Javier Garcia Fernandez

• Abdullaev, Azat; Fernandez, Javier Garcia; Nozais, Chloé; O’Connell, Jacques; Tlegenov, Rustem & Sekerbayev, Kairolla [Vis alle 17 forfattere av denne artikkelen] (2026). Ions leaving no tracks. Nature Communications. 17(1). doi: 10.1038/s41467-026-78195-4. Fulltekst i vitenarkiv Vis sammendrag

  The paths of swift heavy ions are typically traceable in solids, because of confined electronic interactions along the paths, inducing what is known in literature as “ion tracks”, i.e. nano-sized in cross-section cylindrical zones of modified material extending for microns in length. Such tracks readily form in materials exhibiting low thermal conductivities, in particular insulators or semiconductors, altering the homogeneity of materials. In this work, using recently discovered γ/β-Ga2O3 polymorph heterostructures, we show that, in contrast to the trends in many other materials, including that in β-Ga2O3, swift heavy ions leave no tracks in γ-Ga2O3. We explain this trend in terms of the fast disorder recovery, occurring because of multiple configurations in the γ-Ga2O3 lattice itself, so that the disorder formed by ion impacts gets rapidly erased, giving a perception of ions leaving no tracks. As such, γ-Ga2O3, readily integrated with β-Ga2O3 in polymorph heterostructures, emerges as a promising semiconductor platform for devices capable of operating in harsh radiation environments.

• Petersen, Clemens; Vogt, Sofie; Fernandez, Javier Garcia; Vines, Lasse; Kano, Taisei & Grundmann, Marius [Vis alle 8 forfattere av denne artikkelen] (2026). Single-domain, semiconducting κ-Ga2O3 grown on isostructural ε-GaFeO3 templates. Applied Physics Letters. ISSN 0003-6951. 129(17). doi: 10.1063/5.0343866. Fulltekst i vitenarkiv Vis sammendrag

  The epitaxial growth of a phase-pure, single-domain, semiconducting κ-Ga2O3 thin film is demonstrated. Multiferroic ε-GaFeO3, being isostructural to κ-Ga2O3, was used as a substrate and was initially covered by a high-temperature MIST-CVD κ-Ga2O3 seed layer. Subsequently, tin-assisted pulsed laser deposition at 550 °C was applied to grow the single-crystalline epilayer with a thickness of 400 nm. The structural properties were investigated by x-ray diffraction and transmission electron microscopy. A reciprocal space map revealed that the κ-Ga2O3 layer sustains residual strain and is only partially relaxed. Consistently, misfit dislocations were observed, indicating ongoing but incomplete strain relaxation. Secondary ion mass spectrometry confirmed efficient Sn incorporation and revealed Fe diffusion from the ε-GaFeO3 substrate. The Fe impurities segregate predominantly at the advancing growth front, resulting in only moderate bulk compensation of the κ-Ga2O3/ε-GaFeO3 layer. Temperature-dependent in-plane transport measurements revealed freeze out of free electrons with activation energies of Ea,1=48 meV and Ea,2=253 meV at intermediate and high temperatures, respectively. No measurable in-plane conductivity was observed for a reference sample deposited on c-plane sapphire, which exhibited in-plane rotational domains and nanocolumnar growth. These results demonstrate that epitaxial growth on isostructural ε-GaFeO3 enables single-crystalline, semiconducting κ-Ga2O3 and establishes the κ-Ga2O3/ε-GaFeO3 heterostructure as a promising platform for exploring polarization-charge-induced two-dimensional electron gases in multifunctional ferroelectric/ferrimagnetic oxide systems.

• Fernandez, Javier Garcia; Kjeldby, Snorre Braathen; Zeng, L. J.; Olsson, E.; Vines, Lasse & Prytz, Øystein (2025). Formation and stability of Ge nanocrystals in polymorph converted Ga2O3: Temperature-driven journey. Applied Physics Letters. ISSN 0003-6951. 126(5). doi: 10.1063/5.0235371. Fulltekst i vitenarkiv Vis sammendrag

  The influence of germanium (Ge) implantation on β-Ga2O3 followed by ex situ annealing through polymorph conversion in air at various temperatures (600–1100 °C) is studied. Atomic resolution aberration-corrected scanning transmission electron microscopy is employed to examine the structural and microstructural changes induced by the annealing process. The results show that the thermal annealing process leads to the formation of Ge nanocrystals, which subsequently disappear, leaving nano-voids inside the Ge-doped Ga2O3 matrix. In addition, the microstructure displays distinct crystallographic relationships in annealed β-Ga2O3 forming layers with well-defined interfaces. This work reveals the unique effects of Ge-implantation, demonstrating the possible functionalization of Ga2O3 with Ge nanocrystals.

• 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
• 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.

• Montag, S.; Splith, D.; Kneiß, M.; Grundmann, M.; Fernandez, Javier Garcia & Prytz, Øystein [Vis alle 7 forfattere av denne artikkelen] (2023). Cation segregation observed in an (In,Ga) 2 O3 material thin film library beyond the miscibility limit of the bixbyite structure. PHYSICAL REVIEW MATERIALS. 7(9). doi: 10.1103/PhysRevMaterials.7.094603. Fulltekst i vitenarkiv Vis sammendrag

  Structural, morphological, and optical properties of (In1−xGax)2O3 thin films are reported as a function of the cation composition. A material library with 0.1≤x≤0.64 was fabricated by discrete combinatorial synthesis on r-plane sapphire substrates using pulsed laser deposition. The samples crystallize in the cubic bixbyite phase for x≤0.35. The lattice constant and absorption edge energy systematically decrease and increase, respectively, with increasing Ga content up to x=0.2. For higher Ga admixtures, both saturate. In addition, a significant change in surface morphology occurs at x∼0.2. Transmission electron microscopy examinations of selected samples show a homogeneous incorporation of Ga2O3 into cubic In2O3 for x=0.11, while a segregation of Ga-rich and In-rich regions can be seen for x=0.22 and x=0.35. In the sample with x=0.35, the Ga-rich regions exhibit a preferred orientation with an angle of 45∘–55∘ with respect to the substrate normal, which has been shown to result from a correspondingly faceted In-rich bixbyite layer at the substrate–thin film interface.

• Fernandez, Javier Garcia; Kjeldby, Snorre Braathen; Nguyen, Phuong Dan; Karlsen, Ole Bjørn; Vines, Lasse & Prytz, Øystein (2022). Formation of γ-Ga2O3 by ion implantation: Polymorphic phase transformation of β-Ga2O3. Applied Physics Letters. ISSN 0003-6951. 121(19). doi: 10.1063/5.0120103. Fulltekst i vitenarkiv Vis sammendrag

  Ion implantation induced phase transformation and the crystal structure of a series of ion implanted β-Ga 2 O 3 samples were studied using electron diffraction, high resolution transmission electron microscopy, and scanning transmission electron microscopy. In contrast to previous reports suggesting an ion implantation induced transformation to the orthorhombic κ-phase, we show that for 28 Si + , 58 Ni + , and stoichiometric 69 Ga + / 16 O + -implantations, the monoclinic β-phase transforms to the cubic γ-phase. The γ-phase was confirmed for implantations over a range of fluences from 10 14 to 10 16  ions/cm 2 , indicating that the transformation is a general phenomenon for β-Ga 2 O 3 due to strain accumulation and/or γ-Ga 2 O 3 being energetically preferred over highly defective β-Ga 2 O 3 .

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• Petersen, Clemens; Rosnes, Andreas; Fernandez, Javier Garcia; Grundmann, Marius & Wenckstern, Holger Von (2026). ETC-PLD for high-throughput mapping of spatially addressable transparent oxide heterostructures. doi: https:/research.uni-leipzig.de/hlp/TCO2026/. Fulltekst i vitenarkiv
• Fernandez, Javier Garcia; Bergsbak, Ingvild; Rosnes, Andreas; Petersen, Clemens; Løvoll, Marte & Øyvik, Lea [Vis alle 7 forfattere av denne artikkelen] (2026). Combinatorial Thin-Film Libraries: Correlating Composition, Structure, and Defects by TEM. doi: https:/research.uni-leipzig.de/hlp/TCO2026/. Fulltekst i vitenarkiv
• Fernandez, Javier Garcia; Pokle, Anuj; Andreassen, Magnus; Prytz, Øystein; Grundmann, Marius & Wenckstern, Holger Von (2025). In situ and ex situ thermal evolution, crystallization and optical properties of Amorphous (In,Ga)2O3. Fulltekst i vitenarkiv
• Fernandez, Javier Garcia; Montag, S.; Splith, D.; Kneiß, M.; Grundmann, M. & Prytz, Øystein [Vis alle 7 forfattere av denne artikkelen] (2024). Combinatorial PLD Synthesis of (In1−xGax)2O3 Thin Films: Structural, Morphological, and Optical Analysis for Transparent Conducting Oxides. Fulltekst i vitenarkiv
• Fernandez, Javier Garcia; Kjeldby, Snorre Braathen; Zeng, L. J.; Olsson, E.; Vines, Lasse & Prytz, Øystein (2024). Structural Transformation in Ge+ Implanted β-Ga2O3: Functionalization with Ge Nanoparticles leading to 'Emmental-type' Nano-architecture. Fulltekst i vitenarkiv
• Fernandez, Javier Garcia; Montag, S.; Andreassen, Magnus; Pokle, Anuj; Splith, D. & Kneiß, M. [Vis alle 9 forfattere av denne artikkelen] (2024). Probing, microstructure, crystallization and phase segregation in (In1-xGax)2O3 thin films by TEM. Fulltekst i vitenarkiv
• Kjeldby, Snorre Braathen; Fernandez, Javier Garcia; Galeckas, Augustinas; Jensen, Ingvild Julie Thue; Prytz, Øystein & Vines, Lasse (2024). Cross-sectional cathodoluminescence measurements in implanted and annealed gallium oxide. Fulltekst i vitenarkiv
• Fernandez, Javier Garcia; Kjeldby, Snorre Braathen; Nguyen, Phuong Dan; Haug, Kristian; Galeckas, Augustinas & Jensen, Ingvild Julie Thue [Vis alle 9 forfattere av denne artikkelen] (2023). Optical properties and structure relationship in ZnO:Fe with inversion domain boundaries and ZnFe2O4/ZnO heterostructures. Fulltekst i vitenarkiv
• Fernandez, Javier Garcia; Kjeldby, Snorre Braathen; Azarov, Alexander; Pokle, Anuj; Nguyen, Phuong Dan & Vines, Lasse [Vis alle 8 forfattere av denne artikkelen] (2023). Polymorph engineering and radiation tolerance in β-Ga2O3. Fulltekst i vitenarkiv
• Fernandez, Javier Garcia; Kjeldby, Snorre Braathen; Nguyen, Phuong Dan; Haug, Kristian; Galeckas, Augustinas & Jensen, Ingvild Julie Thue [Vis alle 9 forfattere av denne artikkelen] (2022). Structural and optical investigation of Fe-ZnO nanoarchitectures: from inversion domain boundaries to ZnO/ZnFe2O4 heterostructure. Fulltekst i vitenarkiv
• Kjeldby, Snorre Braathen; Azarov, Alexander; Nguyen, Phuong Dan; Venkatachalapathy, Vishnukanthan; Mikšová, Romana & Macková, Anna [Vis alle 10 forfattere av denne artikkelen] (2022). Structural Transformation of β-Ga2O3 through Si-implantation. Fulltekst i vitenarkiv
• Kjeldby, Snorre Braathen; Nguyen, Phuong Dan; Fernandez, Javier Garcia; Haug, Kristian; Galeckas, Augustinas & Jensen, Ingvild Julie Thue [Vis alle 9 forfattere av denne artikkelen] (2022). Optical properties of Fe-decorated inversion domain boundaries and embedded spinel zinc ferrite nanoparticles in zinc oxide. Fulltekst i vitenarkiv
• Kjeldby, Snorre Braathen; Azarov, Alexander; Nguyen, Phuong Dan; Fernandez, Javier Garcia; Venkatachalapathy, Vishnukanthan & Mikšová, Romana [Vis alle 10 forfattere av denne artikkelen] (2022). Structure and luminescence properties of Si-implanted β-Ga2O3. Fulltekst i vitenarkiv

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