Subrina Rafique

Homoepitaxial growth of β-Ga2O3 thin films by low pressure chemical vapor deposition

This paper presents the homoepitaxial growth of phase pure (010) β-Ga2O3 thin films on (010) β-Ga2O3 substrate by low pressure chemical vapor deposition. The effects of growth temperature on the surface morphology and crystal quality of the thin films were systematically investigated. The thin films were synthesized using high purity metallic gallium (Ga) and oxygen (O2) as precursors for gallium and oxygen, respectively. The surface morphology and structural properties of the thin films were characterized by atomic force microscopy, X-ray diffraction, and high resolution transmission electron microscopy. Material characterization indicates the growth temperature played an important role in controlling both surface morphology and crystal quality of the β-Ga2O3 thin films. The smallest root-mean-square surface roughness of ∼7 nm was for thin films grown at a temperature of 950 °C, whereas the highest growth rate (∼1.3 μm/h) with a fixed oxygen flow rate was obtained for the epitaxial layers grown at 850 °C.

Heteroepitaxy of N-type β-Ga2O3 thin films on sapphire substrate by low pressure chemical vapor deposition

This paper presents the heteroepitaxial growth of ultrawide bandgap β-Ga2O3 thin films on c-plane sapphire substrates by low pressure chemical vapor deposition. N-type conductivity in silicon (Si)-doped β-Ga2O3 films grown on sapphire substrate is demonstrated. The thin films were synthesized using high purity metallic gallium (Ga) and oxygen (O2) as precursors. The morphology, crystal quality, and properties of the as-grown thin films were characterized and analyzed by field emission scanning electron microscopy, X-ray diffraction, electron backscatter diffraction, photoluminescence and optical, photoluminescence excitation spectroscopy, and temperature dependent van der Pauw/Hall measurement. The optical bandgap is ∼4.76 eV, and room temperature electron mobility of 42.35 cm2/V s was measured for a Si-doped heteroepitaxial β-Ga2O3 film with a doping concentration of 1.32 × 1018 cm−3.

Low-pressure CVD-grown β-Ga2O3 bevel-field-plated Schottky barrier diodes

We report (010)-oriented β-Ga2O3 bevel-field-plated mesa Schottky barrier diodes grown by low-pressure chemical vapor deposition (LPCVD) using a solid Ga precursor and O2 and SiCl4 sources. Schottky diodes with good ideality and low reverse leakage were realized on the epitaxial material. Edge termination using beveled field plates yielded a breakdown voltage of −190 V, and maximum vertical electric fields of 4.2 MV/cm in the center and 5.9 MV/cm at the edge were estimated, with extrinsic R ON of 3.9 mΩcm2 and extracted intrinsic R ON of 0.023 mΩcm2. The reported results demonstrate the high quality of homoepitaxial LPCVD-grown β-Ga2O3 thin films for vertical power electronics applications, and show that this growth method is promising for future β-Ga2O3 technology.

Temperature and doping concentration dependence of the energy band gap in β-Ga 2 O 3 thin films grown on sapphire

This paper presents the effects of temperature and n-type doping concentration on the energy band gap of β-Ga2O3 thin films grown on c-plane sapphire substrates by low pressure chemical vapor deposition (LPCVD). The β-Ga2O3 thin films were grown using high purity gallium (Ga) and oxygen (O2) as precursors, and Si as the n-type dopant. The transmission electron microscopy (TEM) diffraction pattern showed that the thin films are single crystals that have a monoclinic crystal structure. The dependence of the energy band gap on temperature and n-type doping concentration have been experimentally determined from photoluminescence excitation (PLE) and absorbance spectra. The PLE spectra were measured in the temperature range of 77-298 K. The results indicate that both temperature and carrier concentration play important roles in determining the energy band gap of β-Ga2O3 thin films. The optical gap increased with the electron concentration for ne ≤ 2.52x1018 cm−3, which is due to the dominant Burstein-Moss (BM) shift. The sudden decrease in the energy gap at a doping concentration of 6.23x1018 – 3.05x1019 cm−3 is consistent with the theoretical prediction of Mott criterion for Ga2O3 semiconductor-metal transition. The energy band gap shrinks with an increasing temperature from 77 to 298 K.

Donors and deep acceptors in β-Ga2O3

We have studied the properties of Si, Ge shallow donors and Fe, Mg deep acceptors in β-Ga2O3 through temperature dependent van der Pauw and Hall effect measurements of samples grown by a variety of methods, including edge-defined film-fed, Czochralski, molecular beam epitaxy, and low pressure chemical vapor deposition. Through simultaneous, self-consistent fitting of the temperature dependent carrier density and mobility, we are able to accurately estimate the donor energy of Si and Ge to be 30 meV in β-Ga2O3. Additionally, we show that our measured Hall effect data are consistent with Si and Ge acting as typical shallow donors, rather than shallow DX centers. The high temperature Hall effect measurement of Fe doped β-Ga2O3 indicates that the material remains weakly n-type even with the Fe doping, with an acceptor energy of 860 meV relative to the conduction band for the Fe deep acceptor. Van der Pauw measurements of Mg doped Ga2O3 indicate an activation energy of 1.1 eV, as determined from the temperature dependent conductivity.

Synthesis and characterization of Ga2O3 nanosheets on 3C-SiC-on-Si by low pressure chemical vapor deposition

This study presents the synthesis of single crystalline β-Ga2O3 nanosheets on SiC by low pressure chemical vapor deposition. High purity gallium (Ga) metal and oxygen as source materials and argon as carrier gas were utilized for the synthesis of the nanosheets on a 3C-SiC-on-Si substrate. These single-crystal Ga2O3 nanosheets are free-standing 2D extrusions from their 1D rods, typically 1.5–7 μm in lateral size and 20–140 nm in thickness, featuring aspect ratios ranging from ∼10 to 350. Structural studies based on transmission electron microscopy and Raman spectroscopy revealed the monoclinic phase of Ga2O3 with a single crystalline nature. High resolution transmission electron microscopy with a selected area electron diffraction pattern recorded on a single β-Ga2O3 nanosheet further confirmed their single crystalline nature, with a growth direction perpendicular to (111) crystallographic plane. The growth process governing the formation of these nanosheets is a vapor-solid growth mechanism since no meta...

LPCVD homoepitaxy of Si doped β-Ga2O3 thin films on (010) and (001) substrates

This paper presents the homoepitaxy of Si-doped β-Ga2O3 thin films on semi-insulating (010) and (001) Ga2O3 substrates via low pressure chemical vapor deposition with a growth rate of ≥1 μm/h. Both high resolution scanning transmission electron microscopy and X-ray diffraction measurements demonstrated high crystalline quality homoepitaxial growth of these thin films. Atomic resolution STEM images of the as-grown β-Ga2O3 thin films on (010) and (001) substrates show high quality material without extended defects or dislocations. The charge carrier transport properties of the as-grown Si-doped β-Ga2O3 thin films were characterized by the temperature dependent Hall measurement using van der Pauw patterns. The room temperature carrier concentrations achieved for the (010) and (001) homoepitaxial thin films were ∼1.2 × 1018 cm−3 and ∼9.5 × 1017 cm−3 with mobilities of ∼72 cm2/V s and ∼42 cm2/V s, respectively.

Ultrawide Band Gap β-Ga2O3 Nanomechanical Resonators with Spatially Visualized Multimode Motion

Beta gallium oxide (β-Ga2O3) is an emerging ultrawide band gap (4.5 eV-4.9 eV) semiconductor with attractive properties for future power electronics, optoelectronics, and sensors for detecting gases and ultraviolet radiation. β-Ga2O3 thin films made by various methods are being actively studied toward such devices. Here, we report on the experimental demonstration of single-crystal β-Ga2O3 nanomechanical resonators using β-Ga2O3 nanoflakes grown via low-pressure chemical vapor deposition (LPCVD). By investigating β-Ga2O3 circular drumhead structures, we demonstrate multimode nanoresonators up to the sixth mode in high and very high frequency (HF/VHF) bands, and also realize spatial mapping and visualization of the multimode motion. These measurements reveal a Youngs modulus of EY = 261 GPa and anisotropic biaxial built-in tension of 37.5 MPa and 107.5 MPa. We find that thermal annealing can considerably improve the resonance characteristics, including ∼40% upshift in frequency and ∼90% enhancement in quality (Q) factor. This study lays a foundation for future exploration and development of mechanically coupled and tunable β-Ga2O3 electronic, optoelectronic, and physical sensing devices.

Lifetime laser damage performance of β-Ga2O3 for high power applications

Gallium oxide (Ga2O3) is an emerging wide bandgap semiconductor with potential applications in power electronics and high power optical systems where gallium nitride and silicon carbide have already demonstrated unique advantages compared to gallium arsenide and silicon-based devices. Establishing the stability and breakdown conditions of these next-generation materials is critical to assessing their potential performance in devices subjected to large electric fields. Here, using systematic laser damage performance tests, we establish that β-Ga2O3 has the highest lifetime optical damage performance of any conductive material measured to date, above 10 J/cm2 (1.4 GW/cm2). This has direct implications for its use as an active component in high power laser systems and may give insight into its utility for high-power switching applications. Both heteroepitaxial and bulk β-Ga2O3 samples were benchmarked against a heteroepitaxial gallium nitride sample, revealing an order of magnitude higher optical lifetime da...

Ultra-wide bandgap beta-Ga2O3 for deep-UV solar blind photodetectors(Conference Presentation)

Deep-ultraviolet (DUV) photodetectors based on wide bandgap (WB) semiconductor materials have attracted strong interest because of their broad applications in military surveillance, fire detection and ozone hole monitoring. Monoclinic β-Ga2O3 with ultra-wide bandgap of ~4.9 eV is a promising candidate for such application because of its high optical transparency in UV and visible wavelength region, and excellent thermal and chemical stability at elevated temperatures. Synthesis of high qualityβ-Ga2O3 thin films is still at its early stage and knowledge on the origins of defects in this material is lacking. The conventional epitaxy methods used to grow β-Ga2O3 thin films such as molecular beam epitaxy (MBE) and metal organic chemical vapor deposition (MOCVD) still face great challenges such as limited growth rate and relatively high defects levels. In this work, we present the growth of β-Ga2O3 thin films on c-plane (0001) sapphire substrate by our recently developed low pressure chemical vapor deposition (LPCVD) method. The β-Ga2O3 thin films synthesized using high purity metallic gallium and oxygen as the source precursors and argon as carrier gas show controllable N-type doping and high carrier mobility. Metal-semiconductor-metal (MSM) photodetectors (PDs) were fabricated on the as-grown β-Ga2O3 thin films. Au/Ti thin films deposited by e-beam evaporation served as the contact metals. Optimization of the thin film growth conditions and the effects of thermal annealing on the performance of the PDs were investigated. The responsivity of devices under 250 nm UV light irradiation as well as dark light will be characterized and compared.