Quantum magnetoconductance, delta-sigma(B)=sigma(B)-sigma(0), offers a new, unique way to study phonons in heavily-doped, complex semiconductors, including beta-Ga2O3 and Sn-doped In2O3 (ITO). At low temperature, theory predicts delta-sigma(B)=2.908B^(1/2) S/cm, shown to be true for thick, but not thin, samples. We grew ten ITO films by PLD on fused silica, d=13–292 nm. The thickness dependence was explained by a new delta-sigma(d)-vs-d theory based on a second source of disorder, interface-generated defects that decrease exponentially with distance from the ITO/FS interface. A fit of delta-sigma(d)-vs-d gives three parameters, including d*, the thickness above which the surface is not affected by interface damage.
ZnGa2O4 is a ultra-wide band bandgap transparent oxide with electron transport properties similar to those of the popular b-Ga2O3 but with a higher lattice symmetry. Recent experimental work has been producing high-quality ZnGa2O4 crystals. Here we present our ongoing first-principles modeling work on the structural, thermal, and electronic properties of ZnGa2O4. Elastic stiffness tensor modeling based on symmetry-allowed deformations provides an insight into the effect of symmetry on lattice dynamical properties. We obtain thermodynamical properties such as thermal expansion in the quasiharmonic approximation based on phonons from finite-displacement supercell approach. The phonon dispersions and density of states are compared to those of b-Ga2O3. We find a large number of optical phonons at low energies <15 meV but with higher symmetry by comparison to b-Ga2O3. The large number of optical modes has a signature both in quantum magnetoconductance measurements and in the breakdown field, the latter being a metric relevant for applications in power electronics.
Recent developments in growing highly n-doped wide bandgap oxides such as β-gallium oxide (β − Ga2O3) and more recently zinc gallate (ZnGa2O4) have opened avenues toward important applications, such as transparent electrodes and ohmic contacts. Magnetoconductivity measurements provide a unique method to assess the contribution of phonons to mobility over a wide range of temperatures. For β − Ga2O3 and ZnGa2O4, initial attempts to interpret the measured magnetoconductivity raised fundamental questions about the interplay between the large number of phonon modes in these lattices, electron–phonon scattering, and lattice disorder. Here, we use density functional theory modeling of electron–phonon scattering to help rationalize magnetoconductivity measurements for a wide range of electron concentrations n and temperatures in β − Ga2O3 and ZnGa2O4. The results provide a first-principles understanding of dominant low-field mobility features suggested by phenomenological models used traditionally for semiconductors with high lattice symmetry.
We have employed the framework of quantum magnetoconductivity to develop an experimental method of directly measuring the mobility µph representing inelastic electron-phonon scattering at a given temperature T. Further, we relate µph to material parameters via an equation µph(T) = function(T,Tph,m*,eps-0,eps-inf), where m* is the effective mass, eps-0 and eps-inf are the static and high-frequency dielectric constants, and kTph is an effective phonon energy that represents all the phonon interactions contributing to µph at temperature T. We apply this methodology to an “old” material ZnO; an exciting “new” material, β-Ga2O3; and a combination of the two, ZnGa2O4.
Following demonstrations of bulk and epitaxial growth, zinc gallate ZnGa2O4 has been receiving increased attention for power electronics applications due to bandgap and high and high carrier concentration mobility comparable to those of beta-Ga2O3. Here we use first principles calculations to study stability of the direct and inverted spinel structures of ZnGa2O4 as a function of temperature and the formation energies of native defects: Zn, Ga, and O vacancies, the Zn/Ga, Ga/Zn antisites that provide insights into the nature of shallow donors from high temperature growth, and the higher-energy cation-anion antisites. In addition, we discuss aspects of the electronic structure in the presence of aluminum incorporation.
Recent developments in growing highly n-doped wide bandgap oxides like β-Ga2O3 and more recently ZnGa2O4 hint to important applications, such as transparent electrodes and ohmic contacts. Attempts to interpret the phonon contribution to the measured magnetoconductivity raise fundamental questions on the interplay
between the large number of phonon modes in these lattices, electron-phonon scattering, and
lattice disorder. Here, we present density functional theory (DFT) modeling of electron-phonon
scattering for a wide range of n-carrier concentrations and temperatures in β-Ga2O3 and
ZnGa2O4 . The calculated coupling strengths are corroborated with zone-unfolded phonon
spectra extrapolated from a series of simulation supercells of different sizes and with calculated
dielectric functions. The results provide a first-principles understanding of dominant low-field
mobility features suggested by phenomenological models used traditionally for semiconductors
with high lattice symmetry.
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