ABSTRACT
As material size shrinks to a nanometer scale, the materials start to show specific electrical and optical characteristics based on the quantum confinement of carriers and/or excitons in nanoscale. Recently, superlattices with closely packed quantum dots (QDSLs) with high-uniformity and high-density have received great attention to develop high-performance optoelectronic devices, including lasers and solar cells [1–4]. In the superlattices with closely coupled QDs, discrete states of each QD merge to form broadened mini-bands to behave as brand-new materials [5–9]. For photovoltaic applications, such engineered QDs can be used as adjustable absorber layers with well-designed intermediate bandgap energy to build tandem solar cells [9–12]. QDs of III-V compounds formed by a bottom-up process such as Stranski–Krastanov growth based on self-organization [13] have been applied to optoelectronic devices, including high-performance QD lasers [1–4]. In the “bottom-up” process, there are some limitations in control of the density of QDs: The distance between QDs is too narrow to avoid the coupling of wave functions for high-gain lasers. On the other hand, in “top-down” process technologies, there are also limitations in fabrication of defect-free nanostructures through the process sequences such as photolithography, plasma etching, co-sputtering, and annealing [14–16].
