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Abstract Pseudomorphic and relaxed $$\alpha$$ α -(Al$$_x$$ x Ga$$_{1-x}$$ 1 - x )$$_2$$ 2 O$$_3$$ 3 thin films are grown by combinatorial pulsed laser deposition in the entire composition range on prismatic a- and m-plane $$\alpha$$ α -Al$$_2$$ 2 O$$_3$$ 3 substrates. Pseudomorphic growth on m-plane sapphire has been achieved for $$x \ge 0.45$$ x ≥ 0.45 . A distinct difference between the a- and m-epitaxial plane is observed in reciprocal space map measurements being in agreement with continuum elasticity theory for rhombohedral heterostructures. While pseudomorphic layers on m-plane sapphire show a pronounced shear strain $$e'_5$$ e 5 ′ along the c-axis direction, relaxed layers exhibit a global lattice tilt in the same direction. Both effects are not present on the a-epitaxial plane. Out-of-plane lattice constants as well as $$e'_5$$ e 5 ′ are modeled as function of x employing elasticity theory, confirming theoretical values of the elastic stiffness tensor for $$\alpha$$ α -Ga$$_2$$ 2 O$$_3$$ 3 , especially the non-zero value of the $$C_{14}$$ C 14 component. Possible pyramidal slip systems for strain relaxation in c-axis direction are examined to explain and numerically model the difference in lattice tilt for the two substrate orientations. Graphic abstract

The occurrence of rotational domains is a well-known issue for copper iodide (CuI) that naturally occurs for growth on popular substrates like sapphire. However, this has detrimental effects on the thin film quality like increasing surface roughness or deteriorated transport characteristics due to grain boundary scattering. Utilizing pulsed laser deposition and the in situ growth of sodium chloride (NaCl) and sodium bromide (NaBr) template layers, studies were performed on their potential on suppressing the formation of rotational domains of CuI on c-plane sapphire and SrF2(111) substrates. Corresponding samples were investigated concerning their epitaxial properties and further characterized regarding (volume) crystalline, morphological, and electrical properties. Particularly for NaBr template layers, fully single-crystalline growth of CuI thin films was obtained and resulted in significantly reduced surface roughness of the CuI layer.

Sn5O2(PO4)2 is a promising p-type transparent semiconducting oxide. Phase-pure triclinic Sn5O2(PO4)2 thin films were grown by pulsed laser deposition using a Sn2(P2O7) target with higher phosphorus content. The (001)-oriented growth of triclinic Sn5O2(PO4)2 on glass was achieved by means of a (111)-textured Y2O3 buffer layer. STEM-EDX and XPS revealed that the composition of the obtained film is near-stoichiometric, thus indicating a suitable semiconducting material. The bandgap of the triclinic Sn5O2(PO4)2 film was estimated to be as large as 3.87 eV, which is the first experimental evidence verifying a recent theoretical prediction by Q. Xu et al., Chem. Mater., 2017, 29, 2459 quite closely.

Herein, the magnetoelectric (ME) performance of epitaxial multilayer composite films built from nanometer‐thick layers of multiferroic BiFeO3 and ferroelectric BaTiO3 is reviewed. A successful implementation of shadow‐mask pulsed laser deposition considerably reduces the interface and surface roughness of the multilayers. In dependence of double‐layer thickness and the degree of structural perfection, the multilayers show high ME voltage coefficients up to 480 V cm−1 Oe−1 at 300 K and 0 T bias magnetic field. With decreasing double‐layer thickness, an interface‐driven effect critically enhances the ME coupling in this strain and charge-comediated system. Interestingly, the characteristics of temperature and DC magnetic field dependencies of magnetoelectric voltage coefficients change with the transition from the 2D to 3D character of the single layers, i.e., for BiFeO3 layers thicker than 5 nm within the multilayers. These changes are attributed to variations of the contributing ME coupling mechanisms. Furthermore, scanning transmission electron microscopy (STEM) with energy‐dispersive X‐ray (EDX) spectroscopy mapping‐based nanoanalysis indicates that chemical effects at the interfaces play an important role for the ME performance of the BiFeO3-BaTiO3 multilayer thin films.

We investigate the epitaxial growth of (Al,Ga)2O3 alloy thin films in the corundum phase on r-plane (01.2) Al2O3 substrates. We compare films grown by pulsed laser deposition at substrate temperatures of 750 °C and 1000 °C. The initial strongly anisotropic plastic strain relaxation through the a-plane prismatic glide system is directly evidenced by imaging the associated slip lines. We find enhanced plastic relaxation at the higher substrate temperature. Details of the stoichiometry transfer from the target to the film are discussed.

We investigated properties of an (In(x)Ga(1-x))2O3 thin film with laterally varying cation composition that was realized by a large-area offset pulsed laser deposition approach. Within a two inch diameter thin film, the composition varies between 0.01 ≤ x ≤ 0.85, and three crystallographic phases (cubic, hexagonal, and monoclinic) were identified. We observed a correlation between characteristic parameters of Schottky barrier diodes fabricated on the thin film and its chemical and structural material properties. The highest Schottky barriers and rectification of the diodes were found for low indium contents. The thermal stability of the diodes is also best for Ga-rich parts of the sample. Conversely, the series resistance is lowest for large In content. Overall, the (In(x)Ga(1-x))2O3 alloy is well-suited for potential applications such as solar-blind photodetectors with a tunable absorption edge.

We have investigated structural, optical, and electrical properties of MgZnO:(Al/Ga) thin films in dependence on Mg and Al/Ga concentrations. For this purpose, thin films with two perpendicular, lateral composition gradients, i.e., the Mg composition is varied in one direction whereas the Al/Ga concentration is varied in a perpendicular direction were grown at by pulsed-laser deposition (PLD) using a threefold segmented PLD target and 2-inch in diameter c-plane sapphire substrates. It has been found that compensation by intrinsic acceptors limits efficient doping to dopant concentrations of about . Further, the electrical data suggests, that the compensating defect is doubly chargeable hinting to the zinc vacancy as microscopic origin. Increasing the dopant concentration above leads to a degradation of electrical and structural properties.

LaNiO3 (LNO) thin films were grown using pulsed laser deposition. The c-axis, i.e., out-of-plane lattice parameter of the films was controlled reproducibly by using different substrate materials and by variation of oxygen partial pressure and growth temperature. The out-of-plane (c-axis) strain of LNO deposited on LaAlO3 with increasing oxygen pressure changed from positive to negative. All the films show an excellent metallic conductivity with positive resistivity temperature coefficient. Lowest resistivity was about 300 µΩ cm. At high and low temperatures, the resistivity is explained by electron–phonon scattering and electron–electron interaction, respectively. In addition, the resistivity shows a clear dependence on the c-axis strain of LNO films. With increasing strain, the resistivity increases. However, this effect is much more pronounced for negative c-axis strain. Strain-dependent resistivity of LNO films on LAO at the indicated measurement temperatures. The inset is a typical AFM image of the LNO film surface.

Pulsed laser deposition is one of the most flexible growth methods for high-quality epitaxial multifunctional thin films and short-period superlattices. The following examples of current research interest demonstrate the state-of-the art: First, it is shown that the magnetoelectric performance of multiferroic BiFeO3-BaTiO3 (001)-oriented superlattices depends on the crystalline coherence of the different layers at the interfaces. Second, it is exemplified that dielectric-plasmonic superlattices built from the electrically insulating oxide MgO and the metallically conducting nitride TiN are promising metamaterials with hyperbolic dispersion. As a third example, it is demonstrated that LaNiO3- and LaMnO3-based superlattices with (001)-, (011)-, and (111)-out-of-plane orientation and controlled single layer thickness from 2 to 15 atomic monolayers show metal-insulator transitions and tunable gaps, in partial agreement with density functional theory calculations. Underlined by these examples, it is shown that the precise control of an epitaxially coherent, or two-dimensional layer-by-layer growth, named after Jan van der Merwe, is a prerequisite to achieve the desired functionality of oxide-oxide and oxide-nitride superlattices.

We have grown atomically smooth heteroepitaxial TiN thin films on (100) MgO single crystals by pulsed laser deposition (PLD). To generate optimal conditions for two-dimensional growth of TiN on MgO, the substrates were annealed in-situ using a CO2 laser heater. The MgO surface is smooth and uniformly stepped with a terrace height of half a unit cell; a value of 0.21 nm was measured by atomic force microscopy (AFM). In-situ RHEED oscillations during deposition of TiN indicate two-dimensional growth mode up to a film thickness of about 43 nm. This is confirmed by AFM. The TiN films show smooth, stepped surfaces with a uniform terrace height of 0.211 nm. X-ray diffraction measurements indicate a high film quality, as well. High resolution scans feature intensity fringes around the (200) TiN peak, indicating an abrupt interface between MgO and TiN. On asymmetric reciprocal space maps the peak broadening only in due to finite film thickness proves a high crystalline quality. Maximum electrical conductivity of the films is 2 × 106 S/m at 300 K. AFM topography images (5 × 5 μm2) of the surface of a (100) MgO substrate with a miscut angle of γ = 0.10°, annealed at 950 °C (height scale is 0 to 3.2 nm, left) and a 43 nm thick TiN/MgO thin film grown by PLD (height scale is 0 to 2.4 nm, right). Step height is half a unit cell in both cases.

55 Mn pulsed electron nuclear double resonance (ENDOR) experiments were performed at X-band on high spin S = 5/2 Mn 2+ ions incorporated at zinc lattice sites in heteroepitaxial ZnO thin films. The films have been prepared by pulsed laser deposition and the manganese ions were doped during the growth process. We examine how the c / a lattice axes ratio of the ZnO films influences the 55 Mn hyperfine (hf) and nuclear quadrupole (nq) coupling parameters of the Mn 2+ probe ions. The results are compared with those obtained for Mn 2+ ions present as impurities in ZnO single crystals and revealed that the 55 Mn nq coupling monitors sensitively the structural distortions in the bonding environment of the Mn 2+ ions. The experiments provided the full axially symmetric 55 Mn hf and nq interaction tensors. The latter is found to be very sensitive to small axial distortions of the MnO 4 tetrahedrons. In particular, the 55 Mn pulsed ENDOR spectra of the ZnO:Mn thin films are strongly subjected to strain effects in the nq coupling parameter indicating a variation of the local structural parameters for the heteroepitaxial films. In the analysis of the 55 Mn pulsed ENDOR spectra the axial and cubic zero field splitting of the Mn 2+ ions was taken into account and intensity effects in the ENDOR spectra due to the zero field splitting effects were discussed.

We investigate Umweganregung reflections (X-ray multiple reflections) for , (00.3), and (00.5) for a number of wurtzite structure ZnO samples, namely various bulk crystals (substrates), two ZnO thin films and a ZnO:Mg thin film, grown by pulsed laser deposition on a-plane sapphire. Using a low divergence setup, we achieve a high angular resolution of 0.1 or better. We derive an analytical formula for peak positions as a function of wavelength and the lattice parameters a and c. The three investigated ZnO substrates exhibit all expected (kinematically allowed) reflections for Cu Kα1, Kα2, and Kβ. A lineshape analysis of the multiple reflection peaks shows that the width of certain reflections is determined by the spectral linewidth of the exciting copper lines. The thin films exhibit only slight broadening of the peaks but disappearance of several reflections. The a lattice constant can be determined from the azimuthal peak positions. Umweganregung scan for () of a hydrothermally grown ZnO substrate, excited with Cu Kα1 radiation. The pattern is symmetric with respect to zero angle representing the (10.0) azimuth.

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.

Atomically smooth, pseudomorphic (Al1−xGax)2O3 thin films were grown for 0 ≤ x

Growth control of nonpolar and polar ZnO / Mg x Zn 1 − x O quantum wells (QWs) is demonstrated by in situ RHEED during the pulsed laser deposition process. Nonpolar QWs were grown homoepitaxially on m -plane and on a -plane ZnO single crystals. For m -plane ( 10 1 ¯ 0 ) ZnO QWs we report a change of growth mode from a two dimensional layer by layer growth evidenced by RHEED oscillations to the formation of surface nanostripes as observed by atomic force microscopy. The aspect ratio of the self organized nanostripes depends on the oxygen partial pressure. a -lane ( 11 2 ¯ 0 ) ZnO QW-structures show a smooth surface with a rms-roughness of 0.3 nm. Homoepitaxial nonpolar QWs do not show the quantum-confined Stark effect while polar quantum wells on a -plane sapphire does with an internal electric field of approximately 0.53 MV/cm. Furthermore, by implementing a low temperature Mg x Zn 1 − x O buffer layer, the interface quality of heteroepitaxially grown polar ZnO / Mg x Zn 1 − x O QWs on a -plane sapphire substrates is considerably improved. RHEED oscillations were observed during the whole growth of such QWs.

Epitaxial zinc ferrite thin films are grown on (001) SrTiO3 substrates by pulsed laser deposition. The magnetic anisotropy of the films is examined by superconducting quantum interference device magnetometry and ferromagnetic resonance, with various in-plane and out-of-plane orientations of the magnetic field. The volume magnetization of the films is found to be around 55 kA/m. The existence of a cubic magnetic anisotropy is shown, with the easy axes oriented along the [111] crystallographic directions. The films show ferrimagnetic properties with a Curie temperature higher than 350 K. The nature of a strong “easy-plane”-type bimodal anisotropy in the films is discussed.

Epitaxial multiferroic BaTiO₃-BiFeO₃ composite thin films exhibit a correlation between the magnetoelectric (ME) voltage coefficient αME and the oxygen partial pressure during growth. The ME coefficient αME reaches high values up to 43 V/(cm·Oe) at 300 K and at 0.25 mbar oxygen growth pressure. The temperature dependence of αME of the composite films is opposite that of recently-reported BaTiO₃-BiFeO₃ superlattices, indicating that strain-mediated ME coupling alone cannot explain its origin. Probably, charge-mediated ME coupling may play a role in the composite films. Furthermore, the chemically-homogeneous composite films show an oxygen vacancy superstructure, which arises from vacancy ordering on the {111} planes of the pseudocubic BaTiO₃-type structure. This work contributes to the understanding of magnetoelectric coupling as a complex and sensitive interplay of chemical, structural and geometrical issues of the BaTiO₃-BiFeO₃ composite system and, thus, paves the way to practical exploitation of magnetoelectric composites. ispartof: Materials vol:9 issue:1 pages:1-13 ispartof: location:Switzerland status: published

Phosphorus-doped MgxZn1−xO thin films with various Mg content (0.07 ≤ x ≤ 0.28) and up to 0.7 at% P were grown on ZnO(000-1) single crystals by pulsed laser deposition (PLD). In dependence on the dopant concentrations, tensile or compressively strained, in-plane lattice matched, i.e., pseudomorphic growth of the MgZnO:P films was confirmed by high-resolution X-ray diffraction (HR-XRD). All investigated, high-quality MgZnO:P films are n-type, also after post-growth annealing at 850 °C. From the Hall analysis the activation energy of the dominating donors was derived, yielding values of up to 345 meV for high oxygen partial pressure during growth, and shallow donors with an activation energy of about 30 meV for low oxygen pressure. The Hall-mobility of the MgxZn1−xO:P films at a temperature of 65 K was as high as 740 cm2/Vs, and reached values up to 190 cm2/Vs at room temperature, close to the best values reported for bulk ZnO.

The thermal stability of ZnO / Zn 1 − x Cd x O / ZnO double heterostructures grown by pulsed-laser deposition was studied by annealing them for 30 min in air at temperatures from 620 to 970 °C. In photoluminescence spectroscopy measurements on the as-grown samples emission with energies between 3.122 and 3.044 eV were observed, corresponding approximately to Cd-contents between 2.7% and 3.6%. Due to the annealing a monotonic blue-shift of the Zn 1 − x Cd x O - related luminescence was found with increasing annealing temperature. The shift was small for annealing temperatures of up to 720 °C, thus a stability of the heterostructures up to this temperature was concluded. The underlying diffusion process was investigated to determine the diffusion coefficient for the different annealing temperatures. The temperature dependence of the diffusion coefficient shows an activation enthalpy in the range between 2.1 and 3.5 eV, depending on the growth conditions.

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 (YBa2Cu3O7−δ), semiconducting (ZnO-based), and ferroelectric (BaTiO3-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.

Homoepitaxial MgZnO thin films were grown from PLD targets with 1, 2, 4, and 10 wt.% MgO content on ZnO-buffered ZnO(001) substrates. The resulting Mg content of the films was determined from the blue-shift of the excitonic peak in low-temperature photoluminescence spectra. With increasing Mg content a considerable increase of film mosaicity was observed from HR-XRD (002) rocking curves. Triple-axis reciprocal space maps of symmetric (002) and asymmetric (104) reflections show the structural development in terms of tilt and perpendicular and parallel strain. For more than 1% Mg the films exhibit increasing tensile out-of-plane strain. Very high electron mobilities of 200 cm²/Vs at 300 K and 900 cm²/Vs at 65 K were measured in the homoepitaxial MgZnO/ZnO thin films with free electron concentrations around 10 18 and 10 17 cm − 3 , respectively. The homoepitaxial ZnO template film deposited from a melt-grown ZnO single crystal as PLD target shows two orders of magnitude lower carrier concentration due to high compensation.

The optical properties of ZnO / Mg 0.2 Zn 0.8 O single quantum wells grown by pulsed laser deposition on sapphire substrates as well as on ZnO substrates were studied by photoluminescence measurements. The transition energy of the excitons shows a clear blue shift with decreasing well width due to the quantum confinement. A detailed comparison of our results with the literature indicates the lack of a stringent theory to calculate the exciton transition energies. The experimentally determined full width of half maximum (FWHM) of the luminescence of hetero- and homoepitaxial grown ZnO quantum wells compared with the calculated ones suggests that an additional contribution has to be taken into account to determine the luminescence linewidth correctly. We assume an additional alloy disorder in the quantum well itself due to the diffusion of Mg. Using time-resolved photoluminescence we found a monoexponential decay in the quantum well.

ZnO-based nano- and micro-structures with controlled orientation, size, and lateral density were grown by specially designed two-step pulsed laser deposition and direct carbothermal growth, respectively. Different substrate orientations and nucleation layers allow well-defined tuning of growth direction and lateral arrangement of the wires. As a result, axial MgZnO-ZnO nano quantum dots and homogeneous radial nano coreshell quantum well structures are demonstrated. Donor and acceptor doping of nano- and microwires influence considerably both electrical characteristics as well as morphology including branching. Field effect transistors with n- and p-type wire channel and nanowire p-n junctions seem to prove a reproducible p-type conductivity of phosphorous-doped ZnO wires.

The properties of ZnO/MgZnO heterostructures grown by pulsed-laser deposition on sapphire (1120) and ZnO (0001) have been compared. Electron accumulation layers have been observed for ZnO/MgZnO heterostructures grown on sapphire by capacitance–voltage ( C – V ) spectroscopy. The formation of a two-dimensional electron gas (2DEG) in these structures has been confirmed by temperature dependent Hall effect measurements. From C – V measurements the sheet carrier density in a Zn 0.8 Mg 0.2 O/ZnO/Zn 0.8 Mg 0.2 O quantum well (QW) structure with a well width of about 5 nm is calculated to be only about 9.0 × 10 10 cm − 2 . For the films deposited on sapphire 2D growth is observed in the Burton–Cabrera–Frank mode, as confirmed by atomic force microscopy. Step flow growth mode was achieved for the homoepitaxial thin films. Quantum confinement effects have been confirmed by photoluminescence (PL) measurements. Homoepitaxial QWs are more homogeneous (smaller inhomogeneous recombination broadening) than heteroepitaxial QWs.

The authors used highly rectifying PdOy/MgxZn1−xO Schottky barriers to determine the apparent capture cross-section and the thermal activation energy of the E3 defect in MgxZn1−xO thin films by thermal admittance spectroscopy and deep-level transient spectroscopy for x < 0.08. The samples were grown by pulsed-laser deposition. It is observed that both the apparent capture cross-section and the thermal activation energy increase with increasing Mg mole fraction.

We prepared conductive, polycrystalline or amorphous Cu1.05Cr0.89 Mg0.05O2 films on a-plane sapphire substrates by pulsed laser deposition under different O2 partial pressure and substrate temperature. Hall measurements were performed to study the majority carrier type in these films. Polycrystalline Cu1.05Cr0.89 Mg0.05O2 is n-type conducting at 290 K, while in amorphous Cu1.05Cr0.89 Mg0.05O2 the type of majority charge carriers changes from electrons to holes at around 270 K. Interestingly, the structure has little influence on the magnetic properties of the films. A clear antiferromagnetic to paramagnetic transition was observed in both polycrystalline and amorphous Cu1.05Cr0.89 Mg0.05O2 films at 25 K. Similar electrical properties to Cu1.05Cr0.89 Mg0.05O2 film were observed for Cu0.96Cr0.95 Mg0.05Mn0.04O2 in dependence on the structure, while only paramagnetic without antiferromagnetic ordering was observed down to 5 K. Large negative magnetoresistance of 27% at 20 K was observed at 6 T in amorphous Cu1.05Cr0.89 Mg0.05O2 film.

Homoepitaxial ZnO films deposited on annealed hydrothermal O-face ZnO single crystals show superior structural quality. This is demonstrated by narrow ZnO(00.2) rocking curves with FWHM of typically 23 to 35 arcsec, and nearly dislocation-free TEM cross sections. Nominally undoped ZnO films indicate a minor in-plane strain of about 250 ppm and no out-of-plane strain. Target doping by 0.01% P2O5 or 0.5% Li3N results in pseudomorphic film growth without in-plane strain. Increasing doping concentration of 0.1 and 1% P2O5 results in both in-plane and out-of-plane strain up to 0.9% indicating relaxed films. The O-face polarity of the homoepitaxial ZnO films is confirmed by convergent beam electron diffraction. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

ZrO2-Al2O3 Bragg reflectors for future application in ZnO based resonators grown by pulsed laser deposition on c-plane sapphire and silicon substrates show a reflectivity up to 99.8% with a layer pair number of 12.5. Layer thicknesses obtained from standard ellipsometry agree very well with thicknesses observed by transmission scanning electron microscopy. A very smooth surface roughness of Ra = 0.5 nm was observed for these Bragg reflectors. ZnO thin films grown on bulk c-plane sapphire and on Bragg reflectors revealed comparable high photoluminescence intensity. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Heterojunctions composed of wurtzite-structure (piezoelectric) ZnO and perovskite-structure (ferroelectric) BaTiO3 are very interesting because of the previously observed ionic lattice polarization coupling at their interfaces. We report electric Sawyer-Tower polarization hysteresis measurements and analysis of a ZnO-BaTiO3 heterostructure with Pt front and back contacts deposited by pulsed laser deposition onto a (001) silicon substrate. The ZnO layer is n-type (Nc = 5.5 × 1016 cm−3), and the BaTiO3 (BTO) layer is highly resistive. We observe a strong asymmetric ferroelectric hysteresis, which we attribute to a rectifying depletion layer formation between the ZnO and BaTiO3 layers. The coupling between the wurtzite-structure and perovskite-structure interface polarization influences the depletion layer formation. We develop a physical model for the electric Sawyer-Tower measurements. Our model includes the effects of the depletion layer formation inside the ZnO layer, the interface charge coupling between the ZnO and BaTiO3 layers, and the field-dependent ferroelectric polarization inside the BTO. We obtain a very good agreement between our model-generated data and our experiment. We identify voltages in forward and reverse direction at which the depletion layer opens or closes. These voltages are asymmetric, and reveal the effect of the spontaneous piezoelectric (nonswitchable) interface charge of ZnO, which we determine from our analysis here as Psz = −4.1 μC/cm2.

Zn 90 Mn 7.5 Cu 2.5 O 100 films have been prepared on a-plane sapphire substrates by pulsed laser deposition. Paramagnetism mainly caused by Mn 2+ ions was observed in the films from room temperature down to 2 K. Magnetotransport properties (magnetoresistance (MR) and Hall effect) were studied from 5 K to 290 K up to a field of 6 T. Negative MR was observed at temperature above 100 K. Low field positive MR and high field negative MR was observed at 5 K. Clear anomalous Hall effect with a kink at low field was observed below 20 K, indicating that there exist two different scattering mechanisms for the Mn 2+ and Cu 2+ ions.

We have used deep level transient spectroscopy (DLTS) to characterize defects in ZnO grown by pulsed-laser deposition (PLD). Using high resolution Laplace DLTS, we found that at the high temperature side of the commonly observed defect E3 (about 300 meV below the conduction band) another close lying peak (E3′ with thermal activation energy of 370 meV) is also observed. The concentration ratio of E3 and E3′ depends on the annealing history of the samples. It is most prevalent in as-grown samples and samples that had been annealed in an oxygen atmosphere. This suggests that E3′ may be related to the incorporation of oxygen in the lattice. Electron capture onto that E3′ defect strongly increases with increasing temperature.

Due to the small magnetic moments observed for 3d transition metals in ZnO [M. Diaconu, H. Schmidt, H. Hochmuth, M. Lorenz, G. Benndorf, J. Lenzner, D. Spemann, A. Setzer, K.W. Nielsen, P. Esquinazi, M. Grundmann, Thin Solid Films 486 (2005) 117], there is still space for optimizing ZnO-based diluted magnetic semiconductors for spintronics applications. Motivated by the observation of magnetic moments as high as 4000μB/Gd atom in GaN:Gd [S. Dhar, O. Brandt, M. Ramsteiner, V.F. Sapega, K.H. Ploog, Phys. Rev. Lett. 96 (2005) 037205], we investigated ZnO films doped with 0.01, 0.1 or 1 at.% rare earth (RE) metals. The films, with thicknesses between 20 nm and 1 μm, have been grown by pulsed laser deposition on a -plane sapphire or fused silica substrates. The homogenous incorporation of the RE ions in ZnO was investigated by combined Rutherford backscattering and particle induced X-ray emission measurements. Hall measurements revealed an unexpected dependence of the electron concentration on film thickness, proving a non-uniform distribution of electrically active defects. Magnetotransport measurements at different temperatures were performed to study the magnetoresistance and the presence of the anomalous Hall effect. Large negative magnetoresistance was obtained at 5 K, while no anomalous Hall effect was observed. These results indicate that there are no exchange interactions between the RE ions.

MgZnO thin films, MgZnO/ZnO heterostructures (HS) and double heterostructures (DHS) have been prepared on a-plane sapphire substrates by means of pulsed laser deposition (PLD). A linear blueshift of the MgZnO emission with increasing Mg content is observed in photoluminescence spectroscopy (PL) at 2 K. Cathodoluminescence measurements verify the spatial homogeneity of the emission properties of the MgZnO films. The film roughness is evaluated from atomic force microscopy scans. In MgZnO/ZnO HS the ZnO grows on all appearing MgZnO facets. PL investigations of such PLD-grown heterostructures show the high optical quality of thin ZnO films (d≤100 nm) grown on MgZnO. Capping those structures with a thin MgZnO layer further improves their luminescence intensity and enhances the emission of free-exciton luminescence from the ZnO layers. MgZnO/ZnO/MgZnO DHS with nominal ZnO layer thicknesses of dnom≤6 nm show a clear intensification of the ZnO PL. Temperature dependent PL and transmission measurements between 4.4 and 300 K prove the dominating emission to be due to the recombination of excitons localized in the ZnO. At 2 K, due to confinement effects, their emission energy is blueshifted up to 51 meV compared to free excitons in bulk ZnO.

We have investigated donor-like defects in ZnO substrate material grown by three different methods, and in epitaxial ZnO thin films grown on sapphire by pulsed laser deposition. Temperature dependent Hall effect measurements yield information about dominant donors. The thermal activation energies lie in a wide range from about 20 meV to about 370 meV. Deep level transient spectroscopy is used to obtain parameters of deep donor-like defects. For that, a high-speed diode contact configuration was laid out for the epitaxial thin films in order to determine the defect parameters with high precision. The identified levels are E1, E3 and E4, though the level E4 is observed only in single crystals grown by seeded chemical vapor transport.

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.

Co$_3$O$_4$, ZnFe$_2$O$_4$, CoFe$_2$O$_4$, ZnCo$_2$O$_4$, and Fe$_3$O$_4$ thin films were fabricated by pulsed laser deposition at high and low temperatures resulting in crystalline single-phase normal, inverse, as well as disordered spinel oxide thin films with smooth surface morphology. The dielectric function, determined by spectroscopic ellipsometry in a wide spectral range from 0.5 eV to 8.5 eV, is compared with the magneto-optical response of the dielectric tensor, investigated by magneto-optical Kerr effect (MOKE) spectroscopy in the spectral range from 1.7 eV to 5.5 eV with an applied magnetic field of 1.7 T. Crystal field, inter-valence and inter-sublattice charge transfer transitions, and transitions from O$_{2p}$ to metal cation 3d or 4s bands are identified in both the principal diagonal elements and the magneto-optically active off-diagonal elements of the dielectric tensor. Depending on the degree of cation disorder, resulting in local symmetry distortion, the magneto-optical response is found to be strongest for high crystal quality inverse spinels and for disordered normal spinel structure, contrary to the first principle studies of CoFe$_2$O$_4$ and ZnFe$_2$O$_4$. The results presented provide a basis for deeper understanding of light-matter interaction in this material system that is of vital importance for device-related phenomena and engineering., Comment: Physica Status Solidi B (Accepted 30.09.2015)

Ferromagnetic, semi-insulating Mn-alloyed ZnO films with a Curie temperature above 375 K have been grown by pulsed laser deposition on c-plane sapphire substrates. Antiferromagnetic coupling is revealed by temperature-dependent magnetization measurements. The antiferromagnetic coupling would be compatible with the observed weak ferromagnetism by assuming that the magnetic moments order antiferromagnetically but nonparallel (canted). We find a clear correlation between coercivity and mosaicity of the ferromagnetic Mn-alloyed ZnO films and explain it on the basis of a coercivity mechanism known from soft magnetic materials.

ZnO films with different 3d transition metal doping (Ti, Co, Mn) were prepared by pulsed laser deposition. The occurrence of positive magnetoresistance (MR) was studied in dependence on temperature. Large positive MR was observed in Co- and Mn-doped ZnO films at 5 K. Zn0.98Mn0.02O films show a positive MR up to ∼ 31% at 5 K. The positive MR for Co- and Mn-doped ZnO films decreases drastically from 5 K to 50 K. Positive MR was also observed in Zn0.9998Ti0.0002O films at 290 K, while negative MR was observed at 5 K. Clear stripe-like magnetic domain pattern was observed by magnetic force microscopy for the Zn0.9998Ti0.0002O films, indicating the possible ferromagnetism at room temperature due to Ti3+-ions in a polarized spin state.

ZnCoO : Al, ZnCoO : (Al,Cu), and reference samples without Co have been grown by pulsed laser deposition to investigate the influence of Co-dopants and Cu-codopants on the magneto transport properties of ZnO. Positive magnetoresistance and anomalous Hall effect have been observed for ZnCoO : (Al,Cu). Versatile theoretical approaches for modelling positive magnetoresistance in ZnO-based diluted magnetic semiconductors are still to be developed. Negative magnetoresistance due to the scattering of spin-polarized charge carriers at isolated magnetic impurities has been observed in ZnO : (Al,Cu) without Co-dopants and successfully modelled. Cu-codopants enhance the positive and negative magnetoresistance in ZnCoO : Al and ZnO : Al, respectively, by one order of magnitude.

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.

We present magnetic properties of Zn(Mn, P)O thin films, with 5 at% Mn, epitaxially grown by pulsed laser deposition in oxygen atmosphere on a-plane sapphire substrates. Ferromagnetism above room temperature was obtained for films with 0.1 at% P, grown at around 600 °C. Temperature-dependent magnetization curves indicate the presence of an antiferromagnetic coupling between the Mn magnetic moments. After annealing ferromagnetic films for 2 hours at 800 °C in N2 atmosphere, they became paramagnetic at all temperatures.

The ferromagnetism in highly transparent and intrinsically n-type conducting zinc oxide doped with 3d transition metals (TM), is predicted to be defect mediated. We investigate the generation of deep defects in n-conducting 1 μm thick ZnO:TM films (TM=Co, Mn, Ti) with a nominal TM content of 0.02, 0.20 and 2.00 at.% grown by pulsed laser deposition on a-plane sapphire substrates using deep level transient spectroscopy. We find that a defect level is generated, independent of the TM content, located 0.31 and 0.27 eV below the conduction band minimum of ZnO:Mn and ZnO:Ti, respectively. Different defect levels are generated in dependence on the Co content in ZnO:Co. This work shows that an optimization of defect-related ferromagnetism in n-conducting ZnO:TM thin films will only be possible if the preparation sensitive formation of deep defects is controlled in the same time.

C-axis oriented ZnO thin films were grown on silicon (100) and (111) substrates by pulsed laser deposition. Low temperature photoluminescence spectra show besides the peaks of free excitons, of defect bound excitons, and of a donor–acceptor pair transition a new doublet at 3.328/3.332 eV. The doublet seems to originate from the columnar textured ZnO film structure. A corresponding structural dependence of the broadening parameter of the infrared dielectric functions was derived from spectroscopic ellipsometry in the spectral range from 380 to 1200 cm− 1. The wave numbers of the E1 transverse optical and A1 longitudinal optical phonon modes of the ZnO films on silicon are determined to be 406 and 573 cm− 1, respectively. These values are slightly smaller than those of single-crystalline ZnO thin films on sapphire.

Room-temperature ferromagnetic Zn1−x(TM)xO semiconductor thin films have been grown by pulsed laser deposition (PLD) on c -plane sapphire substrates and the effect of the substitution of Zn2+ ions by transition metal ions (Mn2+ or Ti2+) has been investigated. In contrast to ZnTiO thin films, the grain size of the ZnMnO thin films can be controlled in a wide range by the PLD growth conditions. The magnetic properties of the Zn1−x(TM)xO films have been investigated by a superconducting quantum interference device (SQUID). For semi-insulating ZnMnO films with an optimized grain size, we observed a weak ferromagnetism for temperatures up to 400 K [M. Diaconu, H. Schmidt, H. Hochmuth, M. Lorenz, G. Benndorf, J. Lenzner, D. Spemann, A. Setzer, K.-W. Nielsen, P. Esquinazi, M. Grundmann, Thin Solid Films 486 (2005) 117–121], while the ZnTiO films were only paramagnetic or superparamagnetic. This is expected with respect to the micromagnetic model.