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Ultrathin 2.5nm high-k aluminium oxide
(Al2O3)
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
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
substrate.
Contact Potential
Difference
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. |