Cross-sectional scanning tunnelling microscopy (STM) images on a freshly cleaved III-V semiconductor laser facet regularly indicate atomic scale defects, such as steps and vacancies on the non-polar (1 1 0) surface. In this work, studies were made when sub-monolayer quantities of Si were deposited at 280 degreesC on clean cleaved GaAs(1 1 0). Selective adsorption of Si on atomic defects has been observed with STM imaging of the surface. The simultaneous use of STM and scanning tunnelling spectroscopy (STS) allows the quantification of the Fermi shifts present at atomic steps and vacancies before and after Si adsorption. Localised current-voltage measurements, performed using STS, on the atomic steps and vacancy defect clusters of p-type GaAs(1 1 0) surfaces showed Fermi-level shifts of similar to0.8 and 0.7 eV towards mid-gap position, respectively. However, measurements taken from silicon-coated atomic steps and defect clusters showed that the Fermi-level reverted back towards the 'ideal' flat band position by similar to0.4 eV. This behaviour was distinctively different to silicon adsorbed at defect-free surface of GaAs(1 1 0). These results indicated the passivating potential of silicon on the defect sites of GaAs(1 1 0) surface when deposited under these conditions. The ability to measure and control surface properties on the atomic scale is crucial to the success of nano-technology and the STM/STS studies demonstrate the effectiveness of the technique for this role. (C) 2004 Elsevier B.V. All fights reserved.