May 14, 2013
Nanowire team examines emission from radial quantum structures
Researchers in Sweden have examined the optical properties of InP-InAs core-shell nanowires by correlating micro-photoluminescence (µPL) spectroscopy and cathodoluminescence (CL) imaging measurements. The approach brings together the high spectral resolution of µPL and the high spatial resolution of CL, and allows scientists to determine the origin of the luminescence.
The nanowires in the study consist of a long core made of InP, surrounded by an InAs shell and an InP capping layer. By controlling the InAs growth time, a set of samples with radial InAs shell thicknesses ranging from 1–12 monolayers were made. The shell thicknesses are thin enough to confine charge carriers, leading to quantization in the radial direction, that is – radial quantum wells (QWs). We observe the quantization for different thicknesses as distinct peaks ranging from about 850 to 1600 nm in the µPL spectra. These wavelengths are suitable for near infrared light emitting diodes as well as for telecommunication.
From transmission electron microscopy measurements we could confirm that we had a thin smooth InAs shell sandwiched between the InP core and capping layer, with sharp interfaces. We also confirmed that the crystal structure was in the wurtzite phase and estimated the thickness of a single radial monolayer in the shell. However, previously reported X-ray diffraction measurements on InAs nanowires, together with strain calculations, provided us with a more accurate value of the monolayer thickness. By relating the peak values to the corresponding number of monolayers, we showed that available theoretical and experimental data on effective masses and bandgap energy of InAs agrees very well with our observations.
A second type of nanowires were also made and investigated. The structure was almost identical to the first type, but in this case the InAs shell had been modulated. Apart from the QW, we managed to grow quantum dots (QDs) at five 100 nm long segments, equally separated within the shell along the nanowire. Compared with the first sample, the emission range was broader and, most importantly, much sharper peaks were observed. The results are interesting for further studies within the field of quantum information processing.
Full details can be found in the journal Nanotechnology.
About the author
My name is David Lindgren and I am a PhD student in the division of solid state physics and the nanometer structure consortium at Lund University, Sweden. I belong to the optics group in our division and our main focus is optical characterization of III-V nanowires for various applications. For me it is especially light emitting diodes (LEDs) and solar cells. The particular project reported here was initiated by Prof. Lars Samuelson who also is the founder of our consortium. Growth was done by Dr Kenichi Kawaguchi and PhD student Magnus Heurlin, led by assistant professor Magnus Borgström. Magnus Heurlin also performed the TEM measurements. Theory was done by Prof. Mats-Erik Pistol and partly by me. Optical characterization was undertaken by me and Prof. Anders Gustafsson, who also had a leading role of this project.