Energy band alignment and band gap of Al2O3 and HfAlO films grown by atomic layer deposition on 4H–SiC were determined using x-ray photoelectron spectroscopy. Al2O3 exhibited a symmetric band profile with a conduction band offset (ΔEC) of 1.88 eV and a valence band offset (ΔEV) of 1.87 eV. HfAlO yielded a smaller ΔEC of 1.16 eV and ΔEV of 1.59 eV. The higher dielectric constant and higher effective breakdown field of HfAlO compared to Al2O3, coupled with sufficient electron and hole barrier heights, makes it a potential dielectric for use on 4H–SiC, and provokes interest in further investigation of HfAlO/4H–SiC properties.
In order to minimize ac-dc dispersion, reduce gate leakage and maximize ac transconductance, there is a critical need to identify optimal interfaces, low-k passivation dielectrics and high-k gate dielectrics. In this paper, an investigation of different atomic layer deposited (ALD) passivation dielectrics on AlGaN/GaN-based hetero-junction field effect transistors (HFETs) was performed. Angle-resolved x-ray photoelectron spectroscopy revealed that HCl/HF and NH 4 OH cleans resulted in a reduction of native oxide and carbon levels at the GaN surface. The role of high temperature anneals, following the ALD, on the effectiveness of passivation was also explored. Gate-lag measurements on HFETs passivated with a thin ALD high-k Al 2 O 3 or HfAlO layer capped with a thick plasma enhanced chemical vapor deposited (PECVD) low-k SiO 2 layer, annealed at 600-700 • C, were found to be as good as or even better than those with conventional PECVD silicon nitride passivation. Further, it was observed that different passivation dielectric stacks required different anneal temperatures for improved gate-lag behavior compared to the as-deposited case.
In this work, we have demonstrated a normally-off AlGaN/GaN metal-oxide semiconductor heterojunction field effect transistor (MOSHFET) wherein the enhancement mode operation is enabled by charge storage within a metal floating gate embedded in a dielectric stack and negative charges in the tunnel oxide. By combining ALD SiO 2 and TaN floating gate (FG), up to 6V of V T shift after pulse programming (corresponding ~ 1.2x10 13 charges/cm 2 stored within the FG) is obtained which results in a normally-off device with low gate leakage and good transconductance.
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