J. Zippel

Control of interface abruptness of polar MgZnO/ZnO quantum wells grown by pulsed laser deposition

A strong quantum confined Stark effect (QCSE) was observed in wedge shaped MgZnO/ZnO quantum wells (QWs) grown by pulsed laser deposition. A reduced laser fluence of 1.8 J/cm2 was used. Reference samples grown at higher standard fluence 2.4 J/cm2 showed only a negligible QCSE. Using off-axis deposition without substrate rotation, a constant composition of the barriers was maintained while varying the well width in a wedge shaped QW. A redshift of the QW luminescence with increasing QW thickness up to 230 meV below the ZnO emission was found, accompanied by an increase in the exciton lifetime from 0.3 ns up to 4.2 μs.

Oxide Thin Film Heterostructures on Large Area, with Flexible Doping, Low Dislocation Density, and Abrupt Interfaces: Grown by Pulsed Laser Deposition

Advanced Pulsed Laser Deposition (PLD) processes allow the growth of oxide thin film heterostructures on large area substrates up to 4-inch diameter, with flexible and controlled doping, low dislocation density, and abrupt interfaces. These PLD processes are discussed and their capabilities demonstrated using selected results of structural, electrical, and optical characterization of superconducting (YBa 2Cu 3O 7−δ), semiconducting (ZnO-based), and ferroelectric (BaTiO 3-based) and dielectric (wide-gap oxide) thin films and multilayers. Regarding the homogeneity on large area of structure and electrical properties, flexibility of doping, and state-of-the-art electronic and optical performance, the comparably simple PLD processes are now advantageous or at least fully competitive to Metal Organic Chemical Vapor Deposition or Molecular Beam Epitaxy. In particular, the high flexibility connected with high film quality makes PLD a more and more widespread growth technique in oxide research.

Defect-induced magnetism in homoepitaxial manganese-stabilized zirconia thin films

Ferromagnetic ordering is found for both undoped and Mn-doped zirconia (ZrO2 : Mn) thin films with 0 at% ≤ Mn ≤ 50 at% grown homoepitaxially on ZrO2 : Y2O3(0 0 1) substrates. Highly crystalline films show ferromagnetic saturation magnetization and coercive field at room temperature up to 1 emu cm−3 and 50 mT, respectively. The Curie temperature is in all ferromagnetic samples above 300 K. Comparing optimum films with different Mn content, cubic ZrO2 : Mn films with about 25 at% Mn show reproducibly the highest magnetization, in relation to monoclinic and tetragonal films. In contrast, less crystalline films grown heteroepitaxially on LaAlO3(0 0 1) or under non-ideal conditions show negligible magnetic effects.The fraction of paramagnetically active Mn atoms in a ZrO2 film with 27% Mn at 5 K is only about 1/5 of the incorporated Mn atoms, corresponding well to the share of 20% Mn4+ in XPS. Magnetic trace impurities in the 100 ppm range cannot account for the observed effects. Our results indicate that the observed defect-induced magnetic ordering in nominally non-magnetic zirconia thin films requires a certain balance of overall crystallinity, dislocation density and film mosaicity.

Properties of homoepitaxial ZnO and ZnO:P thin films grown by pulsed-laser deposition

We have investigated the morphology, crystalline quality, the transport and electronic properties of homoepitaxial ZnO and ZnO:P thin films grown by pulsed-laser deposition. Atomic surface steps are visible for growth temperatures of 650°C and higher. The unit cell volume of undoped thin films is smaller than that of the hydrothermal substrates. Phosphorous doping increases the unit cell volume such that a perfect lattice match is achieved for a nominal phosphorous content of 0.01 wt.%. Undoped thin films have a net doping concentration below 1015 cm-3, whereas the phosphorous doping increases the free electron concentration at room temperature to 1017 cm-3 and above. Temperature dependent Hall effect measurements show that interstitial zinc with a thermal activation energy of 34 meV is a dominant donor in homoepitaxial ZnO:P thin films. The Hall mobility of such samples is similar to ZnO single crystals grown by seeded chemical vapor transport. Low temperature photoluminescence measurements reveal recombination of free excitons and excitons bound to interstitial zinc and excitons bound to neutral and ionized aluminum donors. Defect related deep luminescence is not observed for undoped homoepitaxial thin films. In contrast phosphorous doping introduces two broad recombination bands centered at 2.9 eV and 1.9 eV.

Two‐dimensional electron gases in MgZnO/ZnO heterostructures

The authors report investigations of two‐dimensional electron gases (2DEG) formed due to the band offset and the change of the polarization at MgZnO/ZnO interfaces grown on sapphire substrates by pulsed‐laser deposition. Capacitance‐voltage measurements provide the means to resolve the electron density at the MgZnO/ZnO interface. The density of the 2DEGs is of the order 1011 cm−2 and much lower than expected from the change of polarization. The free electron density in the MgZnO is not sufficient to compensate the polarization charges.

Martensitic phase transition and subsequent surface corrugation in manganese stabilized zirconia thin films

Abstract A martensitic phase-transition in pulsed-laser deposited manganese stabilized zirconia (MnSZ) thin films is demonstrated. For atomic Mn contents of Mn Zr O thin films between 0.1 and 0.15, the martensitic transformation of the tetragonal high temperature phase to the monoclinic low temperature phase is revealed. (001)-oriented MnSZ thin films exhibit a modification of surface morphology in the form of ordered pyramids. The surface morphology of (111)-oriented MnSZ thin films shows dislocation lines with a sixfold symmetry. Transmission electron microscopy demonstrates the change of the crystallographic phase within the MnSZ thin films. X-ray diffraction measurements reveal the presence of both the monoclinic as well as the tetragonal phases. By heating the MnSZ thin films in an oxygen-deficient atmosphere, i.e vacuum, the martensitic phase transition can be reversed. By growing MnSZ thin films at elevated temperatures, the tetragonal phase is stabilized. During cooling down from the growth temperature, MnSZ thin films change their colour slightly and defoliate partially from the substrate if the thickness is larger than about 600 nm and the Mn content within .