The influence of thermally assisted tunneling on the performance of charge trapping memory

We evaluate the influence of the thermally assisted tunneling (TAT) mechanism on charge trapping memory (CTM) cell performance by numerical simulation, and comprehensively analyse the effects of the temperature, trap depth, distribution of trapped charge, gate voltage and parameters of TAT on erasing/programming speed and retention performance. TAT is an indispensable mechanism in CTM that can increase the detrapping probability of trapped charge. Our results reveal that the TAT effect causes the sensitivity of cell performance to temperature and it could affect the operational speed, especially for the erasing operation. The results show that the retention performance degrades compared with when the TAT mechanism is ignored.     

Silicon nanoparticle charge trapping memory cell

A charge trapping memory with 2 nm silicon nanoparticles (Si NPs) is demonstrated. A zinc oxide (ZnO) active layer is deposited by atomic layer deposition (ALD), preceded by Al₂O₃ which acts as the gate, blocking and tunneling oxide. Spin coating technique is used to deposit Si NPs across the sample between Al₂O₃ steps. The Si nanoparticle memory exhibits a threshold voltage (V_t) shift of 2.9 V at a negative programming voltage of –10 V indicating that holes are emitted from channel to charge trapping layer. The negligible measured V_t shift without the nanoparticles and the good re‐ tention of charges (>10 years) with Si NPs confirm that the Si NPs act as deep energy states within the bandgap of the Al₂O₃ layer. In order to determine the mechanism for hole emission, we study the effect of the electric field across the tunnel oxide on the magnitude and trend of the V_t shift. The V_t shift is only achieved at electric fields above 1 MV/cm. This high field indicates that tunneling is the main mechanism. More specifically, phonon‐assisted tunneling (PAT) dominates at electric fields between 1.2 MV/cm < E < 2.1 MV/cm, while Fowler–Nordheim tunneling leads at higher fields (E > 2.1 MV/cm). (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

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.     

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.     

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.     

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.     

Performance improvement of charge trap flash memory by using a composition-modulated high-k trapping layer

A composition-modulated （HfO2）x（Al2O3）1-x charge trapping layer is proposed for charge trap flash memory by controlling the A1 atom content to form a peak and valley shaped band gap. It is found that the memory device using the composition-modulated （HfO2）x（Al2O3）l-x as the charge trapping layer exhibits a larger memory window of 11.5 V, improves data retention even at high temperature, and enhances the program/erase speed. Improvements of the memory characteristics are attributed to the special band-gap structure resulting from the composition-modulated trapping layer. Therefore, the composition-modulated charge trapping layer may be useful in future nonvolatile flash memory device application.     

Reduction of charge trapping in HfO2 film on a Ge substrate by trimethylaluminum pretreatment

Trimethylaluminum pretreatment prior to HfO₂ deposition is introduced for native oxide reduction. It is identified that the trimethylaluminum pretreatment could effectively reduce native oxide, which is transformed to an aluminum oxide interfacial layer. Formation of the thin aluminum oxide layer suppresses Ge diffusion into HfO₂, reducing hysteresis in the ca‐ pacitance–voltage curve. Moreover, the device reliability of the trimethylaluminum pretreated sample is improved in a constant current stress test. This work indicates that trimethylaluminum pretreatment is an effective in‐situ method for the gate dielectric stack formation to reduce charge trapping in the HfO₂ film on a Ge substrate. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

聚乙烯薄膜中空间电荷短路放电复合率的发光法研究

通过对在直流高压作用下低密度聚乙烯薄膜中注入的空间电荷的短路放电发光测量,研究了聚乙烯薄膜中空间电荷的复合率.通过短路放电光子数的测量及定量分析考察了电压极性、场强大小及加压时间对短路下电荷复合率的影响.结果表明发光强度（复合率）随外加场强的变化明显,而与加压时间的关系不显著.但场强高于80 MV/m时发光强度（复合率）的增大速度变慢.结合本实验结果及他人的相关数据,得出了聚乙烯薄膜样品的发光效率约为5.9×10^-6,短路初始阶段的0.2 s内样品的电荷复合率约为2.8%. 关键词： 空间电荷 短路放电 复合发光 复合率     

Determination of trapping–detrapping events,recombination processes and gap-state parameters by modulated photocurrent measurements on amorphous silicon

Analysis of the out-of-phase modulated photocurrent (MPC) signal, the so-called Y signal, is proposed for determining the trapping–detrapping events, recombination processes and gap-state parameters in amorphous silicon. This is demonstrated by analysing experimental Y spectra obtained on this material from different laboratories including our own. Model simulations are also employed in which the amphoteric nature of the dangling bonds and their distribution according to the defect-pool model are taken into account. From the reconstruction of the Y signal, phase shift and MPC amplitude spectra, several contributions resolved from the frequency dependence of the experimental Y spectra are identified. Two electron trapping–detrapping processes are resolved. These are attributed to hydrogen-related positive defects and to transitions involving the D^+/0 level of the normal dangling bonds from the defect-pool distribution. At lower frequencies a residual contribution is resolved that is attributed to a term related to recombination through the D^+/0 and D^0/? levels. Between 300 and 150?K the above recombination contribution is essentially from the D^0/? and dominates the Y signal at lower frequencies. In this region a characteristic phase lead appears, which is attributed to the existence of safe hole traps in the valence band tail. Around 150?K, trapping–detrapping events in the conduction band tail dominate.     

Dependence of charge trapping of fluorescent and phosphorescent dopants in organic light-emitting diodes on the dye species and current density

This paper utilizes multilayer organic light-emitting diodes with a thin layer of dye molecules to study the mechanism of charge trapping under different electric regimes. It demonstrates that the carrier trapping was independent of the current density in devices using fluorescent material as the emitting molecule while this process was exactly opposite when phosphorescent material was used. The triplet--triplet annihilation and dissociation of excitons into free charge carriers was considered to contribute to the decrease in phosphorescent emission under high electric fields. Moreover, the fluorescent dye molecule with a lower energy gap and ionized potential than the host emitter was observed to facilitate the carrier trapping mechanism, and it would produce photon emission.     

Modification to poloidal charge exchange recombination spectroscopy measurement in JT-60U tokamak

With consideration of the effects of the atomic process and the sight line direction on the charge exchange re-combination spectroscopy （CXRS）, a code used to modify the poloidal CXRS measurement on Tokamak-60 Upgrade （JT-60U） in Japan Atomic Energy Research Institute is developed, offering an effective tool to modify the measurement and analyse experimental results further. The results show that the poloidal velocity of ion is overestimated but the ion temperature is underestimated by the poloidal CXRS measurement, and they also indicate that the effect of observation angle on rotation velocity is a dominant one in a core region （r/a 〈 0.65）, whereas in an edge region where the sight line is nearly normal to the neutral beam, the observation angle effect is very small. The difference between the modified velocity and the neoclassical velocity is not larger than the error in measurement. The difference inside the internal transport barrier （ITB） region is 2-3 times larger than that outside the ITB region, and it increases when the effect of excited components in neutral beam is taken into account. The radial electric field profile is affected greatly by the poloidal rotation term, which possibly indicates the correlation between the poloidal rotation and the transport barrier formation.[第一段]     

点电荷在拓扑绝缘体和导体中感应磁单极

利用电势和磁标势的第一类零阶贝塞尔函数的公式及拓扑绝缘体材料的本构关系, 推导了点电荷在电介质、 拓扑绝缘体和接地导体三个区域的感应电势及感应磁标势. 研究表明: 点电荷 在电介质、 拓扑绝缘体和接地导体中感应了像电荷和像磁单极; 感应像电荷和感应像磁单极的大小和正负除了与场源电荷、 拓扑绝缘体材料参数等因素有关外, 还与像电荷和像磁单极所处的空间位置有关.     

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.     

模拟分析少子复合速率及吸收层对HIT太阳电池性能的影响

采用AMPS-1D程序模拟分析了前后接触少子复合速率以及吸收层的厚度和少子迁移率对非晶硅/单晶硅异质结太阳电池光伏性能的影响.模拟发现,与太阳电池的前接触少子复合速率相比,背接触少子复合速率对太阳电池光伏性能的影响更为显著.吸收层单晶硅的厚度对太阳电池光伏性能的影响要受到单晶硅隙间缺陷态密度以及背接触少子复合速率的制约.当背接触复合占主要地位时,吸收层越厚电池的转换效率越高;当吸收层隙间缺陷复合占主要地位时,电池的转换效率在某一厚度处达到峰值.吸收层的少子迁移率对太阳电池性能的影响,也要受到背接触少子复合速率的制约.当背接触复合速率较低时,少子迁移率越大,电池的转换效率越高;当背接触复合速率较高时,少子迁移率越小,电池的转换效率越高.     

相变存储单元RESET多值存储过程的数值仿真研究

使用数值仿真方法对相变随机存储器存储单元的RESET操作的多值存储过程进行了研究,建立了三维存储单元模型,用有限元法解Laplace方程及热传导方程以模拟电脉冲作用下的存储单元物性变化过程.计算出单元内相变层的相态分布及单元整体电阻,分析了单元内部尺寸变化对多值存储过程及状态的影响.结果表明,通过精确控制输入电脉冲,相变存储单元能够实现4值存储;多值存储状态受单元内相变层厚度及下电极接触尺寸变化的影响较大;存储状态在80℃的环境温度下均可保持10年以上不失效.     

氮氟复合注入对注氧隔离SOI材料埋氧层内固定正电荷密度的影响

为了抑制埋层注氮导致的埋层内正电荷密度的上升, 本文采用氮氟复合注入方式, 向先行注氮的埋层进行了注氮之后的氟离子注入, 并经适当的退火, 对埋层进行改性. 利用高频电容-电压 (C-V) 表征技术, 对复合注入后的埋层进行了正电荷密度的表征. 结果表明, 在大多数情况下, 氮氟复合注入能够有效地降低注氮埋层内的正电荷密度, 且其降低的程度与注氮后的退火时间密切相关. 分析认为, 注氟导致注氮埋层内的正电荷密度降低的原因是在埋层中引入了与氟相关的电子陷阱. 另外, 实验还观察到, 在个别情况下, 氮氟复合注入引起了埋层内正电荷密度的进一步上升. 结合测量结果, 讨论分析了该现象产生的原因. 关键词： 绝缘体上硅(SOI) 材料 注氮 注氟 埋氧层正电荷密度     

调制光/电作用下染料敏化太阳电池中电荷传输和界面转移研究

详细讨论了染料敏化太阳电池(DSC)在稳态光照射或外加偏压下电荷的传输和转移过程,以及在调制光/电作用下电池的频率响应特点.通过电化学阻抗谱、光电化学阻抗谱、强度调制光电流谱和强度调制光电压谱等四种频谱光电测试手段,对DSC中TiO₂薄膜电子传输和界面转移的相关时间常数进行测量.详细分析和比较了电荷的传输及转移过程对时间常数的影响.结果表明,在低光强或低偏压下电荷传输和转移过程对时间常数影响较小,但在高光强或高偏压下对电子寿命影响明显.     

Nuclear charge radii and electromagnetic moments of scandium isotopes and isomers in the f7/2 shell

Collinear laser spectroscopy experiments on the ScII transition 3d4s ³D₂→3d4p ³F₃ at λ ≈ 363.1 nm were performed on the ^42–46Sc isotopic chain using an ion guide isotope separator with a cooler–buncher. Isotope and isomer shifts and hyperfine structures of five ground states and two isomers were measured. Preliminary results on the nuclear moments and charge radii changes deduced from these measurements are reported.