Electron Trapping Mechanism in LaAlO_{3}/SrTiO_{3} Heterostructures.

In LaAlO_{3}/SrTiO_{3} heterostructures, a still poorly understood phenomenon is that of electron trapping in back-gating experiments. Here, by combining magnetotransport measurements and self-consistent Schrödinger-Poisson calculations, we obtain an empirical relation between the amount of trapped electrons and the gate voltage. The amount of trapped electrons decays exponentially away from the interface. However, contrary to earlier observations, we find that the Fermi level remains well within the quantum well. The enhanced trapping of electrons induced by the gate voltage can therefore not be explained by a thermal escape mechanism. Further gate sweeping experiments strengthen that conclusion. We propose a new mechanism which involves the electromigration and clustering of oxygen vacancies in SrTiO_{3} and argue that such electron trapping is a universal phenomenon in SrTiO_{3}-based two-dimensional electron systems.

The frequency-dependent AC photoresistance behavior of ZnO thin films grown on different sapphire substrates.

Zinc oxide (ZnO) thin films were grown by pulsed layer deposition under an N2 atmosphere at low pressures on a- and r-plane sapphire substrates. Structural studies using X-ray diffraction confirmed that all films had a wurtzite phase. ZnO thin films on a- and r-plane sapphire have grown with orientations along the [0002] and [112[combining macron]0] directions, respectively. Room temperature photoluminescence measurements indicate that the presence of native point defects (interstitial zinc, oxygen vacancies, oxygen antisites and zinc vacancies) is more preponderant for ZnO thin films grown on the r-plane sapphire substrate than the sample grown on the a-plane sapphire substrate. Room temperature impedance spectroscopy measurements were performed in an alternating current frequency range from 40 to 105 Hz in the dark and under normal light. An unusual positive photoresistance effect is observed at frequencies above 100 kHz, which we suggest to be due to intrinsic defects present in the ZnO thin films. Furthermore, an analysis of the optical time response revealed that the film grown on the r-plane sapphire substrate responds faster (characteristic relaxation times for τ1, τ2 and τ3 of 0.05, 0.26 and 6.00 min, respectively) than the film grown on the a-plane sapphire substrate (characteristic relaxation times for τ1, τ2 and τ3 of 0.10, 0.73 and 4.02 min, respectively).

Mimicking Antiferroelectrics with Ferroelectric Superlattices.

Antiferroelectric oxides are promising materials for applications in high-density energy storage, solid-state cooling, and negative capacitance devices. However, the range of oxide antiferroelectrics available today is rather limited. In this work, it is demonstrated that antiferroelectric properties can be electrostatically engineered in artificially layered ferroelectric superlattices. Using a combination of synchrotron X-ray nanodiffraction, scanning transmission electron microscopy, macroscopic electrical measurements, and lateral and vertical piezoresponse force microscopy in parallel-plate capacitor geometry, a highly reversible field-induced transition is observed from a stable in-plane polarized state to a state with in-plane and out-of-plane polarized nanodomains that mimics, at the domain level, the nonpolar to polar transition of traditional antiferroelectrics, with corresponding polarization-voltage double hysteresis and comparable energy storage capacity. Furthermore, it is found that such superlattices exhibit large out-of-plane dielectric responses without involving flux-closure domain dynamics. These results demonstrate that electrostatic and strain engineering in artificially layered materials offers a promising route for the creation of synthetic antiferroelectrics.

[Raman spectra analysis of GaN : Er films prepared by ion implantation].

U-, n- and p-GaN : Er films were prepared by ion implantation method. Three carrier types of samples were studied by Raman spectra analysis. After Er+ ion implantation into GaN samples, new Raman peaks at wavenumber of 293, 362 (see text) 670 cm-1 appeared, where 293 cm-1 was considered as disordered activation of Raman scattering (DARS), 362 and 670 cm-1 may be associated with GaN lattice defects formed after ion implantation. The Ez (high) characteristic peak moves to the high frequency before and after GaN : Er samples annealing at 800 C, indicating that GaN lattice is under the compressive stress. The Lorenz fitting was used to analysed the occurrences of A1 (LO) peak in different samples which is composed of the uncoupled mode LO and the plasmon coupling mode LPP+, qualitatively pointing out the carrier concentration variation of a series of GaN : Er samples.

Quantifying work function differences using low-energy electron microscopy: The case of mixed-terminated strontium titanate.

For many applications, it is important to measure the local work function of a surface with high lateral resolution. Low-energy electron microscopy is regularly employed to this end since it is, in principle, very well suited as it combines high-resolution imaging with high sensitivity to local electrostatic potentials. For surfaces with areas of different work function, however, lateral electrostatic fields inevitably associated with work function discontinuities deflect the low-energy electrons and thereby cause artifacts near these discontinuities. We use ray-tracing simulations to show that these artifacts extend over hundreds of nanometers and cause an overestimation of the true work function difference near the discontinuity by a factor of 1.6 if the standard image analysis methods are used. We demonstrate on a mixed-terminated strontium titanate surface that comparing LEEM data with detailed ray-tracing simulations leads to much a more robust estimate of the work function difference.