Electrical and optical properties of degenerate and semi-insulating ZnGa2O4: Electron/phonon scattering elucidated by quantum magnetoconductivity

We study the electrical and optical properties of degenerate ZnGa2O4 films grown by metalorganic chemical vapor deposition (MOCVD) on sapphire and semi-insulating films grown by pulsed laser deposition (PLD) on fused silica. After a forming-gas anneal at 700 degrees C, the MOCVD film is highly conducting, with a room-temperature carrier concentration of 2 x 10(20)cm(-3), a mobility of 20cm(2)/V s, and direct bandgap absorptions at 3.65eV and 4.60eV. Under the same annealing conditions, the PLD film is semi-insulating, with a direct bandgap absorption at 5.25eV. The phonon structure, important for electrical and thermal conduction as well as superconductivity and other quantum phenomena, is very complicated due to the large number of atoms (and, thus, phonon branches) in the unit cell. However, we show that the phonon contributions to electron mobility (mu(ph)) can be directly measured by quantum-based magnetoconductivity over the temperature span T=10-200 K. From an approximate analytical formula, mu(ph) = function (T-ph, T), we calculate an effective phonon energy kT(ph)(T) that takes account of all phonon contributions at temperature T. For T=10-200K, the value of kT(ph) ranges from about 10 to 90meV, consistent with the energy range of the ZnGa2O4 phonon density of states (at 0K) calculated by density functional theory. The total measured mobility can then be modeled by mu(-1)(tot) = mu(-1)(ii) + mu(-1)(ph), where mu(ii) is the mobility due to ionized-impurity scattering. With a high bandgap, controllable conductivity, high breakdown voltage, and bulk-growth capability, ZnGa2O4 offers opportunities for high-power electronics and UV detectors. Published under license by AIP Publishing.