Formation of Ohmic Contacts to n-GaAs at Temperatures Compatible with Indium Flip-Chip Bonding
Authors: Wood Michael G , Hains Christopher P , Finnegan Patrick Sean , Stephenson Chad A , Klem John F. , Looker Quinn 
Published: February 2020

Heterogenous integration of III-V photodetectors with Si readout circuitry is an enabling technology for applications including focal plane arrays [1] and concentrating solar cells [2, 3]. Typical integration schemes include low temperature flip-chip bonding using indium (In) bump bonds, followed by epoxy underfill, and mechanical and/or chemical removal of the III-V substrate to expose the epitaxially grown detector layers [1].
For maximum flexibility in device design and processing, it is desirable to be able to form low resistance illumination-side contacts to either p-type or n-type layers after flip-chip bonding. Formation of non-alloyed contacts to p-GaAs is relatively straightforward and can be readily implemented after flip-chip bonding [4]. Conventional contacts to n-GaAs are formed by evaporation of Ge/Au/Ni followed by a rapid thermal annealing step at 400 °C [4], considerably above the 156 °C melting point of In and above the degradation temperature of many standard underfill epoxies. Several groups have studied the formation of ohmic contacts to n-GaAs through low temperature annealing of Pd/Ge/Au metal stacks [3, 5, 6], however all reports to date have used annealing temperatures near or above the melting point of In.
In this work, we report the formation of ohmic contacts with a specific contact resistivity as low as 5.6×10-6 Ω-cm2 after annealing at a temperature of 140 °C, significantly below the In melting point. Figures 1 and 2 show experimental results of transfer line method (TLM) test structures measured on an epitaxially grown 800 nm thick, 1x1018 cm-3 n-GaAs layer with 7 nm Pd/ 50 nm Ge/ 200 nm Au contacts patterned with photolithography and evaporation. The samples were annealed in an oven with a N2 ambient and measured repeatedly throughout the annealing process. We observe a transition from Schottky to ohmic behavior after 10 hours of annealing followed by a steady decrease in resistance for annealing times out to 48 hours. A companion sample that had been flip-chip bonded with In bumps and underfilled with Epotek 301 epoxy showed no degradation after annealing. The black line in Fig. 1 is provided as a reference of the measured specific contact resistivity of a standard Ge/Au/Ni/Au metal stack deposited on the same n-GaAs layer after a 30 second, 400 °C anneal.
The creation of low-resistance ohmic contacts to n-GaAs at this low temperature is an enabling technology for III-V heterogeneous integration efforts with a wide array of temperature-sensitive materials such as In, epoxies, photoresists, and plastics.
Acknowledgment
Sandia National Laboratories is a multimission laboratory managed and operated by National Technology and Engineering Solutions of Sandia, LLC, a wholly owned subsidiary of Honeywell International, Inc., for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-NA-0003525. This paper describes objective technical results and analysis. Any subjective views or opinions that might be expressed in the paper do not necessarily represent the views of the U.S. Department of Energy or the United States Government.
References
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[6] C. Wang, et al., “Ohmic contact formation of the Au/Ge/Pd/n-GaAs system formed below 200 °C,” J. Appl. Phys., vol. 79, pp. 4216, 1996.

Figure 1