Transport properties, specific heat and thermal conductivity of GaN nanocrystalline ceramic

The structural and transport properties (resistivity, thermopower and Hall effect), specific heat and thermal conductivity have been measured for GaN nanocrystalline ceramic prepared by hot pressing. It was found that the temperature dependence of resistivity in temperature range 10–300 K shows the very low activation energy, which is ascribed to the shallow donor doping originating in amorphous phase of sample. The major charge carriers are electrons, what is indicated by negative sign of Hall constant and Seebeck coefficient. The thermopower attains large values (−58 μV/K at 300 K) and was characterized by linear temperature dependence which suggests the diffusion as a major contribution to Seebeck effect. The high electron concentration of 1.3×1019 cm−3 and high electronic specific heat coefficient determined to be 2.4 mJ/molK2 allow to conclude that GaN ceramic demonstrates the semimetallic-like behavior accompanied by very small mobility of electrons (∼0.1 cm2/V s) which is responsible for its high resistivity. A low heat conductivity of GaN ceramics is associated with partial amorphous phase of GaN grains due to high pressure sintering.