Physical Properties of Cu Doped ZnO Nanocrystiline Thin Films

Thin films of ZnO nano crystalline doped with different concentrations (0, 6, 9, 12, and 18 )wt. % of copper were deposited on a glass substrate via pulsed laser deposition method (PLD). The properties of ZnO: Cu thin-nanofilms have been studied by absorbing UV-VIS, X-ray diffraction (XRD) and atomic force microscopes (AFM). UV-VIS spectroscopy was used to determine the type and value of the optical energy gap, while X-ray diffraction was used to examine the structure and determine the size of the crystals. Atomic force microscopes were used to study the surface formation of precipitated materials. The UV-VIS spectroscopy was used to determine the type and value of the optical energy gap.


Introduction:
Zinc oxide is an environmentally safe material, as it has a high binding energy at room temperature (60 meV) and also has a large direct energy gap (3.37 eV) 1-3 and thus can be used in various hardware applications , such as solar cells , smart windows, gas sensors, piezoelectric transducers, transparent high power electronics, varistors, and ultraviolet (UV) light-emitters [4][5][6] . Sub-Extra energy levels will be generated in the band gap of the semiconductors when it is doped with metal 7,8 . Many minerals can assume valence depending on the surrounding chemicals, for example, any copper salt when doped in ZnO using an organic mineral solution can lead to various oxidation states Cu 9 . ZnO can be manufactured by several technologies such as chemical vapor deposition, thermal evaporation, magnetically splatter, pulse laser deposition (PLD), chemical vapor deposition, and non-vacuum methods can be used, i.e. absorption and interaction of the SILAR gel spin coating, Pyrolysis methods 1,7,[10][11][12][13][14][15][16][17] . The PLD method is characterized by the other technologies, where in films were prepared by this method at low temperatures due to the increased energy of the lower particles in the laser column resulting from the relatively high deposition rates 16 . In this work, ZnO doped with copper films were prepared for this material of great importance in electro-optical applications 16 . PLD method has been used to prepare ZnO and ZnO:Cu films on glass substrates. Zinc oxide was doped with copper to reduce the band gap and to improve the properties of the solar cells.

Materials and Methods:
In the present study, thin films of ZnO: Cu were successfully deposited on glass substrates in the presence of oxygen gas, using pulsed laser deposition (PLD). The target of ZnO:Cu was prepared by mixing zinc acetate (C 4 H 6 O 4 Zn.2H 2 O), copper acetate Cu(CH 3 COO) 2 and sodium hydroxide (NaOH) which were used without further purification. A solution of 0.3 M zinc acetate, 0.001 M copper acetate, and 1M sodium hydroxide were prepared in separated flasks. Cu doped ZnO with concentration 0, 6, 9, 12, and 18 wt. % nanoparticles were synthesized at room temperature in distilled water by the chemical precipitation method. The mixture is magnetically stirred for 30 min to get a homogeneous solution. Both the undoped and doped solutions are aged for one day for obtaining stability. The precipitation was washed with distilled water several times after separating it by filtration.Metallic ZnO-Cu target with different Cu concentrations (0, 6, 9, 12, and 18) wt. % was ablated by an Nd: YAG pulsed laser (Wavelength of laser: 1064/532 nm). The target with 500 no. of pulses, frequency 6 Hz, and energy The crystal phase and crystallinity of the samples were investigated using X-ray diffraction for2 values ranging from 20 to 60∘ using Cu-K radiation ( = 0.154nm). Transmittance spectra were recorded using UV-VIS spectrophotometer, while surface morphology has been obtained using atomic force microscopy (AFM). Van der Pauw (Ecopia HMS-3000) was used to measure the Hall Effect Results and Discussion:

XRD Studies
The crystalline structures of pure ZnO and ZnO:Cu nanofilms deposited on glass substrates have been investigated using XRD Figur 1 shows the X-ray diffraction (XRD) patterns of pure ZnO doped with Cu at different concentration samples. The diffraction peaks are indexed by comparing the data with JCPDS card file no.36-1451. The pattern of diffraction indicates thin films with a high degree were directed along (100) with c -axial direction in contrast to the study of R.K.Shukla et. al. 9 whose found the plane of 002 high intensity and have hexagonal crystal structures with low-intensity peaks correspond to the planes (002), (102), (101) and (110). In b pattern, a trace of an additional Cu-metal peak at 2θ of 36.26• was observed. This means that Cu atoms in these films (b pattern) not only acted as dopants but also formed embedded Cu clusters, which agrees well with a previous work 18 . The XRD patterns show a difference between the angular positions of Cu-doped ZnO thin films and those of undoped ZnO thin films. This can be attributed to the lattice mismatch induced by the difference between the lattice parameters of dopant atoms and that of the ZnO host.In addition we can see from Table 1 a slight shift in 2θ to higher value with increasing Cu content i.e., because the size of Cu ion ( which have been inserted into lattice) is lesser than Zn ion (covalent radii for for Cu= 1.38Å) The XRD patterns described in this work correspond to those described in 19 .
The full width at half maximum (FWHM) (The (FWHM) is calculated by calculating half of the wave height, then projecting two verticals lines on the x-axis (2θ) from the beginning and the end of the mid-width of the wave, then calculating (θ 2 -θ 1 ), crystallites size (Cryst.S), and 2Ө of pure ZnO and ZnO doped with Cu thin films are listed in Table 1. The crystallite size is calculated from XRD data by using Scherer formula eq.1. 20, 21 : D= 0.9λ /β cos Ө 1 Where β is the full width at half maximum of the peaks, λ is the wavelength of incident X-ray (1.54 Å), D is the crystallite size, and Ө is the degree of the diffraction peak.

Surface Morphology
The AFM images of the surface morphologies of ZnO: Cu are shown in Fig. 2 (a Average grain size in diameter, Roughness Average, and Peak to peak of ZnO: Cu with concentration of Cu (0, 6, 9, 12, and 18) wt. % are displayed in Table 2.  Figure 2 shows the atomic force microscopy (AFM) images of the pure ZnO and ZnO: Cu films. No pinholes were observed in the micrographs, indicating the successful deposition of compact films. As noted in Table 2, the root means square roughness (RMS) of the as-pure ZnO films is about 6.55 nm, which is in good agreement with that reported by Raied K. Jamal et al 21 . The grain size of the ZnO:Cu films decreases, as expected, before significantly increasing again at x=9%wt of Cu. It can be observed in Fig.2 and Table 2 that the grain size parallel to the surface is getting smaller, while the grain size vertical with the substrate is larger. The former reflects the (100) crystalline orientation, and the latter corresponds to the (002) crystalline orientation. Hence, the shift of the prominent crystal direction from (100) to (002) corresponds to the results obtained from XRD patterns. Figure 3 illustrates the absorption spectra of ZnO:Cu nano crystalline from (400 -700) nm. Red shift was observed for the doped zinc oxide thin films. This shift could be result due to the following reasons. The energy sub-levels for of the dopants lies below the conduction band edge (CBE) and above valence band edge (VBE) of ZnO. The creation of energy levels into the energy gap leads to a shift in band gap transmission and absorption visible light 20 . In addition, as the Cu content increased, the background absorption across the entire region increased. This may arise from the metallic Cu, which blocked most visible light.

Optical Properties Absorption
When ZnO has been doped, the electron capture cases (the aperture) are created between the valance band edge and conduction band edge of Zinc Oxide. ZnO doped with Cu films have been improved a visible light absorption.
Increasing visible absorption can be attributed to the transmission of the charge, which can be described as an alternative to the excitation of an electron from orbit d of metal ions 20 . Increased absorbance could be the result of increased levels of impurities within the energy gap 20 .  Energy Gap (Eg) The type of transition was found to be direct (21). The optical energy gap values (Eg) for ZnO:Cu thin films prepared by PLD method have been determined from the region of the high absorption at the fundamental absorption edge of these films by using Tauc equation 21 eq.2 αhv = A(hv-Eg) r .…………… 2 where hυ is the photon energy, Eg is the optical energy gap, B is a constant depends on the nature of the material ( properties of its valence and conduction band ) and r ∶ is a constant that depends on the nature of the transition between the top of the valence band and bottom of the conduction band 22 .
The bandgap values were determined from the intercept of the straight-line portion of the (αhν) 2 against the hν which has been explained in Fig.5. When the concentration of doping increases the energy gap for thin films decreases due to the displacement of the absorption edge towards the higher wavelengths 23,24 , which leads to a decrease in the energy gap of ZnO as shown in Table 3 These results are consistent with the results obtained by literature Ziad T. Al-Dahan 20

Absorption Coefficient
The absorption coefficient α is determined from the high absorption area, i.e. at the fundamental absorption edge of the films using eq. 3 25 . Absorption coefficient of the films was calculated from absorption (A) and the film thickness (t) using eq.3 α =2.303 A/ t 3 The absorption coefficient increases with increasing doping copper concentration as shown in Fig. 6. And these results agree with the result 26 .

Electrical Properties: Hall Effect Measurement
Hall effect calculations were performed at room temperature for pure zinc oxide films and doped with copper (6%, 9%, 12%, 18%) to determine the type and concentration of bulk carriers and their movement. The Hall coefficient (R H ) was a negative signal for pure and doped zinc oxide films suggesting that the films are n-type in conductivity. However, the impurity of the films sometimes affects the type of carriers at concentrations (12%, 18%) varying from (n-type to p-type). Carrier concentration N H , mobility μ H and type of charge carriers obtained from eqs. 3, 4, and 5 26 are shown in Table 4

Conclusion:
Nanocrystalline Cu doped ZnO films have been successfully deposited on glass substrates by pulsed laser deposition (PLD) and their structural and optical properties have been investigated. The XRD analysis demonstrates that the nanocrystals have a polycrystalline structure. The average size of the crystals is calculated and it is found that all samples have a nanoscale structure. The average thin film surface roughness has been increased and decreased with increasing Cu concentration. The values of the energy gap of prepared un-doped and ZnO doped with copper are found to be increasing with increasing Cu concentration. Hall measurements show that the conductivity type is transferred from type p to type n when the percentage of copper doping concentration is (12 and 18) %. From the results obtained, it is found that the prepared material can be used as a gas sensor and to improve the properties of solar cells in general and in the manufacture of photovoltaic cells in particular.