Metallurgical and electrical properties of Au-Ge, Au/Au-Ge, Ni/Au-Ge and Ni/Au/Au-Ge ohmic contacts to GaAs

Good quality ohmic contacts to GaAs microwave devices are of vital importance for the efficient operation and long term stabilityof these devices. The Au-Ge eutectic alloy system is widely used for ohmic contacts to these devices, but little work has been donetowards a combined detailed metallurgical and electrical study of this system upon heat-treatment. In this work the metallurgical and electrical properties of Au-Ge, Au/Au-Ge, Ni/Au-Ge and Ni/Au/Au-Ge contacts to (100) n-type epitaxial on semi-insulating substrate GaAs are studied for heattreatment up to 550°C. Auger depth profiling (ADP), electron probe microanalysis, electron beam induced current (EBIC) and scanning electron microscopy (SEM) are employed for the metallurgical evaluation, while currentvoltage (I-V), capacitance-voi tage (C-V) and contact resistance (r^) measurements were performed for the electrical evaluation of the contacts.The Au/Au-Ge contacts heat-treated below the Au-Ge eutectic temperature (356°C or 363°C) are found to be rectifying followingthe thermionic-fi eld emission regime appropriate to an equivalent Au/GaAs contact of at least one order of magnitude higher doping.Ge is responsible for this shallow n’ interfacial layer. Ge is shown to distribute itself on preferential sites at the interface. Heat-treatment above the eutectic temperature, results in preferential melting at sites rich in Ge, and rectangular particles are formed in a matrix of bare GaAs. A simple metallurgical model based on the idea that Au-Ge is the main reactive system at the interface is proposed to explain the rectangular particle formation and it is shown to be in good agreement with the experimental evidenceand theoretical predictions. The contact resistance is related to the progress of the metallurgical reactions upon heat-treatment, and it is shown that optimum values of contact resistance are obtained for optimum particle formation (high density-large size). The particles are considered to be the pathways of the current. An overlayer of Ni on to the Au-Ge and Au/Au-Ge contacts, changes the metallurgical behaviour of the films dramatically. Upon heat-treatment Ni rapidly diffuses to the interface, where it forms NiGe and NiAs compounds. As Ge is engaged in compounds with Ni the Au-Ge is not the main reactive system to initiate melting and themetallurgical reactions proceed by solid state diffusion. Compound formation at the reacted GaAs interface and limited Ni diffusion to the unreacted GaAs substrate, are shown to produce optimum values of contact resistance. Compound decomposition and enhanced Ni in-diffusion to the unreacted substrate, produce high values of contact resistance. Accelerated life tests (high temperature storage) reveal no degradation in the Au/Au-Ge contacts, while limited degradation takes place in the Ni/Au/Au-Ge contacts. Annealing temperature and time are found to be major parameters in the progress of the metallurgical reactions and the control of the contact resistance. Optimum heat-treatment cycle is 450°C for 2 \ minutes. An interesting new arrangement for measuring contact resistance,developed in this work, is also presented. The doping range of the GaAs layers is between 9 x 10 '15 -3 cm and 3 x 1017 cm"3 and the contact resistance values range between 1CT4 and 10~7 ohms·cm2 , A model in which the contact resistance is inversely proportional to the donor concentration of the startine material is shown to be in aereement with our data.