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  1. Epitaxial growth of κ-phase Ga 2 O 3 thin films is investigated on c-plane sapphire, GaN- and AlN-on-sapphire, and (100) oriented yttria stabilized zirconia (YSZ) substrates via metalorganic chemical vapor deposition. The structural and surface morphological properties are investigated by comprehensive material characterization. Phase pure κ-Ga 2 O 3 films are successfully grown on GaN-, AlN-on-sapphire, and YSZ substrates through a systematical tuning of growth parameters including the precursor molar flow rates, chamber pressure, and growth temperature, whereas the growth on c-sapphire substrates leads to a mixture of β- and κ-polymorphs of Ga 2 O 3 under the investigated growth conditions. The influence of the crystalline structure, surface morphology, and roughness of κ-Ga 2 O 3 films grown on different substrates are investigated as a function of precursor flow rate. High-resolution scanning transmission electron microscopy imaging of κ-Ga 2 O 3 films reveals abrupt interfaces between the epitaxial film and the sapphire, GaN, and YSZ substrates. The growth of single crystal orthorhombic κ-Ga 2 O 3 films is confirmed by analyzing the scanning transmission electron microscopy nanodiffraction pattern. The chemical composition, surface stoichiometry, and bandgap energies of κ-Ga 2 O 3 thin films grown on different substrates are studied by high-resolution x-ray photoelectron spectroscopy (XPS) measurements. The type-II (staggered) band alignments at three interfaces between κ-Ga 2 O 3 and c-sapphire, AlN, and YSZ substrates are determined by XPS, with an exception of κ-Ga 2 O 3 /GaN interface, which shows type-I (straddling) band alignment. 
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  2. The in situ metalorganic chemical vapor deposition (MOCVD) growth of Al 2 O 3 dielectrics on β-Ga 2 O 3 and β-(Al x Ga 1−x ) 2 O 3 films is investigated as a function of crystal orientations and Al compositions of β-(Al x Ga 1−x ) 2 O 3 films. The interface and film qualities of Al 2 O 3 dielectrics are evaluated by high-resolution x-ray diffraction and scanning transmission electron microscopy imaging, which indicate the growth of high-quality amorphous Al 2 O 3 dielectrics with abrupt interfaces on (010), (100), and [Formula: see text] oriented β-(Al x Ga 1−x ) 2 O 3 films. The surface stoichiometries of Al 2 O 3 deposited on all orientations of β-(Al x Ga 1−x ) 2 O 3 are found to be well maintained with a bandgap energy of 6.91 eV as evaluated by high-resolution x-ray photoelectron spectroscopy, which is consistent with the atomic layer deposited (ALD) Al 2 O 3 dielectrics. The evolution of band offsets at both in situ MOCVD and ex situ ALD deposited Al 2 O 3 /β-(Al x Ga 1−x ) 2 O 3 is determined as a function of Al composition, indicating the influence of the deposition method, orientation, and Al composition of β-(Al x Ga 1−x ) 2 O 3 films on resulting band alignments. Type II band alignments are determined at the MOCVD grown Al 2 O 3 /β-(Al x Ga 1−x ) 2 O 3 interfaces for the (010) and (100) orientations, whereas type I band alignments with relatively low conduction band offsets are observed along the [Formula: see text] orientation. The results from this study on MOCVD growth and band offsets of amorphous Al 2 O 3 deposited on differently oriented β-Ga 2 O 3 and β-(Al x Ga 1−x ) 2 O 3 films will potentially contribute to the design and fabrication of future high-performance β-Ga 2 O 3 and β-(Al x Ga 1−x ) 2 O 3 based transistors using MOCVD in situ deposited Al 2 O 3 as a gate dielectric. 
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  3. In this work, β-Ga 2 O 3 fin field-effect transistors (FinFETs) with metalorganic chemical vapor deposition grown epitaxial Si-doped channel layer on (010) semi-insulating β-Ga 2 O 3 substrates are demonstrated. β-Ga 2 O 3 fin channels with smooth sidewalls are produced by the plasma-free metal-assisted chemical etching (MacEtch) method. A specific on-resistance (R on,sp ) of 6.5 mΩ·cm 2 and a 370 V breakdown voltage are achieved. In addition, these MacEtch-formed FinFETs demonstrate DC transfer characteristics with near zero (9.7 mV) hysteresis. The effect of channel orientation on threshold voltage, subthreshold swing, hysteresis, and breakdown voltages is also characterized. The FinFET with channel perpendicular to the [102] direction is found to exhibit the lowest subthreshold swing and hysteresis. 
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  4. In this work, the structural and electrical properties of metalorganic chemical vapor deposited Si-doped β-(Al x Ga 1−x ) 2 O 3 thin films grown on (010) β-Ga 2 O 3 substrates are investigated as a function of Al composition. The room temperature Hall mobility of 101 cm 2 /V s and low temperature peak mobility (T = 65 K) of 1157 cm 2 /V s at carrier concentrations of 6.56 × 10 17 and 2.30 × 10 17  cm −3 are measured from 6% Al composition samples, respectively. The quantitative secondary ion mass spectroscopy (SIMS) characterization reveals a strong dependence of Si and other unintentional impurities, such as C, H, and Cl concentrations in β-(Al x Ga 1−x ) 2 O 3 thin films, with different Al compositions. Higher Al compositions in β-(Al x Ga 1−x ) 2 O 3 result in lower net carrier concentrations due to the reduction of Si incorporation efficiency and the increase of C and H impurity levels that act as compensating acceptors in β-(Al x Ga 1−x ) 2 O 3 films. Lowering the growth chamber pressure reduces Si concentrations in β-(Al x Ga 1−x ) 2 O 3 films due to the increase of Al compositions as evidenced by comprehensive SIMS and Hall characterizations. Due to the increase of lattice mismatch between the epifilm and substrate, higher Al compositions lead to cracking in β-(Al x Ga 1−x ) 2 O 3 films grown on β-Ga 2 O 3 substrates. The (100) cleavage plane is identified as a major cracking plane limiting the growth of high-quality Si-doped (010) β-(Al x Ga 1−x ) 2 O 3 films beyond the critical thicknesses, which leads to highly anisotropic and inhomogeneous behaviors in terms of conductivity. 
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  5. null (Ed.)