Ultrathin ZnO Films Research Articles

Effects of free electrons and quantum confinement in ultrathin ZnO films: a comparison between undoped and Al-doped ZnO

Band gaps and exciton binding energies of undoped and Al-doped ZnO thin films were determined from optical absorption measurement based on the Elliott's exciton absorption theory. As compared to the undoped films, the doped films exhibit a band gap expansion and a reduction in the exciton binding energies due to the free electron screening effect, which suppresses the excitonic absorption and results in a blue shift of the absorption edge. The undoped and doped films show the same quantum size dependence, i.e. both the exciton binding energies and band gap energies increase with decreasing grain size of the oxides.

CO oxidation over ZnO films on Pt(1 1 1) at near-atmospheric pressures

Well-ordered ultrathin ZnO(0001) films were grown on Pt(111) in a layer-by-layer mode. The reactivity of the films as a function of the film thickness and coverage was examined by the CO oxidation reaction at near-atmospheric pressures. At low temperatures (∼450K), CO2 production is found to be much higher on the films of partial coverage than on dense ZnO(0001) films and bare Pt(111). Under reaction conditions, monolayer islands and an entire monolayer film transform into two-monolayers-thick islands, which dominate the surface of the active catalysts. The results provide an adequate structural model for elucidating the reaction mechanism on the oxide/metal boundary at technologically relevant conditions.

Enhancement of intrinsic emission from ultrathin ZnO films using Si nanopillar template

Highly efficient room-temperature ultraviolet (UV) luminescence is obtained in heterostructures consisting of 10-nm-thick ultrathin ZnO films grown on Si nanopillars fabricated using self-assembled silver nanoislands as a natural metal nanomask during a subsequent dry etching process. Atomic layer deposition was applied for depositing the ZnO films on the Si nanopillars under an ambient temperature of 200°C. Based on measurements of photoluminescence (PL), an intensive UV emission corresponding to free-exciton recombination (approximately 3.31 eV) was observed with a nearly complete suppression of the defect-associated, broad-range visible emission peak. As compared to the ZnO/Si substrate, the almost five-times-of-magnitude enhancement in the intensity of PL, which peaked around 3.31 eV in the present ultrathin ZnO/Si nanopillars, is presumably attributed to the high surface/volume ratio inherent to the Si nanopillars. This allowed considerably more amount of ZnO material to be grown on the template and led to markedly more efficient intrinsic emission.

Field Emission in Vertically Aligned ZnO/Si-Nanopillars with Ultra Low Turn-On Field

An effective method of fabricating vertically aligned silicon nanopillars (Si-NPs) was realized by using the self-assembled silver (Ag) nanodots as natural metal-nanomask during dry etching process. The obtained Si-NPs were preferentially aligned along the c-axis direction. Ultrathin ZnO films (~9 nm) were subsequently deposited on the Si-NPs by atomic layer deposition (ALD) to enhance the field emission property. The average diameter of the ZnO/Si-NPs is in the order of tens of nanometers, which enables efficient field emission and gives rise to marked improvement in the field enhancement factor, β. The turn-on field defined by the 10 μA/cm(2) current density criterion is ~0.74 V/μm with an estimated β ≈ 1.33×10(4). The low turn-on field and marked enhancement in β were attributed to the small radius of curvature, high aspect ratio, and perhaps more importantly, proper density distribution of the ZnO/Si-NPs.

Enhanced Free Exciton and Direct Band-Edge Emissions at Room Temperature in Ultrathin ZnO Films Grown on Si Nanopillars by Atomic Layer Deposition

Room-temperature ultraviolet (UV) luminescence was investigated for the atomic layer deposited ZnO films grown on silicon nanopillars (Si-NPs) fabricated by self-masking dry etching in hydrogen-containing plasma. For films deposited at 200 °C, an intensive UV emission corresponding to free-exciton recombination (~3.31 eV) was observed with a nearly complete suppression of the defect-associated broad visible range emission peak. On the other hand, for ZnO films grown at 25 °C, albeit the appearance of the defect-associated visible emission, the UV emission peak was observed to shift by ~60 meV to near the direct band edge (3.37 eV) recombination emission. The high-resolution transmission electron microscopy (HRTEM) showed that the ZnO films obtained at 25 °C were consisting of ZnO nanocrystals with a mean radius of 2 nm embedded in a largely amorphous matrix. Because the Bohr radius of free-exictons in bulk ZnO is ~2.3 nm, the size confinement effect may have occurred and resulted in the observed direct band edge electron-hole recombination. Additionally, the results also demonstrate order of magnitude enhancement in emission efficiency for the ZnO/Si-NP structure, as compared to that of ZnO directly deposited on Si substrate under the same conditions.

In situ thermal residual stress evolution in ultrathin ZnO and Ag films studied by synchrotron x-ray diffraction

Residual-stress evolution in sputtered encapsulated ZnO/Ag/ZnO stack has been studied in-situ by synchrotron x-ray diffraction when heat treated. The ZnO/Ag/ZnO stack encapsulated into Si 3N 4 layers and deposited on (001) Si substrates was thermally heated from 25 °C to 600 °C and cooled down to 25 °C. X-ray diffraction 2D patterns captured continuously during the heat treatment allowed monitoring the diffraction peak shifts of both Ag (15 nm thick) and ZnO (10 nm and 50 nm thick) sublayers. Due to the mismatch between the coefficients of thermal expansion, the silicon substrate induced compressive thermal stresses in the films during heating. We first observed a linear increase of the compressive stress state in both Ag and ZnO films and then a more complex elastic-stress evolution starts to operate from about 100 °C for Ag and about 250 °C for ZnO. Thermal contraction upon cooling seems to dominate so that the initial compressive film stresses relax by about 300 and 700 MPa after thermal treatment for ZnO and Ag, respectively. The overall behavior is discussed in terms of structural changes induced by the heat treatment.

Influence of growth temperature on the optical and structural properties of ultrathin ZnO films

This study investigates the effect of growth temperature on the optical and structural properties of ultrathin ZnO films on the polished Si substrate. Thickness of the ultrathin ZnO films deposited by atomic layer deposition (ALD) method was about 10 nm. Photoluminescence (PL), X-ray diffraction (XRD), transmission electron microscopy (TEM) and atomic force microscopy (AFM) techniques were used to measure the properties of ultrathin ZnO films. Experimental results showed that the ultrathin ZnO film deposited at 200 °C had excellent ultraviolet emission intensity, and the average roughness of the film surface was about 0.26 nm.

Langmuir-Blodgett Ultrathin Films Composed of ZnO Nanoparticles: Fabrication and Characterization

By the Langmuir-Blodgett(LB) technique, the zinc ion containing multilayer was prepared by transferring the zinc acetate, spread on the surface of subphase of ultrapure water and stearic acid-chloroform mixtures, onto a hydrophilic silicon wafer or glass plate. Then the multilayer was converted into ZnO ultrathin films after pre-heating and annealing. The optimized parameters for monolayer formation, such as concentration of subphase, barrier speed and spreading volume, were determined by the measurement of the surface pressure-surface area (Π-A) isotherms. The expended areas after deposition with zinc ions inferred the interaction of stearic acid with zinc ion during the formation of monolayer at air-water interface. The optimized parameters for multilayer deposition, such as surface pressure and lifting speed were determined by the measurement of the transfer coefficient. The X-ray diffraction(XRD) was used to investigate the crystal structures of ZnO nanoparticles and ultrathin film. The surface morphologies of the LB multilayer and ultrathin film were observed by the atomic force microscopy (AFM). A uniform and flat surface of ZnO ultrathin film within nanometer ranges(<35nm) were obtained.

Enhanced visible photoluminescence from ultrathin ZnO films grown on Si-nanowires byatomic layer deposition

Bright room temperature visible emission is obtained in heterostructures consisting of ∼ 3.5 nm thick ZnO ultrathin films grown on Si-nanowires produced by means of self-maskingdry etching in hydrogen-containing plasma. The ZnO films were deposited onSi-nanowires by using atomic layer deposition (ALD) under an ambient temperature of25 °C. The orders of magnitude enhancement in the intensity of the roomtemperature photoluminescence peaked around 560 nm in the presentZnO/Si-nanowire heterostructures is presumably due to the high aspect (surface/volume) ratio inherent to the Si-nanowires, which has, in turn, allowed considerably more ZnOmaterial to be grown on the template and led to markedly more efficient visible emission.Moreover, the ordered nanowire structure also features an extremely low reflectance (∼0.15%) at 325 nm, which may further enhance the efficiency of emission by effectively trappingthe excitation light.

Ordered ultra thin ZnO films on metal substrate

Ultra thin ZnO films were prepared on metal Mo(1 1 0) substrate under ultrahigh vacuum conditions either by depositing Zn in ∼10 −5 Pa oxygen or by oxidizing pre-deposited Zn films. The films were characterized in situ by various surface analytical techniques, including Auger electron spectroscopy, X-ray and ultraviolet photoelectron spectroscopies, low energy electron diffraction and high resolution electron energy loss spectroscopy. The results indicate that a long-range ordered and stoichiometric ZnO films are formed along its [0 0 0 1] direction. The annealing experiments show that as-prepared ZnO films are thermal stable until 800 K. This study provides constructive information to further understand the growth mechanism of ZnO films on different substrates.

Effect of electrically degenerated layer on the carrier transport property of ZnO epitaxial thin films

ZnO films were prepared on (1 1 1) YSZ and (0 0 0 1) sapphire by pulsed laser deposition method. Effect of lattice mismatch on the carrier transport properties of ZnO epitaxial thin films was investigated. The carrier mobility of the ZnO films on YSZ was larger than that of ZnO/sapphire due to smaller lattice mismatch when the thickness was below 150 nm. The effect of electrically degenerated layer on the carrier transport property increased with decreasing the film thickness of ZnO film. The carrier density and electron mobility of 20 nm-thick-ZnO film on either substrate were regardless of the temperature. We concluded that the dominant carrier scattering mechanism in ZnO ultra thin films is double Schottky barriers at the grain boundary and that their height depends on the carrier concentration.

Prediction of Uncompensated Polarity in Ultrathin Films

Relying on first principles simulations of stoichiometric MgO, ZnO, and NaCl (1x1) ultrathin (111) films, we demonstrate the existence of a critical thickness below which polarity is uncompensated: the surface charges are bulklike, and the total dipole moment and the formation energy grow linearly with thickness. This study reveals novel facets of the problematics of polarity akin to the nanoscopic size of the objects and opens stimulating perspectives on polar nanostructures with surface properties and reactivity unaffected by charge compensation as in macroscopic samples.

A new approach to the fabrication of ZnO ultrathin films from annealing ZnS nanoparticulate films

The ZnO ultrathin films were fabricated from annealing the ZnS nanoparticulate films. By the layer by layer self-assembly technique, we constructed the ZnS nanoparticulate films from alternating layers of ZnS nanoparticles and polydiallyldimethylammonium chloride (PDDA). The result of the emission spectra indicated that PDDA played a role of passivator. Annealing the ZnS/PDDA films at different temperatures led to the changes in absorption. The SEM images showed that the preannealed film was composed of the uniformly distributing domains, while on the annealed film, there exist some holes formed by the burning of the organic components. The EDS confirmed that ZnS could be converted to ZnO at 500 °C.

Zinc oxide thin films prepared by means of Langmuir-Blodgett multilayers

Continuous ultrathin ZnO films with a surface roughness less then 1 nm have been fabricated by means of an organic precursor consisting of a Zn-arachidate Langmuir-Blodgett film. The organic component was eliminated by means of UV irradiation of the precursor and subsequent heating. The remaining ZnO films are polycrystalline and highly transparent in the visible range. They exhibit a high electrical resistivity.