Influence of nickel layer thickness on the magnetic properties and contact resistance of AuGe/Ni/Au Ohmic contacts to GaAs/AlGaAs heterostructures

The magnetization of alloyed Ohmic contact film structures of the form AuGe/Ni/Au deposited on a GaAs/AlGaAs heterostructure substrate are reported as functions of Ni-layer thickness and alloying temperature. The observations are correlated with contact resistance and surface morphology studies. It is found that drops in magnetization, due to conversion of Ni to a non-magnetic phase or alloy, begin at anneal temperatures as low as 100 °C for all Ni-layer thicknesses. The conversion is completed at an anneal temperature, T _A, that increases with Ni-layer thickness. T_A varies from 100-200 °C to 400-430 °C as Ni-layer thickness is varied from 10 to 100 nm for an AuGe (88 : 12 wt%) layer thickness of 100 nm. The electrical contact formation, however, appears to begin at much higher temperatures than 100 °C. Lowest contact resistance (0.05 0.01 Ω mm) is obtained when Ni thickness is about 25 nm for 100 nm AuGe layer thickness, anneal temperature is 400 °C and anneal duration is 60 s. This contact is non-magnetic. Measurements on samples with other AuGe layer thicknesses suggest that the contact resistances are comparable to this optimum value, if the ratio of AuGe layer thickness to that of Ni is ≥4. Increasing the Ni-layer thickness reduces the roughness of annealed contacts, but also increases contact resistance. The magnetic measurements are suggestive of a transformed Ni-layer thickness proportional to the thickness of the underlying AuGe layer. It is proposed that Ni diffuses into AuGe in a concentration limited diffusive mechanism followed by segregation into Ni₃Ge.