From Si Nanowires to Ge Nanocrystals for VIS-NIR-SWIR Sensors and Non-volatile Memories: A Review

Nanocrystalline Si and Ge are of high interest for integrated Si photonics related to light emission, optical sensors, photodetectors, solar energy harvesting and conversion devices, and also for floating gate non-volatile memories (NVMs). In this review, we have focused on nanocrystalline porous Si (nc-PS) with extension to Si nanodots, and Ge nanocrystals (NCs)/quantum dots (QDs)/nanoparticles (NPs) embedded in oxides (SiO$_2$, TiO$_2$, HfO$_2$, Al$_2$O$_3$). The great asset of nc-PS is its intense photoluminescence in VIS at room temperature (RT), while Ge NCs/NPs embedded in oxides show high photosensitivity in VIS-NIR-SWIR in the spectral photocurrent up to 1325 nm at RT. Ge NCs/NPs/QDs floating gate NVMs present high memory performance, the retention characteristics corresponding to the state of the art for NCs floating gate NVMs. We prove the relevance of controlling the preparation parameters for obtaining films with targeted photoluminescence, photosensitivity and charge storage properties for applications, e.g. VIS-NIR-SWIR optical sensors and photodetectors, and electronic and photoelectric NVMs. We evidence the correlation of preparation conditions, morphology, composition and crystalline structure with optical, electrical, photoelectrical and charge storage properties and also evidence the contribution of quantum confinement effect, localized states and trapping centers.