Mohammad Saleh N Alnassar

Graphitic Schottky Contacts to Si formed by Energetic Deposition

Carbon films deposited by filtered cathodic vacuum arc have been used to form high quality Schottky diodes on p-Si. Energetic deposition with an applied substrate bias of -1 kV and with a substrate temperature of 100 °C has produced carbon diodes with rectification ratios of ~ 3 × 106, saturation currents of ~0.02 nA and ideality factors close to unity (n = 1.05). Simulations were used to estimate the effective work function and the thickness of an interfacial mixed (C/SiO2) layer from the current/voltage characteristics of the diodes.

Optimisation of Schottky electrode geometry

The geometry of the Schottky contact electrode is important in the design of Schottky power diodes. This work focuses on the optimum shape of the Schottky contact geometry and uses finite element modeling to determine the effects of the shape on electrical characteristics of a diode. The investigation considers the typical situation where the contact is smaller than the substrate area. Simulations were run with different shapes ranging from perfect square to perfect circle with the size of the diode substrate (die) and the distance between the edge of the diode and edge of the Schottky contact as a constant. The different models were examined and compared with magnitude the occurrence of the maximum current density (for a particular output current) and hence the breakdown regions at current density approaching the critical value for breakdown (most likely destruction of a diode) due to high current density. There as an optimum geometry determined for the highest current that the given diode substrate could deliver. The results clearly show that the optimum geometry for the Schottky contact should be neither perfect square nor perfect circle, but an exact geometry in between. This optimum geometry gives the optimum distribution of current density around the edge of the Schottky contact. Investigation is done using Synopsys TCAD. The forward and reverse bias situations were investigated to optimize the electrode geometry.

Circular Cross Kelvin Resistor test structure for low specific contact resistivity

In determining the specific contact resistance of an Ohmic contact, using conventional Cross Kelvin Resistor (CKR) test structures, the errors in doing so occur from parasitic resistances around the contact. These parasitic resistances are difficult to determine because no convenient analytical expression is available to calculate this resistance. However, electrical current entering a circular contact uniformly from all directions can be modeled using analytical expressions. Here we present a new test structure where parasitic resistance can easily be calculated because it occurs between concentric equipotentials. This resistance is then subtracted from the total resistance to give the resistance due to the contact interface and hence the specific contact resistance of that interface. Using aspects of the CKR and the Circular Transmission Line Model (CTLM) we have designed a new test structure, here called the Circular Cross Kelvin Resistor (CCKR) test structure for determining specific contact resistance.

Electrical characterisation of metal contacts to 4H-SiC enhanced by pre-metalisation surface treatment

4H-SiC substrate samples were etched on the surface in order to alter the surface stoichiometry, to make the surface carbon rich using solutions of HF/Ethyl Glycol and HF/H2O. The samples were inspected with AFM, XPS, Raman and electrically (current - voltage) tested by applying probes across two contacts each sample. The results show that in comparison to the unetched samples, the current through the etched samples is approximately two times larger in the HF/EG etched sample and approximately twenty times higher in the HF/H2O etched sample. The experiment shows that selective etching has altered the surface electrical conductivity, surface roughness selectively removed silicon atoms leaving a carbon rich surface.

Improvement of Schottky power diode performance by electrode geometry and surround trenching of Schottky contact

This work aims to improve the performance of SiC Schottky power diode with the alternative approach by using trenching around the Schottky electrode. The research was done with PATRAN, a finite modelling method (FEM). The modelling was done with the building of samples with different geometries into two types (the first without trenching around the Schottky electrode and the second was with trenching around Schottky electrode) with the same current through put for every models from both groups). The results show that there is big improvement as the Schottky electrode can take as much as two times the current with the trenching samples in comparison to the non-trenched samples.

Low temperature of formation of nickel germanide on crystalline germanium

Thin films of nickel germanide conveniently form at the relatively low temperature of 300C in a matter of minutes and at even lower temperatures over a longer time. Here we report on the formation of NiGe on crystalline germanium substrates at low temperatures (less than 300C). Ni films deposited on Ge substrates formed NiGe by heating the samples in an atmosphere nearly void of oxygen. Ni films of thickness 50 to 400nm were deposited on crystalline germanium and heat treatments undertaken on samples for time durations at different temperatures of 5 minutes to 12 hours. It was found that the thickness was not a significant factor and that NiGe formed in a few minutes for all thicknesses heated at 300 C. Long durations were required for the lowest temperature of formation.

Investigation of amorphisation of germanium using modeling and experimental processes

Crystalline germanium substrates were amorphised to a depth of one micron by ion implantation of germanium ions at a series of relatively high energies and dose. Using the ion implantation modeling software TRIM, this paper compares the amorphisation results from the ion implantation simulations and experimental results from transmission electron microscopy (TEM) analysis of cross-sections of implanted samples. TEM cross-section micrographs show a clear boundary between amorphous and crystalline germanium. The effect of amorphisation of Ge on the subsequent formation of Nickel germanide is demonstrated and one significant issue is the increased depth of NiGe grains formed on a-Ge compared with c-Ge.