Deep Level Emission Research Articles

Gas sensor properties of surface modification on nebuliser spray coated ZnO nanostructure thin films

Zinc oxide (ZnO) thin films deposited on glass substrates at various temperatures 300, 400 and 500°C by the nebuliser spray pyrolysis technique. X-ray diffraction results indicate that the films are polycrystalline in nature having hexagonal crystal structure with preferred grain growth orientation along (002) plane. The crystallite size increases along (002) plane with temperature increase from 350 to 500°C and attains a maximum 109 nm at 500°C. High resolution scanning electron microscopy study shows the formation of good quality film on total substrate surface with uniformly distributed tiny spherical shaped grains at 350°C. The ultraviolet–visible–near infrared spectroscopy confirms the possibility of good transparent ZnO thin films deposited with an average transmission of ∼85% in the visible region. Optical band gap energy increases from 3.19 to 3.27 eV with the substrate temperature of 350–500°C. The depression of near band edge (NBE) emission and deep level emission of the films were estimated through photoluminescence measurements. As the substrate temperature increases, the peak of NBE emission is found to have a blue shift towards lower wavelength side, and the same result is also found from the band gap energy determination of optical transmittance measurement. The complex impedance behaviour of the pure and ZnO sensors at 175°C with 250 ppm of ammonia has been studied.

Morphological and Optical Properties of ZnO Thin Films Prepared by Spray Pyrolysis on Glass Substrates at Various Temperatures for Integration in Solar Cell

ZnO thin films are widely used as antireflection layer in solar cells and can be grown by various techniques implemented at various temperatures. In this contribution, we present the morphological and optical properties of ZnO thin films prepared by spray pyrolysis on glass substrates at various growth temperatures from 350 to 550°C. The surface morphology of the films, analysed by scanning electron microscopy SEM, is modified with substrate temperature. Optical characterizations of ZnO films as a function of temperature were carried out by transmittance and photoluminescence (PL) measurements. Main optical properties are as follow: All the films are highly transparent in the visible region of the electromagnetic spectrum. The average transmission in the visible range (400–800nm) was greater than 80%. The band gap of ZnO films increases with increasing substrate temperatures attributed to the increase of the grain size of thesample.PL spectrum of ZnO films can be divided into the UV emission and the visible broadband emission attributed to the near band edge emission (NBE) and to the deep level emissions (DLE), respectively. The biggest ratio of UV emission on visible emission of the PL intensity is observed for the ZnO thin film deposited at 550°C by the spray pyrolysis technique. Therefore, we suggest that this original result indicate the most suitable growing conditions for obtaining high quality sprayed ZnO thin films with higher luminescence performances.

Effect of precursor solutions stirring on deep level defects concentration and spatial distribution in low temperature aqueous chemical synthesis of zinc oxide nanorods

Hexagonal c-axis oriented zinc oxide (ZnO) nanorods (NRs) with 120-300 nm diameters are synthesized via the low temperature aqueous chemical route at 80 °C on silver-coated glass substrates. The influence of varying the precursor solutions stirring durations on the concentration and spatial distributions of deep level defects in ZnO NRs is investigated. Room temperature micro-photoluminesnce (μ-PL) spectra were collected for all samples. Cathodoluminescence (CL) spectra of the as-synthesized NRs reveal a significant change in the intensity ratio of the near band edge emission (NBE) to the deep-level emission (DLE) peaks with increasing stirring durations. This is attributed to the variation in the concentration of the oxygen-deficiency with increasing stirring durations as suggested from the X-ray photoelectron spectroscopy analysis. Spatially resolved CL spectra taken along individual NRs revealed that stirring the precursor solutions for relatively short duration (1-3 h), which likely induced high super saturation under thermodynamic equilibrium during the synthesis process, is observed to favor the formation of point defects moving towards the tip of the NRs. In contrary, stirring for longer duration (5-15 h) will induce low super saturation favoring the formation of point defects located at the bottom of the NRs. These findings demonstrate that it is possible to control the concentration and spatial distribution of deep level defects in ZnO NRs by varying the stirring durations of the precursor solutions.

Dense Plasma Focus Device Based High Growth Rate Room Temperature Synthesis of Nanostructured Zinc Oxide Thin Films

High-energy-density plasmas in plasma focus device are now routinely being used as nonconventional plasma nanotechnology tool with novel processing and deposition parameters which are not available in conventional lowtemperature plasma facilities. This paper reports the synthesis of multilayered polycrystalline intrinsic defects-free nanostructured zinc oxide (ZnO) thin film onto room-temperature silicon substrates at high growth rates using pulsed high-energy-density dense plasma focus device. The device was operated in pure oxygen environment and the ablation target used was a pure zinc rod fitted onto the anode top. Thin films of ZnO were deposited at room temperature using different numbers of dense plasma focus shots and at different distances from the anode top. The as-deposited ZnO showed the formation of polycrystalline hexagonal phase ZnO thin films with strong c-axis orientation. The scanning electron microscopy results showed either uniformly laid nanoparticles or cauliflower aggregates of nanoparticles on the surface of the thin-film samples with average nanoparticle size varying between 23.3 ± 6.1 and 69.7 ± 16.5 nm. The average thicknesses of the multilayered thin films were measured to be in a range of 1.06 ± 0.14 to about 2.82 ± 0.32 μm. The complete disappearance of deep level emission band in photoluminescence spectrum of ZnO thin-film sample deposited using 30 focus shots at deposition distance of 15 cm showed successful synthesis of high crystalline quality intrinsic defects-free as-deposited ZnO thin film. One of the key features of the ZnO thin-film synthesis using nonconventional plasma focus device is the very high average growth rate of about 0.1 μm/shot. At one plasma focus shot per minute operation, the average growth rate comes out to be 0.1 μm/min, which is either higher or comparable with commonly used ZnO deposition methods; but at 10 Hz operation extreme growth rates of about 60 μm/min could be achieved.

Defect-assisted tuning of electroluminescence from p-GaN/n-ZnO nanorod heterojunction

Growth of nanostructured ZnO by solution process always lead to the formation of various kinds of defects. Defect states also can aid in improving different properties of the material. In the case of light-emitting diodes (LEDs), major research is focused on tuning the emission colour so as to achieve white emission without the use of any phosphors. Vertically aligned ZnO nanorods were grown over Mg:GaN substrate by hydrothermal process. High-resolution X-ray diffraction (HRXRD) analysis confirms the epitaxial growth of nanorods over the substrate. The photoluminescence (PL) studies revealed a narrow near band edge emission and a broad defect-induced deep level emission. The intensity of deep level emissions related to Zni, Vo, O i defects decreases on annealing. The V–I characteristics of the heterojunction showed excellent rectifying nature with electroluminescence emission on forward bias. Device fabricated by as-grown ZnO nanorods emits in the UV–blue region and broad emission in the visible region. While the annealed device emitted only in UV–blue region. The emission wavelengths closely matched with that of defect state emissions obtained in the PL studies. By annealing, various defect states density can be controlled, thereby emission colour tuned from white to blue.

Growth Mechanism and Properties of Multibranched ZnO Nano/Microstructure

Without any catalysts-assisted, homobranched ZnO nano/microstructure had been synthesized via a simple low temperature water bath method. Such nano/microstructure which consists of shuttle-like nano/microrod backbone was surrounded by six radial oriented branches. The length of the shuttle-like ZnO nano/microrods is about 5–10 μm. The diameter of the single shuttle-like nano/microrods is 1–2.5 μm at the stem and 10–50 nm at the tip while the branches size from top to bottom is 10–50 nm to 0.5–1 μm. A strong ultraviolet emission and weak deep level emission reflect the high optical quality. In addition, a detailed discussion regarding the probable growth mechanism is presented in this work.

Microstructural evolution of sputtered ZnO thin films with rapid thermal annealing

Zinc oxide thin films were deposited on silicon substrates by reactive RF magnetron sputtering technique. Post-deposition rapid thermal annealing of the sputtered thin films was carried out by varying temperatures, annealing duration and oxygen flow rate. The films, annealed at 1000 °C for 150 s in air ambient, have shown highest degree of crystallinity. The surface of the ZnO films, annealed for longer period, was greatly modulated with the evolution of porous surface. The films annealed in oxygen ambient have shown smoother morphology with the reduction in surface roughness. The characteristic absorption band of Zn–O became prominent due to the increase in Zn–O bond density during rapid thermal annealing process. A significant reduction of the deep level emission in the photoluminescence spectra was observed for annealed samples, whereas the near band edge ultraviolet emission was suppressed for the films annealed in oxygen ambient due to the oxygen adsorption at the film surface.

The influence of growth temperature on structural and optical properties of sputtered ZnO QDs embedded in SiO2 matrix

We report the influence of growth temperature on surface morphology, structural and optical properties of ZnO QDs embedded with SiO2/Si matrix fabricated by radio frequency (RF) magnetron sputtering method. The fragmentation due to elastic strain relaxation compensate adatom diffusion length at higher temperature causes the decrement of the ZnO QDs sizes from ∼41nm to ∼12nm and number density enhancement from ∼0.2 to ∼15.4×1010cm−2 by increasing the growth temperature. ZnO QDs shows a well-defined hexagonal close packed wurtzite structure with lattice parameters close to those of bulk ZnO, confirming their high crystalline quality. Increasing growth parameters causes to decrease the lattice parameters due to change in interatomic distances explained by elimination of defects and structural relaxation. The room temperature photoluminescence (PL) spectra shows strong UV accompanied by weaker green peak originated from recombination of free excitons and dominative deep-level emissions respectively. As the growth temperature increased to 500°C, an emission intensity in the ultraviolet and green region enhanced which is due to increment in the numbers of photo-carriers by increasing the number density of QDs. Calculated band gap using an optical transmittance measurement indicates that the bandgap shift to higher energies is not taken placed because of large size of dots. The Urbach energy increases considerably in samples with increasing growth temperature which is attributed to higher degree of surface disorder in smaller QDs. The excellent features of the results suggest that our systematic analysis method may constitute a basis for the tunable growth of ZnO QDs suitable in nanophotonics.

Surface Plasmon Resonance-Enhanced Luminescence in Pd-Functionalized ZnO Nanowires.

Palladium (Pd)-functionalized ZnO nanowires were synthesized by thermal evaporation of a ZnO/graphite powder mixture followed by solution method. The ZnO nanowires had a rod-like morphology with relatively uniform width and length. The widths and lengths of the nanowires ranged from 30 to 100 nm and 5-10 µm, respectively. The diameters of the Pd particles on the nanowires ranged from 5 to 50 nm. Effects of postannealing on the photoluminescence properties of Pd-functionalized ZnO nanowires were examined. Thermal annealing resulted in an increase and decrease in the near-band edge (NBE) and deep level (DL) emission intensities of Pd-capped ZnO nanowires, respectively, whereas both the NBE and DL emission intensities of uncapped ZnO nanowires were increased by annealing. The intensity ratio of NBE emission to DL emission of the Pd-capped ZnO nanowires was increased ~18 fold by annealing in a hydrogen atmosphere. The underlying mechanism for NBE emission enhancement and DL emission suppression of Pd-capped ZnO nanorwires by postannealing is discussed based on the surface plasmon resonance effect of Pd.

Fabrication and Characterization of <I>n</I>-ZnO Hexagonal Nanorods/<I>p</I>-Si Heterojunction Diodes: Temperature-Dependant Electrical Characteristics

This paper reports the temperature-dependant electrical characteristics of n-ZnO hexagonal nanorods/p-Si heterojunction diodes. The n-ZnO hexagonal nanorods were grown on p-Si substrate by a simple thermal evaporation process using metallic zinc powder in the presence of oxygen. The spectroscopic characterization revealed well-crystalline nanorods, quasi-aligned to the substrate and possessing hexagonal shape. The as-grown nanorods exhibited a strong near-band-edge emis- sion with very weak deep-level emission in the room-temperature photoluminescence spectrum, confirming good optical properties. Furthermore, the electrical properties of as-grown ZnO nanorods were examined by fabricating n-ZnO/p-Si heterojunction assembly and the I-V characteristics of the fabricated heterojunction assembly were investigated at different temperatures. The fabricated n-ZnO/p-Si heterojunction diodes exhibited a turn-on voltage of ~5 V at different temperatures with a mean built-in-potential barrier of 1.12 eV. Moreover, the high values of quality factor obtained from I-V analysis suggested a non-ideal behavior of Schottky junction.

Photoluminescence Properties of CdS/ZnO Nano-Composites Structure

Aiming to modification the surface optical properties, CdS/ZnO composites were synthesized by hydrothermal method. The ZnO/CdS heterostructures were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray energy dispersive spectroscopy (EDS), photoluminescence (PL) measurements at room temperature and the temperature-dependent photoluminescence treatment. In the room temperature PL spectra, the defect-related deep level emission can effectively suppress under optimal CdS deposition condition. With the increasing of CdS deposition time to 150 min, the donor bound exciton (D0X) peak showed a significantly red shift at low temperature (77K) PL spectrum. Through analysis the temperature-dependent photoluminescence spectra, we find that it was caused by acceptor bound exciton (A0X). Meanwhile, there appeared a donor-acceptor pair related emission at lower-energy side of the spectrum. The experiment demonstrated that the surface donor defects can effectively suppresse after CdS nanoparticles deposition on ZnO nanorods.

Luminescence and microstructural features of Dy3+-activated KZnPO4 phosphors

A novel Dy3+ activated KZnPO4 phosphor was synthesized by conventional solid-state reaction. The photoluminescence excitation and emission spectra of the phosphor were investigated. The luminescence intensities of KZnPO4:xDy3+ can be enhanced and the chromaticity coordinates were tuned from cold-white to warm-white region with variety of Dy3+ ions dopant. The deep level emissions of ZnO4 presented at different wavelength by different energy excited, that affect the photoluminescence and chromaticity coordinates of KZnPO4:0.8%Dy3+. This paper clearly revealed the effect of ZnO4 on the photoluminescence of KZnPO4:0.8%Dy3+, which is very helpful to the studies on the phosphors containing Zn2+ ion.

Effect of N and Al-N dual doping on optical, photoluminescence and transport properties of ZnO films

N-doped and Al-N dual-doped ZnO thin films have been synthesized on glass substrate by low-cost spray pyrolysis technique. Introduction of N and Al in ZnO affects the surface morphology and electro-optical properties of ZnO. SEM micrographs clearly show the formation of nano-scale semi-spherical and hexagonal crystalline grains. 3D AFM images revealed the formation of crystalline grains with array structure. Transparency of the films increased substantially for N and Al-N dual-doping in ZnO lattice in the measured optical spectrum range (380 to 1000 nm). Optical band gap calculations reveal that the un-doped, N-doped and Al-N dual-doped ZnO thin films are direct band gap semiconductors. The carrier concentration of undoped and Al-N dual-doped ZnO samples is found to be in the order of ∼1015 cm−3. Highly N (>6%) doped ZnO samples shows p-type behavior. Photoluminescence study confirms both near band edge (NBE) and the deep level (DLE) emissions.

Synthesis and Characterization of Zinc Oxide Nanostructures Using <i>Citrus aurantifolia</i> Extracts

Synthesis of ZnO nanostructures has been demonstrated by using fresh extract ofCitrusaurantifoliaand zinc acetate dehydrate as a source material.Citrus aurantifoliaextract was found to be a very effective agent for the reduction of Zn ions. The pH value of growth solution was set up to be 5 (acidic), 7 (neutral) and 9 (alkaline). The structural and optical properties of ZnO nanostructures have been investigated using field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) and Photoluminescence (PL). By varying the pH, all samples exhibit rod-like structure however the length and diameter of the rod was varied. XRD demonstrated that all samples can be indexed to hexagonal wurzite structure with preferential orientation in (101) reflection plane. The room temperature PL spectrum was also analyzed which have shown a deep level emission which related to various intrinsic defects such as Zn interstitials and oxygen vacancies.

Temperature dependent raman and photoluminescence of an individual Sn-doped CdS branched nanostructure

Temperature dependent Raman spectra and photoluminescence, as well the Raman mapping of different parts in an individual Sn-doped CdS comb-like nanostructure reveal that the stronger electron–phonon coupling exist at the trunk-branch junctions than other parts. The Huang–Rhys factor was calculated and further confirms that the strong electron–phonon correlation at the junction. The localized deformation from the Sn dopant in the lattice leads to strong electron–phonon coupling at the local junction, which is proved by the scanning transmission electron microscope and energy dispersion spectrum. Moreover, the lifetime of near-band-edge emission and deep-level emission drastically increase with decreasing temperature, which both relate to the localized electron–phonon coupling and relaxation process. This work provides a clear description of the localized carrier correlation in the cross junction part of these branched nanostructures, which can be used to modulate the optoelectronic performance of micro/nanodevices.

Annealing effects on the structural and optical properties of ZnO nanoparticles with PVA and CA as chelating agents

The effects of annealing temperatures and chelating agents on the structural and optical properties of ZnO nanoparticles were investigated. The average particle size of ZnO nanoparticles increased with increase of annealing temperatures. The decrease of the full width at half maximum (FWHM) with increasing annealing temperatures inferred increase of particle/grain growth. The grain sizes were also observed to be increased with increase of annealing temperatures. From the absorption spectra of the samples, the absorption was red-shifted and the energy band gap was blue-shifted with increase of annealing temperatures. A sharp UV emission peak was observed and the intensity of this peak increased with annealing temperatures corresponding to the high crystallinity in the samples. At high annealing temperature of 700°C, ZnO exhibited a less intense deep level emission. This negligible deep level emission was attributed to the oxygen vacancies created at higher annealing temperatures.

Defect based violet–blue emission of Mg doped ZnO annealed at different temperatures

This paper describes structural and optical characterization of Mg doped ZnO nanoparticles. The system Zn1−xMgxO with x = 0.0, 0.01, 0.03, 0.05, 0.1 were prepared by a simple chemical method and annealing at different temperatures, T = 500, 700 and 900 °C. XRD spectra and the Rietveld refinement show that the crystalline phase of these samples belongs to the wurtzite hexagonal structure of space group P63mc. The PL results show three main emission bands in the violet and blue emission regions. The presence of several emission bands are attributed to free excitation transitions, interstitial Zn, local zinc vacancy, single ionized oxygen vacancy, and deep level emissions. PL studies at 500 °C revealed that visible PL emission was enhanced with doping concentration (x). Mg doped ZnO structures have changed the optical properties significantly.

Annealing-induced change of hydrogen behavior in ZnO nanorods revealed by photoluminescence

We carry out a comprehensive photoluminescence (PL) study on hydrothermally grown ZnO nanorods to provide insights into the role of hydrogen impurity in ZnO materials. Annealing at 1223K greatly modifies the PL properties of the ZnO nanorods, both for the excitonic and deep level emissions. The suppression of excitons bound to H donors as well as enhancement of deep level emission after annealing suggests the removal of H from ZnO lattice. The temperature-dependent behavior of the deep level emissions suggests the formation of VZn–Hn complexes.

Influence of potassium permanganate on the anisotropic growth and enhanced UV emission of ZnO nanostructures using hydrothermal process for optoelectronic applications

The effect of in situ addition of potassium permanganate (KMnO4) in controlling morphology, composition, structural and optical properties of the ZnO nanostructures prepared by hydrothermal technique has been investigated. The influence of synthesis conditions on the growth of ZnO nanorods was meticulously studied by field-emission scanning electron microscope, X-ray diffractometer, transmission electron microscopy (TEM) and high-resolution TEM. It is demonstrated that the KMnO4 concentration has great influence on the morphology and on the alignment of ZnO nanorods. Further the optical properties of nanostructures were investigated by photoluminescence (PL) spectroscopy and ultraviolet–visible diffuse reflectance spectroscopy. The PL spectrum divulged a continuous suppression of defect-related broadband emission by increasing the concentration of the KMnO4, which produced the quenching of surface defects present in the nanorods. The intensity ratio of the peaks corresponding to near-band emission (NBE) to that of deep-level emission of the KMnO4-modified ZnO nanorods was found to increase by eightfold of magnitude. Further it must be noted that nearly 17-fold enhancement in the PL emission of the peak corresponding to NBE was observed in KMnO4-modified ZnO compared to the ZnO grown without any additive. The I–V plot showed dependence of current values under dark and illumination over the amount of KMnO4 added during the growth stage.

Self-assembled ZnO nanoparticles on ZnO microsheet: ultrafast synthesis and tunable photoluminescence properties

We report on the tunable photoluminescence characteristics of porous ZnO microsheets fabricated within 1–5 min of microwave irradiation in the presence of a capping agent such as citric acid, and mixture of citric acid with polyvinylpyrrolidone (PVP). The UV emission intensity reduces to 60% and visible emission increases tenfold when the molar concentration of citric acid is doubled. Further diminution of the intensity of UV emission (25%) is observed when PVP is mixed with citric acid. The addition of nitrogen donor ligands to the parent precursor leads to a red shift in the visible luminescence. The deep level emission covers the entire visible spectrum and gives an impression of white light emission from these ZnO samples. The detailed luminescence mechanism of our ZnO samples is described with the help of a band diagram constructed by using the theoretical models that describe the formation energy of the defect energy levels within the energy band structure. Oxygen vacancies play the key role in the variation of the green luminescence in the ZnO microsheets. Our research findings provide an insight that it is possible to retain the microstructure and simultaneously introduce defects into ZnO. The growth of the ZnO microsheets may be due to the self assembly of the fine sheets formed during the initial stage of nucleation.