Epitaxial Growth of AlN-Based Heterostructures for Electronics

Abstract

The aluminum nitride (AlN) platform is positioned to be a top contender for the next-generation of communication systems, by enabling integration of both p-channel and n-channel active RF transistors, with passive components such as filters and waveguides. The advantages of the platform primarily come from the AlN material properties of high thermal conductivity, excellent electrical insulation, and the ability to generate high-density polarization-induced carriers. Naturally, any AlN-based electronic device is only as good as the quality of crystal material used to fabricate it. This makes the crystal synthesis of high-quality AlN stack the critical first step in realizing the vision of the integrated AlN platform. This chapter provides the detailed recipes developed by the author to epitaxially grow AlN-based heterostructures with 2D electron (2DEG) and hole gases (2DHG) using plasma assisted molecular beam epitaxy (PA-MBE). The polarization differences at the heterointerfaces are expected to generate high-density 2DEGs and 2DHGs in undoped metal-polar GaN/AlN and AlN/GaN/AlN heterostructures on thick AlN buffer layers. However to demonstrate these experimentally, careful material engineering and epitaxial crystal growth optimization are necessary to ensure high chemical and structural purity. First the epitaxial growth process of the metal-polar GaN/AlN 2DHG heterostructure is presented. Impurity blocking layers (IBLs) are incorporated in the AlN buffer layer to ensure an impurity-free heterointerface. Growth studies are performed to determine the optimal conditions for sharp, abrupt heterointerfaces and uniform, large area growth across full wafers. Next, the epitaxial growth of the AlN/GaN/AlN heterostructure for best 2DEG transport is detailed. In-situ cleaning of the 6H-SiC starting substrates is critical for a clean nucleation and high crystal quality of the subsequent epitaxial layers. The growth of thick AlN layers for in-situ passivation schemes is also presented. These growth techniques result in some of the purest forms of these novel AlN-based heterostructures, as evidenced by the ensuing device performance. These epitaxial growth techniques will allow researchers to reproduce the results presented in this work, and to gain important insights that can be used for epitaxial growth of other heterostructures such as DUV LED/LASER stacks.