Microstructural analysis of Au/Pt/Ti contacts to p-type InGaAs

Abstract

A detailed study on the microstructural changes that occur on annealing of Au/Pt/Ti ohmiic contacts to n-type InGaAs has been carried out. The metal layers were deposited sequentially by electron beam evaporation onto InGaAs, doped with Zn to a level of 7 x 1018 cm -3, that was epitaxially grown on ( 1 0 0~> InP substrates. The deposition sequenc.e and metal layer thicknesses were: Ti (25 or 30 nm), Pt (25 or 30 nm) and Au (250 or 300 nm). Samples were annealed at temperatures ranging from 250-425 °C in a nitrogen atmosphere. As-deposited contacts were Schottky barriers, while a minimum contact resistance of 2 × 10 -5 O cm 2 was obtained by annealing in the 375-425 °C range. Annealing resulted in the inward diffusion of Ti and outward diffusion of In and As, leading to the formation of TiAs, metallic In and Ga-rich InGaAs at the Ti/InGaAs interface. The Pt diffusion barrier was effective in preventing In diffusion into the outer Au layer and minimizing Au diffusion to the semiconductor. IntroductionDuring device operation, electrical communication links between the active regions of semiconductor devices and the external circuit are provided by metal/ semiconductor contacts. Ohmic contacts on InPbased materials are essential for electronic devices such as field effect transistors (FET), junction field effect transistors (JFET), high electron mobility transistors (HEMT) and heterojunction bipolar transistors (HBT), as well as for photonic devices, such as long-wavelength laser diodes, light-emitting diodes (LED) and photoelectronic and solar cells. Semiconductor devices are being continually scaled down in size, and operated at higher current density and elevated temperature, placing ever-increasing demands on ohmic contact performance. The main ohmic contact requirements are low contact resistance and high thermal stability, both of which are affected by the metallization microstructures, as well as good adhesion and a shallow reacted zone at the metallization/semiconductor interface. It is essential to understand the relationship between the microstructure and the electrical properties of the metal/semiconductor contacts in order to fabricate high-quality and reliable ohmic contacts. Transmission electron microscopy (TEM) combined with energy dispersive X-ray spectroscopy is one of the best techniques to monitor and analyse the microstructural changes that occur during annealing. It allows direct observation of interfaces and provides both crystallographic and chemical composition information with high spacial resolution in three dimensions.

Thermodynamic assessment of the Pt–Ti system

“…The intermediate phases Ti 3 Pt, ␣TiPt and ␤TiPt, each of them having a homogeneity range, were treated as (Pt, Ti%) m (Pt%, Ti) n by a two-sublattice model [20,21]. The symbol % denotes the major component in the corresponding sublattice.…”

“…Ti-Pt metallizations are commonly used as ohmic contacts to InGaAs [1][2][3][4] and Schottky contacts to SiC [5]. Interfacial reactions between Ti/Pt metallizations and semiconductor compounds are of interest, because of the technological importance for successful semiconductor device applications under extreme conditions [5].…”

Indium Phosphide Heterobipolar Transistor Technology Beyond 1-THz Bandwidth

“…If the Pt/Pd layer is too thick, ρ c is increased because of deep interdiffusion [13] into the doping-graded base; a very deep penetration will cause basecollector junction failure. If the Pt/Pd layer is too thin, the Ti layer can reach the base semiconductor, forming alloys with As [14] driven by thermal processing and increasing ρ c . If the base-emitter spacing is insufficient, lateral Pd/Pt interdiffusion toward the emitter will cause emitter-base junction failure.…”

Integrated detectors for embedded optical interconnections on electrical boards, modules, and integrated circuits

“…Schottky contacts of 40-Å Pt/ 350-Å Ti/ 400-Å Pt/ 2500-Å Au were deposited, and interdigitated fingers 100 m long with 2-m finger width and 2-m finger spacing were patterned on a 100 m 150 m detection area. The 400-Å-thick Pt layer is a diffusion barrier for the Au into the Ti during the polymer waveguide thermal curing process [21]. By utilizing the Pt diffusion barrier on the MSM, the Schottky barriers are not degraded by the waveguide thermal process and, thus, the dark current is not degraded by the integration process.…”