Journal of ELECTRICAL ENGINEERING, VOL. 58, NO. 6, 2007, 347–350 TEM INVESTIGATIONS OF Au–NiO NANOCRYSTALLINE THIN FILMS AS GAS SENSING MATERIAL Ivan Hotov´ y * — Jozef Liday * — Lothar Spiess ** — Henry Romanus ** — M´ aria ˇ Caploviˇ cov´ a *** — Dalibor B´ uc * — Helmuth Sitter **** — Alberta Bonanni **** — Peter Vogrinˇ ciˇ c * Nanocrystalline NiO thin films were deposited by dc reactive magnetron sputtering in a mixture of oxygen and argon and subsequently coated by Au on a NiO film surface. Very thin Au overlayers with a thickness of about 1 and 7 nm have been prepared by magnetron sputtering. Then, the surface modified NiO films have been analysed by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected electron diffraction (SAED) and energy dispersive X-ray (EDX) methods. NiO thin films had a polycrystalline structure (FCC NiO phase) with the size of the nanocrystals ranging from a few nanometers to 10 nm. The electrical response of NiO-based structure, as a function of hydrogen concentration has been measured. Keywords: thin films, nanocrystals, TEM, nickel oxide 1 INTRODUCTION Nickel oxide (NiO), as a model system for p-type ma- terials, is attractive for its chemical stability as well as for its excellent optical and electrical properties. Indeed, NiO thin films have been studied for applications in elec- trochromic devices and also as functional layers for so- lar cells [1, 2]. In particular, the field of gas sensing has benefited from the production of prospective materials characterized by a high surface-to-volume ratio. The gas- sensing properties of metal oxides are more or less related to the material surface, its high porosity and a nanostruc- ture with small particles. Also, these properties can be essentially improved by doping the surfaces by catalytic metals as gold (Au) [3, 4]. Considerable efforts have been undertaken to investigate thin film materials based on metal oxide, but there is no available information about nanostructured films with surface modification for gas de- tection. The approach is different from the heuristic one because our purpose was not to obtain the best sensors, but to modify and control the metal oxide surface by fab- ricating small Au particles or clusters using magnetron sputtering. This fabrication technique, which facilitates the control of the particle properties such as size and composition on a nanometer scale, allows the tight control over critical process parameters and therefore contributes greatly to the reproducibility of the nanostructure films. We have successfully prepared nanocrystalline NiO thin films with the mean crystal size of ∼ 10 nm by dc reactive magnetron sputtering from a metallic Ni tar- get in a mixture of oxygen and argon. To improve the sensing characteristics of the nanocrystalline NiO films, we deposited very thin Au overlayers with a thickness of about 1 and 7 nm on top of the NiO surface by magnetron sputtering. Then, the modified NiO films have been an- alyzed with TEM, HRTEM, SAED and EDX. Electrical responses of the NiO-based sensors towards H 2 concen- tration have been measured. 2 EXPERIMENTAL DETAILS The NiO films were deposited by dc reactive mag- netron sputtering from a Ni target (101.2 mm in diam- eter, thickness of 3 mm and 99.95 % pure) in a mixture of oxygen and argon. A sputtering power of 600 W was used. Both the inert argon flow and reactive oxygen flow were controlled by mass flow controllers. The relative par- tial pressure of oxygen in the reactive mixture O 2 -Ar was 20 %. The total working sputtering gas pressure was kept at 0.5 Pa and adjusted by a piezoceramic valve. The films thickness as measured by a Talystep were about 100 nm for all the samples. NiO films were deposited onto un- heated KCl for physical characterization. On top of these base films, thin Au overlayers (1 and 7 nm thick) were deposited by magnetron sputtering. The amount of Au deposited on the surface of NiO thin films was controlled by measuring the sputtering time and the thickness of the Au layer was measured by AFM. In order to stabilize the ∗ Department of Microelectronics, Slovak University of Technology, Ilkoviˇ cova 3, 812 19 Bratislava, Slovakia; ivan.hotovy@stuba.sk; ∗∗ Department of Materials Technology, Technical University Ilmenau, PF 100565, D-98684 Ilmenau, Germany; ∗∗∗ Department of Geology of Mineral Deposits, Comenius University, Mlynsk´a dolina, 842 15 Bratislava; ∗∗∗∗ Institute of Semiconductor and Solid State Physics, Johannes Kepler University, Altenbergerstr. 69, A-4040 Linz, Austria ISSN 1335-3632 c  2007 FEI STU