Patterned zero-dimensional nanostructures: fabrication and characterization

quantum dots;nanopores

Dias, Neville L.

关键词: quantum dots;    nanopores;   

Others  :  https://www.ideals.illinois.edu/bitstream/handle/2142/29771/Dias_Neville.pdf?sequence=1&isAllowed=y

美国|英语

来源: The Illinois Digital Environment for Access to Learning and Scholarship

【 摘 要 】

Due to the advantages arising from low-dimensional electronic systems, considerable effort hasbeenputintotheuseofquantumdotsandwiresastheactivemediainoptoelectronic devices.Therealizationofquantumdotbaseddeviceshasbeenplaguedwithnumerous 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 onequantumdot,theissueofuniformitybecomescritical.Evenifthedevicehasonlyone quantumdot,uniformityisessentialtoobtainreproduciblecharacteristicsacrossmultiple 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 thiswork, nanoscale selective areametal-organic chemicalvapor 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-defineddots.Thefabricationapproachavoidsmodificationoftheunderlyingsurface, 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. Althoughnano-fabricationmethodsimposeadegreeofdeterminismonthequantumdot size,thelackofcouplingbetweenindividualdotsinanarraystructurecoupledwiththesize 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. Weconductedaphotoluminescence(PL)studyofnanoporelatticesasafunctionofpore diameterwhilekeepingthepitchandmaterialcompositionsconstant.Goodagreementis 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.