GOX 2022 Session TM-MoA: Characterization & Modelling II

The first reports of Ga₂O₃ MESFETs and MOSFETs by the group of Higashiwaki demonstrated the potential of β-Ga₂O₃ for high breakdown voltage, low on-resistance power electronics due to its ultra-wide bandgap and large breakdown electric field. Realizing that potential requires development of high-quality β-Ga₂O₃ material, best guided by an understanding of the electronic transport properties which directly correlate to device performance. In this talk, through a combination of temperature dependent Hall effect and admittance spectroscopy measurements, I will begin by presenting our work characterizing electrically active defects which may ultimately limit the maximally achievable breakdown voltages in Ga₂O₃ devices. Following that, I will present analysis of transport in plasma-MBE grown β-Ga₂O₃ produced at Air Force Research Laboratory, towards understanding the contributions of various scattering mechanisms limiting the electron mobility in our films. Informed by transport studies such as these, material growers and device engineers can continue to push β-Ga₂O₃ to the limits of its performance.

A good radiation hardness of Ga₂O₃ has been suggested, though its susceptibility to radiation damage is higher than in GaN. To better assess gallium oxide device reliability for nuclear and space applications, more understanding of the structural changes in the material as a result of irradiation is needed. We propose a model for the structural deformation of beta gallium oxide under ion irradiation. Assuming displacements confined primarily to the Ga-atom sublattice, we explain the main features of TEM diffraction patterns from in-situ irradiated gallium oxide using 400 eV Ar ions of fluence 4 x 10¹⁵ cm^-1 (equivalent to 2 displacements per atom) (Fig. 1). We propose that displacements of gallium atoms are confined between close-packed O-atom layers (which in beta gallium oxide exist parallel to the (101), (-201), (-3-10), (-310) planes) with a preference for octahedral interstitial positions and recombination. We thus demonstrate the anisotropic evolution of the octahedral-to-tetrahedral Ga-site ratio in the irradiated gallium oxide as a function of displacements per atom, finding it to increase the most along the [-3-10] direction and the least along the [-201] (Fig. 2). The similarity of the structure post irradiation with that of kappa and alpha gallium oxide is examined. We conclude that while the structure post irradiation shares some features similar to kappa gallium oxide (specifically the octahedral-to-tetrahedral site ratio), ion irradiation does not cause a phase transition of beta gallium oxide into kappa as previously thought (Fig. 3).

The authors gratefully acknowledge the NSUF funding (#1393) for beamtime at Argonne IVEM-Tandem User Facility. The authors also thank Mr Peter Baldo (ANL, USA) for dedication on the operation of the ion accelerator during the experiment.

Ga₂O₃ is a candidate for development of power electronics components that are faster, smaller and more energy efficient than Si-based technology, permitting devices capable of operating at higher voltages, frequencies and temperatures.[1] The κ-phase of Ga₂O₃ (sometimes labeled ε-phase) is a meta-stable orthorhombic phase where large spontaneous polarization has been predicted by density functional theory.[2] By partial substitution of Ga by In or Al, the original bandgap (~4.9 eV) can be decreased or increased, in principle within the range spanned by the bandgaps of In₂O₃ (2.9 eV) and Al₂O₃ (8.8 eV).This provides a wide parameter space for device development through tailoring of properties such as bandgaps and band offsets. It is believed that interface-localized two-dimensional electron gas (2DEG) may be achieved within this materials system, which opens for potential applications in so-called high-electron-mobility transistors (HEMTs).

In the present work the band structure of thin film κ-(In_xGa_1-x)₂O₃/κ-(Al_yGa_1-y)₂O₃ heterostructures are investigated experimentally to evaluate if formation of 2DEG can be within reach. Samples with a selection of x and y compositions were fabricated by Pulsed Laser Deposition (PLD) on sapphire substrates. Both synchrotron and in-house X-ray Photoelectron Spectroscopy (XPS) were used to investigate the position of the valence band edges relative to the Fermi level in the heterostructure layers and corresponding reference samples. Extraction of valence band maxima from XPS was aided by Density functional theory (DFT) calculations. Local bandgap information was provided by Scanning Transmission Electron Microscope Electron Energy Loss Spectroscopy (STEM EELS), which can determine wide bandgaps with a spatial resolution < 10 nm. Valence band offsets across the heterojunctions were obtained by XPS and combined with bandgap information to find the corresponding conduction band offsets and provide a comprehensive overview of band discontinuities across κ-(In_xGa_1-x)₂O₃/κ-(Al_yGa_1-y)₂O₃ interfaces.

High aluminum content striations were observed in (-201) (Al_xGa_1-x)₂O₃ thin films (~500 nm) grown on (0001) and 6° miscut (0001) sapphire substrates. A modulated Al composition structure was observed whose orientation depended on the substrate miscut. High resolution x-ray diffraction (XRD) and transmission electron microscopy (TEM) were used to investigate the structural and chemical properties of the (Al_xGa_1-x)₂O₃ films (with 0<x<0.3) and the in-plane relationship with the underlying sapphire substrate. (-201) (Al_xGa_1-x)₂O₃ films were grown by metalorganic vapor phase epitaxy and the Al composition was controlled by tuning the ratio of trimethylaluminum to triethylgallium flow. The growth was carried out at a substrate temperature of 810 °C and a reduced growth pressure of 15 Torr to minimize Al precursor prereactions.

Scanning transmission electron microscopy measurements (sensitive to Z-contrast) reveal alternating layers of high and low contrasting features throughout the (Al_xGa_1-x)₂O₃ film. In conjunction with energy dispersive spectroscopy, the striations are identified as regions of high Al content. In the films that were grown on c-plane sapphire, the high Al content striations run parallel to the (-201) surface. However, in the case of the 6° miscut sapphire substrates, the high Al content striations are oriented about 8-10° from the surface. In addition, the average period of the striations is smaller with higher Al content ranging from 25 to 7 nm periods for x = 0.04 and x = 0.3 respectively.

XRD (-401) pole figures were measured for (Al_xGa_1-x)₂O₃ films (with 0<x<0.3) and a commercially available (-201) β-Ga₂O₃ substrate as a reference. XRD (-401) pole figure for the (-201) β-Ga₂O₃ reference substrate shows both (-401) at χ = ~23° and (-400), at χ = ~50° (the 2θ angle difference is less than 0.5°) with no symmetry. However, the pole figures from the (Al_xGa_1-x)₂O₃ thin films grown on sapphire (0<x<0.3) all show six-fold symmetry for both (-401) and (-400). We believe the six-fold symmetry is a result of three sets of twins (120° away from each other), plus the existence of anti-parallel domains (0° and 180° pairs). On the other hand, additional 12 spots at χ = ~58° were observed in the (Al_xGa_1-x)₂O₃ films on sapphire. These do not correspond to any planes with (-201) surface orientation, suggesting grains with different orientation exist. We speculate that the anisotropy of the monoclinic structures, the surface energy differences associated with the miscut substrate, and step edge features impact the formation of the composition striations.