Doping of nano-crystalline silicon powders by mechanical alloying: the process and characterization

Abstract

Silicon is the most used semiconductor material in photovoltaic applications. Doping of silicon with III and V group elements to synthesize p and n-type Si requires high temperature (~1000 °C). In the present studies, doping was accomplished in nanocrystalline Si powder at room temperature using mechanical alloying (MA) technique. Aqueous solution of boric acid or phosphoric acid in requisite amount for 50 ppm doping was added to the pristine Si powder and dried. Mechanical alloying of the dried Si powder was performed in the attrition mill in argon atmosphere. MA Si powders were then degassed to remove entrapped argon and other volatiles in a vacuum of 10−2 torr at 200 °C. Scanning electron microscopy and energy dispersive X-ray analysis were performed to study the morphology and composition of ball milled powder. XRD analysis was done to ascertain phases present and crystallite size in the ball milled powders. Micro-Raman results showed an asymmetric peak in terms of Fano type interference and due to phonon scattering, confirming the doping. Degree of crystallinity was studied by deconvoluting Raman peaks. Synchrotron XRF of phosphorus doped Si powder confirms phosphorus doping, in coherence with the results of Raman spectra. XPS spectrum confirms the presence of bounded boron in doped Si powder with binding energy of 186 eV. Charge carrier concentrations and hall mobilities were determined at room temperature. The carrier concentration was found to be matched with the dopants concentration added initially.