Modeling of programming time of nanocrystal flash memory cells

Charging dynamics of nanocrystals in the emerging silicon nanocrystals-based flash memory is explained using a simplified engineering model based on effective mass approximation. The model includes the presence of discrete energy levels in the nanocrystals and also the effect of shift in energy levels in the nanocrystals with more than one electron. The simulated results of shift in threshold voltage as a function of programming time, and the effect of gate voltage and drain voltage on the programming speed agree very well with the experimental results. The tunnel mass of electron in the oxide, which is used as a fit parameter, takes into account the effect of strain and crystallographic orientational effects. Assumptions and limitations of the model are discussed.     

Effects of charge storage dielectric thickness on hybrid gadolinium oxide nanocrystal and charge trapping nonvolatile memory

The characteristics of hybrid gadolinium oxide nanocrystal (Gd₂O₃-NC) and gadolinium oxide charge trapping (Gd₂O₃-CT) memories were investigated with different Gd₂O₃ film thickness. By performing the rapid thermal annealing on Gd₂O₃ films with different thickness, the Gd₂O₃-NCs with the diameter of 6–9 nm for charge storage, surrounded by the amorphous Gd₂O₃ (α-Gd₂O₃) layer, were formed. The α-Gd₂O₃ layer was considered to be the charge trapping layer, resulting in the large memory window of Gd₂O₃-NC/CT memories with thick Gd₂O₃ film. The charge trapping energy level of the Gd₂O₃-NCs and α-Gd₂O₃ layer was extracted to be 0.16 and 0.45 eV respectively by using the temperature-dependent retention measurement. Further, after a 10⁶ program/erase cycling operation, the memory with thin Gd₂O₃ film can be predicted to sustain a 94% memory window of the first cycling one while the memory with thick Gd₂O₃ film suffered from a 30% charge loss because of the traps within the α-Gd₂O₃ layer. The Gd₂O₃ film thickness of 10 nm was optimized to exhibit superior performances of the Gd₂O₃-NC/CT memory, which can be applied into the nonvolatile memory.     

Optical properties of silicon nanocrystal LEDs

In this work, we describe how to fabricate good quality 3 nm nc-Si with low size distribution in thermal SiO₂ oxides. Photoluminescence, excited photoluminescence, and photocurrent measurements are discussed on the basis of theoretical calculations of the quantified levels in nc-Si. The impact of shape and size in quantum dots on transition energies has been highlighted, thanks to 2D symmetrical self-consistent Poisson–Schrödinger simulations. Both direct and indirect gaps in silicon have been considered in order to carry out a better comparison between simulations and optical measurements. A good agreement is found between simulations and experimental data for the indirect gap of 3 nm dots which show a threshold energy around 2 eV. However, the optical recombinations seems to be related to lower energy states probably due to interfacial radiative defects around 1.58 eV. On the basis of highly luminescent nc-Si, we have fabricated an optimized light emitting device (LED) with a calculated design in order to favour both electron and hole injection. Stable red electroluminescence has been obtained at room temperature and the I–V measurements confirm that the current is related to a pure tunnelling process. A modelling of I–V curves confirms a Hopping mechanism with an average trap distance between 1.4 and 1.9 nm. The Fowler–Nordheim process is not observed during light emission for electric fields below 5 MV/cm. Finally, we have not hot carrier injection and thus it seems possible to develop Si-based LEDs with a good reliability.     

Silicon nanocrystal synthesis by implantation of natural Si isotopes

Implantations of pure , , and into SiO₂ can provide significant insight into the formation of silicon nanocrystals (Si-nc) and their light emission properties. Si-nc produced with different fractions of the heavier Si isotopes have been characterized by Raman and photoluminescence spectroscopy. Weak Stokes shifts of the Si-nc phonon peaks indicate that both the implanted Si and the native Si from the SiO₂ substrate contribute to Si-nc nucleation. The Raman measurements also indicate that the Si isotopic composition of the Si-nc is similar to the Si isotopic fraction of the implanted SiO₂. The Si-nc photoluminescence (PL) spectra are shifted towards the blue with increasing Si isotope mass, an indication that the increase of the Si-nc effective mass enhances the excitonic bandgap. Measurements from samples implanted with heavy isotopes at high Si excess concentrations indicate that the Si-nc isotope fraction evolves with annealing time such that the heaviest Si isotope are more concentrated in the vicinity of the Si-nc/SiO₂ interface, which can modify the energy states involved in the radiative transitions associated with Si-nc.     

Near infrared absorption of Si nanoparticles embedded in silica films

The absorption coefficient of Si nanoparticles embedded in a silica matrix obtained through thermal annealing at 1000 °C of SiO thin films has been determined by a combination of ellipsometry and photothermal deflection spectroscopy. The high absorption level below 2 eV was explained by the superposition of the contribution of: (i) extended states and distorted bond states (Urbach tail), giving rise to an exponential regime of the variation of the absorption coefficient on energy and (ii) point defect states. The value of the characteristic energy of the exponential regime was found above 200 meV. This high value was partly related to the high stress present at the np-Si/SiO₂ interface. The point defects were attributed to dangling bonds and induced an additional absorption band located near 1.2 eV contributing to above 100 cm⁻¹ to the absorption at this energy.     

黑体辐射中两类粒子的辐射规律

考虑到相对论效应,采用量子统计的方法,得到了满足Bose-Einstein统计分布和Fermi-Dirac统计分布的两类粒子的黑体辐射公式（包括Planck黑体辐射公式）．发现辐射谱不仅与黑体的辐射温度有关,还与辐射粒子的能量、化学势和种类有关．辐射强度与辐射粒子的能量、静质量、简并因子有关。     

Charge storage and electron/light emission properties of silicon nanocrystals

Monodispersed silicon nanocrystals show novel electrical and optical characteristics of silicon quantum dots, such as single-electron tunneling, ballistic electron transport, visible photoluminescence and high-efficiency electron emission.Single-electron memory effects have been studied using a short-channel MOSFET incorporating Si quantum dots as a floating gate. Surface nitridation of Si nanocrystal memory nodes extends the charge-retention time significantly. Single-electron storage in individual Si dots has been evaluated by Kelvin probe force microscopy.Photoluminescence and electron emission are observed for surface-oxidized silicon nanocrystals. Efficiency of the no-phonon-assisted transition increases with decreasing core Si size. Electron emission efficiency as high as 5% has been achieved for the Si-nanocrystal-based cold electron emitter devices. The non-Maxwellian energy distribution of emitted electrons suggests that the mechanism of electron emission is due to ballistic transport through arrays of surface-oxidized Si nanocrystals. Combined with the ballistic electron emission, the quasi-direct light emission properties can be used for developing Si-based lasers.     

Scaling dependence of memory windows and different carrier charging behaviors in Si nanocrystal nonvolatile memory devices

Based on the charge storage mode,it is important to investigate the scaling dependence of memory performance in silicon nanocrystal(Si-NC) nonvolatile memory(NVM) devices for its scaling down limit.In this work,we made eight kinds of test key cells with different gate widths and lengths by 0.13-μm node complementary metal oxide semiconductor(CMOS) technology.It is found that the memory windows of eight kinds of test key cells are almost the same of about1.64 V @ ±7 V/1 ms,which are independent of the gate area,but mainly determined by the average size(12 nm) and areal density(1.8×10~(11)/cm~2) of Si-NCs.The program/erase(P/E) speed characteristics are almost independent of gate widths and lengths.However,the erase speed is faster than the program speed of test key cells,which is due to the different charging behaviors between electrons and holes during the operation processes.Furthermore,the data retention characteristic is also independent of the gate area.Our findings are useful for further scaling down of Si-NC NVM devices to improve the performance and on-chip integration.     

Proton loss of inner radiation belt during geomagnetic storm of 2018 based on CSES satellite observation

The proton distribution in inner radiation belt is often affected by strong geomagnetic storm disturbance. Based on the data of the sun-synchronous CSES satellite, which carries with several high energy particle payloads and was launched in February 2018, we analyzed the extensive proton variations in the inner radiation belt in a wide energy range of 2 MeV-220 MeV during 2018 major geomagnetic storm. The result indicates that the loss mechanism of protons was energy dependence which is consistent with some previous studies. For protons at low energy 2 MeV-20 MeV, the fluxes were decreased during main phase of the storm and did not come back quickly during the recovery phase, which is likely to be caused by Coulomb collision due to neutral atmosphere density variation. At higher energy 30 MeV-100 MeV, it was confirmed that the magnetic field line curvature scattering plays a significant role in the proton loss phenomenon during this storm. At highest energies > 100 MeV, the fluxes of protons kept a stable level and did not exhibit a significant loss during this storm.     

Electrical and memory properties of silicon nitride structures with embedded Si nanocrystals

In this work, the electrical and memory behaviour of metal-silicon nitride-silicon structures with an embedded nanocrystalline silicon layer, which either consists of separated silicon nanocrystals, or is a continuous nanocrystalline layer, are presented. The structures were prepared by low-pressure chemical vapour deposition (LPCVD). The effect of the duration of deposition and the structure of the nanocrystalline layer were studied. The writing/erasing behaviour was similar for all the structures, but the retention properties were much worse in the structure with a continuous nanocrystalline layer, than in the structures with separated Si nanocrystals. This indicates that Si nanocrystals play role in charge storage in the studied structures.     

Proton radiation effects on optical constants of Al film reflector

The Al film reflectors can yield a high-reflectance over a broad wavelength region, and have been widely used in the spacecraft optical instruments for high quality optical applications. Under the irradiation of charged particles in the Earth radiation belt, the reflectors could be deteriorated. In order to reveal the deterioration mechanism, the change in optical constants of Al film reflector induced by proton radiation with 60\,keV was studied in an environment of vacuum with heat sink. Experimental results showed that when the radiation damage primarily occurs in the Al reflecting film, the extinction coefficient k will gradually decrease with increasing radiation fluence, which results in the decrease of the energies of reflective light. Therefore, the proton radiation induced an obvious degradation of spectral reflectance in the wavelength region from 200 to 800\,nm on the Al film reflector.     

Proton radiation effects of conjugated polymer thin films

Polymeric-thin-film-based electronic and optoelectronic materials and devices are attractive for potential space and certain radiation-related applications due to their inherent features such as lightweight, flexible, biocompatible, etc. Proton radiation is a major form of ionizing radiation in space, particularly in the so-called inner Van Allen belt region where most near-earth satellites are orbiting, yet very few literature and data are available on proton radiation effects on conjugated polymer systems. In this report, the proton radiation effects on potential electronic/optoelectronic properties of several conjugated polymers and their composites are briefly evaluated. Specifically, UV–VIS absorption spectra of several conjugated polymers and/or their composite thin films were measured and compared right before and after the proton radiations at different dosages. The results revealed that proton radiation has very little or insignificant impact up to 800?rad on the optoelectronic properties of poly-3-hexyl-thiophene (P3HT), P3HT:PC₆₀BM blend, a light harvesting donor-bridge-acceptor (DBA) and a novel donor-bridge-fluorinated-acceptor (DBfA)-type block copolymer thin films.     

相变存储器失效机理的研究进展

相变存储器由于具有非易失性、高速度、低功耗等优点被认为是最有可能成为下一代存储器的主流产品之一。然而存储器芯片的良率、密度和操作速度受制于性能最差的单元,因此研究相变存储器的失效机理对于存储器芯片成本的降低以及性能的提升至关重要。文章综述了相变存储器失效机理的研究进展,主要讨论和归纳了电性操作和工艺制程所导致的相变存储器失效模型和失效机理,包括电迁移、热动力学效应、相变应力和热应力、电压极性、结晶引发的偏析、浓度梯度、电极材料以及制造工艺引起的失效。     

Proton conductivity of phosphoric acid derivative of fullerene

The proton conductive property of methano [60] fullerene diphosphoric acid has been investigated under various humidity conditions at the temperature range between 15 and 45 °C. It shows proton conductivity as high as 10⁻² S cm⁻¹ at 25 °C under relative humidity of 95%. Thermal analyses including TG–DTA and thermal desorption mass spectroscopy (TDS) confirm that the compound is thermally stable up to 200 °C. Proton conduction of the compound depends very much on humidity or water content. The logarithmic conductivity at 25 °C is increased linearly with increasing relative humidity. The activation energy (E_a) estimated from the slope of log(σT) vs. 1/T is decreased from 1.08 to 0.52 eV, as the relative humidity is increased from 40% to 75%. The humidity dependence of conductivity is discussed in the light of the observed hydration isotherm.     

Experimental and theoretical studies of Si-CN bonds to eliminate interface states at Si/SiO2 interface

Cyanide treatment, which includes the immersion of Si in KCN solutions followed by a rinse, effectively passivates interface states at Si/SiO₂ interfaces by the reaction of CN⁻ ions with interface states to form Si-CN bonds. X-ray photoelectron spectroscopy (XPS) measurements show that the concentration of the CN species in the surface region after the cyanide treatment is ∼0.25 at.%. Take-off angle-dependent measurements of the XPS spectra indicate that the concentration of the CN species increases with the depth from the Si/SiO₂ interface at least up to ∼2 nm when ultrathin SiO₂ layers are formed at 450 °C after the cyanide treatment. When the cyanide treatment is applied to metal-oxide-semiconductor (MOS) solar cells with 〈ITO/SiO₂/n-Si〉 structure, the photovoltage greatly increases, leading to a high conversion efficiency of 16.2% in spite of the simple cell structure with no pn-junction. Si-CN bonds are not ruptured by air mass 1.5 100 mW cm⁻² irradiation for 1000 h, and consequently the solar cells show no degradation. Neither are Si-CN bonds broken by heat treatment at 800 °C performed after the cyanide treatment. The thermal and irradiation stability of the cyanide treatment is attributable to strong Si-CN bonds, whose bond energy is calculated to be 1 eV higher than that of the Si-H bond energy using a density functional method.     

Theory of interface structure, energetics, and electronic properties of embedded Si/a-SiO2 nanocrystals

We examine the interrelation of the structural and bonding alterations, when Si nanocrystals are embedded in amorphous silicon dioxide, with the electronic properties of the resulting nanocomposite system. Monte Carlo simulations using a valence force-field model obtain the equilibrium structure of the interface, and investigate its energetics, stability and disorder as a function of the nanocrystal size. It is found that when the size is smaller than 2 nm, the embedded nanocrystals get heavily distorted. First-principles calculations of such small nanocrystals reveal a drastic reduction of the energy gap compared to the free-standing case. The origin of this pinning is attributed to the structural deformations, while oxygen states at the interface seem to play a minor role.     

Electrical characterization of MOS memory devices containing metallic nanoparticles and a high-k control oxide layer

We study the electrical characteristics of a MOS structure in which Pt nanoparticles are embedded. This structure has a tunneling oxide of 3.5 nm in thickness (a SiO₂ thermal oxide layer) on top of a Si wafer, and a control oxide of 27 nm (HfO₂ layer deposited by electron gun evaporation). The nanoparticles are deposited on the SiO₂ layer with electron gun evaporation, at room temperature. The electrical study of the structures demonstrates that the “write” process is initiated at low electric fields. This indicates that this type of memory structure can be very promising for the fabrication of high speed MOSFET memory devices with low power consumption. Our charge retention measurements also show promising results.     

Effect of gamma radiation on the electrical properties of Polyaniline/silicon carbide heterojunctions

Polyaniline thin films have been deposited by a very simple technique on p-type Silicon Carbide (SiC) substrates to fabricate heterojunctions devices with good electrical properties. In this work two heterojunctions devices of Polyaniline (PANI) on p-type 4H–SiC and 6H–SiC substrates were electrically characterized using current- voltage (I-V) in the temperature range 20–430 K Capacitance–frequency (C-f) measurements. Furthermore, impedance and capacitance measurements are carried out to study the effect of gamma irradiation on these devices. Additionally, we demonstrate not only the ease of fabrication of PANI/p-SiC heterostructures, but also we show strong indication that these heterostructures have potential applications as sensors of gamma irradiation.     

等效电流方法计算单模光纤的辐射损耗

本文将无源的不规则波导看作有源的理想波导,求等效极化电流电场,直接得到单一光波导模间耦合和辐射损耗问题的解。用这个方法计算了阶跃单模光纤折射率轴向不均匀变化的损耗。这种方法完全脱离了耦合波的概念,它不但物理意义明确,而且求解问题的方法简洁。最后以单模光纤弯曲和微弯损耗为例,说明等效电流方法还适用于广义耦合波理论才能求解的问题。     

A new interpretation of the CO state in half-doped manganites: new results from neutron diffraction and synchrotron radiation experiments

In the R_1−xD_xMnO₃ (x0.5) manganites, the structural phase transition at T_CO is commonly interpreted as a concomitant charge and orbital ordering (CO/OO) process driven by a co-operative Jahn–Teller effect and Coulomb repulsion forces. The low-temperature phase is supposed to contain well-separated and ordered Mn³⁺ and Mn⁴⁺ ionic species in an NaCl-like pattern. Structure refinement, from a neutron diffraction experiment below T_CO on a Pr_0.6Ca_0.4MnO₃ single crystal, gives us a model for the displacement of atoms with respect to the high-temperature phase that invalidates the standard model based in the CO/OO picture. Our result is a non-centrosymmetric crystal structure with two non-equivalent MnO₆ octahedra, both being slightly elongated but displaying very similar average Mn–O distances (1.96 and 1.95 Å, respectively) and having off-centered Mn atoms. We argue that this is a proof of the absence of charge ordering in half-doped manganites in the sense of formation of separated Mn³⁺ and Mn⁴⁺ ionic species. A new qualitative interpretation of the CE-type spin ordering (SO) is proposed. The so-called CO transition is, in fact, a structural transition induced by the change in the mean free path of electrons that continue to be thermally activated below T_CO by forming ferromagnetic Mn–Mn pairs stabilized by a local double-exchange process. The CE SO pattern results from the ordering of these pairs formed at T_CO. High-resolution synchrotron powder diffraction shows a complex anisotropic/asymmetric strains appearing at the transition that can be phenomenologically fitted by additional phases. Complementary electron diffraction and microscopy have shown no trace of macroscopic phase separation.