Comparison of GaAs Metallization Systems for High Temperature Applications
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COMPARISON OF GaAs METALLIZATION SYSTEMS FOR HIGH TEMPERATURE APPLICATIONS
J. SCOFIELD*, S. LIU** and S. SMITH* *Aero Propulsion & Power Directorate, Wright Laboratory,Wright Patterson Air Force Base, Dayton, Ohio 45433-6563 "**Universityof Dayton Research Institute, Dayton Ohio 45469-0170
Abstract Transition metal-germanide/silicide based ohmic contact systems to n-type GaAs have been studied using current-voltage (I-V) and transmission line measurements (TLM). The effects of substituting various metals from groups IVB and VIB into the metallization stack have been compared, as well as the effects of using germanium or silicon. Rapid Thermal Annealing (RTA) was used to facilitate the reactions. Titanium and molybdenum were shown to be more effective metallizations than chromium, even though chromium readily forms a silicide or germanide. Germanium has been found to produce an ohmic contact of low resistivity, while metallization with silicon alone does not show ohmic behavior.
INTRODUCTION The ohmic contact is a fundamental part of every semiconductor device; it provides the link between the external circuits and active regions of the semiconductor. A satisfactory ohmic contact should not significantly perturb device performance; it must supply the required current with a voltage drop that is sufficiently small compared to the drop across the active region of the device [1]. Most semiconductor devices are subjected to thermal cycles from room temperature to about 400'C (600°C for solar cells, 800'C for a brief time for some self-aligned devices) during device fabrication. Also, high temperature operation maybe required, especially for power devices. Therefore, both low contact resistivity and good thermal stability are critical requirements for ohmic contact systems. The Au-Ge-Ni system has been extensively used as the ohmic contact to n-type GaAs, yielding reproducible contacts with low contact resistivity of -10-6 Q2 cm 2 [2]. P3-AuGa is considered to be essential to the formation of this low resistance ohmic contact [3]; however, it is this P-AuGa phase which makes the system unstable at 4000 C, because of its low melting temperature of 375'C [4]. In recent years, indium-based Ni-Si-ln-W ohmic contact systems which possess satisfactory low contact resistivity of -10-6 Q cm 2 and thermal stability up to 400'C have been developed at IBM [5,6]. Indium is important to the contact, and it is believed that the InXGa 1.xAs phase has a primary favorable effect on the electric properties of the contact. However, as in the case of Au in the Au-Ge-Ni system, indium is also responsible for the instability of the Ni/Si/In/W contact at temperatures higher than 400'C. Thus, development of new contact metallizations with improved thermal stability at temperatures higher than 400°C is required.
Mat. Res. Soc. Symp. Proc. Vol. 282. 01993 Materials Research Society
248
The principal difficulty in forming a metallic contact to a GaAs semiconductor lies in the fact that since most metals are unstable in contact with GaAs at
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