Thermoelectric heat-to-power generation is an attractive option for robust and environmentally friendly renewable energy production. Historically, the performance of thermoelectric materials has been limited by low efficiencies, related to the thermoelectric figure-of-merit ZT. Nanostructuring thermoelectric materials have shown to enhance ZT primarily via increasing phonon scattering, beneficially reducing lattice thermal conductivity. Conversely, density-of-states (DOS) engineering has also enhanced electronic transport properties. However, successfully joining the two approaches has proved elusive. Herein, we report a thermoelectric materials system whereby we can control both nanostructure formations to effectively reduce thermal conductivity, while concurrently modifying the electronic structure to significantly enhance thermoelectric power factor. We report that the thermoelectric system PbTe-PbS 12% doped with 2% Na produces shape-controlled cubic PbS nanostructures, which help reduce lattice thermal conductivity, while altering the solubility of PbS within the PbTe matrix beneficially modifies the DOS that allow for enhancements in thermoelectric power factor. These concomitant and synergistic effects result in a maximum ZT for 2% Na-doped PbTe-PbS 12% of 1.8 at 800 K.
β-phase
gallium oxide (Ga2O3) is an
emerging ultrawide bandgap (UWBG) semiconductor (E
G ∼ 4.8 eV), which promises generational improvements
in the performance and manufacturing cost over today’s commercial
wide bandgap power electronics based on GaN and SiC. However, overheating
has been identified as a major bottleneck to the performance and commercialization
of Ga2O3 device technologies. In this work,
a novel Ga2O3/4H-SiC composite wafer with high
heat transfer performance and an epi-ready surface finish has been
developed using a fusion-bonding method. By taking advantage of low-temperature
metalorganic vapor phase epitaxy, a Ga2O3 epitaxial
layer was successfully grown on the composite wafer while maintaining
the structural integrity of the composite wafer without causing interface
damage. An atomically smooth homoepitaxial film with a room-temperature
Hall mobility of ∼94 cm2/Vs and a volume charge
of ∼3 × 1017 cm–3 was achieved
at a growth temperature of 600 °C. Phonon transport across the
Ga2O3/4H-SiC interface has been studied using
frequency-domain thermoreflectance and a differential steady-state
thermoreflectance approach. Scanning transmission electron microscopy
analysis suggests that phonon transport across the Ga2O3/4H-SiC interface is dominated by the thickness of the SiN
x
bonding layer and an unintentionally formed
SiO
x
interlayer. Extrinsic effects that
impact the thermal conductivity of the 6.5 μm thick Ga2O3 layer were studied via time-domain thermoreflectance.
Thermal simulation was performed to estimate the improvement of the
thermal performance of a hypothetical single-finger Ga2O3 metal–semiconductor field-effect transistor
fabricated on the composite substrate. This novel power transistor
topology resulted in a ∼4.3× reduction in the junction-to-package
device thermal resistance. Furthermore, an even more pronounced cooling
effect is demonstrated when the composite wafer is implemented into
the device design of practical multifinger devices. These innovations
in device-level thermal management give promise to the full exploitation
of the promising benefits of the UWBG material, which will lead to
significant improvements in the power density and efficiency of power
electronics over current state-of-the-art commercial devices.
We report on the first demonstration of metalorganic vapor phase epitaxy-regrown (MOVPE) ohmic contacts in an all MOVPE-grown β-Ga 2 O 3 metal semiconductor field effect transistor (MESFET). The low-temperature (600 °C) heavy (n + ) Si-doped regrown layers exhibit extremely high conductivity with a sheet resistance of 73 Ω/□ and a record low metal/n + -Ga 2 O 3 contact resistance of 80 mΩ•mm and specific contact resistivity of 8.3 × 10 −7 Ω•cm 2 were achieved. The fabricated MESFETs exhibit a maximum ON current of 130 mA mm −1 and a high I ON /I OFF ratio of >10 10 . Thermal characterization was also performed to assess the device self-heating under the high current and power conditions.
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