First-Principles Calculations on the Electronic and Optical Properties of Polar Functional Materials

Abstract

Advanced computational methods are continually pushing the boundary of modern materials science. They can guide experimental inquiry by surveying a large number of materials for functional properties as well as guide the design and synthesis of new materials with targeted properties, at a much faster speed than experimental approaches. This rise in predictive insight has helped foster the semiconductor revolution, and in turn, created technologies which make a direct impact on global challenges such as the energy crisis and global warming. However, even with this success, there are a wide variety of materials left to be examined in detail.

In this work two-dimensional (2D), hydrogen-passivated III-nitrides are examined. Density functional theory and many-body perturbation theory are applied to produce accurate band structures and use the Bethe-Salpeter Equation to predict exciton binding energies. The strong polarization in III-nitride monolayers results in a significant quantum-confined Stark effect (QCSE), reducing the large band gaps caused by quantum confinement. Using the polarization as a degree of freedom in constructing bilayer heterostructures, it is shown that aligned orientations yield relatively small band gaps (1.9 eV - 3.2 eV), whereas in anti-aligned orientations there is no QCSE and the gap remains large (>4.4 eV). Exciton binding energies are on the order of 1 eV. These results show that UV emission from 2D GaN is possible andthat the optical gaps of 2D III-nitrides span the visible and UV spectra.

The second portion of this work is dedicated to examining the carrier mobilities of various semiconductors using density functional theory, many-body perturbation theory, and the electron-phonon Wannier method. The phonon-limited hole mobility of Cu2O is determined and it is shown that at room temperature it is polar optical modes which are predominantly responsible for carrier scattering. Four ultra-wide-band-gap(UWBG) materials, rs-BeO, wz-BeO, zb-BeO, and MgO, are then examined. Their ultra-wide band gaps (>6 eV) highlight their promise in high-power electronics, and their electron carrier mobilities are high as well (>107cm2/Vs at room temperature). Finally, the carrier mobility of cubic BN and diamond are examined since experimen-tal results on cBN hole mobility span two orders of magnitude. Electron-phonon coupling matrix elements are evaluated to show that acoustic mode coupling is lower in diamond than in cBN. It is also shown that the room temperature scattering rate of holes is much faster in cBN than diamond. Overall, electron mobilities are comparable while cBN hole mobility (80.4 cm2/Vs) is lower than that of diamond (1970 cm2/Vs).

These computational results emphasize the applicability of 2D and UWBG materials to optoelectronic devices and suggest that polar materials provide a wide degree of useful functional properties.