"Interface Controlled Heteroepitaxy of Ionic Crystals and Covalent Crystals Using Ionicity Control Layr"

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Interface control of heteroepitaxy of very different materials was investigated from the viewpoints of ionicity of the composed materials and a concept of "ionicity control layr" in which large difference of the ionicity was accommodated by inserting a material having intermediated ionicity was proposed. In experiments in this project, GaAs/CaF_2 interface was employed and ZnSe was examined as the ionicity control layr.The epitaxial temperature for SrF_2 film grown on ZnSe (100) was evaluated as 250ﾟC using MBE method. Many previous report concerning heteroepitaxy of fluorides on various semiconductor substrates were ordered in ionicity of substrate materials, including the result of this experiment. It was found that epitaxial temperature was decreased as difference of ionicity between epilayr and substrate became small.Growth condition of a few nm ZnSe layr on CaF_2 (111) surface was optimized using RHEED and AFM.Based on these observation, GaAs layr was grown on the ZnSe layr, and the relation between crystallinity of the overgrown GaAs layr and the growth condition of the underlying ZnSe layr was observed. As a result, FWHM of X-ray rocking curve from the GaAs layr was reduced to about 50% compared with direct growth of GaAs on CaF_2 surface, when 1-5nm thick ZnSe was deposited on CaF_2 surface at 200ﾟC and was post annealed at 300ﾟC prior to the GaAs growth. AFM observation for initial stage of ZnSe growth on CaF_2 surface and GaAs growth on the ZnSe layr revealed that nucleation density of GaAs was increased on ZnSe surface compared with CaF_2 surface. It was speculated that this high density nucleation was contribute to the improvement of crystallinity of GaAs layr.