Toshiyuki Takizawa

Structural changes on annealing of MBE grown (Ga, In)(N, As) as measured by X-ray absorption fine structure

Abstract GaInNAs grown on GaAs has recently been found to optically emit at wavelengths longer than previously possible with material grown epitaxially on GaAs substrates. To improve radiative efficiency, material is annealed after growth, after which the band gap is found to significantly blueshift. Structural changes that occur during this annealing process were studied using fluorescent X-ray absorption fine structure. By comparing the absorption data with lattice parameter and band structure simulations, it was found that the number of In atoms surrounding N atoms increased after the high-temperature annealing. According to ab initio simulations this ordering will increase the band gap of the GaInNAs alloy. We believe the reduction of free energy drives the reordering process towards increasing In–N coordination.

High-Voltage Isolation Technique Using Fe Ion Implantation for Monolithic Integration of AlGaN/GaN Transistors

Ion implantation technique can be applied for planar isolation of AlGaN/GaN heterojunction field-effect transistors (HFETs), which enables high-density integration of the power switching transistors. So far, the reported isolation using ion implantation for GaN devices has never maintained high isolation voltages after high-temperature processing over 800 °C which is commonly used for the fabrication. In this paper, we present detailed analysis and mechanism of thermally stable isolation of GaN devices by Fe ion implantation keeping high breakdown voltage between the devices after high-temperature annealing. Ion species forming deep levels at atomic sites in GaN are examined by using first-principle calculation prior to the experiments. The calculation indicates that the Fe ions stay at Ga sites with deep levels in GaN. The following experiments using various ion species well agree with the aforementioned predictions, where implanted regions by other ions than Fe exhibit reduction of the resistivity after high-temperature annealing to recover the processing damage by the ion implantation. As a result, it is experimentally found that Fe is the only choice to serve high resistivity after the annealing. The Fe ion implantation enables high breakdown voltage of 900 V after the annealing at 1200 °C. This technique is indispensable to enable monolithic integration of the lateral AlGaN/GaN HFETs for high-voltage power switching systems.