nc-Si Dots Research Articles

Photoluminescence and Optical Characterizations of Nanocrystalline Silicon Dots Formed by Plasma-Enhanced Chemical Vapor Deposition

We report the photoluminescence (PL) and optical properties of nanocrystalline silicon (nc-Si) dots grown on thermally grown SiO2 by rf (13.56 MHz) plasma-enhanced chemical vapor deposition of tetrachlorosilane (SiCl4) and H2. The nc-Si dots fabricated at temperatures of ∼120°C show relatively strong emissions in the visible range of 1.3–2.0 eV. The PL peak energy systematically shifts to lower energy from 1.75 eV to 1.45 eV by increasing the deposition period, which is due to the size distribution of nc-Si dots. On the other hand, the PL efficiency depends on both the deposition period and subsequent oxidation methods such as the storage in air, and pure water and HCl acid rinse treatments. The PL efficiency is markedly improved by the pure water rinse rather than by the storage in air, although a prolonged water rinse deteriorates the PL efficiency. On the other hand, the HCl acid rinse results in marked enhancement of PL efficiency with high stability. The effects of deposition conditions and surface treatment on the PL efficiency, stability and optical properties are demonstrated.

Photoluminescence of surface‐nitrided nanocrystalline silicon dots

Light-emitting surface-oxidized nanocrystalline-silicon (nc-Si) dots have been nitrided by nitrogen radicals. The durability of the nc-Si dots under laser illumination was improved by nitridation. It was concluded that Si3N4 is formed at the interface between the nc-Si core and SiO2 because a blue shift of the emission of the nc-Si dots was observed after nitridation at room temperature, suggesting that the interface of the nc-Si dots was improved. This result illustrates that nitridation is an effective treatment to improve the durability of the nc-Si dots.

Operation of nanocrystalline-silicon-based few-electron memory devices in the light of electron storage, ejection, and lifetime characteristics

A metal-oxide-semiconductor field-effect transistor memory device using nanocrystalline Si (nc-Si) dots as a floating gate over a short and narrow channel has been fabricated. Its operation at 77 K presents experimental evidence of storing and ejection of electrons associated with the nc-Si dot in the active area of the device. Though the lifetime of a single electron is apparently longer than the case when it is associated with another electron in the same nc-Si dot, a distribution in lifetime has been generally observed for the stored electrons in the nc-Si dots with the present memory devices.

Formation of silicon nano-dots in luminescent silicon nitride

Strong room-temperature photoluminescence (PL) was observed in the hydrogenated silicon-rich silicon nitride (a-SiNx:H) thin films grown by plasma-enhanced chemical vapor deposition technique. After the thermal annealing process, silicon nano-dots were clearly observed in the Si-rich a-SiN0.56:H thin films. From the X-ray photoelectron spectroscopy (XPS), the Si 2p peak shows distinct SiSi4 bonds after thermal annealing indicated the existence of Si clusters. In addition, the existence of nano-crystallized (nc) Si dots was revealed from the image of high-resolution cross-section transmission electron microscopy (XTEM) analysis. The PL spectra exhibited extensive red-shift from 1.97 to 1.33eV along with the annealing temperature suggests that, the Si nano-dot related luminescence dominates in the luminescence centers of the a-SiN0.56:H layer. The reveal of Si nano-dots related luminescence and the high density of Si nano-dots suggest that the a-SiN0.56:H layer is suitable for the stable light-emitting device.

Statistical emission of electrons from nanocrystalline silicon quantum dot memory nodes in a few-electron MOSFET memory device

Operation of a short and narrow channel metal-oxide-semiconductor field-effect transistor (MOSFET) memory device with a few nanocrystalline Si (nc-Si) dots in the active region has been investigated at 300 and 30 K. The discrete shift of the threshold voltage ( V th) in the current-voltage characteristics that arises from the screening effect of the charge stored in the nc-Si dot above the FET channel, suggests memory operation. It is found that the value of V th changes with temperature whereas the magnitude of the shift in V th is independent of temperature. The lifetime of the electrons stored in the floating node has also been investigated at different read voltages.

NeoSilicon materials and silicon nanodevices

We propose a novel material NeoSilicon, in which both particle size and interparticle distance of nanocrystalline silicon (nc-Si) quantum dots are precisely controlled. New functions in electron transport, photon emission and electron emission are expected due to quantum effects at room temperature and large interaction between dots. The bandgap is determined by the particle size. The conductivity is controlled mainly by tunneling processes. The transport characteristics are also controlled by charge quantization effect. We present fabrication and characterization of NeoSilicon. Based on the idea of separation of the nucleation and the growth processes in plasma decomposition of silane, we have successfully prepared nc-Si particles of 8 nm in diameter with size dispersion of 1 nm, whose surfaces are covered by naturally formed oxide of 1.5 nm thickness. Further reduction of dot size is achieved to 4 nm using self-limiting oxidation processes. Single electron transport characteristics of nc-Si are demonstrated in both planar and vertical transistor structures. Highly efficient electron emission is observed in NeoSilicon samples. Photoluminescence intensity enhancement due to no-phonon-assisted indirect transition is observed in strongly confined nc-Si dots covered by oxide.

Capacitance–voltage study of SiO 2/nanocrystalline silicon/SiO 2 double-barrier structures

The SiO 2/nanocrystalline silicon (nc-Si)/SiO 2 double-barrier structure on p-type (1 0 0) silicon substrate was fabricated in situ by plasma oxidation and layer-by-layer deposition at low temperature (250 °C) in a plasma enhanced chemical vapor deposition reactor. Frequency-dependent capacitance–voltage measurements were performed to study the electrical properties and the results can be explained by schematic band diagrams and equivalent circuits. Single electron charging effect (corresponding to the two capacitance peaks) in large ensemble of nc-Si dots (∼10 9) was observed at room temperature, which demonstrates that the Coulomb blockade energy in nc-Si dots is larger than room thermal energy k B T and the nc-Si size fluctuation effects on the quantum confinement of this SiO 2/nc-Si/SiO 2 structure.

Room Temperature Resonant Tunneling and Coulomb Blockade in Nanocrystalline Si with Double SiO2

ABSTRACTWe studied electron resonant tunneling and Coulomb blockade in nanocrystalline Si (nc-Si) with double SiO2 barriers, which is fabricated in-situ in a plasma enhanced chemical vapor deposition system. In capacitance-voltage measurements, discrete capacitance peaks due to electron resonant tunneling into energy levels of nc-Si dots and Coulomb blockade charging in nc-Si dots have been observed at room temperature. For smaller dots, capacitance peaks are more distinct due to the stronger Coulomb blockade effect. Meanwhile, conductance plateau in the region of capacitance peaks are also observed due to the charging effect in nc-Si dots. Experimental results are in agreement with theoretical evaluation based on the model of Coulomb blockade.

Electron trapping, storing, and emission in nanocrystalline Si dots by capacitance–voltage and conductance–voltage measurements

Temperature and frequency dependent electrical properties of SiO2/nanocrystalline Si (nc-Si)/SiO2 sandwich structures have been studied. A clear shift of the capacitance–voltage and conductance–voltage characteristics toward positive gate voltage suggests electron trapping in an nc-Si dot. The role of interface states and deep traps in our devices has also been thoroughly examined and shown to be unimportant on the overall device performance. The discharging process is found to be logarithmic with time and weakly temperature dependent. The long memory retention time and the logarithmic time dependence of charge loss in the dots are explained by a buildup of opposing electric field in the tunnel oxide, which hinders the discharge of electrons remaining in the dots.

Nanocrystalline silicon electron emitter with a high efficiency enhanced by a planarization technique

A cold electron emitter has been fabricated based on nanocrystalline silicon (nc-Si) quantum dots formed in the gas phase by very-high-frequency plasma decomposition of SiH4. A small size of less than 10 nm and the spherical shape of the nc-Si dots facilitated the generation of hot electrons. Electrons with kinetic energies higher than the work function of the top electrode were extracted into vacuum through the electrode. A planarization process of the nc-Si layer by annealing enhanced the electron emission efficiency to 5%. Efficiency was optimized by varying the thicknesses of the nc-Si layer, the SiO2 layer, and the top electrode film.

Formation of nanocrystalline silicon dots from chlorinated materials by RF plasma-enhanced chemical vapor deposition

Low-temperature formation of nanocrystalline silicon (nc-Si) dots is demonstrated by controlling the early stages of a conventional parallel plate RF (13.56 MHz) plasma-enhanced chemical vapor deposition (PE-CVD) in silicon tetrachloride (SiCl 4) and H 2 mixture. The nc-Si dots were directly formed on thermally-grown SiO 2/c-Si at low temperatures of 150–200 °C. The size, height and number density of the dots were controlled by deposition time, pressure, substrate temperature and RF power. These specific features originate from a low sticking probability of deposition precursor, SiCl x and the chemical stability of chlorine-terminated surface under hydrogen plasma. Not only the plasma conditions but also the surface chemistry of th-SiO 2 determine the dot density.

Observation of Quantum Confinement Effects in Nanocrystalline Silicon Dot Floating Gate Single Electron Memory Devices

ABSTRACTElectron charging and discharging processes in floating gate MOS memory based on nanocrystalline silicon (nc-Si) dots were investigated at room temperature using capacitance-voltage (C-V) and conductance-voltage (G-V) measurements. Sequential electron discharging processes from nc-Si dots manifest themselves clearly in G-V spectroscopy after charging of the dots. According to the conductance peak structure resulting from the Coulomb blockade as well as quantum confinement effects of nc-Si dots, electron-addition energy is estimated to be 50 meV. Taking the electron-charging energy between the silicon substrate and the floating dot (30 meV) into account, the quantum confinement energy is found to be comparable to the electron charging energy for an nc-Si dot of 8 nm in diameter embedded in the silicon oxide.

Temperature and Frequency Dependencies of Charging and Discharging Properties in Mos Memory Based on Nanocrystalline Silicon Dot

AbstractTemperature and frequency dependencies of the electrical properties of SiO2/nanocrystalline Si (nc-Si)/SiO2 sandwich structures have been studied. A clear positive shift in capacitance-voltage (C-V) and conductance-voltage (G-V) characteristics suggests electron trapping in nc-Si dots. The role of interface states and deep traps in these devices has also been examined, which shows that they have little effect on the overall device performance.

New Nanocrystalline si Floating Gate Structue for Nonvolatile Memory Application

ABSTRACTWe report a direct experiment evidence for holes and electrons charging in a new nanocrystalline Si (nc-Si) floating gate structure (SiO2/nc-Si/SiO2/c-Si) fabricated in-situ by plasma oxidation and layer by layer deposition technique in a plasma enhanced chemical vapor deposition (PECVD) system. In this nc-Si floating gate structures, the thickness of tunneling SiO2 layer is about 2 nm and the mean grain size of nc-Si is 6 nm obtained from Raman scattering and AFM measurements. The discrete quantum level and Coulomb charging energy for a single electron have been observed in large ensemble of nc-Si dots by frequency dependent capacitance spectroscopy, which demonstrates that the Coulomb blockade for electron in nc-Si dots is larger than size fluctuation effects on the quantum confinement for our nc-Si floating gate structure. Quantitatively, the experiment results of capacitance spectroscopy are in good agreement with the theoretical calculations. By contrasted with silicon single electron transistor memory made by using ultra fancy nanotechnology, nc-Si based memory can be fabricated with a minimum perturbation of conventional silicon technology and may be closest to industrial application.

Emission lifetime of polarizable charge stored in nano-crystalline Si based single-electron memory

The lifetime of the emission of a single electron stored in a nanocrystalline Si (nc-Si) dot has been studied in order to understand the physical processes for memory applications. A small active area field effect transistor channel (50×25 nm) is defined by electron-beam lithography on a thin (20 nm) silicon-on-insulator channel and allows for the electrical isolation of a single nc-Si dot. Remote plasma enhanced chemical vapor deposition is used to form 8±1 nm diameter nc-Si dots in the gas phase from a pulsed SiH4 source. Electrons stored in a dot results in an observed discrete threshold shift of 90 mV. Analysis of lifetime as a function of applied potential and temperature show the dot to be an acceptor site with nearly Poisson time distributions. An observed 1/T2 dependence of lifetime is consistent with a direct tunneling process, and interface states are not the dominant mechanism for electron storage in this device structure. Median emission lifetimes as a function of applied gate bias are readily modeled by the polarizability of an electron in a delocalized bound state over the entire semiconducting dot.

C-V and G-V Measurements Showing Single Electron Trapping in Nanocrystalline Silicon Dot Embedded in MOS Memory Structure

AbstractWe prepared a SiO2/nanocrystalline Si (nc-Si)/SiO2 sandwich structure. A clear positive shift in C-V and G-V curves due to electrons trapped in nc-Si dots has been observed at room temperature. The peak in conductance around flat band condition indicates that a trap event had occurred where an electron is stored per nc-Si dot. A logarithmic charge loss function is found and this discharging process is independent of the thermal activation mechanism. The longer memory retention time and logarithmic charge loss in the dots are explained by a “built-in” electric field through the tunnel oxide, which varies with time, resulting in a variable tunneling probability. The electric repulsion induced by the built-in electric field hinders the discharging of electrons remained in the dots.

Electron transport in a single silicon quantum structure using a vertical silicon probe

We present a method of electrical measurement of single nanocrystalline silicon (nc-Si) particles fabricated by plasma-enhanced chemical vapor deposition (CVD) at very high frequency of 144 MHz. A vertical Si probe structure with a spheroidal shaped hole in SiO2 and a CVD grown polycrystalline Si electrode allows stable measurement of current through a single nc-Si quantum dot. Periodic Coulomb staircases are observed between 5 and 50 K. The temperature dependence of the differential conductance is consistent with these being electron transport through a double junction array. A Monte Carlo simulation further supports the double junction array model where a nc-Si quantum dot is covered by 1.5-nm-thick natural oxide as a tunnel barrier. Moreover, applying a wraparound gate makes it possible to observe Coulomb oscillation.

Selective growth of nanocrystalline Si dots using an ultrathin-Si-oxide/oxynitride mask

We demonstrate the selective growth of nanocrystalline Si (nc–Si) dots by using ultrathin-Si-oxide/oxynitride mask and low-pressure chemical vapor deposition. The oxynitride layer is selectively grown on the Si(001)–2×1 open window formed in the ultrathin oxide layer by electron-beam-induced selective thermal decomposition. The 10-nm-scale hemispherical nc–Si dots grow selectively on the oxynitride-covered window within the incubation period in which Si growth does not occur on the oxide-covered surface.

Fabrication and characterization of the size-controlled and patterned nc-Si dots

A new method of phase-modulated excimer laser crystallization is adopted to fabricate the patterned nanometer-sized crystalline silicon (nc-Si) dots within the sandwiched structure (a-SiNx:H/a-Si:H/a-SiNx:H) films. The results of transmission electron microscopy, electron diffraction and Raman scattering show the ultra-thin and single-layer nc-Si films were patterned in the lateral direction and the size of crystallites is controlled by the thickness of as-deposited a-Si film in the longitudinal direction. The effects of the laser energy density on the structures of the samples and the crystallization mechanism are discussed.

Two-Gate Transistor for the Study of Si/SiO2 Interface in Silicon-on-Insulator Nano-Channel and Nanocrystalline Si Memory Device

A two-gate single electron memory device with active area of 1.5×10-10 cm2 allows the study of Si/SiO2 interface in nano-scale channels of silicon on insulator (SOI) and nanocrystalline Si (nc-Si) dots. The basis of this device is a trench (150–600 nm wide) defined inside a large “inversion” gate electrode into which nc-Si (8±1 nm diameter) can be deposited by remote plasma enhanced chemical vapor deposition. Device structure is analyzed with 2-D and 3-D numerical simulation of Poisson's and continuity equations, which show uniform surface potential of channel. The device design allows for tuning of threshold shifts as well as controlling the length of active area from 65–165nm with applied gate bias. This allows for quantifiable threshold shifts for charged interface defects on the channel as well as charge stored in nc-Si. Threshhold shifts of between 70–120 mV are calculated depending on the location of charged species along the width of the 30nm wide channel. To study energy levels of single defect sites, lifetime studies of charged sites are possible with and without deposited nc-Si.