Pavel Capek

The influence of 180° ferroelectric domain wall width on the threshold field for wall motion

Unlike ideal 180° ferroelectric walls that are a unit cell wide (∼0.5 nm), real walls in ferroelectrics have been reported to be many nanometers wide (1–10 nm). Using scanning nonlinear dielectric microscopy of lithium niobate (LiNbO3) and lithium tantalate (LiTaO3) ferroelectrics, we show that the wall width at surfaces can vary considerably and even reach ∼100 nm in places where polar defects adjoin a wall. The consequence of such variable wall widths is investigated on the specific property of threshold field required for wall motion. Using microscopic phase-field modeling, we show that the threshold field for moving an antiparallel ferroelectric domain wall dramatically drops by two to three orders of magnitude if the wall was diffuse by only ∼1–2 nm, which agrees with experimental wall widths and threshold fields for these materials. Modeling also shows that wall broadening due to its intersection with a surface will influence the threshold field for wall motion only for very thin films (1–10 nm) whe...

Improved Photoluminescence of InGaN Quantum Wells Grown on Nano-Patterned AGOG Sapphire Substrate by Metalorganic Vapor Phase Epitaxy

Metalorganic vapor phase epitaxy of InGaN quantum wells on GaN template grown on nano-patterned AGOG sapphire substrate leads to improved luminescence intensity by 1.89-2.21-times, presumably due to the reduced defect density.

Site Selective Spectroscopy on Erbium Ions in Stoichiometric Lithium Tantalate

Ferroelectric materials such as lithium niobate (LiNbO3) and the isostructural lithium tantalate (LiTaO3) play an important role in integrated optics since they allow the possibility to combine their favourable electro-optical, acousto-optical, and nonlinear properties with the ability to add additional functional groups by doping. Examples are rare earth ions that act as active centres for laser and optical amplifier applications. We present our sites-selective spectroscopic studies on Er3+ doped nearly stoichiometric LiTaO3 that include results about the assignment of excitation and emission peaks to different sites, symmetry properties of these sites, energy transfer among major sites, and up-conversion efficiencies. We compare the results in LiTiO3 with the corresponding ones in the much better studied LiNbO3 host and find that the type of centres and their spectral feature are very similar.

Near Field Optical Imaging of Carrier Localization in Al x Ga 1-x N Alloys

The wide band gap semiconductor alloys have attracted considerable attention for efficient UV emitters and laser applications. In the presence of high dislocation densities associated with growth on lattice mismatched substrates, it has been shown that carrier localization created by nanoscale compositional inhomogeneities can lead to enhanced emission efficiencies, we have investigated the origin of the localization effect using near-field optical spectroscopy using fiber coupled UV laser. We compare the spectral and spatial properties of the localization features under excitation at 244nm (above bandgap of the matrix) and 325nm (below bandgap of the matrix). Even under the latter excitation, the localized emission peak appears, suggesting a direct excitation of the localization regions.

Luminescence and Raman Based Real Time Imaging of Ferroelectric Domain Walls

We studied ferroelectric domain wall regions in lithium niobate using the photoluminescence of intentionally doped rare earth ions (such as Er 3+ ) as well as Raman spectroscopy and present an overview of the current status of our ongoing investigations. We find that the Er emission is a sensitive tool to observe changes in local electric fields as well as reconfiguration of defect dipoles across the domain wall. The Raman spectra, on the other hand can be used to identify charges that accumulate asymmetrically across a domain wall. We further demonstrate that the imaging methods offer sufficient sensitivity to observe the changes associated with a domain in real time while it is moving.