Epitaxial growth of ZnS on GaP by Zn-S-H 2 CVD method

Abstract

Single-crystal layers of ZnS have been grown epitaxially on GaP substrates in a hydrogen flow system by using metallic zinc and powdered sulfur as starting materials. The effects of various factors, such as source transport rates, reactant gas compositions, growth temperatures and substrate orientations on the epitaxial growth are examined, and the grown layers are studied by means of electron diffraction and optical microscopy. A discussion is given about some advantages of using the component elements as the starting materials over using the presynthesized powdered ZnS source. Epitaxial layers are grown under the following conditions: substrate temperature 663–780°C; temperature of the mixing and reaction zone, where the vapors of zinc and sulfur are brought together, 950°C; transport rate of sulfur (0.9–1.3) × 10 -4 mol/min; ratio between the source transport rates Zn/S 1.2–2.2. The layer on the ( 1 1 1 )P face shows a featureless smooth surface and has zincblende structure with the {111} twin 180° rotated about the axis normal to the layer surface. The layer on the (111)Ga face shows a distinct feature consisting of hexagonal pyramids and has wurtzite structure. The layer on the (100) face has zincblende structure and is relatively poor in the surface flatness. Cracks are observed in all ZnS layers, and the crack planes coincide with the cleavage planes of ZnS. The cracking may be due to the thermal expansion mismatch between ZnS and GaP. Typical growth rates are 1–10 μ/h for growth temperatures of 690–740°C, and the growth rate is found to be an exponential function of reciprocal temperature with an activation energy of about 3.1 eV for the growth on the ( 1 1 1 )P face and about 4.6 eV for the growth on the (111)Ga face.