Quantum Dots Grown By Molecular Beam Epitaxy And Their Applications On Solar Cell And Single Photon Source

Quantum dot devices,e.g.single electron transistors,light-emitting diodes,quantum computing,quantum teleportation,medical imaging,lasers,solar cells,and single photon sources,etc.,can pave the way to the next generation quantum information technology due to discrete energy levels and electronic states in quantum dots.However,quantum dot fabrication or synthesis with high controllability is still a challenge.Due to quality limitations of quantum dots,properties of quantum dot devices are not satisfactory as theoretical predictions.In order to clarify technology and methods of high-quality quantum dot growth,this dissertation studies III-V quantum dots using molecular beam epitaxy?MBE?.Furthermore,the MBE grown quantum dots are fabricated as quantum dot solar cells and single photon sources,respectively.The quantum dot structures are characterized by AFM,SEM,TEM,etc.,while the device performances are evaluated by I-V curve,external quantum efficiency,?-PL and second order correlation function.In the fourth chapter of the thesis,a preliminary study on plasma source enhanced quantum dot optoelectronic devices was carried out.Studies have shown that in the limitation of the performance improvement of quantum dot optoelectronic devices,the use of external plasmons can enhance the optical absorption of quantum dots and make up for the shortcomings of weak absorption at room temperature.First,GaAs and In_xGa_1-x-x As were grown by droplet epitaxy in MBE system.The elements percentage in InGaAs quantum dot can be tuned to tailor the lattice match degree between InGaAs and AlGaAs substrate.The quantum dot intermediate band solar cells were fabricated based on these quantum dots and their photovoltaic performances were characterized by a solar simulator and external quantum efficiency system.We compared the performances of GaAs and In_xGa_1-xAs quantum dot solar cells by studying their strain.Moreover,external infrared pumping sources?1064,980,905 nm?were added to investigate their intra and inter band transitions when testing external quantum efficiency.Second,the GaAsP/GaAs nanowire quantum dot were grown by vapor-liquid-solid?VLS?method.Compared with traditional VLS method using metallic catalysts,we used self-catalyzed VLS to avoid deep-level traps introduced by metallic nanoparticles.The interfaces between GaAsP and GaAs were sharp and surface states were removed by ammonium sulfide passivation.Finally,the samples were evaluated by PL and second order correlation function g²???.Third,preliminary study of plasmonic source enhanced quantum dot optoelectronic devices,including metamaterial absorbers,and core-shell nanoparticle structure,was demonstrated in the fourth chapter.The metamaterial absorber is an electromagnetic absorbing material that is not found in nature.It can achieve perfect absorption and provide new ideas for quantum dot optoelectronic devices.The fourth chapter of the thesis discusses the metamaterial absorbers with unabated absorption as size changes.Secondly,the enhancement of the thin-film solar cells by the core-shell plasmonic nanoparticles is studied.This dissertation covers two methods from quantum dot growth to devices applications.In the solar cell section,we study droplet epitaxy grown GaAs and InGaAs quantum dots.In the fourth chapter,the paper discusses the possibilities of quantum dot optoelectronic devices enhanced with plasmonic sources.The GaAs quantum dot cell demonstrated a better performance than that of the InGaAs cell,yielding an efficiency of0.72%and 0.11%,respectively.The novelty of this dissertation is that we distinguish the strain effect for quantum dot solar cells,paving the way to achieve high-efficiency solar cells.When adding infrared sources,we observed negative and positive EQE enhancement due to the presence of trap states.This clarifies the influence of trap states on quantum dot solar cells.In the nanowire quantum dot section,we achieving high-quality quantum dots via self-catalyst and source beam compensation.The nanowire quantum dot demonstrated high light extraction efficiency and linear polarization because of waveguide and microcavity provided by nanowire.The novelty of this dissertation is that we first studied the single photon emission from the“perfect”GaAsP/GaAs nanowire quantum dot.TRPL revealed that the minority carrier lifetime of the unpassivated sample was 0.167 while that of the passivated one was 0.346 at 10 K;their FWHM were 1.2 and 1.0 meV,respectively.Before passivation,the nanowire quantum showed single photon emission up to 60 K.In contrast,the passivated one demonstrated a single photon emission up to110 K and PL signal up to 220 K.Under 10 K,their g²??=0?are 0.16 and 0.05 respectively.This study provides a direction for achieving high efficiency and high temperature single photon source.Finally,the paper discusses the possibility of using quantum source optoelectronic devices enhanced by plasmonic sources.The optical absorption of quantum dots at room temperature is very weak.Therefore,quantum dot optoelectronic devices using plasmonic enhancement have better performances.The paper discusses the enhancement of plasmonic metamaterial absorber and the core-shell nanostructure.These preliminary studies paved the way for high performance quantum dot optoelectronic devices.