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• Hansen, Ethan R.; Niaouris, Vasileios; Matthews, Bethany E.; Zimmermann, Christian; Wang, Xingyi & Kolodka, Roman [Show all 9 contributors for this article] (2024). Isolation of Single Donors in ZnO. Physical Review Letters. ISSN 0031-9007. 133(14). doi: 10.1103/PhysRevLett.133.146902. Full text in Research Archive
• Wang, Xingyi; Zimmermann, Christian; Titze, Michael; Niaouris, Vasileios; Hansen, Ethan R. & D'Ambrosia, Samuel H. [Show all 9 contributors for this article] (2023). Properties of Donor Qubits in ZnO Formed by Indium-Ion Implantation. Physical Review Applied. 19(5), p. 054090-1–054090-12. doi: 10.1103/PhysRevApplied.19.054090. Full text in Research Archive Show summary

  Neutral shallow donors (D0) in ZnO have emerged as a promising candidate for solid-state spin qubits. Here we report on the formation of D0 in ZnO via implantation of In and subsequent annealing. The implanted In donors exhibit optical and spin properties on par with those of in situ–doped donors. The inhomogeneous linewidth of the donor-bound-exciton transition is less than 10 GHz, comparable to the optical linewidth of in situ In. Longitudinal spin relaxation times (T1) exceed reported values for in situ Ga donors, indicating that residual In-implantation damage does not degrade T1. Two-laser Raman spectroscopy of the donor spin reveals the hyperfine interaction of the donor electron with the spin-9/2 In nuclei. This work is an important step toward the deterministic formation of In-donor qubits in ZnO with optical access to a long-lived nuclear-spin memory.

• Langørgen, Amanda; Zimmermann, Christian; Frodason, Ymir Kalmann; Verhoeven, Espen Førdestrøm; Weiser, Philip Michael & Karsthof, Robert Michael [Show all 8 contributors for this article] (2022). Influence of heat treatments in H2 and Ar on the E1 center in β-Ga2O3. Journal of Applied Physics. ISSN 0021-8979. 131(115702). doi: 10.1063/5.0083861. Full text in Research Archive Show summary

  The influence of heat treating [Formula: see text]-type bulk [Formula: see text]-Ga[Formula: see text]O[Formula: see text] in hydrogen (H[Formula: see text]) and argon (Ar) gases on the presence of the defect level commonly labeled as [Formula: see text] was studied. Fourier transform-infrared spectroscopy confirms that hydrogen (H) is incorporated into [Formula: see text]-Ga[Formula: see text]O[Formula: see text] during H[Formula: see text] annealing at 900 °C. Deep-level transient spectroscopy measurements reveal that the concentration of the [Formula: see text] level is promoted by the introduction of H, in contrast to what is observed in samples heat-treated in an Ar flow. We further find the [Formula: see text] level to be stable against heat treatments at 650 K, both with and without an applied reverse-bias voltage. Potential candidates for the defect origin of [Formula: see text] are investigated using hybrid-functional calculations, and three types of defect complexes involving H are found to exhibit charge-state transition levels compatible with [Formula: see text], including substitutional H at one of the threefold coordinated O sites, Ga-substitutional shallow donor impurities passivated by H, and certain configurations of singly hydrogenated Ga–O divacancies. Among these types, only the latter exhibit H binding energies that are consistent with the observed thermal stability of [Formula: see text].

• Bonkerud, Julie; Zimmermann, Christian; Weiser, Philip Michael; Vines, Lasse & Monakhov, Eduard (2021). On the permittivity of titanium dioxide. Scientific Reports. 11. doi: 10.1038/s41598-021-92021-5. Full text in Research Archive Show summary

  Abstract Conductive rutile TiO 2 has received considerable attention recently due to multiple applications. However, the permittivity in conductive, reduced or doped TiO 2 appears to cause controversy with reported values in the range 100–10,000. In this work, we propose a method for measurements of the permittivity in conductive, n-type TiO 2 that involves: (i) hydrogen ion-implantation to form a donor concentration peak at a known depth, and (ii) capacitance–voltage measurements for donor profiling. We cannot confirm the claims stating an extremely high permittivity of single crystalline TiO 2 . On the contrary, the permittivity of conductive, reduced single crystalline TiO 2 is similar to that of insulating TiO 2 established previously, with a Curie–Weiss type temperature dependence and the values in the range 160–240 along with the c-axis.

• Frodason, Ymir Kalmann; Zimmermann, Christian; Verhoeven, Espen Førdestrøm; Weiser, Philip Michael; Vines, Lasse & Varley, Joel B (2021). Multistability of isolated and hydrogenated Ga-O divacancies in beta-Ga2O3. PHYSICAL REVIEW MATERIALS. 5. doi: 10.1103/PhysRevMaterials.5.025402. Full text in Research Archive Show summary

  This work systematically explores 19 unique configurations of the close-associate Ga–O divacancies (VGaVO) in β−Ga2O3, including their complexes with H impurities, using hybrid functional calculations. Interestingly, most configurations are found to retain the negative-U behavior of VO, as they exhibit a thermodynamic (−/3−) charge-state transition level energetically located in the upper part of the band gap, where the 3− charge state is associated with the formation of a Ga–Ga dimer. The energy positions of the thermodynamic (−/3−) charge-state transition levels divide the divacancy configurations into three different groups, which can be understood from the three possible Ga–Ga dimerizations resulting from the tetrahedral and octahedral Ga sites. The relative formation energies of the different divacancy configurations, and hence the electrical activity of the divacancies, is found to depend on the Fermi-level position, and the energy barriers for transformation between different divacancy configurations are explored from nudged elastic band calculations. Hydrogenation of the divacancies is found to either passivate their negative-U charge-state transition levels or shift them down in Fermi level position, depending on whether the H resides at VO or forms an O–H bond at VGa, respectively. Finally, the divacancy is discussed as a potential origin of the so-called E∗2 center previously observed by deep-level transient spectroscopy.

• Bonkerud, Julie; Zimmermann, Christian; Weiser, Philip Michael; Aarholt, Thomas; Verhoeven, Espen Førdestrøm & Vines, Lasse [Show all 8 contributors for this article] (2020). Fabrication and characterization of Schottky barrier diodes on rutile TiO2. Materials Research Express. 7(6). doi: 10.1088/2053-1591/ab9777. Full text in Research Archive
• Zimmermann, Christian; Verhoeven, Espen Førdestrøm; Frodason, Ymir Kalmann; Weiser, Philip Michael; Varley, Joel B & Vines, Lasse (2020). Formation and control of the E2* center in implanted b-Ga2O3 by reverse-bias and zero-bias annealing. Journal of Physics D: Applied Physics. ISSN 0022-3727. 53(46). doi: 10.1088/1361-6463/aba64d. Full text in Research Archive Show summary

  Deep-level transient spectroscopy measurements are conducted onβ-Ga2O3thin-filmsimplanted with helium and hydrogen (H) to study the formation of the defect levelE∗2(EA=0.71 eV) during heat treatments under an applied reverse-bias voltage (reverse-biasannealing). The formation ofE∗2during reverse-bias annealing is a thermally-activated processexhibiting an activation energy of around 1.0 eV to 1.3 eV, and applying larger reverse-biasvoltages during the heat treatment results in a larger concentration ofE∗2. In contrast, heattreatments without an applied reverse-bias voltage (zero-bias annealing) can be used to decreasetheE∗2concentration. The removal ofE∗2is more pronounced if zero-bias anneals are performedin the presence of H. A scenario for the formation ofE∗2is proposed, where the main effect ofreverse-bias annealing is an effective change in the Fermi-level position within the space-chargeregion, and whereE∗2is related to a defect complex involving intrinsic defects that exhibitsseveral different configurations whose relative formation energies depend on the Fermi-levelposition. One of these configurations gives rise toE∗2, and is more likely to form if theFermi-level position is further away from the conduction band edge. The defect complex relatedtoE∗2can become hydrogenated, and the corresponding hydrogenated complex is likely to formwhen the Fermi level is close to the conduction band edge. Di-vacancy defects formed byoxygen and gallium vacancies (VO−VGa) fulfill several of these requirements, and are proposedas potential candidates forE∗2.

• Bonkerud, Julie; Zimmermann, Christian; Herklotz, Frank; Weiser, Philip Michael; Seiffert, Christoph & Verhoeven, Espen Førdestrøm [Show all 8 contributors for this article] (2020). Electrically-active defects in reduced and hydrogenated rutile TiO2. Semiconductor Science and Technology. ISSN 0268-1242. 36(1). doi: 10.1088/1361-6641/abc854. Full text in Research Archive
• Zimmermann, Christian; Rønning, Vegard; Frodason, Ymir Kalmann; Bobal, Viktor; Varley, Joel B & Vines, Lasse (2020). Primary intrinsic defects and their charge transition levels in beta-Ga2O3. PHYSICAL REVIEW MATERIALS. 4(7). doi: 10.1103/PhysRevMaterials.4.074605. Full text in Research Archive Show summary

  A steady-state photocapacitance (SSPC) setup directly connected to the beamline of a MeV ion implanter is utilized to study primary intrinsic defects in β–Ga2O3 generated by He implantation at cryogenic temperatures (120 K). At low temperatures, the migration of defects is suppressed, and hence the generation of primary intrinsic defects is expected to prevail. SSPC measurements reveal defect-related optical transitions in halide vapor-phase epitaxy (HVPE) -grown β–Ga2O3 thin films with onset energies at 1.3 (T1), 1.7 (T2), 1.9 (T3), 2.6 (T4), 3.7 (T5), and 4.2 eV (T6). T2, T4, T5, and T6 were observed in as-received HVPE-grown β–Ga2O3 thin films, whereby T2 is only sporadically observed. The introduction rates for T3, T4, as well as T6 indicate an origin related to primary intrinsic defects. Notably, T1 and T3 are only observed after He implantation at cryogenic temperatures. Hybrid-functional calculations were performed to estimate the optical absorption cross-section spectra for the gallium (Gai) and oxygen (Oi) interstitials as well as the corresponding vacancies (VGa and VO, respectively), and compared with the measured onsets for optical absorption found by SSPC measurements. Indeed, we propose T3 to be associated with Ga(+/+2)i and/or V(−3/−2)GaI, while T4 is suggested to be related to V(0/+)OK (K=I, II, III) and/or V(−3/−2)GaII. Additionally, several further charge-state transition levels associated with VGaI and VGaII may contribute to T4 and T6. We further studied the kinetics of the defects created with He implantation by exposing the sample to room temperature. The kinetics observed for T3 and T4 further support the proposed assignments of the corresponding defect signatures.

• Zimmermann, Christian; Frodason, Ymir Kalmann; Rønning, Vegard; Varley, Joel B & Vines, Lasse (2020). Combining steady-state photo-capacitance spectra with first-principles calculations: the case of Fe and Ti in β-Ga2O3. New Journal of Physics. 22. doi: 10.1088/1367-2630/ab8e5b. Full text in Research Archive
• Zimmermann, Christian; Frodason, Ymir Kalmann; Barnard, Abraham Willem; Varley, Joel B; Irmscher, Klaus & Galazka, Zbigniew [Show all 10 contributors for this article] (2020). Ti- and Fe-related charge transition levels in beta-Ga2O3. Applied Physics Letters. ISSN 0003-6951. 116(7). doi: 10.1063/1.5139402. Full text in Research Archive Show summary

  Deep-level transient spectroscopy measurements on β-Ga2O3 crystals reveal the presence of three defect signatures labeled E2a, E2b, and E3 with activation energies at around 0.66 eV, 0.73 eV, and 0.95 eV below the conduction band edge. Using secondary ion mass spectrometry, a correlation between the defect concentration associated with E3 and the Ti concentration present in the samples was found. Particularly, it is found that E3 is the dominant Ti-related defect in β-Ga2O3 and is associated with a single Ti atom. This finding is further corroborated by hybrid functional calculations that predict Ti substituting on an octahedral Ga site, denoted as TiGaII, to be a good candidate for E3. Moreover, the deep level transient spectroscopy results show that the level previously labeled E2 and attributed to Fe substituting on a gallium site (FeGa) consists of two overlapping signatures labeled E2a and E2b. We tentatively assign E2a and E2b to Fe substituting for Ga on a tetrahedral or an octahedral site, respectively.

• Olsen, Vegard Skiftestad; Baldissera, Gustavo; Zimmermann, Christian; Granerød, Cecilie Skjold; Bazioti, Kalliopi & Galeckas, Augustinas [Show all 11 contributors for this article] (2019). Evidence of defect band mechanism responsible for band gap evolution in (ZnO)1−x(GaN)x alloys. Physical review B (PRB). ISSN 2469-9950. 100(16). doi: 10.1103/PhysRevB.100.165201. Full text in Research Archive Show summary

  It is known that (ZnO)1−x(GaN)x alloys demonstrate remarkable energy band bowing, making the material absorb in the visible range, in spite of the binary components being classical wide band gap semiconductors. However, the origin of this bowing is not settled; two major mechanisms are under debate: Influence of the orbital repulsion and/or formation of a defect band. In the present work, we applied a combination of the absorption and emission measurements on the samples exhibiting an outstanding nanoscale level of (ZnO)1−x(GaN)x homogeneity as monitored by the high resolution electron microscopy equipped with the energy dispersive x-ray analysis and the electron energy loss spectroscopy; moreover the experimental data were set in the context of the computational analysis of the alloys employing density functional theory and quasiparticle GW approximation. A prominent discrepancy in the band gap values as deduced from the absorption and emission experiments was observed systematically for the alloys with different compositions and interpreted as evidence for the absorption gap shrinking due to the defect band formation. Computational data support the argument, revealing only minor variations in the bulk of the conduction and valence band structures of the alloys, except for a characteristic “tail” in the vicinity of the valence band maximum. As such, we conclude that the energy gap bowing in (ZnO)1−x(GaN)x alloys is due to the defect band formation, presumably at the top of the valence band maximum.

• Zimmermann, Christian; Bonkerud, Julie; Herklotz, Frank; Sky, Thomas Neset; Hupfer, Alexander & Monakhov, Eduard [Show all 8 contributors for this article] (2018). Influence of annealing atmosphere on formation of electrically-active defects in rutile TiO2. Journal of Applied Physics. ISSN 0021-8979. 123(16), p. 161572-1–161572-8. doi: 10.1063/1.5011136. Full text in Research Archive Show summary

  Electronic states in the upper part of the bandgap of reduced and/or hydrogenated n-type rutile TiO2 single crystals have been studied by means of thermal admittance and deep-level transient spectroscopy measurements. The studies were performed at sample temperatures between 28 and 300 K. The results reveal limited charge carrier freeze-out even at 28 K and evidence the existence of dominant shallow donors with ionization energies below 25 meV. Interstitial atomic hydrogen is considered to be a major contributor to these shallow donors, substantiated by infrared absorption measurements. Three defect energy levels with positions of about 70 meV, 95 meV, and 120 meV below the conduction band edge occur in all the studied samples, irrespective of the sample production batch and the post-growth heat treatment used. The origin of these levels is discussed in terms of electron polarons, intrinsic point defects, and/or common residual impurities, where especially interstitial titanium atoms, oxygen vacancies, and complexes involving Al atoms appear as likely candidates. In contrast, no common deep-level defect, exhibiting a charge state transition in the 200–700 meV range below the conduction band edge, is found in different samples. This may possibly indicate a strong influence on deep-level defects by the post-growth heat treatments employed.

• Lang, Mario; Zimmermann, Christian; Krämmer, Christoph; Renz, Tobias; Huber, Christian & Kalt, Heinz [Show all 7 contributors for this article] (2017). Luminescence properties of Cu2ZnSn(S,Se)4 solar cell absorbers: State filling versus screening of electrostatic potential fluctuations. Physical review B (PRB). ISSN 2469-9950. 95(15). doi: 10.1103/PhysRevB.95.155202. Full text in Research Archive

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• Langørgen, Amanda; Frodason, Ymir Kalmann; Zimmermann, Christian; Karsthof, Robert Michael; Weiser, Philip Michael & Jensen, Ingvild Julie Thue [Show all 7 contributors for this article] (2025). Seminar talk - Electrically active defects in β-Ga2O3. Full text in Research Archive
• Frodason, Ymir Kalmann; Zimmermann, Christian; Verhoeven, Espen Førdestrøm; Weiser, Philip Michael; Vines, Lasse & Varley, Joel Basile (2021). Multistability of Ga-O divacancies in β-Ga2O3. Full text in Research Archive
• Zimmermann, Christian; Frodason, Ymir Kalmann; Varley, Joel Basile; Verhoeven, Espen Førdestrøm; Rønning, Vegard & Weiser, Philip Michael [Show all 15 contributors for this article] (2021). Identification of Fe-, Ti- and H-related Charge-state Transition Levels in β-Ga2O3. Full text in Research Archive
• Vines, Lasse; Ingebrigtsen, Mads Eide; Zimmermann, Christian; Rønning, Vegard & Kuznetsov, Andrej (2019). Electrically Active Defects in β-Ga2O3. Full text in Research Archive
• Grini, Sigbjørn; Borgersen, Jon Arthur; Sky, Thomas Neset; Frodason, Ymir Kalmann; Riise, Heine Nygard & Zimmermann, Christian (2018). Omvisning MENA. Full text in Research Archive
• Lento, Jennifer; Goedkoop, Willem; Culp, Joseph M; Christoffersen, Kirsten Seestern; Larusson, Kari Fannar & Fefilova, Elena B. [Show all 26 contributors for this article] (2019). State of the Arctic Freshwater Biodiversity. Conservation of Arctic Flora and Fauna International Secretariat. ISSN 9789935431776. Full text in Research Archive

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