电荷俘获存储器数据保持特性第一性原理研究

本文研究HfO₂掺入Al替位Hf杂质和氧空位共同掺杂对电荷俘获型存储器存储特性的影响. HfO₂作为高介电常数材料由于具有缩小器件尺寸、提高器件性能等优势, 被广泛用于CTM的俘获层. 采用MS和VASP研究了HfO₂俘获层中掺入Al对氧空位形成能的影响. 同时计算了两种缺陷在不同距离下的相互作用能. 计算结果表明在HfO₂中掺入Al使得氧空位的形成能降低, 并且三配位氧空位的形成能比四配位氧空位的形成能降低的更多. 通过研究Al和三配位氧空位两种缺陷间不同距离的三种情况, 计算结果表明当缺陷间距为2.107 Å时, 体系的电荷俘获能最大; 量子态数最多; 布居数最小、Al–O键最长. 通过研究三种体系写入空穴后键长的变化, 得出当缺陷间距为2.107 Å时, 写入空穴后体系的Al–O键长变化最小. 以上研究结果表明, 掺入Al后可以有效提高电荷俘获型存储器的数据保持能力. 因而本文的研究为改善电荷俘获型存储器数据保持特性提供一定的理论指导.     

A Charge-Trap Memory Device with a Composition-Modulated Zr-Silicate High-k Dielectric Multilayer Structure

We report a novel charge-trap memory device with a composition-modulated Zr-silicate high-k dielectric mul- tilayer structure prepared by using the pulsed laser deposition technique. The device employs amorphous （ZrO2）0.5（SiO2）0.5 as the tunneling and blocking oxide layers, and ZrO2 nanocrystals as the trapping storage layer. Zr02 nanocrystals are precipitated from the phase separation of （ZrO2）0.5（SiO2）0.2 films annealed at 800℃, and isolated from each other within the amorphous （ZrO2）0.5（SiO2）0.5 matrix. Our charge trapping device shows a memory window of 2.6 V and a stored electron density of 1×10^13/cm2.     

Analysis of Effect of Zn(O,S) Buffer Layer Properties on CZTS Solar Cell Performance Using AMPS

The Cu_2ZnSnS_4(CZTS)-based solar cell is numerically simulated by a one-dimensional solar cell simulation software analysis of microelectronic and photonic structures(AMPS-ID).The device structure used in the simulation is Al/ZnO:Al/nZn(O,S)/pCZTS/Mo.The primary motivation of this simulation work is to optimize the composition in the ZnO_(1-x)S_x buffer layer,which would yield higher conversion efRciency.By varying S/(S+0) ratio x,the conduction band offset(CBO) at CZTS/Zn(0,S) interface can range from-0.23 eV to 1.06 eV if the full range of the ratio is considered.The optimal CBO of 0.23 eV can be achieved when the ZnO_(1-x)S_x buffer has an S/(S+0) ratio of 0.6.The solar cell efRciency Rrst increases with increasing sulfur content and then decreases abruptly for x 0.6,which reaches the highest value of 17.55%by our proposed optimal sulfur content x = 0.6.Our results provide guidance in dealing with the ZnO_(1-x)S_x buffer layer deposition for high efficiency CZTS solar cells.     

Structural and electrical properties of a high-k SmAlO3 charge trapping flash memory

In this study, we proposed the Al/Al₂O₃/SmAlO₃/SiO₂/Si flash memory devices using high-k SmAlO₃ film as a charge trapping layer and high-k Al₂O₃ film as a blocking layer. The structural and morphological features of these films were explored by X-ray diffraction, X-ray photoelectron spectroscopic and atomic force microscopy. The SmAlO₃ flash memory devices annealed at 800 °C showed excellent electrical properties, such as a large memory window of ～2.61 V (measured at a sweep voltage range of ±5 V) and a small charge loss of ～7.1% (measured time up to 10⁴ s). In addition, the charge trap centroid and charge trap density were extracted by constant current stress method.     

化学法制备的HfO2薄膜的激光损伤阈值研究

 采用化学法制备了HfO₂介质膜，研究了热处理、紫外辐照以及Al₂O₃复合对HfO₂介质膜激光损伤阈值的影响。采用红外光谱（FTIR）和X射线衍射仪对薄膜进行了表征，并用输出波长为1.064 μm、脉宽为10 ns的电光调Q激光系统测试薄膜的激光损伤阈值。实验结果表明：采用150 ℃左右的温度对薄膜进行热处理可以提高薄膜的激光损伤阈值，所获得的薄膜的激光损伤阈值高达42.32 J/cm²，比热处理前的激光损伤阈值提高了82%；无机材料Al₂O₃的适量添加能够提高薄膜的激光损伤阈值，其中HfO₂与Al₂O₃的最佳质量配比约为95∶5；另外，对薄膜进行适当的紫外辐照也可改善HfO₂ 薄膜以及HfO₂-Al₂O₃复合薄膜的抗激光损伤性能。紫外辐照对提高HfO₂-Al₂O₃复合薄膜的激光损伤阈值效果尤为显著，辐照40 min后的激光损伤阈值达到44.33 J/cm²，比紫外辐照前的激光损伤阈值提高了90%。     

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.     

Effects of different dopants on switching behavior of HfO_2-based resistive random access memory

In this study the effects of doping atoms(Al, Cu, and N) with different electro-negativities and ionic radii on resistive switching of HfO2-based resistive random access memory(RRAM) are systematically investigated. The results show that forming voltages and set voltages of Al/Cu-doped devices are reduced. Among all devices, Cu-doped device shows the narrowest device-to-device distributions of set voltage and low resistance. The effects of different dopants on switching behavior are explained with deferent types of CFs formed in HfO2 depending on dopants: oxygen vacancy(Vo) filaments for Al-doped HfO2 devices, hybrid filaments composed of oxygen vacancies and Cu atoms for Cu-doped HfO2 devices,and nitrogen/oxygen vacancy filaments for N-doped HfO2 devices. The results suggest that a metal dopant with a larger electro-negativity than host metal atom offers the best comprehensive performance.     

Interfaces between Al2O3 and beta-Ni(1-x)Al(x) alloys: complex structures and Ab initio thermodynamics

A methodology to study the structural stability of binary alloy/Al2O3 interfaces is developed by expanding the conventional ab initio thermodynamics to include the dependence on alloy composition. Results on beta-Ni(1-x)Al(x)/Al2O3 interfaces predict the existence of two types of stable interfaces. The stable interface at equilibrium with Al-rich or strictly 1:1 alloy contains an Al2-terminated Al2O3 surface and continues with NiAl layers, and the interface at equilibrium with Ni-rich alloy has Al accumulation but continues with a Ni-rich and then NiAl layers. Works of separation for the two interfaces are close to each other.     

Characterisation of sol-gel prepared (HfO2)x(SiO2)1-x (x=0.1, 0.2 and 0.4) by 1H, 13C, 17 O and 29Si MAS NMR, FTIR and TGA

The HfO2-SiO2 system is attracting interest as a possible new dielectric material in semiconductor devices. Knowledge of the location of hafnium within the silica network and the effect hafnium has on the structure will be central to the successful use of this material system in this application. Here, sol-gel techniques have been used to manufacture (HfO2)x(SiO2)1-x samples (x=0.1, 0.2 and 0.4, each heat treated at 250, 500 and 750 degrees C) and these have been characterised by magic angle spinning (MAS) NMR (1H, 13C, 17 O, 29Si), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis. 29Si MAS NMR showed that increasing the hafnia content decreases the connectivity of the silicate network, i.e. increases the range of differently connected SiO4 (Qn) units with more having increased numbers of non-bridging oxygens (i.e. lower n). FTIR and 17 O MAS NMR showed unequivocally that the x=0.4 sample phase-separated at higher temperatures, while in the x=0.1 sample the hafnium was homogeneously mixed into the SiO2 phase without any phase separation.     

Random telegraph noise on the threshold voltage of multi-level flash memory

We investigate the impact of random telegraph noise (RTN) on the threshold voltage of multi-level NOR flash memory. It is found that the threshold voltage variation (ΔV_th) and the distribution due to RTN increase with the programmed level (V_th) of flash cells. The gate voltage dependence of RTN amplitude and the variability of RTN time constants suggest that the large RTN amplitude and distribution at the high program level is attributed to the charge trapping in the tunneling oxide layer induced by the high programming voltages. A three-dimensional TCAD simulation based on a percolation path model further reveals the contribution of those trapped charges to the threshold voltage variation and distribution in flash memory.     

Wet thermal annealing effect on TaN/HfO2/Ge metal-oxide-semiconductor capacitors with and without a GeO2 passivation layer

Wet thermal annealing effects on the properties of TaN/HfO2/Ge metal-oxide-semiconductor(MOS) structures with and without a GeO2 passivation layer are investigated.The physical and the electrical properties are characterized by X-ray photoemission spectroscopy,high-resolution transmission electron microscopy,capacitance-voltage(C-V) and current-voltage characteristics.It is demonstrated that wet thermal annealing at relatively higher temperature such as 550℃ can lead to Ge incorporation in HfO2 and the partial crystallization of HfO2,which should be responsible for the serious degradation of the electrical characteristics of the TaN/HfO2/Ge MOS capacitors.However,wet thermal annealing at 400℃ can decrease the GeO x interlayer thickness at the HfO2/Ge interface,resulting in a significant reduction of the interface states and a smaller effective oxide thickness,along with the introduction of a positive charge in the dielectrics due to the hydrolyzable property of GeO x in the wet ambient.The pre-growth of a thin GeO2 passivation layer can effectively suppress the interface states and improve the C-V characteristics for the as-prepared HfO2 gated Ge MOS capacitors,but it also dissembles the benefits of wet thermal annealing to a certain extent.     

Anomalous behavior of the dielectric constant of hafnium silicates: a first principles study

We present an extensive ab initio study of the structural and dielectric properties of hafnium silicates Hf(x)Si(1-x)O(2) that accounts for the observed anomalous dependence on composition of the static dielectric constant in the entire x range. The results reveal that this complex behavior reflects that of the structural development with x, from silica to hafnia, and clarify how different growth processes can also lead to scattered sets of data. Several simple models proposed thus far to explain part of the experimental data are shown to be inadequate. It is argued that silicate layers with low hafnium content form at the HfO(2)/Si interface and play a crucial role in preserving high electron mobility in the channel.     

Room-temperature ferromagnetism induced by Cu vacancies in Cu_x(Cu_2O)_(1-x) granular films

Cux(Cu2O)1-x(0.09 x 1.00) granular films with thickness about 280 nm have been fabricated by direct current reactive magnetron sputtering. The atomic ratio x can be controlled by the oxygen flow rate during Cux(Cu2O)1-x deposition. Room-temperature ferromagnetism(FM) is found in all of the samples. The saturated magnetization increases at first and then decreases with the decrease of x. The photoluminescence spectra show that the magnetization is closely correlated with the Cu vacancies in the Cux(Cu2O)1-x granular films. Fundamentally, the FM could be understood by the Stoner model based on the charge transfer mechanism. These results may provide solid evidence and physical insights on the origin of FM in the Cu2O-based oxides diluted magnetic semiconductors, especially for systems without intentional magnetic atom doping.     

Epitaxial growth and band alignment of (Gd(x)La(1-x))(2)O(3) films on n-GaAs (001)

Herein we demonstrate the epitaxial stabilization of single-crystalline (Gd(x)La(1-x))(2)O(3) films on n-GaAs (001) with a controlled lattice match. (Gd(x)La(1-x))(2)O(3) films have an in-plane epitaxial relationship with a twofold rotation on GaAs (001). Spectroscopic characterization by photoemission and absorption confirms that the band gap of (Gd(x)La(1-x))(2)O(3) film is approximately approximately 5.8eV. However, the conduction band offset is increased by the unpinned Fermi level of the n-GaAs in the (Gd(x)La(1-x))(2)O(3) film (x=0.97). The correlation of the crystalline property and the interfacial band offset by the electrical properties, as probed by capacitance and leakage current measurements, is also discussed.     

Electrical and material characterization of tantalum pentoxide (Ta2O5) charge trapping layer memory

In this experiment, tantalum pentoxide (Ta₂O₅) was used in a metal/oxide/high-k Ta₂O₅/oxide/silicon (MOHOS) novel nanocrystal memory as a trapping layer. Post-annealing treatment, which can passivate defects and improve the material quality of the high-k dielectric, was applied to optimize device performance for a better memory window and faster P/E (program/erase) cycle. Material and electrical characterization techniques including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and electrical measurements were performed to analyze the device under different annealing conditions. The Ta₂O₅ charge trapping layer memory annealed at 900 °C had a higher window of 3.3 V in the current-voltage (C-V) hysteresis loop, and a higher charge retention capability than the samples prepared under various annealing conditions. These higher levels were due to the higher probability of deep-level charge trapping and lower leakage current.     

HfO2基铁电场效应晶体管读写电路的单粒子翻转效应模拟

使用器件-电路仿真方法搭建了氧化铪基铁电场效应晶体管读写电路,研究了单粒子入射铁电场效应晶体管存储单元和外围灵敏放大器敏感节点后读写数据的变化情况,分析了读写数据波动的内在机制.结果表明:高能粒子入射该读写电路中的铁电存储单元漏极时,处于"0"状态的存储单元产生的电子空穴对在器件内部堆积,使得栅极的电场强度和铁电极化增大,而处于"1"状态的存储单元由于源极的电荷注入作用使得输出的瞬态脉冲电压信号有较大波动;高能粒子入射放大器灵敏节点时,产生的收集电流使处于读"0"状态的放大器开启,导致输出数据波动,但是其波动时间仅为0.4 ns,数据没有发生单粒子翻转能正常读出.两束高能粒子时间间隔0.5 ns先后作用铁电存储单元漏极,比单束高能粒子产生更大的输出数据信号波动,读写"1"状态的最终输出电压差变小.     

Influence of different oxidants on the band alignment of HfO₂ films deposited by atomic layer deposition

Based on X-ray photoelectron spectroscopy(XPS),influences of different oxidants on band alignment of HfO2 films deposited by atomic layer deposition(ALD) are investigated in this paper.The measured valence band offset(VBO) value for H2 O-based HfO2 increases from 3.17 eV to 3.32 eV after annealing,whereas the VBO value for O 3-based HfO2 decreases from 3.57 eV to 3.46 eV.The research results indicate that the silicate layer changes in different ways for H2 O-based and O3-based HfO2 films after the annealing process,which plays a key role in generating the internal electric field formed by the dipoles.The variations of the dipoles at the interface between the HfO2 and SiO2 after annealing may lead the VBO values of H2 O-based and O 3-based HfO2 to vary in different ways,which fits with the variation of flat band(VFB) voltage.     

Mg-Al-Ca合金中三元Laves相的稳定性和电子结构

采用基于密度泛函理论的第一性原理方法,应用VASP (Vienna Ab-initio Simulation Package) 计算软件,研究了Mg-Al-Ca合金中三元Laves相,即Ca(Mg1-x,Alx)2和Al2(Ca1-x,Mgx) (x=0, 0.25, 0.50, 0.75, 1)在不同形态结构(C14, C15和C36)下的相稳定性及电子结构。计算所得的晶格常数和实验值吻合很好,形成能和相关能的计算用来研究三元Laves相的合金化能力和稳定性,结果表明：C14-Ca(Mg0.25,Al0.75)2具有很好的合金化能力,而C15- CaAl2具有很好的结构稳定性。态密度和电荷密度的计算用来研究Mg-Al-Ca合金中三元Laves相稳定性的内在微观机制。     

Magnetic properties of multiferroics-semiconductors Eu(1-x)Ce(x)Mn2O5

Studies of magnetization, magnetoresistance, and magnetic oscillations in semiconductor-multiferroics Eu(1-x)Ce(x)Mn2O5 (x = 0.2-0.25) (ECMO) at temperatures ranging from 5 to 350 K in magnetic fields up to 6 T are presented. It is shown that phase separation and charge carrier self-organization in the crystals give rise to a layered superstructure perpendicular to the c axis. An effect of magnetic field cycling on the superstructure, magnetization, and magnetoresistance is demonstrated. X-ray diffraction studies of ECMO demonstrating the effect of magnetic field on the superstructure are presented. The de Haas-van Alphen magnetization oscillations in high magnetic fields and the temperature-induced magnetic oscillations in a fixed magnetic field are observed at low temperatures. Below 10 K the quantum corrections to magnetization due to the weak charge carrier localization in 2D superlattice layers occur. It is shown that at all the temperatures the Eu(1-x)Ce(x)Mn2O5 magnetic state is dictated by superparamagnetism of isolated ferromagnetic domains.     

Highly efficient, 0.84 slope efficiency, 901 nm, quasi-two-level laser emission of Nd in strontium lanthanum aluminate

The possibility of using direct pumping into the emitting level of the Nd3+ ion in magnesium-compensated strontium lanthanum aluminate (Sr(1-x)La(x-y)Nd(y)Mg(x)Al(12-x)O19) to improve 900 nm 4F(3/2) --> 4I(9/2) laser emission is discussed. Selection of the composition parameters x and y for optimization of laser emission and reduction of heat generation is based on the spectroscopic and crystal growth characteristics. Pumping in the 865.5 nm absorption band 4I(9/2)(Z1) --> 4F(3/2)(R1) transforms the laser process into a quasi-two-level scheme of very low (below 4%) quantum defects. A very high slope efficiency (over 84%) for 901 nm continuous-wave laser emission is demonstrated with Ti:sapphire laser pumping in this band for a crystal with x = 0.4 and y = 0.05.