Vacancies In ZnO Research Articles

An analysis on optical degradation of ZnO/silicone white paint under proton exposure

The optical degradation mechanism of ZnO/silicone white paint caused by 50 keV protons was investigated in terms of X-ray photoelectron spectroscopy (XPS). The results show that the change in solar absorptance Δ α s increases with increasing proton fluence, and the variation of Δ α s has a good relationship with the formation of the oxygen vacancies in ZnO and the Si–C O functional groups in silicone binder. With increasing the proton fluence, both the area ratio of lattice oxygen peak to XPS O 1s spectrum and the peak area in Zn 3d spectra decrease, indicating that the percentage of lattice oxygen decreases and the amount of oxygen vacancies increases. Meanwhile, the area ratio of the Si–C O peak to XPS Si 2p spectrum increases, demonstrating the formation of Si–C O chromophores in silicone binder. Both the oxygen vacancies and the Si–C O chromophores can lead to an optical degradation of ZnO/silicone white paint under proton exposure.

A comparison of zinc oxide thin-film transistors on silicon oxide and silicon nitride gate dielectrics

We compared the properties of ZnO thin-film transistors in the inverted coplanar geometry on thermally grown silicon oxide gate dielectric to devices on silicon nitride, grown by plasma-enhanced chemical vapor deposition at 150 °C or magnetron sputtering at room temperature. The ZnO semiconductor was sputtered without substrate heating at oxygen partial pressures in the range of 10−5−10−4 Torr. At the lowest oxygen partial pressure, transistor characteristics were similar for all dielectrics. The field-effect mobility approached ∼5 cm2/V s and devices generally operated in depletion mode. With increasing oxygen partial pressure, the mobility decreased by 1000× on SiO2, whereas the decrease on silicon nitride was considerably smaller. On SiO2 the threshold voltage was >30 V but <5 V on silicon nitride. Devices on SiO2 operated in enhancement mode, whereas they operated in depletion mode on PECVD silicon nitride. Photoluminescence of ZnO revealed that deep-level emission depended on the specific dielectric on which ZnO was grown. Green emission for ZnO on chemically vapor-deposited silicon nitride was consistent with the presence of oxygen vacancies in ZnO. Yellow emission for ZnO on silicon oxide was associated with electron acceptor defects, such as oxygen adsorbed in grain boundaries or interstitially in ZnO. We concluded that differences in transistor properties were attributable to modification of ZnO defect chemistry, mediated by growth on a specific dielectric.

The magnetic interaction of Fe doped ZnO with intrinsic defects: A first principles study

By density functional supercell calculations we have studied the magnetic properties of intrinsic vacancies in ZnO doped with diluted Fe impurities. We found that the more energetically favored Fe Zn – V O configuration corresponds to the one in which the two defects are far apart. On the contrary, among the investigated configurations of Fe Zn – V Zn , the one having the lower formation energy corresponds to the complex in which one defect is in the next nearest neighbor position with respect to the other. For Zn-poor growth condition the thermal population of the latter complex provide a key to understand the magnetism of the system.

Bonding of H in O vacancies ofZnO: Density functional calculations

We investigate the bonding of H in O vacancies in ZnO using density functional calculations. We find that H is anionic and does not form multicenter bonds with Zn in this compound.Received 2 April 2007DOI:https://doi.org/10.1103/PhysRevB.75.241102©2007 American Physical Society

Vacancy defects in (Zn, Mn)O

We have used positron annihilation spectroscopy to study the vacancy defects in (Zn, Mn)O crystals grown by chemical vapor transport (CVT). Our results show that Zn vacancies are present in both as-grown and high temperature annealed ZnO and Zn0.985Mn0.015O. In addition, we observe O vacancies in ZnO with no Mn. After annealing in O2 at 1000 ∘C, there is no change in the vacancy distribution in ZnO, while the Zn vacancy concentration increases by an order of magnitude in Zn0.985Mn0.015O.

Properties of the oxygen vacancy in ZnO

As-grown ZnO bulk crystals and crystals annealed in vacuum, oxygen, or zinc vapour were characterized by electrical, optical and magnetic resonance spectroscopy. The experiments show that the residual carrier concentration is caused by residual H, Al, Ga and oxygen vacancies (VO) in the material. Annealing the samples in O2 at about 1000 °C (2 atm, 20 h) reduces the H and VO donor concentration by typically one order of magnitude. The photoluminescence and deep level transient spectroscopy (DLTS) results suggest a correlation between the broad unstructured emission at 2.45 eV (“green band”) and a donor level 530 meV below the conduction band, it is attributed to the VO 0/++transition. By using DLTS experiments with optical excitation it is possible to observe a metastable level 140 meV below the conduction band which is assigned to the VO 2+/+ recharging. The results give evidence for the “negative-U” properties of the oxygen vacancy defects predicted by recent theoretical calculations.

Evidence of oxygen vacancy enhanced room-temperature ferromagnetism in Co-doped ZnO

The annealing effects on structure and magnetism for Co-doped ZnO films under air, Ar, and Ar∕H2 atmospheres at 250°C have been systematically investigated. Room-temperature ferromagnetism has been observed for the as-deposited and annealed films. However, the saturation magnetization (Ms) varied drastically for different annealing processes with Ms∼0.5, 0.2, 0.9, and 1.5μB∕Co for the as-deposited, air-annealed, Ar-annealed, and Ar∕H2-annealed films, respectively. The x-ray absorption spectra indicate all these samples show good diluted magnetic semiconductor structures. By comparison of the x-ray near edge spectra with the simulation on Zn K edge, an additional preedge peak appears due likely to the formation of oxygen vacancies. The results show that enhancement (suppression) of ferromagnetism is strongly correlated with the increase (decrease) of oxygen vacancies in ZnO. The upper limit of the oxygen vacancy density of the Ar∕H2-annealed film can be estimated by simulation to be about 1×1021cm−3.

Negative U‐properties of the oxygen‐vacancy in ZnO

AbstractIt is shown that the intensity of the oxygen vacancy (VO) related emission in ZnO at 2.45 eV correlates to the concentration of the donor level E4. E4 is located 530 meV below the conduction band and attributed to the VO0/++ recharging. Deep level transient spectroscopy (DLTS) experiments with optical excitation locate the VO2+/+ level position 140 meV below the conduction band and give evidence for the “negative‐ U” properties of the oxygen vacancies in ZnO. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Observation of Zn vacancies in ZnO grown by chemical vapor transport

AbstractWe have used positron annihilation spectroscopy to study the vacancy defects in ZnO crystals grown by both the conventional and contactless chemical vapor transport (CVT and CCVT). Our results show that Zn vacancies or Zn vacancy related defects are present in as‐grown ZnO, irrespective of the growth method. Zn vacancies are observed in CVT‐grown undoped ZnO and (Zn,Mn)O. The Zn vacancies present in undoped CCVT‐ZnO are the dominant negatively charged point defect in the material. Doping the material with As introduces also Zn vacancy‐related defect complexes with larger open volume. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoluminescence of ZnO nanoparticles prepared by a novel gel-template combustion process

ZnO nanoparticles have been synthesized by mean of a novel gel-template combustion method. The products were characterized by using TG-DTA, XRD and TEM techniques. The products appeared to be regularly spherical or elliptical and their sizes ranged from 20 to 30 nm.The average particle size increased with increasing sintering temperature. The photoluminescence spectra of the resulting ZnO nanoparticles showed a near-UV emission and a green emission. Furthermore, the dependence of the optical properties on the annealing temperatures and annealing time were also investigated. With the increase of the annealing temperature, the intensity of the green emission at 500 nm caused by oxygen vacancies in ZnO rapidly increased, but almost disappeared only at 600 °C.The probable mechanism was discussed.

Oxygen vacancies in ZnO

The electronic properties of ZnO have traditionally been explained by invoking intrinsic defects. In particular, the frequently observed unintentional n-type conductivity has often been attributed to oxygen vacancies. We report first-principles calculations showing that the oxygen vacancy VO is not a shallow donor, but has a deep ε(2+∕0) level at ∼1.0eV below the conduction band. The negative-U behavior that causes the 1+charge state to be unstable is associated with large local lattice relaxations. We present a detailed configuration coordinate diagram, which allows us to provide a detailed interpretation of recently reported ODEPR (optically detected electron paramagnetic resonance) measurements [L. S. Vlasenko and G. D. Watkins, Phys. Rev. B 71, 125210 (2005)].

Electrical characterization of growth-induced defects in bulk-grown ZnO

We have investigated and compared the defects in ZnO grown by two methods, namely seeded chemical vapour transport (SCVT) and melt-growth (MG), using conventional deep level transient spectroscopy (DLTS) and high resolution (Laplace) DLTS with Au and Ru Schottky barrier diodes. Both materials contained two prominent defects, E1, at E C –0.12 eV and E3 at E C –0.29 eV. The SCVT ZnO has E1 as the main defect with a concentration of about 10 16 cm −3, while the MG ZnO has E3 as the main defect with a concentration of above 10 16 cm −3. It has been speculated that this defect is the oxygen vacancy in ZnO. High resolution Laplace DLTS suggests that this level could consist of two closely spaced levels but with different capture cross-sections. The relative concentrations of these defects were found to vary across the region probed by DLTS. The E1 and E3 defects also showed opposite trends in an electric field: an increase in electric field enhanced emission from E1 whereas it slowed down emission from E3. The peak splitting and field dependence may, however, be a consequence of a non-exponential transient. Finally, etching in HCl:H 2O did not affect the defect concentrations or introduce additional defects in ZnO in the MG ZnO.

HRTEM observation of interface states between ZnO epitaxial film and Si(1 1 1) substrate

ZnO epitaxial thin films could be formed by oxidation of ZnS epitaxial thin films deposited on Si substrates by electron-beam evaporation. The orientation relation of the ZnO film was (0 0 0 2), [1 1 −2 0]ZnO//( 1 1 1), [1 −1 0]Si. The ZnS films were oxidized from its surface toward the surface of the Si substrate gradually. The ZnS film with a thickness of about 100 nm was completely changed to ZnO by annealing at 720 °C for 10 min in O 2 atmosphere. By excess annealing, longer than 30 min, an intermediate layer was formed at the interface between the ZnO layer and Si(1 1 1) substrate. Exciton emission with a peak at 3.27 eV from ZnO became dominant and visible emission due to oxygen vacancy in ZnO disappeared by the annealing of the film at 800 °C for 5 h in O 2 flow.

Suppression of the green photoluminescence band in ZnO embedded into porous opal by spray pyrolysis

The photoluminescence (PL) and transmittance characteristics of the zinc oxide embedded into voids of FCC sub-micron packed silicon dioxide spheres by using technologically simple and inexpensive spray pyrolysis are reported. The uniform formation of ZnO nanocrystalline particles inside of the porous opal takes place after deposition in aqueous solution with zinc nitrite hexahydride precursor followed by thermal annealing. The decrease of green PL is observed due to the inhibition of spontaneous emission through oxygen vacancies in ZnO. The strong red shift of the transmittance characteristics signifies the essential filling of voids in the opal matrix.

Suppression of the green photoluminescence band in ZnO embedded into porous opal by spray pyrolysis

The photoluminescence (PL) and transmittance characteristics of the zinc oxide embedded into voids of FCC sub-micron packed silicon dioxide spheres by using technologically simple and inexpensive spray pyrolysis are reported. The uniform formation of ZnO nanocrystalline particles inside of the porous opal takes place after deposition in aqueous solution with zinc nitrite hexahydride precursor followed by thermal annealing. The decrease of green PL is observed due to the inhibition of spontaneous emission through oxygen vacancies in ZnO. The strong red shift of the transmittance characteristics signifies the essential filling of voids in the opal matrix.

Ultraviolet detection with ultrathin ZnO epitaxial films treated with oxygen plasma

We have investigated the effects of oxygen plasma treatment on the UV detection properties of ultrathin (∼20-nm-thick) ZnO epitaxial films. Highly epitaxial ZnO films grown on sapphire were exposed to oxygen-radical-rich, inductively coupled plasma, and then their UV detection properties were characterized at 325 nm wavelength using a photoconductor structure. The oxygen plasma treatment is found to dramatically enhance the UV detection properties of ZnO, reducing the decay time constant (to below 50 μs) and increasing the on/off ratio of photocurrent (to over 1000) with high UV responsivity (1–10 A/W). This result, in conjunction with the microstructural and electrical characterization results, indicates that the plasma treatment efficiently suppresses the chemisorption sites (primarily the oxygen deficiency sites) on surface and also the oxygen vacancies in ZnO, therefore results in major reduction of the chemisorption effects and the dark current, respectively.

The visible luminescent characteristics of ZnO supported on SiO2powder

In situ laser-induced luminescence spectroscopy is used to study the visible luminescent characteristics of ZnO during the preparation process of ZnO supported on SiO2 by the pyrolysis of different Zn precursors in N2 or O2 atmosphere. The excitation source is 325 nm light, which is above the band gap (3.37 eV) of ZnO. In N2 atmosphere, it is shown that green (centered at ca. 520 nm), yellow (centered at ca. 580 nm) and orange (centered at ca. 640 nm) luminescence bands appear for ZnO produced from zinc acetate, zinc hydroxide and zinc nitrate, respectively. After these samples are treated by O2, green band is changed into yellow band and yellow band is changed into orange band. On the other hand, it is also found that the laser irradiation on the sample could alter the luminescent behavior of ZnO produced at the beginning decomposition temperature of the Zn precursors. While this sample is irradiated, the orange band is gradually changed to a yellow band, the luminescent intensity finally increases more than 30 times that at the beginning of irradiation. However, irradiation hardly affects the luminescent properties of ZnO after calcination above 160 °C. The results indicate that the visible luminescence from ZnO is associated with the oxygen vacancies in ZnO, and the electronic state levels responsible for the visible luminescence bands are changing with the density of oxygen vacancies in ZnO. The green, yellow and orange bands are ascribed to the state of ZnO with high density of oxygen vacancies, with moderate density of oxygen vacancies and with less oxygen vacancies, respectively.

Oxygen vacancies in ZnO

We present our investigations on the absorption and emission properties of the neutral oxygen vacancy in ZnO. Temperature-dependent photoluminescence (PL) experiments allow to determine the zero-phonon-energy of the emission and its phonon-coupling parameters. The absorption to the singlet excited state is observed in PL-excitation experiments at energies above 3.1eV. Relaxation drives the system from the singlet, diamagnetic, excited state to the emissive, paramagnetic triplet state. Applying the Mollwo–Ivey relation for the energetical position of the F-centre absorption as a function of the anion–cation distance, we find that the oxygen vacancies in ZnO fit perfectly in the line given by MgO, CaO, SrO and BaO.

The effect of NiO doping on the stoichiometry of ZnO

The additions of NiO (0–2.0 mol%) to ZnO result in a decrease of the parameter c of its crystal lattice and its high-frequency electrical conductivity and in an elimination of the ESR band with g = 1.96. The observed changes were ascribed to the formation of Zn 1− x Ni x O solid solution, where NiO additions decrease the concentration of oxygen vacancies in ZnO.

An effect of heat treatment on the real structure of components of ZnO-SnO2 mixtures

The effect of heat treatment (24 h at 500° C) on the structure and properties of ZnO-SnO2 mixture with 0 to 10 mol % SnO2 was studied. An increase in the SnO2 content results in a decrease of the lattice parameters of ZnO, increase of electrical conductivity and a decrease of the intensity of green luminiscence in the heat-treated ZnO-SnO2 samples. These results are explained in terms of the interaction of interstitial Zni, and Zni' atoms with oxygen atoms from the SnO2 layer formed on ZnO grains, which results in a decrease of ZnO lattice parameters. Simultaneously, oxygen atoms from SnO2 occupy uncharged oxygen vacancies in ZnO, thus decreasing its green luminiscence. The described interaction is supposed to result in the formation of amorphous SnO2-x on the surface of ZnO crystallites.