Polarization-Induced 2D Electron and Hole Gases Homoepitaxially Grown on Single-Crystal AlN Substrates

Abstract

Abstract Body: Owing to their noncentrosymmetric crystal structure, III-nitride semiconductors exhibit internal polarization fields which can be exploited to dramatically expand the design space of polar heterostructures. Due to their strength (1-10 MV/cm), these built-in electric fields can be harnessed for the realization of novel device functionalities via band-structure engineering and polarization-induced doping. This fundamentally different doping technique completely dispenses with the need of impurity donors or acceptors to generate free carriers, enabling the realization of highly mobile 2D electron and hole gases (2DEGs and 2DHGs) in nitride heterostructures. While polarization-induced doping has been previously engineered in nitride structures grown atop sapphire templates, SiC wafers, and single-crystal GaN substrates; further improvements in device performance are expected when 2DEGs and 2DHGs are grown on high-quality single-crystal AlN substrates. This is possible thanks to the high thermal conductivity and high breakdown electric field of bulk AlN crystals. These outstanding material properties allow not only improved thermal management for high-power devices but also takes advantage of the widest bandgap within the family of nitride semiconductors. This last benefit, however, also prevents efficient generation of free carriers via traditional impurity doping techniques. In this scenario, we leverage the intense built-in spontaneous and piezoelectric polarization fields to generate both types free carriers via polarization engineering. In this presentation, we report, for the first time, the epitaxial growth and transport properties of polarization-induced 2DEGs and 2DHGs in GaN/AlN heterostructures, homoepitaxially grown on single-crystal AlN substrates. The simultaneous demonstration of complementary 2DEG and 2DHG in undoped heterostructures and on a single substrate platform, enables an unmatched level of integration for high-power and high-frequency nitride-based electronics, raising hopes for all-nitride CMOS technology.