Liquid-phase explosive crystallization of electron-beam-evaporated a-Si films induced by flash lamp annealing

2014

Can. J. Phys.

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Flash lamp annealing (FLA) of micrometre-order thick amorphous silicon (a-Si) films can induce explosive crystallization (EC), high-speed lateral crystallization driven by the release of latent heat. We develop multipulse FLA system, which emits a quasi-millisecond pulse consisting of a number of subpulses. The emission frequency of the subpulses can be systematically controlled, and the emission of subpulses leads to the periodic modulation of the temperature of a Si film and the resulting formation of macroscopic stripe patterns. The relationship between a subpulse emission frequency and the width of the macroscopic stripe patterns yields EC velocity. Two kinds of EC modes can be observed, depending on the methods of precursor a-Si deposition and (or) a-Si film thickness.Résumé : Le recuit par lampe à décharge (FLA) de films de silicium amorphe (a-Si) peut causer une cristallisation spontanée (explosive) (EC) ou une cristallisation latérale induite, soutenue par le rejet de chaleur latente. Nous développons un système de recuit par lampe à décharge multi-pulse qui émet des impulsions presque milliseconde consistant en un nombre de sousimpulsions. La fréquence d'émission de ces sous-impulsions peut être contrôlée et l'émission de ces sous-impulsions mène à une modulation de la température du film de Si, d'où il résulte la formation d'un patron en bandes macroscopiques. La relation entre la fréquence d'émission des sous-impulsions et la largeur de ces bandes donne la vitesse de EC. Nous observons deux types de modes EC, dépendant de la méthode de déposition du précurseur Si et (ou) de l'épaisseur du film de a-Si. [Traduit par la Rédaction]

Section: Introductionmentioning

confidence: 99%

A method to evaluate explosive crystallization velocity of amorphous silicon films during flash lamp annealing

2014

Can. J. Phys.

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“…Cat-CVD can yield a-Si films with lower film stress than conventional plasma-enhanced CVD (PECVD). We have also revealed that the crystallization of a-Si films induced by FLA is based on explosive crystallization (EC), self-catalytic lateral crystallization driven by the release of latent heat [8][9][10][11][12]. To date, we have observed at least two modes of EC: 1) EC forming poly-Si films with periodic microstructures along a lateral crystallization direction containing solid-phase-nucleated (SPN) 10-nm-sized fine grains, and 2) EC based on liquid-phase epitaxy (LPE) which results in the formation of poly-Si films consisting only of large grains with a length of N10 μm stretching along an EC direction.…”

Section: Introductionmentioning

confidence: 99%

“…A variety of methods have been investigated to obtain thin poly-Si [1][2][3][4][5], and the crystallization of precursor amorphous Si (a-Si) films by post-annealing is one of the most successful approaches to form poly-Si films [1][2][3]. We have so far investigated the utilization of flash lamp annealing (FLA), millisecond-order discharge from Xe lamps, to crystallize precursor a-Si films prepared on glass substrates [6][7][8][9][10][11][12]. Due to its proper annealing duration, FLA can form more than 4 μm-thick poly-Si films without serious thermal damage onto entire glass substrates [6,7].…”

Section: Introductionmentioning

confidence: 99%

The control of the film stress of Cat-CVD a-Si films and its impact on explosive crystallization induced by flash lamp annealing

2015

Thin Solid Films

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a b s t r a c tCatalytic chemical vapor deposition (Cat-CVD) can produce amorphous silicon (a-Si) films with low film stress, in general, compared to plasma-enhanced CVD, and is thus suited for the preparation of precursor a-Si films for thick poly-Si films applied for solar cells. The stress of a-Si films is known to sometimes play an important role for the crystallization of a-Si films and resulting grain size of polycrystalline Si (poly-Si) films formed. I investigate the impact of the stress of Cat-CVD a-Si films on the mechanism of explosive crystallization (EC) induced by flash lamp annealing (FLA). The stress of Cat-CVD a-Si films can be controlled by changing the temperatures of substrates and/or a catalyzing wire during film deposition. Cat-CVD a-Si films with tensile stress (~200 MPa) can be deposited as well as films with compressive stress. The enlargement of grain size is observed in a part of flash-lamp-crystallized (FLC) poly-Si films formed from Cat-CVD films with tensile stress compared to those with compressive stress, which might be an indication of a certain degree of impact of film stress on poly-Si formation. The grain size is, however, much smaller than that of FLC poly-Si films formed from electron-beam-(EB-) evaporated a-Si films with similar tensile stress. This fact may indicate the existence of other critical determinants of EC mechanism.

“…Of a variety of process techniques to form poly-Si films, the annealing of precursor a-Si films for crystallization by rapid treatment has been expected to be one of the most productive methods. We have so far investigated the crystallization of a-Si films on glass substrates by FLA, an annealing technique using millisecond-order discharge from Xe lamps [1][2][3][4][5]. Due to its proper annealing duration, µm-order-thick a-Si films can be crystallized without serious thermal damage onto whole glass substrates.…”

Section: Introductionmentioning

confidence: 99%

“…Depending on the method of a-Si preparation, at least two kinds of EC can occur: one is EC governed by frequent solid-phase nucleation leaving behind periodic microstructures, and the other is EC only through LPE. The former EC occurs when sputtered or catalytic chemical vapor deposited (Cat-CVD) a-Si films are used [1][2][3], while the latter EC is seen if we use EB-evaporated a-Si films as precursors [4,5]. We can obtain larger grains in the latter case due to less frequent nucleation, which would lead to reduction in carrier recombination on grain boundaries and thus contribute to improvement in solar cell properties.…”

Section: Introductionmentioning

confidence: 99%

Formation of polycrystalline silicon films with μm-order-long grains through liquid-phase explosive crystallization by flash lamp annealing

2012 38th IEEE Photovoltaic Specialists Conference

Varlamov

Usami

et al. 2012

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Flash lamp annealing (FLA) is a short-time annealing technique which can heat and crystallize µm-orderthick amorphous silicon (a-Si) films without inducing serious thermal damage onto whole glass substrates thanks to its proper fluence on the order of several tens of J/cm 2 and millisecondorder duration. The FLA of a-Si films leads to lateral explosive crystallization (EC), driven by the release of latent heat, with a lateral crystallization speed on the order of m/s. When we use electron-beam-(EB-) evaporated a-Si films as precursors, EC only through liquid-phase epitaxy (LPE) occurs, resulting in the formation of polycrystalline Si (poly-Si) films with µm-order-long grains stretching along lateral crystallization directions. We see no remarkable difference in the microstructure of poly-Si films on various glass substrates, and this crystallization can take place also when doped EB-evaporated a-Si films are used. We have confirmed the simultaneous crystallization of pn-stacked a-Si films by FLA, and according to the data of scanning spread resistance microscopy (SSRM), the diffusion of dopants, B and P, can be sufficiently suppressed to a level at which the pn-stacked poly-Si films can be used as solar cells. The advantages of rapid crystallization and the formation of large grains would contribute to the establishment of the low-cost fabrication process of thinfilm poly-Si solar cells.