Computation for Crystalline Alloys – Carrier Mobility and Other Properties

Electronic structure theory provides detailed information about atomic coordinates and vibrational restoring forces of atoms in crystalline solids.   Alloys can be modeled as crystals with very large and disordered unit cells. We are interested in a particular semiconductor alloy (GaN alloyed with ZnO) because of its use as a catalyst for solar water splitting (sunlight plus water à H₂ and O₂;  a clean way to store and use solar energy).  This talk will discuss physical information we may extract from the “data base” available from on-going* electronic structure calculations. Our calculations so far tell us about the short-range order when the cations (Ga and Zn) and the anions (N and O) arrange on the wurtzite lattice during a high temperature synthesis.  Atomic positions deviate in curious ways from the perfectly ordered wurtzite structure. We would like to know whether simple rules can be discovered to account for these deviations. The calculations also give vibrational normal modes in harmonic approximation. I will show how these can be used to predict heat conductivity, when there is sufficient disorder that the usual propagating phonon picture of Peierls, Debye, and Boltzmann breaks down. We will also apply the Kubo-Greenwood theory to study carrier mobility of the photo-excited electrons and holes, in a regime where Bloch’s theorem does not apply. One of the messages is that transport can actually be more easily modeled in alloys than in their simpler crystalline counterparts, because scattering is dominated by static disorder, well accounted by DFT theory, rather than by interactions that are harder to model.

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*Luana Pedroza, Jian Liu, Marivi Fernandez Serra, Carissa Misch, Neerav Kharche