Ultrathin 2.5nm high-k aluminium oxide
films on p-type silicon (001) deposited by atomic layer deposition (ALD) were
investigated with noncontact atomic force microscopy (NC-AFM) in ultrahigh
vacuum, using a conductive tip.
Constant force gradient images revealed the
presence of oxide charges and experimental observations as different tip-sample
potentiality were compared with calculations of the electric force gradient
based on a spherical tip model. this model could be substantially improved by
the incorporation of the image of the tip in the semiconductor
Based on the signals of different oxide
charges observed, a homogenous depth distribution of those charges was derived.
Application of a potential difference between sample and tip was found to result
in a net electric force depending on the contact potential difference (CPD) and
effective tip-sample capacitance, which depends on the depletion or accumulation
layer that is induced by the bias voltage. CPD images could be constructed from
height-voltage spectra with active feedback.
Figure 1. 1ìm x 1ìm topography image of a 2.5 nm Al2O3 film on Si
Apart from oxide charges large-scale (150 -
300 nm lateral size) and small-scale (50 - 100 nm) CPD fluctuations were
observed, the latter showing a high degree of correlation with topography
features. This correlation might be a result from the surface-inhibited growth
mode of the investigated layers.