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

摘要

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 Al_xGa_(1-x)N (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/cm~2 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.