Postdeposition annealing of thin nickel films synthesized using R.F. magnetron sputtering technique is carried in this study. The XRD analysis indicates that annealing of the nickel films leads to the formation of nickel oxide with a preferential growth along (200) plane. The oxidation mechanism is observed with a phase transformation and results in polycrystalline NiO films. The surface morphology of the thin films was investigated by scanning electron microscopy (SEM) and atomic force microscopy (AFM) as a function of annealing temperature. The studies indicate the formation of well-defined grain boundaries due to agglomeration of nanocrystallites. The films annealed in the range 573–773 K are found to be porous. The optical transmission spectra of the films annealed at 773 K exhibit interference effects for photon energies below the fundamental absorption edge. The optical studies indicate the existence of direct interband transition across a bandgap of 3.7 eV in confirmation with earlier band structure calculations.
Nickel oxide (NiO) is considered as a model p-type semiconductor. It is a wide bandgap (
Thin nickel films are prepared by R.F. magnetron sputtering in a plasma focusing magnetic field using 99.99% pure nickel powder as the target, purchased from Sigma-Aldrich company. Clean soda lime glass slides are used as substrate material and kept perpendicular to the target surface at a distance of 4.5 cm. The sputter deposition is performed using R.F. magnetron sputtering equipment (Hind Hivac. Planar magnetron sputtering, model-12′′ MSPT) with a frequency of 13.56 MHz and an R.F. sputtering power of 200 W. Before sputtering, the chamber is evacuated to a base pressure of
Sample codes with preparation conditions.
Sample code | S1 | S2 | S3 | S4 | S5 | S6 | S7 | S8 | S9 |
---|---|---|---|---|---|---|---|---|---|
Dep. time (Minutes) | 45 | 45 | 45 | 45 | 45 | 15 | 15 | 15 | 15 |
Annealing temp. (K) | 413 | 473 | 573 | 773 | 773 | 413 | 473 | 573 | 773 |
Annealing time (h) | 0 | 3 | 3 | 3 | 5 | 0 | 3 | 3 | 3 |
The crystalline structural changes of the sputtered films were determined by X-ray diffraction (XRD) in reflection geometry with a Philips P W 1710 diffractometer using Cu
In order to investigate the dependence of crystalline structural changes on annealing, the X-ray diffractograms of the samples annealed at different conditions are taken and are shown in Figure
XRD pattern of Ni powder (S0), Ni/NiO thin film samples (S1, S2, S3, S4, and S5) peak positions for S0Ni powder, and Ni thin film at Ni (111) and Ni (200). For NiO thin films S2, S3, S4, and S5 at NiO (111), NiO (200), and NiO (220).
Only the crystalline phases of the fcc structure of Ni and the rock salt of NiO were identified. The peaks are consistent with the respective ASTM data and indicate the formation of NiO with a preferential growth along (200) plane. The peaks obtained at
The crystallite size can be calculated using Scherrer’s equation as follows:
The crystallite size determined from the (200) diffraction peak for the samples as a function of annealing temperature and time is given in Table
Lattice parameters of annealed NiO thin films with respect to Ni powder.
Parameter | Ni powder | S2 | S3 | S4 | S5 |
---|---|---|---|---|---|
|
4.174 | 4.183 | 4.1829 | 4.161 | 4.160 |
|
4.176 | 4.166 | 4.162 | 4.168 | 4.168 |
|
— | 4.186 | 4.178 | 4.179 | 4.170 |
|
— | 0.002 | 0.016 | 0.011 | 0.010 |
|
0.002 | 0.017 | 0.12 | 0.0073 | 0.006 |
Grain size (nm) | 80 | 14 | 15 | 16 | 15 |
The lattice parameter can be calculated using the following:
The lattice parameter “
Variation of lattice parameters with temperature.
The study of texture grade (Tc) for different planes is helpful in determining the crystalline nature of samples under different preparation conditions. The theoretical intensities for texture grade are taken from JCPDS file 4-835 of cubic NiO. The texture grade of samples is given by
Annealing effect on texture grade of
The annealed samples with preferential orientation along (200) plane show values greater than one which implies the texture nature as well. The same annealed samples with peak orientations along (111) and (220) plane indicate “Tc” values smaller than unity, which means there is a depletion of grains in this direction [
The SEM micrographs of nickel films deposited for 45 minutes (S1) and subjected to annealing temperatures of 573 K for 3 h (S3), and 773 K for 5 h (S5) are shown in Figure
SEM micrographs of NiO thin film samples (a) S1, (b) S3, (c) S5.
AFM image of NiO thin film sample (S4).
The AFM image of sample annealed at 773 K for 3 hours (S4) indicates the agglomeration of nanocrystals and the voids in between the grains. The peaks in between vacant sites in the AFM image account for the high concentration of nucleation centers while annealing the film. The characteristics of height profile indicate the smooth and uniform nature of the film surface on annealing.
Energy dispersive spectroscopy was done for the elemental analysis of the film surface. The EDX spectrum of the film annealed at 773 K for 5 h is shown in Figure
Energy dispersive spectroscopy of Ni film coated for 45 minutes and annealed at 773 K for 5 hours.
There are no other prominent peaks besides those corresponding to nickel and oxygen detected in the spectrum. The nonstoichiometric atomic ratio (Ni/O-70/30) shows the effect of controlled oxygen mechanism on annealing.
The transmission spectra of samples S4, S5, S8, and S9 in the wavelength range 300–900 nm are shown in Figure
Transmission spectra of different thin film samples.
The optical bandgap
Plot of
Thin nickel films are prepared at room temperature by R.F. magnetron sputtering technique from pure nickel powder source. The effect of annealing temperature and deposition time on the properties of these films has been investigated. The NiO films obtained showed enhanced transparency and high surface uniformity. The structural analysis using X-rays reveals the formation of cubic NiO with a well-defined preferential growth along (200) plane. The decrease of full width at half maximum (FWHM) and sharpness of peaks with increase of annealing temperature show that average grain size of NiO thin films increase due to heat treatment. The average grain size decreases with annealing and it shows a microlevel nature on thin films with respect to nickel powder sample. Texture grade analysis on NiO thin films shows its deviation from unity that leads to polycrystalline nature in its prominent peak (200) and a depletion of grains along (220) and (111) peaks. Annealing effects on the SEM and AFM studies on thin film result in the increase of oxidation kinetics, grain rotations, and enhanced crystallization which in turn produces nanocrystalline NiO film with well-defined grain boundaries. AFM analysis on annealed thin nickel film results in an agglomeration of NiO nanocrystals with voids in between grains. The controlled oxidation mechanism is well explained by the nonstoichiometric composition rates of Ni and oxygen in EDX analysis. The fringes on the transmission spectra are a measure of the thickness uniformity and high optical quality of the film. The annealed sample S5 shows a high transparency of 80% with appreciable sheet resistance. The bandgap energies for NiO samples S4 and S8 have comparable values with the reported ones. The conductivity was found to increase more with sputter deposition time than annealing.