Ion-Implanted Evaporated Germanium Layers as n.sup.+ Contacts to GaAs

This invention relates to GaAs devices and more particularly to a means and method for providing a simple, reliable n.sup.+ contact for such devices.

Heretofore various methods have been used to form contacts on GaAs devices. These include an epitaxy n.sup.+ layer deposited on a n.sup.+ GaAs substrate. The n.sup.+ layer is etched in areas other than in the contact regions to get down to the n layer. This process involves complicated chemical deposition equipment which has inherent limitations for large surface devices. Another method involves implanting ions directly into the n GaAs to form an n.sup.+ layer in a planar format. High-temperature annealing is required to activate the implanted n.sup.+ species and an adequte encapsultant has not been developed to provide contacts of about 10.sup.18 ions/cc doping in reproducible format. N.sup.+ dopants may be diffused directly into the GaAs surface. This technique has not been applied successfully due to the high-temperature drive required and to the associated deleterious loss of As. Another method is to make use of an alloy system such as AuGeNi to form the n.sup.+ contact. AuGe is evaporated in an 88/12 composition ratio followed by a thin Ni overcoat to prevent "balling up" during sintering with the substrate. This process is not reliable and has reproducibility problems.

This invention provides a simple, reliable n.sup.+ contact technology for discrete GaAs devices and for the emerging GaAs integrated circuit technology. Super-abrupt contacts are made because of a vacuum deposition process which initially forms the film on the substrate and ion implantation which dopes the film n.sup.+ over its entire area. These allow a much lower anneal temperature than required in the prior art.

FIGS. 1-6 illustrate the separate steps of the process for aming n.sup.+ contacts on GaAs.

The invention will be described referring to FIGS. 1-6. FIG. 1 illustrates a GaAs substrte 10 with a thin film of Ge (p-type) 12 having a thickness of about 3000 A deposited on the substrate. The substrate is placed in an ultra-high vacuum system evacuated to about 1.times.10.sup.-9 Torr. The substrate is heated to a temperture of about 575.degree. C. for about 15 minutes to desorb the gases within the GaAs substrate. Subsequent to heating the substrate, the substrate is cooled to 475.degree. C. and the Ge film is deposited evenly over the surface of the GaAs sunstrate to form a super-abrupt interface. The Ge-coated GaAs substrate is cooled in the vacuum chamber and then removed from the vacuum chamber.

As shown in FIG. 2, phosphorous or arsenic ions 14 are implanted in the Ge film to a depth of about 1500 A with a dosage of about 5.times.10.sup.18 ions/cc. Any well-known equipment can be used for the ion implantation.

As shown in FIG. 3, the ion-planted Ge-GaAs is capped over the Ge film and the periphery with a chemical vapor deposit of oxide, (SiO.sub.2)16 at a temperature of about 400.degree. C. Subsequent to capping the Ge-GaAs, the composite is annealed at a temperture of up to about 500.degree. C. for about one hour in an inert gas ambient. Annealing the composite over-compensates the initial p-type Ge layer which results in the germanium film 18 becoming n.sup.+. The oxide cap on the periphery prevents the arsenide from evaporating or dissociating during the anneal process.

As shown in FIG. 4, the CVD oxide cap has been removed by any well-known chemical etch process which does not attach the Ge film-GaAs.

FIG. 5 illustrates the finished GaAs device. Once the CVD oxide cap has been removed, as shown in FIG. 4, photolithographic processes can be used to etch the Ge film 18 to form a simple contact configuration. Once the simple contact configuration has been formed, the device is prepared by well-known processes and placed in a vacuum in which NiAu, Ni, or Au ohmic metalization is deposited onto the germanium contact configurations to form contacts 20 and 22.

The finished GaAs device shown in FIG. 5 may be capped again with a CVD oxide 24 as shown in FIG. 6 and annealed at a temperature of about 500.degree. C. for a time of about one hour to drive a shallow 200-500 A n.sup.+ germanium diffusion into the GaAs substrate. Subsequent to the anneal, the oxide cap 24 is removed.

Germanium is a material that lattice and thermally matches GaAs. One of the unique features of this invention is using a Ge evaporated film as the host for the n.sup.+ implant which will activate in Ge at a temperature of about 500.degree. C. The process enables one to form large-area, uniform super-abrupt contacts because the process utilizes a vacuum deposition process to initially form the Ge film and an ion implantation which dopes the Ge film uniformly over its entire area. The anneal step required to activate the layer is at a significantly lower temperature than that of the CVD deposition of n.sup.+ layers or the activation of ion implant directly into prior-art GaAs substrates.

Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.