Michinobu Tsuda

Flat (1120) GaN Thin Film on Precisely Offset-Controlled (1102) Sapphire Substrate

A high-quality thin (1120) GaN layer with an atomically flat surface has been successfully grown on a precisely offset-angle-controlled (1102) sapphire substrate by metal-organic vapor phase epitaxy. The insertion of an AlGaN layer between the underlying AlN layer and the GaN layer was found to improve the crystalline quality of the upper GaN layer.

Anisotropically biaxial strain in a-plane AlGaN on GaN grown on r-plane sapphire

In this study, the anisotropically biaxial strain in a-plane AlGaN on GaN is investigated by X-ray diffraction (XRD) analysis using an AlGaN/GaN heterostructure grown on r-plane sapphire. In accordance with XRD reciprocal lattice space mapping, when the AlN molar fraction x in the AlGaN layer is 0.18, the AlGaN layer is fully strained under tensile stress and grows coherently on the underlying GaN layer. However, when x is as large as 0.31, partial relaxation is observed only in the c-axis direction. The tensile stress in the AlGaN layer is calculated taking the actual in-plane lattice constants of the underlying GaN layer into account, and it was found that the stress in the a-plane AlGaN layer in the c-axis direction is approximately 1.7 times larger than that in the m-axis direction.

Impact of H2-Preannealing of the sapphire substrate on the crystallinity of low-temperature-deposited AlN buffer layer

Effect of H2-preannealing of sapphire substrate on low-temperature-deposited (LT) AlN buffer layer deposited by metalorganic vapor epitaxy has been investigated. Crystallinity of LT-AlN buffer layer drastically changes with preannealing temperature variation. It is found that H2-preannealing of sapphire substrate is a requisite to get a better quality LT-AlN and as a consequence it leads to growth of better quality GaN epi-layer on it.

Influence of Ohmic Contact Resistance on Transconductance in AlGaN/GaN HEMT

The correlation between ohmic contact resistivity (ρ c ) and transconductance (g m ) in AlGaN/GaN high-electron-mobility transistors (HEMTs) was investigated. To characterize p c precisely, we fabricated a circular transmission line model (c-TLM) pattern adjoined to a field-effect transistor (FET) pattern on an HEMT. By measuring ohmic contact resistance and sheet resistance using the adjoined c-TLM, intrinsic transconductance (g m0 ), which is not influenced by the source resistance, can be estimated. The g m0 thus obtained is between 179 and 206 mS/mm. Then, it became possible to calculate the correlation between g m and ρ c . We found that ρ c should be below 10 -5 Ωcm 2 for the improvement of g m in AlGaN/GaN HEMT when R sh 400 Ω/□.

Impact of H 2 -preannealing of the Sapphire Substrate on the Crystallization of Low-Temperature-Deposited AlN Buffer Layer

The effect of H2-preannealing of sapphire substrate on low-temperature (LT- ) AlN buffer layer deposited by metalorganic vapor phase epitaxy is investigated. Crystallinity of LT-AlN drastically changes with preannealing temperature variation. It is found that H2-preannealing of sapphire substrate is a requisite to get a better quality LT-AlN and as a consequence it leads to growth of better quality GaN epi-layer on it.