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Enhanced Mobility in InAlN/AlN/GaN HEMTs Using a GaN Interlayer

Abstract: An enhancement of the electron mobility (μ) in InAlN/AlN/GaN heterostructures is demonstrated by the incorporation of a thin GaN interlayer (IL) between the InAlN and AlN. The introduction of a GaN IL increases μ at room temperature (RT) from 1600 to 1930 cm2/Vs. The effect is further enhanced at cryogenic temperature (5 K), where the GaN IL sample exhibits a μ of 16000 cm2/Vs, compared to 6900 cm2/Vs without IL. The results indicate the reduction of one or more scattering mechanisms normally present in InAlN/AlN/GaN heterostructures. We propose that the improvement in μ is either due to the suppression of fluctuations in the quantum well subband energies or to reduced Coulomb scattering, both related to compositional variations in the InAlN. HEMTs fabricated on the GaN IL sample demonstrate larger improvement in dcand high-frequency performance at 5 K; fmax increases by 25 GHz to 153 GHz, compared to an increase of 6 GHz to 133 GHz without IL. The difference in improvement was associated mainly with the drop in the access resistances.

Published in: IEEE Transactions on Electron Devices ( Volume: 66, Issue: 7, July 2019)

Transmorphic epitaxial growth of AlN nucleation layers on SiC substrates for high breakdown thin GaN transistors

Abstract: Interfaces containing misfit dislocations deteriorate electronic properties of heteroepitaxial wide bandgap III-nitride semiconductors grown on foreign substrates, as a result of lattice and thermal expansion mismatches and incompatible chemical bonding. We report grain-boundary-free AlN nucleation layers (NLs) grown by metalorganic chemical vapor deposition on SiC (0001) substrates mediated by an interface extending over two atomic layers L1 and L2 with composition (Al1/3Si2/3)2/3N and (Al2/3Si1/3)N, respectively. It is remarkable that the interfaces have ordered vacancies on one-third of the Al/Si position in L1, as shown here by analytical scanning transmission electron microscopy and ab initio calculations. This unique interface is coined the out-of-plane compositional-gradient with in-plane vacancy-ordering and can perfectly transform the in-plane lattice atomic configuration from the SiC substrate to the AlN NL within 1 nm thick transition. This transmorphic epitaxial scheme enables a critical breakdown field of ∼2 MV/cm achieved in thin GaN-based transistor heterostructures grown on top. Lateral breakdown voltages of 900 V and 1800 V are demonstrated at contact distances of 5 and 20 μm, respectively, and the vertical breakdown voltage is ≥3 kV. These results suggest that the transmorphic epitaxially grown AlN layer on SiC may become the next paradigm for GaN power electronics.

Published in: Applied Physics Letters 115, 221601 (2019)

Microwave Performance of ‘Buffer-Free’ GaN-on-SiC High Electron Mobility Transistors

Abstract: High performance microwave GaN-on-SiC HEMTs are demonstrated on a heterostructure without a conventional thick doped buffer. The HEMT is fabricated on a high-quality 0.25 μm unintentional doped GaN layer grown directly on a transmorphic epitaxially grown AlN nucleation layer. This approach allows the AlN-nucleation layer to act as a back-barrier, limiting short channel effects and removing buffer leakage. The devices with the `buffer-free’ heterostructure show competitive DC and RF characteristics, as benchmarked against the devices made on a commercial Fe-doped epi-wafer. Peak transconductances of 500 mS/mm and a maximum saturated drain current of ~1 A/mm are obtained. An extrinsic f T of 70 GHz and f max of 130 GHz are achieved for transistors with a gate length of 100 nm. Pulsed-IV measurements reveal a lower current slump and a smaller knee walkout. The dynamic IV performance translates to an output power of 4.1 W/mm, as measured with active load-pull at 3 GHz. These devices suggest that the `buffer-free’ concept may offer an alternative route for high frequency GaN HEMTs with less electron trapping effects.

Published in: IEEE Electron Device Letters ( Volume: 41, Issue: 6, June 2020)

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