Study on the polarity control of hexagonal-structure widegap compound semiconductors and its effect on the material control

点击翻译按钮获取中文摘要

Widegap compound semiconductors such as GaN have the hexagonal wurtzite structure and are lack in symmetry along the c-axis. Reflecting this property, the epitaxy process itself and many properties of these materials were greatly affected by the polarity/direction during growth. However, the mechanism for the polarity selection and its control during growth of these materials were not clear.In this research, first we investigated the polarity control mechanism in both MOVPE and MBE of GaN, and we proposed a successful method for the polarity inversion from N-polarity to Ga-polarity, where Al-bilayer played an important role. It was found that the All-covered surface was chemically/thermally stable and Al-bilayer could be frozen into the crystal during growth.Next, the polarity control of ZnO was investigated. The polarity control of oxides was complicated compared to that of nitrides. This was partly attributed to the fact that oxides tended to be amorphous, in particular at low temperatures, the ZnO buffer deposited at low-temperatures sometimes could not keep the epitaxy relationship between the substrate and epilayer. We proposed to use nitrides as buffer layers resulting in successful polarity control.Finally we extended our work to the InN epitaxy for the first time and we clarified that the N-polarity growth regime was preferable in the epitaxy of InN itself and its material control as well. This was because the InN easily decomposes at such low temperatures below 500 C but the epitaxy temperature for the N-polarity growth could be as high as 600 C, which was about 100 deg higher than that of In-polarity growth. On the basis of these results, we succeeded in growth of device-quality InN, InN-based ternary alloys, such as InGaN and InAlN, and very fine structure SQW/MQW consisted of InN/InGaN and InN/InAlN heterostructures for the first time.