Jeffrey A. Klug

A Ferroelectric Oxide Made Directly on Silicon

Metal oxide semiconductor field-effect transistors, formed using silicon dioxide and silicon, have undergone four decades of staggering technological advancement. With fundamental limits to this technology close at hand, alternatives to silicon dioxide are being pursued to enable new functionality and device architectures. We achieved ferroelectric functionality in intimate contact with silicon by growing coherently strained strontium titanate (SrTiO3) films via oxide molecular beam epitaxy in direct contact with silicon, with no interfacial silicon dioxide. We observed ferroelectricity in these ultrathin SrTiO3 layers by means of piezoresponse force microscopy. Stable ferroelectric nanodomains created in SrTiO3 were observed at temperatures as high as 400 kelvin.

Three-dimensional ferroelectric domain imaging of epitaxial BiFeO3 thin films using angle-resolved piezoresponse force microscopy

Here we introduce angle-resolved piezoresponse force microscopy (AR-PFM), whereby the sample is rotated by 30° increments around the surface normal vector and the in-plane PFM phase signals are collected at each angle. We obtained the AR-PFM images of BaTiO3 single crystal and cube-on-cube epitaxial (001) BiFeO3 (BFO) thin film on SrRuO3/SrTiO3 substrate, and confirmed that the AR-PFM provides more unambiguous information on the in-plane polarization directions than the conventional PFM method. Moreover, we found eight additional in-plane polarization variants in epitaxial BFO thin films, which are formed to mitigate highly unstable charged domain boundaries.

Nanoscale piezoresponse studies of ferroelectric domains in epitaxial BiFeO3 nanostructures

We report the dependence of the ferroelectric domain configuration and switching behavior on the shape (square versus round) of epitaxial BiFeO3 (BFO) nanostructures. We fabricated (001) oriented BFO(120 nm)/SrRuO3(SRO,125 nm) film layers on (001) SrTiO3 single crystals by rf magnetron sputter deposition, and patterned them to square (500×500 nm2) and round (502 nm in diameter) shaped nanostructures by focused ion-beam lithography. The surface morphology and the crystalline structure of the nanostructures were characterized by scanning electron microscopy and x-ray diffraction, respectively, while the domain configuration was investigated using piezoelectric force microscopy. We found that the square-shaped nanostructures exhibit a single variant domain configuration aligned along the [1¯11¯] direction, whereas the round-shaped nanostructures exhibit seven variants of domain configuration along the [1¯11¯], [11¯1¯], [111¯], [111], [1¯11], [11¯1], and [1¯1¯1] directions. Moreover, local d33 piezoelectric c...

Effects of cantilever buckling on vector piezoresponse force microscopy imaging of ferroelectric domains in BiFeO3 nanostructures

Systematic studies are presented on the effects of cantilever buckling in vector piezoresponse force microscopy (V-PFM) imaging of polarization domains in thin-film based (001)-oriented BiFeO3 nanostructures, as observed through the coupling of out-of-plane and in-plane PFM images. This effect is a strong function of the laser spot position on the cantilever, being strongest at the free end, and insignificant at 60% of the cantilever length from the pivot point. This finding provides a unique approach to V-PFM imaging of ferroelectric polarization domains, yielding three dimensional PFM images without sample rotation in the plane.

Elastic relaxation and correlation of local strain gradients with ferroelectric domains in (001) BiFeO3 nanostructures

We report an elasticrelaxation and increase in local strain variation correlated with ferroelectric domains within epitaxial BiFeO3thin filmnanostructuresfabricated by combined electron-beam and focused ion-beam nanolithography. Nano-focused x-ray diffraction microscopy provided new insights into the relationship between film strain and ferroelectric domains in nanostructures, namely: (i) an out-of-plane (C-axis) elasticrelaxation of as much as −1.8% Δc/c in a BFO film-based nanostructure relative to the planar filmlattice constant; (ii) an out-of-plane rotation trending from the center towards all released edges of the nanostructure; and (iii) an increase of inter-domain strain variation within the nanostructure of approximately 10 times the inter-domain variation found within the planar film, correlated with ferroelectric domain boundaries as confirmed by piezoresponse-force microscopy. These results indicate that the release of in-plane BFO/SRO mismatch strain in a planar film is taken up by the local ferroelectric domain structure after patterning, resulting in greatly increased mechanical strain gradients within the structure.

Tunneling Spectroscopy of Superconducting MoN and NbTiN Grown by Atomic Layer Deposition.

A tunneling spectroscopy study is presented of superconducting MoN and Nb0.8Ti0.2N thin films grown by atomic layer deposition (ALD). The films exhibited a superconducting gap of 2 meV and 2.4 meV, respectively, with a corresponding critical temperature of 11.5 K and 13.4 K, among the highest reported Tc values achieved by the ALD technique. Tunnel junctions were obtained using a mechanical contact method with a Au tip. While the native oxides of these films provided poor tunnel barriers, high quality tunnel junctions with low zero bias conductance (below ∼10%) were obtained using an artificial tunnel barrier of Al2O3 on the films surface grown ex situ by ALD. We find a large critical current density on the order of 4 × 106 A/cm2 at T = 0.8Tc for a 60 nm MoN film and demonstrate conformal coating capabilities of ALD onto high aspect ratio geometries. These results suggest that the ALD technique offers significant promise for thin film superconducting device applications.