The Role of Surface Reconstructions in MBE Growth of GaAs

Abstract

In this paper we discuss our Scanning Tunneling Microscopy studies of the surface structure, growth and electronic properties of GaAs(001) grown by molecular beam epitaxy (MBE). We find that the reconstruction on this surface plays a critical role in determining the MBE growth mechanism and in Fermi-level pinning. During growth the GaAs(001) surface has a (2x4)/c(2x8) reconstruction that arises from the formation of arsenic dimers with a regular array of arsenic dimer-vacancies. At least two forms of the (2x4) unit cell occur: with two arsenic dimers per unit cell, and with three arsenic dimers per unit cell. We evaluate the present understanding of these structures, and show that it is not yet possible to uniquely determine the structure in the dimer-vacancy row of the 2-dimer unit cell. Simple atomistic models for growth on flat planes and for step-flow growth on vicinal GaAs(001) surfaces are found to be inconsistent with the structure of the (2x4) unit cell. For example, our STM images show that nucleation at a step edge involves the formation of complete (2x4) unit cells. We have recently been able to show that surface Fermi-level pinning on n-type GaAs(001) occurs as a result of the formation of defects in the ordering of the (2x4) unit cells. These defects are kinks in the dimer-vacancy rows, which act as surface acceptors. A detailed understanding of the many reconstructions which occur on the GaAs(001) surface may allow changes to be made to growth mechanisms and electronic properties and so impact the development of new device structures