Ohmic Contacts to p‐ and n‐type GaAs Made with Al‐Sn‐Ni

Abstract

We report the results of preliminary investigations into the suitability of the metallization system, aluminum-tinnickel, as an ohmic contact to both p-type and n-type GaAs. We have found that the A1-Sn-Ni contact is similar to A1-Ge-Ni in that it makes an adequate ohmic contact to both types of GaAs (with specific contact resistances as low as 1.0 • 10 4 ~_cm2), but the A1-Sn-Ni metallization has more uniform surface morphology after annealing. This metallization scheme is most appropriate for test structures in which rapid turnaround and low processing costs are essentiM.After thirty years of fabrication of GaAs devices and integrated circuits, it has been established that the best ohmic c~ontacts are made from gold alloys, and different alloys are needed for n-type and p-type contacts. 1 This complicates GaAs processing, requiring separate deposition steps, separate masking or etching procedures, and so on. An additional problem in the extensive use of goldbased contacts is the high cost of the base material. Consequently, there has been a steady search for a low cost metal system that can be used for both n-and p-type contacts. One promising alternative is the A1-Ge-Ni system, first employed by Zuteeg et al. 2 who demonstrated that this metallization can produce ohmic contacts to n-type GaAs with high radiation hardness and specific contact resistance competitive with Au-Ge-Ni. Additional work by Graham et al. 3 revealed that this metallization made an excellent contact to p-type material and maintained an atomically fiat interface with the GaAs after alloying, ~ without the spiking so prevalent with Au-Ge-Ni. Unfortunately, the A1-Ge-Ni contact shows a tendency to produce unusual surface features after alloying, Ge-rich, raised protrusions, which could interfere with later processing steps like wirebonding? An investigation for an alternative to germanium has been carried out, and it has been discovered that tin appears to be a suitable choice. As the results in this paper detail, the A1-Sn-Ni metallization can produce a contact ohmic to both n-type and p-type GaAs, and it has morphological characteristics superior to the A1-Ge-Ni contact. It is especially easy to pattern A1-Sn-Ni with the ]ift-off process and the surface remains remarkably smooth and planar after alloying. Experimental and Measurement DetailsIn the search for an alternative to germanium in the aluminum based contact, the various column IV elements, silicon, tin, and lead, were tested for suitability with a simple procedure. In separate experiments, metallizations consisting of 300 A column IV element, 300 ,~ Ni, and 1000 .~ A1 were evaporated onto n-type GaAs substrates (No = 1 • 1018 cm-3), fashioned into large circular patterns (100 ~m diam) by lift-off photolithography, and tested electrically for ohmic character before and after alloying steps. Curt-* Electrochemical Society Active Member.ously enough, the AI-Si-Ni contact always demonstrated rectifying characteristics, regardless of alloying time, temperature, or ambient. This is con...

Ohmic contact to p-type GaAs using Cu3Ge

“…The NiInW system has been shown to form an ohmic contact to n-type (ϳ3ϫ10 18 cm Ϫ3 ) GaAs with a specific contact resistivity of 3ϫ10 Ϫ6 ⍀ cm 2 , and with the addition of Mn form an ohmic contact to p-type (ϳ2ϫ10 19 cm Ϫ3 ) GaAs with c ϭ1ϫ10 Ϫ5 ⍀ cm 2 . 7 Roedel et al 8 used an Al/Sn/Ni scheme to form ohmic contacts to both n-and p-type GaAs. The specific contact resistivities achieved with this contact scheme were 1ϫ10 Ϫ4 ⍀ cm 2 on n-type (ϳ3ϫ10 18 cm Ϫ3 ) GaAs and 5ϫ10 Ϫ4 ⍀ cm 2 on p-type (ϳ3ϫ10 19 cm Ϫ3 ) GaAs.…”