Ya-Fen Wu

Displacement Reactions Between Metal Ions and Nitride Barrier Layer/Silicon Substrate

. This spontaneous reaction results in the deposition of metal, and a similar reaction takes place between the metal ions and the underlying silicon substrate. Three possible mechanisms are proposed and discussed in this study. The proposed mechanisms and the details of displacement reaction were elucidated by using Auger electron spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, nuclear magnetic resonance spectroscopy, inductively coupled plasma atomic emission spectrometer, Raman and infrared spectroscopy. TiN was found to oxidize with the formation of TiF6 22

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.

Effect of carrier hopping and relaxing on photoluminescence line shape in self-organized inas quantum dot heterostructures

A steady-state thermal model for carrier dynamics in self-organized InAs∕GaAs quantum dot heterostructure is presented. The effect of quantum-dot size distribution, density of states random population, thermal emission, retrapping, and relaxation are investigated. The temperature-dependent photoluminescence spectra obtained from two samples with different dot densities and size uniformities are simulated in this model and show a good agreement with experiment. The corresponding mechanisms of carrier dynamics related to the thermal redistribution and lateral transition of excited carriers as well as the observed phenomena resulting from these mechanisms, are discussed in detail.

Characterization of Nanocrystallites of InGaN/GaN Multiquantum Wells by High-Resolution X-ray Diffraction

We report on the properties of nanocrystallites in InGaN/GaN multiquantum wells with different indium contents. The electroluminescence (EL) spectra are examined over a broad range of temperatures. According to the band-tail-filling model, greater inhomogeneity of nanocrystallites size is obtained from the temperature-dependent EL peak energy for the sample with higher indium content. To verify the results, the measured high-resolution X-ray diffraction curves are analyzed by the Warren-Averbach analysis model. Based on the model, it is found that the sample with higher indium content exhibits a wider nanocrystallite size distribution. In addition, X-ray diffraction line profile analysis shows stronger internal strain in the high-indium-content sample. Injection current-dependent EL measurements are also carried out. An evident blueshift in the EL peak energy is observed with increasing current in the sample with higher indium content, suggesting a stronger quantum-confined-Stark effect and internal strain. The experimental results coincide with the inference given by the X-ray diffraction line profile analysis.

Temperature and Excitation Dependence of Photoluminescence Spectra of InAs/GaAs Quantum Dot Heterostructures

In this study we investigated the effects that the carrier dynamics have on the temperature- and excitation-intensity- dependent photoluminescence (PL) spectra of a self-assembled quantum dot heterostructure. A rate equation model is proposed to take into account the dot size distribution, the random population of density of states, state filling effects, and the important carrier transfer mechanisms for the quantum dot system, including carrier capture, relaxation, thermal emission, and retrapping. This model reproduces the PL spectra quite well. Our quantitative calculations of the behavior of the thermal emitting carriers under various incident power intensities within the temperature range 15 K-240 K explain the carrier transfer process quite reasonably for the quantum dot system. In addition, we discuss the thermal redistribution and state filling effects in detail in our analysis of the dependence of the PL spectra on the temperature and excitation power intensity applied to the sample. Index

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.