【International Papers】Implantation-driven metastable polymorph engineering and local silicon coordination in Ga₂O₃

      Researchers from the Warsaw University of Technology have published a dissertation titled " Implantation-driven metastable polymorph engineering and local silicon coordination in Ga₂O₃" in Materials & Design.

Background

      The impact of impurities on the electronic properties of semiconductors is determined by their lattice localization and their interactions with structural defects. Consequently, understanding and controlling the mechanisms of impurity localization as well as defect content are essential for semiconductor device design. This holds, in particular, for gallium oxide (Ga₂O₃), an ultra-wide bandgap semiconductor that has garnered significant attention for its promising applications in power electronics, solar-blind optoelectronics, gas sensors, etc. Moreover, Ga₂O₃ exhibits strong polymorphism, which could further enhance device functionality, provided that phase transformations occurring during fabrication can be controlled.

      Ion implantation is one of the primary methods for introducing dopants, and it has been used for fabricating such Ga₂O₃-based electronic devices such as metal–oxide–semiconductor field-effect transistor (MOSFET) channels or ohmic contacts. Historically, n-type conductivity in β-Ga₂O₃ was commonly attributed to oxygen vacancies. However, Varley et al. demonstrated that silicon acts as a highly effective shallow donor, establishing Si as one of the most promising dopants for achieving controllable n-type conductivity. Post-implantation annealing is conventionally used for dopant activation and crystal recovery after the implantation process, and its efficiency strongly depends on parameters such as annealing ambient, temperature, and time, demonstrating that kinetic pathways and defect evolution critically shape electrically active dopant populations. Recent studies of dopant localization in Ga₂O₃ have shown that Si atoms have a tendency to occupy Ga-substitutional sites under many growth and annealing conditions. Nevertheless, the substitutional vs. interstitial fraction and the local coordination are highly sensitive to the specific polymorph, implantation/processing temperature and subsequent annealing.

Abstract

      Ion implantation in gallium oxide (Ga₂O₃) can both introduce dopants and kinetically trap non-equilibrium polymorphs. We investigate how implantation temperature governs polymorph formation, silicon localization, and long-term structural stability in Si-implanted β-Ga₂O₃(100). Room-temperature implantation produces pronounced disorder and drives a disorder-mediated β → γ transformation, which is unambiguously identified by HR XRD. In contrast, implantation at 625 °C promotes dynamic defect recovery and preserves a predominantly β-Ga₂O₃ structure. Si K-edge XANES shows that Si is predominantly present as Si⁴⁺ but adopts polymorph-dependent local environments. In β-Ga₂O₃, Si is distributed between substitutional donor sites and interstitial, defect-associated configurations with γ-like coordination. In γ-Ga₂O₃, the Si-related contribution is preferentially associated with the intrinsically partially occupied nonspinel cation sublattice of the defective-spinel structure. A substantial Si fraction also exhibits SiO₂-like coordination, consistent with partial chemical segregation and the formation of a nanoscale oxide composite within the implanted layer. After approximately one year of ambient storage, samples initially retaining predominantly β-type long-range order develop weak γ-related diffraction signatures, demonstrating delayed structural evolution of the implantation-modified state. These results show that implantation temperature, local dopant coordination, kinetic trapping, and time-dependent defect evolution must be considered together when designing implantation and subsequent thermal processing routes for Ga₂O₃-based materials.

Highlights

      Room-temperature implantation creates a metastable structural state that evolves during storage.

      Local silicon coordination exhibits γ-gallium oxide characteristics that remain undetected by diffraction alone.

      Combining local and long-range structural probes enables reliable identification of implantation-induced phase transformations.

      Symmetry-based diffraction analysis resolves ambiguities in gamma gallium oxide phase identification.

Conclusions

      In this work, we demonstrate that ion implantation in β-Ga₂O₃ should be considered not only as a dopant-introduction method but also as a route for creating kinetically trapped and structurally complex material states. HR XRD, supported by RBS/channeling and Si K-edge XANES with theoretical modeling, establishes a consistent relationship between implantation temperature, long-range polymorph formation, implantation-induced disorder, and local silicon coordination. Room-temperature implantation drives a disorder-mediated β → γ transformation that can be identified non-destructively by conventional high-resolution diffraction using multiple independent crystallographic criteria. Implantation at 625 °C instead promotes dynamic defect recovery, suppresses long-range γ-phase formation, and preserves predominantly β-type crystallographic order.

      Si K-edge XANES shows that the local state of implanted silicon cannot be inferred from average crystallographic symmetry alone. In β-Ga₂O₃, Si is distributed between substitutional donor configurations and interstitial, defect-associated configurations with locally γ-like coordination. In γ-Ga₂O₃, the dominant Si-related contribution is associated with the intrinsically partially occupied nonspinel Ga₃/Ga₄ cation environments of the defective-spinel framework. The close correspondence between these γ-phase environments and the Si-interstitial-gallium-vacancy complexes previously proposed for β-Ga₂O₃ provides a structural interpretation of the earlier β-phase model. In both polymorphic states, a significant fraction of Si exhibits SiO₂-like coordination, consistent with partial chemical segregation and the formation of SiO₂-rich nano-regions. The possible electrical consequences of these SiO₂-rich nano-regions (including electrically inactive Si, carrier-scattering interfaces, and trap-related electrical inhomogeneity) are inferred from their structural and chemical character rather than demonstrated directly in the present study, and therefore require validation by dedicated electrical measurements.

      The appearance of weak γ-related diffraction signatures after approximately one year of ambient storage further demonstrates that implantation-modified Ga₂O₃ may continue to evolve after irradiation. Combined with the γ-like interstitial Si environments identified by XANES, this observation is consistent with a slow defect-mediated reorganization of the cation sublattice, in which locally γ-compatible configurations present in the implantation-modified β matrix progressively develop sufficient spatial correlation to become detectable by diffraction. Time should therefore be considered an additional parameter in the assessment of metastable implantation-induced structures. Although the present work concerns the as-implanted state rather than a fully processed device layer, this state defines the non-equilibrium structural starting point for subsequent annealing and can therefore influence defect recovery, dopant activation, phase stability, and the final microstructure.

      Finally, these results demonstrate that implantation temperature, ion fluence, ion flux, and implantation time should be treated as coupled kinetic processing parameters. Reporting fluence alone is insufficient because the balance between defect production and dynamic recovery depends on the rate and duration of irradiation. Reliable optimization of implanted Ga₂O₃ therefore requires combined consideration of long-range phase identity, local dopant coordination, chemical segregation, and time-dependent defect evolution.