Au/SrTiO3/Au界面电阻翻转效应的低频噪声分析

本文研究了Au/SrTiO₃/Au三明治结构中的双极电阻翻转效应, 观察到高、低阻态的电阻弛豫现象. 低频噪声测量表明高、低阻态的电阻涨落表现出1/f行为. 对比试验表明, 高阻态的低频噪声来源于反向偏置肖特基势垒和氧空位的迁移, 强度较大, 低阻态的噪声则源于类欧姆接触底电极区域的氧空位迁移导致的载流子涨落, 强度较低. 同时, 界面上氧空位浓度的弛豫导致了高、低阻态的弛豫过程. 关键词： 电阻翻转效应 低频噪声 氧空位     

EuS/Ta异质结的极大磁电阻效应

在铁磁/超导异质结中,铁磁体的交换场通过近邻效应将导致超导体准粒子态密度的塞曼劈裂.基于该效应,在外磁场不强的情况下,通过外加磁场可以有效地调节铁磁/超导界面处的交换作用,从而实现超导体在正常态和超导态之间转换,产生极大磁电阻.本文利用脉冲激光沉积方法制备了EuS/Ta异质结并研究了其电磁特性.Ta在3.6 K以下为超导态,EuS在20 K以下为铁磁态.在2 K时,EuS/Ta异质结中可观测蝴蝶型磁滞回线,证明在低磁场下(<±0.18 T)异质结中EuS铁磁态和Ta超导态共存.磁输运测试表明,通过施加外磁场可以有效调节EuS的交换场,随着交换场的增大,同时也加强了界面处的交换作用,从而抑制Ta的超导态,实现了Ta在超导态和正常态之间的转变,在EuS/Ta异质结中观测到了高达144000%的磁电阻.本文制备的EuS/Ta异质结具有极大磁电阻效应,在自旋电子学器件中有潜在的应用前景.     

辐射复合截面的扭曲波计算

采用非相对论量子计算方法计算了电子与高电荷态离子碰撞的辐射复合截面, 其中连续态电子的波函数是由扭曲波近似方法(DWBE)获得. 计算了低能电子与中、低核电荷的高电荷态离子(类H和类Li)的辐射复合截面, 并与采用有效电荷的库仑波计算结果进行了比较, 说明了扭曲波计算的重要性.     

Fe/Al2O3/Fe隧道结特性分析

用离子束溅射方法制备磁性隧道结(MTJ). 研究MTJ样品的隧道结磁电阻(TMR)效应.用X射线 光电子能谱分析了MTJ的软、硬磁层和非磁层及其界面的化学组成与微结构.研究了MTJ的微 结构对氧化铝势垒高度与有效宽度和TMR效应的影响. 关键词： 磁性隧道结 X射线光电子能谱 隧道结磁电阻     

大功率GaAs光导开关寿命实验研究

设计并制作了18 mm间隙的半绝缘GaAs光导开关,开关芯片采用标准半导体工艺制作并封装在陶瓷基板上。测试了光导开关在20 kV工作电压、1 kA工作电流时,不同重复频率工作的开关寿命,结果表明,开关寿命4 000次~7 000次,且随着重复频率的提高,开关寿命有所降低。初步分析了导致开关失效的原因为热损伤,包括局部发热导致的连接损伤和材料损伤以及整体发热导致的暗态电阻下降。     

低能氩等离子处理介电层表面对ZnO薄膜晶体管性能影响（英文）北大核心CSCD

采用磁控溅射室温工艺制备了底栅结构ZnO基薄膜晶体管,其介电层表面采用低能Ar等离子体处理.结构表明等离子处理提高了器件的电学性能,即阈值下降36%,亚阈值斜率减少47%,开启电压从-15V变为0V.但是,经等离子处理的器件表现出源漏电流-栅电压(IDS-VG)迟滞和阈值频移现象.这说明Ar等离子体处理在提高器件电学性能的同时会降低器件的电学稳定性.我们基于溅射损伤和态密度变化规律解释了这种矛盾,并认为介电表面的离子损伤对ZnO基薄膜晶体管的电学稳定性会产生重要影响.     

低能氩等离子处理介电层表面对ZnO薄膜晶体管性能影响（英文）

采用磁控溅射室温工艺制备了底栅结构ZnO基薄膜晶体管,其介电层表面采用低能Ar等离子体处理.结构表明等离子处理提高了器件的电学性能,即阈值下降36%,亚阈值斜率减少47%,开启电压从-15V变为0V.但是,经等离子处理的器件表现出源漏电流-栅电压(IDS-VG)迟滞和阈值频移现象.这说明Ar等离子体处理在提高器件电学性能的同时会降低器件的电学稳定性.我们基于溅射损伤和态密度变化规律解释了这种矛盾,并认为介电表面的离子损伤对ZnO基薄膜晶体管的电学稳定性会产生重要影响.     

VB金属离子掺杂TiO_2金红石构型与稳定性的分子动力学研究(英文)

在300 K与101 325 Pa下,用分子动力学模拟研究0.5mol%,2.1mol%,3.8mol%VB金属离子(V5+,b5+,Ta5+)TiO2金红石构型与总能量.当掺杂离子浓度为2.1mol%时,离子位置具有较小的平方位移,及较明显的原子晶面与高的稳定性,金红石构型保持较好的完整性.其中,与V5+,Ta5+掺杂离子相比,Nh5+离子由于具有Ti5+相匹配的价态与离子半径,能够较好地溶入到TiO6八面体中.     

第一性原理计算M2C（M=V、Nb、Ta）的结构性质、弹性性质和热力学性质

通过密度泛函理论的第一原理计算,研究了 M2C (M=V, Nb, Ta)(空间群:pbcn, No: 60)在高压下的电子结构、弹性和热力学性质。该理论是建立在平面波的基础上,该平面波将在 CASTEP 代码中实现。首先,本文计算的晶格常数与已有的实验结果和理论数据吻合较好。其次,计算了过渡金属碳化物的分波态态密度和总态密度,结果表明这三种过渡金属碳化物均为金属,金属丰度由高到低的顺序为:V2C > Nb2C > Ta2C。第三,研究了高压下的弹性常数 C ij 、集料弹性模量(B、G、E)和泊松比。其中计算得到 Ta2C 的体积模量最高(257 GPa)。根据弹性稳定性判据,预测这三种化合物在 100 GPa 以内均具有力学稳定性。计算的 B/G 比值表明,这三种化合物在 100 GPa 范围内具有延展性。最后,利用准谐德拜模型研究了这三种化合物的热力学性质     

类铜铅离子自电离速率与双电子俘获强度的扭曲波计算

 采用相对论扭曲波近似方法对高离化态类铜铅离子的自电离速率系数与双电子俘获强度进行了计算。计算结果表明:旁观电子的变化使自电离速率系数呈现出规律性变化。同时用该方计算了Ta元素类铜离子、Au元素类镍离子的自电离速率系数，将结果同相关文献的相对论多参数势方法、准相对论性Hartree-Fock-Relativistic方法以及自旋-轨道劈裂跃迁组模型方法所得结果进行了比较，符合得很好。     

Magnetic tunnel junction thermocouple for thermoelectric power harvesting

The thermoelectric power generated in magnetic tunnel junctions (MTJs) is determined as a function of the tunnel barrier thickness for a matched electric circuit. This study suggests that lower resistance area product and higher tunnel magnetoresistance will maximize the thermoelectric power output of the MTJ structures. Further, the thermoelectric behavior of a series of two MTJs, a MTJ thermocouple, is investigated as a function of its magnetic configurations. In an alternating magnetic configurations the thermovoltages cancel each other, while the magnetic contribution remains. A large array of MTJ thermocouples could amplify the magnetic thermovoltage signal significantly.     

Stability of magnetic tunnel junctions

When magnetic tunnel junctions (MTJ) are built into a memory device they will be arranged in a matrix; therefore some of the not addressed elements will be exposed to a significant field during the switching of one element. It has to be avoided that the state of not selected MTJ is changed during this process. Here we present data on the stability of MTJ against small fields which occur during a writing process.     

Crystallization characteristics of a middle CoFeB layer in a double MgO barrier magnetic tunnel junction

The crystallization characteristics of a middle CoFeB free layer in a magnetic tunnel junction (MTJ) with double MgO barriers were investigated by tunneling magnetoresistance (TMR) measurements of patterned cells across an 8-inch wafer. The MTJ structure was designed to have two CoFeB free layers and one bottom pinned layer, separated by MgO tunnel barriers. The observed resistance showed three types of TMR curves depending on the crystallization of the middle CoFeB layer. From the analysis of TMR curves, coherent crystallization of the middle CoFeB layer with the top and bottom MgO barriers was found to occur non-uniformly: About 80% of the MTJ cells in the wafer exhibited coherent crystallization of the middle CoFeB layers with the bottom MgO tunnel barrier, while others had coherent crystallization with the top MgO tunnel barrier or both barriers. This non-uniform crystallization of the middle CoFeB layer in a double MTJ was also clearly observed in tunneling electron microscopy images. Thus, control of the crystallization of the middle CoFeB layer is important for optimizing the MTJ with double MgO barriers, and especially for the fabrication of double barrier MTJ on a large area substrate.     

Cro2 particles for DDC tape

CrO₂ particles provide a good basis for proper pulse recording due to their high output at short wavelengths, combined with low noise print-through. Furthermore, one outstanding features is their low electrical resistance. It avoids statics charging of the magnetic coating and thus eliminates both dropouts caused by the attraction of loose particles and tape edge damage cause by a rough tape pack due to tape static.     

Barrier height and negative tunnel magnetoresistance

Based on the free-electron approximation method proposed by Slonczewski, we substitute the finite magnetic zone by a semi-infinite magnet. On this basis, the relationship between the tunnel magnetoresistance (TMR) and the barrier height of magnetic tunnel junction (MTJ) is studied. We find the TMR at small bias is always positive for various barrier heights when the MTJ has a symmetric configuration and the negative TMR can be observed when MTJ is with lower barrier height in the asymmetric condition.     

Nonlinear spin current and magnetoresistance of molecular tunnel junctions

We report on a theoretical study of spin-polarized quantum transport through a Ni-bezenedithiol(BDT)-Ni molecular magnetic tunnel junction (MTJ). Our study is based on carrying out density functional theory within the Keldysh nonequilibrium Green's function formalism, so that microscopic details of the molecular MTJ are taken into account from first principles. A magnetoresistance ratio of approximately 27% is found for the Ni-BDT-Ni MTJ which declines toward zero as bias voltage is increased. The spin currents are nonlinear functions of bias voltage, even changing sign at certain voltages due to specific features of the coupling between molecular states and magnetic leads.     

Electrical conductance properties for magnetic tunnel junctions with MgO barriers

We measured inelastic electron tunneling (IET) spectra and conductance for MgO tunneling magnetoresistance (TMR) films to obtain information on the ferromagnetic/barrier layer interface. The IET spectra showed the difference between amorphous and crystalline structures in the barrier. In the magnetic tunnel junction (MTJ) with a crystalline barrier the IET spectra indicated an Mg-O phonon peak at a low bias voltage by measurement with a parallel magnetization configuration. On the other hand, no peak was observed in the MTJ with an amorphous barrier.     

Influence of thickness and band structure of insulating barriers on resistance and tunneling magnetoresistance properties of magnetic tunnel junctions with Al-oxide and Ti-alloyed Al-oxide barriers

We investigated the influence of insulating barrier thickness and the Ti composition dependence of the band structure of Al-oxide on the resistance and tunneling magnetoresistance (TMR) behavior of the magnetic tunnel junction (MTJ). Low resistance × area (RA) value (1.1 MΩ μm²) was achieved by decreasing the Al-oxide thickness down to 1.0 nm. However, this led to the partial oxidation of the bottom ferromagnetic (FM) electrode of the junction and non-continuous thin barriers by the occurrence of pinholes, with low TMR ratio of 8.3%. For an alternative for low RA value, we developed a new Ti-alloyed Al-oxide (TiAlO_x) that had lower band gap than Al-oxide as an insulating barrier of MTJ. As the Ti concentration increased up to 5.33 at.% Ti in Al, the RA value of the MTJs was reduced from 9.5 to 0.69 MΩ μm², owing to the band-gap reduction of TiAlO_x caused by the formation of extra bands, mainly composed of Ti-3d orbitals, within the band gap. It was analyzed that TiAlO_x has localized d states in the band gap below the conduction band. In addition, the TMR ratio increased with the Ti concentration and reached a maximum of 49% at 5.33 at.% Ti owing to the microstructural evolution of Ti–Al alloy film in the pre-oxidation state.     

Oxygen-induced symmetrization and structural coherency in Fe/MgO/Fe(001) magnetic tunnel junctions

We present x-ray diffraction experiments and multiple-scattering calculations on the structure and transport properties of a Fe/MgO/Fe(001) magnetic tunnel junction (MTJ). Coherent growth of the top Fe electrode on the MgO spacer is observed only for Fe deposition in ambient oxygen atmosphere leading to a coherent and symmetric MTJ structure characterized by FeO layers at both interfaces. This goes in parallel with calculations indicating large positive tunnel magnetoresistance (TMR) values in such symmetric junctions. The results have important implications for achieving giant TMR values.     

Two waveguide layers in lithium niobate crystal formed by swift heavy Kr ion irradiation

We report the formation of two waveguide layers in a lithium niobate crystal by irradiation with swift heavy Kr ions with high(Ge V) energies and ultralow fluences. The micro-Raman spectra are measured at different depths in the irradiated layer and show that the high electronic energy loss can cause lattice damage along the ion trajectory, while the nuclear energy loss causes damage at the end of the ion track. Two waveguide layers are formed by confinement with two barriers associated with decreases in the refractive index that are caused by electronic and nuclear energy losses, respectively.