Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

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 and utilize ion beam-induced polymorph conversion. Controlling the crystalline structure will allow us to establish new fabrication methods of single-phase polymorph layers, buried layers, multilayers, and different nanostructures in Ga2O3 using focused ion beams (FIBs). The research aims to better understand and control the polymorph conversion, emphasizing spatially resolved modifications by utilizing focused ion beams.
Most of the semiconductor materials transform into an amorphous phase under a high dose of ion irradiation, however, gallium oxide is an exceptionally radiation-tolerant material even at high fluences. In a previous study, Kuznetsov et.al. [1] demonstrated the ion-beam-induced β-to-κ phase trans-formation 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. Additional experimental and simulation results suggest that the formed gamma layer is not only stable up to several hundred degrees but also can tolerate high fluencies of additional ion irradiation. The conversion from the stable to the meta-stable phase seems to be enabled by the formation of a defective spinel structure in which the oxygen lattice remains unchanged [3].
Here, we used Helium Ion Microscopy (HIM) and liquid metal alloy ion source (LMAIS) FIBs to locally irradiate the (-201) oriented β-Ga2O3 sample with different ions (Ne, Ga, 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 by using Electron Backscattered Diffraction (EBSD) and indexing the Kikuchi patterns (Fig.2(b)). Furthermore, Doppler broadening variable energy positron annihilation spectroscopy (DB-VEPAS) and Rutherford Backscatter Spectrometry (RBS) were performed for neon 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 Ne+, Co+, and Si+ FIB irradiation leads to a local transformation of β- Ga2O3 to γ- Ga2O3 and the structure maintains its crystallinity even under the high fluence of ion irradiation instead of being amorphized.