Thermal Stability and Phase Transformations of g-/Amorphous-Al 2 O 3 Thin Films Per Eklund, * Madanagurusamy Sridharan, Gurvinder Singh, Jørgen Bøttiger Introduction Alumina (Al 2 O 3 ) exists in a number of variations (poly- morphs) with very different structures and properties. [1] The a and k polymorphs are widely used as wear-resistant coatings due to their high hardness and thermal stability, while g and u-alumina are used as catalysis supports in catalytic applications due to their lower surface energy. [2] a-Al 2 O 3 coatings typically require high synthesis tempera- ture (1 000 8C by chemical vapor deposition [3] ), which limits the choice of substrates. Physical vapor deposition has enabled growth of crystalline alumina at reduced temperatures: growth of a-Al 2 O 3 can be achieved by a crystallographic template such as a-Cr 2 O 3 , [4–6] in solid solution a-(Cr,Al) 2 O 3 , [7–10] or as recently shown by the use of highly ionized sputtering techniques. [11,12] Generally, however, Al 2 O 3 films sputter-deposited below 700 8C typi- cally comprise amorphous, g -, k-, and/or u-alumina. [13–15] We have previously investigated [16] alumina thin films depos- ited by inductively coupled plasma magnetron sputtering, a technique that uses an rf coil to increase the degree of ionization in the deposition flux. [17,18] The alumina films contained g -alumina in an amorphous matrix (for short ‘‘g /a-Al 2 O 3 ’’ where ‘‘a’’ means amorphous); the phase composition of the films could be varied from fully amorphous to essentially only g by varying the substrate temperature, bias, and rf-coil power. [16] Since the metastable alumina phases can be synthesized at relatively low temperatures but transform irreversibly to the a phase at high temperature, the thermal stability of g /a-Al 2 O 3 films is interesting both from a fundamental- understanding point of view and for potential high- temperature applications. Here, we investigate g /a-Al 2 O 3 thin films annealed in the temperature range 600–1 200 8C. Annealing to 1 100–1 150 8C results in a transformation to a-alumina. The transformation takes place along different paths depending on the initial phase fraction of g in the as- deposited films. The intermediate phase u-alumina appears in samples with low initial g fraction, while the results indicate a direct g -a phase transformation for high initial g fraction. Experimental Part The Al 2 O 3 films were deposited by reactive inductively coupled plasma magnetron sputtering, as described elsewhere. [16] Si(001) Full Paper P. Eklund, M. Sridharan, G. Singh, J. Bøttiger Interdisciplinary Nanoscience Center (iNANO) and Department of Physics and Astronomy, University of Aarhus, DK-8000 Aarhus C, Denmark E-mail: perek@ifm.liu.se P. Eklund Present address: Thin Film Physics Division/FunMat, Department of Physics, Chemistry, and Biology (IFM), Linko¨ping University, SE-581 83 Linko ¨ping, Sweden M. Sridharan Present address: Center for Nanotechnology and Advanced Bio- materials (CeNTAB), SASTRA University, Thanjavur-613402, Tamil Nadu, India Magnetron-sputtered Al 2 O 3 thin films were annealed in ambient air. The phase compositions of the as-deposited Al 2 O 3 films were (i) fully amorphous, (ii) nanocrystalline g -Al 2 O 3 in an amorphous Al 2 O 3 matrix, and (iii) fully crystalline g . For all samples, annealing to 1 100– 1 150 8C resulted in a transformation to a-alumina. The transformation paths depend on the phase fraction of g in the as-deposited films. For amorphous films and films with low initial g fraction, the intermediate phase u-Al 2 O 3 appeared in the range of 1 000–1 100 8C. For pre- dominantly crystalline g -Al 2 O 3 as-deposited films no intermediate Al 2 O 3 phases were observed, indicating a direct g -to-a phase transformation at 1 100 8C. Plasma Process. Polym. 2009, 6, S907–S911 ß 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/ppap.200932301 S907