Laser Quenched Metal-Silicon Alloys
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LASER QUENCHED MFTAL-SILICON ALLOYS M. VON AL1IFN, K. AFFOLTEP, Institute of Applied Physics, University of Bern, CH-3012 Bern, Switzerland, and M. WITT¶MER, Prown Boveri Research Center, CR-5405 Baden, Switzerland. ABSTRACT Q-Switch laser pulses are used to convert vapor-derosited metal-Si thin films into the amorphous state by a melting, mixing and quenching process. Experimental findings obtained with the metals Au, Pt, Pd, V and Nb are presented and compared with results from a numerical simulation of the laser induced melting and quenching process. INTRODUCTION Melting and quenching of metal-Si multilayers on top of inert substrates by means of Q-switch laser pulses has recently been demonstrated to be an effective method in achievinq metallic glass formation in the systems Au-Si, Pt-Si and Pd-Si [1,21. As compared to conventional splat coolinq techniques, faster heat extraction is achieved in the process, resultina in extended compositional ranqes of glass formation. In addition, new metastable crystalline phases were obtained upon subseauent thermal decomprsition of the amorphous films. We have applied similar technioues to the systems V-Si and Nh-Si. In order to ohtain some more cuantitative information about the solidification velocities actually achieved by laser-induced melt auenchinq we have carried out numerical calculations which allow at least for a few of the physical processes occurino durinq solidification in a concentrated alloy rea ime. EXPFPIMFNTAL Procedures and results The experiments carried out so far have been explorative in nature and were mainly aimed at 1) establishinq the compositional ranges of glass formation, 2) determining the thermal stability of the glasses produced, and 3) searching for metastable crystalline phases that might nucleate from the amorphous films upon annealing, but before thermal equilibrium is reached. The samples were prepared by high-vacuum vapor deposition of metal and Si onto Sapphire or glass substrates. The elements were deposited in the form of typically 5 - 10 thin (< 30 nm) alternating films, thus forminq a sandwiched layer of given average composition and of a total thickness between 100 and 200 nm (1,2]. If possible, the top layer was chosen to be Si sufficiently thick in order to optimize absorption of laser radiation [3]. The samples were then irradiated with 35 ns pulses from a Q-switched Nd laser with a spot size of 3 mm. Laser intensity was sufficient to melt the sandwich layer down to the substrate, while preserving a smooth surface. This recauired laser fluences 2 below 1 J/om . The composition of the samples after irradiation was routinely checked by RBS as described previously [1,2]. Structural investiqations were based on x-ray diffractometry usinq a Seemann-Bohlin camera. To trace the phase transitions occurina in the films, we monitored their electrical resistivity (four-point method) or their optical reflectivitv as a function of temperature, while the samples were heated in a
560 furnace at a constant rate of about 30 C/min. After
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