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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]

This work demonstrates direct, rapid 2D thermal mapping measurement capabilities of the ultrawide bandgap semiconductor channel of lateral β-(AlxGa1−x)2O3/Ga2O3 transistors without sample contamination, long acquisition times, or sophisticated thermometry such as developing deep-ultra-violet compatible thermoreflectance systems. The temperature rise in the channel of a β-(Al0.21Ga0.79)2O3/Ga2O3 heterostructure field-effect transistor (HFET) was mapped using thermoreflectance imaging at 470 nm. First, the thermoreflectance response of the HFET channel was measured using a monochromator, revealing a maximum of the reflectance change around 470–480 nm. Thermoreflectance calibrations were then performed at 470 nm (peak of the reflectance change) and yielded an average thermoreflectance coefficient of 1.06 ± 0.07 × 10−4 K−1. Subsequent measurements of the device (power densities of 0.15–1.47 W/mm and gate-source voltage of 0 V) enabled extraction of a device-level thermal resistance of 51.1 mm·K/W in the channel at the drain-side of the gate. High-resolution, in situ scanning thermal microscopy measurements of the channel temperature rise show good agreement with and further support the thermoreflectance measurements. Finally, the thermal profile across the entire device length (metal electrodes and semiconductor channel) and width was simultaneously measured using thermoreflectance imaging at 470 nm, and the peak temperature rise was measured in the channel at the drain-side of the gate electrode. [ABSTRACT FROM AUTHOR]

The effect of heteroepitaxial β-(AlxGa1−x)2O3 film thickness and Al content on surface morphology was characterized to experimentally determine the critical thickness limitations of the (010) β-(AlxGa1−x)2O3/Ga2O3 heterostructure. High-resolution x-ray diffraction was used to assess the strain state of the films; reciprocal space mapping (RSM) revealed that even cracked films were still fully strained. In cracked films, diffuse scattering was observed in RSMs, indicating lattice tilting. Cracking of the films was investigated using atomic force microscopy (AFM), x-ray topography (XRT), bright-field scanning transmission electron microscopy (BF-STEM), and high-resolution transmission electron microscopy. Using both AFM and XRT, the [001] direction was observed to be the most prevalent crack direction; however, cracks were also observed in the [100] direction. In uncracked regions of the films, XRT revealed the alignment of threading dislocations along the [001] direction. Cross-sectional imaging of the crack geometry and propagation was performed using BF-STEM, and it was observed that the cracks in the [001] direction extended through the thickness of the β-(AlxGa1−x)2O3 film (∼205 nm) and a further ∼100–200 nm into the β-Ga2O3 substrate. Experimental data for critical film thickness showed good agreement with previous theoretical calculations based on the Griffith criterion for crack propagation. [ABSTRACT FROM AUTHOR]

Ultra-wide bandgap β-gallium oxide (Ga2O3) devices are of considerable interest with potential applications in both power electronics and radio frequency devices. However, current Ga2O3 device technologies are limited by the material's low intrinsic electron mobility and thermal conductivity. The former problem can be addressed by employing modulation-doped β-(AlxGa1−x)2O3/Ga2O3 heterostructures in the device architecture. In this work, (AlxGa1−x)2O3/Ga2O3 modulation-doped field effect transistors (MODFETs) have been investigated from a thermal perspective. Thermoreflectance thermal imaging was used to characterize the self-heating of the MODFETs. The (Al0.18Ga0.82)2O3 thermal conductivity (3.1–3.6 W/mK) was determined using a frequency domain thermoreflectance technique. Electro-thermal modeling was used to discern the effect of design parameters such as substrate orientation and channel length on the device self-heating behavior. Various thermal management schemes were evaluated using the electro-thermal device model. From an electro-thermal co-design perspective, the improvement in electrical performance followed by the mitigation of self-heating was also studied. For example, by employing a Ga2O3-on-SiC composite wafer, which was fabricated in this work, a 50% increase in power handling capability can be achieved as compared to a homoepitaxial device. Furthermore, flip-chip heterointegration and double-sided cooling approaches can lead to more than 2× improvement in the power handling capability. Using an augmented double-sided cooling design that includes nanocrystalline diamond passivation, a 5× improvement in the power handling capability can be accomplished, indicating the potential of the technology upon implementation of a suitable thermal management scheme. [ABSTRACT FROM AUTHOR]

Silicon (Si) is a common n-type donor in AlxGa1-xN; however, it induces bending of edge-type threading dislocations which can generate tensile stress in the film leading to the formation of channeling cracks in thick layers. Germanium (Ge) has previously been investigated as an alternative to Si for n-type doping of GaN, but its impact on film stress in AlxGa1-xN has not been investigated in detail. In this study, we employ in situ wafer curvature measurements combined with postgrowth characterization to investigate Ge doping of AlxGa1-xN (x = 0–0.62) layers grown on 6H-SiC by metalorganic chemical vapor deposition. It was found that Ge doping (n ∼ 1.6 × 1019 cm−3) of Al0.30Ga0.70N does not induce tensile stress during growth in contrast to that observed with a similar level of Si doping. In addition, the average inclination angle of edge dislocations was similar for undoped and Ge doped films indicating that Ge does not promote surface-mediated dislocation climb. High n-type doping was achieved in Ge doped AlxGa1-xN for lower Al fraction range (x < 0.5), but resistivity increased and carrier density decreased significantly for higher Al fractions. The results demonstrate Ge doping as a viable alternative to Si doping of AlxGa1-xN (x < 0.5) for achieving thick, crack-free layers. [ABSTRACT FROM AUTHOR]