23.01.1997
Prof. John Davies, University of Glasgow
"The effects of strain from metal gates on a heterostructure"

Abstract

It is now well established that metal gates deposited on a heterostructure produce an elastic field as well as the intended electrostatic field. The elastic field couples to electrons through two mechanisms, the deformation potential which is isotropic and the piezoelectric coupling which depends strongly on orientation. Recent experiments on lateral surface superlattices confirm that the piezoelectric coupling generally dominates. We have calculated analytically the strain, piezoelectric charge density, and potential induced in the two-dimensional electron gas by single gates and one-dimensional surface superlattices. Typically the piezoelectric potential is equivalent to about 0.1 V on the gates. Different behaviour is seen on (001), (011) and (111) surfaces. Larger strains may be introduced during the manufacture of III-V field-effect transistors, particularly due to nitride passivating layers. These can lead to changes in threshold voltage of 0.5 V. Interesting problems are also posed by the highly stressed nature of pseudomorphic transistors.