Andrej Kuznetsov

• Esteves, Duarte Magalhães; He, Ru; Bazioti, Kalliopi; Magalhães, Sérgio; Sequeira, Miguel Carvalho & Santos, Luís Filipe [Show all 11 contributors for this article] (2026). Microtubes and nanomembranes by ion-beam-induced exfoliation of β-Ga2O3. Surfaces and Interfaces. 88, p. 109066–109066. doi: 10.1016/j.surfin.2026.109066. Full text in Research Archive
• Liu, Huan; Makkonen, Ilja; Bazioti, Kalliopi; Zhao, Junlei; Azarov, Alexander & Kuznetsov, Andrej [Show all 7 contributors for this article] (2026). Band Offsets at β/γ-Ga2O3 Interface: A Comprehensive Ab Initio and Experimental Investigation. Journal of Physical Chemistry C. ISSN 1932-7447. 130(22), p. 7863–7873. doi: 10.1021/acs.jpcc.6c01872. Full text in Research Archive
• Whitlow, Harry James; Nagy, Gyula; Frost, Robert J.W.; Toyen, Donruedee; Bose, Deepanwita & McLaughlin, Aaron [Show all 11 contributors for this article] (2026). Ion microprobe determination of ex vivo trace element molarities: the LINAEUS code suite. Journal of Analytical Atomic Spectrometry. ISSN 0267-9477. doi: 10.1039/d5ja00505a. Full text in Research Archive Show summary

  MeV-ion based analytical methods such as Particle Induced X-ray Emission (PIXE) using a MeV ion microprobe in conjunction with analysis codes such as GeoPIXE represent powerful methods for analysis of trace elements in biomedical samples. In order to make the method truly quantitative for thin biological samples, knowledge of the local composition and thickness is required. Here the development and application of the LINAEUS code package, a complementary companion code to the GeoPIXE code, are described. This provides regional major element (C, H, N and O) composition and thickness data from Elastic Backscattering Spectrometry (EBS) and Off-Axis Scanning Transmission Ion Microscopy (OA-STIM) measurements. The program suite employs the same dynamic analysis method as used in GeoPIXE. LINAEUS incorporates a tissue region detection procedure based on spatial correlation to mitigate bias due to void regions without tissue corresponding to blood vessels, etc. The results for healthy murine small-intestine tissue showed no differences in the C, H, N and O contents but did show differences in thickness that were attributed to morphological distortion during drying of the tissue section. The molarity of trace elements (S, Cl, K, Ca and Fe) exhibited differences between tissue regions. Ca “hot-spots” (10–30 µm regions with much enhanced Ca levels) reported in other simian tissue samples were observed. These could not be attributed to mineralisation and their origin remains unclear.

• Ousdal, Erlend Lemva; Bathen, Marianne Etzelmüller; Medeiros, Helton Goncalves de; Galeckas, Augustinas; Kumar, Piyush & Martins, Maria I.M. [Show all 10 contributors for this article] (2026). Control and tuning of color centers in 4H silicon carbide by application of electric field via Schottky diode. Materials Science in Semiconductor Processing. ISSN 1369-8001. 201. doi: 10.1016/j.mssp.2025.110097. Full text in Research Archive Show summary

  The ability to tune and manipulate the energy and intensity of photons emitted from color centers in semiconductors is of great importance for developing point defect quantum emitters as a platform for future quantum technology (QT) applications. One of the promising materials to realize point defect based QT is silicon carbide (SiC), as it combines a plethora of color center candidates with mature material processing and device fabrication. Here we explore the use of a Schottky diode, fabricated on a highly doped n-type 4H-SiC epitaxial layer, to control and modulate defect-related emission under both forward and reverse bias conditions. Zero phonon lines (ZPLs) from three prominent color centers are investigated: V1, V1’ and V2 assigned to the silicon vacancy, B1 and B2 from the carbon antisite-vacancy pair, and PL4 of the divacancy complex. All the studied defect-related emission wavelengths are found to shift in response to applied bias, but with a varying magnitude and direction of the shift. The electric field-induced variations are assigned to Stark effect and current flow within the device. Furthermore, two unknown defect signatures, labeled K1 and K2, are observed in the vicinity (within 2 meV) of the V2 ZPL and exhibit a strong forward bias dependence. A possible origin related to silicon vacancies perturbed by nearby carbon antisites is discussed.

• Lábár, János L.; Cora, Ildikó; Pécz, Béla; Azarov, Alexander & Kuznetsov, Andrej (2026). Local deformations quantified with the common sublattice method. Nanoscale. ISSN 2040-3364. 18(4), p. 2264–2276. doi: 10.1039/d5nr03934g. Full text in Research Archive Show summary

  Measurement of strain in epitaxial layers frequently faces the problem of localizing “perfectly” unstrained regions to use as a reference for the whole structure. Here, we develop a method for deducing such references in the unstrained substrate (with one crystal structure) to determine the local lattice deformations in the layer (with another crystal structure). The method works if there is a common sublattice in the layer and the substrate, so that the deviations from it may be used to interrelate the experimentally recorded data. The proof-of-concept is obtained using samples of cubic γ-Ga2O3 layers on a monoclinic β-Ga2O3 substrate, confirming that the new method overcomes several limitations of HRTEM-based methods. The precision of the method is ∼0.5% in strain values.

• Ousdal, Erlend Lemva; Ghezellou, Misagh; Ul-Hassan, Jawad; Kuznetsov, Andrej; Vines, Lasse & Bathen, Marianne Etzelmüller (2025). Identification of electrically active defects in 6⁢𝐻-SiC. PHYSICAL REVIEW MATERIALS. 9(12). doi: 10.1103/qzfw-162p. Full text in Research Archive Show summary

  Deep level defects in silicon carbide (SiC) are of great importance for developing both power electronics and quantum technology applications based on SiC material. While the more technologically mature 4⁢𝐻 polytype has been widely explored, electrical identification of deep level defect states in 6⁢𝐻 -SiC has not been completed. This work aims to identify irradiation-induced charge state transition levels arising from intrinsic defects in 6⁢𝐻 -SiC using deep level transient spectroscopy (DLTS) and the formation energy diagram formalism based on density functional theory (DFT) calculations. By comparing experimental results of 6⁢𝐻 -SiC epitaxial layers with measurements of 4⁢𝐻 -SiC material that has been grown under identical conditions, combined with DFT calculations, we assign possible origins for the observed DLTS-signatures.

• 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. Full text in Research Archive Show summary

  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.

• Kjeldby, Snorre Braathen; Nielsen, Dina Marie; Cooil, Simon Phillip; Frodason, Ymir Kalmann; Zacharaki, Eirini & Lee, In Hwan [Show all 10 contributors for this article] (2025). Tuning Properties of II-IV-Nitrides for Optoelectronic Applications. Advanced Optical Materials. 13(36). doi: 10.1002/adom.202502634. Full text in Research Archive Show summary

  The earth-abundant II-IV-nitrides ZnSnN2 and ZnGeN2 are direct bandgap semiconductors with a wurtzite-derived crystal structure. Their alloys, ZnSnxGe1 − xN2, have bandgaps tunable across the full visible spectrum, making them interesting for many optoelectronic applications. Here, electrical, structural, and optical properties of near-stoichiometric ZnSnxGe1 − xN2 alloys, i.e., where [Zn]/([Zn]+[Ge]+[Sn]) ≈ 0.5, are reported, for samples synthesized by reactive magnetron sputtering. These results reveal unprecedentedly high electrical mobilities in Ge-rich alloys, with values of 136 and 400 cm2/Vs at room-temperature and ≈100 K, respectively. The bandgaps are determined from optical absorption measurements combined with hybrid density functional calculations and reveal a significant Burstein–Moss shift in the Sn rich alloys. Finally, band alignments are determined in the sputter-grown thin films by combining optical transmission measurements, hybrid density functional calculations, and UV photoelectron spectroscopy measurements, where the bandgap variation is predominantly caused by a shift of the conduction band edge. This work elucidates in unprecedented detail the tuning of optical and electrical properties in ZnSnxGe1 − xN2 by variation of the chemical composition, where bandgap values of alloys with x ∈ [0.5−0.7] are suitable for top cell absorbers in two-terminal tandem solar cells assuming a Si bottom cell.

• 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
• Azarov, Alexander; Radu, Cristian; Galeckas, Augustinas; Mercioniu, Ionel Florinel; Cernescu, Adrian & Venkatachalapathy, Vishnukanthan [Show all 11 contributors for this article] (2025). Self-Assembling of Multilayered Polymorphs with Ion Beams. Nano Letters. ISSN 1530-6984. 25(4), p. 1637–1643. doi: 10.1021/acs.nanolett.4c05727. Full text in Research Archive
• Abdullaev, Azat; Sekerbayev, Kairolla; Azarov, Alexander; Venkatachalapathy, Vishnukanthan; Chauhan, Vinay S. & Utegulov, Zhandos [Show all 7 contributors for this article] (2024). Thermal conductivity of double polymorph Ga2O3 structures. APL Materials. 12(8). doi: 10.1063/5.0213985. Full text in Research Archive Show summary

  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.

• Slusar, Tetiana; Azarov, Alexander; Galeckas, Augustinas; Hallén, Anders; Ju, Jung Jin & Moon, Kiwon [Show all 7 contributors for this article] (2024). Er:LiNbO3 Quantum Memory Platform Optimized with Dynamic Defect Annealing. Advanced Optical Materials. doi: 10.1002/adom.202401374. Full text in Research Archive Show summary

  Lithium niobate (LiNbO3) exhibits poor radiation resistance when conventionally implanted with Er at room temperature. To repair the structural disorder, high-temperature post-implantation anneals are typically required; however, such treatments can cause sample cracking and dopant redistribution, which are incompatible with device applications. The optimized approach is proposed by performing implants at elevated but acceptably low temperatures to in situ minimize the structural defects generated in the collision cascades, via so-called “dynamic defect annealing”. Thus, a gradual increase in Er optical activity is shown as a function of irradiation temperature in the range of 25–450 °C, in striking correlation with a decreasing trend for the residual disorder. The impact of moderate post-implantation anneals is also investigated by comparing the results of anneals of samples implanted at 25 °C and 450 °C. This comparison further confirms the higher optical activation of Er and lower residual structural disorder when the material is initially implanted at elevated temperatures. Based on these data, it is concluded that the use of dynamic defect annealing during the Er implantation of LiNbO3 is a promising strategy for optimizing it as a quantum memory platform, resolving otherwise inevitable trade-offs.

• Huang, Qiu-Shi; Li, Chuan-Nan; Hao, Mao-Sheng; Liang, Han-Pu; Cai, Xuefen & Yue, Ying [Show all 9 contributors for this article] (2024). Nature of Disordering in γ-Ga2 O3. Physical Review Letters. ISSN 0031-9007. 133(22). doi: 10.1103/PhysRevLett.133.226101. Full text in Research Archive
• Klevtsov, Anton; Karaseov, Platon; Azarov, Alexander; Karabeshkin, Konstantin; Fedorenko, Elizaveta & Titov, Andrei [Show all 7 contributors for this article] (2024). Non-linear effects in α-Ga2O3 radiation phenomena. APL Materials. 12(11). doi: 10.1063/5.0235497. Full text in Research Archive Show summary

  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. [Show all 17 contributors for this article] (2024). Proton damage effects in double polymorph γ/β-Ga2O3 diodes. Journal of Materials Chemistry C. ISSN 2050-7526. 12, p. 1020–1029. doi: 10.1039/d3tc04171a. Full text in Research Archive Show summary

  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.

• Whitlow, Harry James; Kuznetsov, Andrej; Azarov, Alexander; Nagy, Gyula; Frost, Robert J.W. & Henderson, Nicholas [Show all 17 contributors for this article] (2024). Dynamic Analysis of major elements in biological tissue validating quantification of trace life elements in MeV ion beam microscopy. Nuclear Instruments and Methods in Physics Reseach B. ISSN 0168-583X. 554. doi: 10.1016/j.nimb.2024.165411. Full text in Research Archive
• Chen, Xingqian; Li, Haozhen; Chen, Wei; Mei, Zengxia; Azarov, Alexander & Kuznetsov, Andrej [Show all 7 contributors for this article] (2024). Interstitial Doping of SnO2 Film with Li for Indium-Free Transparent Conductor. Chinese Physics Letters. ISSN 0256-307X. 41(3). doi: 10.1088/0256-307X/41/3/037305. Full text in Research Archive Show summary

  SnO2 films exhibit significant potential as cost-effective and high electron mobility substitutes for In2O3 films. In this study, Li is incorporated into the interstitial site of the SnO2 lattice resulting in an exceptionally low resistivity of 2.028 × 10−3 Ω⋅cm along with a high carrier concentration of 1.398 × 1020 cm−3 and carrier mobility of 22.02 cm2/V⋅s. Intriguingly, Lii readily forms in amorphous structures but faces challenges in crystalline formations. Furthermore, it has been experimentally confirmed that Lii acts as a shallow donor in SnO2 with an ionization energy ΔED1 of −0.4 eV, indicating spontaneous occurrence of Lii ionization.

• Azarov, Alexander; Galeckas, Augustinas; Wendler, Elke; Monakhov, Eduard & Kuznetsov, Andrej (2024). Inverse dynamic defect annealing in ZnO. Applied Physics Letters. ISSN 0003-6951. 124(4). doi: 10.1063/5.0184669. Full text in Research Archive Show summary

  Radiation tolerance of semiconductors depends on the dynamic defect annealing efficiency during irradiation. Consequently, it matters at what temperature one keeps the sample during irradiation, so that elevated temperatures typically result in lower remaining disorder. In the present work, we observed an opposite trend for the nitrogen ion implants into zinc oxide. Combining ion channeling technique, x-ray diffraction, and photoluminescence spectroscopy, we demonstrate that the interaction of nitrogen with radiation defects promotes an inverse dynamic annealing process, so that the increase in irradiation temperature leads to a more efficient defect formation. As a result, the residual radiation disorder is maximized at 650 °C and this state is characterized by the appearance of prominent optical signatures associated with zinc interstitials and strongly reduced strain accumulation as compared to the samples implanted at lower temperatures. However, for higher implantation temperatures, the impact of the inverse annealing decreases correlating with the surface degradation and loss of nitrogen.

• Whitlow, Harry James; Nagy, Gyula; Kuznetsov, Andrej; Frost, Robert J.W.; Azarov, Alexander & Smith, Karen M. [Show all 14 contributors for this article] (2024). Major and Trace Element Composition Differences Revealed in Porcine Intestine by Dynamic Analysis and MeV Ion Microscopy. Physica Status Solidi (a) applications and materials science. ISSN 1862-6300. doi: 10.1002/pssa.202400161. Full text in Research Archive
• Huang, Qiu-Shi; Cai, Xuefen; Zhang, Xie; Kuznetsov, Andrej & Wei, Su-Huai (2024). Synergistic effects of pressure and hole doping on polymorphism of Ga2O3. Physical review B (PRB). ISSN 2469-9950. 109(22). doi: 10.1103/PhysRevB.109.224105. Full text in Research Archive
• Samriti, Samriti; Thakur, Sahil; Ojha, Abhijeet; Gupta, Rajeev; Bechelany, Mikhael & Kuznetsov, Andrej [Show all 8 contributors for this article] (2024). Graphene Oxide as Novel Visible Light Active Photocatalyst: Synthesis, Modification by Nitrogen and Boron Doping, and Photocatalytic Application. Physica Status Solidi (a) applications and materials science. ISSN 1862-6300. doi: 10.1002/pssa.202400169. Full text in Research Archive
• Azarov, Alexander; Galeckas, Augustinas; Cora, Ildikó; Fogarassy, Zsolt; Venkatachalapathy, Vishnukanthan & Monakhov, Eduard [Show all 7 contributors for this article] (2024). Optical Activity and Phase Transformations in γ/β Ga2O3 Bilayers Under Annealing. Advanced Optical Materials. doi: 10.1002/adom.202401325. Full text in Research Archive Show summary

  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 [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.

• Ousdal, Erlend Lemva; Bathen, Marianne Etzelmüller; Galeckas, Augustinas; Kuznetsov, Andrej & Vines, Lasse (2024). Formation and stability of point defect color centers in 6H silicon carbide. Journal of Applied Physics. ISSN 0021-8979. 135(22). doi: 10.1063/5.0205832. Full text in Research Archive Show summary

  Point defect color centers acting as single-photon emitters are promising for quantum technology applications and have been extensively studied, e.g., in the 4H polytype of silicon carbide (SiC). However, the physics of such color centers in other SiC polytypes is much less explored. Herein, we study the formation and thermal stability of such color centers in 6H-SiC using photoluminescence spectroscopy. The emissions from typical single-photon emitters, such as silicon vacancies, divacancies, and carbon antisite-vacancy pairs in 6H-SiC, were monitored as a function of the proton irradiation fluence and post-irradiation annealing, and compared to that in 4H-SiC. Overall, at the background of similarities between the emission behavior in 4H- and 6H-SiC polytypes, we observed prominent differences; e.g., for the thermal stability of the carbon antisite-vacancy pair, which exhibited maximized emissions upon 300 and 900  *C anneals in 4H- and 6H-SiC, respectively. Moreover, we observed a range of defect emission signatures not previously reported for 6H-SiC in the literature and discussed their potential origin in the context of the thermal stability. For example, among the PL-lines in 6H-SiC, we detected periodically repeatable emission signatures, resembling the so-called L-lines recently reported in 4H-SiC, even though their exact origin has not yet been settled in the literature. Thus, we use color centers comparison in different polytypes to better understand the nature of the single-photon emitters in SiC.

• Kurbanova, B.; Chakraborty, D.; Abdullaev, A.; Shamatova, A.; Makukha, O. & Belarouci, A. [Show all 11 contributors for this article] (2024). Multiscale phonon thermal transport in nano-porous silicon. Applied Physics Letters. ISSN 0003-6951. 124(25). doi: 10.1063/5.0205455. Full text in Research Archive Show summary

  We performed a comprehensive multi-scale phonon-mediated thermal transport study of nano-porous silicon (np-Si) films with average porosities in the range of  = 30%–70%. This depth-resolved thermal characterization involves a combination of optical methods, including femtosecond laser-based time-domain thermo-reflectance (TDTR) with MHz modulation rates, opto-thermal micro-Raman spectroscopy, and continuum laser wave-based frequency domain thermo-reflectance (FDTR) with kHz modulation rates probing depths of studied samples over 0.5–1.2, 2–3.2, and 23–34 μm, respectively. We revealed a systematic decrease in thermal conductivity (⁠ ⁠) with the rise of ⁠, i.e., with the lowering of the Si crystalline phase volumetric fraction. These data were used to validate our semi-classical phonon Monte Carlo and finite element mesh simulations of heat conduction, taking into account disordered geometry configurations with various and pore size, as well as laser-induced temperature distributions, respectively. At high ⁠, the decrease in is additionally influenced by the disordering of the crystal structure, as evidenced by the near-surface sensitive TDTR and Rutherford backscattering spectroscopy measurements. Importantly, the values measured by FDTR over larger depths inside np-Si were found to be anisotropic and lower than those detected by the near-surface sensitive TDTR and Raman thermal probes. This finding is supported by the cross-sectional scanning electron microscopy image indicating enhanced distribution over these micrometer-scale probed depths. Our study opens an avenue for nano-to-micrometer scale thermal depth profiling of porous semiconducting media with inhomogeneous porosity distributions applicable for efficient thermoelectric and thermal management.

• Chen, Xingqian; Li, Haozhen; Chen, Wei; Mei, Zengxia; Azarov, Alexander & Kuznetsov, Andrej [Show all 7 contributors for this article] (2024). Optimized indium-free transparent conductor by Zn and F co-doping into tin oxide. Solar Energy Materials and Solar Cells. ISSN 0927-0248. 278. doi: 10.1016/j.solmat.2024.113211. Full text in Research Archive
• Bathen, Marianne Etzelmüller; Karsthof, Robert Michael; Galeckas, Augustinas; Kumar, Piyush; Kuznetsov, Andrej & Grossner, Ulrike [Show all 7 contributors for this article] (2024). Impact of carbon injection in 4H-SiC on defect formation and minority carrier lifetime. Materials Science in Semiconductor Processing. ISSN 1369-8001. 176. doi: 10.1016/j.mssp.2024.108316. Full text in Research Archive
• Zhu, Rui; Kuznetsov, Andrej; Galeckas, Augustinas; Zhang, Yonghui; Liang, Huili & Zhang, Guangyu [Show all 7 contributors for this article] (2024). Novel Photosensitive Dielectric-Based Image Sensor with Both High Signal-to-Noise Ratio and High Fill Factor. ACS Photonics. p. 555–562. doi: 10.1021/acsphotonics.4c02229. Full text in Research Archive Show summary

  Driven by the pursuit of a higher signal-to-noise ratio (SNR), CMOS imagers have evolved from initial passive pixel sensor (PPS) with one transistor (1T) to 3T, 4T, or 5T active pixel sensors (APS) and then recently to digital pixel sensors (DPS). However, this evolution inevitably causes a lower and lower fill factor. Here, we innovatively designed a photodetector (PD) based on a photosensitive dielectric (PSD) strategy. The unique PSD layer works as an insulator in the dark and as a semiconductor under illumination, making the PD have a built-in 4T function consisting of memory, address, amplification, and reset. Further, we prepared a 16 × 16 one-PD (1D) APS array demo. The memory imaging, continuous imaging, power-off imaging, and trajectory tracing imaging functionalities were successfully demonstrated. This work provides a novel 1D APS structure with a built-in 4T function well handling the SNR-fill factor trade-off, paving a new path for a high-performance image sensor with good versatility and scalability.

• Thakur, Sahil; Ojha, Abhijeet; Kansal, Sushil Kumar; Gupta, Navneet Kumar; Swart, Hendrik C. & Cho, Junghyun [Show all 9 contributors for this article] (2024). Advances in powder nano-photocatalysts as pollutant removal and as emerging contaminants in water: Analysis of pros and cons on health and environment. Advanced Powder Materials (APM). 3(6). doi: 10.1016/j.apmate.2024.100233. Full text in Research Archive Show summary

  Photocatalysis is an advanced oxidation process where light exposure triggers a semiconducting nanomaterial (nano-photocatalyst) to generate electron-hole (e−/h+) pairs and free radicals. This phenomenon is widely used for the photocatalysis-assisted removal of organic and other contaminants using wide range of nano-photocatalysts, offering an efficient approach to environmental remediation. However, the introduction of powdered nano-photocatalysts into water systems often leads to unintended secondary pollution in the form of residual nano-photocatalysts, ion leaching, free radicals, toxic by-products etc. Such practices potentially introduce emerging secondary contaminants into aquatic environments, posing risks to both aquatic life and human health. The resulting chemical by-products and intermediates can effectively induce chronic toxicity, neurological and developmental disorders, cardiovascular defects, and intestinal ailments in humans and aquatic species. Despite having a range of health and environmental consequences, this dark side of nano-photocatalysts has been comparatively less explored and discussed in the literature. In this review, the pros and cons of powder nano-photocatalysts are discussed in view of their advantages as well as disadvantages in wastewater treatment. The discussion encompasses their classification based on composition, dimensions, structure, and activity, as well as recent advancements in improving their photocatalytic efficiency. The article also explores the recent advances on their applications in photocatalytic removal of various water pollutants/contaminants of emerging concern (i.e., organic pollutants, micro/nano plastics, heavy ions, disinfections, etc.) Furthermore, an emphasis on the role of such nano-photocatalysts as emerging (secondary) contaminants in water system, along with a thorough discussion of latest studies related to the health and environmental issues, has been discussed. Additionally, it addresses critical issues in applying powder nano-photocatalysts for wastewater detoxification and explores potential solutions to these challenges followed by future prospects.

• Wang, Z.P.; Sun, N.; Yu, X.X.; Gong, H.H.; Ji, X.L. & Ren, F.-F. [Show all 11 contributors for this article] (2024). Performance limiting inhomogeneities of defect states in ampere-class Ga2O3 power diodes. Applied Physics Reviews (APR). 11(2). doi: 10.1063/5.0191343. Full text in Research Archive Show summary

  Impacts of spatial charge inhomogeneities on carrier transport fluctuations and premature breakdown were investigated in Schottky ampere-class Ga2O3 power diodes. Three prominent electron traps were detected in Ga2O3 epilayers by a combination of the depth-resolved capacitance spectroscopy profiling and gradual dry etching. The near-surface trap occurring at 1.06 eV below the conduction band minimum (EC), named E3, was found to be confined within a 180 nm surface region of the Ga2O3 epilayers. Two bulk traps at EC − 0.75 eV (E2*) and at EC − 0.82 eV (E2) were identified and interconnected with the VGa- and FeGa-type defects, respectively. In the framework of the impact ionization model, employing the experimental trap parameters, the TCAD simulated breakdown characteristics matched the experimental breakdown properties well, consistently with inverse proportionality to the total trap densities. In particular, the shallowest distributed E3 trap with the deepest level is responsible for higher leakage and premature breakdown. In contrast, Ga2O3 Schottky diodes without E3 trap exhibit enhanced breakdown voltages, and the leakage mechanism evolves from variable range hopping at medium reverse voltages, to the space-charge-limited conduction at high reverse biases. This work bridges the fundamental gap between spatial charge inhomogeneities and diode breakdown features, paving the way for more reliable defect engineering in high-performance Ga2O3 power devices.

• Kaźmierczak-Bałata, Anna; Bodzenta, Jerzy; Szperlich, Piotr; Jesionek, Marcin; Michalewicz, Anna & Domanowska, Alina [Show all 9 contributors for this article] (2024). Impact of Annealing in Various Atmospheres on Characteristics of Tin-Doped Indium Oxide Layers towards Thermoelectric Applications. Materials. 17(18). doi: 10.3390/ma17184606. Full text in Research Archive Show summary

  The aim of this work was to investigate the possibility of modifying the physical properties of indium tin oxide (ITO) layers by annealing them in different atmospheres and temperatures. Samples were annealed in vacuum, air, oxygen, nitrogen, carbon dioxide and a mixture of nitrogen with hydrogen (NHM) at temperatures from 200 °C to 400 °C. Annealing impact on the crystal structure, optical, electrical, thermal and thermoelectric properties was examined. It has been found from XRD measurements that for samples annealed in air, nitrogen and NHM at 400 °C, the In2O3/In4Sn3O12 share ratio decreased, resulting in a significant increase of the In4Sn3O12 phase. The annealing at the highest temperature in air and nitrogen resulted in larger grains and the mean grain size increase, while vacuum, NHM and carbon dioxide atmospheres caused the decrease in the mean grain size. The post-processing in vacuum and oxidizing atmospheres effected in a drop in optical bandgap and poor electrical properties. The carbon dioxide seems to be an optimal atmosphere to obtain good TE generator parameters—high ZT. The general conclusion is that annealing in different atmospheres allows for controlled changes in the structure and physical properties of ITO layers.

• Samriti, None; Kumar, Promod; Kuznetsov, Andrej; Swart, H.C. & Prakash, Jai (2024). Sensitive, Stable, and Recyclable ZnO/Ag Nanohybrid Substrates for Surface-Enhanced Raman Scattering Metrology. ACS Materials Au. p. 413–423. doi: 10.1021/acsmaterialsau.4c00002. Full text in Research Archive Show summary

  Surface-enhanced Raman scattering is a practical, noninvasive spectroscopic technique that measures chemical fingerprints for varieties of molecules in multiple applications. However, synthesizing appropriate substrates for practical, long-term applications of this method has always been a challenging task. In the present study, we show that ZnO/Ag nanohybrid substrates may act as highly stable, sensitive, and recyclable substrates for surface-enhanced Raman scattering, as illustrated by the detection of methylene blue, selected as a test dye molecule with self-cleaning functionalities. Specifically, we demonstrate the detection enhancement factor of 3.7 × 107 along with exceptional long-term stability explained in terms of the localized surface plasmon resonance from the Ag nanocrystals embedded into the chemically inert ZnO nanoparticles, constituting the nanohybrid. Significantly, these substrates can be efficiently cleaned and regenerated while maintaining their high performance upon recycling. As a result, using these substrates, up to 10–12 M detection sensitivity has been demonstrated, enabling the accuracy required in modern environmental monitoring, bioassays, and analytical chemistry. Thus, ZnO nanoparticles with embedded Ag nanocrystals constitute a novel class of advanced nanohybrid substrates for use in multiple applications of surface-enhanced Raman scattering metrology.

• Azarov, Alexander; Galeckas, Augustinas; Mikhaylov, Alexey; Nikolskaya, Alena; Tetelbaum, David & Korolev, Dmitry [Show all 8 contributors for this article] (2024). Defect evolution and nitrogen incorporation in ion-implanted β-Ga2O3. Applied Physics Letters. ISSN 0003-6951. 125(19). doi: 10.1063/5.0239899. Full text in Research Archive Show summary

  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.

• Thakur, Sahil; Badoni, Ayush; Samriti, None; Sharma, Pratibha; Ojha, Abhijeet & Swart, Hendrik C. [Show all 8 contributors for this article] (2024). Standalone Highly Efficient Graphene Oxide as an Emerging Visible Light-Driven Photocatalyst and Recyclable Adsorbent for the Sustainable Removal of Organic Pollutants. Langmuir. ISSN 0743-7463. 40(35), p. 18486–18502. doi: 10.1021/acs.langmuir.4c01727. Full text in Research Archive Show summary

  Carbon-based nanostructures are promising eco-friendly multifunctional nanomaterials because of their tunable surface and optoelectronic properties for a variety of energy and environmental applications. The present study focuses on the synthesis of graphene oxide (GO) with particular emphasis on engineering its surface and optical properties for making it an excellent adsorbent as well as a visible light-active photocatalyst. It was achieved by modifying the improved Hummers method through optimizing the synthesis parameters involved in the oxidation process. This controlled synthesis allows for systematic tailoring of structural, optical, and surface functionality, leading to improved adsorption and photocatalytic properties for the sustainable removal of organic pollutants in water treatment. Several spectroscopic and microscopic characterization techniques, such as XRD, SEM, Raman, UV–visible, FTIR, TEM, XPS, BET, etc. were employed to analyze the degree of oxidation, surface chemistry/functionalization, morphological, optical, and structural properties of the synthesized GO nanostructures. The analyses showed excellent surface functionality with surface active sites for better adsorptive removal and a tunable band gap from 2.51 to 2.76 eV exhibiting excellent natural sunlight activity (>99%) for photocatalytic removal of the organic pollutant. Various adsorption isotherms have been studied with excellent adsorption capability (Qmax = 454.54 mg/g) as compared to the literature. The study introduces GO both as a proficient stand-alone (sole) nanoadsorbent as well as a nanophotocatalyst for the efficient removal of organic dye pollutants in water treatment. Additionally, the article highlights the sustainable solar light-induced green chemistry aspects of GO as an excellent recyclable adsorbent as a result of its self-cleaning ability under natural sunlight, demonstrating its potential in real eco-friendly environmental and practical applications.

• Polyakov, A.Y.; Nikolaev, V.I.; Pechnikov, A.I.; Yakimov, E.B.; Karpov, S. Yu. & Stepanov, S.I. [Show all 12 contributors for this article] (2023). Two-dimensional hole gas formation at the κ-Ga<inf>2</inf>O<inf>3</inf> /AlN heterojunction interface. Journal of Alloys and Compounds. ISSN 0925-8388. 936. doi: 10.1016/j.jallcom.2022.168315. Full text in Research Archive
• Polyakov, Alexander Y.; Yakimov, Eugene B.; Nikolaev, Vladimir I.; Pechnikov, Alexei I.; Miakonkikh, Andrej V. & Azarov, Alexander [Show all 12 contributors for this article] (2023). Impact of Hydrogen Plasma on Electrical Properties and Deep Trap Spectra in Ga2O3 Polymorphs. Crystals. 13(9), p. 1–25. doi: 10.3390/cryst13091400. Full text in Research Archive Show summary

  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.

• Polyakov, A.Y.; Kochkova, A.I.; Azarov, Alexander; Venkatachalapathy, Vishnukanthan; Miakonkikh, A.V. & Vasilev, A.A. [Show all 11 contributors for this article] (2023). Tuning electrical properties in Ga2O3 polymorphs induced with ion beams. Journal of Applied Physics. ISSN 0021-8979. 133(9), p. 095701-1–095701-9. doi: 10.1063/5.0133181. Full text in Research Archive Show summary

  Ion beam fabrication of metastable polymorphs of Ga2O3, assisted by the controllable accumulation of the disorder in the lattice, is an interesting alternative to conventional deposition techniques. However, the adjustability of the electrical properties in such films is unexplored. In this work, we investigated two strategies for tuning the electron concentration in the ion beam created metastable κ-polymorph: adding silicon donors by ion implantation and adding hydrogen via plasma treatments. Importantly, all heat treatments were limited to ≤600 °C, set by the thermal stability of the ion beam fabricated polymorph. Under these conditions, silicon doping did not change the high resistive state caused by the iron acceptors in the initial wafer and residual defects accumulated upon the implants. Conversely, treating samples in a hydrogen plasma converted the ion beam fabricated κ-polymorph to n-type, with a net donor density in the low 1012 cm−3 range and dominating deep traps near 0.6 eV below the conduction band. The mechanism explaining this n-type conductivity change may be due to hydrogen forming shallow donor complexes with gallium vacancies and/or possibly passivating a fraction of the iron acceptors responsible for the high resistivity in the initial wafers.

• Polyakov, A.Y.; Kuznetsov, Andrej; Azarov, Alexander; Miakonkikh, A.V.; Chernykh, A.V. & Vasilev, A.A. [Show all 10 contributors for this article] (2023). The effects of hydrogenation on the properties of heavy ion irradiated β-Ga2O3. Journal of materials science. Materials in electronics. ISSN 0957-4522. 34(15). doi: 10.1007/s10854-023-10628-y. Full text in Research Archive Show summary

  Defects created in lightly Sn-doped (2 × 1016 cm−3) (010)-oriented bulk β-Ga2O3 implanted with 1.2 MeV, 3 × 1015 cm−2 197Au+ ions before and after treatment in hydrogen plasmas at 330 °C were studied by X-ray measurements, Rutherford backscattering spectra, capacitance–voltage, current–voltage, admittance spectra and deep level transient spectroscopy. Au implantation creates defects that produce total depletion of carriers in the top 1.5 µm and introduces electron traps with energy levels at Ec-0.7 eV, Ec-1.05 eV, Ec-0.45 eV, and deep acceptors with optical ionization thresholds near 1.3 eV, 2.3 eV and 3.1 eV, similar to the centers dominating the spectra of deep traps in β-Ga2O3. Hydrogen plasma treatment greatly enhances the photocurrent and photo-capacitance and decreases the width of the insulating layer produced by Au implantation. The results can be explained by hydrogen passivation of the triply charged Ga vacancies and doubly charged split Ga vacancies acceptors in the implanted region, returning part of this region to n-type conductivity.

• Samriti, NN; Rumyantseva, Marina; Sun, Shuhui; Kuznetsov, Andrej & Prakash, Jai (2023). Emerging nanomaterials in the detection and degradation of air pollutants. Current Opinion in Environmental Science & Health. 35. doi: 10.1016/j.coesh.2023.100497. Full text in Research Archive Show summary

  Air pollution implies significant threats for the human health and the surrounding environment. In recent years, a great deal of research has been carried out on the detection and removal of air pollutants through the development of novel multifunctional nanomaterials. In this paper, such emerging nanomaterials (e.g. metal oxide, metal–organic framework, graphene, MXenes, quantum dots, plasmonic, etc. based nanostructures) with high stability, sensitivity down to parts-per-billion level and excellent removal efficiency for air pollutants have been discussed. The focus is on specific strategies and recent highlights of the development of emerging nanomaterials for environmental and health monitoring. Furthermore, the challenges and future prospects are discussed.

• Azarov, Alexander; Fernández, Javier García; Zhao, Junlei; Djurabekova, Flyura; He, Huan & He, Ru [Show all 13 contributors for this article] (2023). Universal radiation tolerant semiconductor. Nature Communications. 14, p. 1–8. doi: 10.1038/s41467-023-40588-0. Full text in Research Archive Show summary

  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.

• Azad, Fahad; Luo, Caiqin; Azeem, Waqar; Cao, Xing-Zhong; Kuznetsov, Andrej & Shih, Kaimin [Show all 8 contributors for this article] (2023). Influence of Er substitution on the properties of ZnO: A comprehensive study. Ceramics International. ISSN 0272-8842. 49(24), p. 41071–41077. doi: 10.1016/j.ceramint.2022.10.224. Full text in Research Archive
• 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. Full text in Research Archive Show summary

  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.

• Nayak, Mrutyunjay; Bergum, Kristin; Stan, George E.; Lee, In-Hwan & Kuznetsov, Andrej (2023). Sub-Stochiometric Nickel Oxide Hole-Selective Contacts in Solar Cells: Comparison of Simulations and Experiments with Sputtered Films. Physica Status Solidi (a) applications and materials science. ISSN 1862-6300. 220(10), p. 1–7. doi: 10.1002/pssa.202200651. Full text in Research Archive Show summary

  Sub-stochiometric nickel oxide (NiOx) films were investigated as a hole selective contact option in silicon (Si) heterojunction solar cells. Numerical simulations were carried out to evaluate the impacts of the NiOx electronic properties variations and the NiOx/Si interface defect density (Dit) on device performance. Simulation data suggest that the best performance is achievable for wide bandgaps (Eg) and corresponding high valence band edge (EVB) positions in the NiOx films. Overall, in simulations, the performance remains practically unchanged for the nickel vacancy concentrations [VNi] = 1017–1021 cm−3, assuming high EVB and low Dit. The experimental data measured using NiOx films prepared by radio-frequency magnetron sputtering reveal that the increase in [VNi] lifts the conductivity, concurrently decreasing Eg and EVB. As a result, we concluded that the performance of the fabricated sputtered NiOx/Si heterojunction solar cell is limited by high Dit as well as narrow Eg and low EVB.

• Lin, Tianxiang; Li, Si-Hua; Ho, Lok-Ping; Kuznetsov, Andrej; Lee, Ho Nam & Chau, Tony [Show all 7 contributors for this article] (2023). Suppression of the Carbon Vacancy Traps and the Corresponding Leakage Current Reduction in 4H-SiC Diodes by Low-Temperature Implant Activation in Combination With Oxidation. IEEE Electron Device Letters. ISSN 0741-3106. 44(4), p. 578–581. doi: 10.1109/LED.2023.3242296. Full text in Research Archive
• 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), p. 182104-1–182104-4. doi: 10.1063/5.0149870. Full text in Research Archive Show summary

  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.

• Borgersen, Jon; Karsthof, Robert Michael; Rønning, Vegard; Vines, Lasse; Wenckstern, Holger Von & Grundmann, Marius [Show all 8 contributors for this article] (2023). Origin of enhanced conductivity in low dose ion irradiated oxides. AIP Advances. 13(1), p. 015211-1–015211-5. doi: 10.1063/5.0134699. Full text in Research Archive Show summary

  Significant resistivity variations have previously been observed in oxides subjected to relatively low ion irradiation doses, nominally insufficient to generate the amount of bulk defects needed to explain the phenomena. In an effort to unveil the underlying mechanisms, we performed a systematic comparative study of the resistivity evolution in In 2 O 3 -based oxides as a function of low ion doses and ultraviolet (UV) illumination, observing striking correlations. Specifically, we found that irradiation with ∼3 × 10 12  Si/cm 2 and ∼18 h UV exposure result in similar resistivity drops, interpreted in terms of irradiation/illumination assisted desorption of oxygen containing species from the surface. This was further proven by post-irradiation exposure of one of the samples to an oxygen atmosphere partially restoring the resistivity. Combining the present results with literature data, we conclude that the radiation tolerance of In 2 O 3 -based and similar oxides depends on the surface charge modifications, individual defect contributions, and contributions from defect complexes at low, intermediate, and high doses, respectively.

• Polyakov, A.Y.; Vasilev, A.A.; Shchemerov, I.V.; Chernykh, A.V.; Shetinin, I.V. & Zhevnerov, E.V. [Show all 14 contributors for this article] (2023). Conducting surface layers formed by hydrogenation of O-implanted β-Ga2O3. Journal of Alloys and Compounds. ISSN 0925-8388. 945, p. 1–6. doi: 10.1016/j.jallcom.2023.169258. Full text in Research Archive Show summary

  Lightly n-type β-Ga2O3 grown by Halide Vapor Phase Epitaxy (HVPE) on heavily n-type doped β-Ga2O3 substrate was implanted with 1 MeV O ions to a fluence of 1016 cm−2. The film remained β-polymorph and showed no broadening of the x-ray rocking curve width after irradiation even though the calculated number of primary defects was very high. The implanted region was characterized by a strong compensation, likely due to the presence of a high density of split Ga vacancy acceptors. Treatment of the irradiated film in dense hydrogen plasma at 330 °C for 0.5 h led to the formation of a conducting surface layer about 0.5 µm-thick with carrier density 1017 cm−3, a suppression of the signal due to Fe acceptors in Deep Level transient Spectroscopy (DLTS) and a strong enhancement of DLTS peak caused by centers at Ec-0.74 eV (so called E2 * traps). The mechanism appears to be that hydrogen plasma treatment leads to creation of a high number of donor states due complexing of hydrogen with Ga vacancies and to passivation of Fe acceptors with hydrogen donors.

• Zhu, Rui; Liang, Huili; Liu, Shangfeng; Yuan, Ye; Wang, Xinqiang & Ling, Francis Chi-Chung [Show all 9 contributors for this article] (2023). Non-volatile optoelectronic memory based on a photosensitive dielectric. Nature Communications. 14(1). doi: 10.1038/s41467-023-40938-y. Full text in Research Archive Show summary

  Abstract Recently, the optoelectronic memory is capturing growing attention due to its integrated function of sense and memory as well as multilevel storage ability. Although tens of states have been reported in literature, there are still three obvious deficiencies in most of the optoelectronic memories: large programming voltage (&gt;20 V), high optical power density (&gt;1 mW cm −2 ), and poor compatibility originating from the over-reliance on channel materials. Here, we firstly propose an optoelectronic memory based on a new photosensitive dielectric (PSD) architecture. Data writing and erasing are realized by using an optical pulse to switch on the PSD. The unique design enables the memory to work with a programming voltage and optical power density as low as 4 V and 160 µW cm −2 , respectively. Meanwhile, this device may be extended to different kinds of transistors for specific applications. Our discovery offers a brand-new direction for non-volatile optoelectronic memories with low energy consumption.

• Huang, Dong; Shi, Yingli; Younas, Muhammad; Khan, Rao Tahir Ali; Nadeem, Muhammad & Shati, Khaqan [Show all 13 contributors for this article] (2022). High Dielectric Transparent Film Tailored by Acceptor and Donor Codoping. Small. ISSN 1613-6810. doi: 10.1002/smll.202107168. Full text in Research Archive
• Ho, Lok-Ping; Younas, Muhammad; Borgersen, Jon; Khan, Rao Tahir Ali; Rezvani, Seyed Javad & Pollastri, Simone [Show all 12 contributors for this article] (2022). Photoluminescence intensity of Cu-doped ZnO modulated via defect occupancy by applying electric bias. Journal of Physics D: Applied Physics. ISSN 0022-3727. 55(31). doi: 10.1088/1361-6463/ac6912. Full text in Research Archive Show summary

  Abstract Discovering multifunctional materials is of paramount importance for advancing the science and technology. Herein, we report on an optical phenomenon modulated by an electrical process that happened at the metal–ZnO:Cu junction, for which the light emission intensity from the photoluminescence is tuned reversibly by applying electric bias to the junction. Importantly, these observations were correlated with the x-ray absorption measurements, detecting prominent flips in Cu + /Cu 2+ oxidation state occupations in ZnO:Cu film as a function of the resistive switching. Moreover, further analysis of the x-ray absorption data revealed an additional prominent correlation—the signals interpreted as the Zn–O bond fingerprints also exhibited the modulations. By considering the whole set of data, we propose a scenario explaining the modulation phenomena.

• Azarov, Alexander; Galeckas, Augustinas; Ling, Francis Chi-Chung & Kuznetsov, Andrej (2022). Tuning defect-related optical bands by channeling implants in semiconductors. Journal of Physics D: Applied Physics. ISSN 0022-3727. 56(3). doi: 10.1088/1361-6463/aca778. Full text in Research Archive Show summary

  Abstract Ion implantation is an excellent method to introduce defects into semiconductors, extending their functionalities in a controllable way. Herein, we investigated an option to use crystallographically aligned implants as an additional route to control the balance between optically active defects, selecting ZnO as a test material. The optical data were correlated with the structural analysis confirming the formation of different dominating crystalline defects in samples implanted along and off [0001] direction. Specifically, we demonstrated that different proportions in the contents of the extended and point defects in the initial as-implanted states of these samples, lead to prominent variations in the defect-related luminescence upon annealing. As such, we conclude that channeling implants may have an added value in the functionalization of defects in semiconductors, e.g. to tune specific spectral contents in the defect-related emission bands.

• Olsen, Vegard Skiftestad; Frodason, Ymir Kalmann; Hommedal, Ylva Knausgård; Nielsen, Dina Marie; Weiser, Philip Michael & Johansen, Klaus Magnus H [Show all 9 contributors for this article] (2022). Li and group-III impurity doping in ZnSnN2: Potential and limitations. PHYSICAL REVIEW MATERIALS. 6(12), p. 1–8. doi: 10.1103/PhysRevMaterials.6.124602. Full text in Research Archive Show summary

  II–IV nitrides and their alloys represent an earth-abundant and potentially cost-efficient alternative to the well-developed AlN-GaN-InN system. A major drawback with the II–IV nitrides is that ZnSnN2, the lowest band gap material, exhibits unfavorably high carrier concentrations for as-grown, stoichiometric material, limiting the material systems potential use in applications such as solar cells and light-emitting diodes. Lithium (Li) has been suggested as a shallow acceptor defect in ZnSnN2 if substituting for Zn, and hence doping with Li has been identified as a possible way to improve the electronic properties. Herein, theoretical calculations by hybrid functional density functional theory have been employed and extended to include defect complexes as well, which to this point remained unexplored. The calculations reveal that even though Li on the Zn site (the LiZn) is an acceptor, the defect may easily complex with the Lii donor, rendering the complex neutral. Our theoretical findings are supported by a Li-doping series of ZnSnN2, where a doping concentration ranging from 2.10×1019cm−3 to 1.85×1020cm−3 was obtained. The n-type carrier concentration was found to be unaffected by the doping concentration, and no systematic change in the absorption onset, probably affected by a Burstein-Moss shift, was observed. Possible group-III dopants, as have been found to yield interesting results for ZnGeN2, such as In, Ga, Al, and B, have also been investigated as an alternative dopant in ZnSnN2.

• Kjeldby, Snorre Braathen; Azarov, Alexander; Nguyen, Phuong Dan; Venkatachalapathy, Vishnukanthan; Mikšová, R. & MacKová, A. [Show all 9 contributors for this article] (2022). Radiation-induced defect accumulation and annealing in Si-implanted gallium oxide. Journal of Applied Physics. ISSN 0021-8979. 131(12). doi: 10.1063/5.0083858. Full text in Research Archive Show summary

  Defect accumulation and annealing phenomena in Si-implanted monoclinic gallium oxide (β-Ga 2 O 3 ) wafers, having [Formula: see text], (010), and (001) orientations, were studied by Rutherford backscattering spectrometry in channeling mode (RBS/c), x-ray diffraction (XRD), and (scanning) transmission electron microscopy [(S)TEM]. Initially, the samples with different surface orientations were implanted with 300 keV 28 Si + -ions, applying fluences in the range of 1 × 10 14 –2 × 10 16  Si/cm 2 , unveiling interesting disorder accumulation kinetics. In particular, the RBS/c, XRD, and (S)TEM combined data suggested that the radiation disorder buildup in Si-implanted β-Ga 2 O 3 is accompanied by significant strain accumulation, assisting crystalline-to-crystalline phase transitions instead of amorphization. Selected samples having [Formula: see text] orientation were subjected to isochronal (30 min) anneals in the range of 300–1300 °C in air. Systematic RBS/c and XRD characterization of these samples suggested complex structural transformations, which occurred as a function of the fluence and the temperature. Moreover, a detailed (S)TEM analysis of the sample implanted with 2 × 10 16  Si/cm 2 and annealed at 1100 °C was enhanced by applying dispersive x-ray and electron energy-loss spectroscopies. The analysis revealed silicon agglomerations in the form of silicon dioxide particles. Signal from silicon was also detected outside of the agglomerates, likely occurring as substitutional Si on Ga sites.

• Galeckas, Augustinas; Karsthof, Robert Michael; Kingsly, Gana; Kok, Angela; Bathen, Marianne Etzelmüller & Vines, Lasse [Show all 7 contributors for this article] (2022). Cross-Sectional Carrier Lifetime Profiling and Deep Level Monitoring in Silicon Carbide Films Exhibiting Variable Carbon Vacancy Concentrations. Physica Status Solidi (a) applications and materials science. ISSN 1862-6300. p. 1–7. doi: 10.1002/pssa.202200449. Full text in Research Archive Show summary

  The carrier lifetime control over 150 μm thick 4H-SiC epitaxial layers via thermal generation and annihilation of carbon vacancy (VC) related Z1/2 lifetime killer sites is reported. The defect developments upon typical SiC processing steps, such as high- and moderate-temperature anneals in the presence of a carbon cap, are monitored by combining electrical characterization techniques capable of VC depth-profiling, capacitance–voltage (CV) and deep-level transient spectroscopy (DLTS), with a novel all-optical approach of cross-sectional carrier lifetime profiling across 4H-SiC epilayer/substrate based on imaging time-resolved photoluminescence (TRPL) spectroscopy in orthogonal pump-probe geometry, which readily exposes in-depth efficacy of defect reduction and surface recombination effects. The lifetime control is realized by initial high-temperature treatment (1800 °C) to increase VC concentration to ≈1013 cm−3 level followed by a moderate-temperature (1500 °C) post-annealing of variable duration under C-rich thermodynamic equilibrium conditions. The post-annealing carried out for 5 h in effect eliminates VC throughout the entire ultra-thick epilayer. The reduction of VC-related Z1/2 sites is proven by a significant lifetime increase from 0.8 to 2.5 μs. The upper limit of lifetimes in terms of carrier surface leakage and the presence of other nonradiative recombination centers besides Z1/2, possibly related to residual impurities such as boron are discussed.

• Azarov, Alexander; Venkatachalapathy, Vishnukanthan; Karaseov, Platon; Titov, Andrei; Karabeshkin, Konstantin & Struchkov, Andrei [Show all 7 contributors for this article] (2022). Interplay of the disorder and strain in gallium oxide. Scientific Reports. 12(1). doi: 10.1038/s41598-022-19191-8. Full text in Research Archive Show summary

  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.

• Wang, Zhengpeng; Yu, Xinxin; Gong, Hehe; Hu, Tiancheng; Zhang, Yijun & Ji, Xiaoli [Show all 12 contributors for this article] (2022). Identification and Suppression of Majority Surface States in the Dry-Etched β-Ga2O3. The Journal of Physical Chemistry Letters. 13(30), p. 7094–7099. doi: 10.1021/acs.jpclett.2c02167. Full text in Research Archive Show summary

  Surface treatment after dry etching is vital to enhance the surface quality of the material and thus improve device performance. In this Letter, we identified the majority surface states induced by the dry etching of β-Ga2O3 and optimized surface treatments to suppress these electrically active defects with the improved performance of Schottky barrier diodes. Transient spectroscopies suggested that the majority traps (EC–0.75 eV) related to divacancies (VGa–VO) were enhanced in the concentration of 3.37 × 1014 cm–3 by dry etching and reduced to 0.90 × 1014 cm–3 by the combined means of oxygen annealing and piranha solution treatment. The trap evolution is supported by the suppressed donor–acceptor pair radiative recombination related to oxygen vacancies, the improved carrier transport (negligible hysteresis current–voltage and unity ideality factor), and the reduced surface band bending. These findings provide a straightforward strategy to improve surface quality for the further performance improvement of Ga2O3 power diodes.

• Azarov, Alexander; Bazioti, Kalliopi; Venkatachalapathy, Vishnukanthan; Ponniah, Vajeeston; Monakhov, Eduard & Kuznetsov, Andrej (2022). Disorder-Induced Ordering in Gallium Oxide Polymorphs. Physical Review Letters. ISSN 0031-9007. 128(1). doi: 10.1103/PhysRevLett.128.015704. Full text in Research Archive Show summary

  olymorphs are common in nature and can be stabilized by applying external pressure in materials. The pressure and strain can also be induced by the gradually accumulated radiation disorder. However, in semiconductors, the radiation disorder accumulation typically results in the amorphization instead of engaging polymorphism. By studying these phenomena in gallium oxide we found that the amorphization may be prominently suppressed by the monoclinic to orthorhombic phase transition. Utilizing this discovery, a highly oriented single-phase orthorhombic film on the top of the monoclinic gallium oxide substrate was fabricated. Exploring this system, a novel mode of the lateral polymorphic regrowth, not previously observed in solids, was detected. In combination, these data envisage a new direction of research on polymorphs in Ga2O3 and, potentially, for similar polymorphic families in other materials.

• Karsthof, Robert Michael; Bathen, Marianne Etzelmüller; Kuznetsov, Andrej & Vines, Lasse (2022). Formation of carbon interstitial-related defect levels by thermal injection of carbon into n-type 4H-SiC. Journal of Applied Physics. ISSN 0021-8979. 131. doi: 10.1063/5.0077308. Full text in Research Archive Show summary

  Electrical properties of point defects in 4𝐻-SiC have been studied extensively, but those related to carbon interstitials (Ci) have remained elusive until now. Indeed, when introduced via ion irradiation or implantation, signatures related to Ci observed by deep level transient spectroscopy tend to overlap with those of other primary defects, making the direct identification of Ci-related levels difficult. Recent literature has suggested to assign the so-called M center, often found in as-irradiated 4𝐻-SiC, to charge state transitions of the Ci defect in different configurations. In this work, we have introduced excess carbon into low-doped 𝑛-type 150 𝜇m thick 4𝐻-SiC epilayers by thermal annealing, with a pyrolyzed carbon cap on the sample surface acting as a carbon source. Because the layers exhibited initially low concentrations of carbon vacancies ([𝑉C]=1011cm), this enabled us to study the case of complete VC annihilation and formation of defects due to excess carbon, i.e., carbon interstitials Ci and their higher-order complexes. We report on the occurrence of several new levels upon C injection, which are likely Ci-related. Their properties are different from those found for the M center, which point toward a different microscopic identity of the detected levels. This suggests the existence of a rich variety of Ci-related defects. The study will also help generating new insights into the microscopic process of VC annihilation during carbon injection processes.

• Borgersen, Jon; Johansen, Klaus Magnus H; Vines, Lasse; Wenckstern, Holger von; Grundmann, Marius & Kuznetsov, Andrej (2021). Fermi level controlled point defect balance in ion irradiated indium oxide. Journal of Applied Physics. ISSN 0021-8979. 130(8). doi: 10.1063/5.0062135. Full text in Research Archive Show summary

  Fermi level controlled point defect balance is demonstrated in ion irradiated indium oxide (In2O3). Specifically, our observations can be sub-divided into the formation of isolated Frenkel pairs and secondary defects, correlated with an increase and decrease in resistance, respectively. Importantly, by considering the net charge contribution from the most energetically stable Frenkel pair configurations, we explain the data trends for low doses and determine an upper limit for the Fermi level pinning. Moreover, by comparing the corresponding number of generated carriers with the ballistic defect generation rates, we estimate the dynamic annealing efficiency. Further irradiation toward higher doses is consistent with the buildup of secondary defects. As such, the present data may be of practical use in a variety of In2O3 device applications requiring predictions of its radiation tolerance. In a broader perspective, the present methodology may be valuable for benchmarking defect simulation data in semiconductors in general.

• Ayedh, Hussein Mohammed Hussein; Kvamsdal, Kjell-Erik; Bobal, V.; Hallén, A.; Ling, Francis Chi-Chung & Kuznetsov, Andrej (2021). Carbon vacancy control in p+-n silicon carbide diodes for high voltage bipolar applications. Journal of Physics D: Applied Physics. ISSN 0022-3727. 54(45). doi: 10.1088/1361-6463/ac19df. Full text in Research Archive
• Azarov, Alexander; Venkatachalapathy, Vishnukanthan; Vines, Lasse; Monakhov, Eduard; Lee, In-Hwan & Kuznetsov, Andrej (2021). Activation energy of silicon diffusion in gallium oxide: Roles of the mediating defects charge states and phase modification. Applied Physics Letters. ISSN 0003-6951. 119(18). doi: 10.1063/5.0070045. Full text in Research Archive Show summary

  Silicon (Si) is an efficient n-type dopant in gallium oxide (Ga2O3)—an ultra-wide bandgap semiconductor promising in a number of applications. However, in spite of the technological importance for device fabrication, the activation energy for Si diffusion in Ga2O3 is missing in the literature. In the present work, we do such measurements in ion implanted monoclinic β-Ga2O3 samples employing anneals in air ambient, also admitting the influence of potential ion beam induced phase modifications on diffusion. Importantly, we show that Si diffusion in β-Ga2O3 fits with the concentration dependent diffusion model, involving neutral and single negatively charged point defects to mediate the process; so that we assumed gallium vacancies in the corresponding charge states to assist Si diffusion in β-Ga2O3 with activation energies of 3.2 ± 0.3 and 5.4 ± 0.4 eV, respectively. Moreover, we also found that a preexisting phase modified surface layer efficiently suppressed Si diffusion in β-Ga2O3 for temperatures up to 1000 °C.

• Hu, Zechen; Lin, Dehang; Yu, Xuegong; Seiffert, Christoph; Kuznetsov, Andrej & Yang, Deren (2021). Role of metal impurities in multicrystalline silicon solar cell degradation. Applied Physics Express (APEX). ISSN 1882-0778. 14(11). doi: 10.35848/1882-0786/ac2ae6. Full text in Research Archive
• Kaźmierczak-Bałata, Anna; Grzadziel, Lucyna; Guziewicz, Marek; Venkatachalapathy, Vishnukanthan; Kuznetsov, Andrej & Krzywiecki, Maciej (2021). Correlations of thermal properties with grain structure, morphology, and defect balance in nanoscale polycrystalline ZnO films. Applied Surface Science. ISSN 0169-4332. doi: 10.1016/j.apsusc.2021.149095. Full text in Research Archive Show summary

  ZnO thin films grown by atomic layer deposition method were investigated to analyse the correlation between morphology, chemical defects and thermal properties. This analysis became possible by measuring series of similarly thick wurtzite ZnO films synthesized using identical number of deposition cycles at two temperatures – 100 °C and 200 °C. By comparing these sample series, exhibiting the thickness variations from 12 to 118 nm, several prominent correlations for the thermal properties was observed - measured by high-resolution scanning thermal microscopy at room temperature. Specifically, comparing thinnest samples in the series, a lower thermal conductivity was revealed in samples grown at 100 °C and these variations were interpreted in terms of changing populations of native donors, affecting the free carrier concentration and, as such, the thermal conductivity. Further, the increase in thickness for the samples grown at 200 °C was accompanied with a continuous shift to Zn-rich conditions and the corresponding increase in the thermal conductivity, specifically from 0.28 Wm−1 K−1 to 2.81 Wm−1 K−1. As such, we have decoupled the thickness effect on the thermal conductivity from the rest of the phenomena, explaining the thermal transport in terms of the bulk thermal contributions within the grains combined with additional thermal resistance introduced by the grain boundaries.

• Tengchaisri, T.; Bootkul, D.; Intarasiri, S.; Tippawan, U. & Kuznetsov, Andrej (2021). Coloration changes in natural ruby induced by oxygen ion implants correlated with cathodoluminescence data. Nuclear Instruments and Methods in Physics Reseach B. ISSN 0168-583X. 502, p. 29–36. doi: 10.1016/j.nimb.2021.06.002. Full text in Research Archive
• Azarov, Alexander; Venkatachalapathy, Vishnukanthan; Monakhov, Eduard & Kuznetsov, Andrej (2021). Dominating migration barrier for intrinsic defects in gallium oxide: Dose-rate effect measurements. Applied Physics Letters. ISSN 0003-6951. 118(23). doi: 10.1063/5.0051047. Full text in Research Archive Show summary

  Ion bombardment provides an opportunity to study basic properties of intrinsic defects in materials since the radiation-induced disorder accumulation depends on the balance between defect generation and migration rates. In particular, variation of such parameters as irradiation temperature and ion flux, known in the literature as dose-rate effect, interconnects the macroscopically measured lattice disorder with the migration barrier of the dominating defects. In this work, we measured the dose-rate effect in monoclinic gallium oxide (β-Ga2O3) and extracted its activation energy of 0.8 ± 0.1 eV in the range of 25–250 °C. Taking into account that the measurements were performed in the Ga-sublattice and considering 0.8 ± 0.1 eV in the context of theoretical data, we interpreted it as the migration barrier for Ga vacancies in β-Ga2O3, limiting the process. Additionally, we observed and took into account an interesting form of the lattice relaxation due to radiation-induced disorder buildup, interpreted in terms of the compressive strain accumulation, potentially trigging phase transitions in Ga2O3 lattice.

• Olsen, Vegard Skiftestad; Øversjøen, Vetle; Gogova-Petrova, Daniela; Pecz, Bela; Galeckas, Augustinas & Borgersen, Jon Arthur [Show all 9 contributors for this article] (2021). ZnSnN2 in Real Space and k-Space: Lattice Constants, Dislocation Density, and Optical Band Gap. Advanced Optical Materials. doi: 10.1002/adom.202100015. Full text in Research Archive
• Durand, A.; Baron, Y.; Redjem, W.; Herzig, T.; Benali, A. & Pezzagna, S. [Show all 14 contributors for this article] (2021). Broad Diversity of Near-Infrared Single-Photon Emitters in Silicon. Physical Review Letters. ISSN 0031-9007. 126(8). doi: 10.1103/PhysRevLett.126.083602. Full text in Research Archive Show summary

  We report the detection of individual emitters in silicon belonging to seven different families of optically active point defects. These fluorescent centers are created by carbon implantation of a commercial silicon-on-insulator wafer usually employed for integrated photonics. Single photon emission is demonstrated over the 1.1–1.55  μm range, spanning the O and C telecom bands. We analyze their photoluminescence spectra, dipolar emissions, and optical relaxation dynamics at 10 K. For a specific family, we show a constant emission intensity at saturation from 10 K to temperatures well above the 77 K liquid nitrogen temperature. Given the advanced control over nanofabrication and integration in silicon, these individual artificial atoms are promising systems to investigate for Si-based quantum technologies.

• Bathen, Marianne Etzelmüller; Galeckas, Augustinas; Karsthof, Robert Michael; Delteil, Aymeric; Sallet, Vincent & Kuznetsov, Andrej [Show all 7 contributors for this article] (2021). Resolving Jahn-Teller induced vibronic fine structure of silicon vacancy quantum emission in silicon carbide. Physical review B (PRB). ISSN 2469-9950. 104. doi: 10.1103/PhysRevB.104.045120. Full text in Research Archive Show summary

  Point defects in semiconductors are promising single-photon emitters (SPEs) for quantum computing, communication, and sensing applications. However, factors such as emission brightness, purity. and indistinguishability are limited by interactions between localized defect states and the surrounding environment. Therefore, it is important to map the full emission spectrum from each SPE, to understand the complex interplay between the different defect configurations, their surroundings, and external perturbations. Herein, we investigate a family of regularly spaced sharp luminescence peaks appearing in the near-infrared portion of photoluminescence (PL) spectra from n-type 4H-SiC samples after irradiation. This periodic emitter family, labeled the L lines, is only observed when the zero-phonon line signatures of the negatively charged Si vacancy (so-called V lines) are present. The L lines appear with 1.45meV and 1.59meV energy spacing after H and He irradiation and increase linearly in intensity with fluence—reminiscent of the intrinsic defect trend. Furthermore, we monitor the dependence of the L-line emission energy and intensity on heat treatments, electric field strength, and PL collection temperature, discussing these data in the context of the L lines. Based on the strong similarity between the irradiation, electric field, and thermal responses of the L and V lines, the L lines are attributed to the Si vacancy in 4H-SiC. The regular and periodic appearance of the L lines provides strong arguments for a vibronic origin explaining the oscillatory multipeak spectrum. To account for the small energy separation of the L lines, we propose a model based on rotations of distortion surrounding the Si vacancy driven by a dynamic Jahn-Teller effect.

• Assmann, Nicole; Persson, Clas; Kuznetsov, Andrej & Monakhov, Eduard (2021). Fine structure in electronic transitions attributed to nitrogen donor in silicon carbide. Applied Physics Letters. ISSN 0003-6951. 119(26). doi: 10.1063/5.0074046. Full text in Research Archive Show summary

  Nitrogen in group-IV semiconductors has become a well-established element of qubits capable of room-temperature operation. In silicon carbide, nitrogen can occupy different nonequivalent lattice sites, giving rise to different shallow donor states. We report a triplet fine structure in electronic transitions of nitrogen donors on the quasi-cubic carbon site in 4H silicon carbide with activation enthalpies of around 100 meV. The intensities of triplet components have a prominent dependence on the voltage bias. The activation enthalpies of the transitions exhibit the Poole–Frenkel effect, while no bias dependence is observed for the magnitude of splitting. A tentative explanation of the fine structure involves local symmetry changes due to stacking faults.

• Gogova-Petrova, Daniela; Olsen, Vegard Skiftestad; Bazioti, Kalliopi; Lee, In-Hwan; Prytz, Øystein & Vines, Lasse [Show all 7 contributors for this article] (2020). High electron mobility single-crystalline ZnSnN2 on ZnO (0001) substrates. CrysteEngComm. 22(38), p. 6268–6274. doi: 10.1039/d0ce00861c. Full text in Research Archive Show summary

  Making a systematic effort, we have developed single-crystalline ZnSnN2 on ZnO (0001) by reactive magnetron co-sputtering. Epitaxial growth was achieved at 350 °C by co-sputtering from metal targets in a nitrogen atmosphere, and confirmed by transmission electron microscopy (TEM) measurements. TEM verified that the layers are single-crystalline of hexagonal phase, exhibiting an epitaxial relationship with the substrate described by: [11−20]ZnSnN2//[11−20]ZnO and [0001]ZnSnN2//[0001]ZnO. The screw-type threading dislocations originating from the ZnSnN2/ZnO interface were identified as the dominant extended defects. More specifically, we report a pioneering measurement of the dislocation density in this material of 1.5 × 1011 cm−2. Even though there are no literature data for direct comparison, such values are typical of heteroepitaxial growth of III-nitride layers without applying defect density reduction strategies. The films demonstrated a high electron mobility of 39 cm2 V−1 s−1 and 63 cm2 V−1 s−1 for stoichiometric and Zn-rich layers, respectively, while the electron carrier density remained in the low 1019 cm−3 range as determined by the Hall effect measurements at room temperature. Optical bandgaps of 1.86 eV and 1.72 eV were determined for the stoichiometric and Zn-rich samples, respectively. As such, we conclude that ZnSnN2 is an earth-abundant, environmentally-friendly semiconductor and is a promising candidate for cost efficient components in optoelectronics and photovoltaics.

• Borgersen, Jon; Vines, Lasse; Frodason, Ymir Kalmann; Kuznetsov, Andrej; Wenckstern, Holger von & Grundmann, Marius [Show all 9 contributors for this article] (2020). Experimental exploration of the amphoteric defect model by cryogenic ion irradiation of a range of wide band gap oxide materials. Journal of Physics: Condensed Matter. ISSN 0953-8984. 32(50). doi: 10.1088/1361-648X/abac8b. Full text in Research Archive Show summary

  The evolution of electrical resistance as function of defect concentration is examined for the unipolar n-conducting oxides CdO, β-Ga2O3, In2O3, SnO2 and ZnO in order to explore the predictions of the amphoteric defect model. Intrinsic defects are introduced by ion irradiation at cryogenic temperatures, and the resistance is measured in-situ by current–voltage sweeps as a function of irradiation dose. Temperature dependent Hall effect measurements are performed to determine the carrier concentration and mobility of the samples before and after irradiation. After the ultimate irradiation step, the Ga2O3 and SnO2 samples have both turned highly resistive. In contrast, the In2O3 and ZnO samples are ultimately found to be less resistive than prior to irradiation, however, they both show an increased resistance at intermediate doses. Based on thermodynamic defect charge state transitions computed by hybrid density functional theory, a model expanding on the current amphoteric defect model is proposed.

• Ellingsen, Josef Gert Åsheim; Venkatachalapathy, Vishnukanthan; Azarov, Alexander; Nilsen, Ola & Kuznetsov, Andrej (2020). Al incorporation during metal organic chemical vapour deposition of aluminium zinc oxide. Thin Solid Films. ISSN 0040-6090. 709. doi: 10.1016/j.tsf.2020.138245. Full text in Research Archive Show summary

  Aluminium zinc oxide (AZO) is a promising transparent conductive oxide this is difficult to fabricate by metal-organic chemical vapour deposition (MOCVD) because of prereactions between the precursors for Al and oxygen. One way of reducing prereactions is to use separate injectors. In this work, we investigated the effect of separate injectors and various flow configurations on the MOCVD fabrication of AZO using diethyl zinc (DEZn), tert-butanol (tBuOH), and trimethyl aluminium (TMAl) precursors. X-ray diffraction revealed that the resulting films maintained a purely wurtzite ZnO structure, and that the Al incorporation changed the c lattice parameters from 5.207 to 5.181 Å in agreement with theoretical models of mixing on the Zn sub-lattice. This indicates that Al was mainly incorporated onto the Zn sub-lattice. The maximum Al concentration was 6 × 1021 cm . The Al concentration was directly influenced by the TMAl flow rates, but the incorporation efficiency of Al also depended on the O/Zn balance as determined by the tBuOH supply and the total flow rate through the TMAl+DEZn injector. The tBuOH supply decreased the incorporation efficiency, while it increased for higher total flow in the TMAl+DEZn injector although both changes nominally increases the O/Zn balance. This dual, but contradicting, dependency on the O/Zn suggested that the incorporation efficiency depended not only on precursor supply, but also on the chemical stability of the Al species present at the growth interface. The concept of a reaction depth of TMAl was introduced to discuss the observed trends, and a qualitative model was suggested to explain how the Al-species present at the growth interface are determined by the relative depths of the reaction depth and the boundary layer. These findings illustrate the limits of the O/Zn parameter when using separate injectors because the properties of the boundary layer can affect Al incorporation during MOCVD growth. This should be relevant for MOCVD growth of other compound semiconductor systems that are grown using separate injectors.

• Son, Hoki; Park, Jee Hye; Uthirakumar, Periyayya; Kuznetsov, Andrej & Lee, In-Hwan (2020). Impact of chloride surface treatment on nano-porous GaN structure for enhanced water-splitting efficiency. Applied Surface Science. ISSN 0169-4332. doi: 10.1016/j.apsusc.2020.147465. Full text in Research Archive
• Uthirakumar, Periyayya; Devendiran, M.; Kuznetsov, Andrej; Kim, Gyu Cheol & Lee, In-Hwan (2020). Efficient, recyclable, and affordable daylight induced Cu/Cu2O/CuI photocatalyst via an inexpensive iodine sublimation process. Applied Surface Science. ISSN 0169-4332. 537. doi: 10.1016/j.apsusc.2020.147007. Full text in Research Archive

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• Bazioti, Kalliopi; Azarov, Alexander; Vines, Lasse; Prytz, Øystein & Kuznetsov, Andrej (2026). Fundamental insights into polymorphism and defect evolution in ion-irradiated semiconductors by employing atomic-scale Transmission Electron Microscopy. Full text in Research Archive
• Azarov, Alexander; Slusar, Tetiana; Galeckas, Augustinas; Moon, Kiwon; Hallen, Anders & Kuznetsov, Andrej (2024). Dynamic defect annealing in Er implanted LiNbO3. Full text in Research Archive
• Azarov, Alexander; Galeckas, Augustinas; Cora, Ildikó; Fogarassy, Zsolt; Venkatachalapathy, Vishnukanthan & Monakhov, Eduard [Show all 7 contributors for this article] (2024). Thermal stability of double-layer gamma/beta Ga2O3 structures fabricated by ion beams. 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
• Kuznetsov, Andrej & Pecharapa, Wisanu (2023). Radiation and Emission Phenomena in Combination. Physica Status Solidi (a) applications and materials science. ISSN 1862-6300. 220(10). doi: 10.1002/pssa.202300317. Full text in Research Archive
• 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. Full text in Research Archive
• Azarov, Alexander; Bazioti, Kalliopi; Venkatachalapathy, Vishnukanthan; Vajeeston, Ponniah; Monakhov, Eduard & Kuznetsov, Andrej (2022). Disorder-induced ordering in Ga2O3 polymorphs. Full text in Research Archive
• Galeckas, Augustinas; Karsthof, Robert Michael; Gana, Kingsly; Kok, Angela; Bathen, Marianne Etzelmüller & Vines, Lasse [Show all 7 contributors for this article] (2022). Cross-sectional carrier lifetime profiling and deep level monitoring in silicon carbide films exhibiting variable carbon vacancy concentrations. Full text in Research Archive Show summary

  We report on the carrier lifetime control over 150 μmthick 4H-SiC epitaxial layers via thermal generation and annihilation of carbon vacancy (VC) related Z1/2lifetime killer sites. The defect developments upon typical SiC processing steps, such as high- and moderate-temperature anneals in the presence of a carbon cap,are monitored by combining electrical characterization techniques capable of VC depth-profiling, capacitance-voltage (CV) and deep-level transient spectroscopy (DLTS), with a novel all-optical approach of cross-sectional carrier lifetime profiling across 4H-SiC epilayer/substrate based on imaging time-resolved photoluminescence (TRPL) spectroscopy in orthogonal pump-probe geometry,which readily exposes in-depth efficacy ofdefect reduction and surface recombination effects.The lifetime control is realized byinitialhigh-temperature treatment (1800ºC) to increase VC concentration to ~1013 cm-3 level followed by a moderate-temperature (1500ºC) post-annealing of variable duration under C-rich thermodynamic equilibrium conditions. The post-annealing carried out for 5 hours in effect eliminates VCthroughout the entire ultra-thick epilayer. The reduction of VC-related Z1/2 sites is proven by a significant lifetime increase from 0.8 μs to 2.5 μs. We discuss the upper limit of lifetimes in terms of carrier surface leakage and presence of other non-radiative recombination centers besides Z1/2, possibly related to residual impurities such as boron.

• 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. Full text in Research Archive
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• Kuznetsov, Andrej; Dangtip, Somsak & Intarasiri, Saweat (2021). Radiation and Emission in Materials: Similarity of Principles and Multi-Functional Applications. Physica Status Solidi (a) applications and materials science. ISSN 1862-6300. 218(1). doi: 10.1002/pssa.202000772. Full text in Research Archive
• Azarov, Alexander; Venkatachalapathy, Vishnukanthan; Monakhov, Eduard & Kuznetsov, Andrej (2021). Dose-rate effect in β-Ga2O3. Full text in Research Archive
• Azarov, Alexander; Venkatachalapathy, Vishnukanthan; Monakhov, Eduard & Kuznetsov, Andrej (2021). Dominating migration barrier for intrinsic defects in gallium oxide at and above room temperature. Full text in Research Archive
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• Azarov, Alexander; Venkatachalapathy, Vishnukanthan; Monakhov, Eduard & Kuznetsov, Andrej (2021). Radiation defect dynamics in β-Ga2O3: ion flux vs irradiation temperature. Full text in Research Archive
• Bazioti, Kalliopi; Olsen, Vegard Skiftestad; Azarov, Alexander; Johansen, Klaus Magnus H; Svensson, Bengt Gunnar & Kuznetsov, Andrej [Show all 8 contributors for this article] (2020). Role of nitrogren in defect evolution in ZnO and (ZnO)₁−ₓ(GaN)ₓ: STEM-EELS nanoscale investigations. Full text in Research Archive
• Tømta, Sindre Ludvigsen; Kuznetsov, Andrej; Gongora, David Rivas & Karsthof, Robert Michael (2021). Single-Photon Emitters in Silicon for Quantum Technology. Universitetet i Oslo. Full text in Research Archive
• Varslott, Kristoffer Wannebo; Kuznetsov, Andrej; Borgersen, Jon & Bazioti, Calliope (2021). The Effect on Plasmonic Behaviour in Quasi Two-Dimensional GeSe by Variation in Thickness & Phases. Universitetet i Oslo. Full text in Research Archive
• Yaseen, Anisa; Karazhanov, Smagul; Kuznetsov, Andrej & Cremades, Ana (2020). Polymer-Silicon Interface Study. Universitetet i Oslo. Full text in Research Archive

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