Formation and luminescence studies of Ge/Si core-shell quantum dots

Si-based quantum dots (QDs) have attracted much attention as an active element in Si-based optoelectronic applications because their light emission properties due to carrier confinement have the potential to combine photonic processing with electronic processing on a single chip. We have focused on CVD formation and characterization of Si-QDs with Ge core and reported their photoluminescence (PL) properties attributable to type II energy-band alignment between the Ge–core and the Si-shell [1-2]. In addition, we have also demonstrated stable electroluminescence in the near–infrared region from diode structures having a 3-fold stacked Si-QDs with Ge core with an areal dot density of ~2.0×1011 cm−2 under pulsed bias applications [3]. To gain fundamental knowledge and better understanding of the PL properties and to enhance the radiative recombination rate in photoexcited QDs, it would be effective to increase electronic states assisting radiative transition with impurity doping into the QDs and to reduce or not to increase in non-radiative centers if any. Please click Download on the upper right corner to see the full abstract

Characterization of Electronic Charged States of Si-Based Quantum Dots and Their Application to Floating Gate Memories

Nanometer-size Si quantum dots (Si-QDs) with and without Ge core were prepared on thermally-grown SiO2 in a self-assembling manner by controlling the early stages of low pressure chemical vapor deposition (LPCVD). The surface potential changes in individual dots caused by charging or discharging of one electron or a few as were measured by using a Kelvin probe technique in an atomic force microscope (AFM). For Si-QDs larger than 20nm in dot height, surface potential images with a characteristic potential profile with a dimple around the center of the charged dots are observable after electron or hole injection, indicating Coulomb repulsion among the charges retained in the dot. For Si-QDs with a Ge core, electrons are retained stably in Si clad while holes in Ge core, reflecting the energy band discontinuity at the interface between the Si clad and the Ge core. The influence of phosphorous doping to Si-QDs on their electron charging and discharging characteristics was also been studied. Electrical characteristics of metal-oxide-semiconductor (MOS) capacitors and n-channel MOS field-effect-transistors (nMOSFETs) with Si-QDs floating gates confirm multiple-step charging to and discharging from the Si-QDs floating gate at room temperature. From the temporal changes in the drain current with gate voltage switching, it is suggested that the change in the electron distribution in the Si-QDs floating gate play an important role to trigger the transition from a metastable charged state to the next charged state

Room Temperature Light Emission from Superatom-like Ge–Core/Si–Shell Quantum Dots

We have demonstrated the high–density formation of super–atom–like Si quantum dots with Ge–core on ultrathin SiO2 with control of high–selective chemical–vapor deposition and applied them to an active layer of light–emitting diodes (LEDs). Through luminescence measurements, we have reported characteristics carrier confinement and recombination properties in the Ge–core, reflecting the type II energy band discontinuity between the Si–clad and Ge–core. Additionally, under forward bias conditions over a threshold bias for LEDs, electroluminescence becomes observable at room temperature in the near–infrared region and is attributed to radiative recombination between quantized states in the Ge–core with a deep potential well for holes caused by electron/hole simultaneous injection from the gate and substrate, respectively. The results will lead to the development of Si–based light–emitting devices that are highly compatible with Si–ultra–large–scale integration processing, which has been believed to have extreme difficulty in realizing silicon photonics

Effect of electric field concentration using nanopeak structures on the current–voltage characteristics of resistive switching memory

An attempt to reduce the SET voltage and RESET current of resistive switching (RS) memory was made using a geometric array of nanopeak (NP) structures. Bottoms of anodic porous alumina were used to form the NP structures that act as guides for the formation of conductive filaments that effectively concentrate the electric field. Samples were fabricated with flat surfaces (FS) and with two types of NP structure with different NP pitch. The NP samples provided SET voltages less than 2 V with narrow distributions and the RESET current was lower than that with the FS sample