Local Polymorph Conversion in Gallium Oxide via Focused Ion Beam Irradiation

Monoclinic gallium oxide (β-Ga2O3) exhibits the highest chemical and thermal
stability among its four other polymorphs, making it a highly promising
semiconductor material for various applications such as power electronics,
optoelectronics, and batteries, thanks to its exceptionally wide bandgap of around
4.7 eV. However, the challenge lies in effectively managing the metastable
polymorph phases and dealing with underdeveloped nanoscale fabrication
techniques. Our objective is to leverage the potential of ion-beam-induced polymorph
conversion to gain a comprehensive understanding and control over the crystalline
structure. By utilizing focused ion beams (FIBs), we aim to pioneer new fabrication
methods for generating single-phase polymorph layers, buried layers, multilayers,
and diverse nanostructures within Ga2O3. The primary focus of this research is to
enhance our knowledge and control of polymorph conversion, with a particular
emphasis on spatially precise modifications using focused ion beams.
Most semiconductor materials transform into an amorphous phase under a high
dose of ion irradiation, however, Ga2O3 is an exceptionally radiation-tolerant material
even at high fluences of ion irradiation. The conversion from the stable to the
metastable phase seems to be enabled by the formation of a defective spinel
structure in which the oxygen lattice remains unchanged [1]. It has been found that
sub-lattice requires a certain level of damage accumulation, specifically
displacement per atom (DPA), to transform into γ-phase [1].
Here, we used Helium Ion Microscopy (HIM) and liquid metal alloy ion source
(LMAIS) FIBs to locally irradiate (-201) -oriented β-Ga2O3 substrate with different
ions (Ne, Ga, Co, Nd, Si, Au, In) to induce the polymorph transition. Locally and
spatially resolved characterization was performed by Electron Backscatter Diffraction
(EBSD) and analyzing the Kikuchi patterns. 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 provide information about
the interfaces between different polymorphs of Ga2O3. The first results indicate that
the damage/strain created by the Ne+, Co+, and Si+ FIB irradiations leads to a local
transformation of β- Ga2O3 to γ- Ga2O3 and the structure maintains its crystallinity
up to high-fluence FIB irradiation instead of being amorphized.