Metal-Exchange Catalysis in the Growth of Sesquioxides: Towards Heterostructures of Transparent Oxide Semiconductors

Molecular Beam Epitaxy of β-(InxGa1-x)2O3 on β-Ga2O3 (010): Compositional Control, Layer Quality, Anisotropic Strain Relaxation, and Prospects for Two-Dimensional Electron Gas Confinement

In this work, we investigate the growth of monoclinic β-(InxGa1-x)2O3 alloys on top of (010) β-Ga2O3 substrates via plasma-assisted molecular beam epitaxy. In particular, using different in situ (reflection high-energy electron diffraction) and ex situ (atomic force microscopy, X-ray diffraction, time-of-flight secondary ion mass spectrometry, and transmission electron microscopy) characterization techniques, we discuss (i) the growth parameters that allow for In incorporation and (ii) the obtainable structural quality of the deposited layers as a function of the alloy composition. In particular, we give experimental evidence of the possibility of coherently growing (010) β-(InxGa1-x)2O3 layers on β-Ga2O3 with good structural quality for x up to ≈ 0.1. Moreover, we show that the monoclinic structure of the underlying (010) β-Ga2O3 substrate can be preserved in the β-(InxGa1-x)2O3 layers for wider concentrations of In (x ≤ 0.19). Nonetheless, the formation of a large amount of structural defects, like unexpected (10 2 ̅ ) oriented twin domains and partial segregation of In is suggested for x > 0.1. Strain relaxes anisotropically, maintaining an elastically strained unit cell along the a* direction vs plastic relaxation along the c* direction. This study provides important guidelines for the low-end side tunability of the energy bandgap of β-Ga2O3-based alloys and provides an estimate of its potential in increasing the confined carrier concentration of two-dimensional electron gases in β-(InxGa1-x)2O3/(AlyGa1-y)2O3 heterostructures.

Observing the microstructure of (001) κ-Ga2O3 thin film grown on a (-201) β-Ga2O3 substrate using automated crystal orientation mapping transmission electron microscopy

We grew (001) κ-Ga2O3 thin films on (-201) β-Ga2O3 substrates using mist chemical vapor deposition. X-ray diffraction analysis revealed that the thin films grown at 450–800 °C showed (004) κ-Ga2O3...

Sn-Induced Phase Stabilization and Enhanced Thermal Stability of κ-Ga2O3 Grown by Mist Chemical Vapor Deposition

Tin (Sn)-doped orthorhombic gallium oxide (κ-Ga₂O₃) films were grown on (0001) sapphire by mist chemical vapor deposition. It is known that κ-Ga₂O₃ is more stable than α-Ga₂O₃ (corundum) but less stable than β-Ga₂O₃ (monoclinic). This thermodynamic stability means an optimal growth temperature (T _g) of the κ-phase (600-650 °C) is also in between the two. At first, it was observed that Sn doping induced the κ-phase during the growth of the β-phase (T _g = 700 °C). Interestingly, Sn could also promote the κ-phase even under the growth condition that strongly favors the α-phase (T _g = 450 °C). The postgrowth annealing tests at 800-1000 °C showed that the thermal stability of the κ-phase depends on the Sn concentration. The higher the Sn concentration, the more stable the phase. The one with the highest Sn content showed no phase transition from κ to β after annealing at 800, 900, and 1000 °C for 30 min each. This enhancement of thermal stability promises more reliable high-power and high-frequency devices for which κ-Ga₂O₃ is suitable. Although there was no correlation between Sn-induced phase stabilization and the crystal quality, cathodoluminescence revealed that increasing Sn concentration led to the strong suppression of the radiative recombination at 340 nm from the vacancy-related donor-acceptor pairs. This observation suggests that the phase stabilization by Sn could be related to a specific Ga site Sn replaces in the orthorhombic structure.

Single-Domain and Atomically Flat Surface of κ-Ga2O3 Thin Films on FZ-Grown ε-GaFeO3 Substrates via Step-Flow Growth Mode

Herein, single-domain κ-Ga₂O₃ thin films were grown on FZ-grown ε-GaFeO₃ substrates via a step-flow growth mode. The ε-GaFeO₃ possessing the same crystal structure and similar lattice parameters as those of the orthorhombic κ-Ga₂O₃ facilitated the growth of κ-Ga₂O₃ thin films, as observed by the X-ray diffraction (XRD) analysis. Furthermore, the surface morphologies of the κ-Ga₂O₃ thin films exhibited a step-terrace and atomically flat structure. XRD φ-scan and transmission electron microscopy with selected area electron diffraction revealed that there is no occurrence of in-plane rotational domains in the κ-Ga₂O₃ thin films on ε-GaFeO₃ substrates and that the κ-Ga₂O₃ thin film comprised a single domain. TEM analysis revealed that there were no clear dislocations in the observation area. Moreover, high-resolution TEM observation showed that the atomic arrangements of the film and the substrate were continuous without the presence of an intermediate layer along the growth direction and were well-aligned in the in-plane direction.

Band Offsets at κ-([Al,In]xGa1-x)2O3/MgO Interfaces

Conduction and valence band offsets are among the most crucial material parameters for semiconductor heterostructure device design, such as for high-electron mobility transistors or quantum well infrared photodetectors (QWIP). Because of its expected high spontaneous electrical polarization and the possibility of polarization doping at heterointerfaces similar to the AlGaN/InGaN/GaN system, the metastable orthorhombic κ-phase of Ga₂O₃ and its indium and aluminum alloy systems are a promising alternative for such device applications. However, respective band offsets to any dielectric are unknown, as well as the evolution of the bands within the alloy systems. We report on the valence and conduction band offsets of orthorhombic κ-(Al_xGa_1-x)₂O₃ and κ-(In_xGa_1-x)₂O₃ thin films to MgO as reference dielectric by X-ray photoelectron spectroscopy. The thin films with compositions x_In ≤ 0.27 and x_Al ≤ 0.55 were grown by pulsed laser deposition utilizing tin-doped and radially segmented targets. The determined band alignments reveal the formation of a type I heterojunction to MgO for all compositions with conduction band offsets of at least 1.4 eV, providing excellent electron confinement. Only low valence band offsets with a maximum of ∼300 meV were observed. Nevertheless, this renders MgO as a promising gate dielectric for metal-oxide-semiconductor transistors in the orthorhombic modification. We further found that the conduction band offsets in the alloy systems are mainly determined by the evolution of the band gaps, which can be tuned by the composition in a wide range between 4.1 and 6.2 eV, because the energy position of the valence band maximum remains almost constant over the complete composition range investigated. Therefore, tunable conduction band offsets of up to 1.1 eV within the alloy systems allow for subniveau transition energies in (Al_xGa_1-x)₂O₃/(In_xGa_1-x)₂O₃/(Al_xGa_1-x)₂O₃ quantum wells from the infrared to the visible regime, which are promising for application in QWIPs.

Semicoherent growth of single-crystal β-In2S3 layers on InP(111) and InAs(111)

The (103) surface of β-In₂S₃ consists of a purely accidental hexagonal-like periodicity despite the tetragonal crystal structure of β-In₂S₃. β-In₂S₃ layers grow, as a consequence, semicoherently on the (111) surfaces of InP and InAs due to small lattice mismatches.