Unexpected current lowering by a low work-function metal contact: Mg/SI–GaAs

The low-bias current–voltage characteristics of surface barrier diodes, based on semi-insulating GaAs (SI–GaAs) with different contact metals and area, are reported and analyzed in order to demonstrate the possibility of tuning their low-bias transport characteristics. Novel applications of SI–GaAs emerge for devices with low-current at low-bias requirements, as follows from the possibility of current lowering by as much as two orders of magnitude, achieved by manipulation of the contact metallization and the contact area. The lowest current is observed in the structure with small Mg top and large-area Ti/Pt bottom contact. Since the observed behavior contradicts the conventional ohmic bulk limited and thermionic emission transport models, alternative explanations are discussed. The strong blocking ability of the low-work function Mg contact was attributed to the downwards band-bending, formation of a quasi-degenerate interface region accumulating charge carriers, and the corresponding lowering of the bulk SI–GaAs free carrier concentration.

► Tuning of SI–GaAs diode low-bias transport is viable using a low work-function Mg metal.
► Vast current lowering (∼100×) is observed in sample with Mg in a role of a blocking contact.
► Data are inconsistent with the ohmic/bulk limited and thermionic emission transport models.
► Explanation involves creation of quasi-degenerate region, Mg-oxide layer and free carriers lowering in SI–GaAs base.