Monoclinic gallium oxide (beta-Ga2O3) single crystals have a Raman mode at similar to 250 cm(-1) that is strongly correlated with free-electron density. Prior work attributed this peak to an electronic excitation of a shallow donor impurity band. However, heavily n-type thin films grown by metalorganic chemical vapor deposition or molecular beam epitaxy do not have the peak. In the present work, an alternate model is proposed: the 250 cm(-1) Raman peak arises from Ga clusters, defined as two or more Ga atoms that form Ga-Ga bonds. Raman mapping reveals variations in the frequency that are consistent with a distribution of cluster sizes. The intensity of the peak decreases as the temperature is raised, attributed to melting of the Ga clusters. First-principles calculations indicate that the 250 cm(-1) mode is due to Ga-Ga bond-stretching vibrations. As the Fermi energy is raised, the formation of Ga-Ga dimers becomes energetically favorable, explaining the correlation between n-type conductivity and the appearance of the Raman peak.

beta-Ga2O3 is an emerging ultra-wide bandgap semiconductor with great promise for power electronics and optoelectronics. Alloys in the In2O3-Ga2O3 system are interesting for optoelectronic applications, particularly where bandgap tuning is desirable. Herein, beta-(InxGa1-x)(2)O-3 alloys with target compositions x = 0.025 or 0.10 are grown from the melt using the Czochralski and vertical gradient freeze techniques. Growth with 10 mol% In yields only small, needle-like crystals, while 2.5 mol% In allows growth of centimeter-sized single crystals. A substantial degree of indium segregation is unveiled by spatial measurements of lattice parameters and the bandgap. The bandgap decreases by a maximum of 0.28 eV in the case of the highest In content crystals. Z-contrast transmission electron microscopy confirms a solely octahedral coordination of In in the beta-Ga2O3 lattice. With indium concentrations higher than 2.5 mol%, samples contain micron-scale voids that impart a dark coloration. All measured crystals are electrically conductive, with carrier concentrations varying 10(16)-10(17) cm(-3) depending upon the location of the sample in the growth. Lastly, a unique luminescence with unknown origin centered around 2.0 eV is revealed by photoluminescence spectroscopy.

Several acceptor dopants have been explored in β-Ga2O3 to produce semi-insulating substrates and epitaxial films. Fe and Mg make up the majority of research thus far; however, other transition metals provide potential alternatives for optimized performance. β-Ga2O3 bulk single crystals were grown by the Czochralski and vertical gradient freeze methods with a nominal dopant concentration of 0.25 at. % Mn. Ultraviolet-visible-near infrared spectroscopy and photoluminescence revealed polarization- and orientation-dependent optical absorptions (pleochroism) coupled with an orange luminescence. All samples were electrically insulating, on the order of 109–1011 ohm cm at room temperature, indicative of acceptor doping. Actual dopant concentrations of the intentionally doped transition metal and background impurities were determined via glow discharge mass spectrometry, indicating the macroscale segregation behavior. High-temperature resistivity measurements indicated an experimental acceptor level of 1.7 ± 0.2 eV. Hydrogenation of samples resulted in an increase in the orange luminescence and O–H stretching modes observable in the infrared spectrum. Density functional theory calculations were performed to determine the likely site-occupancy and acceptor level of Mn in the bandgap.

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 with 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.

beta-Ga2O3 is a promising ultrawide bandgap semiconductor for next-generation power electronics, but the unintended formation of gamma-Ga2O3 in beta-Ga2O3 crystals has been observed in a variety of situations. Such defective inclusions, resulting from growth kinetics or ion-induced damage, can degrade the material performance and alter the local electronic structure. Previous studies have only examined the presence of gamma-Ga2O3 in beta-Ga2O3 thin-film structures. In this work, we observe the ubiquitous formation of a thin gamma-Ga2O3 layer on the surface of mechanically exfoliated melt grown Al- and Sc-alloyed beta-Ga2O3 single crystals and characterize the atomic scale structure across the interface using scanning transmission electron microscopy. Direct imaging paired with electron diffraction confirms gamma-Ga2O3 formation, and orientation relationships are determined across the interface. Electron energy loss spectroscopy identifies the O K-edge spectral fingerprint of gamma-Ga2O3, while many-body perturbation theory on top of density functional theory explains the shift of the spectral intensity between beta- and gamma-Ga2O3 as an interplay of excitonic and electronic effects. Further first-principles studies evaluate the role of strain on phase stability and identify that at an 8.5% tensile strain, gamma-Ga2O3 becomes energetically favored over beta-Ga2O3. Stabilization of the beta phase of Ga(2)O(3 )under compressive stress is further confirmed through electron diffraction studies of the regions surrounding Vickers indentations. Phase stability is also observed to be independent of the alloying element. These findings confirm the capability for gamma-Ga2O3 to occur under extreme environments while also providing evidence that strain is the underlying driving force causing the phase transformation.

Journal Article High Resolution Scanning Transmission Electron Microscopy (S/TEM) Investigation Of Common Defects In Scandium and Aluminum Alloyed β-Ga2O3 Get access Andrew R Balog, Andrew R Balog Department of Materials Science and Engineering, The Pennsylvania State University, State College, PA, United States Search for other works by this author on: Oxford Academic Google Scholar Leixin Miao, Leixin Miao Department of Materials Science and Engineering, The Pennsylvania State University, State College, PA, United States Search for other works by this author on: Oxford Academic Google Scholar Saiphaneendra Bachu, Saiphaneendra Bachu Department of Materials Science and Engineering, The Pennsylvania State University, State College, PA, United States Search for other works by this author on: Oxford Academic Google Scholar Jani Jesenovec, Jani Jesenovec Materials Science and Engineering Program, Washington State University, Pullman, WA, United StatesInstitute of Materials Research, Washington State University, Pullman, WA, United States Search for other works by this author on: Oxford Academic Google Scholar Benjamin Dutton, Benjamin Dutton Materials Science and Engineering Program, Washington State University, Pullman, WA, United StatesInstitute of Materials Research, Washington State University, Pullman, WA, United States Search for other works by this author on: Oxford Academic Google Scholar John McCloy, John McCloy Materials Science and Engineering Program, Washington State University, Pullman, WA, United StatesInstitute of Materials Research, Washington State University, Pullman, WA, United States Search for other works by this author on: Oxford Academic Google Scholar Nasim Alem Nasim Alem Department of Materials Science and Engineering, The Pennsylvania State University, State College, PA, United StatesMaterials Research Institute, The Pennsylvania State University, State College, PA, United States Corresponding author: nua10@psu.edu Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 29, Issue Supplement_1, 1 August 2023, Pages 1779–1780, https://doi.org/10.1093/micmic/ozad067.921 Published: 22 July 2023

Barium calcium titanate (Ba1-xCaxTiO3, BCTO) annealed under a flowing humid 2

Properties of monoclinic β-Ga2O3 are strongly dependent upon orientation. In the case of optical properties, polarization can reveal differences in optical bandgap and absorptions related to transition metal ions. This phenomenon is known as pleochroism and has been extensively studied in minerology. β-Ga2O3 bulk single crystals doped with Zn, Mn, Cr, or Cu were grown by the Czochralski and vertical gradient freeze methods. Ultraviolet-visible-near infrared spectroscopy and photoluminescence (PL) revealed polarization- and orientation-dependent optical absorptions in β-Ga2O3. Crystals were annealed in reducing and oxidizing environments in an attempt to alter the intensities of absorptions characteristic to the different transition metal oxidation states in a given ligand field. Visible pleochroism was strongest in (001) oriented Mndoped samples as shown by polarized optical microscopy. All samples were electrically insulating, indicative of acceptor doping, aside from Cr-doped samples where Cr acts as a deep donor.

beta-Ga2O3 is an emergent ultrawide bandgap material, which has been recently studied with respect to alumina alloying in order to tailor the bandgap for thin film or bulk applications. In this work, bulk Czochralski and vertical gradient freeze crystals of 6 - 8 mol.% Sc2O3 alloyed beta-Ga2O3 (SGO) -beta(Sc0.06Ga0.94)(2)O-3 to -beta-(Sc0.08Ga0.92)(2)O-3 - were obtained, which showed a nominal + 0.07 eV increase in the optical bandgap compared to unintentionally doped (UID) beta-Ga2O3. SGO was characterized for structure (X-ray diffraction, rocking curve, nuclear magnetic resonance, Raman microscopy), purity (glow discharge mass spectrometry, X-ray fluorescence), optical transmission (200 nm - 25,000 nm), resistivity, and luminescence (laser induced luminescence microscopy, photoluminescence). Structural measurements indicate successful incorporation of Sc, although overall lower quality than UID and beta-(Al0.1Ga0.9)(2)O-3 (AGO) material. Purity and optical measurements demonstrated few acceptor impurities and a widened band gap, although not wide enough for some donors to become deep enough to promote insulating behavior. Bulk SGO crystals demonstrated intense Stark luminescence transitions characteristic of Nd3+ , presumably an impurity in the Sc2O3 powder, with some areas of the crystal showing highly concentrated Nd3+. Despite demonstrating an enlarged bandgap, scandia alloyed beta-Ga2O3 retained electrical conduction similar to UID beta-Ga2O3, unlike alumina alloyed beta-Ga2O3 which was electrically insulating.

Much excitement has surrounded the accelerating development of β-Ga2O3 for electronics due to its ultrawide band gap, high breakdown voltage, compatibility with many dopants, and comparative ease of producing large substrates via meltgrowth techniques. Our research has focused on growth and characterization of Czochralski (CZ) and vertical gradient freeze (VGF) single crystals of β-Ga2O3 with various dopants, including donors (Zr, Hf, Cr), acceptors (Mg, Zn, Fe, Ni, Cu), and alloying elements (Al). We find in general that doping in CZ and VGF materials can be different and sometimes non-uniform due to the interaction with crucible material (Ir), selective evaporation, and thermal profile. We have also explored the creation and identification of gallium vacancies (VGa) through annealing, by using positron annihilation spectroscopy (PAS), hydrogenated Fourier Transform Infrared (FTIR) spectroscopy, and electrical measurements. Different analysis techniques probe different spatial and depth averages, and thus careful consideration must be given to correctly interpret results and significance of defect concentrations determined. Insights from our work to date are offered, in terms of their applicability to devices.

Transition-metal ions (Ni, Cu, and Zn) in β-Ga2O3 crystals form deep acceptor levels in the lower half of the bandgap. In the present study, we characterize the Ni acceptors in a Czochralski-grown crystal and find that their (0/−) level is approximately 1.40 eV above the maximum of the valence band. Both Ni2+ (3d8) and Ni3+ (3d7) acceptors are present in the as-grown crystal. Also present are unintentional Ir3+ (5d6) and Ir4+ (5d5) donors. The neutral Ni3+ acceptors have a low-spin S = 1/2 ground state and are easily monitored with electron paramagnetic resonance (EPR). Principal values of the g matrix for these acceptors are 2.131, 2.138, and 2.233. Although paramagnetic, the singly ionized Ni2+ acceptors are not seen with EPR at X band (9.4 GHz). The Ir4+ donors are monitored with EPR and with infrared absorption spectroscopy. Exposing the Ni-doped β-Ga2O3 crystal to 275 nm light at room temperature increases the concentration of Ni3+ ions and reduces the concentration of Ir4+ ions as electrons move from the acceptors to the donors. After illumination, heating the crystal above 375 °C restores the initial concentrations of the Ni3+ and Ir4+ ions. Broad optical absorption bands peaking near 303 and 442 nm are attributed to the Ni3+ acceptors.

In this work, bulk Czochralski-grown single crystals of 10 mol. % Al2O3 alloyed β-Ga2O3—monoclinic 10% AGO or β-(Al0.1Ga0.9)2O3—are obtained, which show +0.20 eV increase in the bandgap compared with unintentionally doped β-Ga2O3. Further, growths of 33% AGO—β-(Al0.33Ga0.67)2O3—and 50% AGO—β-(Al0.5Ga0.5)2O3 or β-AlGaO3—produce polycrystalline single-phase monoclinic material (β-AGO). All three compositions are investigated by x-ray diffraction, Raman spectroscopy, optical absorption, and 27Al nuclear magnetic resonance (NMR). By investigating single phase β-AGO over a large range of Al2O3 concentrations (10–50 mol. %), broad trends in the lattice parameter, vibrational modes, optical bandgap, and crystallographic site preference are determined. All lattice parameters show a linear trend with Al incorporation. According to NMR, aluminum incorporates on both crystallographic sites of β-Ga2O3, with a slight preference for the octahedral (GaII) site, which becomes more disordered with increasing Al. Single crystals of 10% AGO were also characterized by x-ray rocking curve, transmission electron microscopy, purity (glow discharge mass spectroscopy and x-ray fluorescence), optical transmission (200 nm–20  μm wavelengths), and resistivity. These measurements suggest that electrical compensation by impurity acceptor doping is not the likely explanation for high resistivity, but rather the shift of a hydrogen level from a shallow donor to a deep acceptor due to Al alloying. Bulk crystals of β-(Al0.1Ga0.9)2O3 have the potential to be ultra-wide bandgap substrates for thin film growth, with a lattice parameter that may even allow higher Al concentration β-Ga2O3 single crystal thin films to be grown.

beta-Ga2O3 has demonstrated insulating properties with Mg, Fe, and Zn acceptor doping. Here we investigate Cu doping (0.25 at.%) in bulk Czochralski (CZ) and vertical gradient freeze (VGF) beta-Ga2O3, with significant Cu incorporation, even with the expected Cu evaporation. Representative crystals were assessed for orientation, purity, optical, and electrical properties. The solubility and electronic behavior of Cu dopants are consistent with measured concentrations of 1 x 1018-1 x 1019 atoms/cm3 and electrical measurements that show high resistivities of 109-1010 omega center dot cm. Segregation and precipitation of Cu species in part of the VGF material was determined to be Cu2O by analysis with Raman spectroscopy, photoluminescence microscopy, energy-dispersive X-ray spectroscopy, and laser ablation inductively coupled plasma mass spectrometry. With sufficient Cu concentration, beta-Ga2O3 crystals excited with deep ultraviolet light photodarken rapidly and exhibit decreased resistivity. This darkened state remains at room temperature for several days before decaying.

In this work, we investigate a framework for collecting, processing, and registering in-process monitoring data from an additive manufacturing build with post-build inspection data obtained from CT scans of the produced samples. In addition to processing and combining the data, we look at a statistical analysis of the combined data streams using machine learning methods to obtain a quantitative prediction of local void probability in the CT data from local feature metrics extracted from the in-process monitoring data. Results show promise when the in-process monitoring data are processed to extract designed features/metrics, rather than using only the raw signals.