- تطوير اداء خلية سليكونية مصنعة بطريقة تطعيم أوكسيد الزنك ببعض العناصر المعدنية

Abstract

Abstract The structural and optical properties of pure (ZnO and Au , Ag , Cu) doped thin films with doping ratio (3%) , have been prepared by thermal evaporation technique on glass substrate of (300 nm) thickness at room temperature, have been studied. Both the structural and optical measurements were carried out as deposited and doped films. )XRD( spectra revealed that all films have polycrystalline structure except that doped with Cu it has one significant sharp peak at (2Ө=36.4°). The results of the optical studies showed that the optical transition is direct allowed. The energy gap in general decrease as with doped (Au, Ag, Cu) dopant elements respectively.

Keywords:- ZnO, structural and optical properties, Au, Ag, Cu dopant elements.

Introduction ZnO which is one of the most important binary II-VI semiconductor compounds is a natural n-type electrical conductor with a direct energy wide bandgap of (3.37 eV) at room temperature and a large exciton binding energy (approximately 60 meV). ZnO thin films can be doped with a variety of semiconductors to meet the demands of several application fields [1]. There are several works that use dopants such as Co, In, or Al in ZnO to enhance the optical and electrical conductivity. The doped films can be used for various applications such as transparent electronics , piezoelectric devices , gas sensors , and the transparent electrode window layer of thin-film solar cells [1]. Zinc oxide has attracted a significant attention in the last decades because its wide band gap behavior which confers a host of potential applications in gas sensors [1], solar cells, Catalysis , Organic light emitting diodes (OLEDs) , microelectronic devices how transistors, particularly a very interesting use of the ZnO is like electronic devices with highly nonlinear current-voltage relationships called Varistors [2]. Several processes using both chemical and physical methods have been reported for the production of these materials (ZnO and ZnO-composites) like tape casting, sol-gel, vapor-phase transport method , chemical vapor transport (CVT) process, spray pyrolysis (SP) , sputtering, etc., either to produce powders, thin films, nanoparticles, nanorods, composites, thermal evaporation etc. [3]. The aim of this paper is to demonstrate the physical properties of pure )ZnO( and doped thin films, because of the importance of )ZnO( compound on the solar cells, ZnO thin films are used as an anti-reflective coating (ARC) and transparent conductive oxide (TCO) due to their high optical transmittance in the visible light region, high band gap energy (Eg ≈3. 3 eV), optimum refractive index (n ~ 2.0), and natural n-type electrical conductivity. ZnO can be used as a heat mirrors, piezoelectric devices , thin films, and for chemical and gas sensing [4]. So we study The effect of the dopent element (Au , Ag , Cu) of )3%( were study on the improvement of the optical and structural properties of (ZnO) thin films.

Experimental Part Pure (ZnO and ZnO: Au, Ag, Cu) thin films with doping ratio ( 3%) , have been deposited by thermal evaporation technique of (300 nm) thickness at room temperature from bulk sample in a residual pressure of (10-5 mbar). The absorbance and transmittance were studed at room temperature using a double-beam spectrophotometer model (UV-Visible 2601) in the wavelength range (300-1100) nm ,using blank substrate as the reference position. The absorption coefficient (α) was calculated using the formula: α=2. 303 A/t ---------- (1) Where , t is the film thickness and A is the optical absorbance. The optical absorption spectrum used to determine the optical energy gap and absorption coefficient. The energy dependence of the absorption coefficient (α) near the band edge for band to band and excitation transition could be described by Tauc formula equation : αhν=B (hν -Egopt) 1/r ---------- (2) Where, (B) is a constant inversely proportional to amorphousity, (hν) is the photon energy (eV),( Egopt) is the optical energy gap (eV) ,and (r) is constant and may take values ( 2, 3,1/2, 3/2) depending on the material and the type of the optical transition[2] . The values of (αhν)2 plotted against photon energy (hν), the straight line portion extrapolated to zero and the values, which obtained, represent the optical energy gap for direct transition.

Results and Discussions The structure of pure and doped (ZnO) thin films were studied, noticed the structural changes that affected by doped elements. The X-ray Diffraction spectra of the films showed that they have a polycrystalline structure nearly for all samples with diffraction peaks at (2θ =31. 9°, 36.4°, 47.6°, and 56.6°) except that doped with (Cu) elements, the last two peaks disappear as shown in Fig. (1), with one significant peak at (36.4°) , which is properties of polycrystalline materials when there was an improvement in its characteristics, that it approach the single crystalline structure, and the intensity increase in this direction at this diffraction angle, which referred to (101) direction [2], which indicated that the diffusion of (Cu) into (ZnO) can cause the formation of various countries (CuZn, Cui). It is possible that (Cu) atoms can replace either substitution or interstitial (Zn) atoms in the (ZnO) lattice creating structural deformations [3].

Fig, (1) XRD pattern for pure ZnO and Au, Ag, Cu doped thin films. The optical energy gaps of pure (ZnO) and (Au , Ag , Cu) doped thin films have been calculated from the absorption and transmission measurements, they have been drawn in fig. (2), we deduced that there are allowed direct type for all the samples. The region of the absorption edge in all layers due to the transition between the valence band and the conduction band [1]. Also it can be seen from fig.(2) thin films have sharp absorption peaks at around (370 nm) in ultraviolet region which represent intrinsic absorption peaks of (ZnO).

Fig.(2) (αhν)2 as a function of photon energy (hν) of pure ZnO and Au, Ag, Cu doped thin films.

The optical energy gap values of pure (ZnO) decrease as it doped with (Au , Ag , Cu) thin films respectively as shown in table (1).

Table(1) Optical energy gap (eV) of pure (ZnO) and (Au, Ag, Cu) doped thin films.

Optical energy gap (eV) ZnO ZnO: Au ZnO: Ag ZnO: Cu 3.35 3.2 3.1 2.1

Conclusion The X-ray Diffraction spectra of the pure (ZnO) and dopted films showed polycrystalline structure nearly for all samples with diffraction peaks at (2θ =31.9°, 36.4°, 47.6°, and 56.6°) except that doped with (Cu) element which has one significant peak at ( 36.4°), so in the choosing doped process it approached to single crystalline as we doped with (Au , Ag , Cu) respectively. The optical energy gap has direct allowed transition and it decreases with the choice doping process with (Au , Ag , Cu) respectively, this means that we can use it as a source to fabricate solar cell heterojunction because the material, which must be choice for solar cell heterojunction must have an energy gap between (1-2 eV) , and the sample doped with (Cu) elements has this property.

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