Epitaxially grown III-As nanostructures, like quantum well (QW)/ quantum dot (QD) have already been scrutinized rigorously and incorporated into various devices, like light-emitting diode (LED), laser, Photodetector, solar cell, etc. Most of them use buried QW/QD heterostructure, where the as-grown nanostructures are capped with various combination of thick (In/Ga/Al)As matrix. In contrary, near-surface nanostructures have very less attention owing to additional surface states. However, these near-surface nanostructures have the potential to communicate with the external world. Therefore, it might act as a confined channel, which can be probed externally. Assertively, these near -surface nanostructures have immense potential to act as sensors. Before that, we need to passivate the surface states to hold the best communication with the outer environment. In the present study, we have used Thiourea as a source of sulfur and show the effect of passivation in terms of improved luminescence behavior. Near-surface GaAs/In0.15Ga0.85As/GaAs quantum well and self-assembled GaAs/InAs/GaAs SK quantum dots are grown on GaAs wafer through molecular beam epitaxy. The effective thickness of the top GaAs capping layer has been kept around eight nanometers and twelve nanometers to keep the nanostructure (QW/QD) very close to the surface. As grown samples have shown very poor photoluminescence peaks and increased by few orders after passivation. Pre-etching followed by sulfur passivation has shown the best enhancement of luminescence intensity.
Uncapped In(Ga)As quantum dots (QDs) have got very little attention in comparison with its enfolded counterpart. The existence of surface states makes it less attractive to the research community. On the other hand, colloidal QDs have immense recognition in the field of bio-sensing and bio-imaging. Various surface passivated stable colloidal QDs are now commercially available, but only in solution form. Stable solid-state QDs are still a virtue. So, there is a huge demand for stable solid-state surface QDs which can be easily coupled with an electronic device for sensing application. Simply we need to change the ligand, corresponding to a particular target molecule, and we can detect various chemical and biological elements from low molar solution (Nano bio-sensor regime). With this motivation, we have epitaxially grown a simple vertically coupled InAs QD structure, where both seed and top dot layers are of 2.7 ML InAs. In addition, the top QDs are left uncapped to form a surface quantum dot layer. The as-grown sample is acid-etched (to remove the native oxides) and passivated in 0.5 M Thiourea solution for one hour. Significant enhancement of ground state photoluminescence peak has been observed after the passivation for both surface and buried QDs. Atomic force microscopic (AFM) images confirm the modulation of the surface before and after the ex-situ treatment.
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