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.     

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.     

Hydrothermally grown ZnO nanoflowers for environmental remediation and clean energy applications

Well-crystalline flower-shaped ZnO nanostructures were synthesized by simple hydrothermal process at low-temperature of 145 °C and utilized as a photocatalyst and photo-anode material for photocatalytic degradation and dye-sensitized solar cell applications, respectively. The detailed morphological and the structural characterizations revealed that the synthesized products were flower-shaped, grown in very high-density, and possessed well-crystalline wurtzite hexagonal phase. The chemical composition confirmed the pure phase and good optical properties of as-synthesized ZnO flowers. The as-synthesized ZnO flowers were used as an efficient photocatalyst for the photocatalytic degradation of Rhodamine B which exhibit ～84% degradation within 140 min. Moreover, the as-synthesized ZnO flowers were utilized as photo-anode material for the fabrication of dye-sensitized solar cells (DSSCs) which exhibited overall light-to-electricity conversion efficiency of ～1.38%, open-circuit current (V_OC) of 0.621 V, short-circuit current (J_SC) of ～3.52 mA/cm² and fill factor (FF) of 0.64.     

Flame synthesis of zinc oxide nanocrystals

Distinctive zinc oxide (ZnO) nanocrystals were synthesized on the surface of Zn probes using a counter-flow flame medium formed by methane/acetylene and oxygen-enriched air streams. The source material, a zinc wire with a purity of ～99.99% and diameter of 1 mm, was introduced through a sleeve into the oxygen rich region of the flame. The position of the probe/sleeve was varied within the flame medium resulting in growth variation of ZnO nanocrystals on the surface of the probe. The shape and structural parameters of the grown crystals strongly depend on the flame position. Structural variations of the synthesized crystals include single-crystalline ZnO nanorods and microprisms (ZMPs) (the ZMPs have less than a few micrometers in length and several hundred nanometers in cross section) with a large number of facets and complex axial symmetry with a nanorod protruding from their tips. The protruding rods are less than 100 nm in diameter and lengths are less than 1 μm. The protruding nanorods can be elongated several times by increasing the residence time of the probe/sleeve inside the oxygen-rich flame or by varying the flame position. At different flame heights, nanorods having higher length-to-diameter aspect-ratio can be synthesized. A lattice spacing of ～0.26 nm was measured for the synthesized nanorods, which can be closely correlated with the (0 0 2) interplanar spacing of hexagonal ZnO (Wurtzite) cells. The synthesized nanostructures were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution TEM (HR-TEM), X-ray energy dispersive spectroscopy (EDS), and selected area electron diffraction pattern (SAED). The growth mechanism of the ZnO nanostructures is discussed.     

Facile synthesis of snowflake-like ZnO nanostructures at low temperature and their super catalytic activity for the ozone decomposition

Snowflake-like ZnO nanostructures, with different shapes, have been successfully synthesized on a large scale via a facile chemical precipitation method without utilizing any additive agent. The physicochemical features of the product were characterized by X-ray diffraction (XRD), X-ray photoelectron spectra (XPS), scanning electron microscopy (SEM), and nitrogen absorption-desorption. Results show that the obtained ZnO had a snowflake-like structure which was made of nano-platelets with uniform thickness of 20–30 nm. The average pore size and Brunauer–Emmet–Teller (BET) surface area of the as-synthesized ZnO were 31.3 nm and 12.74 m²/g. XPS spectrum predicts the existence of active hydroxyl groups in the prepared ZnO. The snowflake-like ZnO sample exhibited super catalytic performance for ozone decomposition in water, indicating a new effective catalyst for the ozonation of organic contaminants in water treatment.     

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.     

Deposition of nanocrystalline ZnO by wire explosion technique and characterization of the films' properties

ZnO films deposited on glass, quartz and Al on silicon mono-crystal Si (100) substrates by using the wire explosion technique were investigated by X-ray diffraction (XRD), UV–VIS spectroscopy, scanning electron (SEM) and atomic force microscopy (AFM) measurements. X-ray diffraction measurements have shown that ZnO films are mainly composed of (100), (002) and (101) orientation crystallites. The post-deposition thermal treatment at 600 °C temperature in air has shown that the composite of Zn/ZnO film was fully oxidized to ZnO film. The XRD spectra of the film deposited in oxygen atmosphere at room temperature present high intensity dominating peak at 2h = 36, 32° corresponding to the (101) ZnO diffraction peak. The small fraction of the film (7%) corresponds to the (002) peak intensity at 2h = 34, 42°. This result indicates the good crystal quality of the film and hexagonal wurtzite-type structure deposited by zinc wire explosion. The optical absorption spectra shows the bands at 374, 373 and 371 nm corresponding to deposition conditions. The SEM analysis shows that ZnO films presented different morphologies from fractal network to porous films depending on deposition conditions. AFM analysis revealed the grain size ranges from 50 nm to 500 nm. The nanoneedles up to 300 nm in length were found as typical structures in the film. It was demonstrated that the wire explosion technique is a feasible method to produce ZnO crystalline thin films and nanostructures.     

Silver nanocrystals of various morphologies deposited on silicon wafer and their applications in ultrasensitive surface-enhanced Raman scattering

Silver nanostructures with dendritic, flower-like and irregular morphologies were controllably deposited on a silicon substrate in an aqueous hydrogen fluoride solution at room temperature. The morphology of the Ag nanostructures changed from dendritic to urchin-like, flowerlike and pinecone-like with increasing the concentration of polyvinyl pyrrolidone (MW = 55,000) from 2 to 10 mM. The Ag nanostructures were characterized by transmission electron microscopy, high-resolution transmission electron microscopy, scanning electron microscopy, energy-dispersive X-ray, and X-ray diffraction. Through a series of time-dependent morphological evolution studies, the growth processes of Ag nanostructures have been systematically investigated and the corresponding growth mechanisms have been discussed. In addition, the morphology-dependent surface-enhanced Raman scattering of as-synthesized Ag nanostructures were investigated. The results indicated that flower-like Ag nanostructure had the highest activity than the other Ag nanostructures for Rhodamine 6G probe molecules.     

Microstructural and optical properties of nanocrystalline ZnO deposited onto vertically aligned carbon nanotubes by physical vapor deposition

Nanocrystalline ZnO films with thicknesses of 5 nm, 10 nm, 20 nm, and 50 nm were deposited via magnetron sputtering onto the surface of vertically aligned multi-walled carbon nanotubes (MWCNTs). The ZnO/CNTs heterostructures were characterized by scanning electron microscopy, high resolution transmission electron microscopy, and X-ray diffraction studies. No structural degradation of the CNTs was observed and photoluminescence (PL) measurements of the nanostructured ZnO layers show that the optical properties of these films are typical of ZnO deposited at low temperatures. The results indicate that magnetron sputtering is a viable technique for growing heterostructures and depositing functional layers onto CNTs.     

Effect of Nd3+ incorporation on the microstructure and chemical structure of RF sputtered ZnO thin films

The present work aims at investigating the effects that different levels of Nd atoms incorporation can have on the microstructure and chemical structure of ZnO thin films. Undoped and Nd-doped ZnO films were deposited by RF co-sputtering from pure ZnO and metallic Nd targets in Ar plasma onto Si, quartz and glass substrates. The Nd concentration in the ZnO host matrix was varied in the range 0–26 at.% by varying the bias applied to the Nd target. A comprehensive characterization of the films properties was performed by X-ray photoelectron and Auger electron spectroscopies, X-ray fluorescence analysis, X-ray diffraction and scanning electron microscopy. At low Nd atomic concentration (Nd/Zn < 0.07) Nd atoms were successfully incorporated into the ZnO matrix, whose crystalline structure was preserved. A deterioration of the ZnO würtzite phase was observed on the contrary with increasing Nd content in the films together with the precipitation of a second phase, identified as Nd₂O₃.     

Growth of p-type ZnO thin films by using an atomic layer epitaxy technique and NH3 as a doping source

Nitrogen-doped, p-type ZnO thin films have been grown successfully on sapphire (0001) substrates by atomic layer epitaxy (ALE) using Zn(C₂H₅)₂ [Diethylzinc, DEZn], H₂O and NH₃ as a zinc precursor, an oxidant and a doping source gas, respectively. The lowest electrical resistivity of the p-type ZnO films grown by ALE was 210 Ω cm with a hole concentration of 3.41 × 10¹⁶ cm^− 3. Low temperature-photoluminescence analysis results support that the nitrogen ZnO after annealing is a p-type semiconductor. Also a model for change from n-type ZnO to p-type ZnO by annealing is proposed.     

Fabrication of large-scale ultra-fine Cd-doped ZnO nanowires

We demonstrate bulk synthesis of highly crystal Cd-doped ZnO nanowires by using (Cd + Zn) powders at 600 °C. These mass ultra-fine ZnO nanowires with about 0%, 1%, 4% and 8% Cd so obtained have been characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX), selected area electron diffraction (SAED) and high-resolution TEM (HRTEM). They have the uniform diameter of about 20 nm and several hundred microns in length. The growth of the as-synthesized nanowires is suggested for self-catalyzed vapor–liquid–solid.     

New natively textured surface Al-doped ZnO-TCOs for thin film solar cells via magnetron sputtering

Natively textured surface aluminum doped zinc oxide (ZnO:Al) thin films were directly deposited via pulsed direct current (DC) reactive magnetron sputtering on glass substrates. During the reactive sputtering process, the oxygen gas flow rate was varied from 8.5 sccm to 11.0 sccm. The influences of oxygen flow rate on the structural, electrical and optical properties of naturally textured ZnO:Al TCO thin films with milky surface were investigated in detail. Gradual oxygen growth (GOG) technique was developed in the reactive sputtering process for textured ZnO:Al thin films. The light-scattering ability and optical transmittance of the natively textured ZnO:Al TCO thin films can be improved through gradual oxygen growth method while maintaining a low sheet resistance. Typical natively textured ZnO:Al TCO thin film with crater-like surface exhibits low sheet resistance (R_s ～ 4 Ω), high transmittance (T_a > 85%) in visible optical region and high haze value (12.1%).     

Effects of temperature on ZnO hybrids grown by metal-organic chemical vapor deposition

The growth of three-dimensional ZnO hybrid structures by metal-organic chemical vapor deposition was controlled through their growth pressure and temperature. Vertically aligned ZnO nanorods were grown on c-plane of sapphire substrate at 600 °C and 400 Torr. ZnO film was then formed in situ on the ZnO nanorods at 100, 600, and 700 °C and 10 Torr. High-resolution X-ray diffraction measurements showed that the ZnO film on the nanorods/sapphire grew epitaxially, and that the ZnO film/nanorods hybrid structures had well-ordered wurtzite structures. The hybrid ZnO structure was shown to be about 3–5 μm by field-emission scanning electron microscopy. The hybrid formed at 600 °C showed better crystalline quality those formed at 100 °C or 700 °C. These structures have potential applicability as nanobuilding blocks in nanodevices.     

Photoluminescence study of ZnO nanostructures grown on silicon by MOCVD

ZnO nanostructures with a size ranging from 20 to 100 nm were successfully deposited on (1 0 0)-Si substrates at different temperatures (500–800 °C) using MOCVD. It could be confirmed that the size of ZnO nanostructures decreased with increasing growth temperature. From photoluminescence (PL) studies it was found, that intensive band-edge PL of ZnO nanostructures consists of emission lines with maxima at 368.6 nm, 370.1 nm, 373.7 nm, 383.9 nm, 391.7 nm, 400.7 nm and 412 nm. These lines can be dedicated to free excitons and impurity donor-bound excitons, where hydrogen acts as donor impurity with an activation energy of about 65 meV. A UV shift of the band-edge PL line with increasing growth temperature of ZnO nanostructures was observed as a result of the quantum confinement effect. The results suggest that an increase of growth temperature leads to increased band-edge PL intensity. Moreover, the ratio of band-edge PL intensity to green- (red-) band intensity also increases, indicating better crystalline quality of ZnO nanostructures with increasing growth temperature.     

Synthesis and field emission of patterned SnO2 nanoflowers

This article reports the synthesis and field emission of patterned SnO₂ nanoflowers obtained by a simple method. A patterned Au catalyst film was prepared on the silicon wafer by radio frequency (RF) magnetron sputtering and photolithographic patterning processes. The patterned SnO₂ nanoflowers arrays, with a unit diameter of ∼ 50 μm, were synthesized via vapor phase transport method. Field emission scanning electron microscopy (SEM) and X-ray diffraction (XRD) are used to identify the surface morphology and composition of the as-synthesized SnO₂ nanostructures. The mechanism of formation of SnO₂ nanostructures was also discussed. The measurement of field emission (FE) showed that the as-synthesized SnO₂ nanostructure arrays have a lower turn-on field of 2.6 V/μm at the current density of 0.1 μA/cm². This approach must have a wide variety of applications such as fabrications of micro-optical components and micropatterned oxide thin films used in FE-based flat panel displays and sensor arrays.     

Size and defect related broadening of photoluminescence spectra in ZnO:Si nanocomposite films

Nanocomposite films of zinc oxide and silicon were grown by thermal evaporation technique using varying ratios of ZnO:Si in the starting material. Structural analyses reveal the role of ZnO and amorphous silicon interface in contributing to the relatively less common blue photoluminescence emissions (at ～410 and 470 nm). These blue peaks are observed along with the emissions resulting from band edge transition (370 nm) and those related to defects (520 nm) of ZnO. Careful analysis shows that along with the grain size of ZnO, a suitable compositional ratio of ZnO to silicon is critical for the coexistence of all the four peaks. Proper selection of conditions can give comparable photoluminescence peak intensities leading to broad-band emission.     

Fabrication of n-Zn1−xGaxO/p-(ZnO)1−x(GaP)x thin films and homojunction

Ga doped ZnO (GZO) and GaP codoped ZnO (GPZO) thin films of different concentrations (1–4 mol%) have been grown on sapphire substrates by RF sputtering for the fabrication of ZnO homojunction. The grown films have been characterized by X-ray diffraction (XRD), photoluminescence (PL), Hall measurement, energy dispersive spectroscopy (EDS), time-of-flight secondary ion mass spectrometer (ToF-SIMS), UV–Vis–NIR spectroscopy and atomic force microscopy (AFM). Unlike in conventional codoping, here we directly doped (codoped) GaP into ZnO to realize p-ZnO. The Hall measurements indicate that 2 and 4% GPZO films exhibit p-conductivity due to the sufficient amount of phosphorous incorporation while all the monodoped GZO films showed n-conductivity as expected. Among the p-ZnO films, 2% GPZO film shows low resistivity (2.17 Ωcm) and high hole concentration (1.8 × 10¹⁸ cm^?3) by optimum incorporation of phosphorous due to best codoping. Similarly, among the n-type films, 2% GZO shows low resistivity (1.32 Ωcm) and high electron concentration (2.02 × 10¹⁹ cm^?3) by optimum amount of Ga incorporation. The blue shift and red shift in NBE emission observed from PL acknowledged the formation of n- and p-conduction in monodoped and codoped films, respectively. The neutral acceptor bound exciton recombination (A⁰X) observed by low temperature PL for 2% GPZO confirms the p-conductivity. Further, the high concentration of P atoms than Ga observed from ToF-SIMS (2% GPZO) also supports the p-conductivity of the films. The fabricated p–n junction with best codoped p-(ZnO)_0.98(GaP)_0.02 and best monodoped n-Zn_0.98Ga_0.02O films showed typical rectification behavior of a diode. The diode parameters have also been estimated for the fabricated homojunction.     

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.