Nanosecond Resolved Temperature Measurements Following Pulsed Laser Irradiation
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NANOSECOND RESOLVED TEMPERATURE MEASUREMENTS FOLLOWING PULSED LASER IRRADIATION Michael 0. Thompson Department of MaterialsScience, Cornell University, Ithaca, NY 14853 ABSTRACT A transient resistance technique has been developed which allows monitoring the temperature of a thin film sample at a fixed depth from the surface following pulsed laser irradiation. The technique utilizes the temperature dependence of a thin, electrically insulated, semiconducting or metallic layer. Temperature determinations with nanosecond resolution, an absolute accuracy of ±50 K, and a relative accuracy of ±5 K are demonstrated. Combined with simultaneous interface position and velocity measurements, the undercooling at the interface during rapid solidification may be obtained. Preliminary results using this technique during the solidification of thin Ge films are presented. INTRODUCTION Pulsed laser induced melting and solidification of the near surface region has become a powerful experimental technique in the study of kinetics and thermodynamics of rapid phase transformations. In silicon, pulsed laser irradiation can result in surface melts to ;t1.0 Azm, with epitaxial solidification at velocities approaching 15 m/s.' Several recent experiments have attempted to quantitatively determine, in this velocity regime, the function relating the interface temperature to the interface velocity, the interface response function. 2 In Si, this effort is severely limited by the lack of direct measurements of the interface temperature during the melting and solidification processes. Indeed, in almost all reports to date, the interface temperature can only be estimated from other kinetic measurements or from computer based heat flow simulations. While these temperature estimates are valuable, experimental determinations of the temperature is essential to adequately test current theories for the structure of this interface response function.' These temperature measurements need resolutions of the interface tempearture on the order of a few degrees and time resolution of approximately five nanosecond (the time scale for formation of a-Si from the melt).
On the time scale of interest to pulsed laser irradiation (400 MHz.
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For a fixed geometry, the time dependent resistance may be related directly to the temperature dependent conductivity through
-] L [V Lv(t) -2
tL 1
Wt R(t)
u(t)
(2)
t
where L is the length of the resistor, W is the width and tg is the thickness of the layer. If the temperature dependent conductivity a(T) is known, a(t) can be directly converted to an average temperature T(t) versus time. The conductivity as a function of temperature is shown in fig. 2 for Si and Cr. Metal Thermometer Layers For metals, the conductivity may be approximated as a(T) = a(To)[1 - a(T - To)]
= u(To)[1 - aAT]
(3)
where a(To) is the conductivity at To (a reference point), and a is the temperature coefficient of resistivity and AT = (T - To). For typical metals, c(To)N-10 5 n-'cm-' and a.-3-4x10- 3 K-1.' 2 Naive application of eqn. 3, howev
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