Preparation and Optical Characterization of ZnO-Pr films Masters Thesis Kapilanjan Krishan Department of Physics Indian Institute of Technology – Kanpur November 28, 1998 _______________________________________________________________________ 1. Introduction Zinc Oxide based films have found vast applications due to their electrical and optical properties. A large number of techniques have been used in the preparation of such films, however Pulsed Laser Deposition offers distinct advantages. This technique is extremely versatile and the deposition of films may be controlled to a great degree. The contamination of films in this technique is also kept to a minimum during deposition. In this article, we report on films of ZnO doped with varying amounts of Pr and optically characterize them using transmission measurements. The films were prepared under a range of conditions involving the variation in the substrate, substrate temperature as well as oxygen environment during deposition. 2. Experimental details 2.1 Preparation of targets The targets for laser ablation were made starting from sintered ultra pure pellets of ZnO and powder Pr6O11. The pellets were ground and dissolved in a 1N solution of HCl along with Pr6O11. The concentration of ZnO and Pr6O11 was varied so as to achieve a 0.1%, 1.0% and 5% of Praseodymium (by number) relative to the number of Zn and Pr atoms in solution. The carbonates of Zn and Pr were co-precipitated by adding ammonium carbonate. This precipitate was filtered, dried and calcined at 900oC overnight. The homogeneity of the oxides thus recovered was checked using X-ray diffraction. Only in the 5% Pr sample was significant independent crystallization of Pr6O11 noted. The powders were then pressed in a hydraulic press and sintered at 1350oC for four hours. In some cases sintering at 1100oC overnight was found to suffice. 2.2 Film preparation Substrates of quartz(amorphous) and sapphire 1cm by 0.5cm were ultrasonically cleaned with propyl alcohol. They were mounted on a substrate heater that was used to control the substrate temperature upto 600oC. The targets were mounted in a vacuum chamber at a base pressure of 2-3 x 10-5 bar. A Kr-F excimer laser (Lumonics) of wavelength 248nm was focused at the rotating target.

The substrates were placed at a distance of 7cm or 5cm from the target. The pulse duration was 20ns and the laser was operated at 10Hz with an energy of 300mj. Film deposition was carried out over 18000 pulses after five minutes of surface conditioning. The oxygen environment was varied during deposition by maintaining an oxygen flow rate of about 220ccps at a pressure of 200mbar or depleting the chamber at a pressure of 5-6 x 10-6 bar. The deposition conditions of the samples are summarized below.

The films obtained were subjected to X-ray diffraction and were found to be oriented single crystal films. The X-ray diffraction was carried out using Cu as well as Cr Kα radiation. The transmission of the films on quartz substrates was made using a Perkin Elmer Lambda-40 UV-Vis spectrometer. The transmission spectrum between the wavelengths 190nm and 1100nm was measured. 3. Results and discussion 3.1 X-ray diffraction The diffraction sowed a single sharp peak, (corresponding to a lattice spacing between 1.8 to 2.7 angstroms) with a FWHM between 0.4 and 2.2 degrees.

The results suggest the formation of films with a strong preferred orientation. This preferred orientation was seen in all the films deposited, regardless of substrate temperature or atmosphere. The lattice parameters do not compare well with that reported in literature. This is probably due to the use of a XRD setup that is primarily meant for powder diffraction studies. Single crystalline thin films were studied using a crude adaptor for the goniometer. 3.2 Transmission measurements The transmission measurements showed a sharp optical edge of the films at about 350nm. Films that were deposited at 600oC were found to have an extra absorption dip around that wavelength (Figure 1). The transmission at UV is severely cut off in these films. The oxygen environment is seen to play a dominant role in the optical transmission characteristics (Figure 2). The original optical edge is blue shifted in films made at 200oC as compared to those at 600oC.

Acknowledgements This work was carried out under the supervision of Prof. R. C. Budhani with assistance in operating the pulsed laser by Dr. K. J. Singh.

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