Metastable Phase Formation in Laser Processed Al-Ge Thin Films
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METASTABLE PHASE FORMATION IN LASER PROCESSED Al-Ge THIN FILMS F.Catalina (*), C.N.Afonso (*) and C.Ortiz (**) (*) Instituto de Optica, Serrano 121, Madrid 28006, Spain (**) IBM Almaden Research Center, 650 Harry Road, San Jose, Ca95120-6099,USA ABSTRACT This paper presents a comparative study of the microstructures formed in Al.59Ge.41 and A1. 7 0Ge. 3 0 (eutectic composition) free standing films under laser irradiation. An hexagonal metastable phase located between Al lamella, can be formed depending on the film composition and the laser pulse length. INTRODUCTION Rapid solidification processing techniques are suitable to generate nonequilibrium or metastable alloys I1. Among the different techniques, pulsed laser processing is one of the most promising ones due to its versatility and ability to interact with the material on a microscopic scale. The interaction of the laser beam with the material surface is determined by several variables, among which the most important are the incident power density, the pulse length and the wavelength of the radiation. Undercooling values greater than 100 0 C and cooling rates in the range of 107-109 K/s are easily obtained, giving thus a variety of microstructures and new phases 121. Systems such as Si-Al have generated a large amount of interest from the point of view of rapid solidification and casting applications 131. The Ge-Al system can be expected to be similarly interesting. In particular, this system has proved to exhibit several metastable crystalline structures 14,5,6,71 depending on the undercooling and cooling rate 181. U.Koster 161 pioneered the in situ crystallization study of Ge-Al thin films in the heating stage of a Transmission Electron Microscope (TEM). He was able to detect up to four metastable phases by changing the composition and the heating rate. M.J.Kaufman et al. 171 have produced metastable crystalline phases in this system both by melt spinning or by electron beam crystallization of thin films or submicron powders. The few studies of laser irradiation of Ge-Al have been devoted to the mixing of multilayer configurations 19,101. The main result was the formation of an eutectic amorphous layer. In a previous work Jll on the laser irradiation of Al.59Ge.47 amorphous films, we have shown the formation of the hexagonal phase under certain laser irradiation conditions. It was detected in a structure with eutectic composition being formed by alternated lamella of the metastable phase and textured Al. This paper presents the formation of a crystalline metastable phase induced by pulsed laser irradiation in eutectic Al. 7 0 Ge.30 thin films. It is a comparative study of the structural transformations occurring in AlxGeix (x=.70,.59) thin films under laser irradiation. For the two studied compositions, we found the hexagonal metastable phase. Its formation will be discussed as a function of laser power and pulse length. EXPERIMENTAL The thin films were grown onto glass substrates in a co-sputtering triode system by means of two independently biased Ge (99.9
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