Morphology and photoluminescence properties of zinc oxide films grown by pulsed laser deposition

Zinc oxide films with different morphologies have been grown by pulsed laser deposition, varying substrate temperature and oxygen pressure. At low oxygen pressure and low substrate temperature continuous films with different roughness have been obtained, while at high substrate temperature a film with sparse hexagonal pyramids has been observed. Increasing the oxygen pressure the film became rougher and at 100 Pa a rod-array has been deposited. The columns of this rod-array grew along the wurtzite c-axis perpendicularly to the substrate surface as proved by X-ray diffraction measurements. Near to the sample borders the columns were slightly tilted towards the center of the sample. The possible growth mechanisms giving rise to the different morphologies have been discussed. Low-temperature photoluminescence measurements allowed to get information about the film quality, showing the variations of the excitonic peak and two defect bands (green and violet-blue) with the different deposition parameters.     

Study of structural and optical properties of ZnO films grown by pulsed laser deposition

Wurtzite zinc oxides films (ZnO) were deposited on silicon (0 0 1) and corning glass substrates using the pulsed laser deposition technique. The laser fluence, target-substrate distance, substrate temperature of 300 °C were fixed while varying oxygen pressures from 2 to 500 Pa were used. It is observed that the structural properties of ZnO films depend strongly on the oxygen pressure and the substrate nature. The film crystallinity improves with decreasing oxygen pressure. At high oxygen pressure, the films are randomly oriented, whereas, at low oxygen pressures they are well oriented along [0 0 1] axis for Si substrates and along [1 0 3] axis for glass substrates. A honeycomb structure is obtained at low oxygen pressures, whereas microcrystalline structures were obtained at high oxygen pressures. The effect of oxygen pressure on film transparency, band gap E_g and Urbach energies was investigated.     

Orientation selective growth of MgO films on Si (100) by pulsed laser deposition

Selective growth of single-oriented (110), (100) and (111) MgO films on Si (100) substrates without buffer layers was obtained via a two-step method by pulsed laser deposition. It was found that the orientation of the films was determined at the initial deposition stage by the substrate temperature only. The ambient pressure during deposition, the laser fluence and the etching of the Si substrates have no apparent effect on the orientation of the films, but affect their crystalline quality. Under the present deposition conditions, the surfaces of all three different single-oriented films were very smooth and devoid of any particulates. Received: 23 January 2001 / Accepted: 6 June 2001 / Published online: 2 October 2001     

Optical properties of SrTiO<Subscript>3</Subscript> thin films by pulsed laser deposition

SrTiO₃ thin films were prepared on a fused-quartz substrate by pulsed laser deposition (PLD). Dense and homogeneous films with a thickness of 260 nm were prepared. Optical constants (refractive index n and extinction coefficient k) were determined from the transmittance spectra using the envelope method. The optical band gap energy of the films was found to be 3.58 eV, higher than the 3.22 eV for bulk SrTiO₃, attributable to the film stress exerted by the substrate. The dispersion relation of the refractive index vs. wavelength follows the single electronic oscillator model. The refractive index and the packing density for the PLD-prepared SrTiO₃ thin films are higher than those for the SrTiO₃ films prepared by physical vapor deposition, sol–gel and RF sputtering. Received: 18 March 2002 / Accepted: 7 October 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: mszhang@nju.edu.cn     

The orientation-selective growth of LaNiO3 films on Si(100) by pulsed laser deposition using a MgO buffer

Highly (100)-oriented, (110)-oriented and polycrystalline LaNiO₃ (LNO) films were successfully prepared on Si(100) using an oriented MgO film as a buffer. It was somewhat surprising to find that that the orientation relation between the LNO film and the corresponding MgO buffer was: LNO(100)\MgO(110), LNO(110)\MgO(111) and LNO(polycrystalline)\MgO(100). The crystalline quality of the LNO films was shown to be sensitive to the preparation conditions of the MgO buffer. The film surface was very smooth, without micrometer-sized droplets being observed. All LNO films were of metallic conductivity, with a room-temperature resistivities of approximately 250, 280 and 420 μΩ cm for the (110)-oriented, (100)-oriented and polycrystalline LNO, respectively. Received: 2 April 2001 / Accepted: 23 October 2001 / Published online: 3 June 2002     

Particulate assisted growth of ZnO nanorods and microrods by pulsed laser deposition

Zinc oxide (ZnO) thin films were deposited on the gallium nitride (GaN) and sapphire (Al₂O₃) substrates by pulsed laser deposition (PLD) without using any metal catalyst. The experiment was carried out at three different laser wavelengths of Nd:YAG laser (λ = 1064 nm, λ = 532 nm) and KrF excimer laser (λ = 248 nm). The ZnO films grown at λ = 532 nm revealed the presence of ZnO nanorods and microrods. The diameter of the rods varies from 250 nm to 2 μm and the length varies between 9 and 22 μm. The scanning electron microscopy (SEM) images of the rods revealed the absence of frozen balls at the tip of the ZnO rods. The growth of ZnO rods has been explained by vapor-solid (V-S) mechanism. The origin of growth of ZnO rods has been attributed to the ejection of micrometric and sub-micrometric sized particulates from the ZnO target. The ZnO films grown at λ = 1064 nm and λ = 248 nm do not show the rod like morphology. X-ray photoelectron spectroscopy (XPS) has not shown the presence of any impurity except zinc and oxygen.     

Properties of ZnO thin films grown on Si substrates in vacuum and oxygen ambient by pulsed laser deposition

Epitaxial ZnO thin films have been synthesized directly on Si(1 1 1) substrates by pulsed laser deposition (PLD) in vacuum. The reflection high-energy electron diffraction (RHEED) indicates that streaky patterns can be clearly observed from the ZnO epilayers prepared at 600 and 650 °C, revealing a two-dimensional (2D) growth mode. While the ZnO thin film deposited in oxygen ambient shows ring RHEED pattern. There is a compressive in-plane stress existing in the ZnO epitaxial film, but a tensile one in the polycrystalline film. Compared with the ZnO epilayer, the ZnO polycrystalline film shows more intense ultraviolet emission (UVE) with a small full width at half maximum (FWHM) of 89 meV. It is suggested that the atomically flat epilayers may be powerfully used as transitive stratums to grow high-quality ZnO films suitable for the fabrication of optoelectronic devices.     

Growth of Ag thin films on ZnO(0 0 0 −1) investigated by AES and STM

The growth of Ag films on ZnO(0 0 0 −1) has been investigated by Auger electron spectroscopy (AES) and scanning tunneling microscopy (STM). A high density of islands is nucleated at the earliest stages of the growth. An upstepping mechanism causes these islands to coalesce while the uncovered fraction of the ZnO surface remains constant (30%).     

GaN epilayers on nanopatterned GaN/Si(1 1 1) templates: Structural and optical characterization

Template-based nanoscale epitaxy has been explored to realize high-quality GaN on Si(1 1 1) substrates. We have employed polystyrene-based nanosphere lithography to form the nano-hole array patterns on GaN/Si(1 1 1) template and then, subsequent regrowth of GaN is carried out by metalorganic chemical vapor deposition (MOCVD). During the initial growth stage of GaN on such nanopatterned substrates, we have observed formation of nanoislands with hexagonal pyramid shape due to selective area epitaxy. With further epitaxial regrowth, these nanoislands coalesce and form continuous GaN film. The overgrown GaN on patterned and non-patterned regions is characterized by high-resolution X-ray diffraction (HRXRD) and high-spatial resolution optical spectroscopic methods. Micro-photoluminescence (PL), micro-Raman scattering and scanning electron microscopy (SEM) have been used to assess the microstructural and optical properties of GaN. Combined PL and Raman data analyses show improved optical quality when compared to GaN simultaneously grown on non-patterned bulk Si(1 1 1). Such thicker GaN templates would be useful to achieve III-nitride-based opto- and electronic devices integrated on Si substrates.     

Magnetoresistance of magnetite thin films grown by pulsed laser deposition on GaAs(1 0 0) and Al2O3(0 0 0 1)

Magnetotransport properties of magnetite thin films deposited on gallium arsenide and sapphire substrates at growth temperatures between 473 and 673 K are presented. The films were grown by UV pulsed laser ablation in reactive atmospheres of O₂ and Ar, at working pressure of 8 × 10⁻² Pa. Film stoichiometry was determined in the range from Fe_2.95O₄ to Fe_2.97O₄. Randomly oriented polycrystalline thin films were grown on GaAs(1 0 0) while for the Al₂O₃(0 0 0 1) substrates the films developed a (1 1 1) preferred orientation. Interfacial Fe³⁺ diffusion was found for both substrates affecting the magnetic behaviour. The temperature dependence of the resistance and magnetoresistance of the films were measured for fields up to 6 T. Negative magnetoresistance values of ∼5% at room temperature and ∼10% at 90 K were obtained for the as-deposited magnetite films either on GaAs(1 0 0) or Al₂O₃(0 0 0 1).     

Growth of completely (110)-oriented Pt film on Si (100) by using MgO as a buffer by pulsed laser deposition

(110)-textured MgO films were grown on Si (100) with etching and without etching by pulsed laser deposition. The deposited MgO films were shown to be droplets-free. The MgO film was used as a buffer layer to further grow Pt film on Si (100). A completely (110)-oriented Pt film was obtained on such a buffer layer and its surface is very smooth with a roughness of about 7.5 nm over 5×5 μm. This can be used as a new oriented Pt electrode on silicon for devices. Received: 23 January 2001 / Accepted: 27 April 2001 / Published online: 27 June 2001     

Growth of a textured quasicrystalline phase in Ti-Ni-Zr films prepared by pulsed laser deposition

The preparation in thin film form of the known icosahedral phase in Ti-Ni-Zr bulk alloys has been investigated as a function of substrate temperature. Films were deposited by pulsed laser deposition on sapphire substrates at temperatures ranging from room temperature to 350 °C. Morphological and structural modifications have been followed by grazing-incidence and θ–2θ X-ray diffraction, transmission electron diffraction and imaging. Chemical composition has been analyzed by electron probe microanalysis. The in-depth variation of composition has been studied by secondary neutral mass spectroscopy. We show that pulsed laser deposition at 275 °C makes the formation of a 1-μm-thick film of Ti-Ni-Zr quasicrystalline textured nanocrystallites possible. Received: 7 June 2001 / Accepted: 18 February 2002 / Published online: 3 June 2002 RID="*" ID="*"Corresponding author. Fax: +33-3/8357-6300, E-mail: brien@mines.u-nancy.fr     

Highly oriented (1 0 0) ZnO thin films by spray pyrolysis

Undoped ZnO thin films have been deposited onto glass substrates by spray pyrolysis. The structural, electrical and optical properties were studied on thin films, prepared from precursor solutions with varying the ethanol concentrations. X-ray diffraction studies have shown polycrystalline nature of the films with a hexagonal wurtzite-type structure. The preferential orientation plane (1 0 0) of the ZnO thin film is found to be sensitive to ethanol concentration. The texture coefficient (TC) and grain size value have been calculated. Also ethanol concentration was found to have significant effect on sheet resistivity of the films.     

Thickness influence on surface morphology and ozone sensing properties of nanostructured ZnO transparent thin films grown by PLD

Transparent zinc oxide (ZnO) thin films with a thickness from 10 to 200 nm were prepared by the PLD technique onto silicon and Corning glass substrates at 350 °C, using an Excimer Laser XeCl (308 nm). Surface investigations carried out by atomic force microscopy (AFM) and X-ray diffraction (XRD) revealed a strong influence of thickness on film surface topography. Film roughness (RMS), grain shape and dimensions correlate with film thickness. For the 200 nm thick film, the RMS shows a maximum (13.9 nm) due to the presence of hexagonal shaped nanorods on the surface. XRD measurements proved that the films grown by PLD are c-axis textured. It was demonstrated that the gas sensing characteristics of ZnO films are strongly influenced and may be enhanced significantly by the control of film deposition parameters and surface characteristics, i.e. thickness and RMS, grain shape and dimension.     

The morphology and optical properties of Cr-doped ZnO films grown using the magnetron co-sputtering method

Undoped ZnO and Zn_0.9Cr_0.1O films were prepared on Al₂O₃ (0 0 0 1) substrates using the magnetron co-sputtering technique. X-ray diffraction scans show that all films exhibit nearly single-phase wurtzite structure with c-axis orientation. Both chromium doping and growth ambient have a significant impact on the lattice constants and nucleation processes in ZnO film. A large quantity of subgrains (10 nm in size) has been observed on Zn_0.9Cr_0.1O film grown under Ar + O₂, while irregular plateau-like grains 40-50 nm in size were observed on Zn_0.9Cr_0.1O film grown under Ar + N₂. The ultraviolet-visible transmittance and optical bandgap of all films were also examined. The photoluminescence spectra of all films exhibit a broad emission located around 400 nm, which is composed of one weak ultraviolet luminescence and another rather intense near-violet one, as determined by Gaussian peak fitting. The near-violet emission centered on 400 nm might originate from the electron transition between the band tail state levels of surface defects and/or lattice imperfection in the ZnO film.     

Epitaxial growth of Nd2Hf2O7(1 1 1) thin films on Ge(1 1 1) substrates by pulsed laser deposition

Nd₂Hf₂O₇ (NHO) thin films have been epitaxially grown by pulsed laser deposition (PLD) on Ge(1 1 1) substrates. In situ reflection high-energy electron diffraction (RHEED) evolution of the (1 1 1)-oriented NHO during the deposition has been investigated and shows that the epilayer has a twin-free character with type-B stacking. Interfacial structure of NHO/Ge has been examined by high-resolution transmission electron microscopy (HRTEM). The results indicate a highly crystalline film with a very thin interface, and the orientation relationship between NHO and Ge can be denoted as (1 1 1)_NHO//(1 1 1)_Ge and . Finally, twin-free epitaxial growth of NHO with type-B orientation displays temperature dependence and the type-B epitaxy is favored at high temperature.     

Electroluminescence from a ZnO homojunction device grown by pulsed laser deposition

A ZnO homojunction light emitting device was grown on n+ GaAs substrate by pulsed laser deposition. As-doped ZnO film by diffusion of As from the substrate was used for the p-type side and Al-doped ZnO film for the n-type side of the device. A distinct electroluminescence emission consisting of a dominant emission peak at ∼2.5 eV and a weak shoulder centered at ∼3.0 eV was observed at room temperature. The I-V characteristic of the ZnO homojunction showed a good rectifying behavior with a turn-on voltage of ∼4.5 V and a reverse breakdown voltage of ∼9 V.     

Elaboration of (1 1 1)-oriented La-doped PZT thin films on platinized silicon substrates

PLZT thin films with different thickness were deposited in situ on platinum coated silicon substrates using a multi-target sputtering system. The purpose was to grow (1 1 1)-textured PLZT films on Pt (1 1 1). To this aim, the role of some key parameters on both crystalline quality and electrical properties was investigated. An ultra-thin TiO₂ seeding layer was deposited, prior to PLZT, which strongly affected the crystallographic orientation of the films. The relation between temperature deposition and film crystallinity is analysed. TEM observations show the presence of some very small grains of Zr_0.9La_0.1O_1.95 at the film bottom interface. In the range of thickness investigated, the plot of the inverse capacitance as a function of the film thickness split up into two different curves, each with a linear shape, which however allows determination of a single value of interface capacitance. Above a thickness of 400-500 nm a saturation of the dielectric properties seems to be reached.     

Properties of ZnO thin films grown on Si substrates by photo-assisted MOCVD

ZnO thin films were grown on (1 0 0) p-Si substrates by Photo-assisted Metal Organic Chemical Vapor Deposition (PA-MOCVD) using diethylzinc (DEZn) and O₂ as source materials and tungsten-halogen lamp as a light source. The effects of tungsten-halogen lamp irradiation on the surface morphology, structural and optical properties of the deposited ZnO films have been investigated by means of atomic force microscope (AFM), X-ray diffraction and photoluminescence (PL) spectra measurements. Compared with the samples without irradiation, the several characteristics of ZnO films with irradiation are improved, including an improvement in the crystallinity of c-axis orientation, an increase in the grain size and an improvement in optical quality of ZnO films. These results indicated that light irradiation played an important role in the growth of ZnO films by PA-MOCVD.     

Epitaxially grown ZnO thin films on 6H-SiC(0 0 0 1) substrates prepared by spin coating-pyrolysis

Epitaxially grown ZnO thin film on 6H-SiC(0 0 0 1) substrate was prepared by using a spin coating-pyrolysis with a zinc naphthenate precursor. As-deposited film was pyrolyzed at 500 °C for 10 min in air and finally annealed at 800 °C for 30 min in air. In-plane alignment of the film was investigated by X-ray pole-figure analysis. Field emission-scanning electron microscope, scanning probe microscope, and He-Cd laser (325 nm) was used to analyze the surface morphology, the surface roughness and photoluminescence of the films. In the photoluminescence spectra, near-band-edge emission with a broad deep-level emission was observed. The position of the near-band-edge peak was around 3.27 eV.