Growth and optical absorption spectra of ZnO films grown by pulsed laser deposition

ZnO films on Al₂O₃ substrate were grown by using a pulsed laser deposition method. Through photoluminescence (PL) and X-ray diffraction (XRD) measurements, the optimum growth conditions for the ZnO growth were calculated. The results of the XRD measurement indicate that ZnO film was strongly oriented to the c-axis of hexagonal structure and epitaxially crystallized under constraints created by the substrate. The full-width half-maximum for a theta curve of the (0 0 0 2) peak was 0.201°. Also, from the PL measurement, the grown ZnO film was observed to be a free exciton, which indicates a high quality of epilayer. The Hall mobility and carrier density of the ZnO film at 293 K were estimated to be 299 cm²/V sec and , respectively. The absorption spectra revealed that the temperature dependence of the optical band gap on the ZnO films was − .     

Growth of epitaxial TmFeCuO4 thin films by pulsed laser deposition

Epitaxial thin films of TmFeCuO₄ with a two-dimensional triangular lattice structure were successfully grown on yttria-stabilized-zirconia substrates by pulsed laser deposition and ex situ annealing in air. The films as-deposited below 500 °C showed no TmFeCuO₄ phase and the subsequent annealing resulted in the decomposition of film components. On the other hand, as-grown films deposited at 800 °C showed an amorphous nature. Thermal annealing converted the amorphous films into highly (0 0 1)-oriented epitaxial films. The results of scanning electron microscopic analysis suggest that the crystal growth process during thermal annealing is dominated by the regrowth of non-uniformly shaped islands to the distinct uniform islands of hexagonal base.     

Optical and structural properties of epitaxial GaN films grown by pulsed laser deposition

Epitaxial GaN films have been grown on c-cut sapphire substrates by pulsed laser deposition (PLD) using a KrF laser. The properties of GaN films were improved by increasing the growth temperature to 800°C and the nitrogen pressure during growth to 10 mTorr. Room-temperature photoluminescence exhibits a strong band-edge emission at 3.4 eV. From transmission electron microscopy (TEM), the predominant defects in PLD-GaN observed are stacking faults parallel to the interface and screw dislocations along c-axis, the latter differing from the previously published results where most of the threading dislocation in GaN grown by other techniques are of edge type with Burgers vector of .     

Synchrotron X-ray diffraction studies of heteroepitaxial ZnO films grown by pulsed laser deposition

Heteroepitaxial ZnO films were grown by pulsed laser deposition on various substrates such as GaN-buffered C-Al₂O₃, C-Al₂O₃, A-Al₂O₃, and R-Al₂O₃. The epitaxy nature of the films was investigated mainly by synchrotron X-ray diffraction. The results showed that the GaN interlayer plays a positive role in growing an unstrained, well-aligned epitaxial ZnO film on the basal plane of Al₂O₃. Importantly, the ZnO film grown on R-Al₂O₃ has two differently aligned domains. The dominant (1 1 0) oriented domain has much better alignment in the in-plane direction than the minor portion of (0 0 1) oriented domain, while in the out-of-plane direction the two domains have almost the same mosaic distribution.     

The effect of oxygen pressure on the microstructures of Co-doped rutile TiO2 thin films grown by pulsed laser deposition

Comprehensive microstructures of 7% cobalt-doped rutile TiO₂ thin films grown on c-plane sapphire by pulsed laser deposition were characterized using transmission electron microscopy (TEM). The effects of oxygen pressure during growth on the Co distribution inside the films were investigated, and the detailed growth mechanism of both TiO₂ and TiO₂+Co was discussed. The similar oxygen sublattices and low mismatch between (1 0 0) rutile and c-plane sapphire favors the rutile phase. However, the three-fold symmetry of the substrate surface resulted in three rutile domain orientation variants, and they grow adjacent to each other. Cobalt was found to precipitate out as nanocrystals inside the TiO₂ matrix as the growth pressure of oxygen was decreased. At 0.05 mTorr oxygen pressure, almost all of the Co segregates into crystallographically aligned nanocrystals with a particle size of 4.4±0.15 nm. All the samples have magnetic coercivity at room temperature. The magnetic moment per Co atom increased with decreased oxygen pressure, suggesting that the Co that replaced the Ti²⁺ in the TiO₂ lattice does not have a large magnetic moment.     

Growth control of stoichiometry in LaMnO3 epitaxial thin films by pulsed laser deposition

We have studied structural, magnetic, and optical transport properties of LaMnO₃ (LMO) thin films grown on SrTiO₃. While the stoichiometric LMO is an insulating antiferromagnet, it tends to be a ferromagnetic insulator when grown as thin films. By exploring the majority of growth parameters, we have found that the bulk-like electronic and magnetic phases can be stabilized by growing thin films under reducing atmospheres and by using more energetic laser processes. These conditions are found to reduce the La deficiency in the film resulting in the greatly improved cation stoichiometry. Since oxides are prone to reduce the oxygen content and to alter the cation ratio under such growth conditions, it suggests that the cation and oxygen stoichiometries in complex oxide thin films can be improved by properly optimizing the growth parameters.     

Rotary target method to prepare thin films of CdS/SiO2 by pulsed laser deposition

Thin films of CdS-doped SiO₂ glass were prepared by using the conventional pulsed laser deposition (PLD) technique. The laser target consisted of a specially constructed rotary wheel which provided easy control of the exposure-area ratio to expose alternately the two materials to the laser beam. The physical target assembly avoided the potential complications inherent in chemically mixed targets such as in the sol–gel method. Time-of-flight (TOF) spectra confirmed the existence of the SiO₂ and CdS components in the thin-film samples so produced. X-ray diffraction (XRD) and atomic force microscopy(AFM) results showed the different sizes and structures of the as-deposited and annealed films. The wurtzite phase of CdS was found in the 600^oC-annealed sample, while the as-deposited film showed a cubic–hexagonal mixed structure. In the corresponding PL (photoluminescence) spectra, a red shift of the CdS band edge emission was found, which may be a result of the interaction between the CdS nanocrystallite and SiO₂ at their interface.     

Improving the performance of thermoelectric devices by doping Ag in LaPbMnO3 thin films

A series of Ag-doped La_0.6Pb_0.4MnO₃ thin films were grown on vicinal cut substrates by pulsed laser deposition (PLD). Laser-induced thermoelectric voltages (LITV) had been observed in these films, and these LITV signals had been demonstrated to originate from the anisotropic Seebeck effect. By doping Ag to an optimum value, it was found that the peak values (U_P) of the LITV signals were maximized, and the full-width at half-maximum (τ) of the response curves of LITV were minimized at the same time. The figure of merit (F_m) of the device used as photodetector is greatly improved by doping Ag in La_0.6Pb_0.4MnO₃ thin films. The possible reason for these improvements had been well discussed.     

Effects of growth temperature on the properties of ZnO/GaAs prepared by metalorganic chemical vapor deposition

A series of ZnO films were grown on GaAs(0 0 1) substrates at different growth temperatures in the range 250–720°C by metalorganic chemical vapor depostion. Field emission scanning electron microscopy was utilized to investigate the surface morphology of ZnO films. The crystallinity of ZnO films was investigated by the double-crystal X-ray diffractometry. The optical and electrical properties of ZnO films were also investigated using room-temperature photoluminescence and Hall measurements. Arrhenius plots of the growth rate versus reciprocal temperature revealed the kinetically limited growth behavior depending on the growth temperature. It was found that the surface morphology, structural, optical and electrical properties of the films were improved with increasing growth temperature to 650°C. All the properties of the film grown at 720°C were degraded due to the decomposition of ZnO film.     

Preparation of undoped and indium doped ZnO thin films by pulsed laser deposition method

An original modification of the standard Pulse Laser Deposition (PLD) method for preparing both undoped and indium doped zinc oxide (ZnO:In) thin films at low substrate temperature is proposed. This preparation method does not demand any further post‐deposition annealing treatment of the grown films. The developed method allows to grow thin films at low substrate temperature that prevents them from the considerable loss of their intrinsic electrical and optical properties. The influence of deposition parameters on the electrical and optical parameters of the undoped and the indium doped ZnO thin films is also analysed. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Structure and magnetism of ZnO/Co multilayers prepared by pulsed laser deposition

ZnO/Co multilayers were fabricated on silicate (100) substrate by a pulsed laser deposition method at room temperature. The x‐ray diffraction (XRD) results reveal that the as‐deposited multilayer film is composed of amorphous phase, which leads to high saturation magnetization and low coercivity. Higher coercivity is observed in the ZnO/Co films annealed at 390 °C due to the formation of crystalline metallic Co and semiconducting ZnO. But much higher annealing temperature leads to the oxidation of metallic Co and the reaction between Co and ZnO, which decreases the saturation magnetization and coercivity obviously.     

Microstructure and ferroelectric properties of BaTiO3 films on LaNiO3 buffer layers by rf sputtering

We have fabricated LaNiO₃ and BaTiO₃ films using the rf sputtering method. The LaNiO₃ were deposited on Si substrates, demonstrating a (1 0 0) highly oriented structure and nanocrystalline characteristic with a grain size of 30 nm. The BaTiO₃ thin films were deposited on the LaNiO₃ buffer layers, and have exhibited a (1 0 0) texture with a thickness of 400 nm. A smooth interface is obtained between the LaNiO₃ bottom electrode and the BaTiO₃ film from cross-section observations by scanning electron microscopy. The bi-layer films show a dense and column microstructure with a grain size of 60 nm. Ferroelectric characterizations have been obtained for the BaTiO₃ films. The remnant polarization and coercive field are 2.1 μC/cm² and 45 kV/cm, respectively. The leak current measurements have shown a good insulating property.     

Oriented polycrystalline α-HgI2 thick films grown by physical vapor deposition

This work investigates the growth of polycrystalline α-HgI₂ thick films from physical vapor deposition. By varying the growth conditions, the as-deposited thick films are characterized by scanning electron microscopy, X-ray diffraction, current–voltage and photoconductivity measurements. The growth mechanism and its effects to the properties of these polycrystalline α-HgI₂ thick films are then discussed. Finally, the best deposition conditions for polycrystalline α-HgI₂ thick films compactly formed by separated columnar monocrystallines with uniform orientation along c-direction and with good crystallinity are reported.     

Zinc oxide quantum dots embedded films by metal organic chemical vapor deposition

Zinc oxide (ZnO) quantum dots (QDs) were fabricated on silicon substrates by metal organic chemical vapor deposition. Formation of QDs is due to the vigorous reaction of the precursors when a large amount of precursors was introduced during the growth. The size of the QDs ranged from 3 to 12 nm, which was estimated by high-resolution transmission electron microscopy. The photoluminescence measured at 80 K showed that the emission of QDs embedded film ranged from 3.0 to 3.6 eV. The broad near-band-edge emission was due to the quantum confinement effect of the QDs.     

A study of growth and morphological features of TiOxNy thin films prepared by MOCVD

The growth and morphological features of MOCVD TiO_xN_y films have been characterized to evaluate the effect of various process parameters on film growth. XRD analysis of the films deposited at 600°C on Si(1 1 1) and mica show a TiN(1 1 1) peak at 2θ=36.6°, but only anatase peaks are detected below 550°C. Above 650°C, both anatase and rutile peaks are detected. The presence of ammonia is not effective below 550°C as the deposited film is mostly TiO₂. Also, ammonia does not play any role in homogeneous nucleation in the gas phase, as evident by the deposition of anatase/rutile particles above 650°C. The following changes in the morphological features are observed by varying process parameters. By increasing the ratio of titanium-isopropoxide to ammonia flow, the cluster shape changes from angular to rounded; dilution of the flow results in larger elongated clusters; increase in flow rate at constant precursor to ammonia ratios, changes the cluster shape from rounded to elongated and the cluster size deceases. Deposition at higher temperatures results in finer clusters with a slower growth rate and eventually results in a very thin film with particle deposition at 650°C and above.     

Polar dependence of impurity incorporation and yellow luminescence in GaN films grown by metal-organic chemical vapor deposition

We have investigated the unintentional impurities, oxygen and carbon, in GaN films grown on c-plane, r-plane as well as m-plane sapphire by metal-organic chemical vapor deposition. The GaN layer was analyzed by secondary ion mass spectroscopy. The different trend of the incorporation of oxygen and carbon has been explained in the polar (0 0 0 1), nonpolar (1 1 2¯ 0) and semipolar (1 1 2¯ 2) GaN by a combination of the atom bonding structure and the origin direction of the impurities. Furthermore, it has been found that there is a stronger yellow luminescence (YL) in GaN with higher concentration of carbon, suggesting that C-involved defects are originally responsible for the YL.     

Fabrication and characteristics of CeO2 films on Si(1 0 0) substrates by pulsed laser deposition

A detailed investigation about the dependence of film orientation on deposition temperature and ambient oxygen pressure has been carried out for CeO₂ films on Si(1 0 0) substrates using pulsed laser deposition. It has been found that the CeO₂ film orientation varies with increasing oxygen pressure at 750°C deposition temperature. In addition, the recovery of preferential orientation of CeO₂ films grown at 20 Pa ambient oxygen pressure with increasing deposition temperature has also been found for the first time. X-ray photoelectron spectroscopy (XPS) measurements confirm that stoichiometric CeO₂ films can be grown at lower oxygen pressure (5×10⁻³ Pa). HRTEM result also indicates that the CeO₂ films grown at low oxygen pressure are of high crystallinity.