Department of Physics & NanoLund, Lund University , P.O. Box 118, 22 100 Lund, Sweden.
Department of Materials Science and Engineering, Stanford University , Stanford, California 94305, United States.
ACS Nano. 2017 Oct 24;11(10):10519-10528. doi: 10.1021/acsnano.7b05873. Epub 2017 Oct 9.
The perfect switching between crystal phases with different electronic structure in III-V nanowires allows for the design of superstructures with quantum wells only a single atomic layer wide. However, it has only been indirectly inferred how the electronic structure will vary down to the smallest possible crystal segments. We use low-temperature scanning tunneling microscopy and spectroscopy to directly probe the electronic structure of Zinc blende (Zb) segments in Wurtzite (Wz) InAs nanowires with atomic-scale precision. We find that the major features in the band structure change abruptly down to a single atomic layer level. Distinct Zb electronic structure signatures are observed on both the conduction and valence band sides for the smallest possible Zb segment: a single InAs bilayer. We find evidence of confined states in the region of both single and double bilayer Zb segments indicative of the formation of crystal segment quantum wells due to the smaller band gap of Zb as compared to Wz. In contrast to the internal electronic structure of the nanowire, surface states located in the band gap were found to be only weakly influenced by the presence of the smallest Zb segments. Our findings directly demonstrate the feasibility of crystal phase switching for the ultimate limit of atomistic band structure engineering of quantum confined structures. Further, it indicates that band gap values obtained for the bulk are reasonable to use even for the smallest crystal segments. However, we also find that the suppression of surface and interface states could be necessary in the use of this effect for engineering of future electronic devices.
在 III-V 纳米线中，完美地切换具有不同电子结构的晶体相，可以设计出只有单个原子层宽的量子阱超结构。然而，直到最小的可能晶体段，电子结构将如何变化，只是间接地推断出来。我们使用低温扫描隧道显微镜和光谱学，以原子级精度直接探测纤锌矿（Zb）段在纤锌矿（Wz）InAs 纳米线中的电子结构。我们发现，能带结构的主要特征在单个原子层的水平上突然发生变化。对于最小的 Zb 段，即单个 InAs 双层，在导带和价带两侧都观察到明显的 Zb 电子结构特征。我们发现，在单双层 Zb 段的区域都存在受限态的证据，这表明由于 Zb 的能带隙小于 Wz，因此形成了晶体段量子阱。与纳米线的内部电子结构相反，位于能带隙中的表面态仅受到最小 Zb 段存在的微弱影响。我们的发现直接证明了对于量子限制结构的原子带结构工程的最终极限，晶体相切换的可行性。此外，这表明即使对于最小的晶体段，也可以合理地使用体相获得的带隙值。然而，我们还发现，在利用这种效应来设计未来电子器件时，可能需要抑制表面和界面态。
