Transmission electron microscopy of Au-based Ohmic contacts to n-AlxGa1−xAs

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Abstract
Contacts to n‐type AlxGa1−xAs have been previously fabricated by sequential deposition of Al, Sn, and Au, and then heating near 400 °C. Such contacts exhibit excellent electrical properties, but contact adhesion strength was erratic. Transmission electron microscope experiments revealed an almost continuous void separating the metal from the semiconductor. The void is about 100 Å wide, and frequently widens into large pores; it is essentially completely formed after Au deposition, before heat treatment. The voids probably grow by a Kirkendall‐type mechanism during Au deposition, due to rapid diffusion of Sn into the Au. One simple method of eliminating the voids, that of moving the Sn layer away from the metal‐semiconductor interface by depositing a thin layer of Au before putting down the Sn, is shown to produce contacts with no detectable interface void. With this new metallization, adhesion is greatly improved, without any loss of the excellent electrical properties previously achieved.