Ellipsometric characterization of surface freezing in Ga-based alloys

Abstract

We present results on surface freezing of Ga-based alloys, GaBi, GaPb and GaTl, above the liquidus line between the Ga-rich eutectic and the monotectic point. Spectroscopic ellipsometry (0.8 eV ≤hν≤4.2 eV) and kinetic single wavelength ellipsometry (2.75 eV) have been employed to probe the changes of the interfacial electronic structures on surface freezing. To minimize thermal gradients across the sample a heatable cap that covers the sample and crucible was developed. The surface freezing temperature, TSF, for the spontaneous formation of a solid-like film on top of the Ga-rich liquid on cooling the sample from the homogeneous phase region was found to be independent of the temperature difference between the upper and lower furnace (ΔT: +10 to −10 K) and only weakly dependent on the cooling rate (: 2.5–20 K h−1). In the case of GaPb the solid film consists of solid Pb with a thickness h≥400 Å. Comparing with GaBi we draw analogous conclusions for GaPb and GaTl and suggest that the surface freezing transition precedes the bulk phase transition along the liquidus line as the alloy is cooled.

Surface solidification in liquid Ga‐Tl alloys

“…At this temperature an abrupt change in ∆ from 124° to 115° and in Ψ from 40.5° to 38.25° degree is observed within 45 seconds! The solidification temperature was reproducible in several runs within ±1.5 K. This jump was identified for Ga-Bi, Ga-Pb and Ga-Tl with the formation of a smooth solid surface film [6,9]. The quality of the surface and the reflection alignment is only marginally altered as can be seen by the small change in S 0 .…”

“…This transition was detected by different spectroscopic methods (second harmonic generation, SHG, [6], surface light scattering [7], x-ray photoelectron spectroscopy, XPS, [8], ellipsometry [9]) and was also found in a model calculation of surface phase transitions in Ga-Bi alloys [10]. Ellipsometric investigations showed that this surface solidification phenomenon is robust with respect to significant temperature gradients across the sample and to a variation of the cooling rate.…”

“…We use a phase modulation ellipsometer (UVISEL, Jobin-Yvon) mounted stationary as in-situ instrument to anUHV chamber. To control and minimize temperature gradients across the sample a heatable cap covered Mocrucible and sample [9,11]. The reflection geometry is sketched in the inset of Fig.…”

“…The characteristic kinetics of the film formation leads to the conclusion that at least two separate processes have to be considered: there is at first the fast preceding front that leads to the crystalline film (see e. g. [6,9]). The thin liquid like layer of Tl can be regarded as a precursor for the complete wetting transition as well as for the solidification process.…”

“…Ellipsometric investigations showed that this surface solidification phenomenon is robust with respect to significant temperature gradients across the sample and to a variation of the cooling rate. Plausible reasons were given for the assumption that this surface transition has to be distinguished from the bulk liquidus reaction, even though it occurs close to it [9,11]. Besides these findings there are a number of questions still open.…”