Rapid crystallization of thin solid films
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I. INTRODUCTION Conventional differential scanning calorimetry (DSC) studies chemical transformations at heating rates of 0.01-10 K/s and transformation times longer than 1 s. Since the method measures changes in the heat content of the sample under study, a substantial volume of transforming material must be transformed. The method is hence unsuitable to study transformations in thin films at high transition rates. For the case of thin Si films it has been shown1 that the transformation range can be extended by using laser beams to induce and detect the transition. This article describes a more extended experimental technique appropriate for the study of isothermal transformations in solid films with a thickness of less than 1 fira on a thick substrate. Transformation times between 1 //s and 104 s can be accurately measured. The method employs the absorption of light and its subsequent conversion into heat to drive the transformation, in combination with the measurement of the change in optical properties of the film during the heating period. The heating and probing beams are generated by separate light sources. Results from measurements on the amorphous-to-crystalline transition in thin films of InSb and TeSeSb alloys serve to illustrate the method.
Gaussian spatial intensity distribution is produced with a full width at half-maximum (FWHM) of 8-15 //m. The spot position can be manipulated by slight variation of the angle of incidence of the beam. Part of the incident light is absorbed in the film and converted into heat. Thus the light beam acts as the heat pump driving the
DET (T)
IF X2
IF DET
PBS
IT m
MON NA=0.60 SAMPLE NA=0.A5
II. EXPERIMENTAL TECHNIQUE The experimental setup is shown schematically in Fig. 1. A collimated beam of light is focused onto the sample consisting of a thick transparent substrate, upon which the thin film has been deposited by means of evaporation or sputtering techniques. The beam issues from a Kr-ion laser, operating at A t = 752 nm. Collimating optics is chosen such that the beam significantly underfills the objective lens. On the sample a light spot with a 126
J. Mater. Res. 3 (1), Jan/Feb 1988
http://journals.cambridge.org
IFX 2 2
(C.W.) FIG. 1. Schematical drawing of the experimental setup. The modulated beam with wavelength A., is indicated by a dotted line: BS—beam splitter, PBS—polarizing beam splitter, IF—interference filter, DET—detector, MON—monitor.
0003-6951/88/010126-07$01.75
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© 1988 Materials Research Society
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C. J. van der Poel: Crystallization of thin solid films
transition under study. The pump intensity and its temporal dependence, and consequently the heating rate of the film, are controlled by means of an acousto-optical modulator in combination with a programmable pulse generator. The resulting rise time of the pump beam is 15 ns, with an on-off ratio better than 1000:1. Because of the Gaussian profile the temperature distribution induced by the pump beam is spatially nonuniform (see Sec. HI).
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