Conductivity and Hall effect characterization of highly resistive molecular-beam epitaxial GaN layers

Highly resistive molecular beam epitaxial GaN layers are characterized by temperature dependent conductivity and Hall effect measurements. Seven n-type GaN samples with room temperature layer resistivity ranging between 8 and 4.2×106 Ω cm are used in this study. The experimental data are analyzed by considering various transport models such as band and hopping conduction, scattering on charged dislocations and grain boundaries controlled transport. The same defect level of 0.23 eV, attributed to nitrogen vacancy, is found for layers with ρ300⩽3.7×103 Ω cm. The Hall mobility for two lower resistivity layers is influenced mainly by phonon scattering (μH∼Tx, x=−1.4). However, higher resistivity layers show positive mobility power, x=0.5–0.9, which can be explained by dominating scattering on charged dislocations. Properties of layers with the highest resistivity (1×105 and 4.2×106 Ω cm) and extremely low Hall mobility (6 and <0.1 cm2 V−1 s−1) are consistent with grain boundary controlled transport. The barrier height between grains of 0.11 eV and an average grain size of 200 nm are found. Neither nearest-neighbor or variable range single phonon hopping nor multiphonon hopping can be clearly attributed to the conduction of the layers investigated.