Carrier transport properties of flash-lamp-crystallized poly-Si films

Abstract

We have investigated carrier transport properties of µm-order-thick polycrystalline silicon (poly-Si) films formed by flash lamp annealing (FLA) of precursor amorphous Si (a-Si) films on glass substrates. The Hall mobility of flashlamp-crystallized (FLC) poly-Si films decreases as doping concentration increases, and then reversely increase with further increase in doping concentration, both in the cases of p-and n-type poly-Si films. The tendency observed is characteristic of poly-Si materials having a number of grain boundaries at which carriers are trapped. N2-atmosphere furnace annealing of FLC poly-Si films with low doping concentration significantly recovers their carrier concentration and improve their carrier mobility up to more than 10 cm 2 /Vs, which is probably because of the termination of the trapping states by hydrogen (H) atoms that exists in FLC poly-Si films on the order of 10 21 /cm 3 . The realized mobility of FLC poly-Si films exceeds those of conventional chemical-vapor-deposited (CVD) microcrystalline Si (µc-Si) films, and the remarkable carrier transport properties would lead to effective carrier collection and resulting high quantum efficiency of FLC poly-Si solar cells.

Microstructure of Polycrystalline Silicon Films Formed through Explosive Crystallization Induced by Flash Lamp Annealing

“…We have found that these remaining H atoms can be utilized for the termination of Si dangling bonds by furnace annealing, 14) and the defect density can be decreased to $5 Â 10 16 cm À3 . 15) This value is close to those of CVD a-Si and microcrystalline Si films, and further reduction of defect density will hopefully be realized by optimizing the defectterminating annealing conditions and/or applying other passivation techniques.…”

“…Relatively high Hall mobilities of $10 cm 2 V À1 s À1 have actually been observed in the FLC poly-Si films. 13) Effective carrier collection can be expected in solar cells fabricated using FLC poly-Si films. On the other hand, the passivation of grain boundaries would be one of the most important issues in realizing high-efficiency solar cells using FLC poly-Si films, because of the absence of a-Si tissue passivating grain boundaries, unlike CVD microcrystalline Si films.…”

Defect Termination of Flash-Lamp-Crystallized Large-Grain Polycrystalline Silicon Films by High-Pressure Water Vapor Annealing

“…Another likely reason for the difference is large difference in poly-Si film thickness: 50 nm for excimer-laser-crystallized poly-Si films The defect density obtained in this study is much lower than that of FLC poly-Si films formed from hydrogenated Cat-CVD a-Si films, i.e., $5 Â 10 16 /cm 3 , after furnace annealing under N 2 atmosphere. 20) This might also be due to the difference in grain size. We have succeeded in obtaining 3-m-thick poly-Si films with a low defect density equivalent to that of device-grade CVD a-Si and/or microcrystalline Si (c-Si) films.…”

Impedance spectroscopy of polysilicon in SOI structures

“…On the other hand, the presence of dispersed material structure requires the use of certain techniques and methods, that would bring characteristics of polycrystalline silicon in SOI structures closer to those of monocrystalline, for which high values of charge carriers mobility are inhered. In particular, the authors of [17] have made the flash-lamp-crystallized p-and n-type poly-Si films, in which the Hall mobility was decreasing with the increase of the doping concentration and then reversely rising with the further increase of the latter. Among the other known methods for reducing the structural defects, laser recrystallization [18] of initial poly-Si makes it possible to substantially increase the charge carrier mobility in a layer.…”