Nonalloyed and alloyed low-resistance ohmic contacts with good morphology for GaAs using a graded InGaAs cap layer

Abstract

Using a thin graded layer of InGaAs starting on GaAs and becoming InAs on the top, low‐resistance alloyed and nonalloyed ohmic contacts have been achieved on n⁺‐GaAs epilayers grown by molecular‐beam epitaxy on a semi‐insulating GaAs substrate. In addition, by a suitable choice of the multilayer ohmic metals and by an optimization of the alloying process, good surface morphology was obtained. The transmission‐line model is used to extrapolate contact resistances from measurements on test patterns with multiple gap spacings varying from 1 to 10 μm. The nonalloyed contact resistance is found to be 0.025 Ω mm for a GaAs layer doped to 1×10¹⁸ with a 500‐Å graded InGaAs layer. Alloying the contact at 475 °C for 60 s produces a contact resistance of 0.019 Ω mm. This represents a substantial improvement over the contact resistances obtained by just using an ungraded cap layer of InAs on GaAs. Assuming that the sheet resistance under the contact is the same as the sheet resistance in the top semiconductor layer (this is not strictly true), the nominal value of the specific contact resistance for the nonalloyed situation is 5.32×10⁻⁷ Ω cm², while for the alloyed case the specific contact resistance is 2.56×10⁻⁷ Ω cm². The metal scheme used for the contact is Ni/Ge/Au/Ti/Au.