TEM studies of multilayer ohmic contacts to n-type AlGaN/GaN

Ti and Pd barrier layers between the Al/Ti diffusion couple and the Au capping layer of multilayer ohmic contacts to n-type AlGaN/GaN field effect transistors were found to be ineffective in preventing the diffusion of Au to the AlGaN following high temperature rapid thermal annealing. The formation of a band of TiN grains at the contact/AlGaN interface is responsible for the activation of the contact. The presence of interfacial Au and threading dislocations are implicated in the formation of additional Ti-nitride inclusions into the AlGaN, although these do not appear to disrupt the Ti-nitride layer at the original contact/nitride interface, nor significantly influence the contact resistance

Nanotechnology of pinning centres in high temperature superconducting YBa2Cu3O7 films

For cost-efficient power applications of superconducting coated conductors based on YBa$_2$Cu$_3$O$_7$ (YBCO) films, in applied fields or in self-field, further improvement of critical current by artificial flux pinning centres is required. This project investigated the increase in critical current density (J$_c$) and related physical properties of YBCO films by self-assembling nanotechnology of pinning centres, using substrate decoration, quasi-multilayers (using noble metals and Pr$_2$Cu$_3$O$_7$ (PBCO) in both cases), and targets containing BaZrO$_3$ (BZO) nano-inclusions. Samples were prepared by pulsed laser deposition (PLD) on single crystal SrTiO$_3$ (STO) substrates and on Ni-W Rolling-Assisted Biaxially Textured Substrates (RABiTS). Optical lithography and chemical etching were used to prepare samples for transport measurements. The superconducting properties were characterised by AC susceptibility, magnetisation loops and transport measurements using a Magnetic Property Measurement System (MPMS) and a Physical Properties Measurement System (PPMS). Scanning and Transmission Electron Microscopy (SEM) and (TEM), Atomic Force Microscopy (AFM) and X-ray diffraction were also used to characterise the micro-structure of the films and the structure of artificially-induced pinning centres. The optimum conditions for the growth of YBCO films, Ag and PBCO nano-dots, and BZO nano-columns were investigated. Combinations of all three nana-structuring approaches resulted in a maximum J$_c$ in applied fields and self-field. The related physical properties such as angular dependence of J$_c$, vortex melting line, pinning force, frequency dependence of J$_c$, were also investigated to understand pinning mechanisms in the films. The combination of Ag nano-dots and BZO nano-inclusions in the YBCO target provided the greatest improvement of critical currents of the film in applied fields.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

TEM studies of multilayer ohmic contacts to n-type AlGaN/GaN

Ti and Pd barrier layers between the Al/Ti diffusion couple and the Au capping layer of multilayer ohmic contacts to n-type AlGaN/GaN field effect transistors were found to be ineffective in preventing the diffusion of Au to the AlGaN following high temperature rapid thermal annealing. The formation of a band of TiN grains at the contact/AlGaN interface is responsible for the activation of the contact. The presence of interfacial Au and threading dislocations are implicated in the formation of additional Ti-nitride inclusions into the AlGaN, although these do not appear to disrupt the Ti-nitride layer at the original contact/nitride interface, nor significantly influence the contact resistance

Perfluoroalkyl compounds in relation to life-history and reproductive parameters in bottlenose dolphins (Tursiops truncatus) from Sarasota Bay, Florida, USA

Perfluoroalkyl compounds (PFCs) were determined in plasma, milk, and urine of free-ranging bottlenose dolphins (Tursiops truncatus) from Sarasota Bay (FL, USA) during three winter and two summer capture-and-release programs (2002¿ 2005). Plasma and urine samples were extracted using an ion-pairing method. Perfluoroalkyl compounds were extracted from milk samples using acetonitrile, and extracts were cleaned with graphitized nonporous carbon. All extracts were analyzed by high-performance liquid chromatography¿tandem mass spectrometry. Mean seasonal sum of PFCs (¿PFCs) detected in dolphin plasma ranged from 530 to 927 ng/g wet weight. No significant differences (p > 0.05) were found in concentrations between seasons, suggesting a constant exposure to PFCs. Overall, blubber thickness of dolphins did not correlate with PFC concentrations in plasma, suggesting an absence of PFC sequestration in blubber. Sexually immature calves (age

GaN H-FET development at QinetiQ

The AlGaN/GaN HFET project at QinetiQ has the capability to grow high quality layers using MOVPE, fabricate HFETs with 0.8 and 0.25µm gate length and fabricate amplifiers. Here we present recent work on topics as diverse as X-ray determination of aluminium concentration, 0.25Ohm.mm Ohmic contacts, measurement of saturated velocity, current slump and a 57W hybrid amplifier

Detailed analysis of DC-RF dispersion in AlGaN/GaN HFETs using waveform measurements

Detailed time-domain IV waveforms at RF frequencies are employed for characterisation of AlGaN/GaN HFETs in order to steer and advance device development. The IV time-domain data is used to isolate the separate effects of pinch-off and knee-walkout behaviour in limiting device performance. Furthermore, the waveform measurements which are obtained with a previously unseen level of detail, allowed the direct extraction of optimum device operating condition