Polycrystalline ZnO Research Articles

Annealing rate tuned magnetization level in polycrystalline ZnO:Cu films

By combing structural characterizations and comparative annealing experiments, we found that uniformly annealed ZnO:Cu films exhibited robust room temperature ferromagnetism, while non-uniform annealed ZnO:Cu films result in magnetism quenching. Due to inhomogeneous Ostwald ripening during nanocrystal films annealing, Cu2+ ions exist in adsorbed state, depressing nanocrystals surface state. The dissociative Cu2+ ions will compensate d0 ferromagnetism and interference dual-donor (Vo and Zni) mediated indirect double exchange process. Hence, for colloidal chemistry constructed particles thin films, annealing control must be uniform, reducing the introduction of magnetic instability.

Effect of Annealing Temperature on the Physical Properties of the ZnO Thin Films Deposited by Ultrasonic Spray Pyrolysis

In this work optical, electrical, structural and surface properties of polycrystalline ZnO thin films grown from aqueous solutions (with pH = 5) have been reported. The films have been deposited on glass substrates by ultrasonic spray pyrolysis technique at a substrate temperature of 350 ± 5 °C. Zinc acetate dissolved in deionized water has been used as starting solution. The ZnO thin films have been annealed in air at 450 and 500 °C to improve their physical characteristics. X-ray diffraction reveals that the films are polycrystalline in nature having zincite type crystal structure. Electrical resistivity values of the films have been increased after annealing process. Films are highly transparent in the visible region. The dependence of refractive index, n , and extinction coefficient, k , on the wavelength for ZnO films has been also reported. Optical band gap values have been determined using optical method. Finally, it has been concluded that annealing temperature has an important effect on the optical, structural, surface and electrical properties of the deposited films.

Effects of FeCl3 as oxidizing agent on the conduction mechanisms in polypyrrole (PPy)/pc–ZnO hybrid heterojunctions grown by oxidative chemical vapor deposition

ABSTRACTP‐type polypyrrole (PPy) films are deposited on glass and on n‐type polycrystalline ZnO (pc–ZnO) substrates by oxidative chemical vapor deposition under three different amounts of FeCl3 used as oxidizing agents to form hybrid heterojunctions. Their microstructure, morphology, and electrical characteristics are studied. Particularly, current–voltage characteristics of the PPy/pc–ZnO heterojunctions are analyzed by defining an electrical equivalent circuit. The extracted parameters, together with the estimated heterojunction barrier height and the HOMO energy level of the PPy, indicate that a thermionic emission of holes at the heterojunction determines the saturation current of the diode at low voltage. For larger FeCl3 amounts, the diode ideality factor increases indicating an increment of recombination by tunneling of charge carriers occurring at the heterojunction. This is attributed to a narrowing of the space charge region due to an increment of the number of charge carriers with a growing amount of FeCl3. At high voltages, the PPy thickness influences the ohmic and space–charge limited current mechanisms at the bulk region. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 1537–1544

Effect of the Deposition Temperature and Thermal-Annealing on the I-V Characteristics of Low-Temperature-Processed Polycrystalline ZnO and Amorphous InGaZnO TFTs

Transition metal oxide thin-film transistors (TMO-TFTs) have attracted remarkable attention both in academia and industry during the past few years. ZnO and ZnO-based multicomponent amorphous metal oxides, especially InGaZnO (IGZO), have opened a new window to large-area flexible transparent electronic devices due to their unique properties such as high carrier transport and optical transparency. For large-area flexible transparent electronics based on the transition metal oxides are to be fully implemented, it will be critical to tune the crystalline structure and chemical composition of the metal-oxide channel layers using low-temperature-process methods. During the past few years, several research groups have reported the effects of plasma treatment, annealing process, deposition techniques, and deposition recipes on the electrical performance of the polycrystalline ZnO and amorphous IGZO TFTs; these studies have correlated the electrical performance of the fabricated TFTs to the material characteristics of the channel layer. However, the significant effect of the low-temperature vacuum-annealing and the channel layer deposition temperature on the TFT current-voltage (I-V) characteristics and material properties of the polycrystalline ZnO and amorphous IGZO channel layers has not been well studied. In this research, electrical dependence on the materials properties, surface topography, and chemical composition of the polycrystalline ZnO and amorphous IGZO channel layers are investigated by varying the channel layer deposition temperatures (from room-temperature to 150°C) and post processing low-temperature (150°C) vacuum-annealing. In order to exclude the effect of surface roughness of the gate dielectric and the out-diffusion of hydrogen (H) atoms from plasma-enhanced chemical vapor deposition (PECVD) of conventional gate dielectric layers to the TMO channel layer during the fabrication processes, bottom gate TMO-TFTs were fabricated on thermally grown SiO2. A slight improvement in the threshold voltage (VT) of 150°C processed ZnO TFTs after vacuum-annealing at 150°C was observed. On the other hand, 150°C IGZO TFTs with µ=8.8 cm2/V.s, VT= 4.2 V, S.S.= 0.42 V/decade, and Ion/off >106 became highly conductive and did not show switching behavior after vacuum-annealing. In order to determine the effect of the low-temperature vacuum-annealing on the oxygen vacancy concentration, Vo, the chemical composition of the ZnO thin-films was characterized using X-ray photoelectron spectroscopy (XPS). The O 1s spectra of the ZnO film was fitted into three peaks, one located at low-binding energy (OL), the medium-binding energy (OM), and the high-binding energy (OH) through a Gaussian profile. The OL peak is attributed to the oxygen atoms in the oxide lattice without oxygen vacancies (Vo), OM is attributed to the Vo, and the OH peak is surface oxygen. The OM value in the O 1s XPS spectra showed almost the same amount of Vo, with 18.75 % and 19.2% as the as-deposited and the vacuum-annealed ZnO films, respectively. However, a significant increase in Vo concentration (23.06% vs. 28.71%) was observed in the IGZO channel layers upon annealing in vacuum. The improved I-V characteristics of ZnO TFTs after vacuu-annealing was correlated to the better film crystallinity, larger grain size (25 nm vs. 23 nm), and smoother interface roughness (0.615 vs. 0.814 nm), obtained using X-ray diffraction (XRD), scanning electron microscope (SEM), and atomic force microscopy (AFM) measurements, respectively. ZnO TFTs also exhibited a large negative-shift in VT and increased off-current when the deposition temperature of the ZnO channel layer increased from 150°C to 220°C. These findings were correlated to changes in the crystalline structure and the grain size (29 nm vs. 23 nm) rather than a change in the Voconcentration (19.04% vs. 18.75 %). The I-V characteristics of room-temperature (RT) processed ZnO and IGZO TFTs exhibited a similar trend as the 150°C processed TMO-TFTs. The as-deposited RT ZnO and RT IGZO TFTs showed high resistivity and no switching behavior. After vacuum annealing at 150°C, the RT ZnO TFTs did not show any improvements in the I-V characteristics while the RT IGZO TFTs, displayed semiconducting behavior having I-V characteristics with µ=8.1 cm2/V.s, VT=3.5 V, S.S.=0.51 V/decade, and an Ion/off >106 due to an increase in the Vo concentration from 14.68% to 23.42%,. Lastly, the electrical stability of the drain current under dc bias will be presented.

Novel low-temperature fabrication process for integrated high-aspect ratio zinc oxide nanowire sensors

The authors present a new low-temperature nanowire fabrication process that allows high-aspect ratio nanowires to be readily integrated with microelectronic devices for sensor applications. This process relies on a new method of forming a close-packed array of self-assembled high-aspect-ratio nanopores in an anodized aluminum oxide (AAO) template in a thin (2.5 μm) aluminum film deposited on a silicon substrate. This technique is in sharp contrast to the traditional free-standing thick film methods, and the use of an integrated thin aluminum film greatly enhances the utility of such methods. The authors have demonstrated the method by integrating ZnO nanowires onto the metal gate of a metal-oxide-semiconductor (MOS) transistor to form an integrated chemical field-effect transistor (ChemFET) sensor structure. The novel thin film AAO process uses a novel multistage aluminum anodization, alumina barrier layer removal, ZnO atomic layer deposition (ALD), and pH controlled wet release etching. This new process selectively forms the ZnO nanowires on the aluminum gate of the transistor while maintaining the remainder of the aluminum film intact for other integrated device components and interconnects. This self-assembled high-density AAO template was selectively formed in an ultrasmooth 2.5 μm thick aluminum layer deposited through e-beam evaporation without the electropolishing required in AAO template formation in traditional 100 μm thick free standing films. The resulting nanopore AAO template consists of nanopores of 90 nm in diameter and 1 μm in height at an aerial density of 1.3 × 1010 nanopores/cm2. This thin film AAO template was then filled with ZnO using ALD at 200 °C, forming polycrystalline ZnO nanowires inside the pores. The alumina template was then removed with a buffered NaOH solution, leaving free standing ZnO nanowires of 1 μm height and 90 nm diameter, offering an increase in 38× the surface area over a standard flat ZnO film for sensing applications. The aluminum film remains intact (unanodized) in nonselected regions of the device as well as underlying the ZnO nanowires, acting as the gate of the MOS transistor. The ZnO nanowires were characterized by scanning electron microscopy, energy-dispersive x-ray spectroscopy, and transmission electron microscopy to verify stoichiometry and crystal structure. Additionally, the response of a ZnO nanowire ChemFET was measured using ammonia as a target gas. I-V characterization and transient response to ammonia in the range of 25–200 ppm were examined. The ammonia response to the threshold limit value concentration of ammonia (25 ppm) shows a 56 mV shift in threshold voltage, an overall sensitivity of 14%, an 8 min response time, and a 27 min recovery period. The ZnO nanowire fabrication sequence that the authors present is accomplished at low-temperature (<200 °C) and can be accomplished selectively, making it readily amenable to integration with standard metal-oxide-semiconductor field-effect transistor processing as well as other microelectronic sensors such as surface acoustic wave devices. This new process has initially been demonstrated using ZnO, but is also adaptable to a variety of nanowire materials using appropriate deposition methods as well as selective nanowire release methods. This allows the potential to conveniently fabricate a variety of high-aspect ratio nanowire based microelectronic sensors for a range of applications.

NANOSTRUCTURAL AND OPTICAL PROPERTIES OF HIERARCHICAL ZNO GROWN VIA HYDROTHERMAL METHOD

A simple hydrothermal method was employed to synthesize 3D hierarchical ZnO nanorods deposited on Si (100) substrate at different growth temperatures (110 and 90oC) within 3 h. The structure, mode and composition of hierarchical ZnO nanorods were investigated by XRD, FESEM and EDX spectroscopy. The polycrystalline ZnO nanostructures products were indexed as hexagonal wurtzite structured, while the morphology was urchin like ZnO nanorods with different aspect ratio of nanorods and stoichiometric. The photoluminescence (PL) properties were studied of as-grown ZnO samples dependent on various growth temperatures. The PL results after UV excitation source were shown a single broad Vis emission peak for both samples with absents of UV emission peak. The emission edge of Vis peak was exhibited blue-shift due to increase temperature growth, and that suggest enhancement in hierarchical ZnO nanorods crystallinity. Oxygen deficiency is evidence on the creation various defects types in hierarchical ZnO nanorods. It is responsible on Vis emission bands. The results demonstrate promising future for the hierarchal ZnO nanostructure which could be applied in optoelectronics and gas sensing.

Optical properties of ZnO thin films obtained by heat treatment of Zn thin films on amorphous SiO2 substrates and single crystalline GaSe lamellas

Optical absorption and photoluminescence of polycrystalline ZnO films obtained by thermal oxidation of Zn thin films deposited on amorphous SiO2 (quartz) and (0001) surface of single crystalline GaSe lamellas have been investigated. The absorption edge of submicrometric ZnO films on quartz is determined by direct transitions corresponding to an optical band gap of 3.88eV, at 300K. For ZnO films with thickness between 1.5 and 10μm, the absorption threshold is of excitonic nature. Photoluminescence of polycrystalline ZnO films on amorphous quartz reaches its maximum in the orange spectral range, while that of ZnO films on oriented single crystalline GaSe substrate covers the entire visible range.

Temperature Dependence of Structural and Optical Properties of ZnO Nanoparticles Formed by Simple Precipitation Method

ZnO nanoparticles were successfully synthesized by a simple precipitation method. The effect of growth temperature on the structural and optical properties of the resulting nanoparticles was investigated by transmission electron microscopy (TEM), ultraviolet-visible spectroscopy (UV-VIS) and photoluminescence (PL) spectroscopy. TEM images and selected area electron diffraction (SAED) pattern showed that the nanoparticles were polycrystalline ZnO having hexagonal wurtzite structure. The average size of the nanoparticles were 4.72nm to 7.61nm synthesized at room temperature to 80°C. This result is also consistent with the calculated sizes using effective-mass approximation model. From the optical absorption data, the band gap energy of nanoparticles blue-shifted due to the quantum confinement.

Photoluminescence engineering in polycrystalline ZnO and ZnO-based compounds

The results of the investigations of photoluminescence (PL) in ZnO and ZnO-based composite materials are presented. The PL and PL excitation (PLE) spectra of undoped and doped with I group elements or rear earth ions ZnO polycrystalline films, ZnO, Zn1–xMgxO and ZnMgO–TiO2 ceramics were studied. The structural properties of the samples were investigated with X-ray diffraction. Polycrystalline films were prepared by a screen-printing method and annealed at TS = 500–1000 °C. The films annealed at TS < 800 °C exhibited intense UV emission, whereas defect-related one appeared at 800 °C and enhanced with increasing TS. Improvement of the PL and structural characteristics of ZnO films due to Li-doping were achieved. The PL bands caused by Sm and Ho ions were observed under ZnO band-to-band excitation. In the PL spectra of ZnO and Zn1–xMgxO ceramics, two types of PL bands were separated: i) the bands, whose spectral positions were not influenced by the Mg content (green Cu-related as well as self-activated orange and red ones); ii) the bands, spectral positions of which exhibited some blueshift with increasing Mg content (orange Li- and Ag-related and self-activated green ones). In the PL spectra of ZnMgO–TiO2 composites, an intense red emission was found to appear in addition to the broad blue-orange band inherent in ZnMgO alloy. The red emission was ascribed to Mg2TiO4 inclusions in ZnMgO matrix.

The origin of room temperature ferromagnetism mediated by Co–VZn complexes in the ZnO grain boundary

Ferromagnetism in polycrystalline ZnO doped with Co has been observed to be sustainable in recent experiments.

Identification of extended defect and interface related luminescence lines in polycrystalline ZnO thin films grown by sol–gel process

The luminescence lines related to extended defects and interfaces in polycrystalline ZnO thin films grown by sol–gel process are investigated by combining photoluminescence and cathodoluminescence imaging with high-resolution transmission electron microscopy.

Distribution pattern and allocation of defects in hydrogenated ZnO thin films.

A polycrystalline ZnO thin film prepared by atomic layer deposition was annealed in hydrogen at 10 bar and 350-450 °C. Hydrogenation induced simultaneous formation of oxygen and zinc vacancies whose concentrations were closely related to the temperature of treatment. Spatial distributions of these defects were analyzed by photoluminescence confocal mapping which revealed that their localized appearances are linked to each other. It was also demonstrated that nanomechanical mapping of elastic modulus distribution could be used to assess the allocation of accumulated defects on the topmost surface of ZnO with a depth resolution of only several atomic layers. The higher the temperature of hydrogenation, the higher the concentration, and more uniform the distribution of surface defects. In addition, the correlation between the surface morphology and the accumulated defects was established.

Light emission enhancement from ZnO nanostructured films grown on Gr/SiC substrates

We report on the application of a single layer graphene substrates for the growth of polycrystalline ZnO films with advanced light emission properties. Unusually high ultraviolet (UV) and visible (VIS) photoluminesce was observed from the ZnO/Gr/SiC structures in comparison to identical samples without graphene. The photoluminescence intensity depends non-monotonically on the films thickness, reaching its maximum for 150 nm thick films. The phenomena observed is explained as due to the dual graphene role: i) the dangling bond free substrate, providing growth of relaxed thin ZnO layers ii) a back reflector active mirror of the Fabry-Perot cavity that is formed. The reported results demonstrate the potential of two-dimensional carbon materials integration with light emitting wide band gap semiconductors and can be of practical importance for the design of future optoelectronic devices.

Nanoscale heterogeniety and workfunction variations in ZnO thin films

Nano-roughened, sol–gel derived polycrystalline ZnO thin films prepared by a thermal ramping procedure were found to exhibit different work function values on a sub-micrometer scale. By Kelvin probe force microscopy (KPFM) two distinct nanoscale regions with work function differing by over 0.1eV were detected which did not coincide with the nano-roughened surface topography. In contrast, a flat ZnO surface displayed a single, uniform distribution. Ultraviolet photoelectron spectroscopy (UPS) studies showed that the average workfunction across a flat ZnO surface was 3.7eV while ZnO with a nano-roughened morphology had a lower workfunction of 3.4eV with indications of electronic heterogeneity across the surface, supporting the KPFM results. Scanning Auger Nanoprobe measurements showed that the chemical composition was uniform across the surface in all samples, suggesting the work function heterogeneity was due to variations in crystallinity or crystal orientation on the surface of these thin films. Such heterogeneity in the electronic properties of materials in thin film devices can significantly influence the interfacial charge transport across materials.

Study of Structural Properties of N-Doped ZnO Thin Film Prepared by Reactive Gas-Timing RF Magnetron Sputtering

We examined structural properties of nitrogen doped (ZnO:N) thin films prepared by reactive RF magnetron sputtering technique in conjunction with gas timing method. The deposited films were polycrystalline ZnO in wurtzite structure. Morphology of the ZnO:N films could be modified by adjusting gas timing conditions. The x-ray photoelectron spectroscopy (XPS) and extended x-ray absorption fine structure (EXAFS) analysis showed that incorporation of nitrogen may cause structural distortion in the ZnO:N crystal.

Spectroscopic ellipsometry study of the free-carrier and band-edge absorption in ZnO thin films: Effect of non-stoichiometry

The effect of stoichiometry on the complex dielectric function (ε(E) = ε1(E) + iε2(E)), and thereby on the optical and electrical properties, of rf-sputtered polycrystalline ZnO films was investigated using spectroscopic ellipsometry in the UV-VIS-NIR range. The stoichiometry and the density of the films were quantified by Rutherford backscattering spectroscopy. The lineshape of the dielectric function was fitted using the Tauc-Lorentz multi-oscillator and Drude models. The stoichiometric as well as the sub-stoichiometric films showed a high optical transparency in the visible and a bandgap absorption in the UV region. In the NIR region, however, the sub-stoichiometric films showed a significant increase in absorption with decreasing energy while the absorption was negligible for the stoichiometric films. This difference in behavior is attributed to the presence of free-carriers in the sub-stoichiometric films, whose concentration was determined to be around 4 × 1020 cm−3 from spectroscopic ellipsometry. The high carrier concentration induced by non-stoichiometry is also manifested in a larger value of the optical bandgap. This study shows the power of spectroscopic ellipsometry for the simultaneous determination of the electrical and optical properties of ZnO films and is applicable for a wide range of film thickness.

Influence of Substrate Temperature on the Properties of Nanostructured ZnO Thin Films Grown by RF Magnetron Sputtering

Transparent conducting thin films of ZnO:Ga (GZO) have been deposited onto glass substrates and were prepared by RF-magnetron sputtering from nanoparticles synthesized by the sol–gel method. The preheated substrate temperature was changed from room temperature to 300°C. X-ray diffraction spectra showed that the as-deposited films are polycrystalline ZnO with a hexagonal wurtzite structure. Surface morphology, optical properties (such as transmission, reflectance), and conductivity were investigated. The obtained results revealed that the structures and properties of the films were greatly affected by the substrate temperature. Thin films of GZO have a low resistivity, with a minimum value of 2.20 × 10−3 Ω cm deposited at a substrate temperature of 200°C.

Enhancing ethanol detection by heterostructural silver nanoparticles decorated polycrystalline zinc oxide nanosheets

Noble metals decorated two-dimensional nanostructures with a large surface-to-volume ratio raise substantial promise for many technological applications particularly for high-performance chemical sensing. Here, we design and synthesize a hierarchical heterojunction comprising ultrathin two-dimensional polycrystalline ZnO nanosheets on a large scale, which serve as reaction positions for reduced gases, and uniformly dispersed Ag nanoparticles to enhance reaction activity. We find that the Ag nanoparticles in the heterostructural system act as strong electron acceptors, inducing remarkable surface space charge layers. As a result, the heterostructural system shows significantly improved gas-sensing performance to ethanol as compared to pure ZnO. Such metal nanoparticles decorated two-dimensional polycrystalline ZnO nanosheets can be applied in not only as gas-sensing but also catalytic and photocatalytic fields.

Structural, optical, electrical properties, and strain/stress of electrochemically deposited highly doped ZnO layers and nanostructured ZnO antireflective coatings for cost-effective photovoltaic device technology

High quality polycrystalline and nanostructured ZnO thin films were electrochemically deposited from aqueous solution of zinc nitrate (Zn(NO3)2) at negative electrochemical potential (EC=−0.6 to ‑−1.4V) and moderate temperature (75–80°C) on various substrates (Cu, Si, Mo/glass, ZnSe/Mo/glass, F:SnO2/glass). Undoped (i-ZnO) films were grown free of strain on Cu and Mo/glass using intermediate lattice matched buffer (ZnSe). The i-ZnO films on Si exhibited high tensile in-plane stress of σ=4GPa. Growth rates depended on substrate orientation and electrochemical potential varying from EC=−1.1V for deposition on Si(100) to EC=‑−1.3V for optimum deposition on Si(111). Growth rates of undoped and doped (n-ZnO) films with Al and In dopant (Al:ZnO, In:ZnO) on Mo/glass depended on the solute dopant (AlCl3, InCl3) concentration (0.2nm/s for i-ZnO, 0.7nm/s for n-ZnO with n=5mM, 1.2nm/s for n-ZnO with n=9mM). Hydrostatic compressive strains by incorporation of 0.4–12.0 at.% Al were in the range of εh=‑−0.070 to ‑−2.000. The resistivity of metal-pin contacted n-ZnO/i-ZnO films (Me/Al:ZnO/i-ZnO/Mo/glass, Me/In:ZnO/i-ZnO/ZnSe/Mo/glass, Me: Au/Ni) was in the order 105Ω·cm. In the UV–VIS-NIR region (1.5–5.0eV), the reflectivity of the films did not exceed 20% and the (optical) band gap of Eg=3.5eV indicated their high optical quality and material purity. The optical properties of electrochemically grown ZnO nanorod arrays (transparency: 70-80% at 1.5-3.0eV and band-gap: 3.3–3.7eV) were additionally analyzed by photoreflectance spectroscopy. By application of optical modulation techniques, both spectrally (4meV at 300K) and spatially (7meV/cm) resolved gap energies were quantified and the role of nitric (HNO3) and nitrate (NH4NO3) additives was extensively reinforced. Antireflective coatings (ARCs) of ZnO nanorods assembled in Cu(In,Ga)Se2 thin film solar cells were found to quench the surface reflectivity by 5% at least. Integration of electrochemically processed transparent conductive ZnO films and ZnO ARCs in solar cell device technology is anticipated to provide key-solutions for cost effective energy harvesting.

Limiting factors of carrier concentration and transport of polycrystalline Ga-doped ZnO films deposited by ion plating with dc arc discharge

We develop a deposition technology to enhance carrier concentration (N) together with the improvement of carrier transport of transparent conductive Ga-doped ZnO (GZO) polycrystalline films. 200-nm-thick GZO films were deposited on glass substrates at a temperature of 200°C by ion plating with dc arc discharge. We investigated the effects of oxygen (O2) gas flow rates (OFRs) and discharge current (ID) added between the plasma gun and resource pellet on structural and electrical properties of GZO films. The Ga2O3 contents in the resource pellets was 4.0wt.%. During film growth, O2 gas was introduced into the deposition chamber to control point defects. We found that OFR is the main limiting factor of N and the contribution of grain boundaries (GBs) scattering to carrier transport, whereas ID limits the carrier transport in grain bulk. By optimizing OFRs and ID to obtain high N and Hall mobility (μH) limited by intragrain scattering, we achieved GZO films with an electrical resistivity value of 1.96×10−4Ωcm, an N value of 1.25×1021cm−3 and a μH value of 25.4cm2/Vs.