M. R. Holzworth

Nanoscale manipulation of Ge nanowires by ion irradiation

Nanowires have generated considerable interest as nanoscale interconnects and as active components of both electronic and electromechanical devices. However, in many cases, manipulation and modification of nanowires are required to fully realize their potential. It is essential, for instance, to control the orientation and positioning of nanowires in some specific applications. This work demonstrates a simple method to reversibly control the shape and the orientation of Ge nanowires using ion beams. Crystalline nanowires were amorphized by 30 keV Ga+ implantation. Subsequently, viscous flow and plastic deformation occurred causing the nanowires to bend toward the beam direction. The bending was reversed multiple times by ion implanting the opposite side of the nanowires, resulting in straightening and subsequent bending into that opposite direction. This effect demonstrates the detailed manipulation of nanoscale structures is possible through the use of ion irradiation.

Characterization of the gate oxide of an AlGaN/GaN high electron mobility transistor

A subnanometer thick interfacial oxide layer present between the Ni/Au gate metal stack and semiconducting epilayers of an AlGaN/GaN high electron mobility transistor was characterized using high-angle annular dark-field scanning transmission electron microscopy and laser-assisted atom probe tomography. It was revealed that the oxide is composed of distinct Ni-oxide-rich and Al-oxide-rich layers with no Ga-oxide detected. The results provide information that is of potential importance in determining failure mechanisms and improving reliability of AlGaN/GaN high electron mobility transistors.

Investigation of the effect of temperature during off-state degradation of AlGaN/GaN High Electron Mobility Transistors

Abstract AlGaN/GaN High Electron Mobility Transistors were found to exhibit a negative temperature dependence of the critical voltage ( V CRI ) for irreversible device degradation to occur during bias-stressing. At elevated temperatures, devices exhibited similar gate leakage currents before and after biasing to V CRI , independent of both stress temperature and critical voltage. Though no crack formation was observed after stress, cross-sectional TEM indicates a breakdown in the oxide interfacial layer due to high reverse gate bias.

Field-induced defect morphology in Ni-gate AlGaN/GaN high electron mobility transistors

AlGaN/GaN high electron mobility transistors were electrically stressed using off-state high reverse gate biases. In devices demonstrating the largest, most rapid decrease in normalized maximum drain current, defects were found at the gate/AlGaN epilayer interface and characterized using high-angle annular dark-field scanning transmission electron microscopy. These defects appear to be a reaction between the Ni layer of the Ni/Au gate metal stack and the AlGaN epilayer. Additionally, simulations of the electric field lines from the defective devices match the defect morphology. These results provide important insight toward understanding failure mechanisms and improving reliability of Ni-gate AlGaN/GaN high electron mobility transistors.

Under-gate defect formation in Ni-gate AlGaN/GaN high electron mobility transistors

Abstract High electron mobility transistors based on Aluminum Gallium Nitride/Gallium Nitride heterostructures are poised to become the technology of choice for a wide variety of high frequency and high power applications. Their reliability in the field, particularly the reliability of the gate electrode under high reverse bias, remains an ongoing concern, however. Rapid increases in gate leakage current have been observed in devices which have undergone off-state stressing. Scanning Electron Microscopy, scanning probe microscopy, and Transmission Electron Microscopy have been used to evaluate physical changes to the structure of Ni-gated devices as the gate leakage current begins its initial increase. This evaluation indicates the formation of an interfacial defect similar to erosion under the gate observed by other authors. Defect formation appears to be dependent upon electrical field as well as temperature. Transmission Electron Microscopy has been used to demonstrate that a chemical change to the interfacial oxynitride layer present between the semiconductor and gate metal appears to occur during the formation of this defect. The interfacial layer under the gate contact transitions from a mixed oxynitride comprised of gallium and aluminum to an aluminum oxide.

Recovery in dc and rf performance of off-state step-stressed AlGaN/GaN high electron mobility transistors with thermal annealing

The recovery effects of thermal annealing on dc and rf performance of off-state step-stressed AlGaN/GaN high electron mobility transistors were investigated. After stress, reverse gate leakage current and sub-threshold swing increased and drain current on-off ratio decreased. However, these degradations were completely recovered after thermal annealing at 450 °C for 10 mins for devices stressed either once or twice. The trap densities, which were estimated by temperature-dependent drain-current sub-threshold swing measurements, increased after off-state step-stress and were reduced after subsequent thermal annealing. In addition, the small signal rf characteristics of stressed devices were completely recovered after thermal annealing.

Nanocrack formation in AlGaN/GaN high electron mobility transistors utilizing Ti/Al/Ni/Au ohmic contacts

Abstract AlGaN/GaN HEMTs are poised to become the technology of choice in RF and power electronics applications where high operating frequencies and high breakdown voltages are required. The alloyed contacting scheme utilized in the formation of the source and drain contacts of these devices affects the conduction of electrons through the 2DEG from the moment of ohmic contact formation onward to operation in the field. Analysis of the ohmic contacts of as-fabricated and electrically stressed AlGaN/GaN HEMTs, via chemical deprocessing and Scanning Electron Microscopy, indicates the presence of cracks oriented along the [11-20] directions, which nucleate at metal inclusions present under the alloyed ohmic source/drain contact metal. Cracks which form at the edges of these contact regions can extend into the channel region. It appears that electrical biasing induces additional growth in the longest cracks present within the channel regions of these devices.