InAs nanowires grown on silicon substrates have attracted significant research in the recent years owing to their potential application in high-performance silicon-based infrared photodetectors, solar cells and transistors in which band-edge structure as well as carrier recombination play important roles in optoelectronics. InAs nanowires exhibit unique attributes in the axial alternation of the wurtzite and zinc-blend phases that is initiated by the competition of the surface free energy. However, this is different for the InAs bulk.
Theoretical studies indicate that the conduction and valence band edges of the zinc-blend InAs are both lower than those of the wurtzite InAs. Therefore, the zinc blend and wurtzite InAs constitute a type-II interfacial structure hence modulating the band structure along the axial direction of the nanowires. Electrons distribute in the zinc-blend and holes distribute in the wurtzite InAs.
For optoelectronics application, p-n heterojunction is crucial. The InAs nanowires are of the n-type and therefore a potential configuration is to obtain aligned InAs nanowire assemblages on a p-type silicon substrate for the nanowire heterojunction. At this point, the built-in electric field is along the axial direction. However, the questions remain whether the field would be affected by certain zinc-blend/wurtzite alteration and how the difference would be for the photo-induced carrier recombination in the nanowires grown on p- and n-type substrates.
Researchers led by Professor Jun Shao from Shanghai Institute of Technical Physics, China, acquired mid-infrared photoluminescence spectra from InAs nanowires with improved signal to noise ratio in a 8-290K range of temperature by implementing optimized step-scan photoluminescence. This was contrary to the typical photoluminescence measurements, which have been conducted in temperature ranges below room temperature and with significantly poor signal-to-noise ratio. This has therefore clarified the doubts of the identification of photoluminescence mechanisms. Their work is published in peer-reviewed journal, Nano Letters.
The authors prepared two InAs nanowire samples directly on n- and p- type bare silicon substrates by droplet assisted and molecular-beam epitaxy methods respectively. In droplets acting as nucleation sites, initiated the nanowire growth. This was then followed by the simultaneous opening of the As and In shutters for the InAs nanowires growth.
Scanning electron microscopy images indicated that the N- and P- samples had an average length of 1µm and a diameter of 50-100nm. This diameter implied that the radial quantum effect was negligible and the attributes of the nanowires were bulk-like rather than quantum-like.
Three photoluminescence features were resolved with dominant feature, low-energy feature, and high-energy feature. The origins of the three features were clarified by features’ integral intensity, their energy evolutions, and width with temperature that the dominant feature was owing to the type II radiative recombination near the zinc-blend/wurtzite InAs interfaces. The authors noted that the low-energy feature corresponded to impurity-related optical transition. The high-energy feature on the other side corresponded to the wurtzite-InAs inter-band-gap transition.
The researchers also observed that InAs nanowires on the p-type silicon substrate exhibited different nonradiative recombination rate and carrier-phonon interaction as compared to those on the n-type substrate. This was because the built-in electric field formed in sample-P owing to doping type of the substrate and led to carriers’ assembly at the wurtzite-on-zinc-blend interfaces. This interface was of low optical quality.
The outcomes of the study indicate that the temperature-dependent infrared photoluminescence analysis can be a good tool for determining the interfacial attributes and mechanism behind in the vertical alignment of the InAs nanowires.
Xiren Chen, Qiandong Zhuang, H. Alradhi, Zh. M. Jin, Liangqing Zhu, Xin Chen, and Jun Shao. Midinfrared Photoluminescence up to 290 K Reveals Radiative Mechanisms and Substrate Doping-Type Effects of InAs Nanowires. Nano Lett. 2017, 17, 1545−1551.Go To Nano Letters