Studies on Silicon Carbide: Heteroepitaxy on Silicon and Titanium Alloy Ohmic Contacts

The first half of this dissertation describes the design and construction of a new low pressure MOCVD reactor. Various problems and concerns with leakage, fitting and safety were solved to make the reactor more reliable and safer.

For SiC growth on Si substrates, the so-called two-step growth procedure was adopted and a single precursor hexamethyldisilane was used. XRD results showed that the films on Si(100) were polycrystalline, while epilayers on Si(111) are single crystalline. A few GaN films were also epitaxially grown on Si(111) using a buffer layer of SiC. A possible explanation for the advantages of the two-step over the one-step growth procedure is proposed involving the interfacial energy and total energy change during nucleation.

The second half of the dissertation deals with a microstructural investigation of multi-layer Ti alloy contacts to SiC. For this purpose, numerous sitespecific TEM thin foils were prepared for (Au/)TaSi₂/Ti ohmic contacts with different annealing histories using the Focus Ion Beam liftout technique. Subsequently, the thin foils were studied by various imaging and analytical techniques.

On either sides of the Ti layer in the TaSi₂/Ti/SiC multi-layer structure, two interfaces could be clearly observed. While no C or Si were detected in the Ti contact layer of the as-deposited sample, the annealed samples exhibited strong Si and C signals throughout the Ti layer. This is indicative of the formation of new phases such as silicide(s) and/or carbide(s) of titanium. Moreover, the TaSi₂ layer had acted as a diffusion barrier and prevented the diffusion of Ti to the surface and in-diffusion of oxygen.

In samples with four more layers (i.e. Au/Ti/TaSi₂/Ti in addition to TaSi₂/Ti/SiC, making a seven-layer contact), which were annealed from 1 to 50 hours, a thick interface had formed between the upper TaSi₂ layer and the top Au layer. In addition, on the surface of the contacts, cross-linked Au micro-wires had formed under which many irregular dark patches were present with high concentrations of C, O and Si.

Using the microcharacterization information, we have proposed a failure mode and mechanism for the Ti alloy contacts.