Siliziumkarbidelektronik : technologische und werkstoffwissenschaftliche Untersuchungen zur Metallisierung/Kontaktierung
Abstract The wide band gap semiconductor silicon carbide (SiC) has been recognized as a promising material for future applications in optoelectronics, high frequency technologies, power electronics and especially in high temperature electronics. The presented thesis deals with the production of Ohmic contacts on p-doped 4H-SiC and 6H-SiC for future electronic applications. Based on theoretical considerations and comparative calculations of the carrier concentrations it will be shown that the calculation of the electrically active acceptors can be strongly simplified. The SiC properties and their influence on the formation of contacts will be described in detail. The production of Ohmic p-SiC contacts requires a high hole concentration at the interface between p-SiC and the metallization which might be achieved by an aluminum ion implantation. By means of an implantation through a thin Al layer the maximum acceptor concentration shifts towards the substrate surface and thus, a high acceptor concentration near the surface is obtained. No amorphous regions could be detected after the implantation and the subsequent annealing of the highly doped p-SiC. The specific resistance, the carrier concentration and the carrier mobility are estimated from the temperature dependent sheet resistance. The very high hole mobility indicates an almost 100% incorporation of the acceptors. Tungsten silicide and tungsten carbide are used to realize a metallization and to establish a contact with the SiC samples, in order to avoid undesirable interface reactions between the semiconductor and the metallization. For this reason the influence of the substrate and deposition temperature on the formation of tungsten silicide and tungsten carbide by a sputtering (or co-sputtering) process was investigated in detail. The hexagonal WSi2 phase, which is formed on in-vacuo heated substrates, is used as tungsten silicide metallization. WC and W2C are used as tungsten carbide metallization. Pure WC is formed under a high 2% propane concentration in a propane-hydrogen environment at temperatures above 825°C. W2C is formed under a <= 0.02% propane concentration and temperatures between 750°C - 1050°C. A standard Al/Ti metallization serves as a reference material. It is shown that a hexagonal WSi2 and a pure W2C form on the p-4H-SiC and p-6H-SiC samples, while in the WC layers a small amount of the W2C phase is detectable. No reaction with the SiC substrates is observed for the tungsten-based metallizations. The specific contact resistances of the hexagonal WSi2 on the p-4H-SiC and the p-6H-SiC of rho_K = 6 10^-4 Ohm cm^2 and 1.2 10^-3 Ohm cm^2, respectively, represent an improvement by two orders of magnitude to the previous results. The minimum specific contact resistances of WC and W2C on the p-4H-SiC are 8.9 10^-4 Ohm cm^2 and 1.7 10^-3 Ohm cm^2, respectively, and on the p-6H-SiC 1.8 10^-2 Ohm cm^2 and 2.5 10^-3 Ohm cm^2. The smallest contact resistance at all is reached with the reference material Al/Ti on the 4H-SiC sample with the value of 1.9 10^-4 Ohm cm^2. The specific contact resistances of the tungsten based metallizations and the reference materials lie in the same order of magnitude.