X-Ray Diffraction Study of Chalcopyrite ZnSnP 2 Epitaxial Layers
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ABSTRACT We apply the technique of x-ray diffraction to the determination of the crystallographic structure and the quantitative measurement of the order parameter of ZnSnP 2 epitaxial layers. In bulk, ZnSnP 2 it is possible to obtain highly ordered distribution of Zn and Sn atoms in the cation sublattice, but epitaxial growth often produces partially ordered layers. The ordered and disordered phases correspond to the chalcopyrite and sphalerite structures and their respective band gaps are 1.66 and 1.24 eV. Since ZnSnP 2 is almost lattice-matched to GaAs. it is interesting candidate for optoelectronic applications. Samples used in this work were grown by gas source molecular beam epitaxy on GaAs substrates. Slight variations in growth conditions could be induced to produce partially ordered and disordered structures. Chalcopyrite ordering is determined by the observation of several characteristic reflections identifying the lower symmetry of this structure. For example, reflections from (101), (217) and (611) planes, strictly forbidden for sphalerite, were measured. The quantitative determination of the order parameter could be made by comparing intensities of a carefully chosen set of measured and calculated reflections. We show that while kinematic approximation can be used to model weak superstructure reflections, in the calculation of the strong, low-angle, fundamental reflections used for intensity normalization it is necessary to take into account extinction effects. Order parameters varying from 0 to 30% were obtained. INTRODUCTION Spontaneous ordering in semiconductor alloys is a process with interesting theoretical implications and considerable technological importance [1,2]. The process of ordering strongly alters the electronic band structure of the alloy, with resulting changes in its electrical and optical properties. However, it is difficult to obtain completely ordered epitaxial layers and
some degree of disorder usually remains. This is the case even when energetic considerations favor the ordered structure. Bulk chalcopyrite compounds of AlB"tICVI2 and ABIIvCv 2 are being investigated for applications in solar cells [3] and non-linear optics [2]. Bulk ZnSnP 2 is known to exist in two crystalline forms, chalcopyrite with no tetragonal distortion (ordered with c/a=2 and a= 5.651 A) and sphalerite (disordered with c a=5.651 A) [4]. Chalcopyrite ordering can be described as
formation of a natural atomic superlattice of (ZnP) 2(SnP) 2 oriented in the [201] direction [1]. This superlattice produces an elongation of the c-axis, changing the unit cell from face-centered cubic to body-centered tetragonal. ZnSnP 2 can be grown on GaAs substrates with nominal strain due to the small difference in lattice constants, Aa/a-3 x 10-4. Random and ordered ZnSnP2 has been grown in bulk [5] or by liquid phase epitaxy on GaAs [6]. In both of these growth methods, the cooling rate was used to control the degree of ordering. In this work. we present a study of ordered and disordered ZnSnP2 epitaxial layers by high-res
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