Effect of pH on structural and morphological properties of spray deposited p-type transparent conducting oxide CuAlO 2 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 info abstract Article history: Received 4 September 2007 Accepted 19 February 2008 Available online 4 March 2008 CuAlO 2 due to its delafossite structure is known to exhibit p-type conductivity. p-CuAlO 2 thin films have been prepared using spray pyrolysis technique. The films prepared using precursor solutions with different pH values have been characterized for optical, structural and morphological properties. The X-ray diffractograms of the films deposited using precursor solution with pH 3.7 do not show any peaks, these suggest the amorphous nature of the films. On the other hand, the films deposited with pH 1.5 show the peaks of CuAlO 2 with delafossite crystal structure. Thus the pH of the solution is playing a crucial role in giving the crystalline product of the material of interest. Tauc's plot of the film deposited using precursor solution of pH 3.7 shows the absorption edge at 3.71 eV and the film deposited with solution pH 1.5 shows the absorption edge at 3.64 eV, corresponding to the direct band gap of CuAlO 2 . This confirms the formation of CuAlO 2 . The surface morphology of the films shows the novel networking structure. Compositional analysis done using Energy dispersive X-ray spectrum shows an excess of oxygen that should be favorable for giving the p-characteristics. © 2008 Elsevier B.V. All rights reserved. Keywords: p-Transparent conducting oxide Thin films Spray pyrolysis Crystal structure Surface morphology 1. Introduction Transparent conducting oxides (TCOs) are of interest due their applications in infrared reflective coatings and in optical displays such as active matrix liquid crystal displays, flat-panel displays, UV light emitting diodes, heterojunctions for solar cells and transparent semiconductor devices [1,2]. The majority of TCOs are n-type conductors such as indium tin oxide, fluorine-doped tin dioxide SnO 2 :F, or ZnO. A number of p-type TCOs have recently been developed, such as the CuAlO 2 and SrCu 2 O 2 . This has opened up the possibility of making p–n junctions using p- and n-TCOs. Recently, p-type transparent ternary oxide films, with Cu as a major cationic species, have been reported [1–11]. Preparation of the p-type transparent oxide films has been mainly carried out by pulse laser deposition and rf sputtering using the sintered bulk polycrystal targets [1–5,11], by solid-state reaction at temperatures higher than 1000 °C, chemical-vapor deposition technique [7], solution methods [8] and the reactive dc-sputtering method using Cu and Al elemental targets. Spray pyrolysis is a very simple and large area deposition technique and has been used for the preparation of thin films of different materials and devices like solar cells. Earlier spray pyrolysis has been used for the preparation of p-CuAlO 2 films at higher deposition temperatures [9]. The main advantage of this technique is that the properties of the films can be changed easily by changing the parameters like substrate temperature, molar ratio, and pH of the spray solution. In this letter we report on the preparation and characterization of CuAlO 2 thin films deposited using spray pyrolysis technique. The effect of pH on structural studies of CuAlO 2 films is investigated. 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 (CuCl 2 ·2H 2 O) and (AlCl 3 ) with [Cu]/[Al] molar ratio of 1 in triple distilled water. The pH of thus prepared solution is found to be 3.7. The solution is sprayed on the glass and quartz substrates at temperature 375 °C kept in air. Nitrogen is used as carrier gas. The solution and carrier gas flow rate is maintained at 1–1.5 ml/min and 2.2 kgf/cm 2 respectively. Films are deposited with precursor solution pH of 3.7 and 1.5, the pH of the solution is adjusted using HCl. Structural properties of the films have been studied using Giegerflex-D/max-RB-RU200 Rigaku X-ray diffractometer. Perkin- Elmer Lambda 900 UV–VIS–NIR spectrometer has been used for recording optical absorption spectrum. The surface morphology of the 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 majority carrier type in the films. The compositional analysis is done using EDAX instrument attached to the SEM. For simplicity the films deposited are named as follows: with solution pH 3.7 as Film A, the Materials Letters 62 (2008) 3613–3616 ⁎ Corresponding author. Tel.: +91 11 26591261. E-mail address: vdutta@ces.iitd.ernet.in (V. Dutta). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.02.070 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet