Marko J. Tadjer

We report on a phase-based lock-in thermography approach, combined with a multilayered thermal model (often employed in thermoreflectance analysis), to measure the thermal conductivity of a variety of materials. The spatial distribution of the material’s thermal phase is monitored with an infrared camera, which is locked into the frequency of a modulated laser used to heat the material. This phase distribution is then fit with a thermal model, in which properties such as thermal conductivity are extracted as fit parameters. This approach enables non-contact, front-side measurements, which are insensitive to surface roughness. The technique does not strictly require the application of a transducer layer, but we highlight the practical benefits of applying a removable adhesive layer to serve as a near-surface absorber. We demonstrate the efficacy of the method by measuring materials with thermal conductivities that span more than three orders of magnitude (∼1 to >2000 W m−1 K−1).

In this work, sub-bandgap photoluminescence imaging is presented as a means with which to identify defect bands in bulk AlN. Sub-bandgap photoluminescence spectroscopy revealed large variations across different points in the mid-gap between 1.8 and 3.8 eV, which was subsequently investigated using broad-area photoluminescence imaging. Two different LEDs (340 and 450 nm) were used to spatially identify defect bands present in the UV and visible ranges. Bandpass filters were used at wavelengths between 450 and 700 nm to further deconvolve bands of interest at different wafer points. Raman spectroscopy was performed at each wafer point to ensure that changes in the photoluminescence were not due to the presence of different phases or low crystal quality. Measurements on both wafers revealed E ₂ (high) peaks with full-width half maximum (FWHM) values below 10 cm ⁻¹ at all measured points, suggesting a high degree of crystallinity at all points, as would be expected with high quality bulk substrates. HRXRD measurements subsequently confirmed that the substrates were of high quality and phase uniform across the entire wafer surface.

Vertical Ga2O3 heterojunction diodes were fabricated and systematically studied to evaluate the impact of lateral contact geometry on forward and reverse device performance. The devices were formed on 32 μm thick, lightly doped (8.6 × 1015 cm−3) β-Ga2O3 drift layers grown by halide vapor phase epitaxy on heavily doped n+ substrates. A NiO/Ni/Au heterojunction anode was employed together with dielectric edge termination using a 70 nm SiO2/30 nm SiN field-plate structure. Three device designs with different field plate dimensions (D = 10, 20, and 30 μm) between the first- and second-layer contact metals were investigated. Capacitance–voltage measurements indicate a free carrier concentration of approximately (8.6 ± 1.2) × 1015 cm−3 in the drift layer, while forward I–V characteristics showed comparable saturation currents and turn-on voltages for all designs. Increasing D resulted in a reduction in on-resistance and a significant suppression of reverse leakage current at low to moderate reverse bias. The optimized device with D = 30 μm exhibited the highest breakdown voltage of approximately 8.1 kV, compared with 5.5 and 4.5 kV for D = 20 μm and D = 10 μm, respectively. These results demonstrate that lateral contact spacing is a critical design parameter for electric-field management and breakdown enhancement in Ga2O3 heterojunction diodes.

In this work, we demonstrate device fabrication and characterization of ultrawide bandgap (UWBG) β-(AlxGa1−x)2O3 channel metal–semiconductor field-effect transistors (MESFETs) with Si-implanted source/drain contacts. Films of Si-doped β-(AlxGa1−x)2O3 and unintentionally doped (UID) β-Ga2O3 were grown on an Fe-doped (010) β-Ga2O3 substrate using close-injection showerhead metal–organic chemical vapor deposition (CIS-MOCVD). The Al concentration (x) of the β-(AlxGa1−x)2O3 film was estimated to be ≈21% using x-ray diffraction (XRD), and the Si doping concentration of the n-type β-(Al0.21Ga0.79)2O3 film was measured to be 2 × 1018 cm−3 using secondary ion mass spectroscopy (SIMS). Both gate-recessed and non-recessed MESFET structures were fabricated and had on/off ratios of ≈105. The gate-recessed MESFETs had a lower saturation drain current (15 mA/mm) than the non-recessed structures (26 mA/mm), but they also had a slightly lower off-state leakage current. The gate-recessed structures also enabled better modulation of the channel conductivity, which led to a positive threshold voltage shift of +8V compared with the non-recessed structures. The maximum breakdown voltages of 730 and 858 V were measured for the gate-recessed and non-recessed MESFETs, respectively. Because the thermal conductivity of the disordered alloy β-(Al0.21Ga0.79)2O3 channel is expected to be much lower than β-Ga2O3, thermoreflectance imaging was used to assess the device-level thermal performance. At a DC power density of 0.58 W/mm (VGS = 0 V), an area-averaged gate temperature rise of 54 K was measured. This work demonstrates the potential of leveraging the large-area, low-cost, high-quality β-Ga2O3 substrate platform to develop next-generation power electronics using UWBG β-(AlxGa1−x)2O3 as the active semiconductor.

We report the heteroepitaxial growth of spinel Cr2MnO4 thin films on (2¯01) β-Ga2O3 substrates via off-axis RF magnetron sputtering. Structural analysis confirms (111)-oriented epitaxial films with optimized growth at 950 °C. Fabricated Cr2MnO4/β-Ga2O3 vertical diodes exhibit rectifying behavior with high on/off ratios (∼108), turn-on voltages near 0.85 V, and low reverse leakage current. Transmission electron microscopy and atomic force microscopy analyses show the films exhibit a Stranski–Krastanov growth mode, with three-dimensional texturing of potential interest for device performance. Capacitance–voltage measurements reveal intrinsic p-type conductivity with carrier concentration ∼7 × 1017 cm−3. The results suggest that Cr2MnO4 may be a promising candidate for Ga2O3-based bipolar devices.

We report back-end-of-line growth of nanocrystalline diamond (NCD) on ultrawide bandgap (UWBG) high Al content aluminum gallium nitride (AlGaN) channel high electron mobility transistors for thermal management. A thin (∼15 nm) silicon nitride (SiNx) interlayer was deposited to protect the device surface before performing a low temperature (500 °C) NCD growth process in an attempt to protect the gates on these fully fabricated devices. Notably, atomic force microscopy showed that the maximum lateral grain size exceeded 300 nm even though the film thickness was ∼250 nm. Comparing electrical (DC) performance before and after NCD growth, the gate leakage increased by ∼102 after NCD growth. Despite the lower NCD growth temperature, intermixing of the Ni and Au was observed in the Schottky gate metal stack; however, we believe there is another mechanism, possibly hydrogen-related, that is responsible for the measured increase in gate leakage. Regarding thermal management, the device-level thermal resistance (quantified using the average gate temperature rise measured by thermoreflectance imaging) was reduced by 29% through the incorporation of the top-side diamond film. Using time-domain thermoreflectance, the thermal conductivity of the ≈250 nm thick NCD film was measured to be 45 ± 25 W m−1 K−1. This is expected to be at least 5× greater than the thermal conductivity of the thin disordered AlGaN alloy. There could also be a coupled electrothermal component contributing to the reduced temperature rise from electric field spreading and consequent heat spreading. This study demonstrates a promising first step toward device-level thermal management of high power UWBG Al-rich AlGaN devices.

The cracking and local strain relaxation in (010) (AlxGa1−x)2O3 films grown on Ga2O3 substrates are assessed in terms of film composition and thickness. We utilize x-ray diffraction and electron microscopy techniques combined with simulation and modeling to investigate that film cracking on curvature (flatness) has a directly proportional relationship with film thickness and/or aluminum content. Cross section transmission electron microscopy reveals that cracks along both the (001) and (100) cleavage planes penetrate into the substrate. The diffuse scattered intensity observed in reciprocal space maps (RSMs) is directly correlated with the tilt that is introduced due to the change in the deformation conditions near the cracks. While asymmetric RSMs show that the layers are fully strained, the diffuse scattering distribution in reciprocal space can be interpreted to show that the cracking relaxes and locally tilts the lattice ∼350 nm from the crack edges, which is consistent with the larger radius of curvature associated with the films with higher crack densities. For example, a 200 nm (Al0.13Ga0.87)2O3 thick film has an average inter-crack spacing of 3.3 µm, so most of the epitaxial layer is fully strained except near the cracks where it deforms elastically and is consistent with the gallium oxide Poisson ratio. A reciprocal space model was developed, which imports the strain and tilt distributions (based on finite element modeling) to match the features observed in the experimental maps. We also note that previous studies involving (AlxGa1−x)2O3 films may show evidence of cracking as observed in their symmetric and asymmetric RSMs.

Homoepitaxial Si-doped (010) β-Ga2O3 epitaxial layers with thicknesses ranging from 1.2 to 12 μm were grown simultaneously via close-injection showerhead metal-organic chemical vapor deposition on both Czochralski and edge-defined, film-fed grown (010) β-Ga2O3:Fe substrates. Structural characterization was performed via x-ray diffraction [full-width at half maxima of 21–72 arcsec depending on epilayer thickness and substrate, measured from the symmetric (020) reflection], x-ray topography, Raman spectroscopy, cathodoluminescence spectroscopy, and transmission electron microscopy. Surface characterization was performed via atomic force microscope (AFM) and Nomarski imaging, and electrical characterization was performed via electron paramagnetic resonance spectroscopy and Hall effect measurements. In addition, thermal conductivity was measured via steady-state thermoreflectance. AFM imaging revealed a number of extended defects forming in the epilayer, and transmission electron microscopy imaging detailed a V-shaped pit defect formed by misoriented twin defects likely originating from a defect at or near the substrate surface. The Hall effect data (NS = 2.81–3.1 × 1017 cm−3 and μH = 81.5–103.5 cm2/V s), surface roughness (1.78–4.25 nm rms), and thermal conductivity measurements (19.3–19.8 W/m K) yielded comparable results among all samples, independent of epilayer thickness. These results suggested that even thicker, high quality epitaxial (010) β-Ga2O3 could be grown via this technique, provided that the development of the observed extended defects can be mitigated.

Ultrawide bandgap (UWBG) semiconductor materials, such as β−Ga2O3 (gallium oxide), AlN (aluminum nitride), AlxGa1−xN (AlGaN), and diamond, have emerged as essential candidates for components in high-power, high-frequency applications due to their superior electronic properties. However, with the exception of diamond and AlN, these materials present unique thermal management challenges, primarily because of their low thermal conductivities that are incapable of managing the demand for high power densities. Therefore, novel thermal management approaches that feature new device architectures are needed to prevent excessively high peak temperatures in UWBG devices. In parallel, accurate device-level thermal characterization (with high spatial/temporal resolution) is crucial to verify and optimize these designs with an overall goal to improve device performance and reliability. This paper discusses current thermal metrology techniques used for UWBG semiconductor devices covering: optical methods (Raman and thermoreflectance); electrical methods (gate resistance thermometry); and scanning probe methods (scanning thermal microscopy). More specifically, the steady-state and transient capability of each thermal metrology is explored and the limitation of each technique is highlighted. Finally, this perspective outlines potential advances in transient thermoreflectance imaging including a hyperspectral approach for nitride based heterostructures and a sub-bandgap excitation technique for gallium oxide based electronics. Additionally, the development of a future thermoreflectance microscope is presented. This microscope features high optical transmission, in the deep ultra violet wavelength range, for near bandgap thermoreflectance imaging of UWBG devices.