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1. Assignments of vibrational lines to OD-impurity complexes for adventitious impurities in β-Ga2O3

   https://doi.org/10.1063/5.0219979  

   Venzie, Andrew; Stavola, Michael; Fowler, W_Beall; Glaser, Evan_R; Tadjer, Marko_J; Forbus, Jason_I; Zvanut, Mary_Ellen; Pearton, Stephen_J (July 2024, APL Materials)

   Hydrogen in β-Ga2O3 passivates shallow impurities and deep-level defects and can have a strong effect on conductivity. More than a dozen O–D vibrational lines have been reported for β-Ga2O3 treated with the heavy isotope of hydrogen, deuterium. To explain the large number of O–D centers that have been observed, the involvement of additional nearby defects and impurities has been proposed. A few O–H centers have been associated with specific impurities that were introduced intentionally during crystal growth. However, definitive assignments of O–H and O–D vibrational lines associated with important adventitious impurities, such as Si and Fe, have been difficult. A set of well-characterized Si-doped β-Ga2O3 epitaxial layers with different layer thicknesses has been deuterated and investigated by vibrational spectroscopy to provide new evidence for the assignment of a line at 2577 cm−1 to an OD–Si complex. The vibrational properties of several of the reported OD-impurity complexes are consistent with the existence of a family of defects with a VGa1ic−D center at their core that is perturbed by a nearby impurity. 

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2. Classes of O–D centers in unintentionally and Fe-doped β-Ga2O3 annealed in a D2 ambient

   https://doi.org/10.1063/5.0160331  

   Portoff, Amanda; Venzie, Andrew; Stavola, Michael; Fowler, W. Beall; Glaser, Evan; Pearton, Stephen J. (July 2023, Journal of Applied Physics)

   β-Ga2O3 has attracted much recent attention as a promising ultrawide bandgap semiconductor. Hydrogen can affect the conductivity of β-Ga2O3 through the introduction of shallow donors and the passivation of deep acceptors. The introduction of H or D into β-Ga2O3 by annealing in an H2 or D2 ambient at elevated temperature produces different classes of O–H or O–D centers. This work is a study of the interaction of D with VGa1 and VGa2 deep acceptors as well as other impurities and native defects in Ga2O3 by infrared spectroscopy and the complementary theory. (We focus primarily on the deuterium isotope of hydrogen because the vibrational modes of O–D centers can be detected with a higher signal-to-noise ratio than those of O–H.) O–D centers in β-Ga2O3 evolve upon annealing in an inert ambient and are transformed from one type of O–D center into another. These reactions affect the compensation of unintentional shallow donors by deep acceptors that are passivated by D. Defects involving additional impurities in β-Ga2O3 compete with VGa deep acceptors for D and modify the deuterium-related reactions that occur. The defect reactions that occur when D is introduced by annealing in a D2 ambient appear to be simpler than those observed for other introduction methods and provide a foundation for understanding the D-related reactions that can occur in more complicated situations. 

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3. Tuning electrical properties in Ga2O3 polymorphs induced with ion beams

   https://doi.org/10.1063/5.0133181  

   Polyakov, A. Y.; Kochkova, А. I.; Azarov, A.; Venkatachalapathy, V.; Miakonkikh, A. V.; Vasilev, A. A.; Chernykh, A. V.; Shchemerov, I. V.; Romanov, A. A.; Kuznetsov, A.; et al (March 2023, Journal of Applied Physics)

   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. 

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4. Diffusion-controlled annealing kinetics of interstitial H in SnO2

   https://doi.org/10.1063/5.0186047  

   Venzie, Andrew; Stavola, Michael; Fowler, W Beall; Boatner, Lynn A (December 2023, Journal of Applied Physics)

   SnO2 is a prototypical transparent conducting oxide that finds widespread applications as transparent electrodes, gas sensors, and transparent thin-film devices. Hydrogen impurities in SnO2 give rise to unintentional n-type behavior and unexpected changes to conductivity. Interstitial H (Hi) and H at an oxygen vacancy (HO) are both shallow donors in SnO2. An O–H vibrational line at 3155 cm−1, that can be produced by a thermal anneal at 500 °C followed by a rapid quench, has been assigned to the Hi center and is unstable at room temperature on a timescale of weeks. An IR absorption study of the decay kinetics of the 3155 cm−1 O–H line has been performed. The disappearance of Hi upon annealing has been found to follow second-order kinetics. Measurements of the decay rate for a range of temperatures have determined an activation energy for the diffusion of interstitial H in SnO2. These results provide fundamental information about how unintentional hydrogen impurities and their reactions can change the conductivity of SnO2 device materials in processes as simple as thermal annealing in an inert ambient. 

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5. Intermolecular vibrational energy transfer enabled by microcavity strong light–matter coupling

   https://doi.org/10.1126/science.aba3544  

   Xiang, Bo; Ribeiro, Raphael F.; Du, Matthew; Chen, Liying; Yang, Zimo; Wang, Jiaxi; Yuen-Zhou, Joel; Xiong, Wei (May 2020, Science)

   Selective vibrational energy transfer between molecules in the liquid phase, a difficult process hampered by weak intermolecular forces, is achieved through polaritons formed by strong coupling between cavity photon modes and donor and acceptor molecules. Using pump-probe and two-dimensional infrared spectroscopy, we found that the excitation of the upper polariton, which is composed mostly of donors, can efficiently relax to the acceptors within ~5 picoseconds. The energy-transfer efficiency can be further enhanced by increasing the cavity lifetime, suggesting that the energy transfer is a polaritonic process. This vibrational energy-transfer pathway opens doors for applications in remote chemistry, sensing mechanisms, and vibrational polariton condensation. 

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