Effects of cooling rate and post-heat treatment on properties of ZnO thin films deposited by sol-gel method

Sol-gel spin-coated ZnO thin films are cooled with different rates after the pre-heat treatment. Atomic force microscopy (AFM), X-ray diffraction (XRD), Raman, and photoluminescence (PL) were carried out to investigate the effects of the cooling rate during pre-heat treatment on structural and optical properties of the ZnO thin films. The ZnO thin films cooled slowly exhibit mountain chain structure while the ones cooled rapidly have smooth surface. The ZnO thin films cooled rapidly have higher c-axis orientation compared to the ones cooled slowly. The narrower and the higher near-band-edge emission (NBE) peaks are observed in the ZnO thin films cooled rapidly.     

Influence of annealing temperature on structural, optical and magnetic properties of Mn-doped ZnO thin films prepared by sol-gel method

Thin films of Zn_1−xMn_xO (x=0.01) diluted magnetic semiconductor were prepared on Si (1 0 0) substrates by the sol-gel method. The influence of annealing temperature on the structural, optical and magnetic properties was studied by X-ray diffraction (XRD), atom force microscopy (AFM), photoluminescence (PL) and SQUID magnetometer (MPMS, Quantum Design). The XRD spectrum shows that all the films are single crystalline with (0 0 2) preferential orientation along c-axis, indicating there are not any secondary phases. The atomic force microscopy images show the surfaces morphologies change greatly with an increase in annealing temperature. PL spectra reveal that the films marginally shift the near band-edge (NBE) position due to stress. The magnetic measurements of the films using SQUID clearly indicate the room temperature ferromagnetic behavior, and the Curie temperature of the samples is above room temperature. X-ray photoelectron spectroscopy (XPS) patterns suggest that Mn²⁺ ions were successfully incorporated into the lattice position of Zn²⁺ ions in ZnO host. It is also found that the post-annealing treatment can affect the ferromagnetic behavior of the films effectively.     

Optical properties of Al-doped ZnO thin films by ellipsometry

Al-doped ZnO thin films (AZO) were prepared on Si (1 0 0) substrates by using sub-molecule doping technique. The Al content was controlled by varying Al sputtering time. The as-prepared samples were annealed in vacuum chamber at 800 °C for 30 min. From the XRD observations, it is found that all films exhibit only the (0 0 2) peak, suggesting that they have c-axis preferred orientation. The average transmittance of the visible light is above 80%. Spectroscopic ellipsometry was used to extract the optical constants of the films. The absorption coefficient and the energy gap were then calculated. The results show that the absorption edge initially blue-shifts and then red-shifts with increase of Al content.     

The influence of growth parameters on the optical properties and morphology of UHV deposited Ni thin films

Nickel films of different thickness ranging from 15 nm to 350 nm were deposited on glass substrates, at different substrate temperatures (313-600 K) under UHV condition. The nano-structure of the films was obtained, using X-ray diffraction (XRD) and atomic force microscopy (AFM). The nano-strain in these films was obtained using the Warren-Averbach method. Their optical properties were measured by spectrophotometry in the spectral range of 190-2500 nm. Kramers-Kronig method was used for the analysis of the reflectivity curves. The absorption peaks of Ni thin films at ∼1.4 eV (transition between the bands near W and K symmetry points) and ∼5.0 eV (transition from L_2^′ to L₁ upper) are observed, with an additional bump at about 2 eV. The over-layer thickness was calculated to be less than 3.0 nm, using the Transfer Matrix method. The changes in optical data are related to different phenomena, such as different crystallographic orientations of the grains in these polycrystalline films (film texture), nano-strain, and film surface roughness.     

Effect of self assembly on the nonlinear optical characteristics of ZnO thin films

In the present work, we report the third order nonlinear optical properties of ZnO thin films deposited using self assembly, sol gel process as well as pulsed laser ablation by z scan technique. ZnO thin films clearly exhibit a negative nonlinear index of refraction at 532 nm and the observed nonlinear refraction is attributed to two photon absorption followed by free carrier absorption. Although the absolute nonlinear values for these films are comparable, there is a change in the sign of the absorptive nonlinearity of the films. The films developed by dip coating and pulsed laser ablation exhibit reverse saturable absorption whereas the self assembled film exhibits saturable absorption. These different nonlinear characteristics in the self assembled films can be mainly attributed to the saturation of linear absorption of the ZnO defect states.     

Structure and optical properties of ZnS thin films grown by glancing angle deposition

The glancing angle deposition (GLAD) technique was used to deposit ZnS films by electron beam evaporation method. The cross sectional scanning electron microscopy (SEM) image illustrated a highly orientated microstructure composed of slanted column. The atomic force microscopy (AFM) analysis indicated that incident flux angle had significant effects on the nodule size and surface roughness. Under identical nominal thickness, the actual thickness of the GLAD films is related to the incident flux angle. The refractive index and in-plane birefringence of the GLAD ZnS films were discussed, and the maximum birefringence Δn = 0.036 was obtained at incident flux angle of α = 80°. Therefore, the glancing angle deposition technique is a promising way to create a columnar structure with enhanced birefringent property.     

The structure and optical properties of evaporated Samarium fluoride films

Samarium fluoride (SmF₃) films have been deposited on quartz, silicon and germanium substrates by vacuum evaporation method. The crystal structure of the films deposited on silicon substrate is examined by X-ray diffraction (XRD). The films deposited at 100 °C, 150 °C and 250 °C have the (1 1 1) preferred growth orientation, but the film deposited at 200 °C has (3 6 0) growth orientation. The surface morphology evolution of the films with different thickness is investigated with optical microscopy. It is shown that the microcrack density and orientation of thin film is different from that of thick film. The transmission spectrum of SmF₃ films is measured from 200 nm to 20 μm. It is found that this material has good transparency from deep violet to far infrared. The optical constants of SmF₃ films from 200 nm to 12 μm are calculated by fitting the transmission spectrum of the films using Lorentz oscillator model.     

XRD and XPS analysis of laser treated vanadium oxide thin films

In this work, we present X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) analysis of laser treated vanadium oxide sols. The films were also observed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM) to reveal how the original xerogel structure changes into irregular shaped, layer structured V₂O₅ due to the laser radiation. XRD revealed that above 102 W/cm² the original xerogel structure disappears and above 129 W/cm² the films become totally polycrystalline with an orthorhombic structure. XPS spectra showed O/V ratio increment by using higher laser intensities.     

Dependence of film thickness on the structural and optical properties of ZnO thin films

ZnO thin films are prepared on glass substrates by pulsed filtered cathodic vacuum arc deposition (PFCVAD) at room temperature. Optical parameters such as optical transmittance, reflectance, band tail, dielectric coefficient, refractive index, energy band gap have been studied, discussed and correlated to the changes with film thickness. Kramers-Kronig and dispersion relations were employed to determine the complex refractive index and dielectric constants using reflection data in the ultraviolet-visible-near infrared regions. Films with optical transmittance above 90% in the visible range were prepared at pressure of 6.5 × 10⁻⁴ Torr. XRD analysis revealed that all films had a strong ZnO (0 0 2) peak, indicating c-axis orientation. The crystal grain size increased from 14.97 nm to 22.53 nm as the film thickness increased from 139 nm to 427 nm, however no significant change was observed in interplanar distance and crystal lattice constant. Optical energy gap decreased from 3.21 eV to 3.19 eV with increasing the thickness. The transmission in UV region decreased with the increase of film thickness. The refractive index, Urbach tail and real part of complex dielectric constant decreased as the film thickness increased. Oscillator energy of as-deposited films increased from 3.49 eV to 4.78 eV as the thickness increased.     

Study on crystalline properties of Er-Si-O compounds in relation to Er-related 1.54 μm photoluminescence and electrical properties

Er-Si-O crystalline compounds, which exhibit superlattice structures and sharp and strong Er-related 1.54 μm photoluminescence (PL) spectra at room temperature have been formed by self-assembling growth mechanism. Oxidation of the starting materials which have Si and Er at an atomic ratio of 2:1 are prepared and then oxidation and succeeding high-temperature annealing in Ar above 1250 °C cause a self-assembled superlattice-structured Er-Si-O crystalline compounds. The control of the ratio of Si and Er, as well as the following oxidation and annealing processes, is found to be sensitive to the crystalline properties, PL spectra and electrical properties. In this study, Er-Si-O crystalline thin films are formed on Si substrates by sol-gel and MOMBE methods, and their crystalline properties such as crystalline orientation and concentration ratio of Er, Si and O are investigated. Crystalline Er-Si-O films of high orientation are successfully grown on Si(1 0 0) and its inclined surface. The PL and excitation spectra, fluorescence decay and the electrical properties are found to be strongly related to the crystalline properties. Excess O causes a broader 1.54 μm PL spectra, slower fluorescence decay, lower carrier-mediated excitation and higher resistivity. A precise control of O is found to be necessary to grow superlattice-structured Er-Si-O compounds, which are semiconducting and are excitable via carrier-mediated excitation mechanism.     

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.     

Optical properties of nickel(II)-azo complexes thin films for potential application as high-density recordable optical recording media

Smooth thin films of three kinds of nickel(II)-azo complexes were prepared by the spin-coating method. Absorption spectra of the thin films on K9 glass substrate in the 300-600 nm wavelength region were measured. Optical constants (complex refractive index ) and thickness of the thin films prepared on single-crystal silicon substrate in the 300-600 nm wavelength region were investigated on rotating analyzer-polarizer type of scanning ellipsometer, and dielectric constants , absorption coefficients α as well as reflectance R of thin films were then calculated at 405 nm. In addition, in order to examine the possible use of nickel(II)-azo complex thin film as an optical recording medium, one of the nickel(II)-azo complex thin film prepared on K9 glass substrate with an Ag reflective layer was also studied by atomic force microscopy and static optical recording. The results show that the nickel(II)-azo complex thin film is smooth and has a root mean square surface roughness of 2.25 nm, and the recording marks on the nickel(II)-azo complex thin film are very clear and circular, and their size can reach 200 nm or less.     

Effect of indium incorporation on the optical properties of spray pyrolyzed Cd0.22Zn0.78S thin films

In this study, effect of indium incorporation on the optical properties is investigated for the spray pyrolyzed onto glass substrates at 275°C substrate temperature undoped and indium doped Cd_0.22Zn_0.78S thin films. The average optical transmittance of all the films was over 77% in the wavelength range between 450 and 800 nm. The optical band gap energies of the thin films have been investigated by the measurement of the optical absorbance as a function of wavelength. The optical absorption studies reveal that the transitions are direct band gaps of 3.02 and 3.05 eV for undoped and doped indium Cd_0.22Zn_0.78S thin films, respectively. The Urbach tail parameter and optical constants such as refractive index, extinction coefficient, and dielectric constants were calculated for these films. The dispersion parameters such as single-oscillator energy and dispersive energy were discussed in terms oft he single-oscillator Wemple—DiDomenico model.     

Structural and electrical properties of tantalum nitride thin films fabricated by using reactive radio-frequency magnetron sputtering

TaN thin film is an attractive interlayer as well as a diffusion barrier layer in [FeN/TaN]_n multilayers for the application as potential write-head materials in high-density magnetic recording. We synthesized two series of TaN films on glass and Si substrates by using reactive radio-frequency sputtering under 5-mtorr Ar/N₂ processing pressure with varied N₂ partial pressure, and carried out systematic characterization analyses of the films. We observed clear changes of phases in the films from metallic bcc Ta to a mixture of bcc Ta(N) and hexagonal Ta₂N, then sequentially to fcc TaN and a mixture of TaN with N-rich phases when the N₂ partial pressure increased from 0.0% to 30%. The changes were associated with changes in the grain shapes as well as in the preferred crystalline orientation of the films from bcc Ta [100] to [110], then to random and finally to fcc TaN [111], correspondingly. They were also associated with a change in film resistivity from metallic to semiconductor-like behavior in the range of 77–295 K. The films showed a typical polycrystalline textured structure with small, crystallized domains and irregular grain shapes. Clear preferred (111) stacks parallel to the substrate surface with embedded amorphous regions were observed in the film. TaN film with [111]-preferred orientation and a resistivity of 6.0 mΩ cm was obtained at 25% N₂ partial pressure, which may be suitable for the interlayer in [FeN/TaN]_n multilayers. Received: 6 December 1999 / Accepted: 24 July 2000 / Published online: 9 November 2000     

Oxidation temperature dependent optical properties of bismuth oxide thin films: Effect of vapour chopping and air exposure

The optical properties of vapour chopped and nonchopped bismuth oxide thin films of two thicknesses 1500 Å and 2000 Å have been studied. The films were prepared by thermal oxidation in air; of vacuum evaporated vapour chopped and nonchopped bismuth thin films. As revealed by XRD studies, multiphase and polycrystalline bismuth oxide thin films were obtained. The refractive index was found to increase with the thickness and exposure to air for 40 days. The vapour chopped films showed higher refractive index, band gap and lower grain size than those of nonchopped films. The films showed high transmittance in the visible spectrum. The ageing effect on the vapour chopped films was found low.     

Thio sol-gel synthesis of titanium disulfide thin films and nanoparticles using titanium(IV) alkoxide precursors

Titanium tetraisopropoxide reacts with hydrogen sulfide in butylamine solvent at room temperature to form an amorphous titanium alkoxy-sulfide which can be converted to crystalline titanium disulfide by heat treatment in a flowing hydrogen sulfide gas stream. The reaction has been studied using infrared and Raman spectroscopy, gas chromatography-mass spectrometry, X-ray diffractometry and energy-dispersive X-ray analysis measurements. Based on these studies, it is shown that a partially thiolysed alkoxide precursor forms through the replacement of a limited number of alkoxy groups by hydrosulfide moieties. This alkoxy-hydrosulfide is believed to form following a thiolysis-condensation mechanism similar to the hydrolysis-condensation process that occurs during the corresponding oxide sol-gel reaction. The alkoxy-hydrosulfide species can then be completely thiolysed at 800 °C in a stream of hydrogen sulfide to yield pure, hexagonal titanium disulfide in either film or particulate form.     

Influence of the plasma parameters on the properties of aluminum oxide thin films deposited by laser ablation

The plasma produced by the ablation of a high purity Al₂O₃ target, using the fundamental line (1064 nm) of a Nd:YAG laser, was characterized. The laser fluence was varied in order to study its effect on the characteristics of the produced plasma as well as on the properties of the material deposited. Optical emission spectroscopy (OES) was used to determine the type of excited species present in the plasma. The mean kinetic energy of the ions and the maximum plasma density were determined from the time of flight (TOF) curves, obtained with a planar Langmuir probe. The obtained results reveal that the fast peak in the probe curve could be attributed to Al III, while the slow peak corresponds to the Al II. Aluminum oxide thin films were then deposited under the same conditions of the diagnosed plasma, in an attempt to correlate the plasma parameters with the properties of the deposited material. It was found that when Al II ion energies are lower than 461.0 eV the films deposited have structural characteristics similar to that of α-Al₂O₃, whereas at ion energies greater than 461.0 eV amorphous material was obtained.     

Structural and optical properties of thermal evaporated magnesium phthalocyanine (MgPc) thin films

X-ray powder diffraction (XRD) of MgPc indicated that the material in the powder form is polycrystalline with monoclinic structure. Miller indices, h k l, values for each diffraction peak in XRD spectrum were calculated. Thermal evaporation technique was used to deposit MgPc thin films. The XRD studies were carried out for MgPc thin films where the results confirm the amorphous nature for the as-deposited films. While, polycrystalline films orientated preferentially to (1 0 0) plane with an amorphous background were obtained for films annealed at 623 K for 3 h. Optical properties of MgPc thin films were characterised by using spectrophotometric measurements of transmittance and reflectance in the spectral range from 190 to 2500 nm. The refractive index, n, and the absorption index, k, were calculated. According to the analysis of dispersion curves, the parameters, namely; the optical absorption coefficient (α), molar extinction coefficient (?_molar), oscillator energy (E_os), oscillator strength (f), and electric dipole strength (q²) were also evaluated. The recorded absorption measurements in the UV-vis region show two well defined absorption bands of phthalocyanine molecule; namely the Q-band and the Soret (B-band). The Q-band showed its splitting characteristic (Davydov splitting), and ΔQ was obtained as 0.15 eV. The analysis of the spectral behaviour of the absorption coefficient (α), in the absorption region revealed indirect transitions. The transport and the near onset energy gaps were estimated as respectively 2.74 ± 0.02 and 1.34 ± 0.01 eV.     

Effect of deposition parameters on structural, optical and electrical properties of nanocrystalline ZnSe thin films

This paper presents the chemical bath deposition of zinc selenide (n-ZnSe) nanocrystalline thin films on non-conducting glass substrates, in an aqueous alkaline medium using sodium selenosulphate as Se²⁻ ion source. The X-ray diffraction studies show that the deposited ZnSe material is nanocrystalline with a mixture of hexagonal and cubic phase. The direct optical band gap ‘E_g’ for the as-deposited n-ZnSe films is found to be 3.5 eV. TEM studies show that the ZnSe nanocrystals (NCs) are spherical in shape. Formation of ZnSe has been confirmed with the help of infrared (IR) spectroscopy by observing bands corresponding to the multiphonon absorption. We demonstrate the effect of the deposition temperature and reactant concentration on the structural, optical and electrical properties of ZnSe films.     

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