Effect of Ag doping on the performance of ZnO thin film transistor

Ag-doped zinc oxide (SZO) thin film transistors (TFTs) have been fabricated using a back-gate structure on thermally oxidized and heavily doped p-Si (100) substrate. The SZO thin films were deposited via pulsed laser deposition (PLD) from a 1, 3, and 5 wt.% Ag-doped ZnO (1SZO, 3SZO, and 5SZO, respectively) target using a KrF excimer laser (λ, 248 nm) at oxygen pressure of 350 mTorr. The deposition carried out at both room-temperature (RT) and 200 °C. The SZO thin films had polycrystalline phase with preferred growth direction of (002) as well as a wurtzite hexagonal structure. Compare with ZnO thin films, the SZO thin films were characterized by confirming the shift of (002) peak to investigate the substitution of Ag dopants for Zn sites. The as-grown SZO TFTs deposited at RT and 200 °C showed insulator characteristics. However the SZO TFTs annealed at 500 °C showed good n-type TFT performance because Ag was diffused from Zn lattice site and bound themselves at the high temperature, and it caused generation of electron carriers. The post-annealed 5SZO TFT deposited at 500 °C exhibited a threshold voltage (V_th) of 11.5 V, a subthreshold swing (SS) of 2.59 V/decade, an acceptable mobility (μ_SAT) of 0.874 cm²/V s, and on-to-off current ratios (I_on/off) of 1.44 × 10⁸.     

Biomolecular papain thin films growth by laser techniques

Papain thin films were synthesised by matrix assisted and conventional pulsed laser deposition (PLD) techniques. The targets submitted to laser radiation consisted on a frozen composite obtained by dissolving the biomaterials in distilled water. For the deposition of the thin films by conventional PLD pressed biomaterial powder targets were submitted to laser irradiation. An UV KrF* excimer laser source was used in the experiments at 0.5 J/cm² incident fluence value, diminished one order of magnitude as compared to irradiation of inorganic materials. The surface morphology of the obtained thin films was studied by atomic force profilometry and atomic force microscopy. The investigations showed that the growth mode and surface quality of the deposited biomaterial thin films is strongly influenced by the target preparation procedure.     

Effect of ZnO film deposition methods on the photovoltaic properties of ZnO–Cu2O heterojunction devices

The effect of ZnO film depositions using various film deposition methods such as magnetron sputtering (MSP), pulsed laser deposition (PLD) and vacuum arc plasma evaporation (VAPE) on the photovoltaic properties of ZnO–Cu₂O heterojunction solar cells is described in this report. In addition, the relationship between the resulting photovoltaic properties and the film deposition conditions such as supply power and substrate arrangement was investigated in Al-doped ZnO (AZO)–Cu₂O heterojunction devices fabricated using AZO thin films prepared by d.c. magnetron sputtering (d.c.MSP) or r.f. magnetron sputtering (r.f.MSP). The results showed that the measured photovoltaic properties of devices fabricated with films deposited on substrates oriented perpendicular to the target were better than those of devices fabricated with films deposited on substrates oriented parallel to the target. It was also found that ZnO film depositions under conditions where a relatively weaker oxidizing atmosphere was used yield better properties than films derived from MSP, which utilizes a high-density and high-energy plasma. Using VAPE and PLD, for example, high efficiencies of 1.52 and 1.42%, respectively, were obtained under AM2 solar illumination in devices fabricated at a substrate temperature around 200 °C.     

Effects of oxygen pressure on the growth of pulsed laser deposited ZnO films on Si(0 0 1)

ZnO thin films were prepared on Si(0 0 1) substrates using a pulsed laser deposition (PLD) technique and then their growth and properties were investigated particularly as a function of ambient O₂ pressure during film growth. It was found that the microstructure, crystallinity, orientation and optical properties of the films grown are strongly dependent on the O₂ pressures used. Completely c-axis oriented ZnO films are grown in a low O₂ pressure regime (5×10⁻⁴-5×10⁻² Torr), whereas a randomly oriented film with a much lower crystallinity and a rougher grained-surface is grown at an O₂ pressure of 5×10⁻¹ Torr. This deterioration in film quality may be associated with the kinetics of atomic arrangements during deposition. Our results suggest that ambient O₂ pressure is an important processing parameter and should be optimized in a narrow regime in order to grow a ZnO film of good properties in PLD process.     

Relationship between ultraviolet emission and electron concentration of ZnO thin films

ZnO thin films were deposited on (0001) Al₂O₃ substrates depending on oxygen partial pressure by pulsed laser deposition. Optical properties of ZnO were investigated by photoluminescence (PL). The relationship between PL and electron concentration has been investigated. Origin of the dominant ultraviolet (UV) emission in ZnO thin film measured at room temperature was identified as a free electron-neutral-acceptor transition (eA⁰) through temperature dependence of PL measurement. The UV emission intensity at room temperature is related to variation of electron concentration because a free-electron-neutral-acceptor transition (eA⁰) as origin of UV emission at room temperature is related to impurity concentration of ZnO.     

Characteristics of transparent ZnO based thin film transistors with amorphous HfO2 gate insulators and Ga doped ZnO electrodes

Coplanar type transparent thin film transistors (TFTs) have been fabricated on the glass substrates. The devices consist of intrinsic ZnO, Ga doped ZnO (GZO), and amorphous HfO₂ for the semiconductor active channel layer, electrode, and gate insulator, respectively. GZO and HfO₂ layers were prepared by using a pulsed laser deposition (PLD) and intrinsic ZnO layers were fabricated by using an rf-magnetron sputtering. The transparent TFT exhibits n-channel, enhancement mode behavior. The field effect mobility, threshold voltage, and a drain current on-to-off ratio were measured to be 14.7 cm²/Vs, 2 V, and 10⁵, respectively. High optical transmittance (> 85%) in visible region makes ZnO TFTs attractive for transparent electronics.     

Nano Techniques for Enhancing Critical Current in?Superconducting YBCO Films

A range of nano techniques is explored in order to increase the critical current in pulsed laser deposited (PLD) superconducting YBa₂Cu₃O _x (YBCO) thin films. The structural measurements are linked with magnetic and transport measurements of the films. The effectiveness of PLD techniques is analyzed from the point of view of the dimensionality of nanostructures formed prior to and during the film growth. It is shown that a combination of two-dimensional substrate decoration with nanoparticles before the film deposition and one-dimensional growth of external phase nanorods during the deposition offers a high critical current in magnetic field both along the c-axis and in the ab-plane of YBCO.     

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).     

ZnO thin films prepared by atomic layer deposition and rf sputtering as an active layer for thin film transistor

Recently, the application of ZnO thin films as an active channel layer of transparent thin film transistor (TFT) has become of great interest. In this study, we deposited ZnO thin films by atomic layer deposition (ALD) from diethyl Zn (DEZ) as a metal precursor and water as a reactant at growth temperatures between 100 and 250 °C. At typical growth conditions, pure ZnO thin films were obtained without any detectable carbon contamination. For comparison of key film properties including microstructure and chemical and electrical properties, ZnO films were also prepared by rf sputtering at room temperature. The microstructure analyses by X-ray diffraction have shown that both of the ALD and sputtered ZnO thin films have (002) preferred orientation. At low growth temperature T_s ≤ 125 °C, ALD ZnO films have high resistivity (> 10 Ω cm) with small mobility (< 3 cm²/V s), while the ones prepared at higher temperature have lower resistivity (< 0.02 Ω cm) with higher mobility (> 15 cm²/V s). Meanwhile, sputtered ZnO films have much higher resistivity than ALD ZnO at most of the growth conditions studied. Based upon the experimental results, the electrical properties of ZnO thin films depending on the growth conditions for application as an active channel layer of TFT were discussed focusing on the comparisons between ALD and sputtering.     

Calcium phosphate thin film processing by pulsed laser deposition and in situ assisted ultraviolet pulsed laser deposition

Calcium orthophosphates (CaP) and hydroxyapatite (HA) were intensively studied in order to design and develop a new generation of bioactive and osteoconductive bone prostheses. The main drawback now in the CaP and HA thin films processing persists in their poor mechanical characteristics, namely hardness, tensile and cohesive strength, and adherence to the metallic substrate. We report here a critical comparison between the microstructure and mechanical properties of HA and CaP thin films grown by two methods. The films were grown by KrF* pulsed laser deposition (PLD) or KrF* pulsed laser deposition assisted by in situ ultraviolet radiation emitted by a low pressure Hg lamp (UV-assisted PLD). The PLD films were deposited at room temperature, in vacuum on Ti–5Al–2.5Fe alloy substrate previously coated with a TiN buffer layer. After deposition the films were annealed in ambient air at 500–600 °C. The UV-assisted PLD films were grown in (10^–2–10^–1 Pa) oxygen directly on Ti–5Al–2.5Fe substrates heated at 500–600 °C. The films grown by classical PLD are crystalline and stoichiometric. The films grown by UV-assisted PLD were crystalline and exhibit the best mechanical characteristics with values of hardness and Young modulus of 6–7 and 150–170 GPa, respectively, which are unusually high for the calcium phosphate ceramics. To the difference of PLD films, in the case of UV-assisted PLD, the GIXRD spectra show the decomposition of HA in Ca₂P₂O₇, Ca₂P₂O₉ and CaO. The UV lamp radiation enhanced the gas reactivity and atoms mobility during processing, increasing the tensile strength of the film, while the HA structure was destroyed.     

Co-P-B catalyst thin films prepared by electroless and pulsed laser deposition for hydrogen generation by hydrolysis of alkaline sodium borohydride: A comparison

Co-P-B catalyst thin films have been synthesized on Ni-foam and glass substrate by using electroless deposition (ED) and pulsed laser deposition (PLD) respectively. The efficiency of these catalyst films was tested by catalytic hydrolysis of NaBH₄ for H₂ generation. While the chemically produced Co-P-B film on Ni-foam shows similar activity as that of their corresponding powder, the Co-P-B film deposited by PLD exhibits much superior H₂ generation rate as compared to Co-P-B powder. We attribute this increased efficiency to the special features of the Co-P-B films which are in the form of nanoparticle-assembled films, a peculiar characteristic of PLD films for appropriate choice of the pulse laser parameters. The surface nanoparticle-configuration increases the active surface area and also favors electronic exchange mechanisms to promote hydrolysis process for H₂ gas generation. The films deposited by using laser energy density of 3 J/cm² show the highest activity in connection to the best configuration of the ablated nanoparticles. Different numbers of Co-P-B layers were deposited on Ni-foam by ED and it was found that at least four layers are required for complete coverage of the foam to have the best activity.     

Influence of oxygen pressure on the structural, electrical and optical properties of VO2 thin films deposited on ZnO/glass substrates by pulsed laser deposition

The influence of oxygen pressure on the structural and electrical properties of vanadium oxide thin films deposited on glass substrates by pulsed laser deposition, via a 5-nm thick ZnO buffer, was investigated. For the purposes of comparison, VO₂ thin films were also deposited on c-cut sapphire and glass substrates. During laser ablation of the V metal target, the oxygen pressure was varied between 1.33 and 6.67 Pa at 500 °C, and the interaction and reaction of the VO₂ and the ZnO buffer were studied. X-ray diffraction studies showed that the VO₂ thin film deposited on a c-axis oriented ZnO buffer layer under 1.33 Pa oxygen had (020) preferential orientation. However, VO₂ thin films deposited under 5.33 and 6.67 Pa were randomly oriented and showed (011) peaks. Crystalline orientation controlled VO₂ thin films were prepared without such expensive single crystal substrates as c-cut sapphire. The metal-insulator transition properties of the VO₂/ZnO/glass samples were investigated in terms of electrical conductivity and infrared reflectance with varying temperatures, and the surface composition was investigated by X-ray photoelectron spectroscopy.     

Preparation of Li and Er codoped ZnO thin films and their photoluminescence

Li-Er codoped ZnO thin films have been prepared on Si(100) substrates by pulsed laser deposition (PLD). Both the as-grown and post-annealed films exhibit good crystalline quality with preferred c-axis orientation. After post-annealing at 850 °C, the photoluminescence (PL) related to intra-4f shell of Er³⁺ can be clearly observed. The Li-Er codoped ZnO film shows higher intensity of PL around 1.54 μm than the Er monodoped ZnO film. The behavior is attributed to the lowering of the symmetry of the crystal field around Er³⁺ ions by introducing Li⁺ into ZnO lattice, which is also confirmed by Raman scattering spectra.     

Pulsed laser deposition of silicon-substituted hydroxyapatite coatings

Thin films of Si-substituted hydroxyapatite (Si-HA) were deposited on Si and Ti substrates by pulsed laser deposition (PLD), in the presence of a water vapour atmosphere. The PLD ablation targets were made with different mixtures of commercial carbonated HA and Si powder, in order to produce the Si-HA thin films. The physicochemical properties of the coatings and the incorporation of the Si into the HA structure was studied by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The Si atoms were successfully incorporated into the HA structure, and were found to be in the form of SiO₄⁴⁻ groups, principally displacing carbonate groups off the HA structure.     

Effect of Ga doping on micro/structural, electrical and optical properties of pulsed laser deposited ZnO thin films

Undoped and Ga doped ZnO thin films (1% GZO, 3% GZO and 5% GZO) were grown on c-Al₂O₃ substrates using the 1, 3 and 5 at. wt.% Ga doped ZnO targets by pulsed laser deposition. X-ray diffraction studies revealed that highly c-axis oriented, single phase, undoped and Ga doped ZnO thin films with wurtzite structure were deposited. Micro-Raman scattering analysis showed that Ga doping introduces defects in the host lattice. The E₂^High mode of ZnO in Ga doped ZnO thin film was observed to shift to higher wavenumber indicating the presence of residual compressive stress. Appearance of the normally Raman inactive B₁ modes (B₁^Low, 2B₁^Low and B₁^High) due to breaking of local translational symmetry, also indicated that defects were introduced into the host lattice due to Ga incorporation. Band gap of the Ga doped ZnO thin films was observed to shift to higher energy with the increase in doping concentration and is explicated by the Burstein-Moss effect. Electrical resistivity measurements of the undoped and GZO thin films in the temperature range 50 to 300 K revealed the metal to semiconductor transition for 3 and 5% GZO thin films.     

Influence of O2 pressure on the properties of heavily-doped ZnO:Sb thin films grown by pulsed laser deposition

Heavily Sb-doped ZnO films were deposited on the glass substrates by pulsed laser deposition (PLD). X-ray diffraction (XRD) and photoelectron spectroscopy (XPS) were employed to characterize their microstructures and chemical valence states. Transmittance spectra and Hall measurements were used to evaluate their optical and electrical properties. It was found that the as-prepared ZnO:Sb thin films showed a single-hexagonal-phase structure, with the optical band gap tuning from 3.33 to 3.11 eV. The variation in the band gap was attributed to a large co-axis strain in the alloy films induced by Sb incorporation. Besides, the alloy films showed a semi-insulated characteristic with high resistivity of ~10⁴ Ω cm, which was possibly related to a compensation of intrinsic defects.     

Er,Yb:YVO4 waveguides deposited on amorphous SiO2 by PLD and UVPLD

In this work, erbium, and erbium and ytterbium co-doped YVO₄ waveguiding thin films were deposited on amorphous SiO₂ substrates by pulsed laser deposition (PLD) and ultraviolet-assisted pulsed laser deposition (UVPLD). The influence of the deposition technique on the structure, morphology, and optical properties of the films was investigated. At lower dopant concentrations the films prepared by UVPLD show better crystallinity and optical properties. All the samples show preferred orientation of the (001) zone axes parallel to the substrate surface. The polycrystalline samples show difference in the refractive indexes ?n (?n = n_TE − n_TM) for the TE and TM polarizations.     

Low temperature annealing effects on the structure and optical properties of ZnO films grown by pulsed laser deposition

ZnO thin films were deposited on glass substrates at room temperature (RT) ∼500 °C by pulsed laser deposition (PLD) technique and then were annealed at 150-450 °C in air. The effects of annealing temperature on the microstructure and optical properties of the thin films deposited at each substrate temperature were investigated by XRD, SEM, transmittance spectra, and photoluminescence (PL). The results showed that the c-axis orientation of ZnO thin films was not destroyed by annealing treatments; the grain size increased and stress relaxed for the films deposited at 200-500 °C, and thin films densified for the films deposited at RT with increasing annealing temperature. The transmittance spectra indicated that E_g of thin films showed a decreased trend with annealing temperature. From the PL measurements, there was a general trend, that is UV emission enhanced with lower annealing temperature and disappeared at higher annealing temperature for the films deposited at 200-500 °C; no UV emission was observed for the films deposited at RT regardless of annealing treatment. Improvement of grain size and stoichiometric ratio with annealing temperature can be attributed to the enhancement of UV emission, but the adsorbed oxygen species on the surface and grain boundary of films are thought to contribute the annihilation of UV emission. It seems that annealing at lower temperature in air is an effective method to improve the UV emission for thin films deposited on glass substrate at substrate temperature above RT.     

Pulsed laser deposition of crystalline ZrC thin films

ZrC thin films were grown on Si substrates by the pulsed laser deposition (PLD) technique under various conditions. X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), X-ray diffraction and reflectivity, spectroscopic ellipsometry, and four point probe measurements were used to characterize the properties of the deposited films. It has been found that crystalline films could be grown only by using laser fluences higher than 5 J/cm² and substrate temperatures in excess of 500 °C. For a fluence of 10 J/cm² and a substrate temperature of 700 °C, cubic ZrC films (a = 0.469 nm) exhibiting a (200)-texture were deposited under vacuum or low pressure C₂H₂ atmosphere. These films were smooth, with surface roughness values below 1.0 nm and mass densities around the tabulated value of 6.7 g/cm³. AES depth profiling investigations showed oxygen contamination around 7% in the bulk region. Despite the relatively high levels of oxygen contamination, the deposited ZrC films were very conductive. The use of a low C₂H₂ pressure atmosphere during deposition had a small beneficial effect on crystallinity and stoichiometry of the films.     

Al-doped ZnO films by pulsed laser deposition at room temperature

Polycrystalline Al-doped ZnO films with good photoluminescence property were successfully deposited on quartz glass substrates by pulsed laser deposition (PLD) at room temperature. The films were obtained by ablating a metallic target (Zn:Al 3 wt%) at various laser energy densities (1.0-2.1 J/cm²) in oxygen atmosphere (9 Pa). The structure of the films was characterized by XRD. Ultraviolet photoluminescence centered at 359-361 nm was observed in the room temperature PL spectra of the Al-doped ZnO films.