Electron concentration profiles have been obtained for Al x Ga 1Ϫx N/GaN heterostructure field-effect transistor structures. Analysis of the measured electron distributions demonstrates the influence of piezoelectric effects in coherently strained layers on III-V nitride heterostructure device characteristics. Characterization of a nominally undoped Al 0.15 Ga 0.85 N/GaN transistor structure reveals the presence of a high sheet carrier density in the GaN channel which may be explained as a consequence of piezoelectrically induced charges present at the Al 0.15 Ga 0.85 N/GaN interface. Measurements performed on an Al 0.15 Ga 0.85 N/GaN transistor structure with a buried Al 0.15 Ga 0.85 N isolation layer indicate a reduction in electron sheet concentration in the transistor channel and accumulation of carriers below the Al 0.15 Ga 0.85 N isolation layer, both of which are attributable to piezoelectric effects.

Disorder induced semiconductor to metal transition and modifications of grain boundaries in nanocrystalline zinc oxide thin film J. Appl. Phys. 112, 073101 (2012) Control of normal and abnormal bipolar resistive switching by interface junction on In/Nb:SrTiO3 interface Appl. Phys. Lett. 101, 133506 (2012) Cross-plane electronic and thermal transport properties of p-type La0.67Sr0.33MnO3/LaMnO3 perovskite oxide metal/semiconductor superlattices J. Appl. Phys. 112, 063714 (2012) Polarization Coulomb field scattering in In0.18Al0.82N/AlN/GaN heterostructure field-effect transistorsThe temperature dependence of the current-voltage characteristics of Ni-GaN Schottky barriers have been measured and analyzed. It was found that the enhanced tunneling component in the transport current of metal-GaN Schottky barrier contacts is a likely explanation for the large scatter in the measured Richardson constant.

Transmission electron microscopy was employed to evaluate the microstructure of Al/Ti ohmic contacts to AlGaN/GaN heterostructure field-effect transistor structures. Contact resistance was found to depend on the structure and composition of the metal and AlGaN layers, and on atomic structure of the interface. A 15–25-nm-thick interfacial AlTi2N layer was observed at the contact-AlGaN interface. Formation of such nitrogen-containing layers appears to be essential for ohmic behavior on n-type III-nitride materials suggesting a tunneling contact mechanism. Contact resistivity was found to increase with Al fraction in the AlGaN layer.

A method for enhancing effective Schottky barrier heights in III-V nitride heterostructures based on the piezoelectric effect is proposed, demonstrated, and analyzed. Two-layer GaN/Al x Ga 1Ϫx N barriers within heterostructure field-effect transistor epitaxial layer structures are shown to possess significantly larger effective barrier heights than those for Al x Ga 1Ϫx N, and the influence of composition, doping, and layer thicknesses is assessed. A GaN/Al 0.25 Ga 0.75 N barrier structure optimized for heterojunction field-effect transistors is shown to yield a barrier height enhancement of 0.37 V over that for Al 0.25 Ga 0.75 N. Corresponding reductions in forward-bias current and reverse-bias leakage are observed in current-voltage measurements performed on Schottky diodes.

The dependence of the Schottky barrier height of Ni/Al x Ga 1Ϫx N contact on the Al mole fraction up to xϭ0.23 was studied. The barrier heights were measured by I -V, capacitance-voltage, and the internal photoemission method. The Al mole fractions were estimated from the AlGaN band gap energies measured by photoluminescence. In the range of xϽ0.2 a linear relationship between the barrier height and Al mole fraction was obtained. This was consistent with the slope predicted by the Schottky rule. For xϭ0.23, the measured barrier height was lower than predicted. We believed this was due to crystalline defects at the Ni/AlGaN interface.