Improving the property of ZnO nanorods using hydrogen peroxide solution

Zinc oxide (ZnO) nanorod arrays made by the hydrothermal method were treated with hydrogen peroxide (H₂O₂) solution through two different approaches. One is to immerse ZnO nanorod sample into H₂O₂ solution. The other is a pre-treatment of spin-coating H₂O₂ solution on the seed layer before the growth of the ZnO nanorods. In the first approach, we found that the ultraviolet (UV) emission peak of the ZnO nanorod photoluminescence (PL) spectra was strongly dependent on the immersion time. In the second approach, the H₂O₂ solution influences not only the quality of the seed layer, but also the amount of the oxygen interstitial defects in the ZnO nanorods grown thereon. As a result, the UV emission intensity from the ZnO nanorods is enhanced almost five times. The ZnO nanorod arrays with few oxygen interstitial defects are prepared by the hydrogen peroxide treatment and expected to enable the fabrication of optoelectronic device with excellent performance.     

Synthesis and optical properties of Ce-doped ZnO hexagonal nanoplatelets

Single crystalline Ce-doped ZnO hexagonal nanoplatelets are successfully synthesized. Zinc acetate, cerium nitrate, potassium hydroxide and poly vinyl alcohol were mixed together and transferred to a 100 mL Teflon-lined stainless steel autoclave kept at 150 °C for 24 h. The obtained precipitant is calcined at 600 °C. The morphology and microstructure were determined by field emission scanning electron microscopy (FE-SEM), X-ray diffraction transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX) and photoluminescence (PL) spectroscopy. The investigation confirmed that the products were of the wurtzite structure of ZnO. The doped hexagonal nanoplatelets have edge length 25 nm and thickness 11 nm. EDX result showed that the amount of Ce in the product is about 15%. Photoluminescence of these doped hexagonal nanoplatelets exhibits a blue shift and weak ultraviolet (UV) emission peak, compared with pure ZnO, which may be induced by Ce-doping. The growth mechanism of the doped hexagonal nanoplatelets was also discussed.     

Solvothermal growth of ZnO

The growth of ZnO single crystals and crystalline films by solvothermal techniques is reviewed. Largest ZnO crystals of 3 inch in diameter are grown by a high-pressure medium-temperature hydrothermal process employing alkaline-metal mineralizer for solubility enhancement. Structural, thermal, optical and electrical properties, impurities and annealing effects as well as machining are discussed. Poly- and single-crystalline ZnO films are fabricated from aqueous and non-aqueous solutions on a variety of substrates like glass, (100) silicon, -Al₂O₃, Mg₂AlO₄, ScAlMgO₄, ZnO and even some plastics at temperatures as low as 50 °C and ambient air conditions. Film thickness from a few nanometers up to some tens of micrometers is achieved. Lateral epitaxial overgrowth of thick ZnO films on Mg₂AlO₄ from aqueous solution at 90 °C was recently developed. The best crystallinity with a full-width half-maximum from the (0002) reflection of 26 arcsec has been obtained by liquid phase epitaxy employing alkaline-metal chlorides as solvent. Doping behavior (Cu, Ga, In, Ge) and the formation of solid solutions with MgO and CdO are reported. Photoluminescence and radioluminescence are discussed.     

Structural and infrared properties of zinc oxide film and nanowires

The structural and infrared properties of the highly (00.2) oriented ZnO film, randomly grown Au-catalyzed ZnO nanowires (NWs) and vertically aligned self-catalyzed ZnO NWs were compared. In the XRD analysis, (0 0 2) diffraction intensity of self-catalyzed ZnO NWs was enhanced mainly attributed to the preferential growth of NWs in [0 0 0 1] as compared to the ZnO film and the randomly grown Au-catalyzed ZnO NWs. The high UV-to-green emission ratio of self-catalyzed ZnO NWs in room temperature PL measurement indicates that they had a better crystal quality as compared to Au-catalyzed ZnO NWs and ZnO film. Infrared spectroscopy has been used to characterize these films and nanowires too. The phonon peak 407 cm⁻¹ which related to the transverse optical (TO) vibrations perpendicular to the optical axis was observed in the IR reflectivity measurements on the highly c-oriented ZnO film. The IR peaks that appeared in the 550–580 cm⁻¹ region of the spectra of the specimens could be assigned to the ZnO NWs as it was not observed in the ZnO film. These peaks were observed in the 550–580 cm⁻¹ region in both s- and p-polarized light for the randomly grown Au-catalyzed ZnO NWs. In contrast, the IR peak at 580 cm⁻¹ was clearly shown in p-polarized light but not in the s-polarized light for vertically aligned ZnO NWs. This indicated that the vibration was polarized along the vertically aligned ZnO NWs. The (00.2) orientation of the ZnO specimens could be identified by comparing the p- and s-polarized IR spectra.     

Optimization of hydrothermal growth ZnO Nanorods for enhancement of light extraction from GaN blue LEDs

In this study, we report on the enhancement in the light extraction efficiency of GaN blue LEDs topped with ZnO nanorods. The ZnO nanorods were grown by a two-step hydrothermal synthesis with pre-coated ZnO nanoparticles under optimized condition to give the appropriate size and quality, giving an increase in the light output efficiency of 66%. This improvement is attributed to the optimal rod size and spacing with improved thermal dissipation as compared to light extraction from plain GaN surface. During the ZnO growth on the LEDs, 0.55 M of NH₃ was added and the ZnO sample was later annealed at 475 °C in N₂ ambient, to drive out interstitial oxygen atoms from the tetrahedral unstable site. As a result, a high ratio of UV to orange defect band emission was achieved. The two-step growth of ZnO nanorods on GaN LEDs was effective in generating array of ZnO nanorods which serve as reflector to enhance light extraction from LEDs.     

Decisive role of Au layer on the oriented growth of ZnO nanorod arrays via a simple aqueous solution method

Vertically aligned arrays of ZnO nanorod were synthesized on the Au/SiO₂/Si(1 0 0) substrate by a simple aqueous solution growth process, without pre-prepared ZnO seed layer. For comparison, glass and SiO₂/Si were also used as substrates, and the results show that the Au layer plays a decisive role in orienting the growth of the ZnO nanorod. The effects of other growth parameters, including Zn²⁺ concentration and growth time, on morphology, density, and orientation of the ZnO nanostructure were also studied and with longer reaction time, a new structure namely ZnO nanotip was obtained. Moreover, the growth mechanism of ZnO nanorod arrays grown on the Au/SiO₂/Si substrate was proposed.     

Low temperature II–VI nanowire growth using Au–Sn catalysts

Epitaxial lateral overgrowth is reported for semi-polar (Al,Ga)N(1 1 .2) layers. The mask pattern consisted of periodic stripes of SiO₂ oriented parallel to either the GaN[1 1 .0] or the GaN[1 1 .1] direction. Lateral growth occurred either along GaN[1 1 .1] or along GaN[1 1 .0]. For growth along the [1 1 .0] direction, coalescence was achieved for layer thicknesses >4 μm. However, planarization was not observed yielding extremely corrugated surfaces. For growth in [1 1 .1] direction, coalescence was delayed by a diminishing lateral growth rate. Growth of AlGaN during ELOG resulted in coalescence. Improvement in crystal quality of such buffer layers for the growth of InGaN/GaN quantum wells was confirmed by X-ray diffraction and photoluminescence spectroscopy.     

Growth of hexagonal and cubic InN nanowires using MOCVD with different growth temperatures

We have performed a detailed investigation of the metal-organic chemical vapor deposition (MOCVD) growth and characterization of InN nanowires formed on Si(1 1 1) substrates under nitrogen rich conditions. The growth of InN nanowires has been demonstrated by using an ion beam sputtered (∼10 nm) Au seeding layer prior to the initiation of growth. We tried to vary the growth temperature and pressure in order to obtain an optimum growth condition for InN nanowires. The InN nanowires were grown on the Au+In solid solution droplets caused by annealing in a nitrogen ambient at 700 °C. By applying this technique, we have achieved the formation of InN nanowires that are relatively free of dislocations and stacking faults. Scanning electron microscopy (SEM) showed wires with diameters of 90–200 nm and lengths varying between 3 and 5 μm. Hexagonal and cubic structure is verified by high resolution X-ray diffraction (HR-XRD) spectrum. Raman measurements show that these wurtzite InN nanowires have sharp peaks E₂ (high) at 491 cm⁻¹ and A₁ (LO) at 591 cm⁻¹.     

Laser-induced thermomigration of Te precipitates in CdZnTe crystals

The thermomigration of tellurium precipitates in a cadmium zinc telluride (CdZnTe) crystal was observed using an infrared (IR) CO₂ laser beam. Te precipitates present in CdZnTe have been shown to thermally migrate along a temperature gradient. Selective energy deposition from an IR laser beam in Te precipitates was investigated as a potentially advantageous approach to speed the annealing of Te precipitates in CdZnTe. Initial results indicate Te precipitates do thermally migrate under IR laser heating.     

Effect of substrate temperature on the room-temperature ferromagnetism of Cu-doped ZnO films

Wurtzite structure ZnO films doped with ~2 at% Cu were deposited at substrate temperatures (T_s) from 350 to 600 °C by helicon magnetron sputtering. All the films exhibited room-temperature (RT) ferromagnetism (FM) and the maximum saturation magnetization (M_s) was 1.2 emu/cm³ (~0.15 μ_B/Cu). Cu ions were mainly in a divalent state as identified by X-ray photoelectron spectroscopy. FM tended to increase with decreasing T_s, and vacuum annealing enhanced the M_s. These results suggested that oxygen vacancies and/or zinc interstitials might contribute to the ferromagnetic performance. Thus, the observed FM was explained in terms of the defect related mechanism.     

Growth and optical absorption spectra of ZnO films grown by pulsed laser deposition

ZnO films on Al₂O₃ substrate were grown by using a pulsed laser deposition method. Through photoluminescence (PL) and X-ray diffraction (XRD) measurements, the optimum growth conditions for the ZnO growth were calculated. The results of the XRD measurement indicate that ZnO film was strongly oriented to the c-axis of hexagonal structure and epitaxially crystallized under constraints created by the substrate. The full-width half-maximum for a theta curve of the (0 0 0 2) peak was 0.201°. Also, from the PL measurement, the grown ZnO film was observed to be a free exciton, which indicates a high quality of epilayer. The Hall mobility and carrier density of the ZnO film at 293 K were estimated to be 299 cm²/V sec and , respectively. The absorption spectra revealed that the temperature dependence of the optical band gap on the ZnO films was − .     

Growth and photoluminescence properties of vertically aligned SnO2 nanowires

Vertically aligned SnO₂ nanowires (NWs) were grown for the first time by a vapor–liquid–solid method on c-sapphire with gold as a catalyst under Ar gas flow. Electron backscatter diffraction analysis indicated the NWs are single crystalline having the rutile structure, grow vertically along the [1 0 0] direction, and exhibit a consistent epitaxial relationship where lattice mismatch is estimated to be 0.3% along the SnO₂ [0 1 0] direction. The growth of these NWs is sensitive to many parameters, including growth duration, substrate type, source vapor concentration, and the thickness of the catalyst layer. Photoluminescence measurements at room temperature showed that the vertically aligned NWs exhibit an intense transition at 3.64 eV, a near band-edge transition which is rarely observed in SnO₂.     

Study of the ZnO crystal growth by vapour transport methods

The crystal growth of ZnO by vapour transport is classically made with the assistance of additional species that produce a gaseous mixture, the role of which remains often uncertain in the transport and growth process. Initially, in order to study the mass transport process, a numerical simulation is made to analyse which are the requirements to have an effective transport. As the pressure of each gaseous species is generally unknown, the numerical study has been performed for different total pressures. It is found that, if congruent and equilibrium conditions are assumed at the sublimation and crystallisation interfaces, effective growth conditions can only be attained for a narrow range of total pressures. Nevertheless, it is well known that ZnO growth by vapour transport is possible for a wide range of pressures of gaseous species. As a consequence, partial pressures higher than the equilibrium ones must be present in order to justify the experimental results. We suggest that the thermal decomposition of ZnO is given by an activated process. The analysis of different mechanisms that could justify the activated decomposition, in accord with a systematic set of growth experiments, suggests that some additional species in the growth of ZnO by vapour transport promote the generation of an additional Zn pressure. This zinc pressure would act autocatalytically inducing O₂ and Zn partial pressures higher than the equilibrium ones and promoting thermal decomposition. The above-cited set of experimental growth experiences, that include the presence of C, Zn, Fe, Cu and H₂, will be analysed and interpreted according to this approach.     

Enhancement of near-band edge photoluminescence of ZnO thin films by employing MgF2 buffer layer

ZnO/MgF₂/ZnO sandwich structure films were fabricated. The effects of a buffer layer on structure and optical properties of ZnO films were investigated by X-ray diffraction, photoluminescence, optical transmittance and absorption measurements. Measurement results showed that the buffer layer had the effects of improving the quality of ZnO films and releasing the residual stresses in the films. The near-band edge emissions of ZnO films deposited on the MgF₂ buffer layer were significantly enhanced compared with those deposited on bare substrate due to the smaller lattice mismatch between MgF₂ and ZnO than that between fused silica and ZnO.     

Dense vertical nanoplates arrays and nanobelts of indium doped ZnO grown by thermal treatment of ZnS–In2O3 powders

Dense vertical arrays of indium doped ZnO nanoplates have been grown by thermal treatment of compacted ZnS–In₂O₃ powders with 0.35 at% of In. The distribution of nanoplates is related to the grain structure of the substrate. Only a small content of In has been detected in the plates by energy dispersive X-ray spectroscopy, but comparison with previous works shows that its presence in the precursor determines the growth of the nanoplates. Increase in the amount of In in the precursor leads to the growth of long indium doped ZnO nanobelts. Cathodoluminescence spectra of the nanobelts show a 23 meV blue shift of the band edge emission.     

Structural control and magnetic properties of electrodeposited Co nanowires

Co nanowires with a preferred orientation were fabricated by direct current electrodeposition into the pores of porous anodic alumina membrane, and the structure of Co nanowires was studied by X-ray diffraction and high-resolution transmission electron microscopy with selected-area electron diffraction. It is found that the crystal structure of Co nanowires lies on the deposition potential. When electrodeposition is performed far from equilibrium conditions, i.e., at a high potential, face-centered cubic Co nanowires are deposited, while hexagonal close packing Co nanowires are formed at the low potential. The experimental results indicate that the orientation of the nanowires has effects on the coercivity for both hexagonal close packing (hcp) and face-centered cubic (fcc) Co.     

Tip-growth mode and base-growth mode of Au-catalyzed zinc oxide nanowires using chemical vapor deposition technique

Tip-growth and base-growth modes of Au-catalyzed zinc oxide nanowires (ZnO NWs) were synthesized on Au-film pre-deposited silicon substrates using Chemical Vapor Deposition (CVD) technique. The diameter of tip-growth Au-catalyzed ZnO NWs was proportional to the Au film thickness, whereas the areal density of these NWs was inversely proportional to the Au film thickness. It would be more appropriate to explain the growth of Au-catalyzed ZnO NWs by a combination of Vapor–Liquid–Solid and Vapor–Solid (VLS–VS) mechanisms instead of the conventional VLS mechanism, regardless of tip-growth or base-growth mode of Au-catalyzed ZnO NWs. The competition between the VLS and VS mechanism in the effectiveness of capturing the adsorbed Zn and O atoms would determine the final morphology of ZnO NWs. In addition, Au catalyst promoted the growth rate of NWs as compared to the self-catalyzed ZnO NWs.     

Growth of epitaxial ZnO films on Si (1 1 1) substrates with Cr compound buffer layer by plasma-assisted molecular beam epitaxy

Single crystalline ZnO film was grown on (1 1 1) Si substrate through employing an oxidized CrN buffer layer by plasma-assisted molecular beam epitaxy. Single crystalline characteristics were confirmed from in-situ reflection high energy electron diffraction, X-ray pole figure measurement, and transmission electron diffraction pattern, consistently. Epitaxial relationship between ZnO film and Si substrate is determined to be (0 0 0 1)_ZnO‖(1 1 1)_Si and [1 1 2¯ 0]_ZnO‖[0 1 1]_Si. Full-width at half-maximums (FWHMs) of (0 0 0 2) and (1 0 1¯ 1) X-ray rocking curves (XRCs) were 1.379° and 3.634°, respectively, which were significantly smaller than the FWHMs (4.532° and 32.8°, respectively) of the ZnO film grown directly on Si (1 1 1) substrate without any buffer. Total dislocation density in the top region of film was estimated to be ∼5×10⁹ cm⁻². Most of dislocations have a screw type component, which is different from the general cases of ZnO films with the major threading dislocations with an edge component.     

Chemically assisted vapour transport for bulk ZnO crystal growth

A chemically assisted vapour phase transport (CVT) method is proposed for the growth of bulk ZnO crystals. Thermodynamic computations have confirmed the possibility of using CO as a sublimation activator for enhancing the sublimation rate of the feed material in a large range of pressures (10⁻³ to 1 atm) and temperatures (800–1200 °C). Growth runs in a specific and patented design yielded single ZnO crystals up to 46 mm in diameter and 8 mm in thickness, with growth rates up to 400 μm/h. These values are compatible with an industrial production rate. N type ZnO crystals (μ=182 cm²/(V s) and n=7 10¹⁵ cm⁻³) obtained by this CVT method (Chemical Vapour Transport) present a high level of purity (10–30 times better than hydrothermal ZnO crystals), which may be an advantage for obtaining p-type doped layers ([Li] and [Al] <10⁺¹⁵ cm⁻³). Structural (HR-XRD), defect density (EPD), electrical (Hall measurements) and optical (photoluminescence) properties are presented.