Annealing effect on the microstructure and photoluminescence of ZnO thin films

Zinc oxide thin films have been obtained by pulsed laser ablation of a ZnO target in O₂ ambient at a pressure of 0.13 Pa using a pulsed Nd:YAG laser. ZnO thin films deposited on Si (1 1 1) substrates were treated at annealing temperatures from 400 °C up to 800 °C after deposition. The structural and optical properties of deposited thin films have been characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, photoluminescence spectra, resistivity and IR absorption spectra. The results show that the obtained thin films possess good single crystalline with hexagonal structure at annealing temperature 600 °C. Two emission peaks have been observed in photoluminescence spectra. As the post-annealing temperature increase, the UV emission peaks at 368 nm is improved and the intensity of blue emission at 462 nm decreases, which corresponds to the increasing of the optical quality of ZnO film and the decreasing of Zn interstitial defect, respectively. The best optical quality for ZnO thin films emerge at post-annealing temperature 600 °C in our experiment. The measurement of resistivity also proves the decrease of defects of ZnO films. The IR absorption spectra of sample show the typical Zn–O bond bending vibration absorption at wavenumber 418 cm⁻¹.     

Epitaxial growth of non-polar m-plane ZnO thin films by pulsed laser deposition

Non-polar ZnO thin films were deposited on m-plane sapphire substrates by pulsed laser deposition at various temperatures from 300 to 700 °C. The effects of growth temperature on surface morphology, structural, electrical, and optical properties of the films were investigated. All the films exhibited unique m-plane orientation indicated by X-ray diffraction and transmission electron microscopy. Based on the scanning electron microscopy and atomic force microscopy, the obtained films had smooth and highly anisotropic surface, and the root mean square roughness was less than 10 nm above 500 °C. The maximum electron mobility was ～18 cm²/V s, with resistivity of ～0.26 Ω cm for the film grown at 700 °C. Room temperature photoluminescence of the m-plane films was also investigated.     

Titanium oxide thin films synthesis by pulsed – DC magnetron sputtering

Titanium oxide thin films (1–4 μm) were deposited on the porous Hastelloy-X substrates using the pulsed – DC magnetron sputtering technique and characterized by X–ray diffraction (XRD) and scanning electron microscopy (SEM) methods. Firstly, the films were deposited at different distances between the magnetron and the substrate, as magnetron current and pressure in the deposition chamber were constant. The distance between the magnetron and the substrate was changed from 3 cm to 7 cm, and the deposition rate varied between 10.1 nm/min to 6.0 nm/min. Secondly, pressure influence for the deposition rate was investigated. The deposition rate decreased nearly 15% with the decrease of oxygen pressure from 1.3 to 6.0 Pa. Finally, the influence of the bias (applied to the substrate for the increase of deposition rate) on thin films phase and microstructure was investigated.The experimental results showed that formation of pure titanium oxide thin films was observed in all experimental cases. Only crystallite sizes and orientation were changed. The results showed that there is a possibility to change porosity and uniformity of the growing film by changing oxygen partial pressure during deposition or bias application to the substrate. The existence of columnar boundaries and nanocrystalline structure in the films was observed.     

Fabrication and characterization of tetrapod-like ZnO nanostructures prepared by catalyst-free thermal evaporation

Tetrapod-like ZnO nanostructures were fabricated on ZnO-coated sapphire (001) substrates by two steps: pulsed laser deposition (PLD) and catalyst-free thermal evaporation process. First, the ZnO films were pre-deposited on sapphire (001) substrates by PLD. Then the ZnO nanostructures grew on ZnO-coated sapphire (001) substrate by the simple thermal evaporation of the metallic zinc powder at 900 °C in the air without any catalysts. The pre-deposited ZnO films by PLD on the substrates can provide growing sites for the ZnO nanostructures. The as-synthesized ZnO nanostructures were characterized by using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and Fourier transform infrared spectrum (FTIR). The results show that the tetrapod-like ZnO nanostructures are highly crystalline with the wurtzite hexagonal structure. Photoluminescence (PL) spectrum of as-synthesized nanostructures exhibits a UV emission peak at ～ 389 nm and a broad green emission peak at ～ 513 nm. In addition, the growth mechanism of ZnO nanostructures is also briefly discussed.     

Thickness dependent magnetic properties of BiFeO3 thin films prepared by pulsed laser deposition

BiFe_1 ? xMn_xO₃ thin films having thickness 65 and 130 nm was fabricated on LAO substrates using pulsed laser deposition technique and its structural and magnetic properties were examined. Atomic force microscopy images confirmed that, as the thickness of the films increases the particles size also increases resulting in the decrease of magnetization. The possible cause for the lowering of magnetization with film thickness was discussed. Increase of spontaneous magnetization in BiFeO₃ at room temperature was observed with Mn substitution for Fe. The blocking temperature was found to decrease with increasing film thickness.     

Effect of nitrogen environment on NdFeB thin films grown by radio frequency plasma beam assisted pulsed laser deposition

NdFeB is a very attractive material for applications in electrical engineering and in electronics, for high-tech devices where high coercive field and high remanence are needed. In this paper we demonstrate that the deposition of nitrogen doped NdFeB thin films by pulsed laser deposition, in the presence of a nitrogen radiofrequency plasma beam, exhibit improved magnetic properties and surface morphology, when compared to vacuum deposited NdFeB layers. A Nd:YAG pulsed laser (3ω and 4ω) was focused on a NdFeB target, in vacuum, or in the presence of a nitrogen plasma beam. Substrate temperature (RT-850 °C), nitrogen gas pressure, and radiofrequency power (75–150 W), were particularly varied. The thin films were investigated by means of X-ray diffraction, atomic force microscopy, scanning electron microscopy, spectroscopic-ellipsometry, and vibrating sample magnetometry.     

Biomolecular urease thin films grown by laser techniques for blood diagnostic applications

Matrix assisted pulsed laser evaporation (MAPLE) was used for growing urease thin films designed for bio-sensor applications in clinical diagnostics. The targets exposed to laser radiation were made from a frozen composite manufactured by dissolving biomaterials in distilled water. We used a UV KrF* (λ = 248 nm, τ_FWHM ? 30 ns, ν = 10 Hz) excimer source for multipulse laser irradiation of the frozen targets cooled with Peltier elements. The laser source was operated at an incident fluence of 0.4 J/cm². Urease activity and kinetics were assayed by the Worthington method that monitors urea hydrolysis by coupling ammonia production to a glutamate dehydrogenase reaction. A decrease in absorbance was measured at 340 nm and correlated with the enzymatic activity of urease. We show that the urease films obtained by MAPLE techniques remain active up to three months after deposition.     

Crystal growth and photoluminescence characteristics of Ca2MgSi2O7:Eu3+ thin films grown by pulsed laser deposition

Ca₂MgSi₂O₇:Eu³⁺ films were deposited on Al₂O₃ (0 0 0 1) substrates by pulsed laser deposition. The films were grown at various oxygen pressures ranging from 100 to 400 mTorr. The crystallinity and surface morphology of the films were examined by X-ray diffraction (XRD) and atomic force microscopy (AFM), respectively. XRD and AFM respectively showed that the Ca₂MgSi₂O₇:Eu³⁺ films had a zircon structure and consisted of homogeneous grains ranging from 100 to 400 nm depending on the deposition conditions. The radiation emitted was dominated by a red emission peak at 620 nm. The maximum PL intensity of the Ca₂MgSi₂O₇:Eu³⁺ films grown at 300 mTorr was increased by a factor of 1.3 compared to that of Ca₂MgSi₂O₇:Eu³⁺ films grown at 100 mTorr. The crystallinity, surface roughness and photoluminescence of the thin-film phosphors were strongly dependent on the deposition conditions, in particular, the oxygen partial pressure.     

Optical nonlinearities of BaTiO3 matrix-embedded Au nanoparticles

Composite thin films Au/BaTiO₃ comprising nanometer-sized gold particles embedded in BaTiO₃ matrices were synthesized on MgO(1 0 0) substrates by co-depositing Au and BaTiO₃ targets using pulsed laser deposition technique. The nanostructure of the films and the size distributions of the Au particles were analyzed by high-resolution transmission electron microscopy. Crystal lattice fringes from the Au nanocrystals and BaTiO₃ matrices were observed. The nonlinear optical properties of the Au/BaTiO₃ films were measured using z-scan method at the wavelength of 532 nm with a laser duration of 10 ns. The nonlinear refractive index n₂ and the nonlinear absorption coefficient β were determined to be 2.72 × 10⁻⁶ esu and −1.1 × l0⁻⁶ m/W, respectively.     

Spectroscopic studies on DLC/TM (Cr,Ag, Ti,Ni) multilayers

The hydrogen-free diamond-like carbon (DLC) films with transition metal (TM = Cr, Ag, Ti, Ni) interlayer (bilayer and multilayer) were deposited on to stainless steel and silicon substrates using pulsed laser deposition technique. Secondary ion mass spectroscopy (SIMS) confirmed that the films were hydrogen free. Incorporation of chromium inter layer reduced the stress value by about 3 GPa as determined by micro Raman spectroscopy. Incorporation of the TM inter layer enhanced the photoluminescence (PL) intensity as compared to the monolithic DLC films. The optical band gap determined by spectroscopic ellipsometry for DLC/TM films was found to be in the range of 1.56–1.67 eV.     

Effects of substrate parameters on structure and optical properties of ZnO thin films fabricated by pulsed laser deposition

Highly oriented zinc oxide thin films have been grown on quartz, Si (1 1 1) and sapphire substrates by pulsed laser deposition (PLD). The effect of temperature and substrate parameter on structural and optical properties of ZnO thin films has been characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), optical transmission spectra and PL spectra. The experimental results show that the best crystalline thin films grown on different substrate with hexagonal wurtzite structure were achieved at growth temperature 400–500 °C. The growth temperature of ZnO thin film deposited on Si (1 1 1) substrate is lower than that of sapphire and quartz. The band gaps are increasing from 3.2 to 3.31 eV for ZnO thin film fabricated on quartz substrate at growth temperature from 100 to 600 °C. The crystalline quality and UV emission of ZnO thin film grown on sapphire substrate are significantly higher than those of other ZnO thin films grown on different substrates.     

Structural and optical properties of neodymium-doped lutetium fluoride thin films grown by pulsed laser deposition

Neodymium-doped lutetium fluoride (Nd³⁺:LuF₃) thin films were successfully grown on MgF₂ (0 0 1) substrates by pulsed laser deposition (PLD). It is void of cracks that are otherwise prevalent due to structural phase transitions in Nd³⁺:LuF₃ during thin film deposition and bulk crystal growth. Cathodoluminescence (CL) spectra revealed multiple emission peaks, with a dominant peak in the vacuum ultraviolet (VUV) region at 179 nm. This peak has a decay time of 6.7 ns. The ability to grow high quality Nd³⁺-doped fluoride thin films would enable fabrication of VUV light-emitting devices that will enhance applications requiring efficient VUV light sources.     

Structural and biocompatible characterization of TiC/a:C nanocomposite thin films

In this work, sputtered TiC/amorphous C thin films have been developed in order to be applied as potential barrier coating for interfering of Ti ions from pure Ti or Ti alloy implants. Our experiments were based on magnetron sputtering method, because the vacuum deposition provides great flexibility for manipulating material chemistry and structure, leading to films and coatings with special properties. The films have been deposited on silicon (001) substrates with 300 nm thick oxidized silicon sublayer at 200 °C deposition temperature as model substrate. Transmission electron microscopy has been used for structural investigations. Thin films consisted of ~ 20 nm TiC columnar crystals embedded by 5 nm thin amorphous carbon matrix. MG63 osteoblast cells have been applied for in vitro study of TiC nanocomposites. The cell culture tests give strong evidence of thin films biocompatibility.     

Ultra-fine Al–Si hypereutectic alloy fabricated by direct metal deposition

Ultra-fine Al–Si hypereutectic alloy with <10 μm primary Si phase was fabricated by direct metal deposition (DMD). The microstructure and microhardness of the hypereutectic alloys manufactured under different scanning speeds and laser powers during DMD were investigated. Compared with the conventional modified hypereutectic alloy whose primary Si phase is around 40 μm, the primary Si particle obtained by DMD has a much smaller size of about 5–10 μm. With increased scanning speed and laser power, the volume fraction and size of the primary Si increase. The unique hypoeutectic microstructure can be found around the primary Si phase in the hypereutectic alloy. With increased scanning speed, the size of eutectic Si grain decreases and the microhardness of deposition increase. However, the size of eutectic Si reaches the minimum value at a certain power level, and the microhardness of deposition reaches the maximum value at 850 W laser powers. The mircohardness of the deposited hypereutectic alloy is approximately 2.5 times of that of the raw eutectic alloy.     

Characterization of rhenium nitride films produced by reactive pulsed laser deposition

Rhenium nitride (ReN_x) films were grown on (100)-Si substrates by the reactive pulsed laser deposition (PLD) method using a high purity Re rod in an environment of molecular nitrogen. The resulting films are characterized by several techniques, which include in situ Auger electron spectroscopy, X-ray photoelectron spectroscopy and ex situ X-ray diffraction, scanning electron and atomic force microscopy. Additionally, the four-probe method is used to determine the sheet resistance of deposited layers. Results show that films with N/Re ratios (x) lower than 1.3 are very good conductors. In fact, the resistivity of ReN films for 0.2 < x < 1.3 is of the order of 5% of that of Re films, while at x = 1.3 there is an abrupt increment in resistivity, resulting in dielectric films for 1.3 < x < 1.35. These results differ from the prior understanding that in transition metals, resistivity should increase with nitrogen incorporation.     

Polymer–inorganic nano-composite thin film upconversion light emitters prepared by double-beam matrix assisted pulsed laser evaporation (DB-MAPLE) method

The work was to investigate the possibility of making polymer–inorganic nano-composite films with upconversion fluorescence properties using the double beam matrix-assisted pulsed laser evaporation (DB-MAPLE) method. The existing pulsed laser deposition vacuum chamber was modified to accommodate two laser beams of different wavelengths for simultaneous ablation of two separate targets: a polymer host and a rare earth containing rare earth ion enriched upconversion fluoride dopant. The polymer target was prepared in chlorobenzene and kept frozen during the ablation with circulating liquid nitrogen in accordance with the MAPLE procedure. It was ablated with 1064 nm beam from a pulsed Nd:YAG laser. The pellets made of the synthesized powders of inorganic phosphors of NaYF₄:Yb³⁺, Er³⁺ and NaYF₄:Yb³⁺, Ho³⁺were ablated with 532-nm beam from the same laser. The plumes from both targets were kept overlapping on the substrate during the deposition. X-ray diffraction analysis revealed that the most favorable for upconversion emission of the inorganic target materials was the hexagonal, beta phase of the NaYF₄ matrix existing at a baking temperature between 400 and 600 °C. The fabricated nano-composite films were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), transmission electron microscopy (TEM) and optical fluorescence spectroscopy. The polymer nano-composite films generally retained the crystalline structure and the upconversion fluorescence properties of the initial rare earth compounds due to better control of the deposition process of the materials with substantially different properties. The proposed method can be potentially used for making a wide variety of nano-composite films.     

Antibacterial,cytotoxicity and physical properties of laser — Silver doped hydroxyapatite layers

Hydroxyapatite layers with silver doping from 0.06 at.% to 14 at.% were prepared by laser deposition. The films' physical properties such as morphology, composition, crystallinity, Young's modulus and microhardness were measured. Films were amorphous or polycrystalline in dependence on deposition temperature (from RT to 600 °C). Antibacterial properties were tested using Escherichia coli and Bacillus subtilis cells. The antibacterial efficacy changed with silver doping from 4% to 100%. Cytotoxicity was studied by a direct contact test. Depending on doping and crystallinity the films were either non-toxic or mildly toxic.     

Integration of epitaxial permalloy on Si (1 0 0) through domain matching epitaxy paradigm

This paper addresses epitaxial integration of magnetic materials with Si (1 0 0) based solid state devices. Epitaxial Ni_82.5Fe_17.5 (permalloy, Py) thin films have been synthesized by pulsed laser deposition (PLD) on Si (1 0 0) using MgO/TiN as a template buffer. This epitaxial growth of these large lattice misfit systems was achieved through domain matching epitaxy (DME). The in-plane XRD pattern and selective area electron diffraction (SAED) results clearly indicate cube-on-cube epitaxial alignment. The bright field TEM image of Py/MgO/TiN/Si (1 0 0) heterostructure infers a Py layer thickness of ∼30 nm, with a well aligned island (150–200 nm) structure that is consistent with Volmer–Weber type growth. Magnetization data collected at 4 K and 300 K indicates that the easy axis of the magnetization lies in the plane of the Py. In addition, we have observed an intrinsic positive exchange bias (PEB) field of ∼104 Oe, where the magnetic hysteresis loop is shifted toward the positive field axis under zero field cooling conditions.     

Investigation of ZnO films on Si<111> substrate grown by low energy O+ assisted pulse laser deposited technology

Zinc oxide thin films (ZnO) with different thickness were prepared on Si (111) substrates using low energy O⁺ assisted pulse laser deposition (PLD). The structural and morphological properties of the films were investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements, respectively. The quality of ZnO films was also examined by using Rutherford backscattering spectroscopy/ion channeling (RBS/C) techniques. XRD showed that there was only one sharp diffraction peak at 2θ = 34.3° with the full width at the half maximum (FWHM) of around 0.34° for two ZnO samples, which also indicated that ZnO thin films had a good c-axis preferred orientation. Results of Rutherford backscattering and ion channeling clearly indicated that the Zn:O ratio in zinc oxide thin film approached to unity and the ZnO thin film grown by low energy O⁺ assisted pulse laser deposition had a polycrystalline structure. In the case of ZnO film fabricated by low energy O⁺ assisted pulse laser deposited under identical experimental conditions except growth time, AFM analysis has shown that the root mean square (RMS) roughness (2.37 nm) of thinner ZnO film (35 nm) was far below that (13.45 nm) of the thicker ZnO film (72 nm).     

Synthesis,structure, and optical properties of Au–TiO2 composite thin films

Titanium dioxide (TiO₂) films with varying concentrations of gold particles were synthesized using pulsed DC magnetron sputtering, with the intent to develop infrared reflecting films for use on cars and planes to reduce solar heat load. Under our deposition conditions, the films are smooth (RMS roughness on the order of 1.0–2.0 nm) and consist of rutile TiO₂ with embedded gold. The average gold particle diameter on the sample surface was found to change from 60 to 200 nm as the volume fraction of gold in the films increased from 1.9 to 4.3% (3.5 to 7.9 mol% Au). The maximum reflectance of these films in the infrared region (800–2500 nm) is > 50%, compared with 30% for pure TiO₂. The Maxwell–Garnett equation does not model the reflectance data very well, due to the relatively large gold particle size. Instead, by assuming that the contribution of gold particles to the reflectance response is proportional to their projected areal fraction in an effective medium approximation, we were able to fit the observed reflectance data quite well.