Valence band control of ZnO and fabrication of p-n junction for light emitting device

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Epitaxial growth of ZnO thin film on atomically flat sapphire (0001) substrate was achieved by using laser MBE, in addition, control of in-plane orientation and surface polarity were done [I.Ohkubo et al., Surf. Sci. Lett. 443, L1043 (1999)]. High grade ZnO/MgZnO superlattice was made, whose quality was comparable to that of compound semiconductors, and the blue-shift of energy gap due to the quantum effect was observed [A.Ohtomo et al., Appl. Phys. Lett, 75, 980 (1999) ; 75, 4088 (1999)]. Generally, valence band control needs high quality of thin film, we tried to made higher quality thin film, by using ScAlMgO4 substrate, lattice matching of which with ZnO was very good. As a result, very high quality ZnO thin film comparable to single crystal was made [A.Ohtomo et al., Appl. Phys. Lett. 75, 2635 (1999)]. The exciton luminescence properties was increased [T.Makino et al., Appl. Phys. Lett. 76, 3549 (2000)], Also, the room temperature exciton luminescences of high quality ZnO/MgZnO and CdZnO/MgZnO superlattices were observed [T.Makino et at., Appl. Phys. Lett. 77, 975 (2000) ; 77, 1632 (2000) ; H.D.Sun et al., Appl. Phys. Lett. 77, 4250 (2000)]. Further, ZnO/MgZnO superlattice generated stimulated emission of exciton at room temperature [A.Ohtomo et al., Appl. Phys. Lett. 77, 2204 (2000)]. Fabrication of p-type ZnO has not been done, but band engineering of ZnO thin film was greatly developed.