Analysis of the decay time and bound-states energies of a particle in a specific structure GaMnAs/GaAs quantum well

Surface-active agents (surfactants, e.g. washing agents) strongly modifies properties of gas-liquid interface. We have carried out extensive experiments, in which we study effect of surfactants on the shape oscillations of a bubble, which is attached at a tip of a capillary. In the experiments, shape oscillations of a bubble are invoked by a motion of a capillary, to which the bubble is injected. Decaying oscillations are recorded and their frequency and damping are evaluated. By changing the excitation frequency, three lowest oscillation modes are studied. Experiments were repeated in aqueous solution of several surfactants (terpineol, SDS, CTAB, Triton X-100, Triton X-45) at various concentrations. Generally, these features are observed: Initially a surfactant addition leads to an increase of the oscillation frequency (though surface tension is decreasing); this effect can be attributed to the increasing interfacial elasticity. The decay time of oscillation is strongly decreasing, as a consequence of energy dissipation linked with Marangoni stresses. At a certain critical concentration, frequency decreases abruptly and the decay time passes by a minimum. With further addition of surfactant, frequency decreases, and the decay time slightly lengthens. Above critical micelle concentration, all these parameters stabilize. Interestingly, the critical concentration, at which frequency drop occurs, depends on mode order. This clearly shows that the frequency drop and minimum decay time are not a consequence of some abrupt change of interfacial properties, but are a consequence of some phenomena, which still need to be explained.

In this work, pure and Ni-doped ZnO thin films have been deposited onto glass substrates using the spray pyrolysis technique. All films were deposited at constant deposition parameters but the Ni content was changed from 0 to 7 weight (wt) %. XRD results revealed the formation of a hexagonal ZnO phase whilst no other phases were detected. The crystallite size was determined using Scherrer’s equation and found to be 45.9 nm for the pure film. Scanning electron microscope images show the formation of irregular grains with a broad size distribution. The existence of Ni in the deposited films was confirmed using energy dispersive spectroscopy (EDX), where the Ni content in the film increases as the weight % increases in the starting solution. The optical band gap was determined and found to be 3.3 eV for the pure ZnO films, which was reduced with Ni doping. The performance of the deposited films for UV radiation has been examined for the 365 nm wavelength and at different applied potentials and constant power. The rise and decay times for doped films were observed to exhibit faster rise/recovery as compared to pure films. The minimum response time was found to be 0.09 s for Ni-7 wt% film and the minimum decay time is 0.07 s for Ni-1 wt%.

Recombination dynamics of carriers in GaAs-GaAlAs quantum well structures

Investigation of the dimensionality using temperature-dependent decay times in InGaAs-coupled quantum well-quantum dots structures

The measurement of the bias and temperature dependent photoluminescence, photocurrent and their decay times allows to deduce important physical properties such as barrier height, electron-hole overlap and the magnitude of the piezoelectric field in InGaN quantum wells. However the analysis of these experiments demands for a detailed physical model based on a realistic device structure which is able to predict the measured quantities. In this work a selfconsistent model is presented based on a realistic description of the alloy and doping profile of a green InGaN single quantum well light emitting diode. The model succeeds in the quantitative prediction of the quantum confined Stark shift and the associated change in the electron-hole overlap measured via the change in the bimolecular decay rate using literature parameters for the piezoelectric constants. The blue shift of the emission under forward current conditions can be attributed to the carrier induced screening of the piezoelectric charges as predicted by the model. The photocurrent is calculated via thermionic tunneling through the barriers using a WKB-approximation and the calculated potential profile for the tunneling barrier. From the fact that the bias and temperature dependence of the experimentally observed photocurrent cannot be described by the thermionic tunneling model even though the theoretical potential profile fits excellent to the luminescence data, we conclude that the carrier escape is dominated by a different mechanism such as defect- or phonon-assisted tunneling.

The measurement of bias and temperature dependent photoluminescence, photocurrent and their decay times allows to deduce important physical properties such as barrier height, electron-hole overlap and the magnitude of the piezoelectric field in InGaN quantum wells. However the analysis of these experiments demands for a detailed physical model based on a realistic device structure which is able to predict the measured quantities. In this work a self-consistent model is presented based on a realistic description of the alloy and doping profile of a green InGaN single quantum well light emitting diode. The model succeeds in the quantitative prediction of the quantum confined Stark shift and the associated change in the electron-hole overlap measured via the change in the bimolecular decay rate using literature parameters for the piezoelectric constants. The blue shift of the emission under forward current conditions can be attributed to the carrier induced screening of the piezoelectric charges as predicted by the model.

An exact and compact formula for the optical intersubband response of finite-barrier quantum wells, wires and dots

The universal photoluminescence behaviour of yellow light emitting (Ga,In)N/GaN heterostructures

The understanding .of carrier diffusion processes is crucially important for the design of efficient blue-green optoelectronic devices based on GaInN/GaN quantum wells. The optimization of the performance requires careful consideration of the diffusion of carriers injected from the n and p regions into the quantum wells of the active region. It is well known that the inclusion of silicon in the barriers leads to enhanced radiative efficiency. In this work we have performed time-resolved photoluminescence (PL) experiments to study the diffusion of electrons and holes injected optically into silicon doped GaN and subsequently captured by a single GaInN quantum with low indium concentrations. By vafying the silicon doping and modelling the rise times of the PL signals from the quantum well, we have been able to study the effect of the silicon doping on the amhipolar diffusion coefficient. A set of four samples were grown by MOCVD on sapphire substrates with a 3 nm undoped GaInN single quantum well (SQW) inserted within silicon doped GaN as shown in Fig. I. The nominal indium fraction of the SQW was 6%, which gave emission at 400 nm. The doping of the GaN was varied up to 4X 10'7cm-', and a nominally undoped structure was grown for comparison. CW PL characterization of the samples indicated the PL efficiency at room temperature reached a maximum for a doping level of IX 1017cm?. For the time-resolved PL measurements, the samples were excited in the top Si-GaN layer of thickness 200 nm by frequency-doubled pulses at 355 nm from a mode-locked Tisapphire laser as shown in Fig. I. The PL from the quantum well was monochromated and then detected by a synchroscan streak camera. The time-resolution of the system was 3 ps. Figure 2 shows the rise and decay times observed for the GalnN SQW. The rise time is determined by the diffusion of the carriers in the GaN to the SQW and their subsequent capture by the SQW. This is confirmed by the fact that no rise time was observed when exciting the quantum wells directly below the hand edge of the GaN. We observe that the rise time increases with increasing Si concentration. Numerical modelling of these rise times allows the extraction of the carrier diffusion coeftkients, which are found to decrease with increasing silicon doping. The decay time is mainly dominated by the non-radiative recombination rate in the quantum well. The peak at IX I0l7cm corresponds with the maximum in the PL efficiency observed in the CW experiments. We conclude that the optimal doping level of IX 10'7cm-' represents a good compromise between enhanced PL efficiency and reduced mohility in the GaN due to trapping centres.

Temperature dependent time-resolved exciton luminescence in GaAs/AlGaAs quantum wires and dots

Photoreflectance (PR), photoluminescence (PL) and time-resolved PL were applied to study the optical properties, particularly the localized and delocalized states and carrier dynamics, in GaAs1−xBix/GaAs quantum wells. With increasing Bi concentration the ground state transition (i.e., the transition between the first heavy hole and the first electron subband) red shifts due to Bi-related reduction of the GaAs1−xBix energy gap. Additionally, the transition related to the excited states in the quantum wells is clearly observed for the sample with high Bi concentration of 5.6%, confirming these quantum wells are type I. The PL measurements show the S-shape behavior and indicate the strong localization effect below 150 K for all measured samples, while the PL emission above 150 K is related to delocalized states. The localized character of emission at low temperatures is confirmed by time-resolved PL studies. At 10 K the decay time has strong spectral dispersion (i.e. the decay time increases from ∼10 ns to ∼400 ns going from the high to low energy side of the PL peak). This dispersion disappears above 190 K. At room temperature the decay time is in the order of a few ns.

We have investigated the variation of the photoluminescence intensity and decay time as a function of temperature of a series of InGaN/GaN quantum well structures in which the number of quantum wells was varied. All the samples exhibited a decrease in photoluminescence intensity and decay time with increasing temperature with the rate of decrease being reduced as the number of quantum wells was increased. We have compared these results with a theoretical model which describes the effects of thermally excited carrier escape and recapture. We find reasonable agreement with the results of the model and the experiments for the samples incorporating only a few quantum wells supporting the idea that thermally excited carrier loss is the main non-radiative recombination path.

Zn2-xMnxSiO4 green phosphors have been prepared by the solid state reaction, and photoluminescence, color purity and decay time were investigated as a function of both the firing conditions and the activator concentrations (X=0.005-0.12) using MnCO3, MnO, MnSO4 and MnC2O4 as activators. For the phosphors doped with various activator compounds, there were no distinct differences in the color purity, FWHM values of emission peaks and the color coordinate in the CIE chromaticity diagram. However, Manganese concentration affected greatly the photoluminescence and the decay time. The decay time decreased from 30ms to 6ms as Mn concentration increased from X=0.005 to X=0.12. The decay time was measured at about 9ms for the phosphors doped with X=0.08 showing the highest luminance.

We report on the observation of increased photoluminescence (PL) decay time in strain-induced quantum-well dots (SIQWDs), 120 nm in diameter. Lateral confinement was generated in a GaAs quantum well (QW) by etching a doubly exposed grating pattern into a pseudomorphic, strained layer of In0.35Ga0.65As which overlies the QWs. By spacing three QWs of different widths at varying depths from the In0.35Ga0.65As stressor, lateral strain confinement and vertical strain propagation are directly resolved by the PL spectra. The decay time of the heavy-hole-like excitons in the SIQWDs from the uppermost QW is 420 ps at 2 K, which is longer than the 270 ps PL decay time of the heavy-hole exciton in the reference QW sample.

This paper analyzes a simplified rate equation model of localized exciton emission in GaInN. Expressions for temperature dependent photoluminescence (PL) efficiency and decay time are derived and compared with time integrated (TIPL) and time resolved photoluminescence (TRPL) data for a series of multiquantum well light emitting diodes with varying In composition in the active region. Time resolved photoluminescence is measured up to relatively high temperature (540 K) and a decreasing efficiency coupled with a peak energy decay time that is weakly dependent on temperature is observed. The decay time at peak emission energy begins to decrease at a temperature that depends on the In content in the quantum wells. The analysis developed here demonstrates that application of the expressions τr=τpl/η and τnr=τpl/(1−η) is not sufficient to determine radiative and nonradiative lifetimes from TRPL and TIPL data in the GaInN system. (Here τr is the radiative decay time, τnr is the nonradiative decay time, τpl is the measured PL decay time, and η is the measured TIPL intensity normalized to the low temperature intensity.) GaInN with even small amounts of In exhibits highly efficient luminescence due to recombination through localizing centers. As relaxation occurs into both defects and localizing states after initial generation with the above GaInN band gap excitation, the number of carriers arriving at localization centers can change with temperature. This temperature dependent change should be considered when calculating relevant decay times from TRPL and TIPL data. This mechanism is distinct from an increase in the intrinsic radiative decay time obtained by applying conventional analysis to extract radiative and nonradiative lifetimes from TRPL and TIPL data.