VERY-LOW RESISTANCE NONALLOYED AND IN-SITU OHMIC CONTACTS TO N-GAAS USING DELTA-DOPED SURFACE-LAYERS

The electrical and thermal characteristics of three ohmic contact stru ctures: (i) in situ deposited epitaxial Al on delta-doped GaAs, (ii) e vaporated Au-Ge on delta-doped GaAs and (iii) evaporated Au-Ge on GaAs were compared with each other. The I-V characteristics of the non-all oyed Au-Ge and the epitaxial Al, as-deposited, on delta-doped GaAs wer e linear, whereas the non-alloyed Au-Ge contact on GaAs showed a nonli near I-V characteristic, as expected. The specific contact resistivity (rho(c)) of the non-alloyed Au-Ge contacts on delta-doped GaAs was 1. 4 x 10(-6) OMEGA cm2, whereas it was 9 x 10(-7) OMEGA cm2 for the epit axial Al contacts on 6-doped GaAs, which is the lowest ever reported f or a pure metal on delta-doped GaAs. No value of the specific contact resistivity (rho(c)) could be obtained for the Au-Ge contacts on GaAs because of the nonlinearity of the I-V characteristics. After an alloy ing process at 450-degrees-C all samples showed linear I-V characteris tics. The specific contact resistivity of all samples varied inversely with the n+ doping concentration up to 5 x 10(17) cm-3. Above this co ncentration the specific contact resistivity of the Au-Ge and epitaxia l Al contacts on delta-doped GaAs was constant and hardly changed afte r alloying. Our results confirm that the well known double-barrier (me tal-n+-n) model is valid for the ohmic contacts on delta-doped GaAs.