EMFL researchers demonstrate excellent optical properties of semiconductor nanowires

A collaboration of researchers of the LNCMI in Toulouse, the LNCMI Grenoble, the Weizmann Institute in Rehovot and the HFML revealed surprisingly good optical properties of single GaAs nanowires. A problem with the production of these nanowires is that the material can crystallize in different crystal structures, resulting in non-homogeneous structures with poor optical properties. But research in high magnetic fields now showed GaAs nanowires of excellent quality, that even approaches the quality of the best bulk material. This is promising for the application of these nanowires in small electronic devices. The results are published in Nano Letters on 1 May 2013.

Single nanowires have been proposed for several applications in electronics, opto electronics or photo voltaics. In contrast to bulk material, III-V nanowires can crystallize in wurzite, zinc blende or mixed crystal structures. The different crystal structures have different electronic band structures, giving us a tool to manipulate the optical and electronic properties of the nanowires. Therefore, studying the excitonic behavior in GaAs nanowires is an important part of the description and understanding of their electronic properties.

High magnetic fields are a unique and powerful tool to reveal the nature of excitons in single high aspect ratio GaAs/AlAs core/shell nanowires. We show for the first time that emission from a single nanowires exhibits very strong and narrow excitonic lines. This demonstrates the extremely high quality of our nanowires and proves that their quality approaches that of the best bulk material. The application of a magnetic field probes both the orbital and spin degrees of freedom. The magnetic field dependence of the emission allows us to identify unambiguously the emission with the recombination of excitons bound to defect pairs with different separations, previously observed only in very high quality MBE grown GaAs. 

Our nanowires, easily dispersed onto a Si substrate, should open the way for numerous optoelectronic applications based on well-established Si device technology. Additionally, our nanowires have a pure zinc blende structure, which is the well-pursued structure for GaAs devices. Our results should increase the understanding of the band structure of single GaAs nanowires, which is crucial for many possible applications.

Reference
P. Plochocka, A. A. Mitioglu, D. K. Maude, G. L. J. A. Rikken , A. Granados del Águila, P. C. M. Christianen, P. Kacman, and Hadas Shtrikman, High Magnetic Field Reveals the Nature of Excitons in a Single GaAs/AlAs Core/Shell Nanowire, NanoLett., DOI: 10.1021/nl400417x

The authors would like to thank G. Kopnov for some help with the early stages of sample preparation, Ronit Popovitz-Biro for the TEM mesurements and I. Breslavetz for assistance with the mPL measurements. Part of this work was supported by EUROMAGNET II under the SP7 transnational access program of the European Union under contract number 228043. They would also like to acknowledge partial support by the Israeli Science Foundation grant #530/08 and Israeli Ministry of Science grant #3-6799.

(a) Color plot of the differential mPL spectra showing the evolution of the emission as a function of magnetic field applied perpendicular to the nanowire (b) schematic of the micro-photoluminescence setup used to investigate excitons in hexagonal GaAs/AlAs core/shell nanowires. Inset represents the side view SEM image of the top six microns of the as grown core/shell GaAs/AlAs.