We discuss possibilities of adjustment of a threshold voltage VT in normally off GaN high-electron mobility transistors (HEMTs) without compromising a maximal drain current IDSmax. Techniques of a low power plasma or thermal oxidation of 2-nm thick AlN cap over 3-nm thick AlGaN barrier are developed and calibrated for a thorough oxidation of the cap with a minimal density of surface donors at the inherent oxide-semiconductor interface. It has been shown that while a thermal oxidation technique leads to the channel and/or interface degradation, low density of surface donors and scalability of VT with additionally overgrown Al2O3 may be obtained for plasma oxidized HEMTs. With 10-nm thick Al2O3 deposited at 100 °C by atomic-layer deposition, we obtained VT of 1.6 V and IDSmax of 0.48 A/mm at a gate voltage of VGS = 8 V. Density of surface donors was estimated to be about 1.2 × 1013 cm−2, leaving most of the negative polarization charge at the semiconductor surface uncompensated. Further reduction of surface donors may be needed for even higher VT.

In this paper, the effect of the surface treatments on the performance of Ti/Al/Ni/Au ohmic contacts to InAlN/GaN has been investigated by electrical characterization and by normalized Auger electron spectroscopy (AES). Contact resistance and specific contacts resistivity as low as 0.7 Ωmm and 1.1×10–7 Ωcm2, respectively, have been achieved by pre‐treatment using SiCl4 plasma followed by rapid thermal annealing at at 600 °C. This is compared with the case where no plasma pre‐treatment was performed. Contacts without pretreatment and annealed at at 600 °C were still rectifying and at the optimized annealing temperature of 800 °C reach 1.7 Ωmm contact resistance. Moreover, AES investigations showed that the presence of Carbon impurities on the InAlN surface has been significantly reduced after SiCl4 RIE pre‐treatment. That may account for improved ohmic contacts resistance and lowered optimised temperature of annealing for pre‐treated contacts. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

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The indium oxide thin films were deposited at room temperature by reactive magnetron sputtering in the mixture of oxygen and argon on silicon and oxidized silicon substrates. The influence of the oxygen flow in the reactive mixture and post-deposition annealing on the structural properties were investigated. The as deposited In 2 O 3 films showed a dominating randomly oriented nanocrystalline structure of cubic In 2 O 3 . The grain size decreased with increasing oxygen concentration in the plasma. Annealing in reducing atmospheres (vacuum, nitrogen and argon), besides improving the crystallinity, led to a partial cubic to rhombohedral phase transition in the indium oxide films.

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