3 - Ohmic Contacts to II–VI and III–V Compound Semiconductors

Abstract

This chapter clarifies the current understanding of the effects of inter-racial reactions occurring during formation of ohmic contacts, rather than to compile various recipes for their formation. New generations of high speed transistors, such as high electron mobility (HEMTs) and heterojunction bipolar transistors (HBTs), have been developed from the superior transport properties of III–V semiconductors. Epitaxial growth techniques—such as molecular beam epitaxy (MBE) and chemical vapor deposition (CVD)—on an atomic scale, enabled such development of heterojunction devices with different compositions of ternary or quaternary compounds. Therefore, bandgaps offer higher frequency and power characteristics unattainable in Si-based devices. In addition to the superior transport properties, III–V and II–VI semiconductors have direct bandgaps, enabling quantum well heterostructures for optoelectronic applications such as light-emitting diodes (LEDs) and diode lasers (DLs). For Gallium arsenide (GaAs) and Indium phosphide (InP), where Fermi level pinning has a significant effect on ohmic contacts, incorporation of dopants upon epitaxial regrowth of the semiconductor is the dominant method of forming ohmic contacts. The epitaxial regrowth of doped GaAs or InP is achieved by interfacial reactions, which dissociate the semiconductor lattice. Subsequent reactions decompose the phases bonded to the semiconductor elements and allow them to epitaxially regrow in the solid phase. For GaN and ZnSe, the Fermi level is not completely pinned and therefore it should be easier to form ohmic contacts. This is true for n-type materials where metals with work functions less than the electron affinities result in ohmic contacts, often after heat treatment to penetrate inter-racial contamination layers.