Hidekazu Umeda

Drastic Decrease in Dislocations during Liquid Phase Epitaxy Growth of GaN Single Crystals Using Na flux Method without Any Artificial Processes

The behavior of dislocations of GaN single crystals grown using the liquid phase epitaxy (LPE) technique in a Na flux was investigated using a transmission electron microscope (TEM). Almost all dislocations are concentrated around the interface between LPE-GaN and metal organic chemical vapor deposition (MOCVD)-GaN, which was used as a homoepitaxial substrate. It was revealed that the reduction of dislocation is due to an unusual phenomenon in which many (1011) facets are formed in the initial LPE growth period, followed by the development of the GaN(0001) face with further dislocation reduction. On the basis of our TEM observations and an investigation of the surface morphology at each growth stage, we propose a dislocation reduction mechanism during LPE growth.

Effects of hole traps on the temperature dependence of current collapse in a normally-OFF gate-injection transistor

Kinetic studies on the current collapse of a normally-OFF AlGaN/GaN heterostructure gate-injection transistor (GIT) subject to current collapse have been performed above room temperature. The current collapse becomes more severe as the temperature increases, for which we clarified the physical mechanism based on a device simulation study that the hole traps in the epilayer play an important role. As the temperature increases, hole emission from the hole traps is stimulated, which causes sharper potential bending on the drain side in the OFF state, leading to more severe current collapse. The detailed dynamics of holes and the resultant energy profiles in the switching are discussed.

LPE Growth of Bulk GaN Crystal by Alkali-Metal Flux Method

We succeeded in growing a GaN single crystal substrate with diameter of about two inches using the Na flux method. Our success is due to the development of a new apparatus for growing large GaN single crystals. The crystal grown in this study has a low dislocation density of 2.3×105 (cm-2), The secondary ion mass spectrometry (SIMS) technique demonstrates that the Na element is difficult to be taken in the crystal in both the + and – c directions, resulting in a Na concentration of 4.2 × 1014 (cm-3) in the crystal. Our success in growing a two-inch GaN substrate with a low impurity content and low dislocation density should pave the way for the Na flux method to become a practical application.