Band-Gap Engineering in ZnO Thin Films: A Combined Experimental and Theoretical Study (original) (raw)

Confinement effects in ultrathin ZnO polymorph films: Electronic and optical properties

Relying on generalized-gradient and hybrid first-principles simulations, this work provides a complete characterization of the electronic properties of ZnO ultrathin films, cut along the body-centered-tetragonal(010), cubane(100), hexagonal boron nitride(0001), zinc-blende(110), and wurtzite (10 ¯ 10) and (0001) orientations. The characteristics of the local densities of states are analyzed in terms of the reduction of the Madelung potential on undercoordinated atoms and surface states/resonances appearing at the top of the valence band and bottom of the conduction band. The gap width in the films is found to be larger than in the corresponding bulks, which is assigned to quantum confinement effects. The components of the high-frequency dielectric constant are determined and the absorption spectra of the films are computed. They display specific features just above the absorption threshold due to transitions from or to the surface resonances. This study provides a first understanding of finite-size effects on the electronic properties of ZnO thin films and a benchmark which is expected to foster experimental characterization of ultrathin films via spectroscopic techniques.

Dependence of film thickness on the structural and optical properties of ZnO thin films

Applied Surface Science, 2009

ZnO thin films are prepared on glass substrates by pulsed filtered cathodic vacuum arc deposition (PFCVAD) at room temperature. Optical parameters such as optical transmittance, reflectance, band tail, dielectric coefficient, refractive index, energy band gap have been studied, discussed and correlated to the changes with film thickness. Kramers–Kronig and dispersion relations were employed to determine the complex refractive index and dielectric constants using reflection data in the ultraviolet–visible–near infrared regions. Films with optical transmittance above 90% in the visible range were prepared at pressure of 6.5 × 10−4 Torr. XRD analysis revealed that all films had a strong ZnO (0 0 2) peak, indicating c-axis orientation. The crystal grain size increased from 14.97 nm to 22.53 nm as the film thickness increased from 139 nm to 427 nm, however no significant change was observed in interplanar distance and crystal lattice constant. Optical energy gap decreased from 3.21 eV to 3.19 eV with increasing the thickness. The transmission in UV region decreased with the increase of film thickness. The refractive index, Urbach tail and real part of complex dielectric constant decreased as the film thickness increased. Oscillator energy of as-deposited films increased from 3.49 eV to 4.78 eV as the thickness increased.

Elaboration and Characterization of Zno Thin Films Structural and Optical Study

Journal of Physics: Conference Series, 2019

The Nano-topographical structure of the solid surface is known as a necessary parameter in the physicochemical characterization and wetting properties. In this study, the physicochemical properties are evaluated by calculating the surface energy and by measuring the contact angle. The structural proprieties were determined using XDR. The optical proprieties were studied using the UV-visible technics. Substrates used in this study are the zinc oxide thin films deposited on the glass by sputtering under different powers (150, 200 and 250 watt). The Nano-topographic properties were examined using the atomic force microscopy (AFM) in order to calculate the roughness of different substrates. As results, the images obtained by atomic force microscopy showed that the growth of the power causes the growth of the roughness. XRD diagram assessment revealed that the deposited films have a preferential crystallographic direction according to the (002) plane while maintaining the initial orientation. The optical characterization showed that the bandwidth of these films is in the order of 3.28 eV. It is interesting to mention that the increase in RF power has slightly increased the energy of gap.

Structural and optical characterization of ZnO and AZO thin films: the influence of post-annealing

Journal of Materials Science: Materials in Electronics, 2015

ZnO thin films were deposited on glass substrate using a sol-gel method. The structural and optical properties at different annealing temperatures were studied using X-ray diffraction (XRD), ultra-violet-visible spectroscopy and Raman spectroscopy. X-ray diffraction results show that the c-axis orientation became stronger as the annealing temperature increased from 300 to 500 º C. the optical band gap energy was calculated from the optical absorption using UV-Vis spectrophotometer. The optical band gap of ZnO thin films decreases from 3.378 eV to 3.338 eV as the annealing temperature increases from 300 to 500 ºC, the experimental data are in agreement with the calculated results by specific models of refractive index.

Modification of morphological and optical properties of ZnO thin film

Karbala International Journal of Modern Science, 2017

Zink oxide thin films were prepared using different wet-processing techniques to study the morphological and optoelectronic properties. Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM) have been utilized to study the effect of technique and preparation procedure on the morphology of the films. The spin coated ZnO layers have exhibited ripple-shaped morphological features. Using the growth process with ZnCl 2 and Al(NO 3) 3 doping at different time has shown a change in surface morphology. UVeVisible absorption spectroscopy was used to understand the absorption behaviour and so to calculate the energy gap (E g) for the films produced. It has been revealed that E g of the ZnO thin film increases with the increasing of the number of layers spun onto the substrate. ZnCl 2 doping has no quite big change in E g values, however, Al(NO 3) 3 has resulted in a higher E g value.

Zinc Oxide Bulk, Thin Films and Nanostructures, 2006, p.

Foreword Since the invention of the first semiconductor transistor in 1947 by the scientists of Bell Labs, the semiconductor industry has grown at an incredible pace, fabricating faster, smaller, more powerful devices while manufacturing in larger volume at lower costs. Even though the very first semiconductor transistor was made from ger-manium (Ge), silicon (Si) became the semiconductor of choice as a result of the low melting point of Ge that limits high temperature processes and the lack of a natural occurring germanium oxide to prevent the surface from electrical leakage. Due to the maturity of its fabrication technology, silicon continues to dominate the present commercial market in discrete devices and integrated circuits for computing, power switching, data storage and communication. For high-speed and optoelectronic devices such as high-speed integrated circuits and laser diodes, gallium arsenide (GaAs) is the material of choice. It exhibits superior electron transport properties and special optical properties. GaAs has higher carrier mobility and higher effective carrier velocity than Si, which translate to faster devices. GaAs is a direct bandgap semiconductor, whereas Si is indirect, hence making GaAs better suited for optoelectronic devices. However, physical properties required for high power, high temperature electronics and UV/blue light emitter applications are beyond the limits of Si and GaAs. It is essential to investigate alternative materials and their growth and processing techniques in order to achieve these devices. Wide bandgap semiconductors exhibit inherent properties such as larger bandgap, higher electron mobility and higher breakdown field strength. Therefore, they are suitable for high power, high temperature electronic devices and short wavelength optoelectronics. Zinc oxide is a direct, wide bandgap semiconductor material with many promising properties for blue/UV optoelectronics, transparent electronics, spintronic devices and sensor applications. ZnO has been commonly used in its polycrystalline form for over a hundred years in a wide range of applications: facial powders, ointments , sunscreens, catalysts, lubricant additives, paint pigmentation, piezoelectric transducers, varistors, and as transparent conducting electrodes. Its research interest has waxed and waned as new prospective applications revive interest in the material, but the applications have been limited by the technology available at the time. ZnO has numerous attractive characteristics for electronics and optoelectron-ics devices. It has direct bandgap energy of 3.37 eV, which makes it transparent in visible light and operates in the UV to blue wavelengths. The exciton binding

Intensity of Optical Absorption Close to the Band Edge in Strained ZnO Films

Journal of the Korean Physical Society, 2008

Besides other one of the remarkable properties making wurtzite ZnO such an interesting material is its large exciton binding energy of about 60 meV, leading to stable excitons at room-temperature. Also, the Curie temperature of this wide-gap material has been predicted to lie above room temperature, making ZnO alloyed with magnetic ions a possible material for spintronics applications. One big challenge in the fabrication of ZnO-based heterostructure devices is the lattice mismatch between the ZnO lms and the substrates and the dierent thermal expansion coecients inducing biaxial strain. This work reports on the electronic band structure of biaxially strained ZnO for strains along the a-or the c-axis ranging from 1 % to 1 %, as calculated by means of the empirical pseudopotential method. Thereby, we also account for relativistic eects in the form of the spinorbit interaction, as well as for the energy dependence of the crystal potential through the use of nonlocal model potentials. Moreover, the application of a variable plane wave basis set allows us to directly obtain the strain-induced variations of the electronic and the optical properties of wurtzite ZnO.

Electron beam induced nanostructures and band gap tuning of ZnO thin films

The present work deals with the formation of nanostructures and tuning of optical band gap of ZnO thin films by high energy electron bombardment. In first step, Zinc Oxide (ZnO) thin films were deposited on silicon (1 0 0) substrates by pulsed laser deposition (PLD) technique using KrF Excimer laser. In second step, ZnO thin films were irradiated by electron beam of energy 6, 9, 12 and 15 MeV at constant dose. The surface morphology was studied by Atomic Force Microscope (AFM), whereas the optical properties were determined by Spectroscopic Ellipsometry (SE). Atomic force micrographs showed the formation of nanoscale structures on the surface of ZnO thin film. The nanostructures grew in size and reached its maximum size at 12 MeV electron energy. SE analysis revealed the increase in refractive index, appearance of broad absorption peak in visible region, and increase in optical band gap energy of ZnO thin film by 12 MeV electron bombardment. From the results, it can be concluded that the size of nanostructures and the optical band gap energy of ZnO thin films can be tuned by electron irradiation at various energies.

Nano and micro structural studies of thin films of ZnO

Journal of Materials Science, 2006

Zinc oxide thin films grown by sol-gel and RF sputtering methods have been characterized. The characterization techniques used involve ellipsometry, optical absorption, scanning tunneling microscopy, scanning and transmission electron microscopy. The films grown by sol-gel spin method which followed zinc acetate route exhibited a smoother texture than the films, which were deposited by using zinc nitrate route. The later type of films showed a dendritic character. Nano-structured fine grains of size ranging from 20 to 60 nm were observed with zinc nitrate precursor film. Individual grains show a sharp contrast with different facets and boundaries. Crystal planes and lattice parameters calculated by electron diffraction and X-ray diffraction are quite close and in agreement with the reported values in literature. Scanning tunneling microscopy has been used for measuring the average roughness of the surface and estimating the lattice constants. The STM studies of RF sputtered films, although showing a ZnO structure, exhibited a disturbed lattice. This was presumably due to the fact that after deposition the films were not annealed. Nanographs of 2D and 3D view of atomic positions of ZnO have been presented by using scanning tunneling microscopy.