Polymorph Conversion in Gallium Oxide via Focused Ion Beam Irradiation

Monoclinic gallium oxide (β-Ga2O3) is the chemically and thermally most stable compound, compared to its other four polymorphs, with an ultra-wide bandgap of 4.9 eV. It is a promising semiconductor material for power electronics, optoelectronics, and batteries. However, controlling the metastable polymorph phases is quite hard, and the fabrication technology at the nanoscale is immature. Our goal is to understand polymorph conversion. Controlling the crystalline structure will allow us to establish new fabrication methods of single-phase polymorph coatings, buried layers, multilayers, and different nanostructures in Ga2O3 using focused ion beams (FIBs). The research aims to better understand and control the polymorph conversion with special emphasis on laterally resolved modifications by utilizing focused ion beams.
In a previous study, our project partners Kuznetsov et.al. [1] demonstrated the ion-beam-induced β-to-κ phase transformation in Ga2O3 as shown in Fig.1. However, later, Garcia Fernandez et.al. [2] showed that the monoclinic β-phase actually transforms into the cubic γ-phase.
Here, we used Helium Ion Microscopy (HIM) and other focused ion beams (FIBs) to locally irradiate the (-201) oriented β-Ga2O3 sample with different ions (Ne, Co, Nd, Si, Au, In) to induce the polymorph transition. The successful conversion into γ- Ga2O3 under Ne+ irradiation (Fig.2(a)) has been confirmed using Electron Backscattered Diffraction (EBSD) and indexing the Kikuchi patterns (Fig.2(b)). Furthermore, Positron Annihilation Lifetime Spectroscopy (PALS) was performed for broad beam irradiated implants to better understand the fluence-dependent creation and distribution of defects. Transmission Electron Microscopy (TEM) images also provide information about the distinct and sharp interfaces between different polymorphs of Ga2O3. The first results indicate that the damage/strain created by the FIB irradiation leads to a local transformation of β- Ga2O3 to γ- Ga2O3.