FABRICATION AND CHARACTERIZATION OHMIC CONTACTS MADE WITH AU/NI/GE/AU MULTILAYER METALLIZATION SYSTEM ON N-TYPE GALLIUM ARSENIDE

Abstract

One of the most populariy used method in forming ohmic contact on n-type GaAs is to deposit and anneal Au/Ni/Ge/Au multilayer metallization on n-type GaAs. In this research, the author studied the metallurgical and electrical properties of the Au/Ni/Ge/Au system on n-GaAs. Various contact resistivity measurement methods used by different authors were reviewed. These include the transfer length method (TLM) with adjacent rectangular pads, the TLM with strips between two rectangular pads, and the Kelvin's method. Given the constraint of wafer structure, the TLM with strips between two rectangular pads, modified to include current spreading and metal sheet resistance, is selected. A new method which involves a structure simple to fabricate and easy to measure is developed for contact resistivity measurement. Simplicity in fabrication and measurement makes this method easily applicable in the industry for metallization process monitoring. Various parameters which may affect the results of contact resistivity measurement are studied. It is found that thickness of n-layer GaAs, current density, exposure to light intensity and the direction of current flow through the samples have no significant effect on the values of the contact resistivity measured. Metallurgical and electrical properties of the Au/Ni/Ge/Au system on n-GaAs were studied. Experimental results show that by increasing the external Au layer thickness, contact resistivity drops and saturates at a thickness of about 6000 Å. Secondary ion mass spectroscopy (SIMS) indicates that the improvement in contact resistance is due to Au 'regulating' the amount of NiAs formed, leading to an increase in the area fraction covered by the Ni 2 GeAs. Experimental results indicate that the inner-most Au layer has little effect on contact resistivity. Experimental results also indicate that Ni:Ge ratio is critical in obtaining low contact resistivity with the optimal ratio at 1:1. From these two sets of results, it is clear that contact resistivity is controlled by the Ni:Ge ratio but not the Ni:AuGe ratio, SIMS analysis indicates that with a thin layer of Ni, (Ni:Ge ratio< 1) the catalytic effect of Ni in decomposing GaAs is minimal, resulting in a higher contact resistivity. For a thick Ni layer, however, (Ni:Ge ratio > 1) excessive NiAs is formed. This NiAs reduces the effective area of Ni2Ge.As on n-GaAs and hence increases the contact resistivity, When the total metallization system thickness is increased, while maintaining a constant ratio of the elements present, contact resistivity is found to increase, SIMS analysis indicates that with a thicker metallization layer, Ni spreads over a larger region which results in a lesser amount of Ni at the metal/GaAs interface. The catalytic effect of Ni on decomposition of GaAs is reduced, resulting in an increase in contact resistivity. Studies on the effect of different annealing temperatures on contact resistivity indicate that the annealing temperature must be greater than the eutectic temperature of AuGe (363°C) to produce low contact resistivity. It is found that with increasing doping concentration, the contact resistivity reduces. Finally, a metallization system with good contact characteristics is proposed.