Controlling the optical properties of semiconductor nanowires through morphology engineering: a review

Semiconductor nanowires (NWs) are ideal candidates for nanoscale optoelectronics and nanophotonics due to their high aspect ratio and the possibility to confine light at the nanometer scale. In addition, their free-standing geometry enables the heterogeneous integration of materials and thus leads to novel electronic and photonic functionalities. Their optical response depends primarily on their morphology: diameter, length, tapering and cross-sectional geometry. Consequently, the precise control over NW morphology has become an important issue to explore new light–matter interactions and to increase the device performance. In this review, we first provide an overview of the main approaches to control the morphology of semiconductor NWs. We highlight how parameters such as the temperature, precursor flux ratios, catalyst type and growth technique may be adjusted to tune the NW diameter, length and tapering. Then, we describe how post-growth morphological engineering has been developed such as selective chemical etching or top–down methods. In a second part, we discuss the influence of the NW morphology on their optical properties and applications. We showcase how variations of the NW geometry dictate guided optical modes, the light extraction and absorption efficiency and the emission directionality. These properties are crucial for applications such as waveguides, quantum light sources, lasers, imaging and sensing devices and photovoltaic systems. The examples will be drawn mainly from published reports on III–V, II–VI and Si NWs, as these account for a significant proportion of NWs for optical applications. Finally, we provide a prospective outlook on the opportunities and challenges that morphology engineering of NWs can offer.