Playing with Polarity

TLDR

GaN films grown on sapphire by MOCVD or HVPE typically exhibit Ga‑face polarity, and Ga‑ and N‑face samples differ in structural, morphological, and electronic properties. The review examines how GaN crystal polarity affects epitaxial film and device properties and explores the potential of lateral polarity heterostructures with macroscopic domains separated by inversion domain boundaries. N‑face polarity is achieved by backside removal or by switching polarity during MBE growth with a thin AlN buffer, and lateral polarity heterostructures can be fabricated with macroscopic domains separated by inversion domain boundaries. Different polarities yield distinct Schottky barrier heights, varying positions and properties of spontaneously formed two‑dimensional electron gases, and differing gas and ion adsorption, while lateral polarity heterostructures exploit polarity as a new degree of freedom for studying III‑nitride electronic properties and enabling novel devices.

Abstract

We review the influence of GaN crystal polarity on various properties of epitaxial films and electronic devices. GaN films grown on sapphire by MOCVD or HVPE usually exhibit Ga-face polarity. N-face polarity is obtained either on the backside of such layers after removal from the substrate, or by turning the crystal polarity in MBE growth via a thin AlN buffer layer. In addition to rather obvious differences in their structural and morphological features, Ga- and N-face samples differ also in their electronic properties. Thus, different Schottky barrier heights are observed for both polarities, the position and detailed properties of spontaneously formed two-dimensional electron gases vary with polarity, and the adsorption of gases and ions also show an influence of the two different surfaces. A particular interesting possibility is the growth of lateral polarity heterostructures with predetermined macroscopic domains of different polarity separated by inversion domain boundaries. These structures make use of the crystal polarity as a new degree of freedom for the investigation of electronic properties of III-nitrides and for novel devices.