Robert S. Sposili

Sequential lateral solidification of thin silicon films on SiO2

We report on a low‐temperature excimer‐laser‐crystallization process that produces a previously unattainable directionally solidified microstructure in thin Si films. The process involves (1) inducing complete melting of selected regions of the film via irradiation through a patterned mask, and (2) precisely controlled between‐pulse microtranslation of the sample with respect to the mask over a distance shorter than the single‐pulse lateral solidification distance, so that lateral growth can be extended over a number of iterative steps. Grains up to 200 μm in length were demonstrated; in principle, grains of unlimited length can be produced. We discuss how the technique can be extended to produce large single‐crystal regions on glass substrates.

Low-temperature single-crystal Si TFTs fabricated on Si films processed via sequential lateral solidification

Nonhydrogenated, n-channel, low-temperature-processed, single-crystal Si thin-film transistors (TFTs) have been fabricated on Si thin films prepared via sequential lateral solidification (SLS). The device characteristics of the resulting SLS TFTs exhibit properties and a level of performance that are superior to polycrystalline Si-based TFTs and are comparable to similar devices fabricated on silicon-on-insulator (SOI) substrates or bulk-Si wafers. We attribute these high-performance device characteristics to the absence of high-angle grain-boundaries within the active channel portion of the TFTs.

Single-crystal Si films for thin-film transistor devices

The fact that single-crystal Si would make an ideal material for thin-film transistor devices has long been recognized. Despite this awareness, a viable method by which such a material could be directly produced on a glass substrate has never been formulated. In this letter, it is shown experimentally that location-controlled single-crystal Si regions on a SiO2 surface can be obtained in a glass-substrate compatible manner, via excimer-laser-based sequential lateral solidification of thin Si films using a beamlet shape that self-selects and extends a single grain over an arbitrarily large area. This is accomplished by controlling the locations, shape, and extent of melting induced by the incident excimer-laser pulses, in such a manner as to induce interface-contour-affected sequential super-lateral growth of crystals, during which the tendency of grain boundaries to align approximately orthogonal to the solidifying interface is systematically exploited.

Sequential Lateral Solidification of PECVD and Sputter Deposited a-Si Films

We have investigated sequential lateral solidification (SLS) of amorphous Si films that have been prepared via PECVD and sputter deposition methods. The focus of the work was on identifying and analyzing the energy density and per-pulse translation distance parameter space that permits SLS of these films. Experimental details include the use of a two-axisprojection irradiation system to image a straight-slit beamlet pattern onto the sample, and analyzing the resulting microstructures by SEM and optical microscopy of Secco-etched samples. High-temperature-deposited LPCVD films were also examined to enable further comparative analysis. We conclude from these results that there are no major differences in both the SLS process characteristics and the resulting microstructure among the investigated films (provided that the films are dehydrogenated in the case of PECVD a-Si). Based on the controlled super-lateral growth (C-SLG) model of the SLS process, we attribute these findings to the fact that the SLS method involves—as one of its essential features—complete melting of the Si film at fluences that are sufficient to thoroughly melt crystalline Si films, during which all pre-irradiation phase and microstructural details are erased.

A Novel Agglomerated-Silicon Thin-Film Transistor

This letter discusses the fabrication and electrical characteristics of a novel thin-film transistor (TFT) architecture based on intentionally agglomerated silicon for the active (island) region. Although the agglomeration of irradiated semiconductor is undesirable during the laser crystallization of polycrystalline-silicon TFTs, it is shown that precisely controlled wirelike structures can be obtained for certain conditions. Their width and pitch are maintained over very long distances, and their crystal structure is almost single crystal. Fabricated n- and p-channel TFT characteristics with maximum effective mobility values of 360 and 70 cm2/V ·s, respectively, are presented, with on/off current ratios exceeding ten decades.