Solidification of highly undercooled liquid silicon produced by pulsed laser melting of ion-implanted amorphous silicon: Time-resolved and microstructural studies

Abstract

Nanosecond resolution time-resolved visible (632.8 nm) and infrared (1152 nm) reflectivity measurements, together with structural and Z-contrast transmission electron microscope (TEM) imaging, have been used to study pulsed laser melting and subsequent solidification of thick (190–410 nm) amorphous (a) Si layers produced by ion implantation. Melting was initiated using a KrF (248 nm) excimer laser of relatively long [45 ns full width half maximum (FWHM)] pulse duration; the microstructural and time-resolved measurements cover the entire energy density (E1) range from the onset of melting (at ∼ 0.12J/cm2) up to the onset of epitaxial regrowth (at ∼ 1.1 J/cm2). At low E1 the infrared reflectivity measurements were used to determine the time of formation, the velocity, and the final depth of “explosively” propagating buried liquid layers in 410 nm thick a-Si specimens that had been uniformly implanted with Si, Ge, or Cu over their upper ∼ 300 nm. Measured velocities lie in the 8–14 m/s range, with generally higher velocities obtained for the Ge- and Cu-implanted “a-Si alloys.” The velocity measurements result in an upper limit of 17 (± 3) K on the undercooling versus velocity relationship for an undercooled solidfying liquid-crystalline Si interface. The Z-contrast scanning TEM measurements of the final buried layer depth were in excellent agreement with the optical measurements. The TEM study also shows that the “fine-grained polycrystalline Si” region produced by explosive crystallization of a-Si actually contains large numbers of disk-shaped Si flakes that can be seen only in plan view. These Si flakes have highly amorphous centers and laterally increasing crystallinity; they apparently grow primarily in the lateral direction. Flakes having this structure were found both at the surface, at low laser E1, and also deep beneath the surface, throughout the “fine-grained poly-Si” region formed by explosive crystallization, at higher E1. Our conclusion that this region is partially amorphous (the centers of flakes) differs from earlier results. The combined structural and optical measurements suggest that Si flakes nucleate at the undercooled liquid-amorphous interface and are the crystallization events that initiate explosive crystallization. Time-resolved reflectivity measurements reveal that the surface melt duration of the 410 nm thick a-Si specimens increases rapidly for 0.3E1 <0.6 J/cm2, but then remains nearly constant for E1 up to ∼ 1.0 J/cm2. For 0.3 < E1 < 0.6 J/cm2 the reflectivity exhibits a slowly decaying behavior as the near-surface pool of liquid Si fills up with growing large grains of Si. For higher E1, a flat-topped reflectivity signal is obtained and the microstructural and optical studies together show that the principal process occurring is increasingly deep melting followed by more uniform regrowth of large grains back to the surface. However, cross-section TEM shows that a thin layer of fine-grained poly-Si still is formed deep beneath the surface for E1<0.9 J/cm2, implying that explosive crystallization occurs (probably early in the laser pulse) even at these high E1 values. The onset of epitaxial regrowth at E1 = 1.1 J/cm2 is marked by a slight decrease in surface melt duration.

Excimer-Laser-Induced Melting and Solidification of PECVD a-Si films under Partial-Melting Conditions

“…We note that the above model can be recognized as being distinct from a number of arguments and models that were previously suggested by other investigators in dealing with the research topics that are related to the present work [3,15,16]. Our model appears to be singularly consistent with essentially all experimental results that have been obtained, and, in the process, addresses some of the questions and details that are associated with the initiation and propagation of explosive crystallization.…”

Nonequilibrium model of laser-induced phase change processes in amorphous silicon thin films

“…18. For n pc and k pc at 633 nm of FG and LG pc-Si, the estimates by Lowndes et al 9 were used. For the coefficients C 1 , C 2 of the interface response functions from Eqs.…”

Cristallisation du silicium amorphe par laser à excimères

“…Cependant, sous l'effet d'une impulsion laser suffisamment brève, l'apport de chaleur est tel que le matériau fond (par une transition de phase du premier ordre) avant d'avoir cristallisé. La température de fusion du a-Si étant inférieure de ~ 250°C à la température d'équilibre solide-liquide (température de fusion du c-Si, T c ), il s'ensuit que le liquide ainsi formé est très surfondu, ce qui va favoriser la nucléation rapide de cristallites en son sein (dans les conditions d'irradiation utilisées par Lowndes et al, les premières cristallites apparaissent dans le liquide environ 10 ns après le début de la fusion de la surface [13]). La nucléation et la croissance de ces cristallites libère la chaleur latente de changement d'état du cristal (L c ) dans le liquide, ce qui provoque réchauffement de ce dernier au dessus de la température de fusion du a-Si.…”

“…Il s'ensuit que le a-Si contigu au liquide est amené à fusion, et que cette nouvelle transformation absorbe la chaleur latente de liquéfaction du matériau amorphe, L^ ce qui empêche le liquide de se réchauffer vers T c . La fine couche de liquide nouvellement formée sous les cristallites reste donc en surfusion importante, ce qui entraîne la rapide nucléation de nouvelles cristallites etc.. Comme L c > L a , (voir tableau 1), la fusion de a-Si sur une fine couche (couche liquide enterrée) traversant progressivement toute l'épaisseur du film est autoentretenue, et ce phénomène est appelé cristallisation explosive [13]. Le taux de nucléation étant très élevé dans la fine couche de liquide surfondu, la cristallisation explosive transforme le film de a-Si en un film de Si polycristallin à très petits grains, sur l'épaisseur intéressée par le phénomène.…”

“…Le taux de nucléation étant très élevé dans la fine couche de liquide surfondu, la cristallisation explosive transforme le film de a-Si en un film de Si polycristallin à très petits grains, sur l'épaisseur intéressée par le phénomène. La vitesse initiale de propagation de la couche liquide enterrée a été mesurée autour de 12-14 m/s [13,14]. grains.…”