ALBERT

Notes: We have investigated in detail the influence of interlayer structures on nonalloyed ohmic contact resistance (ρc), in terms of the crystalline defects and the potential barrier at the interlayer/GaAs interface. The interlayer structures are a graded-band-gap InAs/GaAs strained-layer superlattice (graded SLS), a graded-band-gap InGaAs, and conventional SLSs without graded band gaps. A two-layer transmission line model indicates that the barrier resistance in the interlayer highly depends on the interlayer structure: ≤5×10−8 Ω cm2 for the graded SLS and graded InGaAs interlayers and 10−5–10−6 Ω cm2 for the conventional SLS interlayers. To explain the large dependence of the interlayer structure, first, the density and distribution of the misfit dislocations and stacking faults caused by the large lattice mismatch between InAs and GaAs have been investigated in detail by high-resolution transmission electron microscopy. In the graded SLS and conventional SLS interlayers, the influence of the high-density depletion regions spread near the crystalline defects is found to be negligible because of the high doping concentrations (∼1019 cm−3) in the interlayers. Second, the potential barrier at the interlayer/GaAs interface has been investigated by simulating the barrier resistance. The potential barrier profile is calculated self-consistently with Poisson's equation and the Schrödinger equation. Tunneling current through the barrier is analyzed using the Wentzel–Kramers–Brillouin approximation or the numerical wave solution to the Schrödinger equation. The graded SLS interlayer has the effectively smooth conduction band profile without the barriers, which is similar to that of the graded InGaAs interlayer, because of its short period SLS. In the conventional SLS interlayers, the reasonable barrier heights of 0.14–0.26 eV obtained by this simulation indicates that these barriers are the dominant factor which increases the contact resistances. For the low-resistance nonalloyed ohmic contact, therefore, a smooth conduction band profile without band discontinuity is more predominant than the reduction in the crystalline defect density.