Evidence for quantum-confined Stark effect in GaN/AlN quantum dots in nanowires

Semiconductor nanowires have the potential to outperform two-dimensional structures, for instance for light-emitting applications. However, the intrinsic fundamental properties of heterostructures in nanowires still remain to be assessed and compared to their two-dimensional counterparts. We show that polar GaN/AlN axial heterostructures in nanowires grown by plasma-assisted molecular-beam epitaxy are subject to a clear quantum-confined Stark effect. However, the magnitude of this effect is smaller than for two-dimensional structures due to the reduction in piezoelectric polarization that occurs thanks to elastic relaxation which is favored by the nanowire free surfaces. Moreover, we show by temperature-dependent photoluminescence measurement and single-photon correlation measurements that these heterostructures behave like quantum dots.

Figure 1

(Color online) Room-temperature macroPL spectra of ensembles of NWs containing single GaN/AlN axial heterostructures with varying QD height. The peak around 3.45 eV stems from the GaN base of the wire. The broad peak from the QD luminescence shifts toward small energies for thicker GaN QDs.

Figure 2

High-resolution TEM pictures from GaN/AlN heterostructures in NWs. In both cases the scale bar is 10 nm. The thicknesses of the GaN QDs are 1.5 (left) and 4.5 nm (right).

Figure 3

(Color online) PL energy (at room temperature) as a function of QD or QW thickness for this work (PL data from Fig. 1) and other references. The error bars correspond to the PL broadening (full width at half maximum) and to the measured QD thickness dispersion.

Figure 4

(Color online) MacroPL spectra for GaN/AlN QDs in NWs as a function of sample temperature. Each NW contains 10 QDs separated by 10 nm AlN barriers. This sample was used to increase the PL signal and obtain better experimental data. We checked that NWs containing several QDs and single QDs behave in the same way. The inset displays the decay time at $1/e$ and the integrated PL intensities as a function of temperature.

Figure 5

(Color online) Hanbury-Brown and Twiss coincidence histogram for the PL at 4 K of a single GaN/AlN QD in a NW. The continuous line is a fit of the experimental data that allows us to extract $g_2\left(t=0\right)$.