【International Papers】Effect of lithium diffusion into Ga₂O₃ thin films

      Researchers from the University of Parma have published a dissertation titled "Effect of lithium diffusion into Ga₂O₃ thin films" in Applied Surface Science.

Background

      Gallium oxide (Ga₂O₃), listed among the ultra-wide bandgap semiconductor materials, has garnered significant attention in the last decade. It can be stabilized as five different polymorphs, i.e., α, β, γ, δ, and κ, all showing an ultra-wide energy band-gap (E_g ≈ 5 eV). The monoclinic β-Ga₂O₃ is the thermodynamically stable phase and, by far, the most investigated one. Indeed, the possibility to synthetize it as a bulk crystal, to tune its resistivity over a wide range through extrinsic doping with deep acceptor as well as shallow donors, together with the large predicted high breakdown field (E_br ≈ 8 MV/cm), high chemical and thermal stability, combined whit a high radiation hardness, makes this material appealing for various electronic and optoelectronic devices, e.g., Schottky barrier diodes (SBDs), metal–semiconductor and metal-oxide field effect transistors (MESFETs and MOSFETs) Nevertheless, the monoclinic polymorph suffers from the intrinsic low symmetry of its crystal structure, resulting in anisotropy of several functional properties and challenging homo- and heteroepitaxial growth. Indeed, its (−201)-oriented heteroepitaxy on various substrates as well as the homoepitaxy on nominally oriented (100) and (−201) substrates, allows for a geometrical twinning of its monoclinic cell, resulting in the formation (due to the presence of the monoclinic angle) of poly-oriented incoherent boundaries. For this reason, so far only limited homoepitaxial orientations can guarantee low concentration of structural defects. For the demanding field of power-electronic devices, the β-Ga₂O₃ (010)-homoepitaxy is currently the most widely employed one.

Abstract

      The integration of lithium based compounds (e.g., Li:NiO/Ga₂O₃, LiGa₅O₈/Ga₂O₃) in Ga₂O₃ based pn-heterojunctions raises concerns about interface stability, i.e., Li diffusion effects on Ga₂O₃ properties. In this work the ex-situ diffusion of Li is investigated in three different Ga₂O₃ epilayers systems [(001) κ-Ga₂O₃ and (−201) β-Ga₂O₃ heteroepitaxy on (001) α-Al₂O₃, and (010) β-Ga₂O₃ homoepitaxy] at relevant temperatures for the synthesis / processing of Li-based epilayers. It is here experimentally demonstrated and quantified the Li diffusion in all the investigated Ga₂O₃ epilayers systems and Li bulk (D_Li,bulk) and 2D defects (D_Li,2D) diffusion coefficients are provided. In the case of the (010) β-Ga₂O₃ homoepitaxial layer (nominally free of structural defects), hybrid functional theory calculations foresee a diffusion mechanism mediated by Ga vacancies (V_Ga). Moreover, in the (010) β-Ga₂O₃ homo-layer a significant effect on its functional properties (e.g., additional Raman vibrational modes, induced conductivity in an otherwise insulating sample) upon the Li-diffusion process is experimentally highlighted and tentatively related to the passivation of acceptor defects (i.e., formation of V_Ga-nLi complexes).

Highlights

      ● Demonstrated Li diffusion in Ga₂O₃ epilayers relevant for pn heterojunctions.

      ● Li-diffusion can induce a carrier system in otherwise insulating β-Ga₂O_3.

      ● Depth-dependent redistribution of deep level defects in Ga₂O₃ upon Li-diffusion.

      ● Additional Raman mode compatible with the presence of Li in the oxide matrix.

      ● Suggested Li diffusion mechanism in β-Ga₂O₃ through experiments and theory.