C-axis Oriented ZnO Thin Films Research Articles

Improved transparent conductive properties of highly c-axis oriented ZnO thin films upon (Ga, Mg) co-doping

The paper reports the growth and characterization of highly c-axis oriented ZnO thin films and their physical properties upon (Ga, Mg) co-doping. The films were produced on glass substrates by sol-gel spin-coating technique. X-ray diffraction (XRD) patterns of all the films reveal the presence of a hexagonal wurtzite crystal structure of ZnO with a c-axis preferred orientation and crystallite sizes in the range of 28–50 nm. Surface morphological studies reveal the granular morphology of the films. The optical transparency in the visible region increases upon doping and co-doping with a maximum (>95%) for the (1 at% Ga +3 at % Mg) co-doped ZnO thin film (assigned with code name 1G3MZO). The band gap is blue-shifted upon co-doping and found to be a maximum of 3.32 eV for the 1G3MZO film. The electrical conductivity improves upon co-doping, and an optimum value of 70.3 (Ω-cm)−1 is obtained in the 1G3MZO film. It has been concluded that the 1G3MZO film has improved transparent conductive properties and, therefore, finds application in optoelectronic devices.

Growth of highly c-axis oriented ZnO thin films by spray pyrolysis for piezoelectric applications

In this work, the piezoelectric response in ZnO thin films deposited by ultrasonic spray pyrolysis (USP) technique with a preferential orientation at the c-axis was explored. The films were fully characterized by X-ray diffraction (XRD), photoluminescence (PL), atomic force microscopy (AFM), and ellipsometry, respectively. The results showed an increase in the crystalline film quality and grain size at higher temperatures. Besides, the angular distribution of the crystals at the plane (002) was obtained from the pole figures in XRD and they have shown a small deviation from the normal of the substrate plane at higher temperatures. Afterwards, to measure the piezoelectric response, a prototype based on ZnO thin films deposited on silicon cantilevers was fabricated. A mechanical deformation was produced by hitting the cantilever with a metallic pellet falling at a constant height. In this way, the piezoelectric response was observed and measured, results have indicated that samples with better crystallinity and high resistivity have shown the most intense measured piezoelectric voltage.

Synthesis, characterization and antibacterial performance of transparent c-axis oriented Al doped ZnO thin films

Highly c-axis oriented ZnO thin films with wurtzite structure were grown onto preheated glass using spray pyrolysis technique with varying Al/Zn ratios. The transmittance of visible photons through these films was more than 90% and the color bands seen at an off angle was a clear indication of good quality surface. There was an abrubt change in the surface morphology from mixed nanorods and hexagonal stacks to pyramid like structure on increase in Al/Zn ratios. The shift in optical band gap towards high photon energy on introducing Al in ZnO lattice was regarded as Burstein-Moss effect. Photoluminisce spectroscopy revealed the presence of oxygen vacancies and defects related to Zn. The films were tested for their antibacterial activity against Escherichia Coli bacteria; their efficacy was enhanced on increasing Al/Zn ratio. These antimicrobial coatings with excellent optical properties have potential capability to be used in surgical instruments, protective hospital apparels, medical implants, storage containers, textile, and food packs.

Effect of gamma ray irradiation on structural and surface properties of c-axis oriented ZnO thin films

The changes in structure and surface morphology of ZnO thin films due to gamma ray irradiation at high doses are reported. Raman spectroscopy measurements indicate improvement in wurtzite ZnO phase by a higher amount with initial dose of 66 kGy (kGy) and by a lower amount for doses of 132 kGy and 200 kGy. Improvement in c-axis orientation was confirmed by X-ray diffraction measurements. X-ray diffraction measurements also showed a decrease in average crystallite size of ZnO. This is attributed to dissociation of Zn-O bonds by gamma rays which was identified using spectroscopy measurements. Gamma ray irradiation induced surface rearrangement and change in surface roughness with dose were identified using atomic force microscopy measurements. The observed results are explained on the basis of displacement damage and partial annealing effects caused by energetic gamma rays in ZnO thin films.

Fabrication of ultra-flat c-axis oriented ZnO thin films on atomically stepped cyclo-olefin polymer (COP) substrates by pulsed laser deposition at RT

Highly crystalline c-axis oriented ZnO thin films with ultra-flat surfaces were fabricated by pulsed laser deposition at RT on 0.3 nm high atomically stepped cyclo-olefin polymer (COP) substrates whose surfaces were thermally nanoimprinted using atomically stepped sapphire molds. X-ray diffraction and atomic force microscopy measurements showed that the atomically-controlled surfaces of COP substrates enhanced the c-axis orientation and surface flatness of the crystalline ZnO thin films. The ZnO thin films grown on the atomically stepped COP substrates exhibited atomically stepped surface morphology, and also the optical bandgap of about 3.3 eV and RT resistivity of 5.0 × 10−2 Ω cm.

RF Sputtering, Post-Annealing Treatment and Characterizations of ZnO (002) Thin Films on 3C-SiC (111)/Si (111) Substrates

We report on the radio frequency (RF) sputtering of c-axis oriented ZnO thin films on top of epitaxial 3C-SiC-on-Si (111) substrates, which were then subjected to post-annealing treatment at 400, 600 and 800 °C. Grazing incident X-ray Diffraction (XRD) data show that the Full Width Half Maximum (FWHM) values for O2/Ar ratios between 30% and 60% are consistent, with a mean of 0.325° and a standard deviation of 0.03°. This is largely attributed to the smaller lattice mismatch of 5% between the ZnO (002) and SiC (111) films. The quality of the ZnO films deteriorated at the post-annealing treatment of 800 °C, as demonstrated by the increasing value of FWHM diffraction peaks, the reducing value of the peak intensity, the reducing percentage of (002) oriented area under the curve, and the increasing value of biaxial stress. We propose a simple growth model to explain the result.

The role of Al, Ba, and Cd dopant elements in tailoring the properties of c-axis oriented ZnO thin films

Highly c-axis oriented un-doped ZnO and Al-, Ba-, and Cd-doped ZnO thin films were successfully deposited on glass substrate employing sol-gel spin coating method. XRD analysis showed that all thin films possess hexagonal wurtzite structure with preferred orientation along c-axis. Field emission scanning electron microscope (FESEM) was used to study the morphology of thin films. The morphology consists of spherical and non-spherical shape grains. EDX analysis confirms the presence of O, Zn, Al, Ba, and Cd in the relevant thin films. The optical properties of thin films were studied using UV–Vis spectrometer. All thin films possess more than 85% optical transmittance in the visible region. Blue shift in optical band gap Eg has been observed on doping with Al, whereas doping with Ba and Cd resulted in red shift of Eg. Urbach energy Eu of all doped ZnO thin films was found to have excellent correlation with their band gap energy Eg. Moreover, Eg increases while Eu decreases on the increase in crystallite size D. Optical parameters Eg and Eu as well as structural parameters lattice strain and stacking fault probability also show excellent correlation with the B-factor or the mean-square amplitude of atomic vibrations <u2> of the dopant elements. Electrical conductivity measurement of the thin films was carried out using two-point probe method. The electrical conductivity was found to increase with the increase in crystallite orientation along c-axis.

RF Sputtering of ZnO (002) Thin Films on Top of 3C-SiC-on-Si (100) Substrates for Low Cost Piezoelectric Devices

In this paper, we deposited c-axis oriented ZnO thin films on top of epitaxial 3C-SiC/Si (100) substrates using RF magnetron sputtering. We investigated the effect of O2/Ar ratio and the post-annealing temperature. The grazing angle incident x-ray diffraction results show that ZnO thin-films are highly oriented along the (002) crystalline direction between the O2/Ar ratio of 30% to 50%, at the post-annealing temperature of 600 ˚C in nitrogen environment. The recipe from this work can be used to develop low cost piezoelectric devices such as an energy harvester.

Detection of Neisseria meningitidis using surface plasmon resonance based DNA biosensor

Herein, we report the development of a surface plasmon resonance (SPR) based biosensor for the detection of Neisseria meningitidis DNA employing Kretschmann configuration. Highly c-axis oriented ZnO thin film of thickness 200nm was deposited on gold coated glass prisms by RF sputtering technique. Single stranded probe DNA was immobilized on the surface of ZnO thin film by physical adsorption method. SPR reflectance curves were recorded as a function of incident angle of He–Ne laser beam using a laboratory assembled SPR setup. The prepared biosensor exhibits a linear response towards target meningitidis DNA over the concentration range from 10 to 180ng/μl with a high sensitivity of about 0.03°/(ng/μl) and a low limit of detection of 5ng/μl. The SPR biosensor demonstrated high specificity and long shelf life thus, pointing towards a promising application in the field of meningitidis diagnosis.

Interfaces of c-axis oriented ZnO thin films on MgO (001) substrates

c-axis oriented ZnO thin films are epitaxially grown on MgO (001) substrates by pulsed laser deposition. The orientation relations between the films and the substrates and the atomic details at the interfaces are investigated by means of conventional and aberration-corrected transmission electron microscopy. The ZnO thin films show a microstructure of columnar grains that follow the two types of in-plane orientation relations with the MgO substrates: [11 2¯ 0]ZnO//[1 1¯ 0]MgO and [1 1¯ 00]ZnO//[1 1¯ 0]MgO. These columnar grains grow with either an oxygen atom plane or a zinc atom plane as the starting plane on the charge-neutral MgO (001) surfaces, forming locally polar down or polar up interface, respectively. The relative positions of different atoms including oxygen at the interface are determined.

Investigation of threshold switching mechanism based on Mn-doped ZnO/GaN heterojunction by photogenerated carrier injection

Highly c-axis oriented ZnO thin films have been prepared on n-type GaN-coated sapphire substrates by radio frequency reactive magnetron sputtering at 100 °C followed by thermal annealing at 740 °C for 2 h. The ZnO/GaN heterojunction devices show a steady threshold switching (TS) characteristic. In addition, Mn-doped ZnO (MZO)/GaN heterojunction has been prepared and it shows lower threshold voltage and higher on/off ratio than those of ZnO/GaN heterojunction. The formation of TS characteristic may be attributed to trapping and detrapping of electrons in heterojunction device. Furthermore, the devices are investigated by the injection of photogenerated carriers, it is illustrated that the external carrier injection may affect the rate of the trapping of electrons and results in the decrease of the threshold voltage.

Effect of solution concentration on the structural, optical and conductive properties of ZnO thin films prepared by sol–gel method

ZnO thin films with different solution concentrations (0.1–0.9 mol/L) were prepared by a simple sol–gel dip-coating technique. X-ray diffraction, ultraviolet–visible spectroscopy, Hall effect measurements and photoluminescence (PL) spectroscopy were employed to investigate the effect of solution concentration on the structural,optical and electrical conductive properties of the ZnO thin films. The results showed that the ZnO thin films preferentially oriented along the (002) direction at higher solution concentration. The careful study of the optical and electrical conductive properties showed that the resistivity decreased monotonously, while the transmittance increased first and then decreased when solution concentrations changed from 0.1 to 0.9 mol/L. Photoluminescence spectra indicated that the defect-related blue emission was increased with the enhancement of solution concentration. The mechanism of the blue emission, and the reasons why high solution concentration was favorable for forming high c-axis oriented ZnO thin films and obtaining low resistivity were also discussed in detail.

The dependence of structural and optical properties of PLD grown ZnO films on ablation parameters

Highly c-axis oriented ZnO thin films have been grown by pulsed laser deposition (PLD) technique on quartz, silicon (100) and Al2O3 〈0001〉 substrates using KrF excimer laser (λab= 248nm) and Q-switched fourth harmonic Nd:YAG laser (λab= 266nm). The crystalline nature, surface morphology and optical properties of the deposited films depend on the oxygen ambience, substrate nature and deposition temperature. The band gap of ZnO thin films varies with increase of substrate temperature despite of the ablation wavelength. Strong UV-PL emission without any deep level emissions confirms the growth of stoichiometric and crystalline ZnO thin films. Raman scattering studies confirms the growth of c-axis oriented ZnO thin films at different substrate temperature.

Realization of non c-axis oriented ZnO thin films on quartz through Mn–Li co-doping

Non c-axis oriented ZnO thin films were grown on quartz substrates via pulsed laser deposition (PLD) through Mn–Li co-doping. Both oxygen pressure (PO2) and growth temperature (Ts) are revealed to have significant effects on the structural properties of the Zn(Mn, Li)O (ZMLO) films. The self-textured film with predominant (101¯1) preferred orientation (PO) is achieved. In addition, using the ZMLO as a buffer layer, the growth of undoped pure ZnO with successive orientation characteristic is realized as well. Emission study was performed for the ZnO layer showing strong near-band-edge emission with minor deep level emission, indicating high optical quality of the ZnO thin film.

An efficient urea biosensor based on laser ablated ZnO thin film

ABSTRACTZinc oxide (ZnO) thin film deposited onto indium tin oxide (ITO) coated Corning glass substrates using pulsed laser deposition (PLD) technique has been used as a matrix for realization of an efficient urea biosensor after immobilization of urease (Urs) enzyme onto the surface of ZnO. The bioelectrode (Urs/ZnO/ITO/glass) is found to be exhibiting an enhanced sensitivity of 22μΑmΜ−1cm−2 towards urea over a wide detection range of 5-200 mg/dl. The relatively low value of Michaelis menten constant (Km= 0.94mM) indicates high affinity of the immobilized urease towards the analyte (urea). The prepared biosensor retains 90% of its activity for more than 10 weeks. The observed enhanced response characteristics of bioelectrode are attributed to the growth of the matrix (highly c-axis oriented ZnO thin film) with desired surface morphology and high electron communication feature. The results confirm the promising application of PLD grown ZnO thin film as an efficient matrix for urea detection.

Band alignment and interfacial structure of ZnO/Ge heterojunction investigated by photoelectron spectroscopy

Studies on band-offset and band-alignment of heterojunction of highly c-axis oriented ZnO thin films grown on n-Ge (1 1 1) by pulsed laser deposition show a type-II band alignment with the valence band offset (ΔEV) of 3.1 ± 0.2 eV. The valence band spectra of this heterojunction show band onsets corresponding to Ge, interfacial GeOx, and ZnO layers. This observation also enabled us to determine ΔEV of ZnO/GeOx heterojunction to be 1.4 ± 0.2 eV. These studies provide further insight into the band alignment of ZnO/GeOx/Ge system wherein the observed large value of ΔEV of ZnO/Ge can be used for heterojunction based optoelectronic devices.

Highly c-axis oriented ZnO thin film using 1-propanol as solvent in sol–gel synthesis

ZnO thin films were prepared by the sol–gel method, using two distinct solvents, ethanol (EtOH) and 1-propanol (1-PrOH). The influence of these two solvents on the crystallographic, morphological and optical properties of ZnO thin films was investigated. Highly c-axis oriented ZnO thin films, having grains with lengthened shapes and good optical absorbance were obtained using 1-PrOH as a solvent in the ZnO precursor sols.

Influence of rinsing temperature on properties of ZnO thin films prepared by SILAR method with propylene glycol

This study investigated the influence of deposition temperature on the growth as well as the structural and optical properties of ZnO thin films on a glass substrate. The thin films were produced using the successive ionic layer adsorption and reaction (SILAR) technique with propylene glycol. Our results reveal the formation of highly c-axis oriented ZnO thin films at temperatures exceeding 125°C. Increasing the temperature of the propylene glycol enhanced the crystallinity and surface smoothness of the film considerably. Moreover, with an increase in rinsing temperature, the optical transmittance spectra of ZnO thin films was as high as 95% in the visible-light region, with a notable decrease in the occurrence of oxygen defects. Results indicate that higher rinsing temperatures can enhance the process of hydrolyzing Zn(OH)2 and improve the quality of films. In addition, the crystallinity of ZnO films can be enhanced by post-annealing treatment.

Dependence of structural and optical properties of sol–gel derived ZnO thin films on sol concentration

In this work, ZnO thin films were prepared by sol–gel method and the dependence of structural and optical properties of these films on sol concentrations was deeply investigated. Unlike the previous studies, the ZnO thin films deposited in this study have approximately equal thickness, which excludes the influence of film thickness on the physical properties. The results show that low sol concentration is favorable for obtaining high c-axis oriented ZnO thin films with good crystalline quality. When the sol concentration is above 0.7mol/L, the degree of c-axis orientation of ZnO thin films decreases and the optical quality is also degraded. Photoluminescence spectra indicate that the defect-related blue emission is increased with the enhancement of sol concentration. The mechanism of the blue emission is analyzed. The reason why high sol concentration is unfavorable for formation of high c-axis oriented ZnO thin films and obtaining high optical quality is also discussed.

XPS Analysis of ZnO Thin Films Obtained by Pulsed Laser Deposition

Highly c-axis oriented ZnO thin films were deposited on n-Si (111) substrate at various oxygen partial pressures by pulsed laser deposition (PLD). X-ray diffraction (XRD), Atomic force microscopy (AFM) were used to analyze the influence of the oxygen partial pressure on the crystallization and morphology of the ZnO thin films. X-ray photoelectron spectroscopy (XPS) was used to analyze relationships between chemical shifts of XPS energy spectra and stoichiometric ratios of ZnO thin films, and quantitative relationships between content of Zn, O and oxygen partial pressures. An optimal crystallized and stoichiometric ZnO thin film was observed at the oxygen partial pressure of 6.5Pa.