磁势垒构中隧穿现象的研究

本用转移矩阵方法研究了具有纳米尺度的非对称磁势垒结构中电子隧穿效应。结果表明和电子穿越对称双磁势垒结构相比，电子隧穿非双磁势垒结构和传输几率和电导都强烈减小，并且非对称磁势垒结构具有重强的波矢过滤特性。     

磁势垒结构中隧穿现象的研究

本文用转移矩阵方法研究了具有纳米尺度的非对称磁势垒结构中电子的隧穿效应．结果表明和电子穿越对称双磁势垒结构相比，电子隧穿非对称双磁势垒结构的传输几率和电导都强烈减小，并且非对称磁势垒结构具有更强的波矢过滤特性．     

量子点接触中的自旋过滤特性

利用可调节自旋过滤器模型,首次计算并讨论了磁场和电子跃迁能量间隔变化对量子点接触结构中自旋电子过滤特性的影响。研究发现,磁场和电子跃迁能量间隔的变化引起了自旋电子隧穿概率和隧穿电导都呈现出量子台阶效应,磁场的增加使电子的回旋频率和电子的Zeeman能级分裂同时加强,从而导致量子点接触结构中的横向约束加强,而自旋过滤效应明显减弱;当磁场一定时,电子跃迁能量间隔越小,电子的自旋过滤效应越明显。电子跃迁能量间隔改变的同时,也改变了鞍形势的势垒形状和自旋过滤的灵敏度。对于不同的材料,同时考虑磁场和电子跃迁能量间隔的作用可以找到自旋过滤器的最佳过滤效果。尤为重要的是过滤器的结构可以用标准的电子束技术很容易得到,所以研究结论为设计新型自旋过滤器提供了理论依据,具有广阔的应用前景和潜在的商业价值。此外,使用朗道因子值较高的材料作自旋过滤器的衬底,可以进一步提高过滤器的性能。     

双量子环的隧穿电导和隧穿磁电阻

研究了双量子环的隧穿电导和隧穿磁电阻,研究结果表明:隧穿电导和隧穿磁电阻都随半导体环增大做周期性等幅振荡。δ势垒、Rashba自旋轨道耦合、AB磁通对隧穿电导或隧穿磁电阻的影响各不相同。隧穿磁电阻随AB磁通增强发生周期性等幅振荡,并随φ0角的减小而增大。两电极磁矩方向反平行时,隧穿磁电阻恒为零。     

稀磁半导体异质结构的自旋极化输运性质

采用格林函数方法和群速近似研究在电场、磁场作用下电子渡越稀磁半导体异质结构的自旋过滤及自旋分离的特征。研究表明具有不同自旋指向的极化电子渡越同一稀磁半导体异质结构 ,不仅隧穿几率存在着显著的差异 ,而且渡越时间的差异可达几个数量级。这种差异随着外磁场的增强而加大 ,而随外电场的增强而减小。结果意味着稀磁半导体异质结构具有很好的自旋过滤效果 ,且极化电子渡越此类结构在时间上是分离的     

CoFe/Cu/MgAl_2O_4(001)/CoFe磁性隧道结的隧穿磁电阻效应

通过基于密度泛函理论和非平衡态格林函数的第一性原理计算,研究了CoFe/Cu/MgAl2O4(001)/CoFe磁性隧道结(MTJ)的隧穿磁电阻(TMR)效应。结果显示,在一个完整的MgAl2O4(001)晶胞上再添加三层绝缘势垒层不仅会得到更大的隧穿磁电阻,且可以得到一个相对较大的半峰值偏压Vhalf。通过在势垒层和电极之间插入非磁性材料Cu,引起隧穿磁电阻发生反转和振荡现象,分别用左端CoFe-Cu电极的态密度(DOS)和电子的自旋传输对这两种现象进行了讨论。前者主要是由于CoFe-Cu电极的态密度随Cu层的增加发生了变化,后者主要是因为隧穿电子在Cu层中形成量子阱态,发生多重散射引起的。     

外加磁场对含双δ势垒的铁磁/半导体/铁磁异质结中自旋输运和散粒噪声的影响

研究了外磁场存在时,含双δ势垒的铁磁/半导体/铁磁异质结中自旋相关的透射概率和散粒噪声,讨论了量子尺寸效应和Rashba自旋轨道耦合效应.研究结果表明:不同自旋取向的电子隧穿异质结时,透射概率和散粒噪声随半导体长度的变化特性是作等幅振荡;外加磁场和Rashba自旋轨道耦合强度的增强都会加大透射概率和散粒噪声的振荡频率;外加磁场角度的改变会改变散粒噪声的振荡频率;双δ势垒的存在增大了自旋电子透射概率的振幅.     

外加磁场对含双δ势垒的铁磁/半导体/铁磁异质结中自旋输运和散粒噪声的影响

研究了外磁场存在时,含双δ势垒的铁磁/半导体/铁磁异质结中自旋相关的透射概率和散粒噪声,讨论了量子尺寸效应和Rashba自旋轨道耦合效应.研究结果表明:不同自旋取向的电子隧穿异质结时,透射概率和散粒噪声随半导体长度的变化特性是作等幅振荡;外加磁场和Rashba自旋轨道耦合强度的增强都会加大透射概率和散粒噪声的振荡频率;外加磁场角度的改变会改变散粒噪声的振荡频率;双δ势垒的存在增大了自旋电子透射概率的振幅.     

ZnCdSe量子阱/CdSe量子点耦合结构中的激子复合

采用MOCVD方法制备了ZnCdSe量子阱/CdSe量子点耦合结构,利用低温(5K)光致发光光谱和变密度发光光谱研究了该结构中的激子隧穿和复合. 观察到在该结构中存在由量子阱到量子点的激子隧穿现象. 改变垒层厚度会对量子阱和量子点的发光产生显著影响. 在垒层较薄的阱/点耦合结构中,隧穿效应可以有效地抑制量子阱中的带填充和饱和效应.     

ZnCdSe量子阱/CdSe量子点耦合结构中的激子复合

采用MOCVD方法制备了ZnCdSe量子阱/CdSe量子点耦合结构,利用低温(5K)光致发光光谱和变密度发光光谱研究了该结构中的激子隧穿和复合.观察到在该结构中存在由量子阱到量子点的激子隧穿现象.改变垒层厚度会对量子阱和量子点的发光产生显著影响.在垒层较薄的阱/点耦合结构中,隧穿效应可以有效地抑制量子阱中的带填充和饱和效应.     

Giant magnetoresistance in a two-dimensional electron gas modulated by ferromagnetic and Schottky metal stripes

In this paper, by using the transfer matrix method, we theoretically investigate the magnetoresistance (MR) effect in a magnetic-electric barrier nanostructure. The detailed numerical results show that a considerable MR effect can be achieved in such a device and the MR ratio strongly depends on the magnetic-electric barrier height and the distance between the ferromagnetic and Schottky metal stripes.     

AlGaN/GaN二维电子气的特性研究

应用 Al Ga N/Ga N异质结中的压电极化和自发极化边界条件自洽求解了薛定谔方程和泊松方程 ,求出了异质结能带和二维电子气分布。研究了势垒层组分比、势垒层宽度、沟道层掺杂和栅电压变化对二维电子气特性的影响。着重研究了栅电压对二维电子气特性的控制作用 ,提出了使用薄势垒和重掺杂沟道的新 HFET结构     

AlGaN/GaN HFET的优化设计

从自洽求解薛定谔方程和泊松方程出发研究了不同掺杂方式下异质结能带和二维电子气的行为。发现掺杂能剪裁异质结能带的弯曲度、控制电子气的二维特性和浓度。在此基础上研究了不同掺杂方式的掺杂效率。通过掺杂和势垒结构的优化设计,得出了用δ掺杂加薄AlN隔离层的结构,既提高了电子气浓度,又保持电子气的强二维特性。从电子气浓度和栅对电子气的控制力度出发,提出了HFET势垒优化设计中的电子气浓度与势垒层厚度乘积规则。依据二维表面态理论,研究了表面态随帽层掺杂结构的变化。从前述乘积规则和表面态变化出发进行了内、外沟道异质结构的优化设计。优化结构既提高了电子气浓度和跨导,降低了欧姆接触电阻,又抑制了电流崩塌。     

Progress toward electrical injection of spin-polarized electrons into semiconductors

The use of carrier spin as a new degree-of-freedom in semiconductor devices offers new functionality and performance. However, efforts to implement semiconductor spintronics have been crippled by the lack of an efficient and practical means to electrically inject spin polarized carriers into a semiconductor device heterostructure. This paper summarizes progress toward that end using magnetic semiconductors and ferromagnetic metals as spin injecting contacts. We describe a very successful approach which employs a ferromagnetic metal/tunnel barrier contact, where the tunnel barrier is simply a tailored Schottky barrier which forms naturally between the ferromagnetic metal and the semiconductor itself. Initial efforts have demonstrated electron spin polarizations of at least 32% in GaAs quantum-well LED heterostructures. Significantly higher spin injection efficiencies are anticipated in optimized structures. These results demonstrate that spin injecting contacts can be formed using a very familiar and widely employed contact methodology, providing a ready pathway for the integration of spin transport into semiconductor processing.     

Hot Electrons and Hot Spins at Metal–Organic Interfaces

A model is developed to describe the electron transport properties of hot electron devices based on organic semiconductors. For the first time, the simulations cover all the different processes the carriers experience in the device, which allows disentangling various effects on the transport characteristics. The model is compared to experimental measurements and excellent agreement is found. In addition, the model includes the electron spin and is thus able to describe a hot spin transistor. In this device, a spatial variation of the spin diffusion length is found, which scales inversely proportional to the variation of the electron density. The spin current can be increased by increasing the hot electron energy and by decreasing the image charge barrier without changing the spin diffusion length. Unprecedented insight into the effect of interfacial disorder at the metal–organic interface on charge and spin transport is provided. Finally, conditions are established, where majority and minority spin carriers propagate in opposite directions, increasing the spin current relative to the charge current and the occurrence of pure spin currents is analyzed.     

Electrical Spin Injection into the 2D Electron Gas in AlN/GaN Heterostructures with Ultrathin AlN Tunnel Barrier

The spin injection into 2D electron gas (2DEG) in AlN/GaN heterostructures is studied by magneto-transport measurements. An ultrathin AlN layer at the hetero-interface acts as a barrier to form high-quality 2DEG in the triangular quantum well and a tunneling barrier for the spin injection to overcome the conductance mismatch issue. In this study, Hanle signals and inversed Hanle signals are observed, proving that the spin injection is achieved in the 2DEG in the AlN/GaN heterostructure rather than in the interfacial states. The spin-relaxation time in 2DEG at 8 K is found to be as long as 860 ps, which almost keeps constant with bias and decreases with increasing temperature. The spin-relaxation process is illustrated as Rashba spin-orbit coupling dominated D'yakonov Perel’ mechanisms above 8 K. These results show the promising potential of 2DEG in AlN/GaN heterostructures for spin field-effect transistor applications.     

Analysis of PTCDA/ITO Surface and Interface Using X-ray Photoelectron Spectroscopy and Atomic Force Microscopy

在低温和强磁场下,通过磁输运测量研究了不同Al组分调制掺杂AlxGa1-xN/GaN异质结二维电子气(2DEG)的磁电阻振荡现象.观察到低Al组分异质结中的2DEG有较低的浓度和较高的迁移率.     

Al组分对AlxGa1-xN/GaN异质结构中二维电子气输运性质的影响

在低温和强磁场下,通过磁输运测量研究了不同Al组分调制掺杂AlxGa1-xN/GaN异质结二维电子气(2DEG)的磁电阻振荡现象.观察到低Al组分异质结中的2DEG有较低的浓度和较高的迁移率.     

Fe/GaAs中自旋注入效率的研究

自旋电子学是近年来发展迅速的一个研究领域,利用了传导电子自旋这一自由度的自旋电子器件以其提高数据处理速度、降低能量消耗、容易增加集成密度等优点正引起人们的空前关注.文中阐述了自旋的漂移-扩散方程,并对以Fe/GaAs为代表的铁磁性金属/半导体结构(FM/SC)进行了简单分析.如果选取参数适当,可以在Fe/GaAs结中获得较大的自旋注入效率.