Dependence of Si+ and Si2+ sputtering yields on residual oxygen impurity

The Si+ and Si2+ sputtering yields have been measured by time-of-flight (TOF) method under the bombardment of a pulsed 11 keV Ar0 beam on a nominally clean Si(1 1 1) surface. Although the residual oxygen impurity is below the detection limit of Auger electron analysis (∼0.01 monolayer), the SiO+ ions are clearly observed and its yield can be regarded as a measure of the residual O impurity on the Si surface region. The Si+ TOF spectrum changes its shape and the Si+ yield increases linearly with the SiO+ yield, while the shape of the Si2+ TOF spectrum and the Si2+ yield remain almost unchanged. The change in the Si+ TOF spectrum is attributed to the increasing SiH+ yield, which may be caused by the H2O adhesion to the Si surface during the TOF measurement. The increase in the adsorbing H2O may also lead to the enhancement of SiO+ and Si+ yields; Si+ ions may be created through the charge exchange process between O and Si due to difference in their electron affinity. The insensitivity of the Si2+ yield to the residual O impurity is consistent with the Si2+ formation by the Auger ionization process of the excited Si+ having the 2p hole, which is produced in the collision cascades.