MBE Growth and Characterization of Epitaxial MnS and ZnSe Heterostructures on GaAs
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EXPERIMENTAL TECHNIQUES MBE Crystal Growth The MBE growths were performed in a Riber 2300 system that had been previously devoted to the growth of narrow-gap Hg chalcogenides. No deleterious effects of this previous use were found in this study, however, and Hg was completely absent in the grown films as evidenced by photoluminescence (PL). All growths were performed on (100) GaAs wafers, which were degreased and then etched in a 4:1:1 solution of H 2 SO4 :H2 0 2 :H2 0 at 40'C, and finally placed in HCI and rinsed in deionized water before loading into the MBE system. The subsequent thermal cleaning is performed in two steps. First, the substrate is heated slowly to 5000 C and then rapidly to 600'C; the gallium oxide desorbs around 580TC. As soon as the RHEED pattern shows the desorption of the oxide, the substrate is cooled down toward the ZnSe growth temperature under a Se flux, which is interrupted below 4000 C. The ZnSe growth is initiated by simultaneously opening the Se and Zn cell shutters at a substrate temperature between 300 and 350 0 C. A 2xl RHEED pattern is observed for Se-rich growth. The thermal cyclic annealing procedure consists of heating the substrate to 700'C under a Se flux periodically during or immediately after the growth, a total of 4-6 times; the first anneal is typically performed about 5 min. into the growth. The ZnSe growth rate is either 0.1 or 1.5 A/s, depending on the Zn flux. The solid Zn, Se, and S sources are of 6N+ purity from Osaka Asahi, while the Mn is of 4N purity from Cerac. The first MnS growths were performed after exposing the GaAs substrate briefly to Se and Zn fluxes at high temperature, with the object of depositing a few monolayers of ZnSe. Direct exposure of GaAs to S leads to roughening of the surface, as observed by the development of a spotty RHEED pattern; this effect might be partly responsible for the failure to achieve pure zinc blende material in the previous study, where spotty RHEED patterns were reported for the growths on (100) GaAs. Our initial growths were performed at 250 0 C, and resulted in zinc blende material that gradually converted to the rock salt phase, as observed by RHEED and subsequently confirmed by transmission electron microscopy (TEM) and optical measurements as discussed below. Subsequent MnS growths were performed at a lower temgerature of 100-150 0 C after first growing ZnSe buffer layers of about 0.25 gm thickness at 350 C, and demonstrated streaky unreconstructed RHEED patterns. In some cases, the RHEED pattern developed off-axis spots during the growth of MnS and then gradually disappeared, which was found by TEM to correspond to growth of an amorphous layer on top of -300 A of single crystal zinc blende material. The first MnS growths were performed by evaporating elemental S from a conventional Knudsen cell; later growths employed a new prototype valved cracker cell developed by EPI especially for S. The source is held at 150 0 C while the cracking zone is held at 9000 C. Characterization Techniques Low temperature PL measuremen
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