Li-Doped NiO Epitaxial Thin Film with Atomically Flat Surface
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H. Ohtab) Hosono Transparent ElectroActive Materials Project, ERATO, JST, KSP C-1232, 3-2-1 Sakado, Takatsu, Kawasaki 213-0012, Japan
M. Kamiya Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 226-8503, Japan
K. Nomura Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 226-8503, Japan; and Hosono Transparent ElectroActive Materials Project, ERATO, JST, KSP C-1232, 3-2-1 Sakado, Takatsu, Kawasaki 213-0012, Japan
K. Uedac) Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 226-8503, Japan
M. Hirano Hosono Transparent ElectroActive Materials Project, ERATO, JST, KSP C-1232, 3-2-1 Sakado, Takatsu, Kawasaki 213-0012, Japan
H. Hosono Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midori, Yokohama 226-8503, Japan; and Hosono Transparent ElectroActive Materials Project, ERATO, JST, KSP C-1232, 3-2-1 Sakado, Takatsu, Kawasaki 213-0012, Japan (Received 30 September 2003; accepted 8 December 2003)
Li-doped NiO epitaxial films with high electrical conductivity and atomically flat stepped surfaces were fabricated by a combined technique of pulsed laser deposition and subsequent annealing. It was determined that subsequently annealing at temperatures as low as 600 °C significantly decreased electrical conductivity due to Li evaporation when the film surface was not protected from Li evaporation. To suppress Li evaporation, a yttria-stabilized-zironia plate was used to cover the film surface, which raised the annealing temperature up to 1300 °C while maintaining a high Li concentration and electrical conductivity. Thermally annealing at this temperature also improved crystal quality and formed epitaxial films with atomically flat stepped surfaces. The films were single crystalline at least in observation areas, 10 m × 10 m. A reasonably large Hall mobility approximately 0.05 cm2/Vs similar to that reported for bulk single-crystal NiO and a visible-light transmission in excess of 75% were obtained on 120-nm-thick films. Although annealing at higher temperatures such as 1400 °C can further improve the structural and optical properties, the Li concentration in the films was decreased to 1000 °C can improve the film structure. However, if used for p-layers in optoelectronic devices, Li+ doping is necessary to increase the hole concentration.6,28 In this case, it is difficult to grow NiO:Li films that have both high crystalline quality and high conductivity because a high-temperature process, which is necessary for high-quality film, causes serious Li evaporation.29 An approach to overcome this 914
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kind of difficulty is to temporally separate the formation of the film and the modification of the film structure. This concept has been applied to other oxides using the reactive solid-phase epitaxy technique.30,31 We recently reported the fabrication and characteristics of NiO:Li/ZnO p/n heteroepitaxial junction d
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