FROM Si NANOWIRES TO Ge NANOCRYSTALS FOR VIS-NIR-SWIR SENSORS AND NON-VOLATILE MEMORIES: A REVIEW

Abstract

"Nanocrystalline Si and Ge are ofhigh interestfor integrated Si photonics related to light emission, opticul sensors, photodetectors, solar energy harvesting and conversion devices, and also forfloating 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)Zquantum dots (QDs)/nanoparticles (NPs) embedded in oxides (SiCh, TiCE, HfCh, AI2O3). The great asset ofnc-PS is its intense photoluminescence in VIS at room temperature (RT), while Ge NCs/NPs embedded in oxides show high photosensitivity in VISNIR-SWIR in the spectral photocurrent up to 1325 nm at RT. Ge NCs/NPs/QDsfloating 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 parametersfor obtainingfilms with targetedphotoluminescence, 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 ofquantum confinement effect, localized States and trapping centers."

Enhancing Short-Wave Infrared Photosensitivity of SiGe Nanocrystals-Based Films through Embedding Matrix-Induced Passivation, Stress, and Nanocrystallization

“…Similar mechanism explains photoconductive gain in photodetectors based on Ge QDs in SiO 2 by trapping of holes on Ge related defects/traps (subsection 2.2). 38,44,45 The photocurrent density J ph (11.3 mW/cm 2 , 300−950 nm illumination) is higher than the dark current density J dk with 4 orders of magnitude (J−V characteristics, reverse bias) for structures with 14 nm Ge QDs (Figure 4c). A positive opencircuit voltage V oc = 0.25 V is evidenced in the J ph −V curve by the presence of a built-in electric field (35 kV/cm) (Figure 4d).…”

“…Electrostatic doping by the field effect enhances the photoconduction, leading to superlinear bias voltage U dependence of the spectral photocurrent intensity I ph evidenced by a power law dependence I ph ∼ U α with α > 1 or exponential increase of photoconduction ( I ph ∼ exp­( eU / E 0 )) . The mechanism is detailed in refs and .…”

“…Usually, the photocurrent spectra of structures of SiGe NCs in oxide/buffer oxide/Si substrate contain a part at wavelengths longer than 1100 nm that is due to photogenerated carriers in SiGe NCs, together with a maximum positioned at 1100 nm achieved through the capacitive coupling between the active layer and Si substrate, by surface photovoltage and gating effect, and another part at smaller wavelengths due to photocarriers excited from interface states ("defects" as mismatch states) located at the SiGe NC/matrix interface and other, localized states in the oxide. 22,44 All these states can act as trapping centers for holes, and in this way the electron lifetime increases leading to high IQE. Photogenerated carriers in SiGe NCs and/or interface states and localized states in oxide, and carriers (holes) injected from Si substrate contribute to the photocurrent.…”

“…There are few reviews in literature dealing with photosensing/photodetection of structures based on Ge NCs/QDs/NPs embedded in oxides. One review, from 2022, highlights the contributions made by our group to the field, e.g., electrostatic doping by field effect leading to exponential increase of photoconduction ( GeNCs-TiO 2 /SiO 2 /n-Si ) and high IQE over 100% ( ITO/Ge NPs-SiO 2 /n-Si/Al ) at RT, and discusses the underlying physical mechanisms that govern the photosensing . Another review from 2019 focuses on the research carried by Siontas et al group on VIS and NIR photodetectors based on Ge QDs in SiO 2 .…”

“…In the literature, two review papers deal with photodetectors and optical sensors based on Ge QDs/NPs in oxides (SiO 2 , TiO 2 ) discussing the underlying physical mechanisms and device operation 44,45. SiGe NCs-SiO 2 -based photodetectors present responsivities of the same order of magnitude (up to 5.23 A/W at RT and 9.35 A/W at 100 K) as those for photodetectors with Ge QDs/NPs/ NCs embedded in SiO 2 (Tables…”