The role of quantum confinement (QC) in Si and Ge nanostructures (NSs) including quantum dots, quantum wires, and quantum wells is assessed under a wide variety of fabrication methods in terms of both their structural and optical properties. Structural properties include interface states, defect states in a matrix material, and stress, all of which alter the electronic states and hence the measured optical properties. We demonstrate how variations in the fabrication method lead to differences in the NS properties, where the most relevant parameters for each type of fabrication method are highlighted. Si embedded in, or layered between, SiO2, and the role of the sub-oxide interface states embodies much of the discussion. Other matrix materials include Si3N4 and Al2O3. Si NSs exhibit a complicated optical spectrum, because the coupling between the interface states and the confined carriers manifests with varying magnitude depending on the dimension of confinement. Ge NSs do not produce well-defined luminescence due to confined carriers, because of the strong influence from oxygen vacancy defect states. Variations in Si and Ge NS properties are considered in terms of different theoretical models of QC (effective mass approximation, tight binding method, and pseudopotential method). For each theoretical model, we discuss the treatment of the relevant experimental parameters.

Quantum confinement in Si and Ge nanostructures: Theory and experiment

Low-Voltage, Optoelectronic CH3NH3PbI3−xClx Memory with Integrated Sensing and Logic Operations

Charge trapping characteristics of high-relative permittivity (high-?) HfO2 films with Al2O3 as a blocking oxide in p-Si/SiO2/HfO2/Al2O3/metal memory structures have been investigated. All high-? films have been grown by atomic layer deposition. A transmission electron microscope image shows that the HfO2 film is polycrystalline, while the Al2O3 film is partially crystalline after a high temperature annealing treatment at 1000 ?C for 10 s in N2 ambient. A well-behaved counter-clockwise capacitance?voltage hysteresis has been observed for all memory capacitors. A large memory window of ~7.4 V and a high charge trapping density of ~1.1 ? 1013 cm?2 have been observed for high-? HfO2 charge trapping memory capacitors. The memory window and charge trapping density can be increased with increasing thickness of the HfO2 film. The charge loss can be decreased using a thick trapping layer or thick tunnelling oxide. A high work function metal gate electrode shows low charge loss and large memory window after 10 years of retention. High-? HfO2 memory devices with high-? Al2O3 as a blocking oxide and a high work function metal gate can be used in future high-density non-volatile memory device applications.

https://iopscience.iop.org/article/10.1088/0268-1242/22/8/010/pdf

Charge trapping characteristics of atomic-layer-deposited HfO2 films with Al2O3 as a blocking oxide for high-density non-volatile memory device applications

Nanocrystals (NCs), representing a zero-dimensional system, are an ideal platform for exploring quantum phenomena on the nanoscale, and are expected to play a major role in future electronic and photonic devices. Here we review recent progress in the growth, characterization and utilization of some group-IV semiconductors (Si and Ge), metal and high-k NCs for silicon planar technology compatible light-emitting and floating gate memory devices. We first introduce the size-dependent electrical and optical properties of Si and Ge NCs. We outline some of the schemes to achieve light emission from indirect band gap Si and Ge NCs embedded in different high band gap oxide matrices. In particular, special emphasis is given on the review of the advances in Ge NCs because of some of their intriguing electronic and optical properties. We then describe the use of semiconductor and metal NCs as floating gates for non-volatile memory devices to achieve high data retention and faster program/erase speeds. The exploitation of high-k oxides with tunable and variable injection barriers for improved charge storage devices is discussed. Finally, the integration of single and multilayer metallic NCs and multilayer high-k oxides as floating gates is explored by the fabrication and testing of memory transistors.

https://iopscience.iop.org/article/10.1088/0022-3727/46/15/153001/pdf

Nanocrystals for silicon-based light-emitting and memory devices

This work was partially supported by the QuantumFunctional
Semiconductor Research Center in Dongguk University
and by the National Program for Tera Level Nano
Devices through MOST. S.-H.C. acknowledges partial support
from the National Research Program for the 0.1 Terabit
Non-Volatile Memory Development sponsored by Korea
Ministry of Science & Technology. R.G.E. additionally acknowledges
the Australian Research Council for their partial
financial support of this work.

/pdf/large-capacitance-voltage-hysteresis-loops-in-sio2-films-44waiocy4z.pdf

Large capacitance-voltage hysteresis loops in SiO2 films containing Ge nanocrystals produced by ion implantation and annealing

Metal-oxide-semiconductor capacitors with a trilayer structure consisting of the cap gate oxide, sputtered SiGe layers and thermally grown tunnel oxide were fabricated on p-Si substrates. The trilayer structures were rapid thermal annealed at 1000 °C in nitrogen atmosphere for different durations. Cross-sectional transmission electron micrographs revealed the complete isolation of Ge nanocrystals in the sandwiched structure annealed for a longer duration. The optical and charge storage characteristics of trilayer structures were studied through photoluminescence spectroscopy and capacitance-voltage measurements, respectively. Under optimized annealing conditions, an enhancement of the charge storage capability of nanocrystals was observed in agreement with the optical emission characteristics.

Charge storage and photoluminescence characteristics of silicon oxide embedded Ge nanocrystal trilayer structures

A vapor-liquid-solid method has been used to study the temperature dependent growth mechanism of Ge nanostructures on Au-coated Si (100) substrates. The formation of Ge nanodots, nanorods, and nanowires has been observed at different growth temperatures. The diameter of grown nanowires is found to be varying from 40 to 80 nm and that of nanorods from 70 to 90 nm, respectively. A comparative study has been done on three types of samples using x-ray diffraction and Raman spectroscopy. Photoluminescence spectra of grown nanostructures exhibit a broad emission band around 2.6 eV due to oxide related defect states.

Temperature dependent shape transformation of Ge nanostructures by the vapor-liquid-solid method

Ge nanocrystals embedded in bulk SiO2 glasses were prepared by the sol–gel process. Er3+ ions of controlled composition were incorporated into the glasses to investigate the emission properties of the 1.54 µm line. The size of the Ge nanocrystals was found to vary from 5 to 10 nm. XRD and Raman measurements showed the formation of crystalline Ge in SiO2 matrix. Elemental composition of Ge and Er in the samples was determined by RBS analysis. Various absorption edges in the optical spectra of the samples due to the atomic transition of Er3+ ions were detected. The samples exhibited intense emissions at 1.54 µm in the presence of Ge nanocrystals.

http://iopscience.iop.org/article/10.1088/0957-4484/18/9/095704/pdf

M. L. NandaGoswami

Papers

Charge storage and photoluminescence characteristics of silicon oxide embedded Ge nanocrystal trilayer structures

Temperature dependent shape transformation of Ge nanostructures by the vapor-liquid-solid method

Optical characteristics of Er3+-doped Ge nanocrystals in sol–gel-derived SiO2 glass