The effect of annealing temperatures on the optical parameters of NiO0.99Cu0.01 thin films

NiO0.99Cu0.01 films have been deposited using thermal evaporationtechnique on glass substrates under vacuum 10-5mbar. The thicknessof the films was 220nm. The as -deposited films were annealed todifferent annealing temperatures (373, 423, and 473) K undervacuum 10-3mbar for 1 h. The structural properties of the films wereexamined using X-ray diffraction (XRD). The results show that noclear diffraction peaks in the range 2θ= (20-50)o for the as depositedfilms. On the other hand, by annealing the films to 423K in vacuumfor 1 h, a weak reflection peak attributable to cubic NiO wasdetected. On heating the films at 473K for 1 h, this peak wasobserved to be stronger. The most intense peak is at 2θ = 37.12o withthe preferential orientation of the films being (111) plane. The opticalproperties of the films have been studied. The effect of annealingtemperature on the optical parameters of NiO0.99Cu0.01 such astransmittance, reflectance, absorption coefficient, refractive index,extinction coefficient, and real and imaginary parts of dielectricconstant has been reported.


Introduction
Transition metal oxides like Nickel oxide are an attractive material which has lots of special properties such as optical, electrical and magnetic properties.
NiO films can be prepared by multiple physical and chemical methods such as: spray pyrolysis [11,12], electron beam evaporation [13] pulsed laser deposition [14], chemical bath deposition [15] and solgel [16], plasma-enhanced chemical vapor deposition [17] and sputtering [18] have been used to obtain nickel oxide films. All these methods offer different advantages depending on the application of interest and many efforts have been conduced to obtain films with the desirable physical and/or chemical properties. In the case of wide-band-gap semiconductors, addition of impurities often induced dramatic change in their optical and electrical properties. It is well known that the physical characteristics of the NiO films depend on the doping, and then a study of the doping effect on the physical characteristics is always interesting. In the present investigation we report the effect of annealing on NiO 0.99 Cu 0.01 thin films prepared by thermal evaporation technique in order to study the optical constants, including refractive index, extinction coefficient, real and imaginary part of dielectric constant.

Experimental details 1. NiO:Cu nanoparticles growth
During this work Cu doped NiO thin films were prepared using thermal evaporation technique onto glass substrates under vacuum 10 -5 mbar by the combination of rotary and diffusion pump. The NiO powder was doped with (0.01wt%) of Cu (99.99% purity). The thickness of films was 220nm. The annealing process was continued for 1 hour was to improve the properties of the films. The glass slides substrate of 3 x 2 cm 2 area were cleaned with dislated water using ultrasonic process for 15 minute to deposit the films.

Characterization of thin films
X-ray diffraction (XRD) pattern was used to determine the structural properties of NiO:Cu using CuKα radiation of wavelength 1.54Å, the current was 20 mA, the voltage was 30 kV, and the scanning angle 2 was varied in the range of (20-50) degree with speed of 2 cm.min -1 . The optical transmittance was recorded with a double beam Shimadzu UVvisible spectrophotometer in the wavelength range 300-800 nm. In this work the main parameter that control the properties of the films are the annealing temperatures (373, 423, and 473)K. The effect of annealing temperatures on the optical constants was examined.

Results and discussion
The XRD pattern and the values related to peaks for NiO 0.99 :Cu 0.01 thin films are shown in Fig.1. As deposited NiO 0.99 Cu 0.01 films were amorphous. After heating the films to 373K the XRD patterns shows no clear diffraction peaks in the range 2θ (20-50) o . On the other hand, increasing the annealing temperature to 423K in vacuum for one hour, a weak reflection peak attributable to cubic NiO was detected. This peak was observed to be stronger when heating the films to 473K for 1 h. The most intense peak is at 2θ = 37.12 o with the preferential orientation of the films being (111) [20,22]. These indicates that at 423K and 473K is the coexistence of β-Ni (OH) 2 and NiO phases. The diffraction peaks were higher than the peaks that were annealed at lower temperature, implying the high orientation and well crystalline [23]. This suggests that these films were apparently amorphous phase and that annealing induced phase transformation changes and crystallization as also confirmed by [24].

thin films with different annealing temperatures (a):as deposited(303K) (b):T =RT (c):T a =423K (d):T a =473K
The transmission spectra of NiO 0.99 Cu 0.01 films prepared at RT and annealed to 373, 423 and 473K is displayed in Fig. 2. From the transmission spectra it is observed that the film samples absorb heavily within UV-VIS regions but moderately in the NIR regions for the film deposited at room temperature. The maximum absorbance for the films occurred within the UV region from where the absorbance decreased with the wavelength towards the NIR region. The as-deposited film showed transmittance of more than 65% within the VIS-NIR regions. However both the as-deposited and annealed films show transmittance that increased exponentially from the UV region towards the NIR region. The properties of poor transmittance in the UV-VIS but moderately high transmittance in the VIS-NIR exhibited by film sample deposit at RT make the film good material for coating eye glasses for protection from sunburn caused by UV radiations. It is clear from the figure that the spectral characterization has been affected by heat treatment. A maximum transparency of about 65% in the near IR region is observed for NiO 0.99 Cu 0.01 thin films at room temperature. Below 600 nm there is a sharp fall in the transmittance of all films, which is due to the strong absorbance of the films in this region while the structure tends to be more transparent in the region above 600nm. The reflectance (R) has been found by using the relationship: R + T + A = 1 (1) where (A) is the absorption. Fig. 3 shows the reflectance spectra for the NiO 0.99 Cu 0.01 thin films as a function of wavelength at different annealing temperatures. It is clear from this figure that the reflectance of the films decreased with the increasing of the annealing temperature. The absorption coefficient (α) could be calculated by using the following relation [25]: where t is the film thickness. Fig. 4 shows the dependence of the absorption coefficient (α) on the wavelength for NiO 0.99 Cu 0.01 thin films at different annealing temperatures. This figure shows that, the absorption coefficient (α) increases with the increasing of annealing temperatures. This behavior suggests that decreasing in the energy gap with increasing annealing temperature. From this figure, the absorption spectra of NiO 0.99 Cu 0.01 films are characterized by strong absorption at higher photon energy (i.e. low wavelength) with a sharp edge on the short photon energy side. In the higher photon energy, the absorption coefficient takes higher values >10 4 cm -1 and then it is decreased sharply with the increasing of wavelength , and beyond that the change becomes slight, this sharp or low increment is attributed to the lattice absorption bands correspond to the electronic transitions between the highest filled energy band to the lowest empty band, or also corresponds to the density of absorbing centers such as impurities absorption, excitation transition, and other defects in the crystal lattice dependent on the conditions of sample preparation. The refractive index is an important parameter for optical materials and applications. Thus, it is important to determine optical constants of the films. The refractive index of the films was determined from the following relation [26].

3
where k is the extinction coefficient (k=αλ/4π). Fig. 5 shows the dependence of the refractive indices of NiO 0.99 Cu 0.01 thin films on the wavelength. It can be seen that the refractive index increases with increasing of wavelength. As it is shown in this figure that the values of the refractive increases in the range of 300-550nm and then decreases with increasing of wavelength only that was for film deposited at room temperature, the behavior increases for the annealed films at all wavelength range. Also, from this figure, it is found that the value of the refractive index decreases with the increasing of annealing temperature. This behavior may be due to the change in the bond length due to the decrease in the packing density because of increases in degree of amorphosity of films. Fig. 6 shows the variation of extinction coefficient with the wavelength in the range of (300-800) nm. The overall extinction coefficient spectra increase with increasing annealing temperatures. It can be observed from this figure that k decreases with increasing of wavelength. This is attributed to the same reason mentioned previously in absorption coefficient.

Conclusions
NiO 0.99 Cu 0.01 thin films were prepared by thermal evaporation technique. The structure analysis shows that the films are polycrystalline and has cubic structure. The crystalline orientation of NiO 0.99 Cu 0.01 films being (111) and the most intense peak is at 2θ=24.61 o and 37.12 o . The spectral investigations of the transmittance of NiO 0.99 Cu 0.01 films at (300-800) nm region are affected by heat treatment and noticed that the transmission decreases with the increasing of annealing temperatures. The absorption coefficient increases with increasing of annealing temperatures and this may be attributed to the increasing in the localized state and caused a shift in absorption edge to higher wavelength. The refractive index (n), extinction coefficient (k), and dielectric constant () were observed to be decrease with increasing Ta.