The influence of glow-discharge cleaning of Ge/Pd-GaAs on the specific contact resistivity

Compound semiconductors formed from elements in groups 111 and V (111-V) of the periodic table, such as GaAs, have become important materials for microelectronic and optoelectronic applications. These devices are operated at high current densities and are continually undergoing reduction in size. In order to link the active regions of the semiconductor devices to the external circuit, contacts with low electrical resistance (ohmic contacts) are required. This shldy is part of a wide Cffort oriented on improving the electrical properties of ohmic contacts and thus their electrical resistance (specific contact resistivity). Our investigation is focused on capping layer (mainly Pt)/(Sn or Ge)/Pd contact structure on . ' n-type GaAs. Optimization of doping elements [1,2], deposition methods [2], inert atmosphere during annealing [3], absorption-capping layers [1,2,4], cleaning solutions (wet etching) [4] and alloying methods [4] was carried out previously. This paper deals with dry etching of GaAs in situ prior to the deposition of Ge and Pd. Glow-discharge cleaning should be principally applied prior to every vacuum coating process, as every substrate surface, even after careful mechanical cleaning, will retain a water film of several molecular layers. Glow-discharge in this application may be defined as the removal of the adsorbed impurity layers. The substrate to be coated is primarily submitted to a cleaning process. At the same time the substrate temperature rises which is desirable in most cases. An additional positive effect of the glow-discharge is the increased formation of nuclei on the substrate surface. All the above effects contribute to increase the adhesive strength and purity of vapour deposition layers [SI. Annealing is a very important step in the preparation of ohmic contacts. The most used is rapid thermal annealing (RTA) using halogen or discharge lamps. In this rapid heating, only solid-phase reactions occur at the interface. RTA method can also reduce the loss of volatile group V materials, the intermixing of metals and semiconductors, phase segregation and grain growth [6] . Experiments were made in order to optimize the parameters of the glow-discharge cleaning such as polarity of electrode resulting in sputter process or sputter etching, time of cleaning and applied current density of glow-discharge (in range from 56pA.ctK2 to 616 pA.mi2).