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Ohmic Contacts to GaAs Disclosure Number: IPCOM000107198D
Original Publication Date: 1992-Jan-01
Included in the Prior Art Database: 2005-Mar-21
Document File: 6 page(s) / 324K

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Masselink, WT: AUTHOR [+1]


This article describes novel ways to achieve low resistance ohmic contacts to either n- or p-doped GaAs devices.

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This is the abbreviated version, containing approximately 18% of the total text.

Ohmic Contacts to GaAs

       This article describes novel ways to achieve low
resistance ohmic contacts to either n- or p-doped GaAs devices.

      Despite a large body of published ideas the study of ohmic
contacts to GaAs devices is still an active area of research.  This
is due in large part to the continuing miniaturization of device
structures that are approaching the submicron regime.  Current
contact schemes have difficulties to form low resistance and
reproducible ohmic contacts to small areas in the order of 1 mm2.

      The most widely used contact structure employs a bilayer of two
metallic films:  the first layer is gold alloyed with a suitable
dopant (~27 at.% Ge for n-type GaAs, ~25 at.% Zn for p-type GaAs),
the second layer is nickel (see, e.g., (1)).  Alternatives for the
two metals have been proposed, such as aluminum for gold and platinum
for nickel. Upon deposition the bilayer film is annealed at about
450~C to form a good ohmic contact.  The current understanding of the
processes occuring during the heat treatment is the following:  the
annealing temperature is slightly above the Au-GaAs eutectic
temperature.  Thus, a liquid layer forms that consists mainly of Au
and GaAs in the eutectic composition with the above mentioned dopant
dissolved in it. During cool down, the GaAs regrowth epitaxially onto
the substrate by incorporating a certain amount of the dopant. This
creates a thin surface layer of GaAs that is highly doped and thus
yields a good ohmic contact.  The role of the nickel layer is mainly
to prevent balling up of the alloyed gold layer.  Additionally, the
presence of small Ge-Ni-rich clusters in large numbers provides a
lower contact resistivity than can be obtained with Au-Ge alone (2).

      The problems with this contact structure are twofold: many
compounds form between the constituent elements such that the contact
is laterally non-uniform in terms of doping as well as compound
formation.  Consequently, current injection occurs at selective spots
in the contact area leading to current crowding, hot spots and, in
general, impaired electrical performance (3).  In addition, the
nodular or cluster form of these ohmic contacts has been associated
with degradation of the contacts during aging (4).  Last, but not
least, the nickel overlayer easily forms compounds with the dopant
which results in increased resistivity of the GaAs surface layer
after alloying and subsequent device performance (5).  These problems
are amplified if the contact area is very small as is the case with
state of the art devices.

      This article presents three different methods to fabricate good
ohmic contacts to GaAs that do not have the above-mentioned
shortcomings.  The methods are based on the formation of an epitaxial
layer on GaAs that establishes a low contact resistivity between the
GaAs substrate and the interconnect metallization. It is understood
that the present invention covers any po...