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Effect of pH on structural and morphological properties of spray deposited p-typetransparent conducting oxide CuAlO2 thin films
Madhav Singh, A. Ranga Rao, Viresh Dutta ⁎Photovoltaic Laboratory, Centre for Energy Studies, Indian Institute of Technology Delhi, Hauz Khas, New Delhi-110016, India
Article history:Received 4 September 2007Accepted 19 February 2008Available online 4 March 2008
CuAlO2 due to its delafossite structure is known to exhibit p-type conductivity. p-CuAlO2 thin films havebeen prepared using spray pyrolysis technique. The films prepared using precursor solutions with differentpH values have been characterized for optical, structural and morphological properties. The X-raydiffractograms of the films deposited using precursor solution with pH 3.7 do not show any peaks, thesesuggest the amorphous nature of the films. On the other hand, the films deposited with pH 1.5 show thepeaks of CuAlO2 with delafossite crystal structure. Thus the pH of the solution is playing a crucial role ingiving the crystalline product of the material of interest. Tauc's plot of the film deposited using precursorsolution of pH 3.7 shows the absorption edge at 3.71 eV and the film deposited with solution pH 1.5 showsthe absorption edge at 3.64 eV, corresponding to the direct band gap of CuAlO2. This confirms the formationof CuAlO2. The surface morphology of the films shows the novel networking structure. Compositionalanalysis done using Energy dispersive X-ray spectrum shows an excess of oxygen that should be favorable forgiving the p-characteristics.
Transparent conducting oxides (TCOs) are of interest due theirapplications in infrared reflective coatings and in optical displayssuch as active matrix liquid crystal displays, flat-panel displays, UVlight emitting diodes, heterojunctions for solar cells and transparentsemiconductor devices [1,2]. The majority of TCOs are n-typeconductors such as indium tin oxide, fluorine-doped tin dioxideSnO2:F, or ZnO. A number of p-type TCOs have recently beendeveloped, such as the CuAlO2 and SrCu2O2. This has opened up thepossibility of making p–n junctions using p- and n-TCOs.
Recently, p-type transparent ternary oxide films, with Cu as amajor cationic species, have been reported [1–11]. Preparation of thep-type transparent oxide films has been mainly carried out by pulselaser deposition and rf sputtering using the sintered bulk polycrystaltargets [1–5,11], by solid-state reaction at temperatures higher than1000 °C, chemical-vapor deposition technique [7], solution methods[8] and the reactive dc-sputtering method using Cu and Al elementaltargets.
Spray pyrolysis is a very simple and large area depositiontechnique and has been used for the preparation of thin films ofdifferent materials and devices like solar cells. Earlier spray pyrolysishas been used for the preparation of p-CuAlO2 films at higherdeposition temperatures [9]. The main advantage of this technique is
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that the properties of the films can be changed easily by changingthe parameters like substrate temperature, molar ratio, and pH ofthe spray solution. In this letter we report on the preparation andcharacterization of CuAlO2 thin films deposited using spray pyrolysistechnique. The effect of pH on structural studies of CuAlO2 films isinvestigated. For the first time a novel networking surface morphol-ogy in the films is reported.
2. Experimental section
The precursor solution is prepared by dissolving salts of copper(CuCl2·2H2O) and (AlCl3) with [Cu]/[Al] molar ratio of 1 in tripledistilled water. The pH of thus prepared solution is found to be 3.7.The solution is sprayed on the glass and quartz substrates attemperature 375 °C kept in air. Nitrogen is used as carrier gas. Thesolution and carrier gas flow rate is maintained at 1–1.5 ml/min and2.2 kgf/cm2 respectively. Films are deposited with precursorsolution pH of 3.7 and 1.5, the pH of the solution is adjustedusing HCl. Structural properties of the films have been studied usingGiegerflex-D/max-RB-RU200 Rigaku X-ray diffractometer. Perkin-Elmer Lambda 900 UV–VIS–NIR spectrometer has been used forrecording optical absorption spectrum. The surface morphology ofthe films is studied using LEO 435 VP scanning electron microscopy(SEM) and Nanoscope IIIa digital instruments Atomic Force micro-scopy. Hot probe method is used in order to find the majoritycarrier type in the films. The compositional analysis is done usingEDAX instrument attached to the SEM. For simplicity the filmsdeposited are named as follows: with solution pH 3.7 as Film A, the
Fig. 1. (αhv) 2 versus hv plot of Films A, B and C.
Fig. 2. X-ray diffractogram of Films A, B and C.
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film annealed at 475 °C for 1 h as Film B, film deposited withsolution pH 1.5 as Film C.
3. Results and discussion
3.1. Majority carrier determination
The thermo-voltage generated in the hot probe method for all the films shows thatthe hot end is at negative voltage compared to the cold end. This confirms that holes arethe majority carriers in the films. This means that all our spray deposited CuAlO2 filmsare of p-type.
3.2. Optical properties
Fig. 1 shows the plots of (αhv)2 versus hv (Tauc's plots) of the films deposited underdifferent conditions. From the plot the absorption edge for Film A is found to be 3.71 eV,corresponding to the direct band gap of CuAlO2. For Films B and C absorption edges arefound to be at 3.64 eV and 3.65 eV, respectively. Earlier reports suggest that the directband gap of CuAlO2 varies from 3.5–3.75 eV depending on the preparation conditionsand composition of the elements. This confirms the formation of CuAlO2 films by sprayat low deposition temperatures.
3.3. Structural and morphological studies
Fig. 2 shows the X-ray diffractograms of Films A, B and C. Film A does not show anypeaks, which can be due to the amorphous nature of the films. Film B shows the peaksthat can be assigned to CuAlO2 with (101) and (012) as dominant peaks. The peakscorrespond to CuAlO2 only; there is no other peak from other oxides like CuO and Al2O3.This is a very important result, because CuAlO2 films deposited by other methods alsoshow impurity phases (CuO and Al2O3) in addition to CuAlO2 peaks [7].
Fig. 3 shows the SEM images of Film A and Film B. The SEM images of both the filmsshow the novel networking structure. Film B shows an expected increase in the grainsize keeping the networking structure unmodified. Fig. 4 shows the AFM image of FilmA. The AFM image shows same networking structure; this supports the SEM studies.
Compositional analysis from the EDAX spectra (Fig. 5) of Film B (both spot and areaanalysis) shows that the Cu:Al:O are in the ratio of 1:1:2+δ (δ = excess oxygen),thisconfirms the formation of CuAlO2 films with excess oxygen. This excess oxygen isrequired to get good p-type characteristics. Hence, it should be possible to optimize thedeposition conditions in order to improve the electrical characteristics of the spraydeposited films. The EDAX spectra also show the presence of other elements (Si, Ca, Baetc.) that are present due to the contribution from the glass substrate.
X-ray diffractogram of Film C shows (Fig. 2, Film C) the peaks that can be assigned tothat of CuAlO2 with (101) as the dominant peak along with the other peaks. The latticeparameters calculated from XRD data are found to be a=0.286 nm and c=1.695 nm,these values are well matched with the standard data. X-ray diffractograms of Film Ashow the amorphous nature, but Film C shows the crystalline nature of the films, thissuggests that pH of the solution is playing a major role in giving rise to a crystallineproduct. At pH 3.7 the required reaction is not taking place to give the exactcomposition of CuAlO2.
Earlier Bouzidi et al., has reported on preparation and characterization of p-CuAlO2
films using spray pyrolysis [9]. In this process the films were deposited at hightemperatures (450–525 °C), and X-ray diffractograms show the amorphous product.The films were annealed at 575 °C for 1 h to get the crystalline product. But in our casefilms deposited at relatively low temperature (375 °C) with precursor solution of pH 1.5show the crystalline product with better peak intensities.
Our proposed reaction mechanism for the formation of CuAlO2 is as follows:
CuCl2d2H2Oþ AlCl3 þ H2O YOn hot substrate
CuAlO2 þ Gaseous products ð1Þ
CuðOHÞ2→CuO þ H2O ð2Þ
2AlðOHÞ3→Al2O3 þ 3H2O ð3Þ
2CuO þ Al2O3→2CuAlO2 : ð4Þ
The above mechanism suggests that the final product formation goes through aseries of reaction steps. Firstly the formation of hydroxides is taking place, nextformation of corresponding oxides of Cu and Al occurs, these oxides will undergo asolid-state reaction on the hot substrate and form the required product of specificcomposition. The final composition depends on the stability constants of the
Fig. 3. SEM images of Films A, B and C.
3615M. Singh et al. / Materials Letters 62 (2008) 3613–3616
intermediate products. The solution pH of 3.7 is not good enough to give the requiredproduct, but pH qqpls check conversion:b2.5 is found to be suitable to give the requiredproduct with good crystallinity and composition.
Fig. 3 (c) shows the SEM image of Film C. SEM image clearly shows the isolatedparticles joining together on the novel networking structure (shown in the inset).The formation of networking structure is due to the interactions like van derWaals, dipole–dipole, electrostatic forces and hydrophilic interactions. The naturalforces in the system increase the surface energy. In order to minimize the surfaceenergy, the system goes to its most stable phase, this results in the creation ofnetworking morphology.
Fig. 4. AFM imag
4. Conclusions
Thin films of CuAlO2 have been successfully deposited using spraypyrolysis technique at a relatively low substrate temperature of 375 °C.pH is found to play a major role in giving rise to polycrystalline CuAlO2
films. Films deposited with spray solution of pH 1.5 were found to givepolycrystalline CuAlO2 films. For the first time a novel networkingsurface morphology has been reported in these films. EDAX spectra
e of Film A.
Fig. 5. EDAX spectra of Film B.
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show the composition in the ratio of 1:1:2+δ for Cu, Al and oxygen,respectively. The presence of excess oxygen in the films should help inobtaining better p-type conductivity in spray deposited CuAlO2 films.
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