[论文解读] Gunn Effect in Silicon Nanowires: Charge Transport under High Electric Field

Daryoush Shiri, Amit Verma+4 more|arXiv (Cornell University)|Sep 6, 2017

Semiconductor materials and interfaces参考文献 60被引用 4

一句话总结

本研究首次证明，在3.1 nm直径的硅纳米线（SiNWs）中，当施加3%的拉伸应变和5000 V/cm的电场时，可诱导出现吉尔效应——其特征为负微分电阻（NDR）。通过密度泛函理论（DFT）、紧束缚模型和系综蒙特卡洛模拟，作者表明应变可促使NDR起始，并实现电阻率2.3倍的可逆调控，从而在可调谐微波振荡器和电磁机械传感器中具有应用潜力。

ABSTRACT

Gunn (or Gunn-Hilsum) Effect and its associated negative differential resistivity (NDR) emanates from transfer of electrons between two different energy bands in a semiconductor. If applying a voltage (electric field) transfers electrons from an energy sub band of a low effective mass to a second one with higher effective mass, then the current drops. This manifests itself as a negative slope or NDR in the I-V characteristics of the device which is in essence due to the reduction of electron mobility. Recalling that mobility is inversely proportional to electron effective mass or curvature of the energy sub band. This effect was observed in semiconductors like GaAs which has direct bandgap of very low effective mass and its second indirect sub band is about 300 meV above the former. More importantly a self-repeating oscillation of spatially accumulated charge carriers along the transport direction occurs which is the artifact of NDR, a process which is called Gunn oscillation and was observed by J. B. Gunn. In sharp contrast to GaAs, bulk silicon has a very high energy spacing (~1 eV) which renders the initiation of transfer-induced NDR unobservable. Using Density Functional Theory (DFT), semi-empirical 10 orbital ($sp^{3}d^{5}s^{*}$) Tight Binding (TB) method and Ensemble Monte Carlo (EMC) simulations we show for the first time that (a) Gunn Effect can be induced in narrow silicon nanowires with diameters of 3.1 nm under 3 % tensile strain and an electric field of 5000 V/cm, (b) the onset of NDR in I-V characteristics is reversibly adjustable by strain and (c) strain can modulate the value of resistivity by a factor 2.3 for SiNWs of normal I-V characteristics i.e. those without NDR. These observations are promising for applications of SiNWs in electromechanical sensors and adjustable microwave oscillators.

研究动机与目标

• 探究在通常缺乏NDR所需小带隙分裂的硅纳米线中，是否可诱导吉尔效应。
• 探讨应变与电场在硅纳米线中促进电子在能带间转移的作用。
• 确定负微分电阻（NDR）起始点是否可通过应变工程实现可逆调控。
• 评估硅纳米线在可调谐微波振荡器和电磁机械传感器中的应用潜力。

提出的方法

• 采用密度泛函理论（DFT）计算应变硅纳米线中的电子能带结构与有效质量。
• 采用半经验的10轨道 $sp^{3}d^{5}s^{*}$ 紧束缚模型，以原子尺度精度模拟电子性质。
• 利用系综蒙特卡洛（EMC）模拟在高电场下电荷输运行为。
• 模拟中纳入与应变相关的能带结构变化及电子散射机制，以预测I-V特性。
• 系统性地改变电场，观察I-V曲线中NDR的起始点。
• 以3%为增量施加应变，评估其对电阻率与NDR起始点的可逆调控效应。

实验结果

研究问题

• RQ1 尽管硅纳米线的带隙分离较大（约1 eV），是否仍可在其中诱导吉尔效应？
• RQ2 拉伸应变如何影响硅纳米线中能带对齐及子带间电子转移？
• RQ3 硅纳米线中负微分电阻（NDR）的起始点是否可通过应变实现可逆调控？
• RQ4 在无NDR情况下，应变对硅纳米线电阻率的调制程度如何？
• RQ5 使用应变硅纳米线作为可调谐微波振荡器或电磁机械传感器是否具有可行性？

主要发现

• 在3.1 nm直径的硅纳米线中，于3%拉伸应变和5000 V/cm电场下成功诱导出吉尔效应。
• I-V特性中负微分电阻（NDR）的起始点可通过外加应变实现可逆调控。
• 应变调制使硅纳米线在正常I-V区域的电阻率实现2.3倍的变化，且无NDR效应。
• NDR效应源于高电场下电子从低有效质量子带向高有效质量子带的转移。
• DFT、紧束缚模型与系综蒙特卡洛模拟的结合证实了该效应在真实纳米线几何结构中的可行性与稳定性。
• 结果表明，应变硅纳米线可作为可调谐微波振荡器与纳米机械传感器中的可调谐有源元件。