| Co-Investigator(Kenkyū-buntansha) |
OIGAWA Haruhiro University of Tsukuba, Inst.Mat.Sci., Lecturer, 物質工学系, 講師 (60223715)
SHIGEKAWA Hidemi University of Tsukuba, Inst.Mat.Sci., Assistant Professor, 物質工学系, 助教授 (20134489)
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| Research Abstract |
The surface/interface properties of GaAs remain uncontrollable to date, except for the use of heterojunctions, initially, of GaAs/AlGaAs. In particular, the presence of oxygen induces the surface defects, and degrades the performance and reliability of surface devices. In recent years, we found that the surface treatment of GaAs using ammonium polysulfide ((NH_4)_2S_x) solution markedly decreases its surface state densities. For the development of high-performance devices, therefore, it is necessary to epitaxially grow metal and insulator films on the (NH_4)_2S_x-treated surface. So we tried the epitaxial growth of Al and CaF_2 on the (NH_4)_2S_x-treated surface of GaAs with a molecular beam epitaxy (MBE) technique. Here it is important not to collapse a passivating monomolecular layr of sulfur during the deposition, because the effect of (NH_4)_2S_x treatment is brought about by a surface coverage with its layr. Accordingly, we also studied the change in surface/interface structure and chemical state of sulfur with increasing the film thickness by means of reflection high-energy electron diffraction (RHEED), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). As a result, the significant effect of (NH_4)_2S_x treatment was observed as follows :
1)S atoms occupy the near-site of As on top of the surface, where S-Ga bonds are formed.
2)Such a surface is stabilized through charge transfer of excess electrons into the dangling bond states of S,resulting in the reconstruction with S-S dimer structure.
3)The films of Al and CaF_2 are epitaxially grown on the (NH_4)_2S_x-treated surface.
4)When the Al and CaF_2 are deposited, the Al-S and Ca-S bonds are respectively formed at the interface.
5)These interface structures are also stable against the heat treatment.
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