Carriers recombination processes in charge trapping memory cell by simulation

We have evaluated the effects of recombination processes in a charge storage layer, either between trapped electrons and trapped holes or between trapped carriers and free carriers, on charge trapping memory cell＇s performances by numerical simulation. Recombination is an indispensable mechanism in charge trapping memory. It helps charge convert process between negative and positive charges in the charge storage layer during charge trapping memory programming/erasing operation. It can affect the speed of programming and erasing operations.     

Modeling of trap-assisted tunneling on performance of charge trapping memory with consideration of trap position and energy level

In this work, the trap-assisted tunneling(TAT) mechanism is modeled as a two-step physical process for charge trapping memory(CTM). The influence of the TAT mechanism on CTM performance is investigated in consideration of various trap positions and energy levels. For the simulated CTM structure, simulation results indicate that the positions of oxide traps related to the maximum TAT current contribution shift towards the substrate interface and charge storage layer interface during time evolutions in programming and retention operations, respectively. Lower programming voltage and retention operations under higher temperature are found to be more sensitive to tunneling oxide degradation.     

Non-volatile memory effect of a high-density NiSi nano-dots floating gate memory using single triangular-shaped Si nanowire channel

A back-gated nonplanar floating gate device based on buried single triangular-shaped Si nanowire channel (width ～40 nm) and embedded high-density uniform NiSi nano-dots (～1.5×10¹² cm^?2) is demonstrated. Memory properties including memory window, programming/erasing, and retention are evaluated. The transfer and transient characteristics show clear charge injection, storage and removal effects and the associated programming/erasing mechanism based on fringing electric field is studied. Robust room and high temperature retention performance is observed.     

High-performance amorphous In-Ga-Zn-O thin-film transistor nonvolatile memory with a novel p-SnO/n-SnO2 heterojunction charge trapping stack

Amorphous In-Ga-Zn-O (a-IGZO) thin-film transistor (TFT) memories with novel p-SnO/n-SnO₂ heterojunction charge trapping stacks (CTSs) are investigated comparatively under a maximum fabrication temperature of 280 ℃. Compared to a single p-SnO or n-SnO₂ charge trapping layer (CTL), the heterojunction CTSs can achieve electrically programmable and erasable characteristics as well as good data retention. Of the two CTSs, the tunneling layer/p-SnO/n-SnO₂/blocking layer architecture demonstrates much higher program efficiency, more robust data retention, and comparably superior erase characteristics. The resulting memory window is as large as 6.66 V after programming at 13 V/1 ms and erasing at -8 V/1 ms, and the ten-year memory window is extrapolated to be 4.41 V. This is attributed to shallow traps in p-SnO and deep traps in n-SnO₂, and the formation of a built-in electric field in the heterojunction.     

缺陷对电荷俘获存储器写速度影响

基于密度泛理论的第一性原理以及VASP软件,研究了电荷俘获存储器(CTM)中俘获层HfO2在不同缺陷下(3价氧空位(VO3)、4价氧空位(VO4)、铪空位(VHf)以及间隙掺杂氧原子(IO))对写速度的影响.对比计算了HfO2在不同缺陷下对电荷的俘获能、能带偏移值以及电荷俘获密度.计算结果表明:VO3,VO4与VHf为单性俘获,IO则是双性俘获,HfO2在VHf时俘获能最大,最有利于俘获电荷;VHf时能带偏移最小,电荷隧穿进入俘获层最容易,即隧穿时间最短;同时对电荷俘获密度进行对比,表明VHf对电荷的俘获密度最大,即电荷被俘获的概率最大.通过对CTM的写操作分析以及计算结果可知,CTM俘获层m-HfO2在VHf时的写速度比其他缺陷时的写速度快.本文的研究将为提高CTM操作速度提供理论指导.     

Investigation of light programmable non‐volatile memory in an organic phototransistor

Excellent non‐volatile memory characteristics have been demonstrated under the optoelectric conditions for organic phototransistors (OPTs). The high photosensitivity shown as reversible shifts in light‐induced V_TH exhibits a large memory window for programming caused by the excited immobile carriers (electron) trapped as a function of the electrical bias and the light intensity. The long life span of stored electrons also reveals promising behavior with respect to data retention as well as the electrical reliability to serve as a data storage medium with the non‐volatile memory characteristic in OPTs. The V_TH recovery accelerated by the reversible bias stress for the stored charges under irradiation shows that the erasing behavior is clearly brought by the discharge process of long‐lived electrons occupied in deep states. Plausible mechanisms in the energy band are discussed for the programming and erasing process, which provides a fundamental understanding of the intrinsic charge storage behavior in OPTs. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Total Ionization Dose Effects on Charge Storage Capability of Al_2O_3/HfO_2/Al_2O_3-Based Charge Trapping Memory Cell

Because of the discrete charge storage mechanism, charge trapping memory(CTM) technique is a good candidate for aerospace and military missions. The total ionization dose(TID) effects on CTM cells with Al_2O_3/HfO_2/Al_2O_3(AHA) high-k gate stack structure under in-situ 10 keV x-rays are studied. The C-V characteristics at different radiation doses demonstrate that charge stored in the device continues to be leaked away during the irradiation,thereby inducing the shift of flat band voltage(V_(fb)). The dc memory window shows insignificant changes, suggesting the existence of good P/E ability. Furthermore, the physical mechanisms of TID induced radiation damages in AHA-based CTM are analyzed.     

锗/硅异质纳米结构中空穴存储特性研究

对p沟道锗/硅异质纳米结构存储器空穴隧穿的物理过程作了详细的分析，并对器件的擦写和保留时间特性进行了数值模拟.研究结果表明：由于异质纳米结构的台阶状隧穿势垒和较高价带带边差的影响，与传统的硅纳米结构存储器和n沟道锗/硅异质纳米结构存储器相比，当前器件的保留时间分别提高到10⁸和10⁵s以上，同时器件的擦写时间特性基本保持不变.这种存储器结构单元有效地解决了快速擦写编程和长久存储之间的矛盾，极大地提高了器件的存储性能. 关键词： 锗/硅 纳米结构 存储器 空穴存储 数值模拟     

电荷俘获存储器的过擦现象

基于密度泛函理论的第一性原理平面波超软赝势方法和VASP软件对电荷俘获存储器过擦现象进行了分析研究.通过形成能的计算,确定了含有氮空位缺陷的Si3N4和含有间隙氧缺陷的Hf O2作为研究的对象;俘获能的计算结果表明两种体系对电子的俘获能力比对空穴的大,因而对两体系擦写载流子确定为电子.分别计算了Hf O2和Si3N4擦写前后的能量、擦写前后电荷分布变化、吸附能和态密度,以说明过擦的微观机理.对能量和擦写电荷变化的研究,表明Si3N4相比于Hf O2,其可靠性较差,且Si3N4作为俘获层,在一个擦写周期后,晶胞中电子出现减少现象;界面吸附能的研究表明,Si3N4相比于Hf O2在缺陷处更容易与氧进行电子交换;最后,通过对态密度的分析表明Si3N4和Hf O2在对应的缺陷中均有缺陷能级俘获电子,前者为浅能级俘获,后者为深能级俘获.综上分析表明,Si3N4在氮空位的作用下,缺陷附近原子对电子的局域作用变弱,使得Si3N4作为俘获层时,材料本身的电子被擦出,使得擦操作时的平带偏移电压增大,导致存储器发生过擦.本文的研究结果揭示了过擦的本质,对提高电荷俘获存储器的可靠性以及存储特性有着重要的指导意义.     

Multilevel memory characteristics by light-assisted programming in floating-gate organic thin-film transistor nonvolatile memory

In this work, we study charge trapping in floating-gate organic thin-film transistor nonvolatile memories (FG-OTFT-NVMs) fabricated by a simple method. The inner discrete distribution aluminum nanoparticles (Al-Nps) and the continuous compact thin alumina film were formed to act as the floating-gate and the tunneling dielectric layer, respectively by thermally evaporated Al at a slow rate and then heat annealed in dry air. The devices exhibited remarkable photoresponse and memory effect. Compared with the unidirectional threshold voltage (V_T) shifts of memories by programming/erasing (P/E) in dark, larger bidirectional V_T shifts were obtained by light-assisted programming, and therefore the memory performances were enhanced. A multilevel memory behavior was observed in our memories, which depended on programming conditions. The charge trapping mechanisms of memories operated in dark and under illumination are discussed, respectively. The results indicate that optimal memory performance requires charge carriers of both polarities, because it is a very efficient method to enlarge the memory window and to lower the P/E voltage by overwriting trapped charges by injected charges of opposite polarity.     

Charge accumulation in silicon mis structures with dysprosium oxide dielectrics exposed to UV radiation

We study charge accumulation processes in silicon MIS structures with a dysprosium oxide dielectric layer, when the structure is exposed to UV radiation. The dependence of the trapped charge on the exposure time, the applied voltage, and the charge passing through the MIS structure is determined. It is shown that there are deep-seated capture centers for electrons and holes in the dysprosium oxide and these effects can be exploited in devices capable of optically recording and erasing information.Samarkand State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 7, pp. 99–105, July, 1994.     

Probing charge transport in manganite film through switching parameters

We have investigated the bipolar resistive switching of Y_0.95Ca_0.05MnO₃ (YCMO) thin film on Si substrate using pulsed laser deposition. Simulation of Mn L_3,2 near-edge X-ray absorption fine structure has been executed by CTM4XAS to corroborate the presence of a mixed-valence state of Mn ions and oxygen vacancies. The charge transport in the film is described by the space charge limited mechanism. Murgatroyd and space charge limited mechanism relations are used to calculate the mobility and other switching parameters at high resistance state. With a decrease in the switching layer (near to positively biased electrode) thickness, better resistive switching was observed. This work indicates that the localized switching thickness and temperature strongly affect the resistive switching of the YCMO film.     

Formation of high density TiN nanocrystals and its application in non-volatile memories

Non-volatile memory based on TiN nanocrystal （TiN-NC） charge storage nodes embedded in SiO2 has been fabricated and its electrical properties have been measured. It was found that the density and size distribution of TiN-NCs can be controlled by annealing temperature. The formation of well separated crystalline TiN nano-dots with an average size of 5 nm is confirmed by transmission electron microscopy and x-ray diffraction, x-ray photoelectron spectroscopy confirms the existence of a transition layer of TiNxOy/SiON oxide between TiN-NC and SiO2, which reduces the barrier height of tunnel oxide and thereby enhances programming/erasing speed. The memory device shows a memory window of 2.5V and an endurance cycle throughout 10^5. Its charging mechanism, which is interpreted from the analysis of programming speed （dVth/dt） and the gate leakage versus voltage characteristics （Ig vs Vg）, has been explained by direct tunnelling for tunnel oxide and Fowler Nordheim tunnelling for control oxide at programming voltages lower than 9V, and by Fowler-Nordheim tunnelling for both the oxides at programming voltages higher than 9V.     

Al掺杂对HfO2俘获层可靠性影响第一性原理研究

采用基于MS(Materials Studio)软件和密度泛函理论的第一性原理方法, 研究了HfO₂ 俘获层的电荷俘获式存储器(Charge Trapping Memory, CTM)中电荷的保持特性以及耐擦写性. 在对单斜晶HfO₂中四配位氧空位(V_O4) 缺陷和V_O4 与Al替位Hf掺杂的共存缺陷体(Al+V_O4)两种超晶胞模型进行优化之后, 分别计算了其相互作用能、形成能、Bader电荷、态密度以及缺陷俘获能. 相互作用能和形成能的计算结果表明共存缺陷体中当两种缺陷之间的距离为2.216 Å时, 结构最稳定、缺陷最容易形成; 俘获能计算结果表明, 共存缺陷体为双性俘获, 且与V_O4缺陷相比, 俘获能显著增大; Bader电荷分析表明共存缺陷体更有利于电荷保持; 态密度的结果说明共存缺陷体对空穴的局域能影响较强; 计算两种模型擦写电子前后的能量变化表明共存缺陷体的耐擦写性明显得到了改善. 因此在HfO₂俘获层中可以通过加入Al杂质来改善存储器的保持特性和耐擦写性. 本文的研究可为改善CTM数据保持特性和耐擦写性提供一定的理论指导.     

Effect of heating treatment on trap level distribution in polyamide 66 film electrets

The effect of heating treatment on the trap level distribution in polyamide 66 film electret is studied by thermally stimulated depolarization current （TSDC） technique. For annealed polyamide 66, there are three trap levels that respectively originate from space charge trapped in amorphous phase, interphase and crystalline phase. There is one peak that originates from space charge trapped in amorphous phase for quenched one. Using multi-point method to fit the experimental curves, the detrapping current peaks can be separated and the trap depth is obtained. The shallower trap levels trapped in amorphous phase and interphase are obviously close to the deeper trap level trapped in crystalline phase for annealed polyamide 66 as the polarization temperature increases, while the trap level distribution remains unaffected by polarization temperature for quenched one.     

Interplay of parallel electric field and trapped electrons in kappa-Maxwellian auroral plasma for EMEC instability

In this paper, propagation characteristics of electromagnetic electron cyclotron(EMEC) waves based on kappa-Maxwellian distribution have been investigated to invoke the interplay of the electric field parallel to the Earth's magnetic field and auroral trapped electrons. The dispersion relation for EMEC waves in kappa-Maxwellian distributed plasma has been derived using the contribution of the parallel electric field and trapped electron speed. Numerical results show that the presence of the electric field has a stimulating effect on growth rate, which is more pronounced at low values of wave number. It is also observed that as the threshold value of trapped electron speed is surpassed, it dominates the effect of the parallel electric field and EMEC instability is enhanced significantly. The electric field acts as another source of free energy, and growth can be obtained even in the absence of trapped electron drift speed and for very small values of temperature anisotropy. Thus the present study reveals the interplay of the parallel electric field and trapped electron speed on the excitation of EMEC waves in the auroral region.     

部分耗尽SOI MOSFET总剂量效应与偏置状态的关系

实验表明SOI MOSFET掩埋氧化层中的总剂量辐射效应与辐射过程中的偏置状态有关. 对诱发背沟道泄漏电流的陷阱电荷进行了研究. 建立一个数值模型来模拟不同偏置下陷进电荷的建立, 它包括辐射产生的载流子复合和俘获的过程. 模拟结果与实验结果相符, 解释了总剂量辐射效应受偏置状态影响的机理.     

ELDRS and dose-rate dependence of vertical NPN transistor

The enhanced low-dose-rate sensitivity (ELDRS) and dose-rate dependence of vertical NPN transistors are investigated in this article.The results show that the vertical NPN transistors exhibit more degradation at low dose rate,and that this degradation is attributed to the increase on base current.The oxide trapped positive charge near the SiO2-Si interface and interface traps at the interface can contribute to the increase on base current and the two-stage hydrogen mechanism associated with space charge effect can well explain the experimental results.     

DFT方法研究掺杂氮化硅对SONOS器件保持性能的作用

可通过对氮化硅层掺杂来改变俘获电荷的缺陷种类和数量的方法，改善SONOS非挥发性存储器件的保持性能.建立无定形氮化硅和氧、硫、磷、氟或氯掺杂氮化硅中缺陷的簇模型；根据第一性原理的密度泛函理论(DFT)，对缺陷的簇模型结构优化并计算能量，得到缺陷俘获电荷过程的能量变化.发现缺陷俘获电子的能力比俘获空穴的能力好，电子释放过程应对温度敏感，而空穴释放过程主要由隧穿机理控制.预测与氧氮化硅一样，硫或磷掺杂氮化硅代替氮化硅作为SONOS器件的电荷储存层，可改善器件的保持性能.     

Effect of Polarization Temperature on the Chain Segment Motion and Space Charge Detrapping in Polyamide 610 Film Electrets

The effect of polarization temperature on the chain segment motion and charge trapping and detrapping in polyamide 610 films has been investigated by means of thermally stimulated depolarization current (TSDC) and wide-angle X-ray diffraction (WAXD). A small part of the amorphous phase of quenched polyamide 610 changes into the crystalline state with increasing polarization temperature. There are three current peaks (named α, ρ₁, and ρ₂ peak, respectively) in the TSDC spectra. The α peak corresponds to the glass transition, the ρ ₁ peak is attributed to space charge trapped in the amorphous phase, and interphase between crystalline and amorphous phases, and the ρ ₂ peak originates from space charge trapped in the crystalline phase. By analyzing the characteristic parameters of these peaks, it was found that the increase of polarization temperature induced a decrease of the chain segment mobility and promoted the creation of structural traps in polyamide 610. The decrease of the chain segment mobility in the amorphous phase made the intensity of the α peak weak and the activation energy increased. The higher the polarization temperature, the higher the degree of crystallinity and the more charge carriers trapped in the crystalline phase. So, the increase of polarization temperature made the intensity of the ρ ₂ peak strong and increased the stability of trapped charge in the crystalline phase. The increase of polarization temperature also made the intensity of the ρ ₁ peak strong and decreased the stability of trapped charge in the amorphous phase and interphase.