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The low thermal conductivity of β-Ga2O3 is a significant concern for maximizing the potential of this ultra-wide bandgap semiconductor as a power switching device technology. Here, we report on the use of nanocrystalline diamond (NCD) deposited via microwave plasma enhanced chemical vapor deposition (MP-CVD) as a top-side, device-level thermal management solution on a lateral β-Ga2O3 transistor. NCD was grown via MP-CVD on β-(AlxGa1−x)2O3/β-Ga2O3 heterostructures prior to the gate formation of the field-effect transistor. A reduced growth temperature of 400 °C and a SiNx barrier layer were used to protect the oxide semiconductors from etching in the MP-CVD H2 plasma environment. Raman spectroscopy showed a highly sp3-bonded NCD film was obtained at 400 °C, with grain size of about 50–100 nm imaged via atomic force microscopy. The incorporation of the NCD heat-spreading layer resulted in a β-(AlxGa1−x)2O3/β-Ga2O3 heterostructure field-effect transistor showing a decrease in the total thermal resistance at the gate by 42%. The fabrication process, including the NCD etch in the gate region, will need to be improved to minimize the impact of these processes on important device characteristics (i.e., drain current, threshold voltage, and leakage current). [ABSTRACT FROM AUTHOR]

Thin monoclinic Ga2O3 films were deposited on c-plane sapphire substrates by low pressure chemical vapor deposition. The thin films were synthesized using high purity metallic gallium (Ga) and oxygen gas (O2) as precursors. The effect of oxygen volume percentage on the growth rate of thin films was observed at two growth temperatures. Within the investigated growth window, a maximum growth rate of ∼2.9 μm/h was obtained for an oxygen volume percentage of 4.8% with a growth temperature at 800 °C. The film growth rate decreased as growth temperature increased when other growth parameters were kept the same. X-ray diffraction indicates that all films have the β-Ga2O3 structure with (−201) orientation, and those deposited with higher oxygen partial pressure are thicker and have improved crystalline quality. Polarized micro-Raman scattering is consistent with small grains of (−201) β-Ga2O3 having random in-plane orientations. The large variation of the relative intensities of overlapping emission bands contributing to the broad luminescence emission extending between 1.5 and 4.5 eV (∼825 and 275 nm) suggest that deposition conditions strongly affect different defect concentrations. Films deposited at 800 °C with a higher oxygen partial pressure yielded higher resistance, which may result from the incorporation of gallium vacancies, identified as a compensating point defect affecting the electrical conductivity of bulk monoclinic Ga2O3. [ABSTRACT FROM AUTHOR]

One of the major challenges in β-Ga2O3-based high power and high frequency devices is anticipated to be related to the low thermal conductivity of the material which is on the order of 10–30 W/m K. The use of diamond (thermal conductivity ∼2000 W/m K) as a substrate can be one effective approach for achieving better thermal management in β-Ga2O3-based devices. In this work, low pressure chemical vapor deposition was used to grow β-Ga2O3 films on (100) oriented, single-crystalline diamond substrates. A two-step growth technique was employed to avoid the oxidation of the diamond surface at high temperatures. From x-ray diffraction measurements, the β-Ga2O3 films grew along the ⟨ − 201 ⟩ crystalline axis with the β-Ga2O3 (002) planes rotated by ±24.3–27° with respect to the diamond (111) planes. High-magnification scanning transmission electron microscopy imaging revealed an abrupt β-Ga2O3/diamond interface without any voids which is essential for the high rate of heat transfer across the interface. N-type electrical conductivity was measured in a Si-doped β-Ga2O3 film with 1.4 × 1019 cm−3 electron concentration and ∼3 cm2/V s electron mobility. This work demonstrates the feasibility of heteroepitaxy of β-Ga2O3 films on diamond substrates for potential device design and device fabrication with efficient thermal management. [ABSTRACT FROM AUTHOR]

The ultra-wide bandgap semiconductor gallium oxide (Ga2O3) offers substantial promise to significantly advance power electronic devices as a result of its high breakdown electric field and maturing substrate technology. A key remaining challenge is the ability to grow electronic-grade epitaxial layers at rates consistent with 20–40 μm thick drift regions needed for 20 kV and above technologies. This work reports on extensive characterization of epitaxial layers grown in a novel metalorganic chemical vapor deposition tool that permits growth rates of 1.0–4.0 μm h−1. Specifically, optical, structural and electrical properties of epilayers grown at ∼1 μm h−1 are reported, including employment in an operating MOSFET. The films demonstrate relatively smooth surfaces with a high degree of structural order, limited point defectivity (Nd − Na ≈ 5 × 1015 cm−3) and an optical bandgap of 4.50 eV. Further, when employed in a MOSFET test structure with an n+ doped channel, a record high mobility for a transistor structure with a doped channel of 170 cm2 V−1 s−1 was measured via the Hall technique at room temperature. This work reports for the first time a β-Ga2O3 MOSFET grown using Agnitron Technology's high growth rate MOCVD homoepitaxial process. These results clearly establish a significant improvement in epilayer quality at growth rates that can support future high voltage power device technologies. [ABSTRACT FROM AUTHOR]

Recent breakthroughs in bulk crystal growth of the thermodynamically stable beta phase of gallium oxide (β-Ga2O3) have led to the commercialization of large-area β-Ga2O3 substrates with subsequent epitaxy on (010) substrates producing high-quality films. Still, metalorganic chemical vapor deposition, molecular beam epitaxy, and processing of the (010) β-Ga2O3 surface are known to form subnanometer-scale facets along the [001] direction as well as larger ridges with features perpendicular to the [001] direction. A density function theory calculation of the (010) surface shows an ordering of the surface as a subnanometer-scale feature along the [001] direction. Additionally, the general crystal structure of β-Ga2O3 is presented, and recommendations are presented for standardizing (010) substrates to account for and control the larger-scale ridge formation. [ABSTRACT FROM AUTHOR]

Record DC power has been demonstrated in AlGaN/GaN high electron mobility transistors fabricated using a substrate replacement process in which a thick diamond substrate is grown by chemical vapor deposition following removal of the original Si substrate. Crucial to the process is a ~30 nm thick SiN interlayer that has been optimized for thermal resistance. The reductions obtained in self-heating have been quantified by transient thermoreflectance imaging and interpreted using 3D numerical simulation. With a DC power dissipation level of 56 W/mm, the measured average and maximum temperatures in the gate-drain access region were 176 °C and 205 °C, respectively. [ABSTRACT FROM AUTHOR]

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 lm/h) with a fixed oxygen flow rate was obtained for the epitaxial layers grown at 850 °C. [ABSTRACT FROM AUTHOR]

Enhancements in AlGaN/GaN high-electron-mobility transistor (HEMT) performance have been realized through ultrathin (4 nm) AlN passivation layers, formed by atomic layer epitaxy (ALE). A combination of ex situ and in situ surface cleans prepare the surface for deposition of ALE AlN. HEMTs passivated by high crystallinity AlN, grown at 500^\circC, show improvements in 2-D electron gas sheet carrier density, gate leakage current, off-state drain leakage current, subthreshold slope, and breakdown voltage. In addition, degradation of dynamic on resistance during pulsed off-state voltage switching stress is suppressed by \sim50\% compared with HEMTs passivated by conventional plasma enhanced chemical vapor deposition SiNx. [ABSTRACT FROM PUBLISHER]

A novel device structure incorporating an ultrathin AIGaN barrier layer capped by an AIN layer in the source-drain access regions has been implemented to reliably control threshold voltage in AIGaN/GaN high-electron-mobility transistors. A recessed-gate structure has been used to decrease 2-D electron gas (2DEG) density under the gate, thus controlling threshold voltage while maintaining low on-resistance and high current density. The structure presented in this letter implements an ultrathin AIGaN structure grown by metal-organic chemical vapor deposition capped with AIN to maintain a high 2DEG density in the access regions. A selective wet etch using heated photoresist developer is used to selectively etch the AIN layer in the gate region to the AIGaN barrier. We have demonstrated a repeatable threshold voltage of +0.21 V with 4-nim AIGaN barrier layer thickness. [ABSTRACT FROM AUTHOR]