We report manufacturing and characterization of low cost ZnO thin films grown on glass substrates by sol-gel spin coating method. For structural properties, X-ray diffraction measurements have been utilized for evaluating the dominant orientation of the thin films. For optical properties, reflectance and transmittance spectrophotometric measurements have been done in the spectral range from 350 nm to 2000 nm. The transmittance of the prepared thin films is 92.4% and 88.4%. Determination of the optical constants such as refractive index, absorption coefficient, and dielectric constant in this wavelength range has been evaluated. Further, normal dispersion of the refractive index has been analyzed in terms of single oscillator model of free carrier absorption to estimate the dispersion and oscillation energy. The lattice dielectric constant and the ratio of free carrier concentration to free carrier effective mass have been determined. Moreover, photoluminescence measurements of the thin films in the spectral range from 350 nm to 900 nm have been presented. Electrical measurements for resistivity evaluation of the films have been done. An analysis in terms of order-disorder of the material has been presented to provide more consistency in the results.
One of the most important wide bandgap oxide semiconductors that draw worldwide attention of researchers is zinc oxide (ZnO) [
More growth methods have been utilized for ZnO thin films including molecular beam epitaxy [
In this effort, transparent and semiconducting nanocrystalline ZnO thin films have been grown by low cost sol-gel spin coating method on glass substrates. Investigations of the effect of the growth parameters such as preheating temperature, spin speed, initial concentration on the crystallization, optical constants, photoluminescence, and electrical resistivity of ZnO thin films are presented. In addition, an analysis in terms of order-disorder of the material to provide consistency in the results is reported.
Zinc acetate dihydrate, (CH3COO)2 Zn·2H2O (ZAD), diethanolamine (NH(CH2CH2OH)2), and n-butyl alcohol (C4H9OH) were purchased from Oxford Laboratory Reagent, Scharlau, and British Drug Houses, respectively. All chemicals have been utilized without further purification. Thin films have been spun on a microscope glass substrates using spin coater, model Spin-1200D, MIDAS system.
The preparation method of ZnO thin films was reported elsewhere [
The ZnO films were characterized by XRD for the structural properties using a Philips (PW1710 BASED) diffractometer. The surface morphology of the ZnO thin films was measured by scanning electron microscope, model Quanta 250 FEG.
Optical transmittance and reflectance spectra have been recorded by Jasco V-670 double-beam recording spectrophotometer in the range from 350 nm to 2000 nm. The absolute values of transmittance and reflectance have been calculated from the measured transmittance and reflectance [
Figure
X-ray diffraction (XRD) patterns obtained for ZnO thin films S1 and S2.
Its value for S1 is 6.13 × 10−3 nm−2 and for S2 is 8.31 × 10−3 nm−2. The lower the dislocation density is, the better is the quality of the crystallized thin film. Further, the lattice strain
This gives a lattice strain of 2.9 × 10−2 for S1 and 3.3 × 10−2 for S2. Further, the lattice constants
The values of
Figure
Top view of ZnO thin films S1 and S2 using SEM.
Transmission and reflection spectra from 350 to 2000 nm of the thin films are shown in Figure
Optical transmittance and reflectance spectra obtained for ZnO thin films S1 and S2.
The absorption coefficient is related to the transmittance
Figure
Absorption coefficient versus wavelength for ZnO thin films S1 and S2.
The film thickness is estimated from cross-sectional view of SEM and is 292 nm for S1 and 52 nm for S2. The peak value of the absorption coefficient is about 2.5 × 105 cm−1 for S1 and 2.2 × 105 cm−1 for S2. This value is compared to that obtained by pulsed laser deposition and other reported sol-gel technique. In addition, the relation between the absorption coefficient and the incident photon energy
Dependence of the
Figure
Dependence of
The refractive index is related to reflectance, transmittance, and extinction coefficient
Figure
Refractive index versus wavelength for ZnO thin films S1 and S2.
Real dielectric constant
Real part of dielectric constant versus wavelength for ZnO thin films S1 and S2.
Imaginary dielectric constant
Imaginary part of dielectric constant versus wavelength for ZnO thin films S1 and S2.
The volume energy loss function velf and surface energy loss function self, which are measures for the rate of energy loss of electrons as they traverse the bulk or the surface is plotted for the prepared thin films and depicted in Figure
Volume energy loss function and surface energy loss function versus photon energy for ZnO thin films S1 and S2.
For S2 at wavelengths longer than the bandgap wavelength, the refractive index decreases with increasing the wavelength. This suggests that the sample shows normal dispersion behavior. Thus, the dispersion data of the refractive index can be analyzed by single oscillator model [
In the weak absorption region, the square of refractive index
(
Disordering in ZnO thin films arises from two main sources, namely, point defects and line defects (also called dislocation). The point defects include oxygen vacancy
Schematic illustration of band structure of ZnO thin film [
Figure
Photoluminescence spectra of ZnO thin films S1 and S2.
The variation of photoluminescence from S1 to S2 can be analyzed in terms of order-disorder of the material. S1 shows emission at UV with FWHM of about 20 nm, whereas S2 does not show this band, but it shows emission at the violet/blue band. In fact, the interband UV emission is a measure of the ordering in ZnO, the higher and narrower this band, the better the ordering of the ZnO thin film. Besides, the violet/blue band in S2 is broad with a FWHM of about 60 nm, and it shows pronounced band shape asymmetry in the low energy side, which is an indication of the disordering in S2 compared to S1. Further, the photoluminescence intensity was measured as a function of the excitation intensity to explain order-disorder for both samples at both low excitation regime using CW He-Cd laser with power up to 1 W/cm2 and at high excitation regime using pulsed third harmonic Nd:YAG laser with power up to 1.3 MW/cm2. The photoluminescence intensity
Photoluminescence intensity (a) of S1, (b) of S2, and (c) as a function of excitation intensity. He-Cd laser was used for excitation.
Photoluminescence intensity (a) of S1, (b) of S2, and (c) as a function of excitation intensity. Pulsed third harmonic Nd:YAG laser was used for excitation.
Peak photoluminescence wavelength of S1 and S2 using excitation with (a) He-Cd laser and (b) pulsed third harmonic Nd:YAG laser.
For electrical measurements of the films, the resistivity
Figure
Measured
Growth of ZnO nanocrystalline thin films has been prepared by sol-gel method. Different preparation parameters such as initial zinc concentration, spin speed, and heating rate have been changed to obtain different properties of ZnO thin films. Structural characterization using X-ray diffraction for the samples has been reported. The transmittance of the thin films is 92.4% and 88.4% which is relatively high. Optical bandgap of the prepared samples has been estimated, and it varied from 3.2 to 3.28 eV. Real and imaginary dielectric constants of the samples have been reported. Using single oscillator model, the oscillation energy and the dispersion energy of the thin films have been reported, and they were found to be 4.1 eV and 9.7 eV, respectively. Velf and self of the prepared samples have been reported as a function of energy. They increase with increasing the initial zinc concentration. Photoluminescence measurements in the range from 350 nm to 900 nm have been investigated. Electrical resistivity for the films has been measured and explained. An analysis in terms of order-disorder of the material has been presented to provide more consistency in the results. It is believed that the low cost method of preparation and tuning the properties of the films is crucial in advancement of ZnO in different fields.
Abdel-Sattar Gadallah would like to thank G. Lérondel and Campus France for photoluminescence measurements and financial support, respectively.