Band Edge Emission Of ZnO Research Articles

Effect of substrate temperature on the morphology, structural and optical properties of Zn1−xCoxO thin films

Zn1−xCoxO thin films with c-axis preferred orientation were deposited on sapphire (0001) by pulsed laser deposition (PLD) technique at different substrate temperatures in an oxygen-deficient ambient. The effect of substrate temperature on the microstructure, morphology and the optical properties of the Zn1−xCoxO thin films was studied by means of X-ray diffraction (XRD), atomic force microscopy (AFM), UV–visible–NIR spectrophotometer, fluorescence spectrophotometer. The results showed that the crystallization of the films was promoted as substrate temperature rose. The structure of the samples was not distorted by the Co incorporating into ZnO lattice. The surface roughness of all samples decreased as substrate temperature increased. The Co concentration in the film was higher than in the target. Emission peak near band edge emission of ZnO from the PL spectra of the all samples was quenched because the dopant complexes acted as non-radiative centers. While three emission bands located at 409nm (3.03eV), 496nm (2.5eV) and 513nm (2.4eV) were, respectively, observed from the PL spectra of the four samples. The three emission bands were in relation to Zn interstitials, Zn vacancies and the complex of VO and Zni (VOZni). The quantity of the Zn interstitials maintained invariable basically, while the quantity of the VOZni slightly decreased as substrate temperature increased.

Characterization of zinc oxide nanoparticles synthesized by polymer assisted deposition method

Zinc oxide nanoparticles (ZNPs) are synthesized onto glass substrates by employing simple and low cost solution based modified polymer assisted deposition (PAD) method. Trionx100 is used as a capping agent and zinc acetate as the zinc source. TritonX100 concentration is varied from 0.02 to 0.45 M for the synthesis of pure ZnO NPs. TG–DTA analysis was employed to determine the decomposition temperature of TritonX100 and zinc acetate, which lead to the formation of ZnO. The films were further characterized by powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscope (HR-TEM), Fourier transforms infrared spectroscopy (FT-IR) and room temperature photoluminescence (PL). The results indicate that the synthesized nanoparticles (NPs) exhibits the room temperature PL with two emission peaks, one corresponding to ZnO band edge emission and the other one to point defect states created due to oxygen deficiency. The first peak undergoes blue-shift due to change in NPs size while there is no shift in the second peak. Nevertheless, with increase in TritonX100 concentration the peak intensity of defect peak decreases, indicating that the highly pure NPs have been successfully synthesized by PAD method.

The improvement of near-ultraviolet electroluminescence of ZnO nanorods/MEH-PPV heterostructure by using a ZnS buffer layer

A near-ultraviolet light emitting diode (LED) consisting of ZnO nanorods/MEH-PPV heterostructure and a layer of ZnS as the modification buffer layer is reported. Specially, the ZnO nanorods are grown on the ZnS hole-injecting buffer layer by hydrothermal method. Then the devices of ITO/ZnS/ZnO/ZnOnanorods/MEH-PPV/Al are prepared. Under forward bias, not only strong near-ultraviolet (N-UV) electroluminescence (EL) at 380 nm of ZnO band edge emission and weak defect-related emissions of ZnO nanorods at 410, 510 and 760 nm are observed, but also the exciton emission of MEH-PPV at 580 nm is detected. Along with increasing the thickness of ZnS film, the emission at 580 nm strengthens. Compared with the device ITO/ZnO/ZnOnanorods/MEH-PPV/Al we have reported in the past, the experimental results show that the suitable insertion of a ZnS thin layer can enhance the N-UV EL. Under the same voltage, the light intensity can be increased up to 10 times more than that of the device without a ZnS layer. Moreover, the turn-on voltage is reduced remarkably. The effect of ZnS as the hole-injecting buffer layer with different thickness on the EL of ZnO nanorods/MEH-PPV heterostructure is investigated.

Structure and photoluminescence of SiC/ZnO nanocomposites prepared by radio frequency alternate sputtering

SiC/ZnO nanocomposites were prepared by radio frequency alternate sputtering followed by annealing in N2 ambient. Well-crystallized ZnO matrix was obtained after annealed at 750 °C according to X-ray diffractometer patterns. Transmission electron microscopy analyses indicated that the SiC thin layer aggregated to form SiC nanoclusters with the average size of 7.2 nm when the annealing temperature was 600 °C. When the annealing temperatures increased above 900 °C, some of the SiC nanoclusters changed into SiC nanocrystals and surfacial atoms of the SiC nanoparticles were surrounded by a layer of SiOx (x ≤ 2) according to the Fourier transform infrared spectrums. The SiC/ZnO nanocomposites annealed at 750 °C exhibit strong photoluminescence bands ranging from 250 to 600 nm. UV light originates from the near band edge emission of ZnO and the blue emission peaked at around 465 nm (2.7 eV) may be due to the formation of emission centers caused by the defects in Si–O network, while the green-emission peak at around 550 nm (2.3 eV) may be attributed to the deep level recombination luminescence caused by the vacancies of oxygen and zinc.

Anodic electrodeposition of ZnO onto p-Si substrates assisted by light irradiation

ZnO was electrodeposited on p-type silicon substrates with the assistance of light irradiation in order to allow the presence of electrons at the interface of the semiconductor/electrolyte. The electrodeposition was performed at −0.9 V versus Standard Hydrogen Electrode (SHE). Scanning electron analysis revealed that the ZnO deposit consisted of small islands of ZnO randomly dispersed on the p-Si surface. The small islands are in the order of the micron size, and cover ~37% of the whole surface after 4 min of deposition. The characteristic E2 vibration mode in the Raman spectroscopy was clearly observed from the as-deposit ZnO. Photoluminescence measurements of the deposit reveal that the near band edge emission of ZnO is ~12 times more intense than the defects emission band around 543 nm, which is attributed to the presence of interstitial oxygen, and may come from the electrodeposition process in an oxygen rich medium.

Characterization of a-plane orientation ZnO film grown on GaN/Sapphire template by pulsed laser deposition

In this study, the authors have investigated the structural and optical properties of ZnO layer grown by pulsed laser deposition on GaN/ r-plane sapphire. X-ray diffraction results demonstrate the ZnO film to be highly preferentially deposited at a-axis orientation; the different rocking curve values along the two orthogonal directions indicate the low C 2 v symmetry in the growth a-plane ZnO. From free stress to large tensile stress (about 1.34 × 10 9 Pa) distribution along the growth direction of ZnO is revealed by visible Raman mapping spectra. The enhanced significantly high-order longitudinal-optical (LO) phonon modes up to 4th and no TO phonons have been observed in Raman spectrum under UV 325 nm by resonance conditions; an intense and broad disorder activated surface phonon mode is also observed, resulting from the increased disorder on the film surface with stripe-like growth features. Low-temperature photoluminescence measurements reveal that the band-edge emission of ZnO is dominated by neutral donor-bound exciton and free electrons to neutral acceptor emissions. Interfacial microstructure of ZnO/GaN has been examined by transmission electron microscopy, with the epitaxial relationship ( 1 0 1 ¯ 0 ) ZnO//( 0 0 0 2 ¯ ) GaN. All these results indicated that GaN template played an important role in the growth of ZnO film, with full advantage of small lattice mismatch.

Electroluminescence of ZnO nanorods/MEH-PPV heterostructure devices

ZnO nanorods are synthesised by a hydrothermal method on ITO glass. Then ITO/ZnO/ZnO nanorod/MEH-PPV/Al heterostructure devices are fabricated. Under DC bias, ultraviolet (UV) electroluminescence (EL) at 380 nm from ZnO band edge emission is observed. In addition, exciton emission of MEH-PPV at 580 nm and defect related emission of ZnO nanorods at 640 nm and 747 nm are also detected. Upon increasing the applied voltage, the UV EL becomes predominant and is much stronger than other emissions at 28 V. The mechanisms of ZnO band edge emission and background emission are discussed.

Spray pyrolytic deposition of ZnO thin layers composed of low dimensional nanostructures

ZnO nanolayers composed of fine nanostructures have been successively grown by spray pyrolytic deposition at 300∘C over amorphous glass substrates. As deposited samples were analysed by scanning electron microscopy (SEM), showing a granular morphology with grain size in the limit of the microscope resolution. CL measurement shows a broad near band edge (3.4 eV) emission of ZnO in the UV region and the defect level emissions in the green region of the spectrum. The use of intermittent spray pyrolytic deposition is shown as an alternative to increase the homogeneity of the samples when temperatures near to the precursor pyrolytic decomposition is selected, long depositions times are involved, and low thermal conductive substrates are used. We have focused on one of these low thermal conductive substrates, glass, on which spheroid shaped microstructures and inhomogeneities appear.

Ultraviolet electroluminescence from ZnO/NiO-based heterojunction light-emitting diodes

Ultraviolet light-emitting diodes based on ZnO/NiO heterojunctions were fabricated on commercially available n+-GaN/sapphire substrates using a radio frequency magnetron sputtering system. Near band edge emission of ZnO peaking at ∼370 nm with a full-width at half maximum of ∼7 nm was achieved at room temperature when the devices were under sufficient forward bias. With the help of an electron blocking i-Mg1−xZnxO(0<x<1) layer inserted between the ZnO and NiO layers, the emission intensity has been much enhanced and the threshold current drops down to ∼23 from ∼70 mA. The results were discussed in terms of the band diagrams of the heterojunctions.

Enhancement of ZnO photoluminescence by localized and propagating surface plasmons

Insulating spacer layers of MgO were used to identify the enhancement mechanisms of the ZnO band-edge and visible luminescence in ZnO-MgO-Ag and ZnO-MgO-Au multilayers. Purcell enhancement of the ZnO band-edge emission by both Ag and Au surface plasmon polaritons is confirmed by demonstrating that the exponential decay of this emission as a function of increasing MgO thickness is consistent with the Ag and Au SPP evanescent decay lengths. Local surface plasmons excited in Ag and Au nanoparticles and rough films are also shown to enhance the ZnO visible donor-acceptor-pair photoluminescence by dipole-dipole scattering, again with an appropriate dependence on the thickness of the MgO spacer layer. We also confirm that both Ag and Au nanoparticles enhance the ZnO band-edge emission by charge transfer when the MgO spacer layer is absent.

NH3 Doping in MOCVD Growth of ZnO Thin Films

AbstractIn this paper effects of NH3 doping on ZnO thin films grown by metal organic chemical vapor deposition (MOCVD) on c-plane sapphire substrates using diethyl zinc (DEZn) and O2 precursors and N2 as the carrier gas have been studied. NH3 flow rates were varied from 0.1% to 4% in the growth runs. All the runs were done at 500°C at 10 Torr pressure.The XRD measurements show a single ZnO (002) peak. Raman data for the samples confirms presence of ZnO:N modes at 275cm−1, 510cm−1 and 575 cm−1 and 645cm−1. The PL results for Zn rich films show weak broad peaks centered at 480nm and 650nm with no ZnO band edge emission, while oxygen rich films show weak ZnO band edge emission and a strong broad orange peak centered at 650nm. Hall effect measurements indicate that all of the as-grown films are highly resistive. Some are weakly p-type with carrier concentration of 4.24 × 1014 cm−3 and mobility of 16.55 cm2/Vs. Annealing in N2 ambient for 60 minutes at 800°C enhances the PL band edge emission and converts all the films to highly conducting n-type, with carrier concentration on the order of 8 × 1018 cm−3, mobility on the order of 12 cm2/Vs and resistivity of 0.063 Ω-cm.

Observation of enhanced defect emission and excitonic quenching from spherically indented ZnO

The influence of spherical nanoindentation on the band edge and deep level emission of single crystal c-axis ZnO has been studied by cathodoluminescence (CL) spectroscopy and monochromatic imaging. Excitonic emission is quenched at the indent site and defect emission in the range of 450–720nm is enhanced. Analysis of CL monochromatic images and spectra suggests that at least two different defect states are responsible for the broad defect emission band. Additionally, the indents result in a strong crystallographic dependence of the defect emission, producing a rosette feature with [112¯0] [21¯1¯0], and [12¯10] orientations that reflect the star-shaped luminescence quenching observed at the excitonic peak (390nm).

Electroluminescence from ZnO nanowire/polymer composite p-n junction

The characteristics of a hybrid p-n junction consisting of the hole-conducting polymer poly(3,4-ethylene-dioxythiophene)-poly(styrene-sulfonate) (PEDOT/PSS) and n-ZnO nanorods grown on an n-GaN layer on sapphire are reported. Spin coating of polystyrene was used to electrically isolate neighboring nanorods and a top layer of transparent conducting indium tin oxide (ITO) was used to contact the PEDOT/PSS. Multiple peaks are observed in the electroluminescence spectrum from the structure under forward bias, including ZnO band edge emission at ∼383nm as well as peaks at 430, 640, and 748nm. The threshold bias for UV light emission was <3V, corresponding to a current density of 6.08Acm−2 through the PEDOT/PSS at 3V.

Optical Properties of Porous ZnO Nanorods Grown by Aqueous Solution Method

ABSTRACTZnO nanorods were synthesized by chemical solution method at low temperature. The surface of nanorods was changed to porous by using thermal annealing and chemical etching. Surface morphology and their optical properties were changed according to annealing and etching condition. Photoluminescence from as-grown ZnO nanorods almost showed defect related emission in wide range from 450∼900 nm. After annealing at 500°C, the band-edge emission of ZnO was observed and emission at visible range was changed to green with decreasing red-orange. The surface morphology of ZnO nanorods was transformed to porous by chemical etching and it led to increase the intensity of band-edge emission about three times. The internal quantum efficiency for porous ZnO nanorods, which was calculated from ratio PL intensity at 10K and 300K, is about 21%. Also, the random lasing at porous ZnO nanorods was occurred at high optical excitation by a photon with traveling inside or outside of porous ZnO nanorod gets amplified by injection second photon before leaving porous ZnO nanorods.

Vertically well aligned P-doped ZnO nanowires synthesized on ZnO–Ga/glass templates

Vertically well aligned P-doped ZnO nanowires were prepared on ZnO-Ga/glass templates at 550 degrees C by reactive evaporation without metal catalysts and the nanowires were found to be single crystalline with the würtzite structure, oriented in the c-axis direction; the P-doping shortened the physical lengths of the ZnO nanowires without changing their diameter, and furthermore, the introduction of P atoms resulted in a much weaker and broader ZnO band edge emission.

Band-edge emission of undoped and doped ZnO single crystals at room temperature

Band-edge emission of ZnO at around room temperature was investigated by measuring the temperature dependence of cathodoluminescence spectra at 20–300 K. Undoped crystals grown by a vapor transport method and Al-doped crystals by flux method were employed to elucidate the effect of doping on luminescence properties. For the Al-doped crystals, the free-exciton emission was weak through out the temperature range T<300 K. The most intense emission peak of the Al-doped crystal was energetically close to bound exciton annihilation emission. On the other hand, for undoped crystals, it was found that the most intense emission peak at room temperature was at E≈Eg−60 meV and this peak was not assignable to free-exciton annihilation emission. It was also found that this peak is not a reason for the reduction in emission efficiency.