Silicon Nanowires Light Emitting Devices at Room Temperature

Abstract

Group-IV semiconductor nanowires (NWs) are attracting interest among the scientific community as building blocks for a wide range of future nanoscaled devices. Vapor-liquid-Solid (VLS) is the most used technique for semiconductor NWs growth. Si NWs are promising as building blocks for photovoltaic elements, sensors and high-performance batteries; however, Si NWs are less explored for photonic applications, probably since there are many drawbacks due to the NW structure obtained by VLS. In fact, there is a minimum obtainable size which reduces the possibility to have quantum confinement effects without high temperature oxidation processes; metal used as a catalyst may be incorporated inside the NW thus affecting its electrical and optical properties. Moreover, by VLS method the doping is no easily controllable because of the segregation of the dopants at the NWs interface. Indeed, the possibility of obtaining light from silicon at room temperature under optical and electrical pumping is strategic for the communication technology.

Metal-assisted chemical etching (MacEtch) is a powerful technique to obtain high density and low-cost Si NWs with high and controllable aspect ratio. NWs obtained by this technique have exactly the same structure and doping properties of the substrate; their main size is less than 10 nm allowing quantum confinement effects. We will show that Si NWs made by MacEtch are suitable for photonic applications. Although some evidence of luminescence properties of axial nanostructures have been reported in literature, there are no detailed studies on PL properties of Si NWs. In this work we will show a detailed and complete study of the excitation and de-excitation properties as a function of the temperature and of the pump power, determining the excitation cross section and the presence and the origin of possible non-radiative phenomena. Moreover, this detailed study shows the influence of the structural properties on the mechanisms of light emission, in such a way to optimize the emission properties. Based on these results, we performed a further investigation and we designed a light emitting device based on Si NWs, showing the EL emission at room temperature under low voltage pumping. These results have a great impact on the possibility to use Si NWs for photonic applications.

Furthermore, we investigated size-scaling in optical trapping of ultrathin silicon nanowires showing how length influences their Brownian dynamics, since optical trapping is fundamental for the characterization and manipulation of the single Si NW (e.g. for the Photonic Force Microscopy technique). Force and torque constants have been measured on Si NWs of different lengths through correlation function analysis of their tracking signals. Results are compared with a full electromagnetic theory of optical trapping developed in the transition matrix framework, finding good agreement [1].