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

Abstract

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.

Crystallization of catalytic CVD hydrogenated n-a-Si films on textured glass substrates by flash lamp annealing

“…The n-poly-Si films show a relatively low (519.3 cm −1 ) c-Si peak, indicating the existence of tensile stress. 23) The a-Si films deposited at 450 °C are compressively stressed, and the stress reduces at a lower deposition temperature. 23,24) Despite the compressive stress in Cat-CVD a-Si:H films, the compressive stress in the poly-Si is reduced during FLA, resulting in a tensilely-stressed poly-Si surface.…”

“…23) The a-Si films deposited at 450 °C are compressively stressed, and the stress reduces at a lower deposition temperature. 23,24) Despite the compressive stress in Cat-CVD a-Si:H films, the compressive stress in the poly-Si is reduced during FLA, resulting in a tensilely-stressed poly-Si surface. 23,24) c-Si peaks have a full width at half maximum of 7.6-9.6 cm −1 , indicating that the n-poly-Si films are composed of fine grains.…”

“…23,24) Despite the compressive stress in Cat-CVD a-Si:H films, the compressive stress in the poly-Si is reduced during FLA, resulting in a tensilely-stressed poly-Si surface. 23,24) c-Si peaks have a full width at half maximum of 7.6-9.6 cm −1 , indicating that the n-poly-Si films are composed of fine grains. 25) The crystalline fraction of the n-poly-Si films was estimated to be 0.78-0.98, which is lower than that of the i-poly-Si films of 0.88-0.98.…”

“…14) We have attempted to use precursor a-Si films formed by various types of deposition methods, such as catalytic chemical vapor deposition (Cat-CVD), sputtering, and electron-beam (EB) evaporation. [15][16][17][18][19][20][21][22][23][24] The crystallization of EBevaporated a-Si films on flat glass substrates is driven by liquid-phase explosive crystallization (EC), which leads to the formation of crystal grains with a size of several tens of μm. 17) A different type of EC occurs for the crystallization of Cat-CVD a-Si:H films, through which two types of regions with different microstructures appear alternately in the direction of crystallization with a period of ∼1 μm.…”

Effect of starting point formation on the crystallization of amorphous silicon films by flash lamp annealing

“…[1][2][3][4][5][6][7][8][9] There are many methods used to form poly-Si on glass substrates, such as the crystallization of precursor amorphous Si (a-Si) films and the direct deposition of microcrystalline Si (µc-Si) films. Among these methods, we have focused on the crystallization of a-Si by flash lamp annealing (FLA), [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] millisecond-order annealing using pulse light from xenon lamps. [29][30][31] FLA can crystallize µmorder-thick a-Si films without serious thermal damage to glass substrates with poor thermal tolerance because of the thermal diffusion length being several tens of µm both for a-Si and glass.…”