Temperature dependence of transient and conventional annealed AlGaAs/GaAs MODFET ohmic contacts

Au/Ge/Ni ohmic contacts to AlGaAs/GaAs MODFET structures produced by conventional furnace and transient furnace annealing are examined. TLM measurements, as a function of ambient temperature, revealed that in both types of contact the contact resistance increased as the ambient temperature decreased. The transient-furnace-annealed samples consistently exhibited lower contact resistances than the conventionally annealed ones over the temperature range 80-300 K. An improved TLM technique that allows for corrections that take into account the finite metal layer sheet resistance, as well as the modified sheet resistance of the underlying alloyed semiconductor layer, is described and implemented. The specific contact resistivity, corrected for these effects, is an order of magnitude larger than the one extracted using the traditional TLM technique. In addition, it increases with reduced ambient temperature and levels off at low ambient temperatures. This observation is in good agreement with the classical thermionic-field emission model of the specific contact resistivity, which changes to a dominating field emission (tunneling) dependence at low temperatures