ION YIELD EFFECTS IN THE SIMS ANALYSIS OF SILICON DELTA-DOPED LAYERS IN GAAS

Enhancements in the secondary ion yields of Si+ and As+ ions were obse rved in SIMS depth profiles of Si delta-doped layers in GaAs when usin g oxygen primary ions. A systematic SIMS study of the enhancement in t he intensity of the Si+ ion was carried out using a special test struc ture consisting of a series of layers with Si areal densities ranging from 0.01 monolayer (ML) to 2 ML (1 ML = 6.3 x 10(14) Si cm(-2)), The enhancement effect was observed with layers above about 0.2 ML coverag e and leads to erroneous measurements of the Si areal density for such layers. It was found that the level of the enhancement depended upon both the energy and angle of incidence of the oxygen ions, For a 2 ML delta, the level of the enhancement obtained with 5 keV O-2(+) ions at normal incidence was a factor of 1.2. However, using the same ion ene rgy but changing the angle of incidence to 60 degrees from the normal the enhancement obtained was almost a factor of 5. The enhancement was greatest when conditions for high depth resolution were used so that it was not possible simultaneously to achieve high depth resolution an d accurate areal density measurements. The enhancements could not be r emoved by ratioing the silicon intensities to the As matrix lines (for which the enhancements are different), The enhancement effect was not observed when Xe+ or Cs+ primary ions were used and is predominantly an oxygen-induced artefact.