Crystallinity Of ZnO Films Research Articles

Fabrication of surface-patterned ZnO thin films using sol–gel methods and nanoimprint lithography

Surface-patterned ZnO thin films were fabricated by direct imprinting on ZnO sol and subsequent annealing process. The polymer-based ZnO sols were deposited on various substrates for the nanoimprint lithography and converted to surface-patterned ZnO gel films during the thermal curing nanoimprint process. Finally, crystalline ZnO films were obtained by subsequent annealing of the patterned ZnO gel films. The optical characterization indicates that the surface patterning of ZnO thin films can lead to an enhanced transmittance. Large-scale ZnO thin films with different patterns can be fabricated by various easy-made ordered templates using this combination of sol-gel and nanoimprint lithography techniques.

Design of an Interfacial Layer to Block Chemical Reaction for Epitaxial ZnO Growth on a Si Substrate

Epitaxial growth of ZnO film on Si(111) substrates using an MgO interfacial layer has been investigated. The MgO layer acts as a thermodynamically stable buffer layer to suppress interfacial reaction between ZnO and Si, producing a clear interface. A domain-matched structure with mixed 4/3 and 5/4 domains was formed at the interface of Si and MgO to achieve the lowest domain mismatch. Moreover, the epitaxial ZnO film was grown on a MgO buffer layer with a domain matched structure of 11/12 domains, resulting in an (0001)[12̅10]ZnO∥(111)[11̅0]MgO∥(111)[11̅0]Si epitaxial relation. The high crystallinity of ZnO film grown on MgO/Si shows good optical performance with strong photoluminescence and improved Hall mobility.

Effects of gallium doping on properties of a-plane ZnO films on r-plane sapphire substrates by plasma-assisted molecular beam epitaxy

The authors report on the structural, optical, and electrical properties of Ga-doped a-plane (112¯0) ZnO films grown by plasma-assisted molecular beam epitaxy. Ga doping level was controlled by changing the Ga cell temperatures from 350 to 470 °C with an interval of 30 °C. With up to Ga cell temperatures of 440 °C, single crystalline Ga-doped a-plane ZnO films were grown; however, the sample with a Ga cell temperature of 470 °C showed polycrystalline features. The typical striated surface morphology normally observed from undoped ZnO films disappeared with Ga doping. ZnO films doped with Ga cell temperatures up to 440 °C did not show a significant change in full width at half maximum (FWHM) values of (112¯0) x-ray rocking curves by doping. The smallest FWHM values were 0.433° (ϕ=90°) and 0.522° (ϕ=0°) for the sample with a Ga cell temperature of 350 °C. The polycrystalline ZnO film with excessive Ga doping at the Ga cell temperature of 470 °C showed significantly increased FWHM values. Hall measurements at room temperature (RT) revealed that electron concentration began to be saturated at the Ga cell temperature of 440 °C and electron mobility was drastically reduced at the Ga cell temperature of 470 °C. The carrier concentration of Ga-doped ZnO films were controlled from 7.2×1018 to 3.6×1020 cm−3. Anisotropic electrical properties (carrier concentration and Hall mobility) were observed in measurements by the van der Pauw method depending on the direction (c- or m-direction) for the undoped sample but not observed for the doped samples. RT photoluminescence (PL) spectra from the Ga-doped single crystalline ZnO films showed dominant near band edge (NBE) emissions with negligibly deep level emission. The NBE intensity in PL spectra increases with Ga doping.

Effects of post-annealing on Schottky contacts of Pt/ZnO films toward UV photodetector

The Schottky contact of Pt/ZnO was formed by depositing ZnO films oriented along c-axis by pulsed-laser deposition on Pt/Ti buffer layer supported by SiO2/Si substrate. Effects of the post-annealing on the crystallinity, uniformity and native defects of ZnO film as well as Schottky contacts of Pt/ZnO films were investigated. Results show that the annealing can improve the crystallinity of ZnO film, suppress the native defects, and enhance the performance of Pt/ZnO Schottky contacts dramatically. The best Schottky diode shows the largest barrier height of 0.8eV with reverse leakage current of 1.5×10−5A/cm2. The zero-biased photodetector based on the best Schottky diode possesses responsivity of 0.265A/W at 378nm, fast photo-response component with rise time of 10ns and fall time of 17ns. This report demonstrates possibility of ZnO films/Pt hetero-junction with large area Schottky contact, and establishes the potential of this material for use in UV photodetector devices.

Properties of (11–20) a-plane ZnO films on sapphire substrates grown at different temperatures by plasma-assisted molecular beam epitaxy

The effects of growth temperatures (in a wide range from 100 to 800 °C) on properties of a-plane (11 2 ¯ 0) ZnO films grown on r-plane (1 1 ¯ 02) sapphire substrates by plasma-assisted molecular beam epitaxy were investigated. The film was grown as polycrystalline at 100 °C, but grown as single crystalline at the temperatures from 200 to 800 °C without any mixture of c-plane (0001) ZnO and m-plane (10 1 ¯ 0) ZnO. The single crystalline ZnO films showed anisotropic surface morphology with the conglomerated granules or striations along the [0001] ZnO direction. The ZnO film grown at 400 °C showed better crystal quality than others. It showed the smallest full width at half maximums of 0.450° and 0.297° for (11 2 ¯ 0) x-ray omega rocking curves under the measuring configuration with the omega axis parallel and perpendicular to the <0001> ZnO and directions, respectively, and 0.387° for (10 1 ¯ 1) omega rocking curve. The threading dislocation and the stacking fault densities were determined to be ~ 3.7 × 10 10 cm −2 and ~ 8.5 × 10 4 cm −1 for the ZnO film grown at 400 °C.

Role of the crystallinity of ZnO films in the electrical properties of bottom-gate thin film transistors

The strong correlation between the microstructural characteristics of ZnO channel layers grown at various temperatures by radio-frequency magnetron sputtering and the electrical performances of resulting bottom-gate thin film transistors (TFTs) was reported. Transmission electron microscopy revealed that increasing growth temperature enhanced degree of c-axis preferred orientation and enlarged width of columns in the ZnO films. The ZnO channel layers grown at 250 and 350 °C exhibited TFT saturation behavior. However, growing them at ≥ 350 °C produced small grains in the junctions of ZnO/SiO 2 interface and grain boundaries, which led to hump behavior in TFT transfer curve caused by formation of additional boundaries.

Influence of organozinc ligand design on growth and material properties of ZnS and ZnO deposited by atomic layer deposition

Deposition of ZnS and ZnO by the atomic layer deposition technique is performed using both dimethylzinc (DMZn) and diethylzinc (DEZn) as the metal source and H2S or H2O as the counter-reactant. The deposited films are characterized by x-ray diffraction (XRD), x-ray photoelectron spectroscopy, and ultraviolet-visible measurements, and particular emphasis is placed on the influence of the metal precursor on material growth and properties. The use of DMZn as the Zn source results in faster material deposition than growth with DEZn due to a less significant steric factor with DMZn. The material properties of the deposited ZnS films are nearly identical for the DMZn/H2S and DEZn/H2S processes, whereas XRD provided evidence for slight variations in the material properties of the DMZn/H2O and DEZn/H2O grown films. Overall, pure and crystalline ZnS and ZnO films can be deposited via either DMZn or DEZn, and ZnO growth is more affected by the modification of the ligand of the Zn precursor from methyl to ethyl.

The role of pH and boron doping on the characteristics of sol gel derived ZnO films

Undoped and boron doped ZnO films (nominal volume B/Zn ratio = 1%, 3% and 5%) were deposited onto glass substrates by the sol gel method using spin coating technique. Zinc acetate dihydrate (ZnAc), 2-methoxyethanol and monoethanolamine (MEA) were used as a starting material, solvent and stabilizer, respectively. Trimethyl borate (TMB) was used as a dopant source. The pH value of the sol was adjusted with glacial acetic acid and ammonia and it changed from acid to base in nature. The effect of pH value and B dopant on the structural, morphological and optical properties of the films was investigated. The crystalline structure and orientation of the films were investigated using XRD study. The crystallite size of the films was determined. The XRD analyses showed that the undoped ZnO film crystallinity enhanced when pH of the precursor sol was increased from 5.05 to 7.00. Surface morphology was studied by a field emission scanning electron microscope (FESEM), and the effects of the doping concentration and pH values on the microstructure of the films were investigated. The optical band gap values on the surface morphology of the films were determined. The absorption edge shifted depending on the pH values and the B dopant.

The thermal treatment effects of CrN buffer layer on crystal quality of Zn-polar ZnO films

We report on the annealing effects of CrN buffer layers on the crystal quality of Zn-polar ZnO films grown by plasma assisted molecular beam epitaxy. The high-temperature (HT) annealing of CrN buffer layer improved the crystallinity of ZnO films. The full width at half maximums of (0002) and (10–11) ZnO ω-scan X-ray diffraction show 574 and 1296 arcsec, respectively, which show the 3 and 2 times narrower values than those of ZnO films without the annealing process. Moreover, the HT annealing can be effective method to improve the surface smoothness of ZnO film and reduce the crystal tilting.

Effects of Sputtering Parameters on the Properties of Sputtered ZnO Films

The ZnO films were deposited on quartz glass substrate by RF magnetron sputtering. The influences of the deposition power, the Ar/O2 ratio and the total press on the crystallinity of ZnO films were analyzed by X-ray diffraction. The results show that the films deposited at Ar/O2 ratio of 2:3 have better crystalline quality under our experimental conditions. The optimum power is about 120-160 W. The crystal structure was significantly influenced by the total pressure in the chamber. The total press should below 1 Pa in order to get high quality ZnO films.

Texture control of ZnO films by inductively coupled plasma assisted chemical vapor deposition

ZnO films were grown on Si (100) and quartz substrates by inductively coupled plasma-assisted chemical vapor deposition using diethylzinc, O 2, and Ar. ZnO films with the (002) preferred orientation (PO) were formed at substrate temperatures > 250 °C regardless of any other changes made to process variables, since the (002) plane has the lowest formation energy with the highest number of unsaturated Zn–ZnO or O–ZnO bonds. At temperatures < 250 °C, the a-axis plane PO such as (100), (110), and (101) as well as the c-axis (002) plane PO were able to form by varying the temperature, plasma power, and deposition rate. The a-axis PO was formed when the radio frequency power was high enough to produce a crystalline ZnO film but was insufficient to form a (002) PO. The a-axis PO was also formed at higher deposition rates ≥ 20 nm/min when the radio frequency power was high enough to produce crystalline ZnO film. Since the (002) plane grew slowly, the grain exposing (002) plane was overgrown by the grains of the a-axis plane at higher deposition rates.

Structural and electrical properties of single crystalline and bi-crystalline ZnO thin films grown by molecular beam epitaxy

C-oriented ZnO epitaxial thin films are grown separately on the a-plane and c-plane sapphire substrates by using a molecular-beam epitaxy technique. In contrast to single crystalline ZnO films grown on a-plane sapphire, the films grown on c-plane sapphire are found to be bi-crystalline; some domains have a 30° rotation to reduce the large mismatch between the film and the substrate. The presence of these rotation domains in the bi-crystalline ZnO thin film causes much more carrier scatterings at the boundaries, leading to much lower mobility and smaller mean free path of the mobile carriers than those of the single crystalline one. In addition, the complex impedance spectra are also studied to identify relaxation mechanisms due to the domains and/or domain boundaries in both the single crystalline and bi-crystalline ZnO thin films.

Optical and Electrical Properties of Ga-Doped ZnO Single Crystalline Films Grown on MgAl2O4(111) by Low Temperature Hydrothermal Synthesis

Single crystalline gallium-doped ZnO films have been hydrothermally grown at using a ZnO seed layer on (111) substrates. High resolution X-ray diffraction showed an epitaxial relationship between the ZnO single crystalline film and the spinel substrate with an out-of-plane orientation of and an in-plane orientation of and . The effects of the doping concentration as well as the postannealing treatments on the electrical and optical properties were examined. After thermal treatment, the carrier concentration in the 2.48% Ga-doped ZnO films was 2 orders of magnitude higher at with a carrier mobility of . The blueshift of the optical bandgap was also observed from the photoluminescence and optical absorption measurements as the doping concentration increased. This can be explained in the framework of the induced stress and Burstein–Moss effects at high carrier concentration.

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 ∼5×10 9 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.

Investigation of room temperature UV emission of ZnO films with different defect densities induced by laser irradiation

We studied the room temperature UV emission of ZnO films with different defect densities which is fabricated by KrF laser irradiation process. It is shown room temperature UV photoluminescence of ZnO film is composed of contribution from free-exciton (FX) recombination and its longitudinal-optical phonon replica (FX-LO) (1LO, 2LO). With increase of the defect density, the FX emission decreased and FX-LO emission increased dramatically; and the relative strengths of FX to FX-LO emission intensities determine the peak position and intensity of UV emission. What is more, laser irradiation with moderate energy density could induce the crystalline ZnO film with very flat and smooth surface. This investigation indicates that KrF laser irradiation could effectively modulate the exciton emission and surface morphology, which is important for the application of high performance of UV emitting optoelectronic devices.

Room-temperature deposition of crystalline patterned ZnO films by confined dewetting lithography

In this work patterned ZnO films were prepared at room-temperature by deposition of ∼5 nm size ZnO nanoparticles using confined dewetting lithography, a process which induces their assembly, by drying a drop of ZnO colloidal dispersion between a floating template and the substrate. Crystalline ZnO nanoparticles exhibit a strong visible (525 nm) light emission upon UV excitation ( λ = 350 nm). The resulting films were characterized by scanning electron microscopy (SEM) and atomic force microscope (AFM). The method described herein presents a simple and low cost method to prepare crystalline ZnO films with geometric patterns without additional annealing. Such transparent conducting films are attractive for applications like light emitting diodes (LEDs). As the process is carried out at room temperature, the patterned crystalline ZnO films can even be deposited on flexible substrates.

Preparation of N-Doped ZnO Films by MOCVD

Crystalline ZnO films were grown on Y-stabilized ZrO2 substrates heated at 300 - 600 °C in NH3 atmosphere. It is clarified from Fourier transform infrared measurements that N-doped ZnO films grown at 350 and 400 °C contain N-C and Zn-H bonds. In the devices of n-type ZnO/N-doped ZnO/Au, a good rectification characteristic is attained for an N-doped ZnO film deposited at 300°C, whereas a linear current vs. voltage characteristic is seen for a film deposited at 500 °C.

ZnO Wurtzite Single Crystals Prepared by Nanorod-Assisted Epitaxial Lateral Overgrowth

We have developed a novel method to grow thick single crystalline ZnO films for the use of supporting layers by employing nanorod-assisted epitaxial lateral overgrowth (NRELO). The NRELO ZnO films were epitaxially grown on vertically arrayed nanorods at reduced temperatures by metalorganic chemical vapor deposition. The resultant films had epitaxial structures similar to the conventional ZnO films on sapphire and relatively low dislocation density. During the growth evolution of the NRELO films, the dominant stress was changed from nearly stress-free in the nanorods to strong in-plane tensile stress in the top region, and the rotation of the (0002) plane clearly disappeared in the NRELO ZnO film. This study shows that ZnO NRELO has the possibility to be used as supporting layers in optoelectronic devices with transparent and vertical stack structures because it exhibited high transparency, electrically semiconducting, and low defect density at the same time.

One-step growth of structured ZnO thin films by chemical bath deposition in aqueous ammonia solution

Structured ZnO films have been fabricated on soda-lime glass slides at a low temperature (80–85 °C) by a chemical bath deposition method in one step without seed layers. Mixed aqueous solutions of zinc sulfate, ammonia and thiourea were used at alkaline conditions. The influence of the ammonia concentration in the initial solution on the property of the deposited film was investigated systematically. The morphology, structural and optical properties of the deposited films were examined and characterized by x-ray diffraction (XRD), energy-dispersive spectroscopy x-ray diffraction (EDX), scanning electron microscopy (SEM), Raman spectroscopy and photoluminescence (PL) spectroscopy. Structural analyses with XRD, EDX and SEM revealed that the formed films exhibit a wurtzite hexagonal phase. The deposited film was more preferentially oriented in the (0 0 2) direction with an increase in the ammonia concentration from 0.75 to 2 mol l−1. The optical-phonon E2 mode at 437 cm−1 in the Raman spectrum, together with the XRD and EDX analyses, showed that flower-like and columnar crystalline ZnO films were formed in two ammonia concentration ranges, 0.75–1.4 mol l−1 and 1.6–2.0 mol l−1, respectively. Furthermore, PL spectra showed strong and high intensity peaks of UV emission with suppressed green emission for these deposited ZnO films. ZnS films were formed with a high ammonia concentration of 3.0 M. The formation mechanisms of ZnO, Zn(OH)2 and ZnS phases were discussed.

Growth behavior and microstructure of ZnO epilayer on γ-LiAlO2 (100) substrate by chemical vapor deposition

Low lattice-mismatched γ-LiAlO2(100) substrates were employed to grow ZnO epitaxial films by chemical vapor deposition. The influence of growth temperature on growth behavior of ZnO was investigated. Results indicated that the low lattice-matched (100) crystallites nucleate on substrate at all growth temperatures employed. However, a second type of crystallites having an (0001) orientation can also nucleate on substrate at low growth temperature of 575 °C and 640 °C. The growth rate of the later crystallite is, however, higher than that of the (100) one and finally results in a single crystalline ZnO film having an [0001] azimuthal orientation. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)