Characterization of AuGe / Ni / Au Contacts on GaAs / AlGaAs Heterostructures for Low‐Temperature Applications

Abstract

We show that the Ge-to-Ni ratio in the three-level AuGe/Ni/Au contact metallization is an important parameter which determines the value of the contact resistance. An optimum atomic ratio Xc~,./XN~ around 0.8-1.0 is found to reproducibly yield contact resistivities in the range of 10 '-10 2 ~l-mm. The role of the Ge/Ni ratio was observed for contacts to heavily doped GaAs surface layers and for contacts to A1GaAs/GaAs heterostructures used for two-dimensional electron gas (2DEG) transistors and quantum Hall resistance standards. The contacts obtained on 2DEG heterostructure samples maintain their characteristics down to the lowest temperatures tested (0.3 K), showing that this system is suitable for contacts to critical structures such as quantum Hall resistance standards used for metrological purposes.The Au-Ge/Ni/Au metallization system has been intensively investigated for its use as low-resistance ohmic contacts to n-type GaAs and other III-V compounds (1-4) and is still widely used today for the fabrication of a wide range of electronic and optoelectronic devices (5). Because of the recent interest in two-and one-dimensional transport phenomena in semiconductors, the study of the contact behavior at low temperatures has taken on new significance. The Au-Ge/Ni/Au metallization system is suitable for contacts to critical structures such as quantum Hall resistance standards used for metrological purposes at temperatures down to 0.3 K.

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

“…A correlation was reported between the formation of Ni 2 GeAs (between Au-Ga and GaAs) layers and low contact resistance [11]. Thick Ni layers are beneficial in improving morphology [12][13][14], which in turn, influences contact area. However, increasing the Ni-layer thickness may have other undesirable consequences: (a) unreacted Ni may make the structure ferromagnetic, (b) formation of compounds with Ni and Ge may deplete Ge and hence influence contact resistance.…”

Degradation of GaAs/AlGaAs quantized Hall resistors with alloyed AuGe/Ni contacts

“…If the parallel conduction is indeed due to current flow in the cap layer, one would have to etch away the cap layer after alloying the contacts and before depositing the passivating nitride coating in order to eliminate the parallel conduction (as was done by Bühlmann, Ref. [ 16 ]). Alternatively, a heterostructure with a lower donor density in the cap layer, or possibly even an undoped cap layer could be used to prepare the devices.…”

“…The high tunneling resistance of this barrier will cause the contact resistances to be very large. Minimum contact resistances are obtained when the alloying time is long enough for the metal to reach the proximity of the interface between the AlGaAs donor layer and the GaAs channel in which the 2 DEG resides [ 15 , 16 ]. The penetration of the metal to this interface, however, alters the potential well in which the electron gas is confined and reduces the density of electrons in the 2 DEG immediately below the contact.…”

Dependence of melting, roughness and contact resistances on Ge and Ni content in alloyed AuGe/Ni/Au-type electrical contacts to GaAs/AlGaAs multilayer structures

“…The recipe gives low contact resistances in the region of 0.03-0.1 mm; however, this is achieved at the expense of an increased surface roughness [7], a factor that influences the transistor gate fabrication. The surface roughness can be reduced by increasing the Nilayer thickness or decreasing the Ge content below that of the eutectic composition (88:12 wt%), albeit at the expense of increasing the contact resistance [8][9][10][11][12].…”