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  1. Resonant interactions involving local vibrational modes in crystals

    When an impurity with a light mass is inserted into a crystal, it can undergo a high-frequency oscillation referred to as a local vibrational mode (LVM). A Fermi resonance may occur between the LVM and lower-frequency modes of the defect. The LVM may also interact with phonons or the electromagnetic field. Understanding these interactions can help model and control diffusion, defect reactions, and thermal conductivity. LVMs have been probed in semiconductors using pressure and alloying as experimental parameters, resulting in anticrossing between localized and extended vibrational modes. Furthermore, these types of vibrational interactions could play an important role in themore » stability and thermal properties of organic–inorganic hybrid semiconductors. The coupling between an LVM and electromagnetic wave yields an “LVM polariton,” an excitation that has significant vibrational and electric-field amplitudes.« less
  2. Phase Transition of Heptane under Pressure

    A solid-to-solid phase transition was discovered in n-heptane with an onset at a pressure of 1.7 ± 0.1 GPa, evidenced by a discontinuity in the pressure–volume equation of state obtained from synchrotron X-ray diffraction experiments conducted at room temperature. The phase transition also resulted in discontinuous shifts of vibrational peaks in the infrared spectra. No additional phase transitions were detected up to 10 GPa, contradicting an earlier claim of a phase transition at 3 GPa. Furthermore, the hydrostatic limit of heptane in a diamond anvil cell was estimated to be 3.5 GPa. Above that pressure, the stress state is notmore » well-defined, which may explain prior reports of a phase transition at 7.5 GPa.« less
  3. Color center in β-Ga2O3 emitting at the telecom range

    Transition metal (TM) ions incorporated into a host from a wide bandgap semiconductor are recognized as a promising system for quantum technologies with enormous potential. In this work, we report on a TM color center in β-Ga2O3 with physical properties attractive for quantum information applications. The center is found to emit at 1.316 μm and exhibits weak coupling to phonons, with optically addressable higher-lying excited states, beneficial for single-photon emission within the telecom range (O-band). Using magneto-photoluminescence (PL) complemented by time-resolved PL measurements, we identify the monitored emission to be internal 1E→3A2 spin-forbidden transitions of a 3d8 TM ion withmore » a spin-triplet ground state—a possible candidate for a spin qubit. We tentatively attribute this color center to a complex involving a sixfold coordinated Cu3+ ion.« less
  4. Photoluminescence of Cr3+ in β-Ga2O3 and (Al0.1Ga0.9)2O3 under pressure

    The effects of pressure on single crystals of Cr-doped gallium oxide (β-Ga2O3:Cr3+) and aluminum gallium oxide [(Al0.1Ga0.9)2O3] were examined by measuring the wavelength shift in the spectral R lines. Photoluminescence (PL) spectra of these materials were collected from samples in diamond anvil cells at pressures up to 9 GPa. The β-Ga2O3:Cr3+ R lines were found to shift linearly under hydrostatic pressure. The (Al0.1Ga0.9)2O3 R lines also show a linear shift but the R1 line shifted less than for β-Ga2O3:Cr3+. The ratio of R2 to R1 peak areas vs pressure is dominated by nonradiative recombination. X-ray diffraction measurements of (Al0.1Ga0.9)2O3 indicatemore » that its equation of state is similar to that of β-Ga2O3. β-Ga2O3:Cr3+ was examined under non-hydrostatic conditions by using mineral oil as a pressure transmitting medium. Similar to the case in ruby, the R1 line is much more sensitive to non-hydrostatic stress than R2. Spatially resolved PL of a sample at 8 GPa in mineral oil showed significant variations in the R1 emission wavelength. Furthermore, these results suggest that the R1 line can serve as a sensitive probe of alloy composition and non-hydrostatic stress, while the R2 line is insensitive to these perturbations.« less
  5. Photoluminescence spectroscopy of Cr3+ in β-Ga2O3 and (Al0.1Ga0.9)2O3

    Alloying β-Ga2O3 with Al2O3 to create (Al xGa1- x)2O3 enables ultra-wide bandgap materials suitable for applications deep into ultraviolet. In this work, photoluminescence (PL) spectra of Cr3+ were investigated in monoclinic single crystal β-Ga2O3, and 10 mol. % Al2O3 alloyed with β-Ga2O3, denoted β-(Al0.1Ga0.9)2O3 or AGO. Temperature-dependent PL properties were studied for Cr3+ in AGO and β-Ga2O3 from 295 to 16 K. For both materials at room temperature, the red-line emission doublet R1 and R2 occurs at 696 nm (1.78 eV) and 690 nm (1.80 eV), respectively, along with a broad emission band at 709 nm (1.75 eV). The linewidthsmore » for AGO are larger for all temperatures due to alloy broadening. For both materials, the R-lines blue-shift with decreasing temperature. The (lowest energy) R1 line is dominant at low temperatures due to the thermal population of the levels. For temperatures above ~50 K, however, the ratio of R2 to R1 peak areas is dominated by nonradiative combination.« less
  6. Selectively patterned Mg-doped GaN by SiNx-driven hydrogen injection

    In this work, we demonstrate a method to achieve selectively patterned Mg-doped GaN layers using hydrogen drive-in through plasma-enhanced chemical vapor deposition (PECVD) silicon nitride (SiNx) films. Activated Mg-doped GaN layers were selectively deactivated by patterned PECVD SiNx films with low-temperature annealing and showed high-resistive behavior. Spatially resolved photoluminescence measurements were used to optically verify the deactivation of Mg acceptors and showed distinct features corresponding to activated and deactivated Mg in GaN. The method suggested here provides a simple and effective method to achieve patterned Mg-doped GaN regions without thermal and plasma damage, which could cause degradation of device performance.more » The proposed method could provide a way to achieve future high-performance GaN lateral and vertical devices that rely on laterally patterned doping.« less
  7. Classification of Semiconductors Using Photoluminescence Spectroscopy and Machine Learning

    Photoluminescence spectroscopy is a nondestructive optical method that is widely used to characterize semiconductors. In the photoluminescence process, a substance absorbs photons and emits light with longer wavelengths via electronic transitions. This paper discusses a method for identifying substances from their photoluminescence spectra using machine learning, a technique that is efficient in making classifications. Neural networks were constructed by taking simulated photoluminescence spectra as the input and the identity of the substance as the output. Here, six different semiconductors were chosen as categories: gallium oxide (Ga2O3), zinc oxide (ZnO), gallium nitride (GaN), cadmium sulfide (CdS), tungsten disulfide (WS2), and cesiummore » lead bromide (CsPbBr3). The developed algorithm has a high accuracy (>90%) for assigning a substance to one of these six categories from its photoluminescence spectrum and correctly identified a mixed Ga2O3/ZnO sample.« less
  8. Persistent Room-Temperature Photodarkening in Cu-Doped β - Ga 2 O 3

    Beta-Ga2O3 is an ultra-wide bandgap semiconductor with emerging applications in power electronics. The introduction of acceptor dopants yields semi-insulating substrates necessary for thin-film devices. In the present work, exposure of Cu-doped beta-Ga2O3 to UV light > 4 eV is shown to cause large, persistent photo-induced darkening at room temperature. Electron paramagnetic resonance spectroscopy indicates that light exposure converts Cu2+ to Cu3+, a rare oxidation state that is responsible for the optical absorption. The photodarkening is accompanied by the appearance of O-H vibrational modes in the infrared spectrum. Hybrid function calculations show that Cu acceptors can favorably complex with hydrogen donorsmore » incorporated as interstitial (Hi) or substitutional (HO) defects. When CuGa-HO complexes absorb light, hydrogen is released, contributing to the observed Cu3+ species and O-H modes.« less
  9. Photoluminescence and Raman mapping of β-Ga2O3

    Semi-insulating single crystal β-Ga2O3 is becoming increasingly useful as a substrate for device fabrication. Fe doping is a method for producing such substrates. Along with Fe dopants, β-Ga2O3:Fe also contains Cr3+. Photoluminescence (PL) emission peaks at 690 nm (1.80 eV) and 696 nm (1.78 eV), as well as a broad feature around 709 nm (1.75 eV), are observed in β-Ga2O3:Fe. PL mapping of the 690 nm emission showed high and low intensity bands due to impurity striations introduced during crystal growth. PL mapping also revealed surface defects showing broad emissions around 983 nm (1.26 eV) and 886 nm (1.40 eV)more » that were spatially localized, occurring at discrete spots on the sample surface. Raman mapping of an 886 nm emission center revealed peaks at 2878 and 2930 cm-1, consistent with an organometallic or hydrocarbon compound. Raman mapping of the 983 nm center showed a peak at 2892 cm-1. Bright UV emission centers showed Raman peaks at 2910 and 2968 cm-1, which are attributed to Si-CH3 groups that may originate from silica polishing compounds or annealing in a silica ampoule.« less
  10. Zinc–hydrogen and zinc–iridium pairs in β-Ga2O3

    Zinc-doped monoclinic gallium oxide (β-Ga2O3:Zn) has semi-insulating properties that could make it a preferred material as a substrate for power devices. In this work, infrared and UV/Visible spectroscopy were used to investigate the defect properties of bulk β-Ga2O3:Zn crystals. As-grown crystals contain a single O-H stretching mode at 3486.7 cm-1 due to a neutral ZnH complex. A deuterium-annealed sample displays the corresponding O-D stretching mode at 2582.9 cm-1, confirming the O-H assignment. A strong Ir4+ electronic transition at 5147.6 cm-1 is also observed, along with sidebands attributed to ZnIr pairs. These sidebands show distinct differences compared with Mg-doped samples; mostmore » importantly, several peaks are attributed to Ir4+ paired with a Zn on the tetrahedral Ga(I) site. Annealing under an oxygen atmosphere produced insulating material with a resistance above 1 TΩ.« less
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