OPTICAL PROPERTIES OF TRANSPARENT NANOCRYSTALLINE Cu 2 O THIN FILMS SYNTHESIZED BY HIGH PRESSURE GAS SPUTTERING Ramesh Chandra, Praveen Taneja and Pushan Ayyub Department of Condensed Matter Physics and Materials Science, Tata Institute of Fundamental Research, Homi Bhabha Road, Mumbai 400 005, India. (Received April 13, 1999) (Accepted June 9, 1999) Abstract—We report the controlled synthesis of nanocrystalline thin films of Cu 2 O as well as metallic Cu on glass substrates at room temperature by the versatile high pressure magnetron sputtering technique. Deposition of nanophase materials takes place in a relatively high pressure of inert gas (2– 600 mTorr) at 300 K. The average primary grain size of the materials thus synthesized was found to lie in the 4 –14 nm range and could be controlled by proper choice of process parameters (nature and the pressure of the sputtering gas, applied power, substrate temperature, and system geometry). In general, nanocrystalline thin films were formed at 300K, while loosely aggregated nanoparticles deposited at 77K. We define the conditions under which it is possible to deposit optically transparent, nanocrystalline thin films of semiconducting, single phase Cu 2 O on glass or quartz. ©1999 Acta Metallurgica Inc. Introduction A material is usually termed “nanophase” when it’s average crystallographic domain size lies in the approximate range of 1–50 nm. These include sintered materials with an ultrafine grain structure, loosely aggregated nanoparticles, and nanocrystalline thin films. Their novel properties, coupled with numerous exciting applications, many of them futuristic, have invigorated the field tremendously in recent years (1,2). The classes of synthetic methods used to prepare nanophase materials include chemical processes, spray techniques and physical vapor deposition. Here, we report the synthesis of optically transparent, nanocrystalline thin films of semiconducting Cu 2 O as well as metallic Cu by dc magnetron sputtering in an inert gas atmosphere at relatively high pressures with the substrate (glass or quartz) being held between room temperature and 77K. Nanocrystalline semiconductors are interesting from the point of view of basic physics as well as applications. In such systems, quantum size effects (which essentially result in a blue shift of the energy gap) are manifested when the system size is comparable to the Bohr radius of the exciton that is characteristic of the system. A reduction in the size may also lead to enhanced luminescence and non-linear optical properties. A thin film with a nanocrystalline structure is often transparent when the grain size is much smaller than optical wavelengths. Transparent conducting films have been previously made from a number of semiconducting oxides such as SnO 2 , In 2 O 3 , CdO, etc. Such films find important applications as windows for photovoltaic devices, anti-reflective coatings, displays, trans- parent heaters and electrodes, thin film resistors, vidicons, and so on. To the best of our knowledge, this is the first successful attempt at depositing transparent, nanocrystalline thin films of single phase, semiconducting Cu 2 O. Two other factors are also significant: (a) the deposition was done on commonly Pergamon NanoStructured Materials, Vol. 11, No. 4, pp. 505–512, 1999 Elsevier Science Ltd Copyright © 1999 Acta Metallurgica Inc. Printed in the USA. All rights reserved. 0965-9773/99/$–see front matter PII S0965-9773(99)00336-0 505