Oxygen Vacancies In ZnO Films Research Articles

Study on photoelectric properties of Si supported ZnO

The ZnO films were deposited on p-Si(100) substrate to show the stoichiometric coefficient ratio of Zn and O is greater than 1:1, indicating the existence of oxygen vacancies in ZnO films, moreover, the oxidation state of Si2p shows the formation of silicon oxide in the vicinity of Si/ZnO interface (ZnO side), which result from Si diffusion into ZnO film. Different models of Si supported ZnO were constructed based on experimental results to study its photoelectric properties, the numerical investigations reveal that Fermi level of ZnO:Si, ZnO:Si-int, and ZnO-V O all move to conduction band minimum, which means they are beneficial to n type semiconductor. The dielectric function of ZnO–Si-int moves to the lower energy direction (red shift), the conductivity of which is higher in the range of 3eV∼7eV, and its absorption and reflectivity both are higher with wavelength more than 300 nm; however, the optical properties of ZnO:Si are poor, implying that the degradation of photoelectrical properties of Si/ZnO heterojunction result from Si diffusion in ZnO films. • We perform a test on photoelectric property of Si supported ZnO nanofilm. • We perform a numerical approach on photoelectric property of Si supported ZnO nanofilm. • We detected the fantastic change on optical properties of Si supported ZnO nanofilm. • It implies a potential method for design photovoltaic devices.

Analysis of the carrier transport mechanism and barrier height formation of graphene/Zr–ZnO Schottky contact by I-V-T

The graphene/Zr–ZnO Schottky contacts were prepared by sol-gel method, and the electrical properties of Schottky contact were analyzed by thermionic emission model. The results showed the oxygen vacancies in ZnO films decrease due to Zr ion doping. Meanwhile, the leakage current of graphene/Zr–ZnO Schottky contact reduces and barrier height increases, which is mainly related to the reduction of defect levels at the interface owing to Zr ion doping. By the I-V-T measurement, the result showed that the barrier height increases and the ideal factor decreases with the raising of temperature from 160 K to 300 K, which can be explained by the lateral inhomogeneities of barrier height that was confirmed with gauss distribution function.

Study on interfacial interaction between Si and ZnO

The interfacial reaction and bonding mechanism of Si based undoped and Al-doped ZnO films were investigated experimentally and numerically. Si/ZnO heterojunctions were fabricated onto p-Si(100) wafers by RF magnetron sputtering, then the undoped and Al doped ZnO (AZO) films coated Si substrates were annealed in argon atmosphere (ZnO-Ar and AZO-Ar) and air atmosphere (ZnO-Air and AZO-Air) respectively. X-ray diffraction (XRD) and Fourier transform infrared spectra (FTIR) show the prepared Si-based films have wurtzite structure with (002) preferential growth, and demonstrate several Si oxides exist in both cases. X-ray photoelectron spectroscopy (XPS) finds that oxygen vacancies exist in ZnO-Ar, moreover, the stoichiometry between Zn and O in AZO-Ar film to be close to 1:1, and high oxygen content in ZnO-Air, which are also supported by Hall measurement. Furthermore, the Si 2p of XPS spectrum shows the width of interfacial interaction between Si and ZnO become weaker with the order of ZnO-Ar, AZO-Ar and ZnO-Air. To elucidate the joining and bonding mechanism between Si substrate and ZnO coating layer, molecular dynamics and first principle simulations were performed in this work, the simulation results show certain interstitial Si atoms coming from Si substrate interact with O atoms related to the Zn–O bonding at the Si/ZnO interface, and resulting in oxygen vacancies in ZnO film. The experiment and simulation show the interfacial interaction between Si and ZnO depends on oxygen content coming from ZnO coating layer, the limited oxygen coming from ZnO film could increase the width of Si/ZnO interfacial interaction and leave oxygen vacancies in ZnO film due to Si robbing O atoms related to Zn–O bonding.

Oxygen-defective ZnO films with various nanostructures prepared via a rapid one-step process and corresponding photocatalytic degradation applications

The deposition of oxygen-defective ZnO films exhibiting varied nanostructures via Solution Precursor Plasma Spray (SPPS) route, a one-step, minute-scaled duration and large scale method, is reported. The in situ formation of oxygen vacancies in ZnO films was confirmed by UV–Visible, Raman and photoluminescence (PL) spectroscopy and the as-prepared samples exhibit a bandgap as low as 3.02 eV. Density functional theory (DFT) simulation demonstrates that the polarization of ZnO is enhanced by the created oxygen vacancies, leading to substantially improved photocatalytic activity. The comparative experiments also revealed that forming and preserving appropriate ZnO precursor clusters inside the plasma plume is requisite for obtaining propitious ZnO nanostructures, which was followed by the in situ transfer and growth of the clusters on the preheated substrate. The ZnO-NRs films fully degrade the aqueous Orange II dye solutions within 120 min and maintain a quasi-intact activity (95.8% retention) after five test runs, which highlight their good stability. The oxygen vacancies and the narrowing of the bandgap also enable a visible light-driven photodegradation activity with conversions as high as 54.1%. In summary, this work not only reveals that the photocatalytic activity of SPPS-deposited ZnO films benefit from oxygen vacancies and well nanostructures, but also suggests that the SPPS route is of high potential for preparing metal oxides films destined to functional applications.

Substrate effect on the room-temperature ferromagnetism in un-doped ZnO films

Room-temperature ferromagnetism was achieved in un-doped ZnO films on silicon and quartz substrates. Photoluminescence measurement and positron annihilation analysis suggested that the ferromagnetism was originated from singly occupied oxygen vacancies (roughly estimated as ∼0.55 μB/vacancy), created in ZnO films by annealing in argon. The saturated magnetization of ZnO films was enhanced from ∼0.44 emu/g (on quartz) to ∼1.18 emu/g (on silicon) after annealing at 600 °C, as silicon acted as oxygen getter and created more oxygen vacancies in ZnO films. This study clarified the origin of ferromagnetism in un-doped ZnO and provides an idea to enhance the ferromagnetism.

Bipolar switching behavior in TiN/ZnO/Pt resistive nonvolatile memory with fast switching and long retention

Highly stable bipolar resistive switching behaviors of TiN/ZnO/Pt devices were demonstrated for the first time. The excellent memory characteristics including fast switching speed (<20 ns for set and <60 ns for reset), long retention (in the order of 105 s) and non-electroforming process were demonstrated. The bipolar switching behaviors can be explained by formation and rupture of the filamentary conductive path consisting of oxygen vacancies. The excellent bipolar switching behavior can be attributed to the significant amount of oxygen vacancies in ZnO film and the effect of TiN layer serving as an oxygen reservoir.

Structure and UV emission of nanocrystal ZnO films by thermal oxidation of ZnS films

ZnO films prepared by the thermal oxidation of the ZnS films through thermal evaporation are reported. The as-deposited ZnS films have transformed to ZnO films completely at 400 °C. The 400–700 °C annealed films with a preferential c-axis (0 0 2) orientation have a hexagonal wurtzite structure. The band gap of ZnO films shifts towards longer wavelength with the increase of the annealing temperature. The relationship between the band gap energy of ZnO films and the grain size is discussed. The shift of the band gap energy can be ascribed to the quantum confinement effect in nanocrystal ZnO films. The photoluminescence spectra of ZnO films show a dominant ultraviolet emission and no deep level or trap state defect emission in the green region. It confirms the absence of interstitial zinc or oxygen vacancies in ZnO films. These results indicate that ZnO film prepared by this simple thermal oxidation method is a promising candidate for optoelectronic devices and UV laser.

Hydrogen-doped high conductivity ZnO films deposited by radio-frequency magnetron sputtering

Hydrogen-doped zinc oxide (ZnO:H) films were deposited by rf magnetron sputtering as transparent conductive films. The resistivity of ZnO:H film was significantly reduced by the addition of H2 in Ar during rf sputtering. The electrical resistivity of ZnO:H films reached 2×10−4Ωcm. The carrier concentration increased with increasing H2 concentration during deposition. X-ray diffraction results showed that the d0002 interplanar spacing increased with increasing H2 concentrations. The carrier concentration was significantly reduced in two orders of magnitude by increasing the substrate temperature from 150 to 250 °C during deposition. Both results suggested that the increase of carrier concentration by adding H2 during sputtering was due to the hydrogen donor rather than the oxygen vacancies in ZnO films, consistent with the theoretical predictions by Van de Walle. UV–visible spectroscopy further showed that the transmittance is high up to 100% in the visible range. The band gap determined by optical absorption increased with increasing H2 composition. The phenomenon is interpreted as the filling of conduction band by electrons in n-type semiconductor.