Delivered by Publishing Technology to: Guest User IP: 202.220.240.248 On: Thu, 18 Apr 2013 14:20:19 Copyright American Scientific Publishers Materials Express Article Copyright © 2013 by American Scientific Publishers All rights reserved. Printed in the United States of America 2158-5849/2013/3/085/007 doi:10.1166/mex.2013.1097 www.aspbs.com/mex Diameter-controlled growth and impurity doping of silver colloid-seeded silicon microwires to nanowires for the realization of solar cell materials Mrinal Dutta * , Lavanya Thirugnanam, Keisuke Sato, and Naoki Fukata * International Center for Materials Nanoarchitectonics (MANA), National Institute for Materials Science, 1-1 Namiki, Tsukuba, 305-0044, Japan ABSTRACT Silicon (Si) microwires to nanowires were synthesized by chemical vapour deposition using silver (Ag) colloid catalyzed growth techniques. Control over diameter was achieved by varying the growth temperature while remaining far below the eutectic temperature of Ag colloids-Si system. Low-temperature growth of these nanowires demonstrates a vapour-solid–solid growth mechanism rather than a vapour-liquid–solid growth mech- anism. Boron (B) acceptors and phosphorus (P) donor doping were carried out to make p-type and n-type Si wires. B local vibrational peaks and Fano broadening in optical phonon peaks of micro Raman scattering and electron spin resonance signals from the conduction electrons were used to demonstrate B and P atom doping respectively. These results will be important for future integration where Si wires can be combined as p-n junction for light harvesting material in next generation Si based solar cells. Keywords: Silicon, Microwires, Nanowires, Doping, n-Type, p-Type. 1. INTRODUCTION Silicon (Si) wires have attracted significant attention in the last few years for a potentially wide variety of appli- cations, ranging from field-effect transistors (FETs), solar cells and other nano-electronics to chemical and biological sensors. 1 2 Most Si wires grown using the vapour-liquid- solid (VLS) process reported so far are made using Au as a catalyst. 3 However, Au creates unfavorable properties, as it introduces two impurity energy levels within the Si band-gap: one acceptor level 0.54 eV below the conduction band and another donor level 0.29 eV above the valance band, which act as deep level traps that decrease carrier mobility, lifetime and diffusion length. 4 5 Gold present on the tip and side walls of Si wires 6 is difficult to remove due to its chemical inertness, which causes a serious metal ∗ Authors to whom correspondence should be addressed. Emails: DUTTA.Mrinal@nims.go.jp, FUKATA.Naoki@nims.go.jp contamination problem in photovoltaic devices. However, the use of different metals has been attempted, such as Ag, Al, Ni, Pt, Cu, etc., as alternative catalysts for the growth of Si wires. 7 8 Silver shows distinct advantages, such as ease of removal of superficial Ag using chemical methods. 9 Hitherto, though, little attention has been paid to the Ag-catalyzed growth of Si wires. There are only a few reports which describe the growth of large-diameters Si whiskers by the VLS mechanism. 9–11 Another key challenge is the doping of these Si wires to control conductivity and carrier type to obtain n or p-type Si wires, which are essential for Si nano or micro wire p-n junctions for the application of photovoltaic and opto-electronic devices. To realize these, it is important to develop characterization techniques as well as doping methods. Raman scattering methods are very useful for boron (B)-doped p-type Si materials. One of our authors has applied this method to Si nanowires (SiNWs) and Mater. Express, Vol. 3, No. 1, 2013 85