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

Nanosecond timescale thermal dynamics of AlGaN/GaN electronic devices

Time-resolved Raman thermography, with a temporal resolution of , was used to study the thermal dynamics of AlGaN/GaN electronic devices (high-electron mobility transistors and ungated devices). Heat diffusion from the device active region into the substrate and within the devices was studied. Delays in the thermal response with respect to the electrical pulse were determined at different locations in the devices. Quasi-adiabatic heating of the AlGaN/GaN devices is illustrated within the first of device operation. The temperature of devices on SiC was found to reach of the dc temperature when operated with -long electrical pulses

Social networking: a collaborative open educational resource

Studies undertaken since the introduction of Web 2.0 have focussed mainly on open educational resources (OERs) such as email, blogging and virtual learning environments. No consistent efforts have been undertaken to study the use of social networking sites as a tool for learning in the second language classroom. This study examined the use of social networking sites as a means of establishing an asynchronous online environment for use in language learning in a classroom setting. The findings described are based on the use of a social networking site called Ning in a classroom during a summer school project, which was conducted at a further education college in the UK. The project focussed in particular on informal language practice when using social networking sites. The paper then examines the effect the use of the social networking tool had on group cohesion and learner-to-learner interaction, and how these, in turn, enhanced informal language learning due to an increase in learner collaboration. The study found that the use of Ning enhanced group cohesion and that learners started working in different groups once Ning was introduced. Finally, it highlights the potential technical and administrative barriers that can impede an institution in implementing its educational strategy in regards to OERs – in this case, social networking sites in the classroom. The further education college in which this study took place had no policies in place in regards to the use of OERs in the classroom, thus the paper concludes with recommendations in regards to training and policies so that researchers and practitioners can learn from the project

X-Band GaN SPDT MMIC with over 25 Watt Linear Power Handling

Single pole double throw (SPDT) switches are becoming more and more key components in phased-array radar transmit/receive modules. An SPDT switch must be able to handle the output power of a high power amplifier and must provide enough isolation to protect the low noise amplifier in the receive chain when the T/R module is transmitting. Therefore gallium nitride technology seems to become a key technology for high power SPDT switch design. The technology shows good performance on microwave frequencies and is able to handle high power. An X-band SPDT switch, with a linear power handling of over 25 W, has been designed, measured and evaluated. The circuit is designed in the coplanar waveguide AlGaN/GaN technology established at QinetiQ.\ud \u

Utilization of waveform measurements for degradation analysis of AlGaN/GaN HFETs

This paper employs, for the first time, RF waveform engineering to monitor device degradation over an RF "burn in" period. Measured RF current and voltage waveforms are used to monitor the degradation effects seen in GaN HFET transistors during large signal CW RF stress testing. The technique provides extra information on device performance compared with standard RF performance measures, demonstrating clearly where on the output IV plane the degradation is occurring and allowing device designers advanced insight into the degradation mechanisms limiting RF performance

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

Reducing thermal resistance of AlGaN/GaN electronic devices using novel nucleation layers

Currently, up to 50% of the channel temperature in AlGaN/GaN electronic devices is due to the thermal-boundary resistance (TBR) associated with the nucleation layer (NL) needed between GaN and SiC substrates for high-quality heteroepitaxy. Using 3-D time-resolved Raman thermography, it is shown that modifying the NL used for GaN on SiC epitaxy from the metal-organic chemical vapor deposition (MOCVD)-grown standard AlN-NL to a hot-wall MOCVD-grown AlN-NL reduces NL TBR by 25%, resulting in ~10% reduction of the operating temperature of AlGaN/GaN HEMTs. Considering the exponential relationship between device lifetime and temperature, lower TBR NLs open new opportunities for improving the reliability of AlGaN/GaN devices