Developing a hybrid single band carrier transport model for (Al,Ga)N heterostructures

Abstract

Aluminium gallium nitride (Al,Ga)N alloys and heterostructures are used in the development of UV light emitting devices, and can emit at energies extending into the UV-C spectral range. In the UV-C wavelength window and thus at high AlN content, devices exhibit poor quantum efficiencies. In order to aid the development of these devices, simulation techniques which capture the essential physics of these materials and heterostructures should be used. Due to a change in band ordering in a quantum well at compositions close to Al\(_{0.75}\)Ga\(_{0.25}\)N, special attention should be given to the treatment of valence band states in device simulation. In this work we develop a hybrid single band effective mass model which is informed by degree of optical polarization data obtained from atomistic multi-band calculations. Overall, the hybrid single band effective mass model is benchmarked against tight-binding electronic structure calculations. To do so a confining energy landscape is extracted from the tight-binding model and used as input for the single band effective mass calculations. Moreover, the extracted tight-binding energy landscape is transferred to a drift-diffusion model, allowing therefore for a multi-scale study of transport properties of a single (Al,Ga)N quantum well embedded in a p-i-n junction. Our results show that wider wells lead to a lower turn-on voltage due to a reduction of the band gap, but the internal quantum efficiency of these wells is lower than in narrower wells. Alloy disorder leads to carrier localization and an uneven distribution of recombination within the quantum well plane, which gives rise to percolation currents. A comparison of results with ‘pure’ band simulations shows that when TE emission dominated, the heavy hole mass is a good approximation. In contrast, where band mixing was strong between heavy hole and split-off bands the mass from the split off band was very effective.