Role of steps in epitaxial growth

The role of steps in the epitaxial growth of quantum structures is discussed. We present experimental results and theoretical predictions of growth on stepped surfaces. Scanning tunneling microscopy (STM) images of molecular beam epitaxy grown GaAs(001) surfaces misoriented by 1° and 2° towards the (111) A direction show non-uniform terraces with a peak in the terrace width distribution at 40 Å. Simple models of atoms landing on a step and attaching at the ascending step edge, however, predict an equalization of terrace widths. A thermodynamic model which allows the steps to move freely with the constraint that it costs energy to form a kink predicts step bunching for high kink energies. Steps on vicinal surfaces have been utilized for growing quantum wire structures using a technique where fractional monolayers of different materials are deposited on a stepped surface, leading to the creation of a lateral superlattice (LSL). The terrace width uniformity is observed by STM to improve dramatically with the growth of an AlAsGaAs LSL. Cross-sectional transmission electron microscopy of LSLs shows good segregation of the composite layers.