Browsing by Author "Yu, K. M."

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    Compensating point defects in 4He+ -irradiated InN
    (American Physical Society (APS), 2007) Tuomisto, Filip; Pelli, A.; Yu, K. M.; Walukiewicz, W.; Schaff, W. J.; Teknillisen fysiikan laitos; Department of Applied Physics; Perustieteiden korkeakoulu; School of Science
    We use positron annihilation spectroscopy to study 2 MeV 4He+ -irradiated InN grown by molecular-beam epitaxy and GaN grown by metal-organic chemical-vapor deposition. In GaN, the Ga vacancies act as important compensating centers in the irradiated material, introduced at a rate of 3600 cm exp −1. The In vacancies are introduced at a significantly lower rate of 100cm−1, making them negligible in the compensation of the irradiation-induced additional n-type conductivity in InN. On the other hand, negative non-open volume defects are introduced at a rate higher than 2000cm exp −1. These defects are tentatively attributed to interstitial nitrogen and may ultimately limit the free-electron concentration at high irradiation fluences.
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    Undoped p-type GaN1-xSbx alloys: Effects of annealing
    (2016-12-19) Segercrantz, N.; Baumgartner, Y.; Ting, M.; Yu, K. M.; Mao, S. S.; Sarney, W. L.; Svensson, S. P.; Walukiewicz, W.; Department of Applied Physics; Antimatter and Nuclear Engineering; Swiss Federal Institute of Technology Lausanne; University of California, Berkeley; City University of Hong Kong; United States Army Research Laboratory; Lawrence Berkeley National Laboratory
    We report p-type behavior for undoped GaN1-xSbx alloys with x ≥ 0.06 grown by molecular beam epitaxy at low temperatures (≤400 °C). Rapid thermal annealing of the GaN1-xSbx films at temperatures >400 °C is shown to generate hole concentrations greater than 1019 cm-3, an order of magnitude higher than typical p-type GaN achieved by Mg doping. The p-type conductivity is attributed to a large upward shift of the valence band edge resulting from the band anticrossing interaction between localized Sb levels and extended states of the host matrix.
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    ZnO 1-x Te x highly mismatched alloys beyond the dilute alloy limit: Synthesis and electronic band structure
    (AMER INST PHYSICS, 2019-04-21) Ting, M.; Yu, K. M.; Jaquez, M.; Sharp, I. D.; Ye, Yifan; Segercrantz, N.; Greif, R.; Mao, S. S.; Liu, Chao Ping; Walukiewicz, W.; Department of Applied Physics; University of California, Berkeley; City University of Hong Kong; Technical University of Munich; Lawrence Berkeley National Laboratory; Shantou University; University of California, Irvine
    We have synthesized ZnO1-xTex highly mismatched alloys (HMAs) with high, up to x = 0.34, Te content using pulsed laser deposition. We have found that the film composition is strongly dependent on the growth temperature and background pressure during growth. Lowering the growth temperature and/or increasing the background Ar or N-2 pressure increases the Te content in the film. When grown in O-2 atmosphere, however, oxidation of the Te species occurred, resulting in much less Te incorporation in the O sublattice. While a lower substrate temperature is needed for the incorporation of more Te in the ZnO1-xTex film, the crystallinity of the film degrades at low growth temperature. X-ray photoemission, soft x-ray absorption, and x-ray emission measurements reveal that the observed drastic narrowing of the optical bandgap with increasing Te content is primarily due to the modification of the valence band. The experimentally observed evolution of the optical properties of ZnO1-xTex HMAs from dilute to mid-composition range (x > 0.3) is analyzed within the framework of a modified band anticrossing model with composition dependent coupling parameters describing the anticrossing interaction between the valence band of the matrix and Te localized states. Electrically, we found that adding Te in ZnO increases the film resistivity. When doped with N, a drastic drop in n from mid-10(19) to 10(15) cm(-3) is observed for ZnO1-xTex with similar to 2%-4% of Te. These electrical behaviors can be understood as the effect of the upward shift of the valence band, which favors the formation of native as well as N acceptors.