The Influence of Phonon Emission on Electron Transport in Hexagonal and Cubic Gallium Nitride

Electron transport characteristics of GaN crystals in high electric fields are shown to be essentially influenced by different optical phonon modes inherent to the hexagonal and cubic phases of these compound crystals. Additional optical phonon modes (≈ 26 meV) competing with the higher-energy ones (≈ 92 meV) in hexagonal GaN, together with the low-lying satellite valley (Γ3), dramatically reduce the drift mobility of electrons in comparison with conventional models. Presented Monte Carlo data are in excellent agreement with the time-of-flight experiment. The cubic GaN crystal phase with its satellite electron energy valleys shifted well above the main (Γ1) valley is chosen as a convenient medium for elucidating the role of high-energy longitudinal optical phonon accumulation. A simple efficient one-particle Monte Carlo method is proposed for an account of excess phonons. Phonon heating is shown to bring about the moderate additional reduction of the drift mobility and an increase in mean electron energy.