High-pressure Pulsed Laser Deposition Research Articles

Pulsed-laser deposition and characterization of ZnO nanowires with regular lateral arrangement

We report on the high-pressure pulsed-laser deposition growth of periodic arrays of free-standing single zinc oxide nanowires with uniform hexagonal arrangement and cross-section with thickness of less than 100 nm. In order to achieve the wire alignment, we prepared an ordered array of catalytic gold seed particles by a nanosphere lithography mask transfer technique using monodisperse spherical polystyrol nanoparticles. These templates were investigated by scanning electron microscopy and atomic force microscopy prior to nanowire growth. X-ray diffraction revealed the epitaxial relationships between the nanostructures and the a-plane sapphire substrate and excellent crystal quality. The optical properties of the ZnO nanowire arrays were measured by cathodoluminescence.

Ordered growth of tilted ZnO nanowires: morphological, structural and opticalcharacterization

Tilted ZnO nanowires have been grown by high-pressure pulsed laser deposition (PLD) onm-plane sapphire without employing any catalyst species. The nanowires are inclined30° with respect to the substrate normal and show well-defined epitaxial relationships with them-plane sapphire substrate, the projection of the nanowires’ axis being parallel to the in-plane sapphire direction. Two sets of nanowires, differing in diameter and length, coexist within theinitial growth stages, but only the narrowest and longest nanowires are found to keep ongrowing as deposition proceeds. A systematic study of the effects of growth conditions,including number of pulses, temperature, total pressure and oxygen partial pressure, hasbeen carried out to determine those conditions under which a completely ordered array oftilted nanowires, with a minimum angular distribution, is obtained. Among thegrowth conditions, temperature and oxygen partial pressure are seen to mainlyaffect the nanowires’ axial and lateral growth rates, respectively, while the totalchamber pressure allows monitoring of the evolution from thin film growth, at lowpressure, to a mixture of nanowire/nanobelt growth, at high pressure. Finally,cathodoluminescence (CL) shows that low growth temperatures and high oxygenpartial pressures improve the overall optical quality of the ZnO nanowire arrays.

Electron paramagnetic resonance in transition metal-doped ZnO nanowires

The wide-band-gap zinc oxide-based diluted magnetic semiconductors currently attract considerable attention due to their possible use in spintronic devices. In this work, we studied ZnO nanowire samples synthesized on 10×10 mm2 a-plane sapphire substrates by high-pressure pulsed laser deposition. The samples were characterized by scanning electron microscopy (SEM) and electron paramagnetic resonance (EPR) in the X-band (≃9.3 GHz) from T=4 to 300 K. According to the SEM pictures, the nanowires exhibit a length of about 1 μm and are aligned perpendicular to the substrate surface. The structures have a hexagonal cross section and their diameter ranges from 60 nm up to 150 nm. For the lowest nominal concentrations of xMn=3 at. % and xCo=5 at. %, we detect the anisotropic EPR spectra of isolated Mn2+ (3d5, S6) and Co2+ (3d7, F4), respectively, on Zn sites. The detection of the well-resolved anisotropic spectra proves a coherent crystallographic orientation of the nanowires. The linewidth was larger than the best values reported in the literature. Nevertheless, it was possible to identify two different components, A and B, of the reported spectra. From the temperature dependence of the EPR intensity, we found that both components exhibit paramagnetic behavior and are present in a concentration ratio of NB∕NA=1.4. In the case of the Mn-doped ZnO wires, the linewidth increases with increasing Mn concentration due to the dipole-dipole interaction of the paramagnetic ions. At the highest used nominal concentration, xMn=10 at. %, an additional broad single line is observed.

Growth and characterization of Mn- and Co-doped ZnO nanowires

We report on the high-pressure pulsed laser deposition growth of zinc oxide nanowires containing about 0.2 at.% Co and 0.5 at.% Mn by NiO and Au catalyst. Scanning electron microscopy and X-ray diffraction measurements revealed arrays of parallel-standing nanowires with hexagonal cross section and uniform in-plane epitaxial relations without rotational domains. Elemental analysis was carried out using particle induced X-ray emission and Q-band electron spin resonance. The valence of the incorporated Mn was determined to be 2+. Atomic and magnetic force microscopy measurements indicate that Mn is incorporated preferentially at the nanowire boundaries.

Growth and Characterization of Optical and Electrical Properties of ZnO Nano- and Microwires

ABSTRACTZnO nanowires are readily fabricated with a novel and unique, high-pressure pulsed laser deposition process developed recently by us. It allows the flexible and reproducible control of morphology, diameter, lenght and composition. Furthermore, we report on the electrical characterization as well as stimulated emission from a zinc oxide (ZnO) microcrystals grown by carbothermal evaporation observed by spatially resolved photoluminescence (PL) and high excitation spectroscopy (HES).

Mg x Zn 1 − x O ( 0 ⩽ x < 0.2 ) nanowire arrays on sapphire grown by high-pressure pulsed-laser deposition

Mg x Zn 1 − x O nanowires with Mg-content x from 0 to 0.2 have been grown by high-pressure pulsed-laser deposition (PLD) on gold-covered sapphire single crystals. The PLD process allows for a unique wide-range control of morphology, diameter, and composition of the MgxZn1−xO nanowires. The diameter of single ZnO wires could be varied between about 50 and 3000 nm, and the Mg content x of MgxZn1−xO wire arrays was controlled via the PLD gas pressure. The microscopic homogeneity of Mg content is displayed by cathodoluminescence (CL) imaging of the excitonic peak energy. The fluctuation of CL peak energy between individual wires is about an order of magnitude smaller than the alloy broadening.

Strong polarization enhancement in asymmetric three-component ferroelectric superlattices

Theoretical predictions--motivated by recent advances in epitaxial engineering--indicate a wealth of complex behaviour arising in superlattices of perovskite-type metal oxides. These include the enhancement of polarization by strain and the possibility of asymmetric properties in three-component superlattices. Here we fabricate superlattices consisting of barium titanate (BaTiO3), strontium titanate (SrTiO3) and calcium titanate (CaTiO3) with atomic-scale control by high-pressure pulsed laser deposition on conducting, atomically flat strontium ruthenate (SrRuO3) layers. The strain in BaTiO3 layers is fully maintained as long as the BaTiO3 thickness does not exceed the combined thicknesses of the CaTiO3 and SrTiO3 layers. By preserving full strain and combining heterointerfacial couplings, we find an overall 50% enhancement of the superlattice global polarization with respect to similarly grown pure BaTiO3, despite the fact that half the layers in the superlattice are nominally non-ferroelectric. We further show that even superlattices containing only single-unit-cell layers of BaTiO3 in a paraelectric matrix remain ferroelectric. Our data reveal that the specific interface structure and local asymmetries play an unexpected role in the polarization enhancement.

Spatially Inhomogeneous Impurity Distribution in ZnO Micropillars

Using ZnO a variety of micro- and nanostructures can be fabricated. We investigate hexagonal micropillars grown by high pressure pulsed laser deposition on sapphire with spatially and spectrally resolved cathodoluminescence. We demonstrate that the impurity and defect incorporation is strongly inhomogeneous, at least under the present growth conditions, dramatically influencing the optical properties.

Comparison of high-pressure dc-sputtering and pulsed laser deposition of superconducting YBa2Cu3O x thin films

Superconducting YBa2Cu3Ox thin films were deposited on NdGaO3 (110) substrates using two different techniques: dc sputtering at high oxygen pressure and pulsed laser deposition. The structure, electrical properties, and surface morphology of the obtained films were compared. The superior crystal quality of dc-sputtered films fabricated at the same temperature and at oxygen pressure of the same range as for laser-deposited films can be explained by a lower deposition rate providing time for recrystallization processes. The re-evaporation becomes significant for dc sputtering at high deposition temperatures and results in Badeficient films. The high mobility of atoms on the surface of the growing film during laser deposition helps in the formation of smoothc-oriented areas of the film.