Preparation and characteristics study of ZnO : ( Al , Cu , I ) thin films by chemical spray pyrolysis

Spray pyrolysis deposition technique has been used to grow ZnO thin films doped with different dopant species (Al, Cu, I). The optical and electrical properties of films were investigated as function of dopant type and concentration. The structural characteristics of undoped and doped ZnO films were studied using X-ray diffraction (XRD). The electrical resistivity as low as 4 × 10−2 Ωcm was obtained for ZnO:Al with 5 at.% dopant concentration. ZnO:Cu films prepared at specific conditions exhibited p-type conductivity. The optical band gap of doped ZnO films varied from 3.09 eV to 3.2 eV. XRD investigation confirmed that the doped ZnO films had preferred orientation in the direction of (101) plane. [DOI: 10.1380/ejssnt.2006.636]


I. INTRODUCTION
Zinc oxide films have found extensive applications in heterojunction solar cells [1], gas sensors [2] ,varistors [3] and a phosphor for color displays [4].It has many attractive properties such as low toxicity, high conductivity, high transmittance in visible region, and high infrared reflectance.A variety of methods have been used to prepare ZnO films such as thermal evaporation [5], rf sputtering [6], CVD [7], and pulsed laser deposition [8].In addition to these techniques chemical spray pyrolysis (CSP) technique has received attention due to its simplicity and consequent economics aspect as it does not require high vacuum equipments, and another attractive point is the possibility of production of large-area films.Low resistive ZnO films have been achieved by doping with various third group elements like Al, B, Ga or with fifth group elements like F. Extensive studies were focused on grown of ZnO films by CSP [9,10].The possibility to obtain p-type ZnO film opens up novel applications in optoelectronic light-emitting devices, including laser operating in UV and blue regions [11].Extensive studies were carried out on ZnO:Cu films [12][13][14], which act as room-temperature ferromagnetism due to Cu ions substituted into ZnO lattice.Iodine is a useful n-type dopant because it is expected to have a lower diffusion coefficient compared to third group elements .No significant data were published on optical, structural and electrical properties of ZnO films doped with Cu and I dopants.Furthermore, no comparative study was reported on ZnO film doped with Cu, Al, and I dopants.The main objective of the present work was focused on the structural, optical, and electrical properties of ZnO films doped with various dopants (Al, Cu, I) grown by CSP.

II. EXPRIMENT
Home-made chemical spray pyrolysis system was used to grow ZnO:(Al, Cu, I) films on cleaned coring glass substrates All films are prepared at substrate temperature of 400 • C and some ZnO:Cu films were grown at 250 • C.The description of chemical spray system used in this study was presented elsewhere [15].The deposition process involves decomposition of an aqueous solution of 0.1 M zinc nitrate Zn (NO 3 ) 2 with spray rate of 2.5 mL/h.To performed doping Al(NO 3 ) 3 , NH 4 I, and CuCl 2 were added to solution separately.The Al/Zn, Cu/Zn , and I/Zn ratios varied from zero to 5 at.%. Compressed air was used as carrier gas.Film thickness was measured using interferometer and it was 400 nm for all samples.The structure of grown films was investigated using CuK α XRD system.The optical transmittance of films in the spectral range 300-900 nm was studied using a Shimadzu UV-VIS spectrophotometer.The electrical resistivity of grown films and carrier concentrations have been measure using Hall measurements.The conduction type was investigated using thermoelectric measurements.

III. RESULTS AND DISCUSSION
Figure 1 shows XRD patterns of pure ZnO and doped ZnO films.All diffracted peaks belong to hexagonal structure of ZnO and are indexed on the basis of ASTM data card no.5-664.The spectra confirmed that no existence of peaks correspond to dopants and/or dopants oxides.The structure of undoped ZnO shows the domination of (002) plane which means that the c-axis is perpendicular to the substrate [16] and the lattice constants were found to be a o = 0.3245 nm and c o = 0.5208 nm .These values are very close to that of ASTM data a o = 0.3249 nm and c o = 0.5205 nm.Table I tabulates the lattice constants of all diffracted planes for undoped and doped ZnO films.XRD spectra of ZnO films are changed after doping, the peak intensity of I(002) crystal plane was decreased and increase of peak intensity of I(100) and I(101) crystal planes.These results are in good agreement with published results [17].The I(002)/I(101) ratio of undoped and doped ZnO films can be evaluated from Table II.It is obvious that a significant reduction in I(002)/I(101) ratio was observed after doping, the highest value was noticed for ZnO:I film, however all films have diffracted peaks intensity ratio higher than that for ASTM data and these results are agree well with recently reported data particularly for high doping concentration [18].The low electrical resistivity in TCO's(transparent conductive oxide)    Al +3 atoms will substitute Zn +2 and act as donors [19].Seebeck measurements confirmed that all prepared ZnO:Al films were n-type.In the case of iodine impurities which belong to seventh group elements, the I −1 dopant is substituted by O −2 and causes an additional free electron in conduction band.The mechanism of doping with transition elements (3d-ions) such as Cu +1or+2 depends on the substitution between Cu and oxygen vacancies.Sometimes, the Cu ion act as acceptor.The obtained results confirmed that all ZnO:Cu films made at 250 • C were p-type while the other that prepared at higher substrate temperature were n-type ,the reason for these results is unknown and need more investigation.Buchholz [12] shows recently the possibility of produced p-type ZnO:Cu using pulsed laser ablation under certain condition.On the other hand, Hartmann, et.al [20] show that the resistivity of ZnO decreased after doping with Cu dopant at 5 at.% doping concentration and decreased more after vacuum annealing due to reduced Cu +2 ions to Cu +1 (Cu + oxidation state).Figure 2 shows the variation of electrical resistivity with dopant concentrations.The undoped n-ZnO film has resistivity around 8.5×10 −2 Ωcm, it is clear that the resistivity of all doped ZnO films decreases with increasing doping concentration in similar manner.At certain doping concentration the ZnO:Al films have the lowest resistivity.The effective doping is obtained when the ionic radius of the dopant is the same or less than the ionic radius of Zn +2 .All the dopants used in this work have ionic radii larger than that for host which explains the reason that the obtained results of resistivity for ZnO doped with Al, Cu, I technique [17].
Figure 3 shows the variation of carrier concentration with dopant concentration for different dopant species, the resistivity of ZnO:Al films are little bit higher than that for ZnO:In films prepared by the same method [17].
Figure 4 displays the optical transmittance plots for undoped and doped ZnO films, the transmittance taken for different doping concentrations.It is obvious from the these spectra that the heavily doped ZnO films have low transmittance in the visible region due to scattering of photons by crystal defects produced by doping and also due to free carriers absorption of photon.From optical transmittance data the absorption coefficient α was calculated for all films.
where m * ve is the reduced effective mass of the valence band and the conduction band.After gathering the results of transmittance (T ) and sheet resistance (R s ) ,the figure of merit (Φ) was determined from the following relationship and listed in Table III.
Table III presents the Φ of ZnO:(Al, Cu, I) films.The highest value of Φ obtained was 1.29 × 10 −4 Ω −1 for ZnO:Al film with doping concentration of 1 at.%. Increasing doping concentration resulted in a significant decreasing in figure of merit.

IV. CONCLUSION
Optical, electrical, and structural properties of ZnO and ZnO:(Al , Cu , I) films prepared by spray pyrolysis were presented.The dopant type strongly affects ZnO properties.The I(002)/I(101) ratio for diffraction peaks was decreased after doping.ZnO:Al film exhibited the lowest resistivity at doping concentration of 5%.ZnO:Cu film prepared at 250 • C was p-type ,while the other conditions and dopants demonstrated n-type.The optical band gap was ranged from 3.09 to 3.18 eV depending on dopant type and concentration.Figure of merit has the highest value for ZnO:Al film.The effect of post-deposition annealing on films characteristics is underway.

TABLE I :
XRD data for ZnO and ZnO:(Al, I, Cu) films.

Electrical Resistivity ( cm) Dopant Concentration (at.%)
FIG. 2: Electrical resistivity of doped ZnO films as a function of doping concentration.films is due to high electron concentration comes from the

TABLE II :
Band gap of ZnO and ZnO:(Al, I, Cu)films.

TABLE III :
Values of Figure of merit of pure and doped ZnO films.