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.