【International Papers】Substrate-orientation dependence of β-Ga₂O₃ nitridation by RF nitrogen plasma for GaN heterojunction diodes

      Researchers from the University of Illinois Urbana-Champaign have published a dissertation titled "Substrate-orientation dependence of β-Ga₂O₃ nitridation by RF nitrogen plasma for GaN heterojunction diodes" in Journal of Applied Physics.

Background

      β-Gallium oxide (Ga₂O₃) has been implemented for power and optical microelectronic devices, demonstrating high purity and high crystallinity with controllable n-type doping. However, p-type doping has not been demonstrated to the degree of control required for power devices. Given the extremely low dispersion leading to large hole effective mass, generated holes are polaronic in nature and are unable to contribute meaningfully to conduction. As an alternative, heterojunction devices have been explored, with demonstrated success in p-type gallium nitride (p-GaN). GaN-β-Ga₂O₃ heterojunctions have complementary properties, both are classified as wide (or ultra-wide) bandgap, and have high electron saturation velocity and large critical field strength. Furthermore, GaN has a robust p-type dopant in the form of magnesium (Mg).

Abstract

      The influence of substrate orientation on the structural properties of heteroepitaxial GaN films grown on nitridated β-Ga₂O₃ (BGO) is investigated. The nitridation of (100), (001), and (-201) β-Ga₂O₃ was monitored with reflection high energy electron diffraction and x-ray photoelectron spectroscopy, suggesting the reaction proceeds through the formation of an oxynitride phase in all cases. The nitridation rate (Γ) under fixed nitridation conditions was observed to increase monotonically with the inverse of the surface free energy of the starting substrate, Γ(001) < Γ(-201) < Γ(100). Hexagonal GaN is realized on all orientations, however, (001) BGO nucleates an undesired (101) plane GaN interface. The presence of restructured BGO grains is also observed on (100) BGO. Analysis of XRD pole figures suggests that twinned lamellae are not translated into the GaN epitaxy after nitridation of (-201) or (100), though may be present when grown on (001), attributed to large misfit strain in this orientation. Mg:GaN grown on (201) BGO retained p-type conductivity and low FHWM of the 0002 x-ray rocking curve, indicating (-201) BGO being the most conducive of the tested interfaces for GaN heteroepitaxial device fabrication.