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

Abstract

Ultrathin 2.5 nm high-k aluminum oxide ͑Al 2 O 3 ͒ films on p-type silicon ͑001͒ deposited by atomic layer deposition ͑ALD͒ were investigated with noncontact atomic force microscopy ͑NC-AFM͒ 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 ͑CPD͒ 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 size͒ and small-scale ͑50-100 nm͒ 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.

Recent Trends in Surface Characterization and Chemistry with High‐Resolution Scanning Force Methods

“…On such films, charging experiments can be performed, where charges can be created by different methods with the tip: contacting the tip with the surface, by corona discharge from the tip, or by inducing tunneling of electrons from the conducting support into the thin‐film or vice versa. Most experiments of this type were done in air by EFS and EFM on SiO 2 ,304 Si 2 N 3 ,305 Al 2 O 3 ,306, 307 GaN,308 and PMMA309 films and also on nanoclusters embedded into or supported by thin SiO 2 films 310–313. In UHV, such experiments can be more precise, such that single electron processes can be studied 299, 314…”

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

“…The measurement of the CPD fluctuations on the Si/ Al 2 O 3 sample, measured with lock-in techniques, are in line with our previous work with height-voltage spectroscopy 6 for the magnitude, lateral size, and correlation with the topography. The correlation between CPD and topography is probably a result of the ALD process, since the correlation is significantly stronger than for the thermal SiO 2 reference sample.…”

“…5 Recently we showed a correlation between CPD and topography for 2.5 nm atomic layer deposited Al 2 O 3 films on hydrogenterminated Si. 6 Although this correlation might be partly related to the presence of nuclei in the film, it is probably difficult to observe substrate inhibition from the CPD signal, since CPD fluctuations can also be caused by variations in the interface state density and doping concentration. 5 In this communication we present CPD and differential capacitance measurements on thermal SiO 2 and ALD grown Al 2 O 3 films, deposited directly on Si ͑with substrate inhibition͒ and on SiO 2 ͑noninhibited͒.…”

Processing and Interconnections of Finely Segmented Semiconductor Pixel Detectors for Applications in Particle Physics and Photon Detection

“…We used the method of applying a thin-film insulator with a negative oxide charge on the top of the n + segmentations. We have successfully prepared thin aluminum oxide (Al 2 O 3 ) layers that naturally have a negative oxide charge by using the method of atomic layer deposition (ALD) [7][8][9].…”