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The carrier recombination behavior of GaN-based blue laser diodes (LDs) is studied and analyzed by experiments and simulation calculations before lasing, with a particular focus on the role of Auger recombination. It is found that Auger recombination plays a crucial role in the decrease in differential efficiency and threshold current of GaN-based blue LDs. The theoretical calculation results show that a large Auger recombination rate may lead to a dominant recombination channel before lasing, which could exceed the radiation recombination and result in an obvious decrease in the differential efficiency. Such a high Auger recombination will dissipate a large number of carriers in the quantum well, resulting in deterioration of device performance, a higher threshold current and a lower efficiency. This work presents a method to evaluate Auger recombination through differential efficiency and also provides evidence that suppressing the Auger recombination rate is beneficial to improve the performance of blue LDs. [ABSTRACT FROM AUTHOR]

The optical properties of InGaN/GaN violet light-emitting multiple quantum wells with different thicknesses of GaN quantum barriers are investigated experimentally. When the barrier thickness decreases from 20 to 10 nm, the photoluminescence intensity at room temperature increases, which can be attributed to the reduced polarization field in the thin-barrier sample. However, with a further reduction in the thickness to 5 nm, the sample's luminescence intensity decreases significantly. It is found that the strong nonradiative loss process induced by the deteriorated crystal quality and the quantum-tunneling-assisted leakage of carriers may jointly contribute to the enhanced nonradiative loss of photogenerated electrons and holes, leading to a significant reduction in photoluminescence intensity of the sample with nanoscale ultrathin GaN quantum barriers. [ABSTRACT FROM AUTHOR]

We demonstrate an ultra-low efficiency droop in c-plane polar InGaN blue light-emitting diodes (LEDs) by reducing the carrier density using a wide InGaN last quantum well (LQW). It is found that the LEDs with a 5.2 nm thick LQW show a negligible efficiency droop, with an external quantum efficiency (EQE) reducing from a peak value of 38.8% to 36.4% at 100 A/cm2 and the onset-droop current density is raised from 3 A/cm2 to 40 A/cm2 as the LQW thickness increases from 3.0 nm to 5.2 nm. The analysis based on the ABC model indicates that small efficiency droop is caused by the reduced carrier density using a wide LQW. The peak efficiency is reduced with a wide LQW, which is caused by the reduction of the electron-hole wavefunction overlap and the deterioration of the crystal quality of the InGaN layer. This study suggests that the application of the InGaN LEDs with a wide LQW can be a promising and simple remedy for achieving high efficiency at a high current density. [ABSTRACT FROM AUTHOR]

Carrier recombination and electron spin relaxation dynamics in asymmetric n-doped (110) GaAs/AlGaAs quantum wells are investigated with time-resolved pump-probe spectroscopy. The experiment results reveal that the measured carrier recombination time depends strongly on the polarization of pump pulse. With the same pump photon flux densities, the recombination time of spin-polarized carriers is always longer than that of the spin-balanced carriers except at low pump photon flux densities, this anomaly originates from the polarization-sensitive nonlinear absorption effect. Differing from the traditional views, in the low carrier density regime, the D'yakonov–Perel' (DP) mechanism can be more important than the Bir–Aronov–Pikus (BAP) mechanism, since the DP mechanism takes effect, the spin relaxation time in (110) GaAs QWs is shortened obviously via asymmetric doping. [ABSTRACT FROM AUTHOR]

Via temperature-dependent time-resolved photoluminescence spectroscopy, we investigate the radiative and nonradiative recombination processes in thin (quantum well widths of about 1.5 nm) a-plane, m-plane, ( [ABSTRACT FROM AUTHOR]

We present results on photoexcited carrier lifetimes in few-layer transition metal dichalcogenide MoS2 using nondegenerate ultrafast optical pump-probe technique. Our results show a sharp increase of the carrier lifetimes with the number of layers in the sample. Carrier lifetimes increase from few tens of picoseconds in monolayer samples to more than a nanosecond in 10-layer samples. The inverse carrier lifetime was found to scale according to the probability of the carriers being present at the surface layers, as given by the carrier wave function in few layer samples, which can be treated as quantum wells. The carrier lifetimes were found to be largely independent of the temperature, and the inverse carrier lifetimes scaled linearly with the photoexcited carrier density. These observations are consistent with defect-assisted carrier recombination, in which the capture of electrons and holes by defects occurs via Auger scatterings. Our results suggest that carrier lifetimes in few-layer samples are surface recombination limited due to the much larger defect densities at surface layers compared with the inner layers. [ABSTRACT FROM AUTHOR]

Enhancement of internal quantum efficiency (IQE) in InGaN quantum wells by insertion of a superlattice interlayer and applying the pulsed growth regime is investigated by a set of time-resolved optical techniques. A threefold IQE increase was achieved in the structure with the superlattice. It was ascribed to the net effect of decreased internal electrical field due to lower strain and altered carrier localization conditions. Pulsed MOCVD growth also resulted in twice higher IQE, presumably due to better control of defects in the structure. An LED (light emitting diode) structure with a top p-type contact GaN layer was manufactured by using both growth techniques with the peak IQE equal to that in the underlying quantum well structure. The linear recombination coefficient was found to gradually increase with excitation due to carrier delocalization, and the latter dependence was successfully used to fit the IQE droop. [ABSTRACT FROM AUTHOR]

The combined temporally, spatially and spectrally-resolved optical techniques, namely the photoluminescence, light induced transient grating, and differential reflectivity were used for investigation of excitation-dependent PL efficiency, exciton lifetime, and diffusion coefficient in Si-doped Al-rich multiple quantum wells and epilayers at various temperatures. Novel features of carrier recombination and in-plane diffusion were observed. Low-excitation radiative lifetime of 1-2 ns was found temperature-independent in 80-150 K interval, while it sublinearly decreased with excitation at excess carrier densities above 1018 cm−3. The lifetime decrease correlated with the increase of diffusion coefficient, indicating excitation-enhanced delocalization of localized excitons and therefore enhanced capture to nonradiative centers. The droop of photoluminescence efficiency with excitation was the strongest at 80-150 K due to strong delocalisation at low-temperatures, while at higher temperatures the thermal activation prevailed in photoluminescence excitation dependence. The photoluminescence efficiency quenching at T > 200 K provided rather high activation energies of ∼100 and 160 meV for Al-rich multiple quantum wells and epilayers, correspondingly. [ABSTRACT FROM AUTHOR]

We present a comprehensive study of photoexcited carrier dynamics in differently grown InGaN/InGaN multiple quantum well (MQW) structures, modified by insertion of a wide interlayer structure and subsequent growth of differently shaped quantum wells (rectangular, triangular, trapezoidal). This approach of strain management allowed the reduction of dislocation density due to gradually increasing In content in the interlayer and shaping the smooth quantum well/barrier interfaces. A set of c-oriented MQW structures emitting at 470 nm were grown at Vilnius University, Institute of Applied Research, using a closed coupled showerhead type MOCVD reactor. Photoluminescence (PL) spectra of MQW structures were analysed combining continuous wave and pulsed PL measurements. Reactive ion etching of the structures enabled discrimination of PL signals originated in the InGaN interlayer structure, underlying quantum wells, and quantum barriers, thus providing growth-related conditions for enhanced carrier localization in the wells. Time-resolved PL and differential transmission kinetics provided carrier lifetimes and their spectral distribution, being the longest in triangular-shape QWs which exhibited the highest PL intensity. The light-induced transient grating (LITG) technique was used to determine the spatially averaged carrier lifetime in the entire heterostructure, in this way unravelling the electronic quality of the LED internal structure at conditions similar to device performance. LITG decay rates at low and high excitation energy densities revealed increasing with photoexcitation nonradiative recombination rate in the triangular and trapezoidal wells. [ABSTRACT FROM AUTHOR]

Carrier localization effect has been observed in GaN/AlGaN multiquantum wells (MQWs) by analyzing the temperature dependent photoluminescence (PL) results. Specifically, two neighbouring peaks can be seen in the PL spectra with an abnormal variation when increasing the temperature. A conduction band model with two minimal potentials corresponding to two carrier localizations is proposed to explain the PL spectra. [ABSTRACT FROM AUTHOR]

Motivated by possible application of InGaN/GaN multi-quantum well solar cells in hybrid concentrated photovoltaic / solar thermal power conversion systems, we have analyzed the temperature and intensity dependence of the open-circuit voltage of such devices up to 725 K and more than 1000 suns. We show that the simple ABC model routinely used to analyze the measured quantum efficiency data of InGaN/GaN LEDs can accurately reproduce the temperature and intensity dependence of the measured open-circuit voltage if a temperature-dependent Shockley–Read–Hall lifetime is used and device heating is taken into account. • Open-circuit voltage of InGaN quantum-well solar cells is well described by recombination models. • Temperature dependence is captured by a temperature dependent Shockley–Read–Hall model. • Self-heating becomes relevant at high solar concentration. • A transition to a radiatively limited regime is predicted below 200 K. [ABSTRACT FROM AUTHOR]

Interband photoluminescence (PL) and stimulation emission (SE) from HgTe/HgCdTe quantum well (QW) heterostructures are studied in 5–20 µm wavelength range in regard to long‐wavelength lasing applications. The authors obtain carrier lifetimes using time‐resolved photoconductivity measurements and show that the dominating mechanism of carrier recombination changes from the radiative process to the non‐radiative one as the bandgap is decreased, limiting the "operating" temperature for SE. The authors suggest that decreasing the QW width should reverse the balance in carrier recombination in favor of radiative processes and demonstrate 75 K improvement in the "operating" temperature in structure with narrower QW. [ABSTRACT FROM AUTHOR]