Search

AlInAs/AlGaAs quantum dots (QDs) have emerged as excellent emitters across the visible spectral range, showcasing highly tunable electronic properties through variations in composition and size. This versatility allows for diverse band alignments within the same system. In this study, we present compelling evidence for the coexistence of type-I indirect and direct emissions from QDs, supported by comprehensive analyses of their photoluminescence responses to excitation power, temperature, and time, along with band structure calculations. The high-density QD system exhibits signs of lateral coupling, facilitated by carrier transfer between dots, modulated by energy barriers and recombination times. Additionally, we can unequivocally prove that the QDs act as carrier reservoirs that progressively feed optically active states in the bulk GaAs at low temperaturesan attractive prospect for hot-carrier photovoltaics. Above certain temperatures, the bulk system reverts to the anticipated predominantly radiative recombination dynamics. Our theoretical framework, accounting for the coexistence of QD specimens with varying recombination times, successfully elucidates the optical response at different temperatures, emphasizing the pivotal role of QD excitation in enhancing the effective lifetime of carriers in the bulk. [ABSTRACT FROM AUTHOR]

The authors describe district-specific disability-adjusted life-year (DALY) of lung cancer attributable to ambient particulate matter < 2.5 um in diameter (PM2.5) for Guangzhou city in China, so as to help prioritizing environmental health action from geospatial perspective. Comparative risk assessment and satellite-derived PM2.5 concentrations were used to investigate the spatial-temporal distribution of DALY attributable to ambient PM2.5 for lung cancer. Integrated exposure-response model and kriging model were constructed based on estimated relative risk (RR) from risk assessment. Annual mean PM2.5 increased by 25.9% from 2010 (71.1 ug/m3) to 2013 (89.5 ug/m3). Estimated RRs ranged from 1.37 (95% uncertainty interval [UI]: 1.04 ~ 1.86) to 1.99 (95% UI: 1.12 ~ 3.19) among the districts. For lung cancer, DALY attributable to PM2.5 increased by 26.8% from 2010 to 2013, reaching 43352.7 DALYs (95% UI: 8157.9 ~ 62371.7) in 2013. The paper showed that population residing in highly-polluted and aged districts might suffer a higher relative risk for developing lung cancer. Our validated analysis framework also showed that population suffered from a higher loss of lung cancer DALYs, partly due to the higher PM2.5 exposure in some subareas within the city. We reveal that ambient PM2.5 pollution contributed substantially to lung cancer burden, both locally and sub-locally. These results suggest the need for enhanced environmental health policies in the city. [ABSTRACT FROM AUTHOR]

We investigate optical transitions and carrier dynamics in hybrid structures containing type-I GaAs/AlGaAs quantum wells (QWs) and type-II GaSb/AlGaAs quantum dots (QDs). We show that the optical recombination of photocreated electrons confined in the QWs with holes in the QDs and wetting layer can be modified according to the QW/QD spatial separation. In particular, for low spacer thicknesses, the QW optical emission can be suppressed due to the transference of holes from the QW to the GaSb layer, favoring the optical recombination of spatially separated carriers, which can be useful for optical memory and solar cell applications. Time-resolved photoluminescence (PL) measurements reveal non-exponential recombination dynamics. We demonstrate that the PL transients can only be quantitatively described by considering both linear and quadratic terms of the carrier density in the bimolecular recombination approximation for type-II semiconductor nanostructures. We extract long exciton lifetimes from 700 ns to 5 μs for QDs depending on the spacer layer thickness. [ABSTRACT FROM AUTHOR]

InAs quantum-dot structures were grown using a GaAs1-xSbx matrix on a GaAs(001) substrate. The use of GaAs1-xSbx for the buffer and cap layers effectively suppressed coalescence between dots and significantly increased the dot density. The highest density (~3.5 x 1011/cm²) was obtained for a nominal 3.0 monolayer deposition of InAs with an Sb composition of x = 13-14% in the GaAs1-xSbx matrix. When the Sb composition was increased to 18%, the resulting large photoluminescent red shift (~90 meV) indicated the release of compressive strain inside the quantum dots. For x > 13%, we observed a significant decrease in photoluminescence intensity and an increase in the carrier lifetime (≥4.0 ns). This is attributed to the type-II band alignment between the quantum dots and matrix material. [ABSTRACT FROM AUTHOR]

Spectroscopic study of the InAs quantum dot (QD) formation in GaAs ringlike nanostructures is carried out. Ga droplet epitaxy is used to form GaAs ringlike nanostructures. Subsequently InAs is deposited to obtain InAs QDs by self-assembly inside the holes of the nanostructures. Regularly spaced bands in the photoluminescence spectra exhibit state filling properties under increased excitation power. However, it is demonstrated that these bands do not represent excited states of a single ensemble of dots, but are separate ensembles with individual ground state energies, which are coupled through the GaAs ring structure on which they form. The most likely cause of these uniformly spaced ensemble energies is monolayer differences in the effective height of the dots. Temperature, excitation power density, and time dependent photoluminescence measurements are used to demonstrate the significance of the interdot coupling. The photoluminescent properties of these novel nanostructures make them candidates for optoelectronic applications. [ABSTRACT FROM AUTHOR]

Crossed transitions between the wetting layer valence band and the quantum dot (QD) electron states are revealed in the photoluminescence from self-assembled In0.4Ga0.6As/GaAs QDs. The strength of these transitions becomes comparable with the excitonic transitions for below-GaAs barrier excitation and decreases significantly with below wetting layer excitation. The observed peculiar QD photoluminescence dependences on temperature and excitation density are due partly to interdot carrier transfer through the continuum states related to the wetting layer morphology and to phonon-assisted processes. [ABSTRACT FROM AUTHOR]

Time-resolved photoluminescence measurements are carried out for InAs quantum dots (QDs) coupled through a GaAs barrier to an In0.3Ga0.7As quantum well (QW). It is found that decay time of photoluminescence response from the QW is significantly shortened when compared to a reference sample only containing a QW with similar parameters while the decay time of photoluminescence response for the QD layer is significantly lengthened compared to reference sample only containing a QD layer. A rate equation model is developed to describe the observed behavior via carrier capture from QW to QD states in the QD:QW structure. [ABSTRACT FROM AUTHOR]

Time-resolved photoluminescence (PL) measurements are carried out for the InGaAs quantum dots (QDs) grown on (001) and (311) oriented GaAs substrates. The detection energies scan the spectral region from the energy of the QD excitonic transition up to the wetting layer absorption edge. A convex-shaped decay of the PL signal in this region gives evidence of carrier relaxation through the continuum states below the wetting layer similar to a diffusion process. Strong dependence of the decay time on the excitation density observed for the QD ground-state PL is consistent with a presented theoretical model. [ABSTRACT FROM AUTHOR]