Joshua T. Buontempo

We report the observation of Shubnikov–de Haas (SdH) oscillations in metalorganic chemical vapor deposition (MOCVD)-grown (010)-oriented β-(AlxGa1−x)2O3/β-Ga2O3 modulation δ-doped heterostructures. Triethylgallium (TEGa) and triethylaluminum (TEAl) were used as precursors, which enabled the growth to occur at a relatively low substrate temperature of 650 °C. The as-grown films exhibit sub-nanometer roughness and show distinct Laue oscillations, confirming a high-quality, sharp heterointerface. Additionally, we report the highest β-(AlxGa1−x)2O3/β-Ga2O3 two-dimensional electron gas (2DEG) low-temperature mobility to date of 2914 cm2/Vs at 45 K. From the SdH oscillations below 10 K, we extract a 2DEG density of 1.7×1012 cm−2, a cyclotron effective mass of 0.305±0.007 me, a quantum scattering time of 178±11 fs, and an estimated effective g-factor of 3.1. The effective mass is in reasonable agreement with the calculated conduction band non-parabolicity in β-Ga2O3 given the estimated position of the Fermi level above the conduction band minimum. Temperature-dependent transport is further analyzed to determine that background and remote ionized impurity scattering are the likely dominant scattering mechanisms at low temperatures. This work affirms the viability of MOCVD using TEAl and TEGa for growing high-quality gallium oxide-based heterostructures.

Net carrier densities in highly Si-doped β-Ga2O3 decrease during thermal annealing as a function of temperature, time, and PO2 for both in situ and implant-doped films. The mechanism for this loss has been attributed to the formation of gallium vacancies (VGa3−), which act as compensating acceptors and form passivating defect complexes. Samples doped above 3 × 1019 cm−3, annealed at the same temperature and PO2, ultimately approach a similar net carrier density of 1–3 × 1019 cm−3 (for 950 °C and PO2 < 2 × 10−7 bar) independent of the starting dopant concentration or doping method. These highly doped samples, metastable under these conditions, generate VGa3− to reach the steady state |ND − NA|, determined by the Fermi level. An activation energy of 1.6 ± 0.2 eV was observed for the rate of net carrier loss in high concentration in situ doped films. Scanning transmission electron microscopy and electrical results for implanted samples show that dopant activation, lattice recovery, and compensation are all active within the first minutes of annealing. Understanding the metastability of higher doping levels and VGa3− formation allows for the development of mitigation strategies to achieve highly Si-doped β-Ga2O3 by implant (8 × 1019 cm−3 with >70 cm2/V s mobility), including using the minimum required anneal time to achieve lattice recovery and dopant activation, reducing vacancy formation. Compensation initially limited full activation of deeper implants to 1 × 1020 cm−3; by protecting the free surface with a SiO2 cap during annealing, the activation efficiency doubled, and the mobility increased by 30%, highlighting the importance of controlling VGa3− formation and compensation.

Preserving a contamination-free metal–semiconductor interface in β-Ga2O3 is critical to achieve consistently low resistance (< 1 Ω-mm) ohmic contacts. Here, we report a scanning transmission electron microscopy study on the variation in Ti/Au ohmic contact quality to (010) β-Ga2O3 in a conventional lift-off vs a metal-first process. We observe a thin ∼1 nm carbon barrier between the Ti and Ga2O3 in a non-conductive contact fabricated by a conventional lift-off process, which we attribute to photoresist residue, not previously detected by x-ray photoelectron spectroscopy due to the thinness and patchy coverage of the carbon layer, as well as roughness of the Ga2O3 surface. This thin carbon barrier is confirmed by electron energy loss spectroscopy and atomic force microscopy-infrared spectroscopy. We believe that the presence of the thin and patchy carbon layer leads to the highly inconsistent contact behavior in previous reports on non-alloyed contacts. Adventitious carbon is also observed in a conductive ohmic contact metal-first processing on an as-grown sample. We find that a five minute active oxygen descum is sufficient to remove this carbon on as-grown samples, further improving the ohmic behavior and reducing the contact resistance Rc to 0.06 Ω-mm. We also show that an hour long UV-ozone treatment of the Ga2O3 surface can eliminate carbon residue from the lift-off processing, resulting in a low Rc of 0.05 Ω-mm.

In this study, we investigate in situ etching of β-Ga2O3 in a metalorganic chemical vapor deposition system using tert-butyl chloride (TBCl). We report etching of both heteroepitaxial 2¯01-oriented and homoepitaxial (010)-oriented β-Ga2O3 films over a wide range of substrate temperatures, TBCl molar flows, and reactor pressures. We infer that the likely etchant is HCl (g), formed by the pyrolysis of TBCl in the hydrodynamic boundary layer above the substrate. The temperature dependence of the etch rate reveals two distinct regimes characterized by markedly different apparent activation energies. The extracted apparent activation energies suggest that at temperatures below ∼800 °C, the etch rate is likely limited by desorption of etch products. The relative etch rates of heteroepitaxial 2¯01 and homoepitaxial (010) β-Ga2O3 were observed to scale by the ratio of the surface energies, indicating an anisotropic etch. Relatively smooth post-etch surface morphology was achieved by tuning the etching parameters for (010) homoepitaxial films.

Metal-first non-alloyed ohmic and Schottky contacts are fabricated on β-Ga2O3 with a range of metal work functions (ϕM). The resulting ohmic contacts are of high quality with a contact resistance (Rc) as low as 0.069 ± 0.003 Ω mm. Measurements of the barrier heights (ϕB) indicate that metal-first processing, which preserves the as-grown/bare-substrate surface, also partially un-pins the Fermi-level in (010) and (2¯01) oriented Ga2O3. Depth-resolved XPS (x-ray photoelectron spectroscopy) measurements of the oxidation state throughout the contact metal at the contact–Ga2O3 interface indicate that most non-alloyed contact metals are at least partially oxidized by room temperature redox reactions with the underlying Ga2O3, with metals with a lower ϕM also demonstrating the greatest level of oxidation. As oxidation has been previously observed to enhance a metal’s work function, this may imply that to-date observations of indices of surface behavior << 1 on β-Ga2O3, which have been attributed to severe Fermi-level pinning, may need to be corrected to account for this partial oxidation in addition to other surface modifications during device processing demonstrated in this work.

Magnetic field sensing has the potential to become necessary as a critical tool for long-term subsurface geophysical monitoring. The success of distributed fiber optic sensing for geophysical characterization provides a template for the development of next generation downhole magnetic sensors. In this study, Sentek Instrument’s picoDAS is coupled with a multi-material single mode optical fiber with Metglas^® 2605SC cladding wire inclusions for magnetic field detection. The response of acoustic sensing fibers with one and two Metglas^® 2605SC cladding wires was evaluated upon exposure to lateral AC magnetic fields. An improved response was demonstrated for a sensing fiber with in-cladding wire following thermal magnetic annealing (~400 °C) under a constant static transverse magnetic field (~200 μT). A minimal detectable magnetic field of ~500 nT was confirmed for a sensing fiber with two 10 μm cladding wires. The successful demonstration of a magnetic field sensing fiber with Metglas^® cladding wires fabricated via traditional draw processes sets the stage for distributed measurements and joint inversion as a compliment to distributed fiber optic acoustic sensors.