GoFIB Gallium Oxide Fabrication with Ion Beams

• Titov, Andrei; Klevtsov, Anton; Fedorenko, Elizaveta; Azarov, Alexander; Kuznetsov, Andrej & Karaseov, Platon (2025). Density and fractality of collision cascades in α-Ga2O3. Applied Physics Letters. ISSN 0003-6951. 127(15). doi: 10.1063/5.0288966. Fulltekst i vitenarkiv Vis sammendrag

  Control over radiation defect generation is essential while applying ion-beam technology to materials. In this work, we study disorder formation in rhombohedral polymorph of gallium oxide (α-Ga2O3) as a function of the bombarding ion mass. As such, we discriminate the impact of the collision cascades density, specifically its impact on the surface amorphization rates in α-Ga2O3. Importantly, along with collecting experimental data, we calculated collision cascades parameters using different algorithms based on the binary collision approximation predicting disorder accumulation trends observed in the experiments. Furthermore, we showed that collision cascades in α-Ga2O3 can be modeled within a fractal geometry framework, correlating fractal dimension with the collision cascade density. These results enhance the understanding of radiation defects in Ga2O3 polymorphs, applicable in the development of the ion beam technology in these interesting materials.

• Azarov, Alexander; Fernandez, Javier Garcia; Zhao, Junlei; He, Ru; Park, Ji-Hyeon & Jeon, Dae-Woo [Vis alle 9 forfattere av denne artikkelen] (2025). Phase glides and self-organization of atomically abrupt interfaces out of stochastic disorder in α-Ga2O3. Nature Communications. 16. doi: 10.1038/s41467-025-58516-9. Fulltekst i vitenarkiv Vis sammendrag

  Disorder-induced ordering and remarkably high radiation tolerance in γ-phase of gallium oxide is a recent spectacular discovery at the intersection of the fundamental physics and electronic applications. Importantly, by far, these data were collected with initial samples in form of the thermodynamically stable β-phase of this material. Here, we investigate these phenomena starting from metastable α-phase and explain radically new trend occurring in the system. We argue that in contrast to that in β-to-γ disorder-induced transitions, the O sublattice in α-phase exhibits hexagonal close-packed structure, so that to activate α-to-γ transformation significant structural rearrangements are required in both Ga and O sublattices. Moreover, consistent with theoretical predictions, α-to-γ phase transformation requires accumulation of the substantial tensile strain to initiate otherwise impossible lattice glides. Thus, we explain the experimentally observed trends in term of the combination of disorder and strain governed process. Finally, we demonstrate atomically abrupt α/γ interfaces paradoxically self-organized out of the stochastic disorder.

• Zhao, Junlei; Fernandez, Javier Garcia; Azarov, Alexander; He, Ru; Prytz, Øystein & Nordlund, Kai [Vis alle 9 forfattere av denne artikkelen] (2025). Crystallization Instead of Amorphization in Collision Cascades in Gallium Oxide. Physical Review Letters. ISSN 0031-9007. 134. doi: 10.1103/PhysRevLett.134.126101. Fulltekst i vitenarkiv
• Azarov, Alexander; Radu, Cristian; Galeckas, Augustinas; Mercioniu, Ionel Florinel; Cernescu, Adrian & Venkatachalapathy, Vishnukanthan [Vis alle 11 forfattere av denne artikkelen] (2025). Self-Assembling of Multilayered Polymorphs with Ion Beams. Nano Letters. ISSN 1530-6984. 25(4), s. 1637–1643. doi: 10.1021/acs.nanolett.4c05727. Fulltekst i vitenarkiv
• Abdullaev, Azat; Sekerbayev, Kairolla; Azarov, Alexander; Venkatachalapathy, Vishnukanthan; Chauhan, Vinay S. & Utegulov, Zhandos [Vis alle 7 forfattere av denne artikkelen] (2024). Thermal conductivity of double polymorph Ga2O3 structures. APL Materials. 12(8). doi: 10.1063/5.0213985. Fulltekst i vitenarkiv Vis sammendrag

  Recently discovered double gamma/beta (γ/β) polymorph Ga2O3 structures constitute a class of novel materials providing an option to modulate functional properties across interfaces without changing the chemical compositions of materials, in contrast to that in conventional heterostructures. In this work, for the first time, we investigate thermal transport in such homo-interface structures as an example of their physical properties. In particular, the cross-plane thermal conductivity (k) was measured by femtosecond laser-based time-domain thermoreflectance with MHz modulation rates, effectively obtaining depth profiles of the thermal conductivity across the γ-/β-Ga2O3 structures. In this way, the thermal conductivity of γ-Ga2O3 ranging from 1.84 to 2.11 W m−1 K−1 was found to be independent of the initial β-substrates orientations, in accordance with the cubic spinel structure of the γ-phase and consistently with the molecular dynamics simulation data. In turn, the thermal conductivity of monoclinic β-Ga2O3 showed a distinct anisotropy, with values ranging from 10 W m−1 K−1 for [−201] to 20 Wm−1 K−1 for [010] orientations. Thus, for double γ-/β-Ga2O3 polymorph structures formed on [010] β-substrates, there is an order of magnitude difference in thermal conductivity across the γ/β interface, which can potentially be exploited in thermal energy conversion applications.

• Klevtsov, Anton; Karaseov, Platon; Azarov, Alexander; Karabeshkin, Konstantin; Fedorenko, Elizaveta & Titov, Andrei [Vis alle 7 forfattere av denne artikkelen] (2024). Non-linear effects in α-Ga2O3 radiation phenomena. APL Materials. 12(11). doi: 10.1063/5.0235497. Fulltekst i vitenarkiv Vis sammendrag

  The rhombohedral phase of gallium oxide (α-Ga2O3) is of interest because of its highest bandgap among the rest of the Ga2O3 polymorphs, making it particularly attractive in applications. However, even though the ion beam processing is routinely used in device technology, the understanding of radiation phenomena in α-Ga2O3 is not mature. Here, we study non-linear effects for radiation disorder formation in α-Ga2O3 by varying both the defect generation rate and the density of collision cascades, enabled by comparing monoatomic and cluster ion implants, also applying systematic variations of ion fluxes. In particular, we show that the collision cascade density governs the surface amorphization rates, also affected by the ion flux variations. These trends are explained in terms of the non-linear in-cascade and inter-cascade defect interactions occurring during ballistic and dynamic defect annealing stages. As such, these data reveal new physics of the radiation phenomena in α-Ga2O3 and may be applicable for more predictive ion beam processing of α-Ga2O3-based devices.

• Polyakov, Alexander Y.; Vasilev, Anton A.; Kochkova, Anastasiia I.; Shchemerov, Ivan V.; Yakimov, Eugene B. & Miakonkikh, Andrej V. [Vis alle 17 forfattere av denne artikkelen] (2024). Proton damage effects in double polymorph γ/β-Ga2O3 diodes. Journal of Materials Chemistry C. ISSN 2050-7526. 12, s. 1020–1029. doi: 10.1039/d3tc04171a. Fulltekst i vitenarkiv Vis sammendrag

  Double polymorph γ/β-Ga2O3 structures remain crystalline upon unprecedentedly high crystal disorder levels where other semiconductors lose their long-range symmetry and, eventually, become amorphous. However, it is unclear if this radiation tolerance translates to device-like operation, where much lower levels of damage degrade the performance. In this work, we fabricated conducting double polymorph γ/β-Ga2O3 structures using ion implantation and subsequent hydrogenation of the top γ-Ga2O3 layer, instead of conventional impurity doping which is limited by γ-Ga2O3 stability tradeoffs. While not a direct comparison, these structures exhibited much higher radiation tolerance compared to conventional Schottky diodes made of β-Ga2O3. Specifically, using 1.1 MeV proton irradiation at fluences of 1014–1015 cm−2, conventional β-Ga2O3 diodes became unfunctional, while double polymorph γ/β-Ga2O3 diodes remained operational. The centers supplying electrons in γ-Ga2O3 were characterized by prominent DX-like persistent photocapacitance. For samples implanted with Ga+ and Si+ to produce the β → γ transition, annealed at 600 °C and plasma hydrogenated, the net donor concentration was ∼1012 cm−3, with dominant electron traps near EC − 0.65–0.7 eV and photocapacitance and photocurrent spectra determined by deep acceptors with optical ionization thresholds 1.3 eV, 2 eV, 2.3 eV and 2.8 eV. Irradiation with 1 MeV protons increased the net donor density of these conducting γ/β-Ga2O3 structures, with carrier creation rates of (1.5–4.4) × 10−2 cm−2, in sharp contrast to the carrier removal rates of 150–200 cm−1 under identical conditions in the original β-Ga2O3 films.

• Azarov, Alexander; Galeckas, Augustinas; Cora, Ildikó; Fogarassy, Zsolt; Venkatachalapathy, Vishnukanthan & Monakhov, Eduard [Vis alle 7 forfattere av denne artikkelen] (2024). Optical Activity and Phase Transformations in γ/β Ga2O3 Bilayers Under Annealing. Advanced Optical Materials. doi: 10.1002/adom.202401325. Fulltekst i vitenarkiv Vis sammendrag

  Gallium oxide (Ga2O3) can be crystallized in several polymorphs exhibiting different physical properties. In this work, polymorphic structures consisting of the cubic defective spinel (γ) film on the top of the monoclinic (β) substrate are fabricated by disorder-induced ordering, known to be a practical way to stack these polymorphs together. Such bilayer structures are annealed to investigate optical properties and phase transformations. Specifically, photoluminescence and diffuse reflectance spectroscopy are combined with transmission electron microscopy, Rutherford backscattering/channeling spectrometry and X-ray diffraction to monitor the evolutions. As a result, a two-stage annealing kinetics is observed in γ/β Ga2O3 bilayers associated with the epitaxial γ-to-β regrowth at the interface at temperatures below 700 °C and a non-planar γ-to-β phase transformation starting at higher temperatures. Thus, the present data enhance understanding of the polymorphism in Ga2O3, interconnecting the phase transformation kinetics with the evolution of the optical properties.

• Fernandez, Javier Garcia; Kjeldby, Snorre Braathen; Zeng, L. J.; Azarov, Alexander; Pokle, Anuj & Nguyen, Phuong Dan [Vis alle 10 forfattere av denne artikkelen] (2024). In situ atomic-resolution study of transformations in double polymorph γ/β-Ga2O3 structures. Materials Advances. 5, s. 3824–3831. doi: 10.1039/D3MA01011B. Fulltekst i vitenarkiv Vis sammendrag

  Disorder-induced formation of metastable Ga2O3 polymorphs as well as the recovery of the stable state upon annealing attract attention because of the fundamental novelty and rapidly increasing interest in the use of Ga2O3 in practical applications. In this study, double polymorph γ/β-Ga2O3 structures fabricated by the radiation-induced disorder approach were used as a starting point for systematic in situ annealing electron microscopy experiments. We show that, under the conditions of the TEM in situ annealing, double γ/β-Ga2O3 polymorph structures remained stable up to 300 °C, when onsets of the γ-to-β transformation become traceable, leading to a prominent γ- and β-mixture already at 500 °C. Interestingly, the recrystallization of the β-Ga2O3 occurs throughout the whole γ-film and the preferential alignments at the newly emerging γ/β-interfaces are different from that of the initial γ/β-interface formed as a result of the disorder-induced ordering. The alignments of the two polymorphs are maintained as a function of temperature – with a reduction in the volumetric ratio of γ-domains for increasing annealing temperature. Finally, at 1100 °C, γ-Ga2O3 fully transforms into β-Ga2O3, without dominating crystallographic relationships or preferred orientations, indicating that energy barriers are not any longer implied limiting factors, because of a sufficiently high thermal energy supply. Thus, these TEM in situ measurements enable a new level of accuracy for assessing polymorphic transformations in Ga2O3.

• Azarov, Alexander; Galeckas, Augustinas; Mikhaylov, Alexey; Nikolskaya, Alena; Tetelbaum, David & Korolev, Dmitry [Vis alle 8 forfattere av denne artikkelen] (2024). Defect evolution and nitrogen incorporation in ion-implanted β-Ga2O3. Applied Physics Letters. ISSN 0003-6951. 125(19). doi: 10.1063/5.0239899. Fulltekst i vitenarkiv Vis sammendrag

  Nitrogen plays an important role in the Ga2O3-based device fabrication since it demonstrates a deep acceptor behavior and it can be used as a compensation impurity. In the present work, we introduced N into the monoclinic β-Ga2O3 single crystals by ion implantation using different dose-rates. The thermal evolution of the structural and optical properties of the implanted samples was studied by combining Rutherford backscattering/channeling spectrometry and x-ray diffraction with photoluminescence spectroscopy. We demonstrate that crystal disorder in N-implanted β-Ga2O3 is a strong function of ion flux even for room temperature implantation. Unlike other implanted species, the defect annealing kinetics for N exhibit a distinct two-stage behavior, where the low-temperature stage (<400 °C) is characterized by a rapid defect annealing, whereas the radiation defects exhibit a very slow evolution at the elevated temperatures (>400 °C). These trends are discussed in the framework of the disorder stabilization induced by the incorporation of the implanted N atoms into the Ga2O3 lattice. The obtained results can be used to better understand the interaction between N and intrinsic defects as well as to push forward the development of β-Ga2O3-based electronic devices.

• Polyakov, Alexander Y.; Yakimov, Eugene B.; Nikolaev, Vladimir I.; Pechnikov, Alexei I.; Miakonkikh, Andrej V. & Azarov, Alexander [Vis alle 12 forfattere av denne artikkelen] (2023). Impact of Hydrogen Plasma on Electrical Properties and Deep Trap Spectra in Ga2O3 Polymorphs. Crystals. 13(9), s. 1–25. doi: 10.3390/cryst13091400. Fulltekst i vitenarkiv Vis sammendrag

  In this study, the results of hydrogen plasma treatments of β-Ga2O3, α-Ga2O3, κ-Ga2O3 and γ-Ga2O3 polymorphs are analyzed. For all polymorphs, the results strongly suggest an interplay between donor-like hydrogen configurations and acceptor complexes formed by hydrogen with gallium vacancies. A strong anisotropy of hydrogen plasma effects in the most thermodynamically stable β-Ga2O3 are explained by its low-symmetry monoclinic crystal structure. For the metastable, α-, κ- and γ-polymorphs, it is shown that the net result of hydrogenation is often a strong increase in the density of centers supplying electrons in the near-surface regions. These centers are responsible for prominent, persistent photocapacitance and photocurrent effects.

• Azarov, Alexander; Fernández, Javier García; Zhao, Junlei; Djurabekova, Flyura; He, Huan & He, Ru [Vis alle 13 forfattere av denne artikkelen] (2023). Universal radiation tolerant semiconductor. Nature Communications. 14, s. 1–8. doi: 10.1038/s41467-023-40588-0. Fulltekst i vitenarkiv Vis sammendrag

  Abstract Radiation tolerance is determined as the ability of crystalline materials to withstand the accumulation of the radiation induced disorder. Nevertheless, for sufficiently high fluences, in all by far known semiconductors it ends up with either very high disorder levels or amorphization. Here we show that gamma/beta (γ/β) double polymorph Ga 2 O 3 structures exhibit remarkably high radiation tolerance. Specifically, for room temperature experiments, they tolerate a disorder equivalent to hundreds of displacements per atom, without severe degradations of crystallinity; in comparison with, e.g., Si amorphizable already with the lattice atoms displaced just once. We explain this behavior by an interesting combination of the Ga- and O- sublattice properties in γ-Ga 2 O 3 . In particular, O-sublattice exhibits a strong recrystallization trend to recover the face-centered-cubic stacking despite the stronger displacement of O atoms compared to Ga during the active periods of cascades. Notably, we also explained the origin of the β-to-γ Ga 2 O 3 transformation, as a function of the increased disorder in β-Ga 2 O 3 and studied the phenomena as a function of the chemical nature of the implanted atoms. As a result, we conclude that γ/β double polymorph Ga 2 O 3 structures, in terms of their radiation tolerance properties, benchmark a class of universal radiation tolerant semiconductors.

• Azarov, Alexander; Venkatachalapathy, Vishnukanthan; Lee, In-Hwan & Kuznetsov, Andrej (2023). Thermal versus radiation-assisted defect annealing in β-Ga2O3. Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. ISSN 0734-2101. 41(2). doi: 10.1116/6.0002388. Fulltekst i vitenarkiv Vis sammendrag

  Gallium oxide (Ga2O3) exhibits complex behavior under ion irradiation since ion-induced disorder affects not only the functional properties but can provoke polymorphic transformations in Ga2O3. A conventional way used to minimize the lattice disorder is by doing postirradiation anneals. An alternative approach is to prevent the disorder accumulation from the beginning, by doing implants at elevated temperatures, so that a significant fraction of the disorder dynamically anneals out in radiation-assisted processes. Here, we use these two approaches for the minimization of radiation disorder in monoclinic β-Ga2O3 implanted to a dose below the threshold required for the polymorphic transformations. The results obtained by a combination of channeling and x-ray diffraction techniques revealed that implants at 300 °C effectively suppress the defect formation in β-Ga2O3. On the other hand, in order to reach similar crystalline quality in the samples implanted at room temperature, postirradiation anneals in excess of 900 °C are necessary.

• Azarov, Alexander; Park, Ji-Hyeon; Jeon, Dae-Woo & Kuznetsov, Andrej (2023). High mobility of intrinsic defects in α-Ga2O3. Applied Physics Letters. ISSN 0003-6951. 122(18), s. 182104-1–182104-4. doi: 10.1063/5.0149870. Fulltekst i vitenarkiv Vis sammendrag

  Migration properties of the intrinsic defects were investigated in α-Ga2O3 by controllable introduction of the lattice disorder with ion irradiation and monitoring its evolution as a function of ion dose, flux, and temperature. Already the dose dependence acquired at room temperature suggested prominently high mobility of intrinsic defects in α-Ga2O3, since we observed two distinct disordered regions—near the surface and in the bulk—instead of a Gaussian shape following the ballistic defects production process. Moreover, the disorder accumulation has shown to be highly sensitive to the variation of the ion flux and temperature, known in the literature as the dose-rate effect. Therefore, by monitoring the process as a function of the flux and temperature, we observed such dose-rate effect in α-Ga2O3 with an activation energy of 0.33 ± 0.04 eV, which we attributed to the migration barrier of the intrinsic defects in the Ga sublattice, from where we collected the experimental data. By setting these results in the context of the theoretical data available in the literature, we argued that this energy may be attributed to the migration activation of the Ga self-interstitials in α-Ga2O3.

• Azarov, Alexander; Venkatachalapathy, Vishnukanthan; Karaseov, Platon; Titov, Andrei; Karabeshkin, Konstantin & Struchkov, Andrei [Vis alle 7 forfattere av denne artikkelen] (2022). Interplay of the disorder and strain in gallium oxide. Scientific Reports. 12(1). doi: 10.1038/s41598-022-19191-8. Fulltekst i vitenarkiv Vis sammendrag

  Abstract Ion irradiation is a powerful tool to tune properties of semiconductors and, in particular, of gallium oxide (Ga 2 O 3 ) which is a promising ultra-wide bandgap semiconductor exhibiting phase instability for high enough strain/disorder levels. In the present paper we observed an interesting interplay between the disorder and strain in monoclinic β-Ga 2 O 3 single crystals by comparing atomic and cluster ion irradiations as well as atomic ions co-implants. The results obtained by a combination of the channeling technique, X-ray diffraction and theoretical calculations show that the disorder accumulation in β-Ga 2 O 3 exhibits superlinear behavior as a function of the collision cascade density. Moreover, the level of strain in the implanted region can be engineered by changing the disorder conditions in the near surface layer. The results can be used for better understanding of the radiation effects in β-Ga 2 O 3 and imply that disorder/strain interplay provides an additional degree of freedom to maintain desirable strain in Ga 2 O 3 , potentially applicable to modify the rate of the polymorphic transitions in this material.

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• Azarov, Alexander; Park, Ji-Hyeon; Jeon, Dae-Woo; Monakhov, Eduard & Kuznetsov, Andrej (2023). Defect migration energies in Ga2O3 polymorphs measured by variations of temperature and flux under irradiation. Fulltekst i vitenarkiv
• Azarov, Alexander (2023). Radiation disorder induced ordering. Fulltekst i vitenarkiv
• Azarov, Alexander; Bazioti, Kalliopi; Venkatachalapathy, Vishnukanthan; Vajeeston, Ponniah; Monakhov, Eduard & Kuznetsov, Andrej (2022). Disorder-induced ordering in Ga2O3 polymorphs. Fulltekst i vitenarkiv
• Azarov, Alexander; Bazioti, Kalliopi; Venkatachalapathy, Vishnukanthan; Vajeeston, Ponniah; Monakhov, Eduard & Kuznetsov, Andrej (2022). Ion-beam-induced phase transitions in gallium oxide: impact of the implantation temperature. Fulltekst i vitenarkiv

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