Patterned zero-dimensional nanostructures: fabrication and characterization

Due to the advantages arising from low-dimensional electronic systems, considerable effort has been put into the use of quantum dots and wires as the active media in optoelectronic devices. The realization of quantum dot based devices has been plagued with numerous obstacles. Conventional quantum dots are formed by strain-driven self-assembly. The stochastic nature of the process results in a distribution of dot sizes. If a device is composed of more than one quantum dot, the issue of uniformity becomes critical. Even if the device has only one quantum dot, uniformity is essential to obtain reproducible characteristics across multiple devices. Thus, the geometrical parameters of a quantum dot, such as shape and size as well as the chemical composition, need to be controlled. In this work, nanoscale selective area metal-organic chemical vapor deposition (MOCVD) has been used to define InAs dot nucleation sites with highly ordered dot-to-dot pitches down to 80 nm corresponding to densities greater than 10^ 10 cm-2, which are among the highest reported for site-defined dots. The fabrication approach avoids modification of the underlying surface, allowing for easier integration into a variety of devices. Patterning of an oxide film by electron beam lithography also allows for creation of arbitrary closely packed arrangements of quantum dots for novel device designs. The resulting quantum dot array has the potential to be used as a template for fabricating multi-stack structures for use in laser and photodetector applications. Although nano-fabrication methods impose a degree of determinism on the quantum dot size, the lack of coupling between individual dots in an array structure coupled with the size variation is the primary cause for inhomogeneous broadening in quantum dot based devices. In an attempt to address broadening in quantum dots, the nanopore active layer was proposed. The nanopore is in essence an inverse quantum dot structure consisting of a periodically perforated quantum well that has been filled with a higher bandgap material. In the limit of small pores or large lattice spacing, the nanopore electronic properties approach those of a quantum well. At the other extreme, the nanopore behaves like a quantum dot. Thus the novelty in the nanopore active layer is that it presents an opportunity to design devices covering the continuum between fully three-dimensionally confined quantum dots and one-dimensionally confined quantum wells. The in-plane periodicity results in miniband formation due to resonant scattering. Theoretical calculations of the intersubband scattering rate in nanopore lattices predict decreased intersubband scattering rates. This is due to the reduced overlap between in-plane components of the initial and final wavefunctions. We conducted a photoluminescence (PL) study of nanopore lattices as a function of pore diameter while keeping the pitch and material compositions constant. Good agreement is obtained between PL spectra and finite-element calculations of the band structure. We observe increased emission from the higher subbands as the pore diameter is increased, which is a direct experimental verification of theoretical predictions. The decreased carrier cooling rate makes the nanopore useful as a solar cell material in which hot carriers excited by energetic photons can be captured before they decay to lower energy states