Microstructure of Metal-GaAs Interfaces
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T. S. Kuan, P. E. Batson, J. L. Freeouf, T. N. Jackson, and E. L. Wilkie IBM Thomas J. Watson Research Center, Yorktown Heights, N. Y. 10598
ABSTRACT The interfacial reactions and microstructures of metal-GaAs contacts are, in general, much more complicated and difficult to control than the corresponding metal-Si contacts. They are very sensitive to reaction temperature, ambient, metal layer thickness, and the GaAs surface cleaning procedure. Many of the ohmic and gate contacts to GaAs currently in use or under development for GaAs FET devices are comprised of more than one metal species and in some cases a doping element as well. In dealing with these complexities, information about the microstructure at the contact interface is critically needed: for the evaluation of a specific contact metallurgy, for the definition of an optimum fabrication process, and, most important of all, for generating new ideas for better contact schemes. In this paper, our TEM and STEM studies of several ohmic and gate contacts that are of technological interest will be described. Attention will be drawn to the link between the interfacial microstructures and their electrical behavior, the kinetics of interfacial reactions, and thermal stability. The current constraints on obtaining ideal, reliable and controllable metal-GaAs contacts will also be discussed.
INTRODUCTION Good ohmic and Schottky contacts to GaAs are critical to high-performance integrated GaAs FET devices. Most of the contact schemes available today do not have adequate electrical uniformity and thermal stability to meet the stringent requirements for IC applications. There are ample measured data on the characteristics of many of these metal contacts. However, the physics behind their empirical behavior is poorly understood. This is because the thermal reactions that occur at the metal/GaAs interface are complicated and sensitive to subtle variations in process conditions. Furthermore, the structural analysis techniques used in most studies do not have a resolution level capable of revealing the critical microstructure elements that govern the electrical behavior. To illustrate the additional complexities of metal-GaAs contacts, we will compare the thermal reactions occurring at Pd/GaAs and Pd/Si contacts in the first section of this paper. There are of course limited similarities between the GaAs and Si contacts. In the second section we will then examine the microstructure of a typical Au-Ni-Ge ohmic contact to GaAs and discuss the current constraints on the control and improvement of this contact. In the third section the cross-sectional microstructure of a WSi 0/GaAs gate contact used in a self-aligned FET will be examined. The materials requirements for a good gate contact from the microstructural viewpoint will be discussed. Pd/GaAs VERSUS Pd/Si Even a simple metal contact to GaAs such as Pd/GaAs is complicated, since the metal can react with Ga or As or both at different temperatures. The picture is further
Mat. Res. Soc. Symp. Proc. Vol. 54. 1986 Materials Re
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