Dr. Muhammad Y. Bashouti
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.
