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Javier Garcia Fernandez

Researcher - Centre for Materials Science and Nanotechnology Physics

Email j.g.fernandez@smn.uio.no

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Visiting address Oslo Science Park Gaustadalléen 21 0349 Oslo

Postal address Postboks 1048 Blindern 0316 Oslo

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Academic interests

My interests are focused on applications of transmission electron microscopy (TEM) in the study of functional materials. More specifically, my research focuses on structural characterization at the atomic level (phase identification, defects, oxidation states, etc.) by the using of imaging and diffractometric techniques, including HRTEM, STEM imaging, and electron diffraction, alongside analytical spectroscopy like EELS and EDS. I aim to establish fundamental structure-property relationships across a diverse range of material systems.

I am open to establishing new projects and explore opportunities focused on TEM analysis and materials characterization. If you are looking to collaborate feel free to reach out.

Background

Researcher, Centre for Materials Science and Nanotechnology Physics, University of Oslo (2024 - Pres.)
Postdoctoral fellow, Centre for Materials Science and Nanotechnology Physics, University of Oslo (2021 - 2024)
Postdoctoral fellow, CNRS-CEMES (2020-2021)
PhD in "Advanced Chemistry" School of Chemical Sciences. Complutense University of Madrid (2020)
MSc in “Nanophysics and Advanced Materials”, School of Physical Sciences. Complutense University of Madrid (2015)

Tags: SMN, Structure Physics

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. Full text in Research Archive Show summary
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 [Show all 9 contributors for this article] (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. Full text in Research Archive Show summary
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 [Show all 9 contributors for this article] (2025). Crystallization Instead of Amorphization in Collision Cascades in Gallium Oxide. Physical Review Letters. ISSN 0031-9007. 134. doi: 10.1103/PhysRevLett.134.126101. Full text in Research Archive
Fernandez, Javier Garcia; Kjeldby, Snorre Braathen; Zeng, L. J.; Azarov, Alexander; Pokle, Anuj & Nguyen, Phuong Dan [Show all 10 contributors for this article] (2024). In situ atomic-resolution study of transformations in double polymorph γ/β-Ga2O3 structures. Materials Advances. 5, p. 3824–3831. doi: 10.1039/D3MA01011B. Full text in Research Archive Show summary
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 [Show all 7 contributors for this article] (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. Full text in Research Archive Show summary
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. Full text in Research Archive Show summary
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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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. Full text in Research Archive
Fernandez, Javier Garcia; Montag, S.; Splith, D.; Kneiß, M.; Grundmann, M. & Prytz, Øystein [Show all 7 contributors for this article] (2024). Combinatorial PLD Synthesis of (In1−xGax)2O3 Thin Films: Structural, Morphological, and Optical Analysis for Transparent Conducting Oxides. Full text in Research Archive
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. Full text in Research Archive
Fernandez, Javier Garcia; Montag, S.; Andreassen, Magnus; Pokle, Anuj; Splith, D. & Kneiß, M. [Show all 9 contributors for this article] (2024). Probing, microstructure, crystallization and phase segregation in (In1-xGax)2O3 thin films by TEM. Full text in Research Archive
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. Full text in Research Archive
Fernandez, Javier Garcia; Kjeldby, Snorre Braathen; Nguyen, Phuong Dan; Haug, Kristian; Galeckas, Augustinas & Jensen, Ingvild Julie Thue [Show all 9 contributors for this article] (2023). Optical properties and structure relationship in ZnO:Fe with inversion domain boundaries and ZnFe2O4/ZnO heterostructures. Full text in Research Archive
Fernandez, Javier Garcia; Kjeldby, Snorre Braathen; Azarov, Alexander; Pokle, Anuj; Nguyen, Phuong Dan & Vines, Lasse [Show all 8 contributors for this article] (2023). Polymorph engineering and radiation tolerance in β-Ga2O3. Full text in Research Archive
Fernandez, Javier Garcia; Kjeldby, Snorre Braathen; Nguyen, Phuong Dan; Haug, Kristian; Galeckas, Augustinas & Jensen, Ingvild Julie Thue [Show all 9 contributors for this article] (2022). Structural and optical investigation of Fe-ZnO nanoarchitectures: from inversion domain boundaries to ZnO/ZnFe2O4 heterostructure. Full text in Research Archive
Kjeldby, Snorre Braathen; Azarov, Alexander; Nguyen, Phuong Dan; Venkatachalapathy, Vishnukanthan; Mikšová, Romana & Macková, Anna [Show all 10 contributors for this article] (2022). Structural Transformation of β-Ga2O3 through Si-implantation. Full text in Research Archive
Kjeldby, Snorre Braathen; Nguyen, Phuong Dan; Fernandez, Javier Garcia; Haug, Kristian; Galeckas, Augustinas & Jensen, Ingvild Julie Thue [Show all 9 contributors for this article] (2022). Optical properties of Fe-decorated inversion domain boundaries and embedded spinel zinc ferrite nanoparticles in zinc oxide. Full text in Research Archive
Kjeldby, Snorre Braathen; Azarov, Alexander; Nguyen, Phuong Dan; Fernandez, Javier Garcia; Venkatachalapathy, Vishnukanthan & Mikšová, Romana [Show all 10 contributors for this article] (2022). Structure and luminescence properties of Si-implanted β-Ga2O3. Full text in Research Archive

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Published Sep. 5, 2022 4:26 PM - Last modified Mar. 3, 2026 11:30 PM

Projects

Research groups

Solid-state Physics and Quantum Technology