Theoretical and experimental investigations of vertical hole transport through unipolar AlGaN structures: Impacts of random alloy disorder

Abstract

We report on the vertical hole transport through unipolar unintentionally doped (UID) and p-type doped AlGaN heterostructures to evaluate the effectiveness of the UID and doped AlGaN as barriers to the hole transport. Band diagram and current density–voltage (J–V) simulations are conducted in one-dimensional and three-dimensional schemes, with the latter including compositional fluctuations within the alloy AlGaN barrier layer. The simulation results using a self-consistent Poisson-drift diffusion scheme, incorporating the Localization Landscape theory, indicate a large asymmetric barrier to the hole transport by UID AlGaN. The asymmetric J–V characteristics are attributed to the asymmetric band diagrams calculated for the unipolar structure. The simulation results are verified by experiments using unipolar vertical hole transport structures enabled by n-to-p tunnel junctions (TJs) grown by ammonia molecular-beam epitaxy. The TJ structures are utilized to minimize the issues with the high spreading resistance of p-regions and to eliminate the need for its dry etching, which normally results in degraded p-contacts. The experimental results show that even a thin UID AlxGa1−xN (x = 14%, 13 nm) introduces an asymmetric barrier to the hole transport; a nearly 100% increase in the voltage drop induced by a thin UID AlGaN at 50 A/cm2 in the reverse direction is observed compared to an only 25% corresponding increase in the forward direction. Furthermore, p-type doping of the AlGaN layer results in a drastic drop in the potential barrier to hole transport in both directions. The results are beneficial for understanding the behavior of various structure designs within optoelectronics and power electronics.