Thermally Stable, Low Resistance Indium-Based Ohmic Contacts to n and p-Type GaAs

ABSTRACTRecently, thermally stable, low resistance In-based ohmic contacts to n-type GaAs have been developed in our laboratories by depositing a small amount of In with refractory metals in a conventional evaporator, followed by rapid thermal annealing. By correlating the interfacial microstructure to the electrical properties, InxGa1-xAs phases grown epitaxially on the GaAs were found to be essential for reduction of the contact resistance (Rc). This low resistance was believed to be due to separation of the high barrier (φb) at the metal/GaAs contact into two low barriers at the metal/InxGa1-xAs and InxGa1-xAs/GaAs interfaces. In this paper the effects of the In concentration (x) in the InxGa1-xAs phases and addition of dopants to the contact metal are presented. High In concentration is desirable to reduce the φb at the metal/InxGa1-xAs interface. Such contacts were prepared by sputter-depositing InAs with other contact elements, but the low Rc values were not obtained. The reason was explained to be due to an increase in the φb at the InxGa1-xAs/GaAs interface due to the formation of misfit dislocations. However, addition of a small amount of Si to the contact metals reduced significantly the Rc value. This contact demonstrated excellent thermal stability: no deterioration was observed at 400°C for more than 100 hrs. In addition, the use of this Ni(Si)InW contact metal allowed us to fabricate the low resistance ohmic contacts by one-step (simultaneous) annealing for “implant-activation” and “ohmic contact formation”, which simplifies significantly GaAs device fabrication process steps. For p-type ohmic contacts, low resistance contacts were fabricated by depositing the same NilnW contact material to p-type GaAs. This contact was also thermally stable during subsequent annealing at 400°C. Within our knowledge this is believed to be the first demonstration of low resistance, thermally stable ohmic contact fabrication using the same materials for both n and p-type GaAs.