Epitaxial twin growth of ZnSe on semi-insulating ZnSe substrate

Abstract

Recently, the study of ZnSe compounds has been increased again, because it is a potential material for blue-light-emitting devices [1, 2] with a 2.67 eV direct energy gap [3]. From many experiments in the past, it appeared that good-quality single crystals of ZnSe were difficult to grow. However, high-quality material must be prepared for good opto-electronic devices, so it is necessary to try various methods to obtain high-quality ZnSe single crystals. Besides the growth of high-quality ZnSe bulk crystals, there is another method epitaxial growth of ZnSe on GaAs or other substrates which can be used to grow ZnSe epilayers for opto-electronic devices [4, 5]. However, very few authors have used a ZnSe substrate for growing ZnSe epilayers [6]. In order to obtain more knowledge of ZnSe homoepitaxial growth, our laboratory focused on the study of the growth of a ZnSe epilayer on ZnSe substrate. Recently, interesting experimental data showing that an epitaxial twin-epilayer had been grown on a ZnSe substrate were obtained. The results will be discussed in this paper. Fig. 1 shows a photograph of the surface morphology of the (1 1 0) ZnSe substrate with zinc-blende structure. The ZnSe substrate used was sliced from a bulk crystal which was obtained by sublimation growth. The sliced specimen and a high-purity ZnSe (6N) powder source were used for ZnSe epitaxial growth in a closed quartz tube. The cleavage surface of the substrate was smooth. However, after the epilayer was deposited on the substrate many parallel bands appeared as shown in Fig. 2 (the growing temperature of the epilayer was 550 ° C). These bands were introduced by repeated { 1 1 1 } twins. The results obtained were similar to those of Kikuma and Furukoshi [7]. Fig. 3 also shows clearly the surface morphology of the twin epilayer by scanning electron microscopy (SEM, Jeol JXA-840). Many trapezoidal islands appear on the surface of the epilayer. There are two kinds of twin-bands which are repeated on the epilayer. These two kinds of two-bands have the same shape of trapezoidal islands but the directions of the parallel sides of the trapezoidal islands are 110 ° different, as shown in Fig. 3. It appears that the directions of the {1 1 1} planes were rotated 110 ° with reference to the [1 1 0] axis from one twin-band to another. The structure of the twin-epilayer was studied further by its Laue pattern as shown in Fig. 4. It appeared that the epilayer and the substrate were (1 1 0) planes. However, there were two groups of spots in the pattern as shown in Fig. 5 which illustrates clearly the pattern of Fig. 4. The beam spot of X-rays covered some twin-bands on the epilayer. Type I spots as shown in Fig. 5 formed the Group I pattern and Type II spots formed the Group II pattern. However, some spots which were overlapping Type I and Type II spots appeared as circular spots. These two groups of spots were the same pattern if the Group I pattern was rotated 110 ° with reference to the centre axis. These two groups of patterns (Group I and Group II) were introduced from the {1 1 1} twin-planes of the (1 1 0) epilayer and substrate because the interplanar angle of the two {1 1 1} twin-planes could be 110 °. From Figs 2 and 5, it appears that {1 1 1} twins were found on the epilayer. The interplanar angles of the {1 1 1} twin-planes was 110 °. From the above description, a twin-epilayer can be found. However, what is the reason for introducing a twin-epilayer? It may be suggested that the twin growth was introduced by the substrate. The substrate