Shula Chen

Temperature-dependent spin injection dynamics in InGaAs/GaAs quantum well-dot tunnel-coupled nanostructures

Time-resolved optical spin orientation spectroscopy was employed to investigate the temperature-dependent electron spin injection in In0.1Ga0.9As quantum well (QW) and In0.5Ga0.5As quantum dots (QDs) tunnel-coupled nanostructures with 4, 6, and 8 nm-thick GaAs barriers. The fast picosecond-ranged spin injection from QW to QD excited states (ES) was observed to speed up with temperature, as induced by pronounced longitudinal-optical (LO)-phonon-involved multiple scattering process, which contributes to a thermally stable and almost fully spin-conserving injection within 5–180 K. The LO-phonon coupling was also found to cause accelerated electron spin relaxation of QD ES at elevated temperature, mainly via hyperfine interaction with random nuclear field.

Power-dependent spin amplification in (In, Ga)As/GaAs quantum well via Pauli blocking by tunnel-coupled quantum dot ensembles

Power-dependent time-resolved optical spin orientation measurements were performed on In0.1Ga0.9As quantum well (QW) and In0.5Ga0.5As quantum dot (QD) tunnel-coupled structures with an 8-nm-thick GaAs barrier. A fast transient increase of electron spin polarization was observed at the QW ground state after circular-polarized pulse excitation. The temporal maximum of polarization increased with increasing pumping fluence owing to enhanced spin blocking in the QDs, yielding a highest amplification of 174% with respect to the initial spin polarization. Further elevation of the laser power gradually quenched the polarization dynamics, which was induced by saturated spin filling of both the QDs and the QW phase spaces.

Temperature-dependent radiative and non-radiative dynamics of photo-excited carriers in extremely high-density and small InGaN nanodisks fabricated by neutral-beam etching using bio-nano-templates

Temperature-dependent radiative and non-radiative dynamics of photoexcited carriers were studied in In0.3Ga0.7N nanodisks (NDs) fabricated from quantum wells (QWs) by neutral-beam etching using bio-nano-templates. The NDs had a diameter of 5 nm, a thickness of 2 and 3 nm, and a sheet density of 2 × 1011 cm–2. The radiative decay time, reflecting the displacement between the electron and hole wavefunctions, is about 0.2 ns; this value is almost constant as a function of temperature in the NDs and not dependent on their thickness. We observed non-exponential decay curves of photoluminescence (PL) in the NDs, particularly at temperatures above 150 K. The thermal activation energies of PL quenching in the NDs are revealed to be about 110 meV, corresponding to the barrier heights of the valence bands in the disks. Therefore, hole escape is deemed responsible for the PL quenching, while thermal activation energies of 12 meV due to the trapping of carriers by defects were dominant in the mother QWs. The above-mentioned non-exponential PL decay curves can be attributed to variations in the rate of hole escape in the NDs because of fluctuations in the valence-band barrier height, which, in turn, is possibly due to compositional fluctuations in the QWs. We found that non-radiative trapping, characteristic of the original QW, also exists in about 1% of the NDs in a form that is not masked by other newly formable defects. Therefore, we suggest that additional defect formation is not significant during our ND fabrication process.Temperature-dependent radiative and non-radiative dynamics of photoexcited carriers were studied in In0.3Ga0.7N nanodisks (NDs) fabricated from quantum wells (QWs) by neutral-beam etching using bio-nano-templates. The NDs had a diameter of 5 nm, a thickness of 2 and 3 nm, and a sheet density of 2 × 1011 cm–2. The radiative decay time, reflecting the displacement between the electron and hole wavefunctions, is about 0.2 ns; this value is almost constant as a function of temperature in the NDs and not dependent on their thickness. We observed non-exponential decay curves of photoluminescence (PL) in the NDs, particularly at temperatures above 150 K. The thermal activation energies of PL quenching in the NDs are revealed to be about 110 meV, corresponding to the barrier heights of the valence bands in the disks. Therefore, hole escape is deemed responsible for the PL quenching, while thermal activation energies of 12 meV due to the trapping of carriers by defects were dominant in the mother QWs. The above-me...

Transient photoluminescence in InGaN nano-disks fabricated by nano-scale neutral-beam etching utilizing bio-nano templates

We study transient photoluminescence (PL) in In0.2Ga0.8N nano-disks (NDs) fabricated from a 2 or 3 nm-thick quantum well (QW) by damage-free neutral-beam etching utilizing bio-nano-engineered etching templates. A lateral averaged diameter of the ND was controlled to be 9 nm with a high sheet-density up to 2.6×1011 cm-2. Transient PL in the NDs was measured as a function of temperature and compared with that in the mother QWs. Thermal quenching of PL is strongly suppressed in the NDs, while the PL intensity in the QWs rapidly decreases with increasing temperature. A PL-decay time in the NDs is 0.1 ns at 6 K, which is significantly shorter than that of 3.5 ns in the QW. The temperature dependence of the PL decaying property shows that a radiative decay time of 0.1 ns in the NDs is almost constant for temperature, while a non-radiative one decreases with increasing temperature. This significantly faster and relatively temperature-insensitive radiative decay time can be attribute to the strong confinement due to the ND formation in addition to strain relaxation.

Growth optimization of spin-transport barriers used for spin-polarized light-emitting diodes based on InGaAs quantum dots

We have studied GaAs and AlGaAs barriers for the purpose of improving spin-transport performance in spin-polarized light-emitting diodes (LEDs) based on self-assembled quantum dots (QDs) of InGaAs. In the spin-LED utilizing a spin-functional optical active layer of In-based self-assembled QDs, growth temperatures of top barriers of GaAs and AlGaAs were reduced to suppress indium diffusion from the QDs into the barriers after forming the QDs. We show a significant improvement of spin-transport property as well as of carrier-transport one with increasing growth temperature of the Al0.1Ga0.9As barrier from 580 to 640 °C, while luminescent spectral energy and shape of the QDs are not markedly affected.

Transient photoluminescence study on spin dynamics in InGaAs-based coupled nanostructures of quantum dots with quantum wells

We have made transient photoluminescence (PL) study on electron-spin dynamics in InGaAs-based coupled nanostructures of quantum dots (QDs) with quantum wells (QWs). Self-assembled InGaAs QDs were grown integrated with an InGaAs QW through a GaAs tunneling barrier or embedded in a GaAs QW. Time-resolved circularly polarized PL in the QDs was measured as a function of temperature after optical spin excitation selectively in the QW, reflecting electron-spin polarization injected from the QW into QDs. We show the spin injection dynamics induced by spin tunneling and subsequent energy relaxation from the QW into QDs in the former coupled QDs. Spin relaxation at excited states in the QDs after the dynamical spin injection is shown as a function of temperature. These coupled QD samples exhibit thermally persistent spin polarization up to 200 K, originating from ultrafast and thus efficient spin injection as well as longer spin-relaxation times compared to radiative decay times in the QDs after the injection.

Ultrahigh-density self-assembled quantum dots of InGaAs and suppression of optical state-filling effect

We have grown ultrahigh-density self-assembled quantum dots (QDs) of InGaAs with sheet densities up to 2.5×1011 cm-2 and lateral diameters down to 10 nm, where the dot density increases with increasing As pressure during dot growth under optimum growth conditions. A ground-state photoluminescence (PL) spectrum shows a spectral width of 47 meV for the highest-density sample. Optical excitation-density dependences of the PL intensity and time profile are studied. The PL intensity from QD excited states increases with increasing excitation power, originating from a state-filling effect in QDs, which is directly confirmed by a plateau-like behavior on the PL decay curve. We find that the filling effect is significantly suppressed in the above ultrahigh-density dot ensemble, which suggests potential applications to superior energy-saving lasing and spin-functional optical devices.