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Mnx Zr1-x O2 (MnZO) thin films were grown by pulsed-laser deposition on single crystalline yttria-stabilized zirconia (YSZ) and a-plane sapphire substrates with manganese contents from 0 up to about 50 at.%. A fully stabilized cubic structure occurs for Mn contents x equal or larger than 20 at. % on YSZ substrates. For x ≈0.5, phase separation of Mn-oxides occurs. Below 11 at. %, only the monoclinic phase is observed. The thin films are electrically insulating up to x = 0.3. By further increasing the Mn content or by reducing the structural quality, the resistivity ρ decreases from 3×109Ωcm down to 3×104Ωcm. For MnZO thin films on a-plane sapphire substrates, Seebeck-effect measurements verify a transition from p-type conductivity to n-type conductivity around 500 K with increasing temperature, which is probably governed by an enhanced ionic conduction. Cathodoluminescence measurements clearly show a Mn-related emission at about 2.8 eV, correlated to an Mn-induced electronic state in the bandgap of MnZO. From electron paramagnetic resonance measurements and x ray photoelectron spectroscopy, we conclude that both Mn3+ and Mn4+ is present in our MnZO thin films. [ABSTRACT FROM AUTHOR]

We report on the study of hafnium oxide thin films grown by pulsed laser deposition at various partial oxygen pressures by Time Differential Perturbed Angular Correlations using the nuclear probe 181Hf(β-)181Ta to determine the nuclear quadrupole interaction (NQI), and by x-ray diffraction. The samples were neutron activated and measured at room temperature as received as well as after annealing in air. All spectra exhibited two to three inequivalent probe sites, even after annealing. At 0.3 mbar oxygen partial pressure and annealing for 5 hs at 1073 K the majority (88%) of the sites exhibited NQI parameters as reported for the bulk monoclinic phase [ωQ = 125.4(2) Mrad/s, η = 0.335(5)]. We can exclude amorphous as well as cubic and tetragonal hafnium oxide phases in the annealed samples. There was no indication of room-temperature ferromagnetism. [ABSTRACT FROM AUTHOR]

Transparent metal-insulator-semiconductor field-effect transistors (TMISFETs) are commonly designated as one keystone of transparent circuitry. TMISFETs were demonstrated using carbon nanotubes, organics, or oxides. The optimization of their gate-insulator as well as the field-effect mobility and switching voltages is of major interest in this research field. We present an alternative approach based on metal-semiconductor field-effect transistors (MESFETs) circumventing these problems of TMISFET technology. We use ultrathin transparent rectifying contacts (TRCs) consisting of non-insulating AgxO or PtxO layers and a highly conducting capping layer realized by room-temperature sputtering. The process is compatible with other low-temperature, low-cost manufacturing steps. We demonstrate the potential of such TRCs for ZnO-based MESFETs having a transparency of 70% in the visible, on/off-ratios higher than 106 within a gate-voltage sweep of only 2.7 V and mobilities up to 12 cm2/V s. Inverters fabricated from these transistors exhibit maximum gain of 196 and uncertainty level of 0.36 V, outperforming inverters based on TMISFETs. [ABSTRACT FROM AUTHOR]

We report on luminescence properties from T=2 K up to room temperature of ZnO/Zn1-xCdxO/ZnO double heterostructures grown by pulsed-laser deposition on a-plane sapphire substrates. Depending on the growth conditions, the spectral position of the Zn1-xCdxO related maximum has been tuned from 3.19 to 3.056 eV, corresponding approximately to Cd contents between 2.1% and 5.6%. Independent of x we observe intense phonon replicas of the photoluminescence (PL) maximum. The quenching of the luminescence intensity indicates the presence of two thermal activation energies, one of them being assigned to the delocalization of excitons from donors. The temperature-dependent PL spectra exhibit the so-called "S-shape" behavior as function of temperature for the Zn1-xCdxO due to the superposition of the usual S-shape, caused by the alloy, and a change in the peak character from donor-bound exciton to free exciton. [ABSTRACT FROM AUTHOR]

We investigated the surface roughness and the luminescence properties of MgxZn1-xO thin films (0≤x≤0.19). The thin films were grown on a-plane sapphire substrates by pulsed laser deposition. The root-mean-square surface roughness depends on the oxygen partial pressure p(O2) applied during deposition and takes a minimal value of 0.6 nm for p(O2)=1×10-3 mbar. Deposition of the thin films on a ZnO buffer layer further diminishes the surface roughness. The photoluminescence maximum peak from the MgxZn1-xO thin films is due to free excitons, which are localized in potential minima at low temperatures, and shows a linear high-energy shift with increasing x. The full width at half maximum (FWHM) of the photoluminescence depends on p(O2). Samples with the smallest FWHM values were grown at p(O2)=1.6×10-2 mbar. We analyze the contributions of alloy broadening and of the lateral Mg-concentration inhomogeneity to the photoluminescence FWHM. In optimized samples FWHM is limited by random alloy broadening. Scanning cathodoluminescence shows that the lateral inhomogeneity of the Mg distribution decreases with p(O2) if the samples are grown directly on the sapphire substrates. Deposition of the MgxZn1-xO thin films on a ZnO buffer layer leads to a laterally homogeneous Mg distribution in the samples. [ABSTRACT FROM AUTHOR]

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, 6S) and Co2+ (3d7, 4F), 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. [ABSTRACT FROM AUTHOR]

Infrared spectroscopic ellipsometry in the spectral range from ω=360 cm-1 to ω=1500 cm-1 and Raman scattering spectroscopy are applied to study the long-wavelength optical phonon modes and dielectric constants of MgxZn1-xO thin films in the composition range 0<=x<=1. The samples were grown by pulsed laser deposition on sapphire substrates. X-ray diffraction measurements of the thin film samples reveal the hexagonal wurtzite crystal structure for x<=0.53 and the cubic rocksalt crystal structure for x>=0.67. A systematic variation of the phonon mode frequencies with Mg-mole fraction x is found for both hexagonal and cubic MgxZn1-xO thin films. The modified random isodisplacement model matches the observed composition dependence of the phonon mode frequencies for the hexagonal structure thin films [J. Chen and W. Z. Shen, Appl. Phys. Lett. 83, 2154 (2003)], whereas a simple linear approximation scheme is sufficient for the cubic structure part. We observe a discontinuous behavior of the transverse optical phonon modes (decrease), and the static and high-frequency dielectric constants (increase) within the phase transition composition region from the wurtzite structure part to the rocksalt structure part. On the contrary, the longitudinal phonon mode parameters increase almost linearly, and upon phase transition the splitting between the transverse and longitudinal modes increases. We associate this discontinuous behavior with the change of the nearest-neighbor coordination number from fourfold (wurtzite structure) to sixfold (rocksalt structure) in our samples and the associated increase in bond ionicity from ZnO to MgO. Accordingly, we propose that the reduced exciton mass parameter should approximately double upon changing from wurtzite to rocksalt crystal structure. [ABSTRACT FROM AUTHOR]

Zn1-xCdxO(1120) films have been grown on (0112) sapphire (r-plane) substrates by metal-organic vapor phase epitaxy. A 800-nm-thick ZnO buffer, deposited prior to the alloy growth, helps to prevent the formation of pure CdO. A maximum uniform Cd incorporation of 8.5 at. % has been determined by Rutherford backscattering spectrometry. Higher Cd contents lead to the coexistence of Zn1-xCdxO alloys of different compositions within the same film. The near band-edge photoluminescence emission shifts gradually to lower energies as Cd is incorporated and reaches 2.93 eV for the highest Cd concentration (8.5 at. %). The lattice deformation, due to Cd incorporation, has been described using a new reference frame in which the lattice distortions are directly related to the a-plane surface structure. Cd introduction does not affect the c lattice parameter but expands the lattice along the two perpendicular directions, [1120] and [1100], resulting in a quadratic volume increase. [ABSTRACT FROM AUTHOR]

Infrared dielectric function spectra and phonon modes of high-quality, single crystalline, and highly resistive wurtzite ZnO films were obtained from infrared (300-1200 cm[sup -1]) spectroscopic ellipsometry and Raman scattering studies. The ZnO films were deposited by pulsed-laser deposition on c-plane sapphire substrates and investigated by high-resolution x-ray diffraction, high-resolution transmission electron microscopy, and Rutherford backscattering experiments. The crystal structure, phonon modes, and dielectric functions are compared to those obtained from a single-crystal ZnO bulk sample. The film ZnO phonon mode frequencies are highly consistent with those of the bulk material. A small redshift of the longitudinal optical phonon mode frequencies of the ZnO films with respect to the bulk material is observed. This is tentatively assigned to the existence of vacancy point defects within the films. Accurate long-wavelength dielectric constant limits of ZnO are obtained from the infrared ellipsometry analysis and compared with previously measured near-band-gap index-of-refraction data using the Lyddane-Sachs-Teller relation. The ZnO model dielectric function spectra will become useful for future infrared ellipsometry analysis of free-carrier parameters in complex ZnO-based heterostructures. [ABSTRACT FROM AUTHOR]

Several experimental techniques are employed to explain the angular dependence of the ferromagnetic resonance observed in the different metal nanocrystals in Fe, as well as Ni implanted ZnO. The resonance is shown to be perpendicular to the sample surface in case of the Fe nanocrystals.

Several studies are conducted to explain the different luminescence properties of ZnO/[Zn.sub.1-x][Cd.sub.x]O/ZnO double heterostructures. The photoluminescence spectra of the heterostructures are shown to be completely dependent on the temperature.

A series of chemically prepared [Co.sup.2+]-doped ZnO colloids is surface modified either by growing shells of ZnSe or by the in situ encapsulation in poly(styrene) (PS). The studies have shown that as the surface shell is not removed by the PS encapsulation, this effect is not very pronounced for this surface modification procedure.

The surface roughness and the luminescence properties of [Mg.sub.x][Zn.sub.1-x]O thin films, which are grown on a-plane sapphire substrates by pulsed laser deposition, are investigated. The analysis has shown that the deposition of the [Mg.sub.x][Zn.sub.1-x]O thin films on a ZnO buffer layer has led to a laterally homogeneous Mg distribution in the samples.

Raman spectroscopy and transmission electron microscopy (TEM) are used to study the recrystallization upon sample annealing up to T = 1000 degree Celsius within sculptured thin films composed of silicon square screws. The chiral geometry as well as the fiberlike fine structure of the individual square crews has survived the high temperature annealing process and the crystallite formation has occurred within the silicon fibers, which restricts the average grain size diameter.

The structural, surface, and optical properties of phase-pure κ-(AlxGa1−x)2O3 thin films on c-sapphire and STO(111):Nb substrates as well as on MgO(111) and κ-Ga2O3 templates are reported as a function of alloy composition for x < 0.4. The thin films were grown by tin-assisted pulsed laser deposition (PLD). For the variation of the Al-content, we utilized radially segmented PLD targets that enable the deposition of a thin film material library by discrete composition screening. Growth on κ-Ga2O3 (001) thin film templates enhanced the phase pure growth window remarkably up to x = 0.65. The crystallization of the κ-phase was verified by X-ray diffraction 2θ-ω-scans for all samples. Both in- and out-of-plane lattice constants in dependence on the Al-content follow a linear relationship according to Vegard's law over the complete composition range. Atomic force microscope measurements confirm smooth surfaces (Rq ≈ 1.4 nm) for all investigated Al-contents. Furthermore, bandgap tuning from 4.9 eV to 5.8 eV is demonstrated and a linear increase in the bandgap with increasing Al-content was observed. [ABSTRACT FROM AUTHOR]