Synthesis and Characterization of Si Nanoparticles Obtained on Sonication of Porous Silicon Multilayer Films

Abstract

Synthesis of Si quantum dots (QDs), useful for multi-junction crystalline Si solar cells, using porous Silicon (PS) is presented in this paper. Four types of freestanding PS structures are fabricated by anodization method with modulation of current density between two levels. The level-1 current density is kept constant at 20 mA/cm2 (for reference monolayer structure - sample A) and 10 mA/cm2 (for all multilayer structures samples B, C, D). The level-2 is varied between 0 to 50 mA/cm2 (0, 20, 30, 50 mA/cm2 as sample A, B, C and D respectively). In order to obtain Si QDs from PS films, the films are subjected to sonication (120 W, 42 kHz) for 6 hours. HRTEM images confirm presence of Si nanoparticles in the range of 2 to 8 nm. Various spectroscopic analyses of Si nanoparticles are performed in order to evaluate quantum confinement behavior and surface modification observed during sonication. Analysis of de-convoluted Raman peaks shows frequency downshift and increase in full width half maximum due to formation of QDs. After sonication, PL spectroscopy indicates blue shift from 2.54 eV (sample A) to 2.85 eV (sample D_6HR), similar to the observations made by UV-Vis spectroscopy. FTIR spectra show oxidation of Si QDs during sonication. Spectroscopic and microscopic results are explained using quantum confinement and surface modification phenomenon.