed. H.C. Freyhardt (Springer, Berlin, 1980).
Journal of Crystal Growth 54 (1981) 534—540 North-Holland Publishing Company
STRUCTURAL INVESTIGATIONS OF CuInSe2 EPITAXIAL LAYERS ON ~l 1O}- AND 1100}-ORIENTED GaAs SUBSTRATES A. TEMPEL, B. SCHUMANN, K. KOLB and G. KUHN Sektion Chemie, Fachbereich Kristallographie, Karl-Marx-Universitöt, Scharnhorststrasse 20, DDR- 7030 Leipzig, GDR Received 7 January 1981
Epitaxial layers of CuInSe2 on {uo}- and {100}-oriented GaAs substrates were deposited by flash evaporation and investigated using the RHEED technique. The temperature range of epitaxial growth was found to be 720—920 K. The epitaxial relationships expected from geometrical considerations are realized in the growth process, but the kinds of epitaxial relation having the smallest misfit are preferred. In layers grown on {100}-oriented substrates a superlattice structure occurs.
1. Introduction
studied using reflection high energy electron diffraction (RHEED) and the C—Pt-replica technique.
Currently there is considerable interest in the Cu— III—V12 compounds because of their potential application m photovoltaic devices [1—3].For this reason, thin films of these compounds have been grown and investigated by different methods [4] In this connection it is also of interest to investigate thin epitaxial layers with different orientations because their properties are, in general, anisotropic. So far, the epitaxial relationships for CuInSe2 epitaxial layers on [11 1}A and ~11 1} B-oriented GaAs substrates have been studied in detail [5,6]. It is the purpose of this paper to investigate the structural properties of CuInSe2 thin epitaxial ifims grown on ~1l0}- and {100}-oriented substrates and to clear the influence of the substrate orientations on the layer structure. -
3. Basic considerations CuInSe2 isa chalcopyrite-type semiconductor with the space group I 42d. The structure is very similar to that of sphalerite but the cation sublattice is occupied alternately by copper and indium, respectively. So the structure is not cubic but tetragonal with the lattice parameters a0 = 5.782 A and c0 = 11.62 A and the lattice constant ratio co/ao = 2.01 [7]. The lattice parameter a0 = 5.6533 A of GaAs differs only slightly from the value a~1l~Se2and, therefore, the lattice misfit is small. In principle, one can expect three possibilities of epitaxial overgrowth for the deposition of CulnSe2 onto {110} and {iOO} -oriented GaAs substrates, respectively, illustrated schematically in figs. la and 2a. Geometrical considerations allow us to write down immediately the epitaxial relationships expected: -
2. Sample preparation and experimental The growth and the structural investigations of the layers were carried out with the same methods described previously [5]. CuInSe2 thin films were prepared by flash evaporation of prereacted material onto ~1l0}- and ~100}-orientedGaAs. The substrate temperature was varied in the range from 620 to 920 Km 25 K. Theproperties depositionofrate aboutwere 5 ~ 1. steps The of structural thewas layers s~
0022-0248/81 /0000—0000/S02 .50 © 1981 North-Holland
[110}GaAs II {102}cuinsez 100> 11)0> CuInSe2,
(la)
dlujGa~s 1~’”-’fCuiiiSe2, ~l°°>GaAs (001>CuInSe 2,
(ib)
GaAs ,-~
ri
1n~
——
A. Tempel eta!.
~
/ Structural investigations of CuInSe2 epitaxial layers
535
Fig. 1. The three possibilities for epitaxial overgrowth of CuInSe2 on {i 10}-oriented GaAs. Schematical representation (a), calculated RHEED diagrams for CuInSe2 in the azimuths (100> (b), (100> (c), (001) (d), and RHEED pattern expected if all the three possibilities of overgrowth are realized (e). The arrows indicate the direction of the c-axis ofthe deposit and the diameter of the spots characterizes the intensity expected.
~strite
Fig. 2. The three possibilities for cpitaxial overgro~~thofCulnSe2 on ~100}-oriented GaAs. Schematical representation ~a), calculated RI-TEED diagrams for CuInSe2 in the azimuths (100> (b), (100> (c), (001> (d), and RHEED pattern expected if all the three possibilities of overgrowth are realized (e). The arrows indicate the direction of the c-axis of the deposit and the diameter ofthe spots characterizes the intensity expected.
A. Tempel et a!. / Structural investigations of CuInSe
536
2 epitaxial layers
~Il00}GaMII ~00l}~uin~e2 00>CuinSe K l°°>GaAsIlK 1 2
Table 2
(2a)
~iOO}GaAs II {100}CuInSe2 1>CuInSe Ki°°)GaAs (°° 2.
hkl
(2b) Figs. lb—id and 2b—2d give calculated RHEED diagrams for all cases of overgrowth on ~1l0}- and {l00}oriented GaAs substrates, respectively, expected for azimuths (100) with reference to the substrate. The arrows mark the directions of the c-axis of the deposit. If all orientations of the deposit corresponding to figs. la and 2a, are realized in the growth process, an experimental investigation of the layers by RHEED should give characteristic diffraction patterns according to figs. le and 2e consisting ofa superposition of the diagrams lb—id and 2b—2d, respectively. For instance, such types of electron diffraction diagram were obtained by Manolikas et al. [8]. These authors have investigated CuIn 5Se8 bulk material in transmission electron diffraction and have explained these diffraction patterns by domains with different (perpendicular) directions of the c-axes. For an exact analysis ofthe diffraction pattern obtamed experimentally we have calculated the structure factors for (hkl) reflections in CuInSe2. The calculation of F~kl
~
[f~ exp((u0h +v~k+ w~l)2iri)]
n Table 1 The five types of structure factor of CulnSe2
_______________________________________________
F~kl
Additional hkl conditions
6(fCu ~fin
Intensity order of magnitude
220 400 204 440 008
F~k1
hkl
5 3.35 x 10~ 3.30x10 3.38 X 10~
240 200 402 600 208
2.70x103 1.80 x l0~ 2.46 X lO~ 3.74 x l0~ 1.80 x l0~
202 130 134 402 242
5.80 x 10° 2.30 10~ 2.30 xx 10~ 9.l2xlOi 5.80 x 100
3.19 3.41
xx l0~ 10~
112 132 116 332 512 101
1.71 x 10~ 1.68 l0~ 1.71 xx 10~ 1.66x105 1.63 x 10~ 4.81 x l0~
103 301
1.32 x l0~ 3.47 X 102
211 213
2.57 x 103 5.46x103
-
-
-
-
-
gives five types of reflection listed in table 1. Here, j;, is the atomic scattering factor of the ,ith atom and u, v, w are the atomic coordinates. In table 1, A is the free parameter of the chalcopyrite structure and means here the deviation of the Se atoms from the position (n/4, n/4, z), ii = 1, 3 in x- and y-direction, respectively [9]. Table 2 gives some values ofF~kjfor CuInSe2. The values of table 2 were calculated with A = 0.015 [7] and putting the atomic numbers of Cu, In, Se forfcu,fin,fse. If the F~kl values and, therefore, the reflection intensities expected are known it is possible to decide whether the epita.xial relationships according to figs. la and 2a will be realized in the same strength or not by analysing the RHEED diagrams experimentally obtamed.
h + k +1/2 = 4n, h, k even
1O~
4. Experimental results
‘6~fCu+f~~)2 + 64f~e
h 1k +1/2 odd, h, k odd
1O~
4.].
8(fcU
h + k +1 even, 1 odd
10~
h + k + 1/2 = 4n + 2,h, k even
1i11
h, k, 1/2 mixed
100 1o2
CuInSe 2 epitaxial layers were obtained in the whole temperature range of 720—920 K. Below 720 K the layers are partly polycrystalline. The structure of all layers investigated was the chalcopyrite type, layers with a cubic structure were not found. Twins
+
2f~e)2
Calculated F~k1values for sonic chosen reflections
‘
—
fin)2
6(fCu ~fIn — 2f~e)2 ‘ ~ 0 due to X ~ 0
~1]O}-Oriented
substrates
/ Structural investigations of CuInSe2 epitaxial layers
A. Tempel et a!.
~
537
up to 795 K as it follows from spikes of the spots in [221] (fIg. 3a). At growth temperatures higher than 870 K stacking faults do not occur. Since the specific spots caused by crystallites according to fig. lb are very weak in the RHEED diagrams it follows that the epitaxial relationships (1 a) {l 10}GaM II
{102}cuin~e2, 00)GaAs UK l°°>CuInSe
K1
2,
are preferred in the layer growth process(fig. 3b). Furthermore, because of the occurrence of spikes the faces {OOi}, ~i00} and ~102}, ~1lO}, respectively, should be well developed in the layer morphology. At high growth temperatures the layers become very smooth. Table 3 gives a summary of the experimental results obtained for CuInSe2 deposition onto {110}oriented GaAs.
~
4.2. ~1OO}-Orientedsubstrates -
,
.
.
-
hg. .,. RHELD diagrams ot a C ulnSe2 epitaxlal 1dm on [110]. -oriented GaAs substrate, T = 795 K: (a) azimuth (110) with regard to the substrate; (b) azimuth (100) with regard to the substrate.
of the type T221 and T22~were observed in the RHEED diagrams. Twins around [221] appear only in layers grown at temperatures of 695 K. The tendency to stacking fault formation seems to increase
In the case of {iOO}-oriented substrates the sub. strate temperature was varied from 620 to 920 K. The epitaxial temperature was found to be 720 K. Below 720K the layers were polycrystalline but in all cases the films had the chalcopyrite structure. With regard to twin formation we found similar results as in the case of ~1l0} .oriented substrates. The tendency to twin formation increases up to temperatures of about 795 K, at higher temperatures the twin con-
Table 3 Results of epitaxy of CulnSe2 on ~1I0}-oriented GaAs T(K)
Epitaxial overgrowth
Twins T221
620 695 720 745 770 795 820 845 870 895 920
Polycrystalline Epitaxial and polycrystalline Epitaxial Epitaxial Epitaxial Epitaxial Epitaxial Epitaxial Epitaxial Epitaxial Polycrystalline
Stacking faults
Preference of (la)
T221
—
—
—
—
++
++
—
—
—
+
+
—
+
+
—
+
+ +
÷
++ ++ ++ ++
—
—
++
—
+ ++
— —
++ +
++
—
++
++
—
—
+
—
—
—
—
538
A. Tempel et at
/ Structural investigations of CuIuSe2
centration decreases. Twins of the type T2~1were observed only at 820 K. It is necessary to remark that stacking faults occur only in layer regions having a twin position with regard to the matrix. In the films investigated the epitaxial relationships (2a) {100}GaAs
K
II {00l}cuinse2,
l°°>GaAsIlK
100>cuinSe 2
are clearly favoured as can be concluded from the
_ _ Fig. 4. RHEED diagrasns of a CulnSe2 epitaxial film on Øoo} -oriented GaAs substrate, T5 = 870 K: (a) azimuth (100) with regard to the substrate; (b) azhnuth (110) with regard to the substrate; (c) azimuth (110) with regard to the substrate (see text).
epitaxial layers
absence of specific spots caused by crystallites grown with prismatic faces parallel to the substrate surface (fig. 4a). A discussion of this factwifi be given in section 5. It should be noted that the RHEED diagrams of CuInSe2 epitaxial layers grown on {lOO} -oriented GaAs show a special feature. The RHEED patterns obtained in the azimuth K 110) with respect to the substrate show additional spots (fig. 4b) which can be understood as due to a periodic structure with the fourfold interplanar distances. The reflections vanish, if the sample is rotated around the surface normal into a neighbouring azimuth of KllO) type (fig. 4c). This result is in accordance with the symmetry of the [100]. faces. In principle, there are two possibilities to explain these additional reflections: (i) Superlattice structures due to a defmed array of point defects (vacancies, interstitials etc.) (see for instance refs. [10,11]). (ii) Superlattice structures due to surface reconstruction (see for instance refs. [12—14]). In this case, the observed patterns can be interpreted as due to a 4 X 1 reconstructed surface structure. Additional spots of the same type were observed in CuGa07In0~Se2thin films and Cu—In—Se layers obtained with source material having a composition near CuIn2 0Se3 For an exact understanding of this effect further investigations including a systematic variation of the deposition parameters, of the composition of the source materials, and of annealing paramaters are needed. In table 4 the experimental results obtained for CuInSe2 deposition onto [100].-oriented GaAs are summarized. ~.
5. Discussion The epitaxial relationships experimentally found are in good agreement with those expected from pure geometrical considerations. However, it is not clear, why the relationships (la) and (2a) are preferred compared to (lb) and (2b). It may be that the difference in the lattice misfit of CuInSe2 on GaAs is responsible for this effect. In table 5 the lattice misfit =
(ddePosit
—
d5ub5t~~te)/d5r~b5tr5te, .
.
.
.
is given for the different epita.xial relationships. As
A. Tempel et al.
/ Structural investigations ofCuinSe2
539
epitaxial layers
Table 4 Results of epitaxy of CulnSe2 on {i 00)-oriented GaAs T (K)
Epitaxial overgrowth
Twins
Stacking faults
T22i 620 670 695 720 745 770 795 820 845 870 895 920
Polycrystalline Epitaxial and polycrystalline Epitaxial and polycrystalline Epitaxial Epitaxial Epitaxial Epitaxial Epitaxial Epitaxial Epitaxial Epitaxial Polycrystalline
T22r —
—
—
—
—
+
—
—
+
+
+ ++
+ ++
—
++
++ ++
+ +
—
—
—
—
++
+
+
+
+
++
+
++
++
—
++
+
+
+
+
++
++
+
+
+
—
+
++
+
—
++
++
+
+
++
+
—
—
—
+
+ —
Misfit for epitaxial growth of CulnSe2 on fi io}- and {ioo)oriented GaAs substrates; the upper indices are related to deposit directions, the lower to substrate directions, respectively Misfit
Average
(~/~)
misfit
(la) (ib) 2 a1 (2b)
l3ioo
= =
2.28, 2.77,
201 13i 10
j31 ~
=
=
2.52 2.28
13 13
ioo
—
i~i00
—
2.28, ~
2.28
j3
=
2.28, 13?~~ = 2.77
13
100
— —
T211
+
Table S
100
Superlattice structure
—
can be seen the misfit in the cases (1 a) and (2a) is smaller than in the cases (ib) and (2b), respectively, Particularly large is the average misfit difference in the case of )100}-oriented substrates. Thus, it can be suggested that the preference of the relationships (2a) compared to (2b) should be more pronounced than the preference of(la) to (ib) for {l 10)-oriented substrates which is in accord~tncewith the trends observed in our experiments. Thus, we conclude that the lattice misfit between deposit and substrate plays an important role hi establishing the epitaxial relationships in those cases
Epitaxial relationships
Preference of (2a)
= =
2.40 2.53
— —
2.28
=
2.53
—
where different relationships with different lattice misfits are possible from geometrical considerations. In order to get further in evidence for this supposition it could be very interesting to investigate epitaxial layers of CuInSe2 or related compounds on substrate materials with different lattice parameters. Acknowledgements The authors would like to thank Dr. H. Neumann for critical reading of the manuscript. This work was performed in the Arbeitsgemeinschaft A111Bv -Halbleiter of the Karl-Marx-University, Leipzig. References [1] S. Wagner, Inst. Phys. Conf. Ser. 35 (1977) 205. [2] L.L. Kazmerski Inst. Phys. Conf. Ser. 35 (1977) 217. [3] E. Bucher, Appi. Phys. 17 (1978) 1. [4] H. Neumann, G. Kuhn and B. Schumann, Progr. Crystal Growth Characterization 3 (1981) 157. [5] B. Schumann, C. Georgi, A. Tempel, G. Kuhn, Nguyen Van Nam, H. Neumann and W. Hong, Thin Solid Films 52 (1978) 45. [6] B. Schumann, H. Neumann, E. Nowak and G. Kuhn, Knistal Tech., to be published. [7] J. Parkes, R.D. Tomlinson and M.J. Hampshire, J. Appi. Cryst. 6 (1973) 414.
540
A. Tempel et a!.
/ Stnictural investigations of CuInSe2 epita.xial layers
[8] L. Manolikas, J. Van Landuyt, R. Dc Ridder and S. Amelinckx, Phys. Status Solidi (a) 55 (1979) 709. [9]A. Tempel and B. Schumann, Kristal Tech. 13 (1978)
389. [10]L. Däweritz, Kristal Tech. 11(1976) 745. [11]D. Van Dyck, C. Conde-Amiano and S. Amehinckx, Phys. Status Solidi (a) 58 (1980) 451.
[12] A.Y. Cho and J.R. Arthur, Progr. Soiid-State Chem. 10 (1975) 157. [13]J. Massies and P. Etienne, Rev. Tech. Thomson CS1’ 8 (1976) 5. [14]K. Ploog, in: Crystals, Vol. 3, Ed. H.C. Freyhardt (Springer, Berlin, 1980).