Atomic-Level Insights into the Radiation Damage and Recovery of β-Ga2O3 for High-performance Semiconductors

β-Ga2O3, the most thermodynamically stable polymorph of Ga2O3, offers immense potential to enhance the operating temperature and lifespan of semiconductors while maintaining high performance levels. Its ultrahigh bandgap energy of nearly 5 eV [1], as compared to GaN (3.4 eV) and Si (1.1 eV), makes it highly suitable for extreme operating conditions. β-Ga2O3's incredibly high electric breakdown field of 8 MV/cm [2] enables transistors to operate as switches at high frequencies up to the megahertz regime. Additionally, as one of the transparent conductive oxides, combining β-Ga2O3 with graphene [3] and other 2D materials can lead to the development of innovative functional devices with photo-response characteristics through ionizing irradiation and doping effect. However, high-energy particle irradiation can cause dominant crystallographic defects, leading to challenges in the nature of radiation damage accumulation, segregation, phase transformation [4], and damage annealing with solid-phase recrystallization in disordered regions that require further investigation. To address these challenges and explore the behavior of β-Ga2O3 under extreme operating conditions (high dose rate and high-temperature annealing), we employed cross-sectional scanning transmission electron microscopy (STEM) imaging to study the defect formation and phase transformation along the depth direction. In situ and ex-situ annealing STEM were also carried out at high temperatures to investigate the radiation damage and structural recovery mechanism upon thermal annealing. These techniques enable us to better understand the atomic-level behavior of β-Ga2O3 in response to high-energy particle irradiation and extreme temperature conditions, providing new insights into its potential for enabling the development of advanced semiconductors that can withstand even the harshest operating environments. We present a comprehensive atomic-level characterization of defect formation and phase transformation in (001) β-Ga2O3 induced by Si+ ion irradiation. By employing state-of-the-art STEM and secondary ion mass spectroscopy (SIMS) techniques, we were able to investigate the impact of Si+ irradiation concentrations up to 1021 cm-3 on the crystal structure of β-Ga2O3. The STEM data revealed that ion irradiation leads to the formation of nanoscale dark flecks which are caused by local lattice distortion of the β phase structure, which progresses to heavily defective structures and ultimately transforms into the γ phase inclusion as the Si+ peak concentration increases. The atomic structure within the dark fleck region represents a <010> screw dislocation based on possible slip systems of β-Ga2O3 [5]. Moreover, the number of γ phase inclusion grows as a function of irradiation dose, leading to the formation of planer defects within the γ phase that can cause lattice shifts within the local regions (Fig. 1). To further our understanding of the atomic structure of irradiation damage and lattice recovery after thermal annealing, we performed in situ and ex situ annealing of β-Ga2O3 at temperatures up to 1,100 °C. Our observations showed that the defective γ phase transformed back to the β phase but left a high concentration of dislocations in the sample with high Si peak concentration (1020 cm-3) (Fig. 2). On the other hand, we noted no extended defect formation when the sample had a lower Si peak concentration (1019 cm-3), indicating that the formation of dislocations is linked to the inhomogeneous Si distribution, corresponding to the irregular peak shapes in SIMS analysis. In summary, our atomic scale STEM analysis offers critical insights that enable precise control of dopant dosage, distribution, and defects induced by ion irradiation in β-Ga2O3. This research has significant implications for the processing, fabrication, and practical applications of future β-Ga2O3-based power electronic devices, thereby paving the way for the development of new and improved semiconductor technologies [6]. High-angle annular dark field (HAADF) STEM images of (001) β-Ga2O3 with Si+ irradiated concentration of 1020 cm-3, revealing various atomic structures. The insets in the images show that (a) a regular β-Ga2O3 which contained a clustering of divacancy interstitial defect complexes indicated by red circles) (b) a combination of both regular (blue) and shifted (green) structures (c) a planar defect of (110) γ-Ga2O3 involved both staggered (middle) and stacking (bottom) configurations. (a) In situ annealing of the Si+ irradiated β-Ga2O3 at the elevated temperatures up to 1100 °C. The diffraction patterns in the insets reveal that both radiated region and substrate have β phases (b) For samples with Si+ irradiation concentration 1020 cm-3, ex situ annealing was carried out at 950°C. (c) The HAADF STEM image displays a region with bright entangled line features, which signifies the recovery of the lattice structure from γ phase back to β phase. However, this recovery process results in the generation of a high concentration of entangled dislocations.