Ion-induced electron emission as a means of studying energy- and angle-dependent compositional changes of solids bombarded with reactive ions: I. Oxygen bombardment of silicon

Abstract

Ion-induced electron yields γ of n-type silicon bombarded with O + and O + 2 ions at energies between 1 and 12.5 keV have been measured at impact angles θ between 0° (normal incidence) and 86°, both for sputter-cleaned samples and under conditions of stationary loading with oxygen. The electron yields were determined by target current measurements. To avoid yield deficits due to charging of fully oxidised samples, it was necessary to use defocused beams in combination with sometimes rather high extraction voltages. Surprisingly, the charging effects were a factor of about three more severe with O + than with O + 2 ions. At normal incidence, the electron yields γ ∞ 0 of oxygen-saturated samples were much higher than the yield γ 0 0 of clean samples. The ratios r 0= γ ∞ 0/ γ 0 0 decreased with increasing energy, from ∼16 at the lowest to ∼7 at the highest energy. This energy dependence of r 0 is attributed to preferential loss of oxygen, the effect being stronger the higher the energy. Pronounced preferential sputtering of oxygen was also observed in sputter profiling of oxygen-implanted samples using Ne + bombardment. The same yield ratios as for normal incidence were measured at off-normal ion impact, provided the sample had been fully oxidised at 0°. With increasing impact angle, the stationary yields of oxygen-bombarded samples first increased, then decreased rather rapidly and finally arrived at a level which was only marginally higher than with clean silicon. The fall-off in yield occurred at larger angles the lower the beam energy: 37–40° at 1 keV and 23–26° at 10 keV. These critical angles θ c for beam-induced oxidation are in good agreement with sparse data previously obtained by other techniques. There is also a close correlation of θ c with angles at which the sputtering yields are equal to 0.5Si atoms/O atom. The very strong enhancement of the electron yields, produced by bombardment-induced oxidation, is difficult to explain by current electron emission models, but there is some evidence that the high yields observed with oxidised silicon are due to a very low surface barrier for electron emission from these samples.