Jen-Cheng Wang

Anomalous luminescence phenomena of indium-doped ZnO nanostructures grown on Si substrates by the hydrothermal method

In recent years, zinc oxide (ZnO) has become one of the most popular research materials due to its unique properties and various applications. ZnO is an intrinsic semiconductor, with a wide bandgap (3.37 eV) and large exciton binding energy (60 meV) making it suitable for many optical applications. In this experiment, the simple hydrothermal method is used to grow indium-doped ZnO nanostructures on a silicon wafer, which are then annealed at different temperatures (400°C to 1,000°C) in an abundant oxygen atmosphere. This study discusses the surface structure and optical characteristic of ZnO nanomaterials. The structure of the ZnO nanostructures is analyzed by X-ray diffraction, the superficial state by scanning electron microscopy, and the optical measurements which are carried out using the temperature-dependent photoluminescence (PL) spectra. In this study, we discuss the broad peak energy of the yellow-orange emission which shows tendency towards a blueshift with the temperature increase in the PL spectra. This differs from other common semiconductors which have an increase in their peak energy of deep-level emission along with measurement temperature.

Cross sections for the investigation of the electroluminescence excitation of InGaN∕GaN quantum wells in blue light-emitting diodes with multiquantum barriers

Cross sections of the electroluminescence (EL) excitation of InGaN∕GaN multiquantum wells (MQWs) in blue light-emitting diodes (LEDs) with multiquantum barriers (MQBs) have been investigated. It was found that a device with an MQB structure exhibited a higher quantum efficiency, as well as higher temperature insensitivity, compared with the conventional MQW LEDs. A total cross section of 5.3×10−15cm2 was obtained for the MQB QWs, by fitting them to the measurement of the spectral intensity at room temperature; the value was 4.5×10−15cm2 for those devices with GaN barriers. Not only the EL excitation cross section for active region, but also the abnormal quantum efficiency evolutions were found to be a function of temperature. Moreover, they were in good agreement with the rate equation model.

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.

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

Exciton wavefunction coupled surface plasmon resonance for In-rich InGaN film with perforated aluminum cylindrical micropillar arrays

The optical characterization of excitons coupled with surface plasmon resonance (SPR) for InGaN/GaN heterostructures with perforated cylindrical micropillar arrays is investigated. We analyze the optical characteristics of excitons coupled with SPR for InGaN/GaN heterostructures with perforated cylindrical micropillars, as shown in measurements of the photoluminescence (PL) spectra over a broad range of temperatures between 20 and 300 K. From the temperature-dependent PL spectra, we observe the better SPR coupling effects, resulting in less carrier confinement in the InGaN energy band. The magnitude of the redshift of the emission peak shown by the sample with the coated aluminum (Al) pattern is larger than that shown by the sample with no metal film. This was due to the presence of more exciton coupling surface plasmons within the Al/InGaN interface. The enhancement of the PL intensity of the sample with the deposited Al pattern film can be attributed to a stronger SPR coupling interaction with the excitons. The experimental results indicate that a perforated Al cylindrical micropillar array can significantly affect carrier confinement, enhancing the quantum efficiency of Al/In-rich InGaN heterostructures due to the interaction of the SPR coupling effect between the InGaN quantum dot-like region and the Al film.

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.