ZnO Band Edge Research Articles

Influence of phosphorus doping and post-growth annealing on electrical and optical properties of ZnO/ c-sapphire thin films grown by sputtering

Electrical and optical properties of phosphorus, P, doped ZnO thin films, which were grown on c-plane sapphire substrates, are studied using the Hall-effect measurement, secondary-ion mass spectroscopy (SIMS), X-ray photoelectron spectroscopy (XPS), and optical absorption spectroscopy. An increase in P doping concentration converts ZnO thin films from insulating to n-type semiconducting with large resistivity (∼10 2 Ω cm) at room temperature. Rapid thermal annealing (RTA) reduces the resistivity by several orders of magnitude. The core levels of Zn3s and Zn2p 3/2, probed by XPS using C1s as reference, are shifted monotonically to larger energies with increasing P doping concentration. An analysis of the optical absorption spectra near the band edge of ZnO reveals an exciton absorption peak at 3.37 eV, which is independent of P doping. RTA also induces a large-energy shift for the XPS core levels; however, the optical absorption spectra show an apparent blue-shift upon annealing. The correlations between the electrical and optical properties are related to an existence of a near-surface electron accumulation layer as well as donors' activation by RTA.

Charge transfer dynamics in Cu-doped ZnO nanowires

Time resolved photoluminescence (TRPL) and transient absorption (TA) spectroscopy reveal an ultrafast charge transfer (CT) process, with an electron localization time constant 39±9 ps, between the ZnO host and the Cu dopants in Cu-doped ZnO nanowires. This CT process effectively competes with the ZnO band edge emission, resulting in the quenching of the ZnO UV emission. TRPL measurements show that the UV decay dynamics coincides with the buildup of the Cu-related green emission. TA measurements probing the state-filling of the band edge and defect states provide further support to the CT model where the bleaching dynamics concur with the TRPL lifetimes.

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.

Tuning Electron Transfer Rates via Systematic Shifts in the Acceptor State Density Using Size-Selected ZnO Colloids

We report direct measurements of the influence of the available density of acceptor states on the rate of near-barrierless electron transfer between a dye sensitizer and an oxide semiconductor. The electron donor was the excited state of a zinc porphyrin, and the acceptors were a series of size-selected ZnO nanocrystals. The available density of states was tuned by controlling the relative position of the ZnO band edge using quantum confinement. The resulting change in the rate was consistent with a simple model of the state density as a function of energy above the ZnO band edge.

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.

Effect of post-annealing on structural and optical properties, and elemental distribution in heavy Eu-implanted ZnO thin films

The effect of post-annealing on ZnO thin films implanted with Eu at 1 × 1017 ions/cm2 was studied to characterize the structural and luminescent properties, and the distribution of Eu in thin films. After post-annealing at 600°C, the broadening of the ZnO(002) diffraction peak was the same as that of the as-implanted sample. The sample showed orange luminescence due to Eu3+. After post-annealing at 900°C, a sharp diffraction peak of ZnO(002) was observed. The orange luminescence due to Eu3+ had disappeared, and instead, the intense luminescence of the band-edge of ZnO was observed in the sample. Ion images of Zn and Eu indicated that the Eu separated from the ZnO during the post-annealing. It is thought that the difference in luminescence is due to the separation of Eu from ZnO.

Cr을 첨가한 ZnO의 결함과 입계 특성

In this study, we investigated the effects of Cr dopant (1.0 at% <TEX>$Cr_2O_3$</TEX> sintered at <TEX>$1000^{\circ}C$</TEX> for 1 h in air) on the bulk trap (i.e. defect) and interface state levels of ZnO using dielectric functions (<TEX>$Z^*$</TEX>, <TEX>$M^*$</TEX>, <TEX>$Y^*$</TEX>, <TEX>$\varepsilon^*$</TEX>, and <TEX>$tan{\delta}$</TEX>), admittance spectroscopy (AS), and impedance-modulus spectroscopy (IS & MS). For the identification of the bulk trap levels, we examine the zero-biased admittance spectroscopy and dielectric functions as a function of frequency and temperature. Impedance and electric modulus spectroscopy is a powerful technique to characterize grain boundaries of electronic ceramic materials as well. As a result, three kinds of bulk defect trap levels were found below the conduction band edge of ZnO in 1.0 at% Cr-doped ZnO (Cr-ZnO) as 0.11 eV, 0.21 eV, and 0.31 eV. The overlapped defect levels (<TEX>$Zn^{..}_i$</TEX> and <TEX>$V^{\cdot}_0$</TEX>) in admittance spectra were successfully separated by the combination of dielectric function such as <TEX>$M^*$</TEX>, <TEX>$\varepsilon^*$</TEX>, and <TEX>$tan{\delta}$</TEX>. In Cr-ZnO, the interfacial state level was about 1.17 eV by IS and MS. Also we measured the resistance (<TEX>$R_{gb}$</TEX>) and capacitance (<TEX>$C_{gb}$</TEX>) of grain boundaries with temperature using impedance-modulus spectroscopy. It have discussed about the stability and homogeneity of grain boundaries using distribution parameter (<TEX>$\alpha$</TEX>) simulated with the Z"-logf plots with temperature.

Effect of ITO buffer layers on the structural, optical and electrical properties of ZnO multilayer thin films prepared by pulsed laser deposition technique

ZnO/ITO/ZnO/ITO/ZnO five layer thin films are fabricated by pulsed laser deposition on quartz substrate kept at a substrate temperature ( T s) 873 K at a background oxygen pressure ( pO 2) of 0.05 mbar for different deposition duration for ZnO layers (5, 10, 15 and 20 min) while keeping a constant deposition time for ITO layers. The structural, optical and electrical properties of the as-deposited thin films have been investigated by GIXRD, AFM, UV–visible spectra, photoluminescence spectra and temperature-dependent electrical resistivity measurements (10–300 K) using four-probe technique. XRD patterns of the films show a polycrystalline nature. From the AFM images, the average grain size and mean surface roughness are estimated and the particles are densely packed in the film. A high transmittance is observed for all the films in the visible and IR region. The PL spectra show that the emission is in the UV and visible region due to the near band edge and deep level transitions of ZnO. The electrical resistivity of all the films are calculated and are very low compared to the reported values of as deposited ZnO thin films. The co-existence of very high transmittance in the visible region and very low dc resistivity enables these films suitable for optoelectronic device fabrications. The structural, optical and electrical studies reveal that ITO buffer layers improve the crystalline quality, optical and electrical properties of ZnO multilayered thin films.

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.

Photoluminescence and built-in electric field in ZnO∕Mg0.1Zn0.9O quantum wells

Photoluminescence study of ZnO∕Mg0.1Zn0.9O quantum wells with graded well width (Lw) was carried out at 4.2K. The emission evolution from quantum confinement regime to quantum-confined Stark regime was observed clearly. For large Lw, the emission splits into two peaks which are attributed to the emissions of ZnO band edge and separately localized carriers, respectively. The internal electric field in the well layer was estimated to be ∼0.3MV∕cm, being similar to previous reports. The results are useful in designing ZnO QW based optoelectronic devices.

Magneto-optical and transport studies of ZnO-based dilute magnetic semiconductors

Thin film samples of ZnO doped with V were grown on sapphire substrates by pulsed laser deposition (PLD). The magnetization was measured by SQUID magnetometry and the films were found to be ferromagnetic at room temperature. The transmission, Faraday rotation and magnetic circular dichroism were measured as a function of frequency at room temperature over an energy range of 1.5–4.0 eV and carrier concentrations were determined from Hall effect measurements. Clear magneto-optical signals that are ferromagnetic in origin were observed at the ZnO band edge and the optimal conditions for observing large ferromagnetic magneto-optic signals are discussed.

Room-Temperature Magneto-Optics of Ferromagnetic Transition-Metal-Doped ZnO Thin Films

Magneto-optic studies of ZnO doped with transition metals Co, Mn, V, and Ti indicate a significant magnetic circular dichroism (MCD) at the ZnO band edge at room temperature, together with an associated dispersive Faraday rotation. Similar spectra occur for each dopant, which implies that the ferromagnetism is an intrinsic property of the bulk ZnO lattice. At 10 K, additional paramagnetic contributions to the MCD are observed, but above about 150 K, the magnitude of the MCD signal is dominated by the ferromagnetism and is almost temperature independent. The MCD at the ZnO band edge shows room temperature hysteretic behavior.

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 &amp;lt;3V, corresponding to a current density of 6.08Acm−2 through the PEDOT/PSS at 3V.

Growth of ZnO nanocrystals in silica by rf co-sputter deposition and post-annealing

Thin films with ZnO nanocrystals in silica were synthesized by rf reactive magnetron co-sputter deposition and post-annealing. The films were deposited from a ZnO/Si composite target in an rf oxygen plasma. The deposited films were annealed in air/vacuum at high temperatures to grow ZnO nanocrystals. The deposited and annealed films were characterized by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), uv–vis spectroscopy (UV–VIS) and photoluminescence (PL) measurements. FT-IR results of the films show the vibrational features of Si–O–Si and Zn–O bonds. UV–VIS spectra of the deposited film shows the band edge of ZnO. The XRD results of the films annealed at 750°C and 1000°C indicate the growth of ZnO nanocrystals with average crystallite sizes between 7nm and 26nm. PL measurements of the deposited film show a broad visible luminescence peak which can be due to ZnO. These results suggest the growth of ZnO nanocrystals in silica matrix.

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.

Impurity band characteristics near the band edge of Al-doped ZnO

The characteristics of impurity bands near the band edge of Al-doped and undoped ZnO ceramics were investigated by photoluminescence, photoluminescence excitation, and x-ray diffraction. We found that Al(0.6%)-doped ZnO had two impurity bands whose binding energies were roughly 13 and 99meV below the effective band edge. Also, we found that Al(1.1%)-doped ZnO had an impurity band of ∼80meV binding energy below the effective band edge. As the doping concentration of ZnO increases, Al-impurity bands degenerated from two localized levels to the single localized level. The green band emission of Al(0.6%)-doped ZnO is stronger than those of the pure and Al(1.1%)-doped ZnO because of the high charge transfer rate to the effective band.

Luminescence of Zinc Oxide Crystals Controlled by Electrode Potential and Electrochemical Reactions

The voltage dependence of the green luminescence of ZnO crystals used as electrodes in an electrochemical cell was investigated and compared with the voltage dependence of the photocurrent. When the exciting light is of shorter wavelength than the band edge of ZnO, this luminescence can be varied from its maximal value to complete extinction by applying small positive voltages. The luminescence resulting from excitation within the long wavelength tail of absorption, however, cannot be influenced. The potential dependence of the luminescence has been investigated with differently doped crystals under various conditions. The addition of formic acid to the electrolyte causes an increase of luminescence; this is found to be a consequence of photoelectrochemically induced radical reaction by which electrons are injected into the conduction band of the electrode. The potential dependence of the green luminescence is explained by a mechanism in which the trapping of the holes in recombination centers competes with their extraction, which leads to the generation of the photocurrent. The average concentration of photogenerated holes in the crystal is therefore rate limiting for the intensity of luminescence, provided that an excess of electrons is available for recombination. At sufficiently large positive potentials, however, when electrons are being depleted in the space charge layer, their concentration also becomes limiting, and electron injection can stimulate luminescence. The experimentally observed potential dependence of luminescence and photocurrent, as well as their interrelation, is shown to be consistent with results which were derived from a theoretical treatment of these effects. It is suggested that this green luminescence may be used as a probe for the study of the space charge layer and of electrochemical reactions.