Role of secondary etching of silicon nanowires towards quantum confinement effect

Abstract

The quantum confinement effect (QCE) plays a significant role in tuning the bandgap of the silicon nanowires (SiNWs) by converting from indirect to direct bandgap semiconductor. The QCE makes the silicon (Si) semiconductor suitable for efficient solar cell and optoelectronic devices. The QCE only occurs for the nanostructure of Si, whose diameter is lesser than Bohr's diameter (≈9.8 nm). Metal-assisted chemical etching (MACE) fabricates columnlike SiNWs array on the Si substrate through primary etching. Length of the fabricated NWs is in the micrometer range, and its diameter is much higher (d > 70 nm) than Bohr's diameter, which contradicts the existence of QCE in the NWs. However, secondary etching etches the sidewalls of the SiNWs forming Si nano-crystals and Si quantum dots on the NWs whose average size is lesser than Bohr's diameter. HRTEM image and mathematical analysis of the SiNWs, using Raman, XRD, and Photoluminescence (PL) characterization, calculate the average size of the nanostructures formed on the sidewalls. The analysis indicates nanostructures of size to be less than five nm, formed on the SiNWs due to secondary etching, the sole reason behind the origin of QCE.