Laser processing of silicon for photovoltaics and structural phase transformation

Abstract

High-power nanosecond-pulse-width laser processing is attracting increasing attention for the manufacturing of low-cost high-performance silicon photovoltaic and microelectronic devices. However, the lack of fundamental understanding of laser induced defect formation and phase transformation hinders the broader application of lasers. To address this, we systematically investigated the laser-induced phase transformation using different laser systems of 532 nm wavelength, 1.3 ns pulse width and 1064 nm wavelength, 50 ns pulse width. In doing this, we carried out cross-sectional transmission electron microscopy (TEM) and Raman spectroscopy line-mapping studies to analyze the local phase information across the laser processed spot. We demonstrate the retention of single-crystalline phase under 1.64 J/cm2 fluence using a 532 nm wavelength laser. This retention of single-crystalline phase is important for ensuring high effective carrier lifetime and hence high photovoltaic conversion efficiency. Moreover, the 1064 nm wavelength laser processed samples under increasing fluences showed a phase evolution from crystalline to amorphous/polycrystalline transformation. After 1064 nm laser processing above 1.47 J/cm2 fluences, microtwins with dislocations were observed, in addition to increasing expansion stress. Additionally, the appearance of extra spots in the (3 1 1) diffraction ring pattern obtained by TEM studies of samples processed at 1.60 J/cm2 fluence using a 1064 nm laser, demonstrates the generation of a high density of dislocations.