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Understanding the unique properties of ultra-wide band gap semiconductors requires detailed information about the exact nature of point defects and their role in determining the properties. Here, we report the first direct microscopic observation of an unusual formation of point defect complexes within the atomic-scale structure of β-Ga2O3 using high resolution scanning transmission electron microscopy (STEM). Each complex involves one cation interstitial atom paired with two cation vacancies. These divacancy-interstitial complexes correlate directly with structures obtained by density functional theory, which predicts them to be compensating acceptors in β-Ga2O3. This prediction is confirmed by a comparison between STEM data and deep level optical spectroscopy results, which reveals that these complexes correspond to a deep trap within the band gap, and that the development of the complexes is facilitated by Sn doping through increased vacancy concentration. These findings provide new insight on this emerging material's unique response to the incorporation of impurities that can critically influence their properties.

Atomic scale details of the formation of point defects and their evolution to phase transformation in silicon (Si) implanted β-Ga2O3 were studied using high resolution scanning transmission electron microscopy (STEM). The effect of Si implantation and the formation of defects was studied as a function of the dose of implanted atoms, and the detailed mechanism of lattice recovery was observed using both in situ and ex situ annealing of the implanted β-Ga2O3. The implantation created nanoscale dark spots in STEM images, which we identified as local γ-Ga2O3 inclusions generated by the relaxation of lattice due to ⟨010⟩ screw dislocations created by the implantation. The number and size of γ-Ga2O3 regions increased as the Si dose increased, and eventually the γ-Ga2O3 crystal phase (with stacking defects) took over the entire implanted volume when the peak Si concentration was over ∼1020 cm−3. Annealing above 1100 °C disintegrates the local γ-Ga2O3 phase and returns the structure to defect-free, single crystal β phase, likely indicating that point defects (such as Si interstitials and cation vacancies) are spatially redistributed by the annealing. However, when the structure is completely transformed to γ-Ga2O3 by the implantation, post-annealing leaves a high concentration of dislocations within the β phase, which relates to the inhomogeneous distribution of Si atoms detected by secondary ion mass spectrometry. [ABSTRACT FROM AUTHOR]

Large area epitaxy of two-dimensional (2D) layered materials with high material quality is a crucial step in realizing novel device applications based on 2D materials. In this work, we report highquality, crystalline, large-area gallium selenide (GaSe) films grown on bulk substrates such as c-plane sapphire and gallium nitride (GaN) using a valved cracker source for Se. (002)-Oriented GaSe with random in-plane orientation of domains was grown on sapphire and GaN substrates at a substrate temperature of 350-450 °C with complete surface coverage. Higher growth temperature (575 °C) resulted in the formation of single-crystalline ε-GaSe triangular domains with six-fold symmetry confirmed by in-situ reflection high electron energy diffraction and off-axis x-ray diffraction. A two-step growth method involving high temperature nucleation of single crystalline domains and low temperature growth to enhance coalescence was adopted to obtain continuous (002)-oriented GaSe with an epitaxial relationship with the substrate. While six-fold symmetry was maintained in the two step growth, β-GaSe phase was observed in addition to the dominant ε-GaSe in cross-sectional scanning transmission electron microscopy images. This work demonstrates the potential of growing high quality 2D-layered materials using molecular beam epitaxy and can be extended to the growth of other transition metal chalcogenides. [ABSTRACT FROM AUTHOR]

The valence and conduction band offsets at (100) β-(AlxGa1−x)2O3/β-Ga2O3 heterointerfaces with the increasing Al composition are determined via x-ray photoelectron spectroscopy. The (100) β-(AlxGa1−x)2O3 thin films with the Al composition of 0.10 < x < 0.52 are grown on (100) β-Ga2O3 substrates by the metalorganic chemical vapor deposition method. By examining the onset of inelastic energy loss in core-level atomic spectra, the bandgaps of β-Ga2O3 and β-(AlxGa1−x)2O3 alloys with different Al compositions are measured from 4.83 ± 0.12 eV (x = 0) to 5.85 ± 0.08 eV (x = 0.52). The valence band offsets are determined to be −0.06 ± 0.06 eV (x = 0.10), −0.11 ± 0.06 eV (x = 0.33), and −0.19 ± 0.06 eV (x = 0.52). The conduction band offsets of 0.34 ± 0.17 eV (x = 0.10), 0.62 ± 0.17 eV (x = 0.33), and 1.21 ± 0.16 eV (x = 0.52) are determined from the extracted bandgaps of β-(AlxGa1−x)2O3 alloys. The determined band alignments at β-(AlxGa1−x)2O3/β-Ga2O3 interfaces reveal the formation of type-II (staggered gap) heterojunctions for all Al compositions investigated. The bowing parameters obtained from the quadratic fitting of both conduction band minimum and valence band maximum values are estimated to be 1.25 eV and 0.005 eV, respectively. [ABSTRACT FROM AUTHOR]

β-(AlxGa1−x)2O3 thin films are grown on ( 2 ¯ 01) β-Ga2O3 substrates via metal organic chemical vapor deposition to investigate the solubility of Al in β-phase Ga2O3. The x-ray diffraction (XRD) spectra reveal crystalline quality ( 2 ¯ 01) β-(AlxGa1−x)2O3 thin films with Al compositions up to 48%. The Al compositions are further confirmed by high resolution x-ray spectroscopy measurements and energy-dispersive x-ray spectra (EDS) mapping. The bandgap energies extracted from XPS spectra range between 5.20 ± 0.06 eV and 5.72 ± 0.08 eV for x = 21%–48%. The surface morphology evaluated by both scanning electron microscopy and atomic force microscopy shows elongated features with granules along the [010] direction, which are suppressed with the increasing Al content. A systematic growth study through tuning growth parameters indicates that the chamber pressure plays an important role in both surface morphology and Al incorporation. Material characterization via high resolution scanning transmission electron microscopy and STEM-EDS reveals Al fluctuations in the sample with the 48% Al composition. Atomic resolution STEM imaging and XRD spectra for ( 2 ¯ 01) β-(AlxGa1−x)2O3/Ga2O3 superlattice structures confirm that the periodicity of the β-(AlxGa1−x)2O3/Ga2O3 sub-layers is well maintained with high-Al compositions. [ABSTRACT FROM AUTHOR]

We investigated atomic scale chemical heterogeneity in β-(AlxGa1−x)2O3 thin films with different aluminum (Al) concentrations using atom probe tomography. Two film samples with an Al molar concentration of x = 0.2 and x = 0.5, grown by plasma assisted molecular beam epitaxy, were analyzed and compared. The measured overall compositions were found to be in agreement with the target compositions in both cases. The film with the higher Al content showed a significant chemical heterogeneity, which could be attributed to the low growth temperature. [ABSTRACT FROM AUTHOR]

(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. [ABSTRACT FROM AUTHOR]