Record-high electron mobilities were achieved for silicon-doped (010) β-Ga₂O₃ homoepitaxial films grown via metalorganic chemical vapor deposition (MOCVD). Key growth parameters were investigated to reduce the background doping and compensation concentration. Controllable n-type Si doping was achieved as low as low-10¹⁶ cm⁻³. Record carrier mobilities of 184 cm²/V s at room temperature and 4984 cm²/V s at low temperature (45 K) were measured for β-Ga₂O₃ thin films with room-temperature doping concentrations of 2.5 × 10¹⁶ and 2.75 × 10¹⁶ cm⁻³, respectively. Analysis of temperature-dependent Hall mobility and carrier concentration data revealed a low compensation concentration of 9.4 × 10¹⁴ cm⁻³. Using the two-donor model, Si on the tetrahedrally coordinated Ga(I) site represented the primary shallow donor state, and the secondary donor state was found to possess an activation energy of 120 meV. The demonstration of high-purity and high-quality β-Ga₂O₃ thin films with uniform and smooth surface morphology via MOCVD will harness its advantages as an ultrawide-bandgap semiconductor for power electronic and short-wavelength optoelectronic device applications.

(010) β-(Al_xGa_1−x)₂O₃ thin films were grown on (010) β-Ga₂O₃ substrates via metalorganic chemical vapor deposition with up to 40% Al incorporation by systematic tuning of the Trimethylaluminum (TMAl)/triethylgallium molar flow rate ratio and growth temperature. High crystalline quality with pure β-phase (Al_xGa_1−x)₂O₃ was achieved for films with Al composition x < 27%, while a higher Al composition induced phase segregation which was observed via X-ray diffraction spectra. Al incorporation was highly dependent on the growth temperature, chamber pressure, oxygen partial pressure, and TMAl molar flow rate. Atomic resolution scanning transmission electron microscopy (STEM) imaging demonstrated a high crystalline quality β-(Al_0.15Ga_0.85)₂O₃ film with an epitaxial interface. High resolution STEM imaging of (Al_xGa_1−x)₂O₃/Ga₂O₃ superlattice (SL) structures revealed superior crystalline quality for the 23% Al composition. When the Al composition reaches 40%, the SL structure maintained the β-phase, but the interfaces became rough with inhomogeneous Al distribution. N-type doping using Si in β-(Al_xGa_1−x)₂O₃ films with the Al composition up to 33.4% was demonstrated.

This paper investigated the growth of (Al_xGa_1−x)₂O₃ thin films on semi-insulating (010) Ga₂O₃ substrates over the entire Al composition range (0% < x ≤ 100%) via metalorganic chemical vapor deposition (MOCVD). For the Al composition x < 27%, high quality single phase β-(Al_xGa_1−x)₂O₃ was achieved. A mixture of β and γ phases existed in (Al_xGa_1−x)₂O₃ when Al composition ranged between 27% and 40%, whereas a single γ-phase was observed for the films with Al composition x > 40%. The transition from the β to γ phase in AlGaO alloys was observed from x-ray diffraction spectra. The growth of γ-phase AlGaO with higher Al content was further confirmed via atomic resolution scanning transmission electron microscopy imaging and nanodiffraction. Compositional and statistical analyses performed on data acquired from atom probe tomography provided insight on the local compositional homogeneity in AlGaO films with different Al compositions. For AlGaO with pure β or γ phases, the Al composition distribution showed homogeneity with similar Al composition values as extracted from the x-ray diffraction peak positions. For AlGaO films with mixed β and γ phases, inhomogeneity in the Al composition distribution became more obvious in the nm scale. A mechanism was proposed for the observed phase transformation between β and γ phases in MOCVD growth of AlGaO films.

The results of a detailed investigation of electrically active defects in metal-organic chemical vapor deposition (MOCVD)-grown β-Ga₂O₃ (010) epitaxial layers are described. A combination of deep level optical spectroscopy (DLOS), deep level transient (thermal) spectroscopy (DLTS), and admittance spectroscopy (AS) is used to quantitatively map the energy levels, cross sections, and concentrations of traps across the entire ∼4.8 eV bandgap. States are observed at E_C-0.12 eV by AS; at E_C-0.4 eV by DLTS; and at E_C-1.2 eV, E_C-2.0 eV, and E_C-4.4 eV by DLOS. While each of these states have been reported for β-Ga₂O₃ grown by molecular-beam epitaxy (MBE) and edge-defined film fed grown (EFG), with the exception of the E_C-0.4 eV trap, there is both a significantly different distribution in the concentration of these states and an overall ∼10× reduction in the total trap concentration. This reduction is consistent with the high mobility and low background compensating acceptor concentrations that have been reported for MOCVD-grown (010) β-Ga₂O₃. Here, it is observed that the E_C-0.12 eV state dominates the overall trap concentration, in marked contrast with prior studies of EFG and MBE material where the state at E_C-4.4 eV has dominated the trap spectrum. This sheds light on possible physical sources for this ubiquitous DLOS feature in β-Ga₂O₃. The substantial reduction in trap concentration for MOCVD material implies great promise for future high performance MOCVD-grown β-Ga₂O₃ devices.

Single α-phase (Al_xGa_1−x)₂O₃ thin films are grown on m-plane sapphire (α-Al₂O₃) substrates via metalorganic chemical vapor deposition. By systematically tuning the growth parameters including the precursor molar flow rates, chamber pressure, and growth temperature, the epitaxial growth of high-quality phase pure α-(Al_xGa_1−x)₂O₃ films (0 ≤ x ≤ 1) is demonstrated with smooth surface morphologies and alloy homogeneities by comprehensive material characterization. The asymmetrical reciprocal space mapping reveals fully relaxed films for α-(Al_xGa_1−x)₂O₃ films with x ≤ 0.5. The coherent growth of α-(Al_xGa_1−x)₂O₃/α-Al₂O₃ superlattice structures is demonstrated with abrupt interfaces and uniform Al distribution for higher Al compositions at x = 0.78 in the α-(Al_xGa_1−x)₂O₃ layer. The influence of growth parameters, such as growth temperature and chamber pressure, on the phase stabilization and Al incorporation in the α-(Al_xGa_1−x)₂O₃ films is investigated. While lower growth temperatures facilitate the phase stabilization of α-Ga₂O₃ thin films, lower chamber pressure leads to higher Al incorporation in α-(Al_xGa_1−x)₂O₃ films. High resolution x-ray photoelectron spectroscopy was utilized for determining the Al compositions and bandgaps of α-(Al_xGa_1−x)₂O₃. Furthermore, the evolution of the valance and conduction band offsets at α-Al₂O₃/α-(Al_xGa_1−x)₂O₃ heterojunctions is evaluated with the variation of Al compositions, which reveals the formation of type-I (straddling) band alignment between α-Al₂O₃ and α-(Al_xGa_1−x)₂O₃.