Microstructural characterisation of TiAlTiAu and TiAlPdAu ohmic contacts to AlGaN/GaN

Ti/Al/Ti/Au and Ti/Al/Pd/Au contacts to AlGaN/GaN have been investigated to ascertain the effect of annealing temperature on the structural evolution of the contacts. Ti/Al/Ti/Au contacts become ohmic after rapid thermal annealing at 750°C or higher, corresponding to the formation of an interfacial TiN phase, with inclusions penetrating through the AlGaN layer observed after annealing at 950°C. The Pd layer is shown to be more efficient at inhibiting diffusion of Au to the interface than Ti. Ohmic behaviour was not seen with the Ti/Al/Pd/Au scheme. Either the presence of Au at the interface may improve ohmic behaviour, or the Ti:Al ratio is insufficient in this scheme

Microstructural characterisation of TiAlTiAu and TiAlPdAu ohmic contacts to AlGaN/GaN

Ti/Al/Ti/Au and Ti/Al/Pd/Au contacts to AlGaN/GaN have been investigated to ascertain the effect of annealing temperature on the structural evolution of the contacts. Ti/Al/Ti/Au contacts become ohmic after rapid thermal annealing at 750°C or higher, corresponding to the formation of an interfacial TiN phase, with inclusions penetrating through the AlGaN layer observed after annealing at 950°C. The Pd layer is shown to be more efficient at inhibiting diffusion of Au to the interface than Ti. Ohmic behaviour was not seen with the Ti/Al/Pd/Au scheme. Either the presence of Au at the interface may improve ohmic behaviour, or the Ti:Al ratio is insufficient in this scheme

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

Field plate designs in all-GaN cascode heterojunction field-effect transistors

Different source field plate (FP) connections are compared for the all-GaN integrated cascode device to address the capacitance matching and turn-off controllability issues reported in the conventional GaN plus Si cascode. The experimental results suggest that the cascode device with an FP connected to the source terminal can significantly suppress the off-state internode voltage, leading to minimized capacitive energy loss and reduced overvoltage stress at the internode. This is attributed to the reduced ratio of the drain-source capacitance of the depletion mode cascode part to the total capacitance at the cascode internode. An additional FP on the E-mode cascode part is proposed to further suppress the off-state internode voltage and benefit the device. Cascode devices with the source FP connecting to the enhancement mode gate have an improved switching controllability via gate resistance during turn-off and hence enhanced dv/dt immunity in the drain loop

The relative importance of electron-electron interactions compared to disorder in the two-dimensional "metallic" state

The effect of substrate bias and surface gate voltage on the low temperature resistivity of a Si-MOSFET is studied for electron concentrations where the resistivity increases with increasing temperature. This technique offers two degrees of freedom for controlling the electron concentration and the device mobility, thereby providing a means to evaluate the relative importance of electron-electron interactions and disorder in this so-called ``metallic'' regime. For temperatures well below the Fermi temperature, the data obey a scaling law where the disorder parameter ($k_{\rm{F}}l$), and not the concentration, appears explicitly. This suggests that interactions, although present, do not alter the Fermi-liquid properties of the system fundamentally. Furthermore, this experimental observation is reproduced in results of calculations based on temperature-dependent screening, in the context of Drude-Boltzmann theory.Comment: 5 pages, 6 figure

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

Vertical field inhomogeneity associated with threading dislocations in GaN high electron mobility transistor epitaxial stacks

A measurement technique combining Kelvin-probe force microscopy with substrate bias is developed and demonstrated on AlGaN/GaN-on-Si device structures under conditions relevant to the effect of off-state drain bias stress in transistors. For a high substrate bias, the measurements show a significantly lowered surface potential surrounding a small proportion of dislocations imaged with atomic force microscopy (AFM), laterally extending on a scale of up to a micrometer. Both the density and the size of those features increase with substrate bias; however, conductive AFM measurements under the same bias conditions showed no leakage reaching the surface associated with those features. Our model considers localized conductive paths that end a certain distance below the 2D electron gas electrically “thinning” the epitaxy and, therefore, deforming the potential and increasing the electric field under off-state stress bias. The conclusion is that the vertical electric field in the buffer is laterally highly non-uniform with an enhanced vertical field in the vicinity of those dislocations. This non-uniformity redirects the substrate bias stress from the buffer to the channel with potential consequences for breakdown

Enhancement-mode metal-insulator-semiconductor GaN/AlInN/GaN heterostructure field-effect transistors on Si with a threshold voltage of +3.0V and blocking voltage above 1000V

Enhancement-mode AlInN/GaN metal–insulator–semiconductor heterostructure field-effect transistors on silicon are reported. A fluorine-based plasma treatment and gate dielectric are employed, and the devices exhibit a threshold voltage of +3 V. A drain current density of 295 mA/mm for a gate bias of +10 V is measured. An excellent off-state blocking voltage capability of 630 V for a leakage current of 1 µA/mm, and over 1000 V for 10 µA/mm are achieved on a 20-µm-gate–drain separation device at gate bias of 0 V. The dynamic on-resistance is ~2.2 times the DC on-resistance when pulsing from an off-state drain bias of 500 V