Hybrid Silicon Nanowire: From Basic Science to Applied Nanotechnology

Max Planck Institute for the Science of Light, Erlangen

Abstract

The ability to manipulate the properties of silicon nanowires (SiNWs) through controlling their surfaces is important for the realization of SiNW-based devices in the fields of electronics and (bio)sensors. However, the presence of oxide at the SiNW surfaces is undesirable because a defective oxide layer (e.g., native SiO2) is thought to induce uncontrolled interface states in the silicon. Thus, there is a considerable interest in learning how to control surface properties through chemical methods. These protection strategies should prevent extensive oxidation of the SiNW surfaces. We develop a method for functionalization SiNWs with alkyl chains using a versatile two step chlorination/alkylation process, while preserving the original length and diameter of the NWs. We show that Si NWs terminated with alkyl molecules, through Si-C bonds, provide surface stability that depends on the chain length and molecular coverage. Our results provide evidence that alkyl-SiNWs provide stronger Si-C bonds and higher surface stability in ambient conditions than equivalent two-dimensional (2D) Si surfaces having similar or higher initial coverage.
To utilize the new properties, hybrid SiNWs was integrated in field effect transistor (FETs) and solar cells. In particular FETs fabricated from these SiNWs displayed characteristics that depended critically on the type of molecular termination. Without molecules the source–drain conduction is unable to be turned off by negative gate voltages as large as 20 V. Upon adsorption of organic molecules there is an observed increase in the ‘‘on’’ current at large positive gate voltages and also a reduction, by several orders of magnitude, of the ‘‘off’’ current at large negative gate voltage. Solar cells show increased their open-circuit voltage, Voc, an increased short-circuit current, Jsc and a higher fill factor, FF, become possible. Jsc, Voc, and FF are the three key parameters characterizing solar cell performance. For example, we improved the efficiency four times and the photoelectron yield show new band threshold emission.