On the atomic structure of the Al-GaAs(100) interface

Abstract

Transmission electron microscopy has been used to investigate the atomic structure of the Al-GaAs(100) interface. Samples grown by molecular beam epitaxy (MBE) on the GaAs c(2 × 8) and (4 × 6) surfaces showed the epitaxial relationship (100)Al||(100)GaAs, [010]Al||[011]GaAs. Samples grown by MBE on the c(4 × 4) surface and also in a conventional evaporator on an ion-etched (100) surface showed the additional relationship (110)Al||(100)GaAs with either [110]Al||[011]GaAs or [001]Al||[011]GaAs. (110) epitaxy was attributed to nucleation at surface steps and the marked predominance of one crystallographic variant was attributed to the non-equivalence of the 〈011〉 directions in the GaAs(100) surface.

Studies on plan-view and cross-sectional specimens showed that a 1·4% mismatch in regions of Al(100)/GaAs(100) was usually accommodated by misfit dislocations with Burger's vectors b=(a/4) 〈011〉 referred to the GaAs. Occasionally misfit dislocations of b=(a/2) 〈010〉 type were observed. It is shown that these misfit dislocations arrays imply, and may induce, an interface where rows of Al atoms project midway between rows of Ga and rows of As atoms, confirming the result obtained recently by Eaglesham et al. using convergent-beam electron diffraction. Studies of cross-sections of MBE samples showed an Al-GaAs(100) interface which was atomically smooth but which contained interfacial regions of Al_xGa₁ -_xAs or AlAs up to about 50 Å thick in some places. The significance of these observations for understanding the electrical properties of Al/GaAs(100) contacts is also discussed.