Magnus Andreassen

• Andreassen, Magnus; Rosnes, Andreas; Petersen, Clemens; Schenk, Richard Justin; Both, Kevin Gregor & Bergum, Kristin [Vis alle 7 forfattere av denne artikkelen] (2026). Combinatorial Synthesis and Characterization of Lattice-Matched Ga2O3–Cr2O3 and Ga2O3–V2O3 Sesquioxide Alloys for Ultrawide-Bandgap Semiconductor Applications. Physica Status Solidi (a) applications and materials science. ISSN 1862-6300. 223(15). doi: 10.1002/pssa.70437. Fulltekst i vitenarkiv Vis sammendrag

  Gallium sesquioxide is a wide‐bandgap semiconductor of high interest for power electronics and deep UV‐sensing applications. The α‐phase ( E g  = 5.6 eV) can be synthesized with excellent crystal quality on isostructural Al 2 O 3 substrates, removing the need for expensive homoepitaxial growth that is required for the thermodynamically stable β‐phase ( E g  = 4.9 eV). Here, continuous composition spread material libraries of (Ga 1‐ x V x ) 2 O 3 and (Ga 1‐ x Cr x ) 2 O 3 with 0.05 <  x  < 0.45 are synthesized by pulsed laser deposition, and characterized structurally and optically with the aim of decreasing the optical absorption edge of α‐Ga 2 O 3 to expand the range of potential optoelectronic applications. The (Cr x Ga 1‐ x ) 2 O 3 material library remained stable in the rhombohedral α‐phase throughout the investigated compositional range, exhibiting high crystal quality with minimal changes to the unit cell parameters. A systematic shift in optical absorption edge from 5.5 to 3.9 eV is observed, as determined by spectrophotometry. The (Ga 1‐ x V x ) 2 O 3 material library exhibits a systematic shift from 5.2 to 2.9 eV with increasing x V , but a region of decreased crystal quality is observed for 0.11 ≤  x V  ≤ 0.20. Additionally, a secondary V 2 O 5 phase is observed for x V  > 0.23. Overall, bandgap engineering of α‐(Ga 1‐ x V x ) 2 O 3 and α‐(Ga 1‐ x Cr x ) 2 O 3 is demonstrated, and chromium substitution is identified as especially promising due to high crystallinity and systematic change of optical absorption edge.

• Fernández, Javier García; Pokle, Anuj; Andreassen, Magnus; Prytz, Øystein; Grundmann, Marius & Wenckstern, Holger Von (2026). Ex Situ and In Situ Thermal Evolution and Crystallization of Amorphous (In,Ga)2O3. ACS Applied Electronic Materials. 8(11), s. 4582–4588. doi: 10.1021/acsaelm.6c00284. Fulltekst i vitenarkiv Vis sammendrag

  The present study investigates the structural evolution of amorphous (In,Ga)2O3 thin films grown by pulsed laser deposition and thermal annealing under vacuum (ex situ) and in situ transmission electron microscopy (TEM). Ex situ annealing results showed that the crystallization starts close to the substrate interface at 500 °C with randomly oriented nanocrystals, nanovoids, and on top of that, columnar grains of cubic c-(In,Ga)2O3. For the in situ TEM studies, we observed the formation of a heterogeneous polycrystalline mixture of elemental indium (In), monoclinic β-Ga2O3, and a minor contribution of c-In2O3 when annealed at 400 °C. Further annealing at 700 °C led to gradual evaporation of indium, resulting in a β-Ga2O3-dominated sample with nanovoids. These findings provide insights into phase stability, evolution, and elemental redistribution, which are critical for optimizing (In,Ga)2O3 thin films for optoelectronic applications.

• Petersen, Clemens; Schultz, Thorsten; Andreassen, Magnus; Vogt, Sofie; Koch, Norbert & Grundmann, Marius [Vis alle 7 forfattere av denne artikkelen] (2025). Toward Ultrawide Bandgap Engineering: Physical Properties of an α-(TixGa1-x)2O3 Material Library. Physica Status Solidi. Rapid Research Letters. ISSN 1862-6254. 19. doi: 10.1002/pssr.202500033. Fulltekst i vitenarkiv Vis sammendrag

  Due to its high bandgap of 5.3–5.6 eV and high predicted breakdown field of 10 MV cm−1, much attention is drawn to the ultrawide bandgap semiconductor α-Ga2O3 for applications in high-power and solar blind optoelectronic devices. In contrast to the thermodynamically most stable β-phase of Ga2O3, various transition metal sesquioxides with rhombohedral crystal structure and similar lattice constants to α-Ga2O3 are available for bandgap engineering toward lower bandgap energies. Therefore the material system α-(TixGa1−x)2O3 in principle offers the possibility to tune the materials bandgap for wavelength selective optoelectronics over an extremely wide range from 5.6 eV (α-Ga2O3) down to 0.14 eV (α-Ti2O3). In this work, high-throughput combinatorial synthesis by pulsed laser deposition is employed to realize a spatially addressable material library covering almost the entire composition range within the ternary (TixGa1−x)yOz solid solution. Phase-pure growth of (TixGa1−x)2O3 up to x = 0.25 is reported, exceeding previously found miscibility limits by a factor of 5. The physical properties of the material system are investigated in relation to x and bandgap engineering within the rhombohedral α-(TixGa1−x)2O3 material system is demonstrated over an up to now unprecedented large spectral range from 4.4 to 5.3 eV.

• Dahl, Øystein; Sunding, Martin Fleissner; Killi, Veronica Anne-Line Kathrine; Svenum, Ingeborg-Helene; Grandcolas, Mathieu & Andreassen, Magnus [Vis alle 10 forfattere av denne artikkelen] (2023). Interrogation of the Interfacial Energetics at a Tantalum Nitride/Electrolyte Heterojunction during Photoelectrochemical Water Splitting by Operando Ambient Pressure X-ray Photoelectron Spectroscopy. ACS Catalysis. 13(17), s. 11762–11770. doi: 10.1021/acscatal.3c02423. Fulltekst i vitenarkiv Vis sammendrag

  Photoelectrochemical (PEC) water electrolysis is an important energy conversion (power-to-chemical) method, providing a solution to the intermittent nature of solar energy. However, as PEC systems usually suffer from low operational stability, they are seriously lagging in up-scaled demonstrations and viability. PEC systems are based on semiconductor/liquid interfaces, which have been extensively studied by experiments and theory, but there is a significant knowledge gap in the energetics of such interfaces during operation. In this work, operando ambient pressure X-ray photoelectron spectroscopy (AP-XPS) has been used to characterize the electrical and spectroscopic properties of a pristine Ta3N5 photoelectrode and a Ta3N5/NiOx protection/passivation layer system, which stabilizes an otherwise quickly corroding pristine photoelectrode. We directly observed Fermi-level pinning of Ta3N5 within the applied potential window under both dark and illumination conditions, detrimental to the performance and stability of the photoelectrode. Interestingly, in the Ta3N5/NiOx protection/passivation layer system, the Fermi level gets unpinned under illumination, allowing quasi-Fermi-level splitting and sustaining a significant PEC performance as well as high stability.

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• Svenum, Ingeborg-Helene; Dahl, Øystein; Sunding, Martin Fleissner; Killi, Veronica Anne-Line Kathrine; Grandcolas, Mathieu & Andreassen, Magnus [Vis alle 10 forfattere av denne artikkelen] (2026). Operando insight into Fermi-level (un)pinning at Ta3N5/NiOx/electrolyte interfaces during photoelectrochemical water splitting. 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.; 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
• Dahl, Øystein; Sunding, Martin Fleissner; Thøgersen, Annett; Killi, Veronica Anne-Line Kathrine; Svenum, Ingeborg-Helene & Grandcolas, Mathieu [Vis alle 10 forfattere av denne artikkelen] (2023). Direct observation of interfacial energetics at Ta3N5/electrolyte and Ta3N5/NiOx/electrolyte heterojunctions by operando ambient pressure X-ray photoelectron spectroscopy during photoelectrochemical water splitting. Fulltekst i vitenarkiv
• Dahl, Øystein; Sunding, Martin Fleissner; Thøgersen, Annett; Killi, Veronica Anne-Line Kathrine; Svenum, Ingeborg-Helene & Grandcolas, Mathieu [Vis alle 10 forfattere av denne artikkelen] (2023). Operando observation of interfacial energetics at the Ta3N5/electrolyte heterojunction by ambient pressure X-ray photoelectron spectroscopy for photoelectrochemical water splitting. Fulltekst i vitenarkiv
• Andreassen, Magnus (2026). Pulsed Laser Deposition of Sesquioxide Thin Films for Combinatorial Materials Science. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  Combinatorial materials science is an experimental methodology in which highthroughput synthesis and characterization techniques are used to discover and develop materials more efficiently than conventional one-by-one synthesis techniques. High-throughput synthesis involves deposition techniques where multiple samples can be synthesized simultaneously, either with discrete compositions or continuous compositional gradients (CCS). This time- and resource-efficient methodology enables experimental mapping of large compositional spaces, e.g. for the development of novel semiconductor materials. In this work, efforts are directed towards methodological development of pulsed laser deposition (PLD) techniques for combinatorial materials science. A study of segmented PLD targets showed that, for many materials, individually sintered pellets that are cut and assembled yielded better results than co-pressing and -sintering segmented targets due to compatibility issues at elevated temperatures. Additionally, a substrate holder that facilitates an in-situ temperature gradient of more than 100 K across a 2" wafer has been developed. Moreover, principal component analysis was used to improve signal-to-noise ratios in X-ray diffraction data from combinatorial samples, which enabled reliable feature extraction with an 8-fold increase in scan speed compared to non-processed datasets. In addition to this, PLD was used to synthesize ultra-wide bandgap oxides, which are increasingly relevant for applications such as power electronics, transparent conductive oxides, and solar-blind photodetectors. Gallium sesquioxide, Ga2O3, is one of the ultra-wide bandgap semiconductors of high scientific interest for such applications. With the aim to decrease the bandgap of the rhombohedral α-phase Ga2O3 (Eg = 5.3–5.6 eV), alloying with isostructural transition metal oxides with lower bandgaps has been investigated. Here, CCS thin films consisting of (CrxGa1–x)2O3, (Ga1–xVx)2O3, and (Ga1–xTix)2O3 were grown on m-Al2O3 substrates and characterized optically and structurally to investigate the accessible alloying range where the structure remains in the α-phase. The (CrxGa1–x)2O3 sample remained in the α-phase throughout the compositional range, while phase segregation was observed for (Ga1–xVx)2O3 and (Ga1–xTix)2O3. Systematically declining optical bandgaps were observed for the α-phase of all three materials libraries, where cationic substitution ranges of 6–9 at.% for V, 9–25 at.% for Ti, and 6–44 at.% for Cr yielded bandgap ranges of 5.2–5.0 eV, 5.3–4.4 eV, and 5.5–3.9 eV in single-phase regions, respectively. Finally, an annealing study of a PLD-grown (Ga0.37In0.67)2O3 thin film deposited at ambient temperature was conducted to investigate the crystallization process. Reduction and evaporation of metallic In was observed in samples ex-situ annealed in vacuum and during an in-situ TEM annealing experiment. Spectrophotometry investigations of a sample ex-situ annealed at 800 ◦C revealed an optical bandgap of 4.2 eV, which is 0.45 eV higher than In2O3 and currently the highest bandgap observed for bixbyite-structure (Ga,In)2O3. This work contributes to the development of exploratory combinatorial materials science techniques, and elucidates important material properties for several ultra-wide bandgap semiconductor oxides. The materials described herein are of high scientific and technological interest due to their structural and electronic properties.

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