Genomic design of strong direct-gap optical transition in Si/Ge core/multishell nanowires.
Genomic design of strong direct-gap optical transition in Si/Ge core/multishell nanowires.
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DOI：
10.1021/nl2040892
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发表时间：
2012-02
期刊：
影响因子：
10.8
通讯作者：
Lijun Zhang;Mayeul d'Avezac;Jun-Wei Luo;A. Zunger
中科院分区：
文献类型：
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作者：
Lijun Zhang;Mayeul d'Avezac;Jun-Wei Luo;A. Zunger
尽管经过几十年的研究，找到一种在带边缘具有强光学活性的硅基材料仍然是一个挑战。人们的兴趣在于将光学和电子功能结合在同一片晶圆上，同时保留为硅开发的非凡技术。然而，硅是一种间接间隙材料。晶体动量守恒规定，在带边缘的光学活动包括一个声子，在电子-空穴对的顶部，因此光子的吸收和发射仍然是相当不可能发生的事件，需要光学相当厚的样品。将纳米结构和合金化对硅基材料能带结构的影响结合起来，是将硅基材料转变为强光吸收/发射材料的一条有前途的途径。然而，可能配置的数量显示出组合爆炸。此外，虽然可以很容易地识别在动量空间中形式上直接的构型（由于能带折叠），但在阈值处没有偶极子允许的跃迁，但问题不仅仅是能带结构的计算，还包括吸收强度的计算。使用遗传算法和半经验伪势哈密顿量来描述电子结构，我们已经探索了数十万种可能的同轴核心/多壳Si/Ge纳米线，其方向为[001]，[110]和[111]，发现了一些“神奇序列”的核心，然后是特定的Si/Ge多壳，可以提供直接带隙和强振荡器强度。研究揭示了几个简单的设计原则：(1)Ge芯在产生强带隙跃迁方面优于Si芯；（ii）[001]和[110]取向具有直接带隙，而[111]取向不具有直接带隙；（iii）多壳纳米线的光学活性比普通纳米线高一个数量级；（iv）获胜构型的主要主题是直接允许跃迁，涉及相当薄的硅壳嵌入在宽的锗壳中。我们讨论了增强的光学活性的物理来源，以及可能的实验结构缺陷对我们的候选核/多壳纳米线的光学活性的影响。
Finding a Si-based material with strong optical activity at the band-edge remains a challenge despite decades of research. The interest lies in combining optical and electronic functions on the same wafer, while retaining the extraordinary know-how developed for Si. However, Si is an indirect-gap material. The conservation of crystal momentum mandates that optical activity at the band-edge includes a phonon, on top of an electron-hole pair, and hence photon absorption and emission remain fairly unlikely events requiring optically rather thick samples. A promising avenue to convert Si-based materials to a strong light-absorber/emitter is to combine the effects on the band-structure of both nanostructuring and alloying. The number of possible configurations, however, shows a combinatorial explosion. Furthermore, whereas it is possible to readily identify the configurations that are formally direct in the momentum space (due to band-folding) yet do not have a dipole-allowed transition at threshold, the problem becomes not just calculation of band structure but also calculation of absorption strength. Using a combination of a genetic algorithm and a semiempirical pseudopotential Hamiltonian for describing the electronic structures, we have explored hundreds of thousands of possible coaxial core/multishell Si/Ge nanowires with the orientation of [001], [110], and [111], discovering some "magic sequences" of core followed by specific Si/Ge multishells, which can offer both a direct bandgap and a strong oscillator strength. The search has revealed a few simple design principles: (i) the Ge core is superior to the Si core in producing strong bandgap transition; (ii) [001] and [110] orientations have direct bandgap, whereas the [111] orientation does not; (iii) multishell nanowires can allow for greater optical activity by as much as an order of magnitude over plain nanowires; (iv) the main motif of the winning configurations giving direct allowed transitions involves rather thin Si shell embedded within wide Ge shells. We discuss the physical origin of the enhanced optical activity, as well as the effect of possible experimental structural imperfections on optical activity in our candidate core/multishell nanowires.