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All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Micro-segregation phenomena and related spectroscopic signals in melt-grown β-Ga2O3 single crystals

One of the primary advantages of β-Ga2O3 over incumbent wide bandgap semiconductors is the ability to grow directly from the melt. Melt growth, using Czochralski or similar methods, results in impurities in the crystal which originate from the crucible, such as iridium and other transition metals like chromium. These impurities exhibit optoelectronic signatures useful for their identification and sensitive to the Fermi energy of a given crystal (i.e., signatures vary with the electrical conductivity of the matrix). In this work, we describe how laser Raman systems can be used to map and spatially correlate Cr3+ photoluminescence, electronic-coupled Raman scattering from Ir4+d–d internal transitions, and the Raman line attributed to hydrogenic shallow donors. Laser ablation inductively coupled plasma mass spectrometry directly measured spatially dependent relative metal concentrations and confirmed spectroscopic signals resulting from heterogeneities in impurity concentrations in β-Ga2O3 boules. Mapping of photoluminescence and Raman-related signatures is, thus, demonstrated as an effective and facile method for spatial measurement of chemical heterogeneities in both insulating and conductive melt-grown β-Ga2O3 crystals.

Dutton, Benjamin L. (ORCID:000000031272130X)↗
DOE OSTI2025

Characterization of chromium impurities in β -Ga2O3

Chromium is a common transition-metal impurity that is easily incorporated during crystal growth. It is perhaps best known for giving rise to the 694.3 nm (1.786 eV) emission in Cr-doped Al2O3, exploited in ruby lasers. Chromium has also been found in monoclinic gallium oxide, a wide-bandgap semiconductor being pursued for power electronics. In this work, we thoroughly characterize the behavior of Cr in Ga2O3 through theoretical and experimental techniques. β-Ga2O3 samples are grown with the floating zone method and show evidence of a sharp photoluminescence signal, reminiscent of ruby. We calculate the energetics of formation of Cr from first principles, demonstrating that Cr preferentially incorporates as a neutral impurity on the octahedral site. Cr possesses a quartet ground-state spin and has an internal transition with a zero-phonon line near 1.8 eV. By comparing the calculated and experimentally measured luminescence lineshape function, we elucidate the role of coupling to phonons and uncover features beyond the Franck–Condon approximation. The combination of strong emission with a small Huang–Rhys factor of 0.05 and a technologically relevant host material renders Cr in Ga2O3 attractive as a quantum defect.

Turiansky, Mark E. (ORCID:0000000291543582)↗
DOE OSTI2026

Epitaxy of Beta-Ga2O3 on Highly-Offcut (Greater than 10 Degrees) Substrates

Beta-Ga2O3 has emerged as a leading candidate for next-generation power electronics, radio frequency (RF) switches, and extreme environment electronics due to a wide band gap (4.6 - 4.9 eV), high dopability (approximately 40 meV activation energy for an isolated silicon donor), and melt growth characteristics resulting in commercially available 4-inch substrates and commercial demonstrations of 6-inch substrates by multiple techniques. The (100) surface of Ga2O3 is highly desirable from a device and epitaxy standpoint - bulk growth of (100) material is more scalable than (010), the surface is nearly lattice-matched to p-type partner NiO, and Al2O3 incorporates at higher concentrations without phase separation. However, the epitaxial growth rate on (100) surfaces is less than 10% of other faces due to weak bonding and favorable desorption. Recent demonstrations have shown growth rate improvements from 0.4 nm/min to 1.5 nm/min by growing on (100) wafers that are offcut 6 degrees in the -c direction.1 These films show step-flow growth from (-201) step-edges and high electron mobility. Despite these exciting results, offcuts greater than 6 degrees have not been explored due to the waste associated with grinding and polishing large offcuts. In this talk we will discuss the molecular beam epitaxy (MBE) growth and properties of Beta-Ga2O3 grown on (100) substrates offcut in the -c direction up to 13.4 degrees. These large offcuts are enabled by edge-fed film-defined growth (EFG) where the offcut is grown into the surface by pulling the crystal through the EFG die with the seed crystal rotated by the desired offcut angle. We will demonstrate that 13.4 degrees offcut substrates still exhibit a terraced (100) surface, and that a >10x increase (4.8 nm/min) in growth rate is achieved. As previously reported on lower offcuts, we observe 100% reversal of substrate twin domains around the (001) direction at the substrate-epilayer interface. We will discuss electrical properties including record-low (by MBE) unintentional doping densities of < 5E15 cm-3.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Polarization-dependent resonant inelastic x-ray scattering of β-Ga2O3: An experimental and computational study

The bulk electronic structure of β-Ga2O3 single crystals was investigated using oxygen K-edge x-ray emission and absorption spectroscopy as well as resonant inelastic soft x-ray scattering. Spectra were obtained for different orientations of the crystal planes with respect to the polarization vector of the incident x-ray beam. The spectra are analyzed with calculations based on density functional theory and using the Bethe–Salpeter equation in the ocean code to take the core–hole interaction into account. These calculations correctly capture all the main features in the experimental spectra, demonstrating the potential of the approach to predict the electronic properties of similar compounds. We find a pronounced anisotropy as a function of the excitation polarization vector and significant differences in the spectral contributions from the inequivalent oxygen atoms of β-Ga2O3.

Pyatenko, Elizaveta↗
DOE OSTI2023

Ultrathin stable Ohmic contacts for high-temperature operation of β -Ga2O3 devices

Beta gallium oxide (β-Ga2O3) shows significant promise in high-temperature, high-power, and sensing electronics applications. However, long-term stable metallization layers for Ohmic contacts at high temperatures present unique thermodynamic challenges. The current most common Ohmic contact design based on 20 nm of Ti has been repeatedly demonstrated to fail at even moderately elevated temperatures (300–400 °C) due to a combination of nonstoichiometric Ti/Ga2O3 interfacial reactions and kinetically favored Ti diffusion processes. Here, we demonstrate stable Ohmic contacts for Ga2O3 devices operating up to 500–600 °C using ultrathin Ti layers with a self-limiting interfacial reaction. The ultrathin Ti layer in the 5 nm Ti/100 nm Au contact stack is designed to fully oxidize while forming an Ohmic contact, thereby limiting both thermodynamic and kinetic instability. This novel contact design strategy results in an epitaxial conductive anatase titanium oxide interface layer that enables low-resistance Ohmic contacts that are stable both under long-term continuous operation (>500 h) at 600 °C in vacuum (≤10−4 Torr), as well as after repeated thermal cycling (15 times) between room temperature and 550 °C in flowing N2. This stable Ohmic contact design will accelerate the development of high-temperature devices by enabling research focus to shift toward rectifying interfaces and other interfacial layers.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

Materials Data on Ga2O3 by Materials Project

Ga2O3 is Corundum structured and crystallizes in the trigonal R-3c space group. The structure is three-dimensional. Ga3+ is bonded to six equivalent O2- atoms to form a mixture of distorted corner, edge, and face-sharing GaO6 octahedra. The corner-sharing octahedra tilt angles range from 49–61°. There are three shorter (1.95 Å) and three longer (2.11 Å) Ga–O bond lengths. O2- is bonded to four equivalent Ga3+ atoms to form a mixture of distorted corner and edge-sharing OGa4 trigonal pyramids.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ga2O3 by Materials Project

Ga2O3 crystallizes in the orthorhombic Cmcm space group. The structure is three-dimensional. there are two inequivalent Ga3+ sites. In the first Ga3+ site, Ga3+ is bonded in a 6-coordinate geometry to six O2- atoms. There are two shorter (1.96 Å) and four longer (2.17 Å) Ga–O bond lengths. In the second Ga3+ site, Ga3+ is bonded to six O2- atoms to form a mixture of edge and corner-sharing GaO6 octahedra. The corner-sharing octahedral tilt angles are 58°. There are four shorter (1.98 Å) and two longer (2.08 Å) Ga–O bond lengths. There are two inequivalent O2- sites. In the first O2- site, O2- is bonded in a 4-coordinate geometry to four Ga3+ atoms. In the second O2- site, O2- is bonded to four Ga3+ atoms to form corner-sharing OGa4 tetrahedra.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ga2O3 by Materials Project

Ga2O3 crystallizes in the triclinic P1 space group. The structure is three-dimensional. there are sixteen inequivalent Ga3+ sites. In the first Ga3+ site, Ga3+ is bonded to four O2- atoms to form distorted GaO4 tetrahedra that share corners with five GaO6 octahedra and an edgeedge with one GaO4 trigonal pyramid. The corner-sharing octahedra tilt angles range from 58–67°. There are a spread of Ga–O bond distances ranging from 1.77–2.13 Å. In the second Ga3+ site, Ga3+ is bonded in a 6-coordinate geometry to six O2- atoms. There are a spread of Ga–O bond distances ranging from 1.81–2.43 Å. In the third Ga3+ site, Ga3+ is bonded to six O2- atoms to form distorted corner-sharing GaO6 octahedra. There are a spread of Ga–O bond distances ranging from 1.86–2.42 Å. In the fourth Ga3+ site, Ga3+ is bonded to four O2- atoms to form corner-sharing GaO4 tetrahedra. The corner-sharing octahedra tilt angles range from 54–67°. There are a spread of Ga–O bond distances ranging from 1.82–2.03 Å. In the fifth Ga3+ site, Ga3+ is bonded to six O2- atoms to form GaO6 octahedra that share corners with six GaO4 tetrahedra, a cornercorner with one GaO4 trigonal pyramid, and edges with three GaO6 octahedra. There are a spread of Ga–O bond distances ranging from 1.88–2.19 Å. In the sixth Ga3+ site, Ga3+ is bonded to six O2- atoms to form GaO6 octahedra that share corners with six GaO4 tetrahedra, a cornercorner with one GaO4 trigonal pyramid, and edges with three GaO6 octahedra. There are a spread of Ga–O bond distances ranging from 1.88–2.13 Å. In the seventh Ga3+ site, Ga3+ is bonded in a rectangular see-saw-like geometry to four O2- atoms. There are a spread of Ga–O bond distances ranging from 1.75–1.93 Å. In the eighth Ga3+ site, Ga3+ is bonded to four O2- atoms to form GaO4 trigonal pyramids that share corners with four GaO6 octahedra, corners with two equivalent GaO4 tetrahedra, and an edgeedge with one GaO4 tetrahedra. The corner-sharing octahedra tilt angles range from 55–67°. There are a spread of Ga–O bond distances ranging from 1.83–2.09 Å. In the ninth Ga3+ site, Ga3+ is bonded to six O2- atoms to form GaO6 octahedra that share corners with seven GaO4 tetrahedra and an edgeedge with one GaO6 octahedra. There are a spread of Ga–O bond distances ranging from 1.94–2.32 Å. In the tenth Ga3+ site, Ga3+ is bonded to six O2- atoms to form GaO6 octahedra that share corners with six GaO4 tetrahedra and edges with three GaO6 octahedra. There are a spread of Ga–O bond distances ranging from 1.92–2.24 Å. In the eleventh Ga3+ site, Ga3+ is bonded to four O2- atoms to form GaO4 tetrahedra that share corners with four GaO6 octahedra and corners with two equivalent GaO4 trigonal pyramids. The corner-sharing octahedra tilt angles range from 48–59°. There are a spread of Ga–O bond distances ranging from 1.79–1.98 Å. In the twelfth Ga3+ site, Ga3+ is bonded in a rectangular see-saw-like geometry to four O2- atoms. There are a spread of Ga–O bond distances ranging from 1.72–1.94 Å. In the thirteenth Ga3+ site, Ga3+ is bonded in a 6-coordinate geometry to six O2- atoms. There are a spread of Ga–O bond distances ranging from 1.84–2.46 Å. In the fourteenth Ga3+ site, Ga3+ is bonded to four O2- atoms to form corner-sharing GaO4 tetrahedra. The corner-sharing octahedra tilt angles range from 47–61°. There are a spread of Ga–O bond distances ranging from 1.86–1.95 Å. In the fifteenth Ga3+ site, Ga3+ is bonded to four O2- atoms to form GaO4 tetrahedra that share corners with three GaO6 octahedra and corners with two equivalent GaO4 tetrahedra. The corner-sharing octahedra tilt angles range from 55–59°. There are a spread of Ga–O bond distances ranging from 1.84–1.92 Å. In the sixteenth Ga3+ site, Ga3+ is bonded to four O2- atoms to form GaO4 tetrahedra that share corners with three GaO6 octahedra and corners with two equivalent GaO4 tetrahedra. The corner-sharing octahedra tilt angles range from 51–67°. There are a spread of Ga–O bond distances ranging from 1.85–1.91 Å. There are twenty-four inequivalent O2- sites. In the first O2- site, O2- is bonded in a bent 120 degrees geometry to two Ga3+ atoms. In the second O2- site, O2- is bonded to four Ga3+ atoms to form a mixture of distorted edge and corner-sharing OGa4 tetrahedra. In the third O2- site, O2- is bonded in a bent 120 degrees geometry to two Ga3+ atoms. In the fourth O2- site, O2- is bonded in a 4-coordinate geometry to four Ga3+ atoms. In the fifth O2- site, O2- is bonded in a bent 120 degrees geometry to two Ga3+ atoms. In the sixth O2- site, O2- is bonded in a trigonal non-coplanar geometry to three Ga3+ atoms. In the seventh O2- site, O2- is bonded in a 4-coordinate geometry to four Ga3+ atoms. In the eighth O2- site, O2- is bonded in a distorted trigonal planar geometry to three Ga3+ atoms. In the ninth O2- site, O2- is bonded in a bent 120 degrees geometry to two Ga3+ atoms. In the tenth O2- site, O2- is bonded in a distorted trigonal planar geometry to three Ga3+ atoms. In the eleventh O2- site, O2- is bonded to four Ga3+ atoms to form a mixture of distorted edge and corner-sharing OGa4 tetrahedra. In the twelfth O2- site, O2- is bonded to four Ga3+ atoms to form distorted corner-sharing OGa4 tetrahedra. In the thirteenth O2- site, O2- is bonded in a distorted trigonal non-coplanar geometry to three Ga3+ atoms. In the fourteenth O2- site, O2- is bonded in a rectangular see-saw-like geometry to four Ga3+ atoms. In the fifteenth O2- site, O2- is bonded in a 4-coordinate geometry to four Ga3+ atoms. In the sixteenth O2- site, O2- is bonded in a distorted trigonal planar geometry to three Ga3+ atoms. In the seventeenth O2- site, O2- is bonded in a distorted trigonal non-coplanar geometry to three Ga3+ atoms. In the eighteenth O2- site, O2- is bonded in a distorted trigonal planar geometry to three Ga3+ atoms. In the nineteenth O2- site, O2- is bonded in a distorted trigonal planar geometry to three Ga3+ atoms. In the twentieth O2- site, O2- is bonded in a distorted trigonal pyramidal geometry to four Ga3+ atoms. In the twenty-first O2- site, O2- is bonded in a 4-coordinate geometry to four Ga3+ atoms. In the twenty-second O2- site, O2- is bonded in a 3-coordinate geometry to four Ga3+ atoms. In the twenty-third O2- site, O2- is bonded in a trigonal non-coplanar geometry to three Ga3+ atoms. In the twenty-fourth O2- site, O2- is bonded in a trigonal planar geometry to three Ga3+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Materials Data on Ga2O3 by Materials Project

Ga2O3 crystallizes in the monoclinic C2/m space group. The structure is three-dimensional. there are two inequivalent Ga3+ sites. In the first Ga3+ site, Ga3+ is bonded to six O2- atoms to form GaO6 octahedra that share corners with seven equivalent GaO4 tetrahedra and edges with four equivalent GaO6 octahedra. There are a spread of Ga–O bond distances ranging from 1.97–2.11 Å. In the second Ga3+ site, Ga3+ is bonded to four O2- atoms to form GaO4 tetrahedra that share corners with seven equivalent GaO6 octahedra and corners with two equivalent GaO4 tetrahedra. The corner-sharing octahedra tilt angles range from 56–64°. There is three shorter (1.86 Å) and one longer (1.89 Å) Ga–O bond length. There are three inequivalent O2- sites. In the first O2- site, O2- is bonded in a trigonal planar geometry to three Ga3+ atoms. In the second O2- site, O2- is bonded to four Ga3+ atoms to form a mixture of distorted edge and corner-sharing OGa4 tetrahedra. In the third O2- site, O2- is bonded in a trigonal planar geometry to three Ga3+ atoms.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Ultrawide bandgap vertical β-(Al x Ga1 −x )2O3 Schottky barrier diodes on free-standing β-Ga2O3 substrates

Ultrawide bandgap β-(AlxGa1−x)2O3 vertical Schottky barrier diodes on (010) β-Ga2O3 substrates are demonstrated. The β-(AlxGa1−x)2O3 epilayer has an Al composition of 21% and a nominal Si doping of 2 × 1017 cm−3 grown by molecular beam epitaxy. Pt/Ti/Au has been employed as the top Schottky contact, whereas Ti/Au has been utilized as the bottom Ohmic contact. The fabricated devices show excellent rectification with a high on/off ratio of ∼109, a turn-on voltage of 1.5 V, and an on-resistance of 3.4 mΩ cm2. Temperature-dependent forward current-voltage characteristics show effective Schottky barrier height varied from 0.91 to 1.18 eV while the ideality factor from 1.8 to 1.1 with increasing temperatures, which is ascribed to the inhomogeneity of the metal/semiconductor interface. The Schottky barrier height was considered a Gaussian distribution of potential, where the extracted mean barrier height and a standard deviation at zero bias were 1.81 and 0.18 eV, respectively. A comprehensive analysis of the device leakage was performed to identify possible leakage mechanisms by studying temperature-dependent reverse current-voltage characteristics. At reverse bias, due to the large Schottky barrier height, the contributions from thermionic emission and thermionic field emission are negligible. By fitting reverse leakage currents at different temperatures, it was identified that Poole–Frenkel emission and trap-assisted tunneling are the main leakage mechanisms at high- and low-temperature regimes, respectively. Electrons can tunnel through the Schottky barrier assisted by traps at low temperatures, while they can escape these traps at high temperatures and be transported under high electric fields. This work can serve as an important reference for the future development of ultrawide bandgap β-(AlxGa1−x)2O3 power electronics, RF electronics, and ultraviolet photonics.

Materials Science↗
DOE OSTI2019

Materials Data on Ga2O3 by Materials Project

Computed materials data using density functional theory calculations. These calculations determine the electronic structure of bulk materials by solving approximations to the Schrodinger equation. For more information, see https://materialsproject.org/docs/calculations

36 MATERIALS SCIENCE↗
DOE OSTI2019

Materials Data on Ga2O3 by Materials Project

Computed materials data using density functional theory calculations. These calculations determine the electronic structure of bulk materials by solving approximations to the Schrodinger equation. For more information, see https://materialsproject.org/docs/calculations

36 MATERIALS SCIENCE↗
DOE OSTI2019

Materials Data on Ga2O3 by Materials Project

Computed materials data using density functional theory calculations. These calculations determine the electronic structure of bulk materials by solving approximations to the Schrodinger equation. For more information, see https://materialsproject.org/docs/calculations

36 MATERIALS SCIENCE↗
DOE OSTI2019

Materials Data on Ga2O3 by Materials Project

Computed materials data using density functional theory calculations. These calculations determine the electronic structure of bulk materials by solving approximations to the Schrodinger equation. For more information, see https://materialsproject.org/docs/calculations

36 MATERIALS SCIENCE↗
DOE OSTI2019

Materials Data on Ga2O3 by Materials Project

Computed materials data using density functional theory calculations. These calculations determine the electronic structure of bulk materials by solving approximations to the Schrodinger equation. For more information, see https://materialsproject.org/docs/calculations

36 MATERIALS SCIENCE↗
DOE OSTI2015

Catalytic propane dehydrogenation over In₂O₃–Ga₂O₃ mixed oxides

We have investigated the catalytic performance of novel In₂O₃–Ga₂O₃ mixed oxides synthesized by the alcoholic-coprecipitation method for propane dehydrogenation (PDH). Reactivity measurements reveal that the activities of In₂O₃–Ga₂O₃ catalysts are 1–3-fold (on an active metal basis) and 12–28-fold (on a surface area basis) higher than an In₂O₃–Al₂O₃ catalyst in terms of C₃H₈ conversion. The structure, composition, and surface properties of the In₂O₃–Ga₂O₃ catalysts are thoroughly characterized. NH₃-TPD shows that the binary oxide system generates more acid sites than the corresponding single-component catalysts. Raman spectroscopy suggests that catalysts that produce coke of a more graphitic nature suppress cracking reactions, leading to higher C₃H₆ selectivity. Lower reaction temperature also leads to higher C₃H₆ selectivity by slowing down the rate of side reactions. XRD, XPS, and XANES measurements, strongly suggest that metallic indium and In₂O₃ clusters are formed on the catalyst surface during the reaction. The agglomeration of In₂O₃ domains and formation of a metallic indium phase are found to be irreversible under O₂ or H₂ treatment conditions used here, and may be responsible for loss of activity with increasing time on stream.

Tan, Shuai↗