Growth Of ZnO Thin Films Research Articles

Structural Analysis of ZnO Thin Films Grown in Room Temperature on PET Film

The structure of ZnO thin films grown in room temperature by reactive DC sputter technique on polyethylene terephthalate film were evaluated by SEM and TEM. The quality of ZnO thin films grown in room temperature were observed to vary widely. ZnO crystals grow without uniform orientation in early stage of growth, and then ZnO crystallinity improves as the ZnO thin films grow up. And ZnO crystallinity is influenced by roughened surface of PET film.

Development of human IgE biosensor using Sezawa-mode SAW devices

This paper reports Sezawa-mode surface acoustic wave (SAW) devices with via-isolated cavity to construct the allergy biosensor. To fabricate Sezawa-mode SAW devices, the RF magnetron sputtering method for the growth of piezoelectric ZnO thin films are adopted and influences of the sputtering parameters are investigated. The optimal substrate temperature of 300 °C, RF power of 120 W and sputtering pressure of 2 Pa were used to deposit piezoelectric ZnO films with a smooth surface, uniform grain size and strongly c-axis-orientated crystallization. A back-etched SAW resonator is used in this study. The wet etching of (100)-oriented silicon wafers is used to form a back-side cavity which is critical to the formation of a hopper cavity for holding bio-analytes. The remaining membrane structure silicon thickness was 25 μm. In this report, the chrome (Cr, 12 nm)/gold (Au, 66 nm) layer was initially deposited onto the sensing area of SAW devices as the binding layer for biochemical sensor. The resonance frequency of the Sezawa-mode SAW device is 1.497 GHz. The maximum sensitivity of the Sezawa-mode is calculated to be 4.44 × 106 cm2/g for human immunoglobulin-E (IgE) detection. The stability for human IgE detection is calculated to be 80% and the variation of the stability ±3% was obtained after several tests.

Double-beam pulsed laser deposition for the growth of Al-incorporated ZnO thin films

Pulsed laser deposition in a delayed-double beam configuration is used to incorporate in situ Al in ZnO thin films. In this configuration, two synchronized pulsed-laser beams are employed to ablate independently a ZnO and an Al target. We investigated the effects of relative time delay of plasma plumes on the composition of the films with the aim of evaluating the performance of this technique to produce doped materials. Relative delay between plumes was found to control the incorporation of Al in the film in the range from 14% to 30%. However, to produce low impurity concentration of Al-doped ZnO (with Al incorporation less than 2%) the fluence used to produce the plasmas has more influence over the film composition than the relative plume delay. The minimum incorporation of Al corresponded to a relative delay of 0μs, due to the interaction between plumes during their expansion.

Highly c-Axis Oriented ZnO Thin Films Grown by Sol–Gel Method for SAW Sensor Application

Highly <i>c</i>-Axis Oriented ZnO Thin Films Grown by Sol–Gel Method for SAW Sensor Application

Laser molecular beam epitaxy of ZnO thin films and heterostructures

We report on the growth of epitaxial ZnO thin films and ZnO-based heterostructures on sapphire substrates by laser molecular beam epitaxy (MBE). We first discuss some recent developments in laser-MBE such as flexible ultraviolet laser beam optics, infrared laser heating systems or the use of atomic oxygen and nitrogen sources, and describe the technical realization of our advanced laser-MBE system. Then we describe the optimization of the deposition parameters for ZnO films such as laser fluence and substrate temperature and the use of buffer layers. The detailed structural characterization by x-ray analysis and transmission electron microscopy shows that epitaxial ZnO thin films with high structural quality can be achieved, as demonstrated by a small out-of-plane and in-plane mosaic spread as well as the absence of rotational domains. We also demonstrate the heteroepitaxial growth of ZnO-based multilayers as a prerequisite for spin transport experiments and the realization of spintronic devices. As an example, we show that TiN/Co/ZnO/Ni/Au multilayer stacks can be grown on (0 0 0 1)-oriented sapphire with good structural quality of all layers and well defined in-plane epitaxial relations.

Growth and optical characterization of diamond-shaped zinc oxide nanostructures deposited by pulsed laser ablation

We report on the growth of zinc oxide diamond-shaped nanostructured thin films by pulsed laser deposition technique on silicon substrate at different substrate temperatures (room temperature to 600 \(^{\circ }\)C) and at fixed background pressure of oxygen using Nd:YAG laser at a wavelength of 532 nm. The influence of substrate temperature on the grain size, surface morphology and optical properties is characterized by x-ray diffraction, field-emission scanning electron microscope, photoluminescence and Raman spectroscopy. x-ray diffraction results show that the grain size increases with increasing substrate temperature during the growth of ZnO thin films due to improved crystallinity but at 450 \(^{\circ }\)C the crystallinity degrades. It is attributed to the formation of diamond-shaped ZnO nanostructures as supported by the field emission scanning electron microscope images. Consequently, increase in photoluminescence and Raman intensities is also attributed to the formation of diamond-shaped structures. The growth of diamond-shaped structure is discussed in the light of growth of various planes in the hexagonal structure of ZnO.

Surface excitons on a ZnO (000-1) thin film

Elementary excitations at the polar (000-1) surface of a 20 nm pseudomorphically grown ZnO thin film are examined by steady state and time-resolved photoluminescence spectroscopy at low temperature. We control the density of emission centers through the deposition of prototypical organic molecules with a carboxylic acid anchor group by the Langmuir-Blodgett technique. Knowledge of the precise film thickness, defect concentrations and number density of deposited molecules leads us to associate the surface exciton emission to defect-related localization centers that are generated through a photochemical process.

Thickness dependence of optoelectronic properties in ALD grown ZnO thin films

ZnO thin films with high conductivity and high transparency were grown on Si (100) substrates by atomic layer deposition. Thickness dependent (43–225nm) changes in crystallographic, optical and electrical properties are reported and discussed. Increase in film thickness caused a decrease in the bandgap by relaxation of stress in the plane of the film and led to an improvement in crystallinity and conductivity. The optical studies showed a noticeable change towards the contribution of excitonic and phonon replica to the UV-emission band.

Electrochemically deposited ZnO films: an XPS study on the evolution of their surface hydroxide and defect composition upon thermal annealing

Electrodeposition from ZnCl2 aqueous solution was performed to grow ZnO thin films on the surface of polycrystalline copper plates. Electrochemical parameters for deposition were optimized by means of cyclic voltammetry (CV). The morphology of the deposits was studied via scanning electron microscopy (SEM), and their chemical composition was ascertained by means of X-ray photoelectron spectroscopy (XPS). The effects of changing the deposition bath temperature (T bath) and the role played by post-deposition treatments, such as temperature and time of annealing in air, were studied. SEM images of freshly deposited vs. annealed samples have shown that in the former case the films display a rough morphology with mixed grain/hexagonal platelets structures and in the latter smaller but more uniformly dispersed cubic grains. T bath is found to be the key parameter to induce the different morphology in the deposited films, which reflects in a different chemical reactivity of surface species, as found on the basis of the binding energies and relative quantitative ratios between Zn 2p and O 1s peaks. In fact, a higher T bath favours a more efficient desorption of OH groups upon annealing, the O 1s peak resulting to much more drastically modified oxide/hydroxide intensity ratio with respect to the case of the sample deposited at lower T bath.

Comparative Study on Structural and Optical Properties of Nanostructured ZnO Thin Films Grown by Thermal Evaporation and Sol–Gel Techniques

Comparative Study on Structural and Optical Properties of Nanostructured ZnO Thin Films Grown by Thermal Evaporation and Sol–Gel Techniques

Transparent and conducting intrinsic ZnO thin films prepared at high growth-rate with c-axis orientation and pyramidal surface texture

The growth of ZnO thin films has been optimized by adjusting the intrinsic ion vacancies, by controlling the RF power applied to the plasma in magnetron sputtering. Preferred c-axis oriented intrinsic ZnO films with largest grain size and a hexagonal wurtzite structure, exhibiting high room temperature conductivity, σ∼1.37S/cm, high transparency, ∼80–90% within 450–800nm and ∼90–96% within 800–1900nm, low reflectance (<5% in the visible range) was obtained at a very high deposition rate ∼214nm/min, at 300°C, by maintaining higher concentration of Zn interstitials or singly ionized oxygen vacancy, corresponding to an optimized RF power of 200W. Films have lowest internal stress, smallest dissipation factor defined as ɛ2/ɛ1, and the specific pyramidal surface texture creates enough surface roughness that helps to improve the light scattering from the surface and makes it suitable for efficient use in thin-film silicon solar cells. With increasing RF power beyond 200W, the Zn–O bond length reduces promptly and the internal stress increases monotonically approaching toward a virtual saturation. The preferred crystallographic alignment shifts from (002) to (103), i.e., from c to a-axis orientation, as the surface energy of ZnO crystal changes due to the increase in the Zn-to-ZnO ion ratio in the plasma caused by the plausible de-oxygenation of ZnO at elevated RF powers. Oxygen deficient ZnO films having the flower-like surface texture prepared with a very high deposition rate ∼554nm/min at 500W could indeed make the material suitable for gas and chemical sensing applications.

The dependence of structural and optical properties of PLD grown ZnO films on ablation parameters

Highly c-axis oriented ZnO thin films have been grown by pulsed laser deposition (PLD) technique on quartz, silicon (100) and Al2O3 〈0001〉 substrates using KrF excimer laser (λab= 248nm) and Q-switched fourth harmonic Nd:YAG laser (λab= 266nm). The crystalline nature, surface morphology and optical properties of the deposited films depend on the oxygen ambience, substrate nature and deposition temperature. The band gap of ZnO thin films varies with increase of substrate temperature despite of the ablation wavelength. Strong UV-PL emission without any deep level emissions confirms the growth of stoichiometric and crystalline ZnO thin films. Raman scattering studies confirms the growth of c-axis oriented ZnO thin films at different substrate temperature.

Improved damp heat stability of Ga-Doped ZnO thin film by pretreatment of the polyethylene terephthalate substrate

A study on the damp heat stability of transparent conducting ZnO thin film grown on a polyethylene terephthalate substrate (PET) is reported. By thermal annealing of the PET substrate at 100°C with Ar flow in a vacuum chamber prior to the sputtering growth of Ga-doped ZnO (GZO) thin film, significantly enhanced damp heat stability was achieved at 60°C with a 90% relative humidity. Electrical and structural characterizations of the GZO thin films were carried out and the effects of the pretreatment on the improved damp heat stability are discussed.

Optical properties and electrical resistivity of boron-doped ZnO thin films grown by sol–gel dip-coating method

Sol–gel dip-coating was used to grow ZnO thin films doped with various concentrations of B ranging from 0 to 2.5at.% on quartz substrates. The effects of B doping on the absorption coefficient (α), optical band gap (Eg), Urbach energy (EU), refractive index (n), refractive index at infinite wavelength (n∞), extinction coefficient (k), single-oscillator energy (Eo), dispersion energy (Ed), average oscillator strength (So), average oscillator wavelength (λo), moments M−1 and M−3, dielectric constant (ε), optical conductivity (σ), and electrical resistivity (ρ) of the BZO thin films were investigated. The transmittance spectra of the ZnO and BZO thin films show that the transmittance of the BZO thin films was significantly higher than that of the ZnO thin films in the visible region of the spectrum and that the absorption edge of the BZO thin films was blue-shifted. The BZO thin films exhibited higher Eg, EU, and Eo and lower Ed, λo, M−1 and M−3 moments, So, n∞, and ρ than the ZnO thin films.

Growth of high quality ZnO thin films with a homonucleation on sapphire

ZnO thin films were epitaxially grown on c-plane sapphire substrates by plasma-assisted molecular beam epitaxy. A low temperature homonucleation ZnO layer was found crucial at the interfacial region to absorb the defects formed by the lattice mismatch between the sapphire and ZnO, resulting in a smooth surface that enables smooth 2D epitaxial growth. High quality ZnO films were achieved after careful optimization of critical growth conditions: the sequence of Zn and O source shutters, growth temperature for both the ZnO nucleation and growth layer, and Zn/O ratio. Oxygen plasma pretreatment was not applied prior to the growth, thus shortening the growth time and reducing oxidation of the metallic sources. Resultant epitaxial ZnO films on sapphire demonstrated a root-mean-square surface roughness of 0.373 nm for 1 μm × 1 μm atomic force microscope images with clear hexagonal structure and terrace steps. The x-ray diffraction full width at half maximum (FWHM) for ω and ω-2θ ZnO (0002) triple-crystal rocking curves were measured to be 13 and 26 arc/s, respectively. This FWHM value is lower than any reported to date in the literature, with ω and ω-2θ values indicating excellent coherence of the epitaxial layer along the interface and the growth direction, accordingly. These x-ray diffraction and surface roughness values are lower than those obtained using common nucleation layers such as MgO, indicating that growth with ZnO nucleation layers on sapphire may lead to higher quality electrical and optical devices.

Optical Characterization of ZnO Thin Films Grown by Thermal Oxidation of Metallic Zinc

-type Si (100) substrates by thermal oxidation of pre-deposited Zn films bythermal evaporation deposition in vacuum. The metallic Zn films were thermally oxidized at different tempera-tures ranging from 300 C to 500 C. The structural as well as optical characterizations of the thin films werecarried out by using XRD, AFM, Raman spectroscopy, UV measurements and photoluminescence (PL) respec-tively. The Raman spectra showed that only

Oxygen Tracer Diffusion in A-Axis Oriented ZnO Thin Films Grown on (01-12) Sapphire by Pulsed Laser Deposition

The a-axis oriented ZnO thin films deposited on sapphire substrates by pulsed laser deposition were studied to investigate the effects of pre-annealing on oxygen diffusion. The effect was as follows: the oxygen diffusion coefficient decreased, and the oxygen concentration in the tailing regions of the profiles reduced. Ion images of an oxygen tracer revealed the high-diffusivity paths for oxygen tracer diffusion. The temperature dependence of oxygen tracer diffusion coefficients (Db) in as-deposited and pre-annealed thin films were determined to be Db [cm2/ = 9.2x102 exp (- 405 [kJ/mo / RT) and Db [cm2/ = 1.8x103 exp (- 418 [kJ/mo / RT), respectively. On basis of these results, the crystal orientation on Db and the mechanism for oxygen diffusion were discussed.

Magnetic and Electrical Properties of Undoped and Holmium Doped ZnO Thin Films Grown by Sol-Gel Method

Undoped and holmium-doped ZnO thin films were obtained, using the sol-gel method. The films were characterized by scanning electron microscopy (SEM), Hall effect measurements, electron paramagnetic resonance (EPR) spectroscopy and superconducting quantum interference device vibrating sample magnetometry (SQUID-VSM). The Hall effect measurements have indicated a n-type conduction with a resistivity of about 3.2 Ω.cm for the undoped ZnO and of about 4.5 Ω.cm for the 5 at. % Ho-doped ZnO thin films. The EPR measurements have indicated the presence of interstitial zinc in the undoped ZnO and the presence of zinc vacancies in 5 at. % Ho-doped ZnO. Ho (5 at. %)-doped ZnO films exhibit superparamagnetism at 5 K, while a low paramagnetic behavior was observed at room temperature.

Morphology and Luminescence of ZnO Films Grown on a Au(111) Support

Using scanning tunneling microscopy and spectroscopy, we have analyzed the growth of ZnO thin films on a Au(111) support. Because of the 12% lattice mismatch with the metal beneath, ZnO develops a (0001)-oriented coincidence lattice that gives rise to a well-ordered hexagonal Moire pattern with 2.2 nm periodicity. This superstructure disappears at 4 ML film thickness, when wide, atomically flat terraces delimited by straight monatomic steps become detectable in the STM. The long-range order of the films is deduced from sharp hexagonal spot patterns in low-energy electron diffraction. STM-based luminescence and conductance spectroscopy reveals that the ZnO band gap approaches the bulk value in films thicker than 10 ML. Additional photon peaks with sub-band-gap energies indicate the presence of defects in the wurzite lattice. The intrinsic polarity of the ZnO(0001) surface is accounted for by a reduced Zn–O interlayer distance and an adsorption-mediated compensation scheme in thin and thick films, respectively.

Local p-type conduction of Na-doped ZnO thin films grown by MOCVD

Growth of Na-doped ZnO thin films by low-pressure metal–organic chemical vapor deposition method was realized using cyclopentadienylsodium as the sodium source. Secondary ion mass spectrometry measurements revealed the existence of Na in the ZnO thin films. Low-temperature scanning tunneling microscopy and scanning tunneling spectroscopy were used for obtaining local density of states (LDOS) of the Na-doped ZnO thin films at nanometer scale. The local measurements indicated a spatial distribution of the LDOS as well as the conductivity type that could be linked to spatial variations of Na doping level.