Residual disorder in low-temperature polycrystalline silicon

Abstract

Polycrystalline silicon, widely used so far as current line material in microelectronics, has recently become a promising candidate for thin films devices (transistor, solar cell). In all these applications, low-temperature fabrication (< 600°C) will represent a significant progress. This was achieved through two ways : (i) by assisting the thermal decomposition of silane with a diode hf plasma ; the temperature threshold of polycrystalline deposition is decreased down to 350°C; (ii) by purely thermal deposition of an amorphous film at 620°C, then in situ recrystallization at 400°C through exposure to a hydrogen plasma, which on the other hand passivates the grain boundaries. We report the correlative variations of the grain size (x-ray), the residual short range disorder (Raman) and the H content (Elastic Recoil Detection) for these various poly-Si preparations. Posthydrogenation is shown to relax the Si matrix. The Raman line widening previously attributed to crystallite size below a few 102 A is shown here to originate from residual short range disorder in the material. At this moment, poly-Si films obtained at low temperature exhibit higher residual disorder than those obtained from pure LPCVD at high temperature. Moreover, depending on the preparation, additional disorder is shown to appear at the surface and interface of all polycrystalline films.