电-声子散射对石墨烯场效应管暂态输运性质的影响

The operational speed is one of the most important indicators for evaluating the merits of emerging nanoscale electronic devices. The most direct understanding of the operational speed comes from analyzing the transient dynamic response of the device, i.e. the time-dependent transient current which is induced by the sudden variation of the external bias voltage. The intrinsic time scale of the transient current in a nanoscale system is largely determined by such factors as the dwell time - which measures how long the electron spends in the system; the first passage time - which measures the time needed for direct transmission, etc.. These scales are the simplest to theoretically analyze. However, in any molecular devices under practical operational conditions, the transient current is not only dependent on the intrinsic time scales, but also critically limited by interactions: the most important one is the electron-phonon (e-p) interaction that provides the e-p inelastic relaxation time scale. On the other hand, for the emerging nanoscale devices where the atomic details of the nanostructures play a dominant role for quantum transport, the dynamics of transient current under e-p scattering has never been investigated from atomistic first principles. This unfortunate state of affair must be resolved before quantitative predictions can be made for transient speed of the multitudes of nanoscale electronic devices. In this project, we want to develop such a theory to describe the phonon-assisted transient transport properties in nanoscale electronic devices using the non-equilibrium Green's function (NEGF) method. In this theory, we consider not only the intrinsic time scales, but also the e-p inelastic relaxation time scale. When combining it with the density functional theory (DFT), we want to investigate the transient transport properties of various graphene field-effect transistors (graphene-FET) to understand the critical parameters that affect the operational speed, including the phonon-branches, the temperature, the width of the graphene ribbon, the length of the FET channel, the effects of N and B doping, the effect of a substrate (i.e. SiC), the strength of the driving pulse, etc.

响应速度是描述纳米电子器件运算性能的一个重要指标。通过测量暂态过程中电流随时间的演化，人们可以获得纳米电子器件的响应速度。理论上，电子器件的响应速度不仅依赖于电子的固有时间尺度，也跟器件中的相互作用密切相关，其中最重要的是电-声子相互作用。在本项目中，我们拟采用非平衡格林函数方法，发展描述纳米体系暂态输运性质的理论。在该理论中，我们既要考虑电子的固有时间尺度，也要考虑由电-声子非弹性散射引起的弛豫时间尺度。另一方面，考虑到电子器件本身的结构特征在输运过程中扮演着非常重要的角色，我们拟将该描述电子器件暂态输运性质的理论和密度泛函理论相结合，从第一性原理计算出发研究不同石墨烯场效应管的暂态输运性质，进而分析各种临界参数，包括声子、偏压、温度、石墨烯条带的宽度、场效应管的沟道长度、杂质（比如N/B掺杂）、衬底（比如SiC）等对器件响应速度的影响。

电流对偏压的响应速度是描述纳米尺寸电子器件性能的一个重要指标。作为最重要的新材料之一，石墨烯被广泛认为是制备新一代纳米器件的主要材料之一。因此，研究基于石墨烯的纳米器件的暂态输运性质具有非常重要的科研价值和现实意义。. 本项目从理论上研究了基于石墨烯的纳米器件的暂态输运性质。首先，我们发展了一套非平衡格林函数理论来描述纳米体系暂态输运性质，并基于该理论开发了一套计算程序。紧接着，我们利用该程序，从第一性原理计算出发研究不同石墨烯场效应管的暂态输运性质。. 以磁性钴-石墨烯纳米器件为例，我们研究了在器件两端施加突然打开的台阶状电压脉冲后电流对电压的响应过程。我们针对了以下几个问题进行了探讨：（1）自旋极化的电流需要多长的演化时间才能够达到稳定状态？（2）响应时间和电压脉冲信号的强度有什么关系？（3）石墨烯的手性对器件响应时间有无影响？（4）由电声子散射对响应时间有无影响？. 计算表明，该磁性纳米器件的接通时间大约为5-20飞秒，但是电流的弛豫时间却长达几个皮秒。对自旋向上和向下的电子皆是如此。响应时间决定于费米能级以下的共振态，而与石墨烯的手性和脉冲偏压的大小无关。电流衰减振荡的频率等于脉冲偏压大小的二分之一，衰减率等于对应共振态的寿命。当考虑了体系的非弹性相位弛豫散射之后，瞬态电流原来较持久的振荡行为得到抑制，弛豫时间减少至几百飞秒。. 该研究结论表明：（1）在以石墨烯作为导电材料制备纳米器件时，石墨烯的手性选择并不重要。换句话说，石墨烯的手性对器件中电流的流动影响不大，这样在石墨烯器件制备中有了更大的选择余地。（2）自旋向上的电子和自旋向下的电子具有相近的响应速度，自旋极化程度对此没有影响。（3）适当的温度和杂质散射可以抑制电子的长时间振荡，有助于提高器件的响应速度。该项目的顺利完成对制备新一代高速度、低功耗的石墨烯电子器件提供了理论指引。