Modeling and characterization of photoresponse of nanowires with asymmetric contacts and bending strain

Abstract

This work investigates (i) contact asymmetry induced response of semiconductors, with emphasis on nanowire photo response, and (ii) strain induced electronic and optoelectronic properties of narrow silicon nanowires. Contact area asymmetry is shown to produce large zero-bias photo response in bulk silicon, which is explained by TCAD modeling. Modeling also shows that both short circuit current and open circuit voltage in nanowire solar cells and photo sensors can be enhanced by using contact metals with large work function difference. This does not need any p-n junction type asymmetric band structure device that requires high and controlled doping. However, two-contact large nanowires are found to suffer from low current density due to minority carrier recombination and inefficient carrier collection at contacts. As a solution, multi-contact and grounded-gate designs are modeled and found to enhance photo response. By simple fabrication and characterization, we show that nanowire networks with dual-metal contacts produce large photo currents. Interestingly, a sparse network gives larger response than a relatively dense network. High resistance nanowire-nanowire contact is shown to place a limit on electrical transport and explains such unintuitive response. Finally, Molecular dynamics and quantum mechanical transport simulation of bent narrow silicon nanowires show that bending strain causes larger decrease in transmission gap compared to the usually investigated uniaxial strain and that a moderate amount of bending strain may be capable of causing a semiconductor to metal transition.