Excimer Laser Amorphous Silicon Film Crystallization: A Study of Time Resolved Reflectivity Measurements
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EXCIMER LASER AMORPHOUS SILICON FILM CRYSTALLIZATION: A STUDY OF TIME RESOLVED REFLECTIVITY MEASUREMENTS C. SUMMONTE, M. BIANCONI AND D. GOVONI, CNR-LAMEL, via Castagnoli 1, 40126 Bologna, Italy ABSTRACT Time Resolved Reflectivity during XeCl pulsed laser irradiation of amorphous silicon films deposited on glass was measured. Simulation of the process by a Heat Flow Calculation in which explosive crystallization was not forced to occur, predicts the coexistence of partial bulk nucleation and a traveling molten layer. Optical simulation of Time Resolved Reflectivity was used to critically examine the Heat Flow Calculation results, substantially confirming the existence of a mixture of thermodynamical phases. INTRODUCTION Excimer laser crystallization of hydrogenated amorphous silicon thin film deposited on glass is a technique that is presently encountering large favour in amorphous silicon processing, and is being analyzed under many aspects. In this investigation, Heat Flow Calculation (HFC) is utilized, though some constraints must be introduced to produce a satisfying picture of the melting-solidification process. The choice between bulk nucleation1 and explosive crystallizationl, 2, the existence of a primary and a secondary melt and its consequence in the production of different grain structures, are physical aspects of the process that still need to be defined. Measurements of Time Resolved Reflectivity (TRR) are widely employed during irradiation, and are mainly used to detect the time scale of events, while very little is normally quantitatively deduced from actual reflectivity values. This is mainly due to poor knowledge of the optical behaviour of Si phases and their mixture above the solid phase crystallization temperature. In this work, we investigate laser heating below and above the melting threshold using TRR, and examine the experimental data by combining HFC with optical calculations (OC), both to draw information about the optical behaviour of amorphous silicon at high temperature, and to deduce some details about the physics of the process. EXPERIMENTAL Hydrogenated amorphous silicon (a-Si:H) films, 0.4 ptm thick, PECVD deposited on Corning glass at 230 C substrate temperature, were irradiated in air. We utilized a XeCI Excimer Laser, X=308 nm, FWHM pulse duration 45 ns. Laser fluence was varied between 0.08 and 0.28 J/cm 2, being the melting threshold about 0.20 J/cm 2 . TRR at 633 nm was measured by making use of an unpolarized HeNe laser beam at an angle of incidence of 230. The laser beam was allowed into an optical fiber, collected by a Digital Signal Analyzer via an O/E converter and filtered at 100 MHz (risetime 3.5 ns) . Some samples were examined by Transmission Electron Microscopy (TEM) at 300 kV. In order to simulate the thermal process, a program based on HFC has been developed. The sample is divided in 12.5 rum layers and a state array contains information about the 3 different states coexisting in any layer at any time. The analysis of the phase diagram of silicon shows that ultrafa
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