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Title: Novel Contact Formation for 4H-SiCPower Devices
Author: Jennings, Michael Robert
ISNI:       0000 0004 2679 4611
Awarding Body: The University of Warwick
Current Institution: University of Warwick
Date of Award: 2007
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SiC is seen as the material of choice for the next generation of power semiconductor devices because of its potential to deliver high power densities whilst maintaining high efficiency levels. The novel contact formation examined in this· thesis is aimed at improving the robustness and ohmicity of contacts to 4H-SiC. Innovative triple and multiple metallisations based on a combination of Al/Ti and Al/Ni layers have been fabricated, which are aimed at reducing the contact resistance and improving the contact morphology to p-type 4H-SiC. The lowest specific contact resistivity obtained was based on a triple layer AI/Ti/Al contact, measured at 5.0 x 10-6 Ocm2 • This value approaches the state of the art. The multiple layered contacts all dis- . played Schottky-like characteristics even after high temperature annealing. X-ray diffraction scans have indicated enhanced formation of metal silicides with respect to the ohmic triple layered contacts based on Al/Ti (Ti3SiC2) and Al/Ni (NiSix ). Low levels of silicide formation have been found in multiple layered contacts. Scanning electron microscope examination and wavelength dispersive analysis point to the fact that multiple layered contacts display a superior morphology, reducing the amount of Al spreading, which is known to be problematic during a high temperature anneal. A physical and electrical investigation of silicon contact layers grown by molecular beam epitaxy on n-type 4H-SiC (Si/SiC) has been conducted. The effect of the silicon doping concentration, growth temperature and SiC surface cleaning prior to growth have been evaluated. X-ray diffraction and scanning electron microscope analysis have been utilised for the purpose of evaluating the morphology and crystal structure of grown silicon layers. I-V and e-V measurements have shown the rectifying properties of the Si/SiC heterojunction along with their corresponding built-in potentials and energy band offsets. Modelling of Si/SiC heterojunction diodes revealed that the current characteristics can be explained by an isojunction drift-diffusion and thermionic emission model. The modelling results point to the fact that I-V properties of Si/SiC n-N heterojunction diodes are relatively independent of the doping concentration of the grown silicon layer. Preliminary . results obtained from p-type silicon grown by molecular beam epitaxy on n-type 4H-SiC have been presented. A possible application for Si/SiC layers is a vertical heterojunction MOSFET. The establishment of 4H-SiC PiN diode technology is discussed. Scanning electron microscope examination has been utilised for the purpose of evaluating PiN diode fabrication based on shadow masking and liftoff techniques. Reverse bias I-V measurements highlight the fact that mesa isolated PiN diodes display around 5 orders of magnitude lower leakage currents with respect to their un-isolated counterparts. Forward bias I-V measurements have compared performances of diodes with different anode metallisations based on AI, Ni and Ti layers. PiN diodes based on Ti show lower turn-on voltages while mesa isolated PiN diodes displayed higher current densities and more ideal forward characteristics when compared to their un-isolated counterparts. __.
Supervisor: Not available Sponsor: Not available
Qualification Name: Not available Qualification Level: Doctoral
EThOS ID:  DOI: Not available