Development of oxide semiconductor and dielectric electronics using helicon-wave-excited-plasma sputtering methods

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To progress 'Oxide Optoelectronics' using the helicon-wave-excited-plasma sputtering (HWPS) method, deposition of polycrystalline thin films, epitaxial growth of semiconductors, deposition of multilayer structures of dielectric materials, and metal oxides were carried out. Significant results are following.i) Effectiveness of the high-temperature-annealed self-buffer layer (HITAB) on the heteroepitaxy of ZnO and MgZnO was investigated for the epitaxial growth using HWPS method, namely HWPSE. Atomically flat HITAB was obtained by depositing 100-nm-thick films of ZnO or MgZnO at 500-600 ℃ followed by high-temperature annealing at 900-1000 ℃ for 1 hr. The ZnO epilayers grown on ZnO HITAB prepared on a-plane Al_2O_3 substrates exhibited a free-excitonic photoluminescence (PL) peak at low temperature, and the PL spectra at 300 K were dominated by the near-band-edge excitonic emissions.ii) As transparent conducting oxide (TCO) films for photovoltaic (PV) applications, indium tin oxide (ITO) as well as polycrystalline ZnO doped by Al or Ga were deposited by the HWPS method. The electron concentration of Ga-doped ZnO (ZnO : Ga) films was as high as 10_<20> cm_<-3>. For the light absorbing layers, good quality Cu (Ga_xIn_<1-x>)Se_2 films were prepared by the selenization technique using metalorganic sources.iii) Distributed Bragg reflectors (DBRs) composed of 8-pair SiO_2/ZrO_2 dielectric multilayers, of which central wavelength was tuned at B-exciton resonance wavelength of ZnO (366.5 nm), were fabricated using the reactive HWPS (R-HWPS) method. The reflectivity at 366. 5 nm was higher than 99. 5% and the stop-band width (R【greater than or equal】95%) was as large as 82 nm.iv) Delafossite structure CuAlO_2 films, which are expected to be p-type TCO films, were deposited by HWPS using Cu and Al metal targets, as well as by R-HWPS method using CuAlO_2 sintered target. Although low resistivity films are not yet obtained, delafossite structure was confirmed.