Effects of Cu-doping on Optical Properties of NiO

This work presents the effect of Cu-doping on some optical properties of Cu:NiO thin film prepared by spray pyrolysis technique. UV-Visible spectrophotometer in the range 380-900 nm used to determine the absorbance spectra for various Cu-doping of Cu:NiO thin film. The transmittance and energy gap are decreased with increasing Cu-doping in the prepared films, while absorption coefficient, extinction coefficient, and skin depth are increased with increasing Cu-doping


INTRODUCTION
Bulk NiO has a cubic rock-salt crystal structure (Fm-3m) undergoing a weak cubic-torhombohedral distortion (R-3m) below the Néel temperature T N =523 K due to the magnetostriction effect [1].The composition of nickel oxide can be represented as NiO x H y but for simplicity, it is referred to as NiO x [2].They are attractive materials which have lots of special properties such as optical, electrical and magnetic properties. They have been employed as an antiferramagnetic material [3], p-type transparent conducting films [4], a material for electro chromic display devices [5] and a part of functional sensor layers in chemical sensors [6].
In the present work, the influence of the Cu-doped on the optical properties of NiO films deposited by chemical spray pyrolysis is considered.

EXPERIMENTAL DETAILS
Dissolved nickel chloride hexahydrate (NiCl 2 .6H 2 O) (from sigma-Aldrich company) with re-distilled water was used as a starting solution for deposition thin films of Cu:NiO by using chemical spray pyrolysis method. The dissolve solution was made with100 ml of redistilled water to make 0.1 M solution. The volumetric ratio of Cu was 2% and 4% and substrate temperature was 380 ºC. The layers have been deposited onto glass substrates that are cleaned in distilled water and then dried using air blower. After that they were cleaned again with acetone in order to remove any strains on it. In order to optimize the deposition arriving at the following conditions; spraying rate3ml /min, substrate to nozzle30 cm, spraying time during each cycle 7 sec, time interval between successive sprays1.5 min, and the carrier gas (filtered compressed air) were maintained at a pressure of 10 5 Nm -2 .
Thickness of the films was measured gravimetrically and the measured thickness is about 300 nm. The prepared films were annealed at 450 and 500 ºC, and then optical transmittance and absorbance were recorded in the wavelength range (380-900nm) usingUV-Visible spectrophotometer (Shimadzu Company Japan) double beam spectrophotometer.

RESULTS AND DISCUSSION
The optical properties of the deposited Cu : NiO thin films on glass substrate temperature 380 ºC with various contain of Cu by using UV-Visible spectrophotometer in the region of 380-900 nm. Fig.1 represent the relationship between transmittance and wavelength of Cu : NiO thin films. From this figure, it can notice that the transmittance decreased with increasing contain of Cu in NiO thin films, this may be attributed to the creation of levels at the energy band by increasing Cu contain.

ILCPA Volume 48
The extinction coefficient (k), which is related to the exponential decay of the wave as it passes through the medium, is defined as [17]: where α is the absorption coefficient and λ is the wavelength. Fig.6 represent the relationship between extinction coefficient and wavelength. From this figure, it can notice the increases of k with increasing content of Cu in the Cu : NiO thin film. The skin depth (χ) represents the electromagnetic wave will have amplitude reduced after traversing a thickness that calculated from the following relation [19]: Fig.8 showing the variation of skin depth with wavelength. Skin depth increased with increasing Cu contain in Cu : NiO thin film.

CONCLUSION
The effect of Cu-doping on some optical properties of Cu : NiO thin film prepared by spray pyrolysis technique is studied. The transmittance is decreased with increasing Cudoping in the prepared films, while absorption coefficient, extinction coefficient, and skin depth are increased with increasing Cu-doping. Energy gap decreased from3.1 eV for a pure NiO thin film to 2.9 eV after 4% addition of Cu for NiO thin film.