First Principles Modeling of High Field Transport in Ultra-Wide Bandgap Materials

Ultra-wide band gap (UWBG) materials offer the potential for greatly improved power electronic device performance due to their predicted higher breakdown fields limited by avalanche breakdown, as well as their favorable transport characteristics such as high mobility and drift velocity, which reduce on-resistance and allow for high frequency operation in power conversion applications. Experimental data on the high field transport properties of UWBG materials such as the impact ionization coefficients are relatively limited, with considerable variability. Hence, to understand the limits of performance of these materials, we report on first principles theoretical calculations of the high field transport properties of UWBG materials using a combination of ab initio calculations of the electronic and phononic structure coupled with particle based full-band Cellular Monte Carlo (CMC) high field transport simulation.<br/><br/>The electronic structure is computed using the GW method based on the BerkeleyGW code, which accurately predicts the bandgaps and excited states of UWBG materials. The phonon dispersion is calculated from DFPT (density functional perturbation theory) using Quantum Espresso. The full wave-vector dependent deformation potentials are computed using the GW eigenvectors as input to the EPW (electron-phonon using Wannier) code, to calculate the electron-phonon scattering rate from first principles. The calculated electronic structure, phonon dispersion, and anisotropic electron-phonon deformation potentials are then input to the CMC code. The CMC code has been developed in-house for a number of years, and simulates the dynamics of an ensemble of charge carriers using scattering rates tabulated in a large look-up table, which allows computationally efficient simulation of non-equilibrium carrier dynamics across the entire Brillouin zone. Besides electron-phonon and defect scattering, we calculate the band-to-band impact ionization scattering rate directly from the GW electronic structure, using a screened Coulomb interaction based on the full band frequency-dependent Lindhard dielectric function. Based on these scattering mechanisms as input, transport quantities such as the velocity-field characteristics and impact ionization coefficients as a function of field are calculated from full band Cellular Monte Carlo (CMC) simulation.<br/><br/>We have applied this framework initially to diamond in comparison to available high field transport data. One important observation is that while the critical field depends strongly on the material bandgap, the relative magnitude of the deformation potential plays an important role as well. Lower values of the deformation potential lead to more energetic electron and hole populations which favor impact ionization, hence reducing the breakdown field. We compare different approximations of the deformation potential in relation to the simulated impact ionization coefficients and their impact on breakdown. The impact of other scattering processes due to defects such as ionized impurities on the high field properties are also under investigation. We also are currently investigating other UWBG materials, specifically BN and AlN, and will report on their high field properties as well.