Rapid Crystallization of Amorophous Gallium Produced by Laser Quenching
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RAPID CRYSTALLIZATION OF AMOROPHOUS GALLIUM PRODUCED BY LASER QUENCHING
J. Fr6hlingsdorf and B. Stritzker Institut fur Festkdrperforschung, Kernforschungsanlage Jilich, P.O. Box 19 13, D-5170 Jilich, FRG
ABSTRACT Transient conductance measurements reveal the metastability of amorphous Gallium (a-Ga): Gallium can be amorphized for a certain "lifetime" at temperatures as high as 60 K, i.e. well above the crystallization temperature of 16 K. For the first time the lower part of the C-shaped transition curve in a TTT-diagram can be measured for a pure metal. The results are in good agreement with undercooling behavior of liquid Ga-droplets.
INTRODUCTION Laser irradiation with nsec-pulses is known to provide insight into processes that control metastable and amorphous phase formation of importance for certain applications
[1] For basic research gallium appears to be an ideal system for several reasons. First, there is no influence of a second species. Second, Ga is the only metal that can be amorphized by vapor quenching [2] and partially by laser quenching [3] or even, in the presence of 4 - 5 at % oxygen, low temperature ion irradiation [4] . Third, appearing phases can be rather well identified by their distinct residual resistivity superconducting transition temperature Tc.
p0 and their
EXPERIMENTAL Ga metal (Koch-light Lab., grade 6N) was evaporated onto quartz substrates kept at liquid He-temperature resulting in a-Ga with a resistivity
of 29 p0 cm and a superconducting
Mat. Res. Soc. Symp. Proc. Vol. 100. 01988 Materials Research Society
610
transition temperature Tc of 8.5 K. Amorphous Gallium is stable until about 16 K. It then transforms into the metastable crystalline a-phase ( -Ga, monoclinic [5]) with a resistivity of 3 ý2cm and a Tc value of 6.3 K. At about 60 K a-Ga transforms into the stable crystalline a -phase ( a -Ga, orthorhombic [6]) with a resistivity of 12 Vocm and a Tc of 1.07 K. These different values of p0 and Tc were used for phase identification. Temperature of the films was measured by a calibrated Ge-resistor (1 K < T < 30 K) and a Pt-resistor (30 K < T). Resistivity was determined by means of a two point probe technique and transient conductance measurements. Superconductivity of Ga-films causes bias voltages of less than 1 V resulting in low signal to noise ratios as compared to transient measurements of silicon [7]. Laser irradiation could be performed in situ immediately after quench condensation and suitable annealing to obtain a or a-Ga. The laser used in the experiments was a KrF-excimerlaser with 248 nm wavelength, 40 nsec total pulse duration and output power of about 0.5 J/cm2 (without focussing elements). RESULTS As described in an earlier paper, both a - and a- crystalline films can be amorphized partially by pulsed laser irradiation [3]. Threshold energy densities for amorphization of s-Ga increase with increasing film thickness whereas the onset of a~a transition occurs at about 225 mJ/cm2 , independent of film thickness. Transient measurements give evidence of amor
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