A. Zinine

Imaging of oxide charges and contact potential difference fluctuations in atomic layer deposited Al2O3 on Si

Ultrathin 2.5 nm high-k aluminum 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 revealed the presence of oxide charges and experimental observations at different tip-sample potentials 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. 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. 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.

Nanoscale topography-capacitance correlation in high-K films : Interface heterogeneity related electrical properties

Kelvin probe force microscopy in ultrahigh vacuum was used to study inhomogeneities of the contact potential difference (CPD) and differential capacitance of thin atomic layer deposited Al2O3 films. CPD fluctuations correlate equally strongly with the surface topography for deposition on hydrogen-terminated Si and thermal SiO2. The correlation of the differential capacitance with the topography clearly distinguishes films based on the starting surface. The lateral electrical homogeneity of these thin oxides depends crucially on their initial nucleation.