Formation of Low-Resistance Ohmic Contact by Damage-Proof Selective-Area Growth of Single-Crystal n +-GaN Using Plasma-Assisted Molecular Beam Epitaxy

Abstract

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.

Ti-based nonalloyed Ohmic contacts for Al0.15Ga0.85N∕GaN high electron mobility transistors using regrown n+-GaN by plasma assisted molecular beam epitaxy

“…A heavily doped n + -GaN layer ͑n = 1.0ϫ 10 19 cm −3 ͒ was grown only in the source/ drain region giving rise to a record low contact resistivity of 1.8ϫ 10 −8 ⍀ cm 2 . 11,12 In this work, we extend the advantages of the regrowth technique to fabricate nonalloyed Ohmic contacts for HEMTs, analyze the performance of the resulting contacts, and compare the effects of regrowth and recessed drain/source on nonalloyed contacts. Thus obtained high dc performance of the nonalloyed GaN-based HEMTs will be presented and discussed.…”

“…Since the KOH solution can etch both single-and polycrystalline GaN layers, before the KOH etching an additional layer of 100 nm thick SiO 2 was deposited on the selectively regrown n + -GaN in an effort to protect the single crystalline GaN contact layers from damage by the heated KOH solution. 12 The residual SiO 2 layers were etched using buffered oxide etchant. The regrowth process was performed only for HEMT-B and -D. For the recessed drain/source ͑HEMT-C and -D͒, dry etching was performed using ICP-RIE with a Cl 2 / Ar plasma before the regrowth processes.…”