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.

This paper investigated the growth of (AlxGa1−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-(AlxGa1−x)2O3 was achieved. A mixture of β and γ phases existed in (AlxGa1−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.

A new record-high room temperature electron Hall mobility (µ RT = 194 cm 2 /V•s at n ~ 8×10 15 cm -3 ) for β-Ga 2 O 3 is demonstrated in the unintentionally doped thin film grown on (010) semi-insulating substrate via metalorganic chemical vapor deposition (MOCVD). A peak electron mobility of ~9500 cm 2 /V•s is achieved at 45 K. Further investigation on the transport properties indicate the existence of sheet charges near the epi-layer/substrate interface. Si is identified as the primary contributor to the background carrier in both the epi-layer and the interface, originated from both surface contamination as well as growth environment. Pre-growth hydrofluoric acid cleaning of the substrate lead to an obvious decrease of Si impurity both at interface and in epi-layer. In addition, the effect of MOCVD growth condition, particularly the chamber pressure, on the Si impurity incorporation is studied. A positive correlation between the background charge concentration and the MOCVD growth pressure is confirmed. It is noteworthy that in a β-Ga 2 O 3 film with very low bulk charge concentration, even a reduced sheet charge density can play an important role in the charge transport properties.

(010) β-(AlxGa1−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 (AlxGa1−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-(Al0.15Ga0.85)2O3 film with an epitaxial interface. High resolution STEM imaging of (AlxGa1−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-(AlxGa1−x)2O3 films with the Al composition up to 33.4% was demonstrated.

Single β-phase (100) (Al x Ga 1−x ) 2 O 3 thin films were successfully grown on (100) oriented β-Ga 2 O 3 substrates via metalorganic chemical vapor deposition (MOCVD). By systematically tuning the precursor molar flow rates and growth conditions including chamber pressure, growth temperature and group VI/III molar ratio, pure β-phase (100) (Al x Ga 1−x ) 2 O 3 films with up to 52% of Al compositions were achieved. Comprehensive material characterization via X-ray diffraction (XRD) and highresolution scanning transmission electron microscopy (HR-STEM) revealed high quality epitaxial growth of (100) β-(Al x Ga 1−x ) 2 O 3 films on (100) native substrates. High resolution X-ray spectroscopy (XPS) was used for determining the AlGaO bandgaps and the Al compositions. Two-dimensional defects in the β-(Al x Ga 1−x ) 2 O 3 films were investigated utilizing atomic resolution STEM imaging. Additionally, film characterization via HR-STEM imaging, XRD, and energy-dispersive X-ray spectroscopy (STEM-EDS) revealed coherent growth of high quality (100) β-(Al x Ga 1−x ) 2 O 3 /Ga 2 O 3 superlattice (SL) structures (x ≤ 50%) with abrupt interfaces and relatively uniform Al distribution.Step flow growth of (100) β-(Al x Ga 1−x ) 2 O 3 with smooth and featureless surface morphology was observed in AlGaO samples with high-Al compositions. A mechanism for the stepflow growth of high-Al content β-(Al x Ga 1−x ) 2 O 3 film is proposed by considering Al adatoms as preferred incorporation sites for AlGaO nucleation and growth.