Properties of Ge-doped, high-quality bulk GaN crystals fabricated by hydride vapor phase epitaxy

We investigated the properties of Ge-doped, high-quality bulk GaN crystals with Ge concentrations up to 2.4×1019 cm−3. The Ge-doped crystals were fabricated by hydride vapor phase epitaxy with GeCl4 as the dopant source. Cathodoluminescence imaging revealed no increase in the dislocation density at even the highest Ge concentration, with values as low as 3.4×106 cm−2. The carrier concentration, as determined by Hall measurement, was almost identical to the combined concentration of Ge and unintentionally incorporated Si. The electron mobilities were 260 and 146 cm2 V−1 s−1 for n=3.3×1018 and 3.35×1019 cm−3, respectively; these values are markedly larger than those reported in the past for Ge-doped GaN thin films. The optical absorption coefficient was quite small below the band gap energy; it slightly increased with increase in Ge concentration. Thermal conductivity, estimated by the laser-flash method, was virtually independent of Ge concentration, maintaining an excellent value around 2.0 W cm−1 K−1. Thermal expansion coefficients along the a- and m-axes were approximately constant at 5.0×10−6 K−1 in the measured doping concentration range.