Evaluation of thermal and electrical properties of GaN grown using oxide vapor-phase epitaxy

In this study, we investigated the thermal and electrical properties of gallium nitride (GaN) crystals grown usingthe oxide vapor-phase epitaxy (OVPE) method. We comprehensively evaluated the thermal conductivity and electrical resistivity of OVPE-GaN specimens across the wide oxygen-concentration range of 1019–1021 cm−3, which included a so far uninvestigated ultrahigh doping range beyond 1020 cm−3. The evaluation results demonstrated that the thermal conductivity of the OVPE-GaN specimens decreased monotonically as the oxygen concentration increased, declining to approximately one-fifth of that of specimens composed of unintentionally doped (UID) GaN whose crystals were grown using hydride vapor-phase epitaxy (HVPE) at the highest doping level. Notably, OVPE-GaN specimens with oxygen concentrations in the 1019 cm−3 range exhibited a c axis thermal diffusivity comparable to that of UID-HVPE-GaN specimens. Furthermore, thermal anisotropy was observed in the OVPE-GaN specimens over the mentioned oxygen-concentration range, where c axis thermal diffusivity was higher than in-plane thermal diffusivity because of the unique crystalline structure of the specimens consisting of spatially separated high- and low-oxygen-concentration regions. Although the electrical resistivity also decreased with increasing oxygen concentration, the doping efficiency significantly decreased as the oxygen level increased despite the continued increase in the carrier concentration. Rutherford backscattering spectrometry/channeling measurements provided direct evidence that a large portion of the oxygen impurities in high-concentration OVPE-GaN occupied interstitial positions. The interstitial oxygen atoms functioned as acceptors, resulting in a reduced doping efficiency. Furthermore, the atoms were expected to induce lattice strain, which enhanced phonon scattering, thereby partly reducing the thermal conductivity. These findings are crucial for the thermal management of devices fabricated on OVPE-GaN substrates.