G. W. Shu

Concentration dependence of carrier localization in InN epilayers

The authors studied the concentration dependence of carrier localization in InN epilayers using time-resolved photoluminescence (PL). Based on the emission-energy dependence of the PL decays and the PL quenching in thermalization, the localization energy of carriers in InN is found to increase with carrier concentration. The dependence of carrier concentration on the localization energy of carriers in InN can be explained by a model based on the transition between free electrons in the conduction band and localized holes in the deeper tail states. They suggest that carrier localization originates from the potential fluctuations of randomly located impurities.

Optical coupling from InGaAs subcell to InGaP subcell in InGaP/InGaAs/Ge multi-junction solar cells

Spatially-resolved electroluminescence (EL) images in the triple-junction InGaP/InGaAs/Ge solar cell have been investigated to demonstrate the subcell coupling effect. Upon irradiating the infrared light with an energy below bandgap of the active layer in the top subcell, but above that in the middle subcell, the EL of the top subcell quenches. By analysis of EL intensity as a function of irradiation level, it is found that the coupled p-n junction structure and the photovoltaic effect are responsible for the observed EL quenching. With optical coupling and photoswitching effects in the multi-junction diode, a concept of infrared image sensors is proposed.

Recombination dynamics of photoluminescence in thiol-protected gold nanoclusters

Recombination dynamics of photoluminescence (PL) in Au nanoclusters (NCs) with different capping molecules were studied with time-resolved PL. Based on the emission-energy of carrier lifetimes; we suggest that the fast and slow PL decay of Au NCs originates from recombination of the linear Au–S bond and the staple motif, respectively. The effect of carrier localization in Au NCs was found to depend on the capping molecules. The zero-dimensionality of carriers in Au NCs was demonstrated by the temperature dependence of the time-resolved PL.

Optical and structural properties of vertically stacked and electronically coupled quantum dots in InAs/GaAs multilayer structures

This work systematically investigated the optical and structural properties of multilayer electronic vertically coupled InAs/GaAs quantum dot (QDs) structures grown by molecular beam epitaxy for long-wavelength applications. A significant energy blue-shift in the photoluminescence (PL) spectra from 30-period InAs/GaAs QDs structures was observed as the GaAs spacer thickness was decreased. Transmission electron microscopy (TEM) and PL measurements indicated that the abnormal blue-shift can be attributed to the strain-driven In/Ga intermixing between QDs and spacer layers, which overcompensates for the effects of electronic and structural couplings between QD layers. Moreover, this study demonstrates that increasing the growth rate of InAs QDs can prevent intermixing. A PL emission wavelength of 1320 nm with strong luminescence at room temperature, which corresponds to an energy red-shift of 50 meV from that of the single QD layer sample, was achieved in a 10-period InAs/GaAs QD superlattice with a spacer thickness of 16 nm.

Distance dependence of energy transfer from InGaN quantum wells to graphene oxide

We report the distance-dependent energy transfer from an InGaN quantum well to graphene oxide (GO) by time-resolved photoluminescence (PL). A pronounced shortening of the PL decay time in the InGaN quantum well was observed when interacting with GO. The nature of the energy-transfer process has been analyzed, and we find the energy-transfer efficiency depends on the 1/d² separation distance, which is dominated by the layer-to-layer dipole coupling.

The photoluminescence decay time of self-assembled InAs quantum dots covered by InGaAs layers

The temperature dependence of the time-resolved photoluminescence (PL) of self-assembled InAs quantum dots (QDs) with InGaAs covering layers was investigated. The PL decay time increases with temperature from 50 to 170 K, and then decreases as the temperature increases further above 170 K. A model based on the phonon-assisted transition between the QD ground state and the continuum state is used to explain the temperature dependence of the PL decay time. This result suggests that the continuum states are important in the carrier capture in self-assembled InAs QDs.

Efficient energy transfer from InGaN quantum wells to Ag nanoparticles

Nonradiative energy transfer from an InGaN quantum well to Ag nanoparticles is unambiguously demonstrated by the time-resolved photoluminescence. The distance dependence of the energy transfer rate is found to be proportional to 1/d(3), in good agreement with the prediction of the dipole interaction calculated from the Joule losses in acceptors. The maximum energy-transfer efficiency of this energy transfer system can be as high as 83%.

Interrelation of transport and optical properties in gold nanoclusters

Temperature dependence of the electrical conductivity and photoluminescence (PL) in Au nanoclusters (NCs) is investigated. The correlation of the conductivity and PL in Au NCs at different temperatures is evident: (i) for T<50 K, both the conductivity and PL intensity decrease with temperature, which suggests thermal structural fluctuations; (ii) for 50 K<T<90 K, conductivity and PL are explained by variable range hopping; (iii) for 90 K<T<170 K, simple thermal activated hopping dominates in conductivity, with a rate-equation model proposed to analyze the carrier transfer in PL.Temperature dependence of the electrical conductivity and photoluminescence (PL) in Au nanoclusters (NCs) is investigated. The correlation of the conductivity and PL in Au NCs at different temperatures is evident: (i) for T<50 K, both the conductivity and PL intensity decrease with temperature, which suggests thermal structural fluctuations; (ii) for 50 K<T<90 K, conductivity and PL are explained by variable range hopping; (iii) for 90 K<T<170 K, simple thermal activated hopping dominates in conductivity, with a rate-equation model proposed to analyze the carrier transfer in PL.

Dependence of Biasing Voltage and Illumination Power on the Built-in Electric Field of InGaP Solar Cells

The electroreflectance spectra of InGaP solar cells at various biasing voltages and illumination levels were studied. The Franz–Keldysh oscillations were observed and the electric field at the junction of solar cells can be extracted. The measured electric field decreases with increasing biasing voltage and illumination level. The theoretical electric fields as a function of the biasing voltage and the photocurrent are calculated on the basis of the depletion approximation in the junction theory and the photovoltaic effect, respectively.

Energy transfer from InGaN quantum wells to Au nanoclusters via optical waveguiding

We present the first observation of resonance energy transfer from InGaN quantum wells to Au nanoclusters via optical waveguiding. Steady-state and time-resolved photoluminescence measurements provide conclusive evidence of resonance energy transfer and obtain an optimum transfer efficiency of ~72%. A set of rate equations is successfully used to model the kinetics of resonance energy transfer.