Sintered ohmic contacts to n-and p-type GaAs

Previously, all known ohmic contacts to n-GaAs have involved a so-called "alloying" procedure which consists of melting a Au-Ge eutectic or Sn-based alloy films on GaAs. We describe here a new contact metallization scheme consisting of Pd/Ge/n-GaAs which requires sintering rather than melting in order to produce ohmic contacts. The sintering is done at temperatures ranging from 350°C, 15 min to 500°C, 2 h depending on the doping level of n-GaAs (10
¹⁸
-10
¹⁶
cm
^-3
. For n-10
¹⁶
cm
^-3
GaAs, a specific contact resistance of 3 × 10
^-4
Ω.cm
²
was achieved. Sintering leads to the formation of some PdGe and intermetallics associated with the Pd/GaAs interaction, namely, PdAs
2
and PdGa. Ohmic behavior is attributed to a combination of the doping action of Ge (as donor) and fast in-diffusion kinetics of Pd. Sintered contacts to n
⁺
and p
⁺
GaAs (N
D.A
∼ 10
¹⁸
cm
^-8
) made by Au, Pt, and Ti Were also investigated for ohmic behavior. Each of these three metals was at least partially effective in forming ohmic contacts to p
⁺
GaAs; the degree of effectiveness increases on going from Ti to Au to Pt. It is proposed that a reasonable guideline to follow When searching for ohmic contacts is X
m
≳ X
d
where X
m
is the width of the metallurgical junction and X
d
is the ideal depletion layer width. This condition should favor ohmic contacts by promoting a micro 3-dimensional current flow at the conductor-semiconductor interface and thereby maximizing field-emission probability.