Hui-Tang Shen

Carrier dynamics study of the temperature- and excitation-dependent photoluminescence of InAs/GaAs quantum dots

We investigate the effects that the carrier dynamics have on the temperature and excitation intensity dependence of the photoluminescence (PL) of self-assembled InAs∕GaAs quantum dot heterostructures having different size uniformities. We propose a rate equation model that takes into account the dot size distribution, the random population of density of states, and all of the important mechanisms of carrier dynamics, including radiative and nonradiative recombinations, thermal escaping and relaxing, and state filling effects. We used this model to simulate the PL spectra obtained from our samples; the results agree well with the measured data. We discuss in detail our quantitative calculations of the corresponding mechanisms of the thermal redistribution and state filling effects. These mathematical analyses provide distinct explanations for the phenomena we observed in the temperature- and incident power-dependent PL spectra of samples having different size uniformities.

Anomalous excitation dependence of electroluminescence in InGaN∕GaN light-emitting diodes

We have systematically investigated the anomalous excitation dependence of the electroluminescence (EL) in InGaN∕GaN multiple-quantum-well light-emitting diodes over a temperature range from 300to20K. Initially, an increase in the emission intensity occurred upon decreasing the temperature, until a maximum was reached at the temperature Tm. A blueshift in the position of the EL peak was followed by a redshift that occurred at the crossover temperature Tc. Both of these characteristic temperatures correlate with the presence of statistic microbarriers arising from potential inhomogeneity. The higher the In content incorporated into the heterobarriers, named multiple quantum barriers, the lower the values of Tm and Tc obtained from the spectral observations; this phenomenon implies an augmentation in the microscopic nonradiative transport through the microbarriers. An increase in the injection current also led to decreases in both of these characteristic temperatures. In addition, a functional correlation e...

Effect of multiquantum barriers on performance of InGaN∕GaN multiple-quantum-well light-emitting diodes

In this paper we demonstrate that the improvement in the emission intensity afforded by the introduction of multiquantum barrier (MQB) structures in an InGaN/GaN multiple-quantum-well (MQW) light-emitting diode (LED) is attributable to increased excitation cross sections. Over the temperature range from 300 to 20 K, the excitation cross sections of the MQW emissions possessing MQB structures were between 9.6 × 10-12cm2and 5.3 × 10-15cm2, while those possessing GaN barriers were between 8.1 × 10-12cm2and 4.5 × 10-15cm2. We found, however, that the figure of merit for the LED light output was the capture fraction of the cross section; we observed that the dependence of the optical intensity on the temperature coincided with the evolution of the capture fraction. This analysis permitted us to assign the capture cross-section ratios at room temperature for the MQWs with MQBs and with GaN barriers as 0.46 and 0.35. Furthermore, the MQW system possessing well-designed MQB structures not only exhibited the thermally insensitive luminescence but also inhibited energetic carrier overflow.

Electroluminescence Phenomena in InGaN/GaN Multiple Quantum Well Light-Emitting Diodes with Electron Tunneling Layer

The phenomena of electroluminescence in InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) with an n-AlGaN layer and a superlattice of 10 periods of InGaN (10 A)/GaN (15 A) serving as the electron tunneling layer (ETL) have been investigated in detail over a broad temperature range from 20 to 300 K at various injection currents. Compared with conventional LEDs with a well-designed ETL, quantum efficiency and temperature insensitivity are found to be improved when an n-AlGaN layer is inserted. This is attributed to the localization effect of the n-AlGaN layer being stronger than that of the ETL layer, as analyzed using the Varshini formula and band-tail model. Nevertheless, the inserted ETL layer with the purpose of improving the carrier injection into the active layer not only increases the carrier recombination quantity, which leads to a marked increase in output light emission intensity, but also reduces the light emission intensity compared with sample with the n-AlGaN layer. Consequently, inserting a blocking layer between an active layer and a p-GaN layer may increase the output light emission intensity of the sample with an ETL.

Characterization of the carrier localization confinement for InGaN/GaN multiple quantum well heterostructures with hydrogen-flow treatments

Major developments in group-III nitride semiconductors have led to the commercial production of InGaN-based blue/green multiple quantum well (MQW) laser diodes (LDs) and light-emitting diodes (LEDs) for use in varied applications. The main approaches have been adopted to meet the increasing demands for improved efficiency in modern optoelectronic devices; enhancing the light extraction and the quantum efficiency. In this work, the improvement of carrier localization confinement in InGaN/GaN multiple quantum well structures has been achieved by introducing hydrogen-flow treatment into the growth procedures. To characterize the radiative recombination mechanisms in the active layers, the temperature-dependent photoluminescence (PL) of InGaN/GaN MQW structures have been measured. It has been found the strong temperature-dependent blueshift of the emission peak energy for the conventional MQW sample due to band filling effect. As the temperature increased, for the MQW sample with hydrogen-flow treatment, it has been found the emission peak of PL spectra exhibited an obvious red-blue-red shift, i.e., S-shaped shift. By introduction of hydrogen flow during the growth procedures, it has been expected not only to encourage atom coherence motions tend to three-dimension cluster formations but also to provide a stronger localization confinement ability to enhance exciton radiative recombinations in the band tail of the density of states. From the Arrhenius plot of PL intensity, compared with the value of 120 meV achieved for the conventional MQW sample, the higher activation energy value of 300 meV for the MQW sample with hydrogen-flow treatment implies that there was better confinement ability for the excess charge carriers.

Anomalous Optical Characteristics of Carrier Transfer Process in Quaternary AlInGaN Multiple Quantum Well Heterostructure

The carrier-transport characteristics of quaternary AlInGaN heterosystems are studied in-depth using photoluminescence measurements. Based on Singhs model, a higher degree of disorder in quaternary AlInGaN heterostructures is observed to manifest not only the extension of static microbarrier width, but also the enhancement of carrier localization effects. To provide a clear picture of the random configuration of the carriers photogenerated in quaternary AlInGaN heterosystems, the thermodynamic quantities, i.e., the transition enthalpy ΔH and the transition entropy ΔS, describing the spontaneous fluctuations in the irreversible generation-recombination processes increased with temperature. It is found that the anomalous temperature-dependent phenomena can be attributed to the carrier-thermalization processes. The narrow interlayer distance of an AlInGaN system facilitates thermally excited carrier redistribution. However, due to the inhibition of photocarrier transfers, AlInGaN heterostructures with wider interlayer spacing exhibit more temperature insensitivity.

Observations of electrical and luminescence anomalies in InGaN∕GaN blue light-emitting diodes

Unique correlations between the electrical and optical characteristics of InGaN∕(In)GaN multiple quantum-well light-emitting diodes (LEDs) were investigated over a broad range of temperatures. The dependence of nonunity ideality factors extracted from the current-voltage analysis on temperature determines the carrier-transport mechanisms in the heterodevices. The pseudotemperatures To for the LEDs with multiquantum barriers and with GaN barriers were found to be 945 and 1385K, respectively, at temperatures of 180–300K while having values of 1195 and 2720K below about 180K. Correspondingly, the temperature-dependent electroluminescence observations suggest that the To anomaly caused the spectral intensity to deteriorate.

Characterization of the carrier dynamics and interface-state charge fluctuations in quaternary AlInGaN multiple quantum well heterostructures

Nitride-based light-emitting diodes (LEDs) have recently attracted to understand the emission mechanisms in novel multiple quantum well (MQW) heterosturctures. To understand substantially the unique spectral response, it is necessary to examine the carrier transport behavior. In this work, we studied the unique correlations between the carrier dynamics and optical characteristics of the quaternary AlInGaN MQW heterostructures with different barrier widths. It has been found that the photoluminescence peak energy of quaternary AlInGaN MQW blueshifts when decreasing the barrier width. This is attributed to the redistribution among the well and barrier of the strong electrostatic fields induced by polarization effect. It resulted in not only the diminutions of the charge density induced by piezoelectric field, but also the increments of the interface-state charge distribution from the collective influence of alloy disorder and interface roughness. We resort the Arrhenius plots to demonstrate the localized effect originated from indium fluctuation. Our results show the exciton-localization effect can be enhanced monotonically by increasing the barrier widths. On the other hand, we corroborated the surface charge density increased while increasing barrier widths between the epitaxial layers in this investigation.

The effect of junction temperature on the optoelectrical properties of green InGaN/GaN multiple quantum well light-emitting diodes

In this work, thermal effects on the optoelectrical characteristics of green InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) have been investigated in detail for a broad temperature range, from 30oC to 100oC. The current-dependent electroluminescence (EL) spectra, current-voltage (I-V) curves and luminescence-current (L-I) curve have been measured to characterize the thermal-related effects on the optoelectrical properties of the InGaN/GaN MQW heterostructures. Experimentally, both the forward voltages decreased with slope of -2.6 mV/K and the emission peak wavelength increased with slope of +4.5 nm/K with increasing temperature, indicating a change in the contact resistance between the metal and GaN layers and the band gap shrinkage effect. With increasing injection current, it has been found the strong current-dependent blueshift of -0.048 nm/mA in EL spectra. It was attributed to not only the stronger band-filling effect but also the enhanced quantum confinement effect, resulted from the piezoelectric polarization and the spontaneous polarization in InGaN/GaN heterostructures. The junction temperature calculated by forward voltage was from 25.6 to 14.5oC and by emission peak shift was from 22.4 to 35.6oC.

Influences of Multiquantum Barriers on Carrier Recombination in InGaN/GaN Multiple-Quantum-Well Light-Emitting Diodes

The influences of multiquantum barriers (MQBs) on the carrier confinement and carrier recombination in blue InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) have been investigated in depth over a broad range of temperatures from 20 to 300 K. Time-resolved photoluminescence (TRPL) temporal decay was measured to examine the dynamics of the carriers in both devices. The exciton recombination times of the blue emission are 2.81 and 4.11 ns for the QWs with MQB and GaN barriers, respectively. The former is a reasonable value for the radiative recombination in the structure with MQBs, and results from the enhancement of the exciton confinement. It was found that a device with an MQB structure exhibited higher emission intensity as well as lower temperature sensitivity than the conventional MQW LEDs. The improvement of the quantum efficiency for the MQB device was attributed to the fact that the enhancement of the excitons was confined in the MQW region and inhibited the carrier overflow into the GaN region.