Hajime Okumura

Improvement of film quality using Si-doping in AlGaN/GaN heterostructure grown by plasma-assisted molecular beam epitaxy

Silicon(Si)-doping was employed to improve the film quality of AlGaN/GaN heterostructure by radio frequency plasma-assisted molecular beam epitaxy. By Si doping during the buffer-layer growth, the electrical and structural characteristics of the films were improved. The electron mobility of the two-dimensional electron gas increases without changing the sheet carrier density. The full-width at half-maximums of (1012) X-ray rocking curves reduces. By the atomic force microscopy measurements, smooth surface morphology with micro-steps lower than unit cell height is found. We consider that the improvement of the film quality was caused by the reduction of the nano-pipes due to the growth-mode change into step-flow mode.

AlN/AlGaN/GaN Metal Insulator Semiconductor Heterostructure Field Effect Transistor

We present the characteristics of a novel AlN/AlGaN/GaN metal insulator semiconductor heterostructure field-effect transistor (MIS-HFET) structure with an AlN cap layer as a gate insulating layer. The gate leakage current for the AlN/AlGaN/GaN MIS-HFET was shown to be more than three orders of magnitude smaller than that for the AlGaN/GaN HFET at around -20 V gate bias. This demonstrates that the wide band gap AlN/AlGaN gate structure suppresses the gate leakage current, resulting in improved device characteristics compared with common HFETs in this study. A maximum gm of 134 mS/mm was observed at a 4 V drain-source bias of a MIS-HFET with a gate length (Lg)=0.85 µm, gate width (Wg)=57 µm, and drain-source spacing (Lds)=3 µm. A threshold voltage (Vt) of -3.06 V and a maximum channel current of 551 mA/mm were observed.

Improvement of DC Characteristics in AlGaN/GaN Heterojunction Field-Effect Transistors Employing AlN Spacer Layer

AlGaN/GaN heterojunction field-effect transistors (HJFETs) employing an AlN spacer layer have been fabricated. By employing the AlN spacer layer, the two-dimensional electron gas (2DEG) mobility increased from 650 cm2/(Vs) to 950 cm2/(Vs). With regard to DC operating characteristics, the maximum transconductance gmmax increased from 100 mS/mm to 125 mS/mm. In addition, the Schottky barrier at the gate electrode improved, and the saturation current Is decreased from 1.0×10-9 A to 6×10-11 A.

Improvement of AlGaN/GaN Heterostructure Field Effect Transistor Characteristics by Using Two-Step Ohmic Contact Process

We attempted a new ohmic contact process for two-step metallization, and demonstrated improvement of the ohmic properties and device characteristics. The contact resistance of the two-step ohmic process was improved from 4.2 Ωmm obtained in a conventional contact to 2.6 Ωmm. The specific contact resistivity of the transmission line method ohmic contacts improved by one order of magnitude, from 10-4 Ω-cm2 in the conventional contact sample to 10-5 Ω-cm2 in the two-step contact sample treated with Cl2 plasma. The dc measurements of the HFETs showed improved current-voltage curves in the two-step contact samples. The values of Ron of the AlGaN/GaN HFET with the two-step contact process and the conventional HFET were calculated to be 0.23 mΩ-cm2 and 0.28 mΩ-cm2 in the linear (ohmic) region, respectively.

Roles of Si Irradiation during the Growth Interruption on GaN Film Qualities in Plasma-Assisted Molecular Beam Epitaxy

Roles of Si irradiation during GaN growth interruption in plasma-assisted molecular beam epitaxy are studied. It is found that irradiated Si can modulate the GaN surface energy to enhance the Ga surface diffusion, resulting in a flat GaN surface with monolayer steps as determined by atomic force microscopy observations. Furthermore, cross-sectional transmission electron microscopy reveals that threading dislocations in a GaN film can be greatly reduced by Si irradiation, which agrees very well with the results of high-resolution X-ray diffraction measurement of asymmetric (102) GaN diffraction. These two positive effects of Si irradiation are promising for growing high-quality GaN with excellent surface morphology and low dislocation density for device applications.

Indium Roles on the GaN Surface Studied Directly by Reflection High-Energy Electron Diffraction Observations

Indium roles on the GaN surface are studied by reflection high-energy electron diffraction (RHEED) during interruption of the molecular beam epitaxial growth. It was found that there are two effects of In on the GaN surface. One effect is the surfactant effect, which can change rough N-polarity GaN surfaces to smooth ones. The other effect is the protection effect where In can suppress the evaporation of Ga atoms from the surface, and reduce the reaction between GaN and N radicals to form Ga and N2. These two effects are particularly important for growth that requires the growth interruption in order to fabricate the designed structures.