Adrian Habanyama

By Adrian Habanyama and Craig M. Comrie

Nickel and palladium germanides are the most promising candidates for nano-electronic contact materials to active areas of germanium-based devices. Solid-state reactions were thermally induced in conventional thin film couples of the Ni/Ge and Pd/Ge systems in order to study the sequence of phase formation. By embedding a thin layer of tantalum or tungsten as an inert marker between coupling thin film layers and observing its movement during phase formation, the dominant diffusing species were identified and monitored. In the Ni/Ge system, Ni5Ge3 was the first phase to form followed by NiGe. The results showed that during Ni5Ge3 formation, Ni was the sole diffusing species. During NiGe formation, both Ni and Ge diffused with the Ge diffusion prominent during the early stages, while the later stage of growth was dominated by Ni diffusion. The only phases observed to form in the Pd/Ge system were PdGe and Pd2Ge, the latter being the first. Palladium was the dominant diffusing species during both phase formations. Lateral diffusion couples were also prepared by the deposition of thick rectangular islands of germanium on to thin films of nickel and palladium. Several aspects of thermally induced lateral (as opposed to vertical) growth of phases were studied.

Part of the book: Intermetallic Compounds

By Adrian Habanyama

We examine the reported interface-based processes used in the modulation of Schottky barrier heights at the nickel germanide/n-type germanium and palladium germanide/n-type germanium junctions. Various sample preparation and characterization methods are discussed. Stable Ni/Ge and Pd/Ge structural phases are identified, and their temperature range of stability is established. Current-voltage (I-V) and capacitance-voltage (C-V) characteristics are analyzed to study the effect of various interface control processes. Sheet resistivity and its stability over various annealing temperature ranges are analyzed. The fundamental mechanisms at play in order to achieve ohmic characteristics are observed and analyzed using various interface control processes. Some interfacial and structural factors that pin the Fermi level are analyzed in relation to experimental results. The different interfacial control processes are analyzed, and their effectiveness is compared. Recommendations are made for the improvement of Ni and Pd contacts in the next generation of n-type germanium-based nanoelectronic devices.

Part of the book: Advanced Material and Device Applications with Germanium

By Adrian Habanyama

Metal-semiconductor interfaces are an essential part of any nano-electronic device. One of the concerns in germanium based technology is the presence of Fermi-level pinning (FLP) which leads to large Schottky barrier heights (SBH) for electrons. Details of the factors that pin the Fermi level will be discussed in this chapter. In an Ohmic contact there is an almost unimpeded transfer of majority carriers across the interface. One way to achieve such a contact is by doping the semiconductor heavily enough so that tunneling is possible. Heavy doping is not always advantageous or possible, depending on the type of device being fabricated. Other ways are to locally incorporate dopant atoms at the metal-germanium interface or to insert an interlayer into the interface. In practice, however, the contact resistivity is very sensitive to the interlayer thickness and the temperature of annealing used during the fabrication process. The latter two ways of achieving an Ohmic contact are interface control processes as opposed to the first way which is a bulk process. In this chapter we present the essential theoretical and experimental details required for the examination of some of the novel interface control processes developed for the fabrication of NiGe/n-Ge and PdGe/n-Ge Schottky and Ohmic contacts.

Part of the book: Advanced Material and Device Applications with Germanium