THE APPLICATION OF SCANNING TUNNELING MICROSCOPY TO THE STUDY OF MOLECULAR-BEAM EPITAXY

A detailed knowledge of the structure of flat planes and the distribution of steps and islands resulting from growth by molecular beam epitaxy (MBE) is essential to the understanding of the growth mechanism and interface formation. The atomic resolution, real space imaging capability of the scanning tunneling microscope (STM) allows such features to be directly seen, providing a new technique for the study of MBE grown surfaces. In order to study the as grown surface by STM it is necessary either to have an STM in an MBE system, or to transfer the specimen from the MBE chamber to the STM while protecting the surface from contamination. This paper will review the work that has been done with STM on MBE grown surfaces together with the difficulties and limitations involved. In particular, the first STM study of an MBE grown surface, namely GaAs(001) will be discussed. In this case, the MBE grown GaAs(001) surface was capped with arsenic and then transferred to the STM chamber where the capping layer was removed by heating. STM images showed that the arsenic rich (2X4) surface present during growth a layer of arsenic on the surface, with the unit cell consisting of 3 arsenic dimers and one dimer vacancy. This has important consequences for the growth mechanism of GaAs(001). Islands on the surface have a preferential orientation, and are built up from complete unit cells. The STM images provide a very clear picture of the overall order and morphology of the grown surface. This initial STM study has shown that the STM can provide new information important in understanding the growth and fabrication of device structures. © 1990.