| Optically active, highly uniform, cylindrical InGaAs quantum dot (QD) arrays have been fabricated using nanosphere lithography (NSL) combined with Bromine ion-beam-assisted-etching (Br-IBAE) and MBE-assisted GaAs mass transport (MT). Nanosphere lithography, a low-cost and high-throughput lithographic technique, is demonstrated here as a self-assembled nano pattern technique on GaAs and GaSb substrates and then used as a dry etch mask to fabricate nanopillar, nanoholes and nanodisk arrays. The photoluminescence (PL) from nanodisk arrays showed the blue shifted emission peaks as compared to that from the original single InGaAs quantum well (SQW) sample from which these arrays were fabricated, demonstrating 3D carrier confinements. However, PL emission intensity from the nanopillar arrays decreased by two orders of magnitude. This degradation of the QD optical properties is a result of the etch damage during the Br-IBAE dry etch process. An additional energy blue shift was also demonstrated by means of an InGaAs selective wet chemical etch that reduced the diameters of the nanodisks. Following QD nanodisk fabrication, a novel GaAs MT process was performed to encapsulate the nanosphere lithographically defined InGaAs disks in a GaAs matrix, resulting in passivation of the etch-damaged QD sidewall layer. The PL following the MT process indicated a significant improvement of the QD optical properties, showing a PL emission intensity increase of a factor of 5-11, when compared to PL from un-transported wet etched nanodisk samples. In addition, a PL peak energy red shift was observed after MT, presumably due to the decrease of lateral barrier potential (from vacuum to GaAs), as well as the elimination of the depletion layer. Furthermore, the MT process in the ultra-high-vacuum (UHV) Molecular Beam Epitaxy (MBE) environment enables GaAs overgrowth following MT for surface planarization. An additional factor of 4 PL emission intensity increase was observed following GaAs overgrowth. The PL spectral linewidth (22meV) was less than that of self assembled quantum dots (SAQDs) (35meV), demonstrating an improved uniformity for these lithographically formed disk QDs. Thus, this work demonstrates the potential of NSL and MT to fabricate high optical quality InGaAs QDs for device applications, such as high performance QD laser and detectors. |
