Selective area growth (SAG) has been demonstrated using plasma-assisted molecular beam epitaxy (PAMBE) resulting in vastly improved Ohmic contacts for GaN-based high-power field-effect transistors (FETs). A heavily doped n-GaN layer was grown only in the Ohmic contact region and the resulting nonalloyed Ti∕Al∕Ti∕Au metal contacts exhibited linear Ohmic behavior. Through rapid thermal annealing, very low specific contact resistivity (1.8×10−8Ωcm2) was obtained at 850°C. Furthermore, contact resistances below 0.8Ωmm were obtained by annealing at a wide range of temperatures (750–950°C). GaN metal-semiconductor FETs were fabricated to investigate the effect of the PAMBE-SAG on device performance, producing great improvement in the dc characteristics.
To achieve very low ohmic contact resistance, an n + -GaN layer was selectively deposited using plasma-assisted molecular beam epitaxy (PAMBE). During this process polycrystalline GaN grew on the patterned SiO 2 region, which was subsequently removed by a heated KOH solution, resulting in damage to the n + -GaN surface. To prevent this damage, an additional SiO 2 layer was selectively deposited only on the n + -GaN region. To optimize the fabrication process the KOH etching time and n + -GaN layer thickness were adjusted. This damage-proof scheme resulted in a specific contact resistance of 4.6 · 10 -7 X cm 2 . In comparison, the resistance with the KOH etching damage was 4.9 · 10 -6 X cm 2 to 24 · 10 -6 X cm 2 . The KOH etching produced a large number of pits (4.1 · 10 8 cm -2 ) and degraded the current transport. X-ray photoelectron spectroscopy (XPS) and secondary-ion mass spectrometry (SIMS) analysis indicated that KOH etching was very effective in removing the oxide from the GaN surface and that the O-H bonding at the GaN surface was likely responsible for the degraded contact performance. The optimum n + -GaN thickness was found to be 54 nm.
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