Formation of Several-Micrometer-Thick Polycrystalline Silicon Films on Soda Lime Glass by Flash Lamp Annealing

Abstract

We have succeeded in forming polycrystalline silicon (poly-Si) films with thicknesses of over 4 mm on soda lime glass by flash lamp annealing (FLA) of precursor amorphous Si (a-Si) films deposited by catalytic chemical vapor deposition (Cat-CVD). The insertion of Cr thin films between glass substrates and a-Si significantly improves the adhesion of Si films to the glass substrates, resulting in uniform crystallization of a-Si in 20 Â 20 mm 2 area. Several types of substrate, such as quartz substrates, are also used instead of soda lime glass to elucidate the effects of the properties of glass substrates on formation of the poly-Si films. a-Si films tend to be crystallized under lower irradiance than those on quartz glass substrates, which can be described by the lower thermal conductivity and the thermal diffusion length of soda lime glass. Raman spectra of the poly-Si films on soda lime glass show high crystallinity close to unity. The utilization of soda lime glass with poor thermal resistivity is of great importance for the cost-effective mass production of thin-film poly-Si solar cells.

Variation of crystallization mechanisms in flash-lamp-irradiated amorphous silicon films

“…1 shows surface appearances of FLC poly-Si films with various film thicknesses. A 2.4-mm-thick poly-Si film reveals a rainbow-colored surface, as previously shown on the surfaces of thicker FLC poly-Si films [12,15,16]. Fig.…”

“…We have already confirmed that there is no significant difference in the crystalline fractions and microstructures of poly-Si films formed on quartz and soda lime glass substrates [12], and in this study, we chose quartz substrate (with a smaller thermal expansion coefficient) because it suppresses Si film peeling. The use of Cr films is also to suppress Si film peeling, and we have confirmed that Cr films do not affect the crystallization mechanisms of a-Si films on them, except for a slight modulation of the irradiance required for crystallization caused by the reflection of irradiated flash lamp light on the Cr films.…”

“…Therefore, FLA can be a candidate method of forming c-Si films with high throughput and low thermal damage to substrates. We have succeeded in forming polycrystalline Si (poly-Si) films of more than 4 mm thickness by FLA of a-Si films even on conventional soda lime glass substrates [12], and demonstrated the operation of solar cells using flashlamp-crystallized (FLC) poly-Si films [13,14]. The solar cell properties obtained so far are short-circuit current density of 8.6 mA/cm 2 , open-circuit voltage of 0.30 V, fill factor of 0.51, and conversion efficiency of 1.3% [14], and they will be improved by optimizing the solar cell process.…”

Polycrystalline Si films with unique microstructures formed from amorphous Si films by non‐thermal equilibrium flash lamp annealing

“…This annealing duration corresponds to the thermal diffusion length of glass and a-Si of approximately 50 µm, meaning sufficient heating of a-Si films a few µm thick, while avoiding thermal damage to glass substrates. We have actually demonstrated that poly-Si films more than 4 µm thick can be formed by FLA even on soda lime glass substrates [8,9], and also succeeded in the operation of solar cells fabricated using the poly-Si films [10]. Furthermore, the crystallization of a-Si films by FLA is found to take place by a unique mechanism, unlike simple solid-phase or liquidphase crystallization, resulting in the formation of unprecedented periodic microstructures.…”

Electrical properties of polycrystalline silicon films formed from amorphous silicon films by flash lamp annealing

“…As one of the crystallization methods, we have studied flash lamp annealing (FLA) mainly for solar cell applications, which is a millisecond-order annealing and can crystallize mm-order-thick a-Si films into poly-Si films without thermal damage to entire glass substrates because of selective annealing of a-Si films due to a 10 mm-order thermal diffusion length. We have already clarified that 4.5 mm thickness a-Si films, deposited by catalytic chemical vapor deposition (Cat-CVD) on Chromium (Cr)-coated quartz or soda lime glass substrates, can be crystallized by FLA with high areal uniformity and without pealing of the poly-Si films [1,2].…”