Multiquantum Barrier Structure Research Articles

Design of Hole-Blocking and Electron-Blocking Layers in AlxGa1-xN-Based UV Light-Emitting Diodes

The band-engineered structure design for electron-blocking layer (EBL) and hole-blocking layer (HBL) in Al x Ga1– x N-based ultraviolet light-emitting diodes (UV LEDs) is performed and analyzed theoretically. Simulation results show that the severe polarization effect is efficiently mitigated and the downward-bended band profile of the EBL is improved when the EBL is designed with a graded-composition and multiquantum barrier (GMQB) structure. As a result, the capabilities of both electron confinement and hole injection, and also the light output power are promoted. On the contrary, for the HBL, the design of composition graded and/or multiquantum barrier structures reduces the effective potential barrier height for holes in the valence band and, consequently, causes a considerable hole overflow. The UV LED, thus, exhibits superior optical performance when the LED structure is simultaneously designed with a GMQB EBL and a bulk HBL.

Numerical Simulation of GaN-Based LEDs With Chirped Multiquantum Barrier Structure

The authors report the numerical simulation of GaN-based light-emitting diodes (LEDs) with either a conventional AlGaN electron blocking layer (EBL), uniform multiquantum barrier (UMQB) structure, or chirped multiquantum barrier (CMQB) structure. It is found that the 102-meV effective barrier height simulated from the LED with CMQB structure is larger than those simulated from the LEDs with a UMQB structure (90 meV) and with conventional AlGaN EBL (60 meV). With the large effective barrier height, it is found that LEDs with a CMQB structure exhibit smaller leakage current. It is also found that the maximum internal quantum efficiencies are 0.703, 0.842, and 0.887, for the LEDs with conventional EBL, UMQB structure, and CMQB structure, respectively. In addition, it is found that forward voltages simulated from the LEDs with CMQB structure and with UMQB structure are both smaller than that simulated from the LED with conventional AlGaN EBL. These results also agree well with the experimental data.

The effect of multi-quantum barrier structure on light-emitting diodes performance by a non-isothermal model

A multi-quantum barrier structure is employed as the electron blocking layer of light-emitting diodes to enhance their performance. Using the non-isothermal multi-physics-field coupling model, the internal quantum efficiency, internal heat source characteristics, spectrum characteristics, and photoelectric conversion efficiency of light-emitting diodes are analyzed systematically. The simulation results show that: introducing multi-quantum barrier electron blocking layer structure significantly increases the internal quantum efficiency and photoelectric conversion efficiency of light-emitting diodes and the intensity of spectrum, and strongly ensures the thermal and light output stability of light-emitting diodes. These results are attributed to the modified energy band diagrams of the electron blocking layer which are responsible for the decreased electron leakage and enhanced carrier concentration in the active region.

GaN-Based LEDs With a Chirped Multiquantum Barrier Structure

We report the fabrication of GaN-based blue light-emitting diodes (LEDs), which separately incorporate the three different electron blocking layers (EBLs), namely, a conventional AlGaN, a uniform multiquantum barrier (UMQB), and a chirped multiquantum barrier (CMQB). On the administration of 20 mA injection current, the corresponding LED output powers measured were 27.5, 27.2, and 25.4 mW for CMQB LED, UMQB LED, and LED, respectively, with a conventional AlGaN EBL. It was also found that the LED with CMQB EBL exhibited a significantly lower drooping effect and a smaller forward bias as compared with LEDs with a conventional AlGaN EBL and UMQB EBL.

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 300to20K, the excitation cross sections of the MQW emissions possessing MQB structures were between 9.6×10−12 and 5.3×10−15cm2, while those possessing GaN barriers were between 8.1×10−12 and 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.

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.

The effect of high temperature on the optical properties of InGaN/GaN blue light-emitting diodes with multiquantum barriers

The effect of the temperature on the characteristics of InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) with multiquantum barriers (MQBs) has been investigated in-depth over a broad range of temperatures from 200 to 370 K. It was found that if the device had an MQB structure it exhibited stronger blue emission as well as had a larger capture cross-section fraction than did the conventional MQW LEDs. This was due to the MQB configuration, which offered a large contribution to the exciton recombination; the adoption of an appropriate heterobarrier allows the achievement of improved LED performance. However, the unavoidable disordering formed in the In-rich MQB heterostructures caused not only EL intensity deterioration, but also rapid quenching rate at temperatures above 300 K.

Thermionic emission perpendicular to bulk and multiquantum AlxGa1−xInP barriers

A study on thermally activated currents across the bulk and multiquantum barrier (MQB) AlxGa1−xInP∕GaInP has been carried out and compared to experimental results from a series of n-i-n diodes over a range of temperatures. By considering the true quantum mechanical nature of the barriers, in contrast to the classical Richardson formalism, it is found that the alloy crossover strongly affects the transport properties of the material. The measured prefactor is found to decrease as Al content is increased. When applied to the MQB structures, the existing model fails to capture the experimental results.

High-power GaN-based blue-violet laser diodes with AlGaN∕GaN multiquantum barriers

AlGaN ∕ GaN multiquantum barriers (MQBs) were introduced into violet AlInGaN laser diodes with an InGaN multiquantum-well structure, resulting in drastic improvements in lasing performance. Comparing with conventional AlGaN single electron blocking layer (EBL), lower threshold current of 32mA and higher slope efficiency of 1.12W∕A at room temperature has been achieved by using the AlGaN∕GaN multiquantum barrier. This improvement implies that p-type AlGaN∕GaN MQBs are more effective in suppressing the overflow of electrons than p-type AlGaN single EBL. Effective barrier heights of the MQBs should be higher than the single EBL due to the quantum effect of MQBs and the enhancement of p-type doping efficiency. Additionally, the effect of strain on InGaN multiquantum wells from the single EBL can be reduced by using the AlGaN∕GaN MQBs structure.

Suppression of TE polarization by a multiquantum-barrier structure in sub-630-nm tensile-strained GaInP-AlGaInP QW lasers

Many applications require laser emission with stable single polarization. While stable transverse-electric (TE) emission is favored by several mechanisms and, consequently, is relatively easy to obtain, achieving stable transverse-magnetic (TM) emission is more difficult. This letter proposes a method for suppressing the TE polarization by using a multiquantum-barrier. In our experiments, the multiquantum-barrier structure was inserted into the p-type cladding of tensile-strained GaInP-AlGaInP quantum-well lasers emitting at wavelengths shorter than 630 nm. As a result, the TE emission was suppressed over a wide range of injection levels, operating temperatures, and device lengths, proving that the method is effective for achieving stable TM emission.

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.

Study of electroluminescence quenching in the InGaN/GaN blue diode with multi-quantum barrier structure

This work has demonstrated electroluminescence (EL) quenching of the InGaN/GaN quantum well (QW) diodes with multi-quantum barrier (MQB) structure at several temperatures from 20 to 300 K. Both high- and low-energy bands indicate a surprising tendency when the temperature and injection current are altered. Increasing temperature reduces the high-energy band and clearly increases the quantum efficiency of the low-energy band. The device with MQB improves the quantum efficiency throughout the whole temperature range and displays good agreement with the EL-integrated intensity in this study. The sample with an optimal MQB structure could significantly improve the quantum efficiency both at low temperature up to one order and room temperature up to 20% compared with the sample without the MQB structure.

Characterization of the carrier confinement for InGaN/GaN light emitting diode with multiquantum barriers

ABSTRACTElectroluminescence (EL) of InGaN/GaN multiquantum wells (MQW) in blue light-emitting diodes (LED) with multiquantum barriers (MQB) has been investigated. It was found that a device with an MQB structure exhibited better performance in carrier confinement, as well as higher temperature insensitivity, compared with the conventional MQW LEDs. The total cross sections ware obtained for those devices with and without MQBs, by fitting to the measurement of the spectral intensity. Not only the cross sections, but also the carrier lifetimes depending on temperature can therefore be mainly attributed to changes in Boltzmann population. All the detailed calculations are also agreement with the observations.

An investigation of multi-quantum barriers for band offset engineering in AlGaInP/GaInP lasers

In this paper, we present a critical study of a multi-quantum barrier (MQB) structure fabricated using the Al 0.5In 0.5P/Ga 0.5In 0.5P material system. The structure was optimised theoretically based on a single Γ band model. When an identical structure was placed in a quantum well laser device, no improvement in the threshold current was observed compared to QW laser structure without an MQB. Cross-sectional scanning tunnelling microscopy (STM) was used to assess the structural quality of the MQB structure for the first time. Factors affecting the limited barrier enhancement are discussed in terms of the interfacial quality of the Al 0.5In 0.5P/Ga 0.5In 0.5P interface in the superlattice of the MQB and the need to consider multi-valley transport. The paper highlights some of the fundamental problems that must be overcome for MQBs to be a viable method to improve red laser efficiency.

Effects of electron effective mass on the multiquantum barrier structure in AlGaInP laser diodes

The effects of electron effective mass on the multiquantum barrier (MQB) structure in AlGaInP LD has been systematically studied. Using a new effective mass model, we found that we can achieve an enhanced barrier height of 1.68U0 or 253 m eV. However, such an increase in barrier height is much smaller than the previous reported value obtained using a different electron effective mass model. We also found that the chirped MQB (CMQB) can only enhance the barrier height by 0.276U0 . On the other hand, the enhanced barrier height can reach 2.2U0 for a properly designed multistack MQB (MSMQB).

Evaluation of multiquantum barriers in bulk double heterostructure visible laser diodes

Three bulk (Al/sub x/Ga/sub 1-x/)/sub 0.5/In/sub 0.5/P double heterostructure (DH) lasers were designed and fabricated to examine the effect of incorporating multiquantum barrier (MQB) structures. The first laser, used as a reference, has a conventional structure, while the remaining lasers include MQB structures, one designed to achieve a virtual barrier of 75 meV, the other having a transmission window up to 100 meV above the bulk barrier height. Measurements show a reduction of up to 31% in the room temperature threshold current and an increase in characteristic temperature of 20 K by the inclusion of the optimized MQB structure in comparison with the reference laser. However, since the leaky MQB design also shows a significant room temperature improvement over the reference laser we suggest that the device improvements produced by the MQB structures are not solely due to the formation of a virtual barrier.

Characteristics of multistack multiquantum barrier and its application to graded-index separate confinement heterostructure lasers

The enhancement of electron barrier height by multistack multiquantum barrier structure is simulated using the transfer matrix method. The validity and feasibility of this concept is verified by the experimental results on GaAs-AlAs multistack multiquantum barriers. Based on the simulated results, both 0.78 and 1.3 /spl mu/m graded-index separate confinement heterostructure (GRIN-SCH) lasers with predicted enhanced carrier and optical confinements using graded multistack multiquantum barriers are designed. Lower threshold current, higher modulation bandwidth as well as higher characteristic temperature are expected for these lasers.

The control and modification of metal-semiconductor interfaces using multi quantum barriers

In this paper we report, for the first time, on the effect of placing a multi quantum barrier (MQB) near the gold-Al 0.3Ga 0.7As interface. A theoretical model has been developed to determine the effective barrier height for a given Schottky-MQB structure directly from the reflectivity calculated as a function of electron energy, RvE. The abruptness and shape of the RvE curve were used to characterise the enhancement efficiency owing to the presence of the MQB. Based on the results of this, a metal-Al 0.3Ga 0.7As sample containing an MQB structure ( Al 0.3 Ga 0.7 As GaAs ) was designed and grown by molecular beam epitaxy. A standard metal-Al 0.3Ga 0.7As Schottky barrier (with no MQB), was also grown for comparison. Gold Schottky contacts were formed and the barrier height measured, using a current-voltage technique, in-situ to avoid degradation on exposure to air. In addition, capacitance-voltage, and ballistic electron emission microscopy were used to further characterise the interfaces ex-situ. The experimentally measured barriers of both samples were found to be in excellent agreement with the theoretical predictions from the reflectivity curves. The dependence of the barrier height enhancement on the structure and composition of the MQB is also discussed.

Sidegating effect improvement of GaAs metal–semiconductor field effect transistor by multiquantum barrier structure

The sidegating effect in a GaAs metal–semiconductor field effect transistor is significantly suppressed by the use of a multiquantum barrier (MQB) structure as a buffer layer. The enhancement of potential barrier height of the MQB buffer layer is confirmed from the comparison with a heterostructure buffer layer device.

GaInAsP/InP Multi-Quantum Barrier (MQB) Grown by Chemical Beam Epitaxy (CBE)

A multi-quantum barrier (MQB) using the GaInAsP/InP system has been demonstrated for the first time. n-i-n tunneling diodes consisting of MQB and bulk barriers were grown by chemical beam epitaxy (CBE). From the measurement of 77 K current-voltage characteristics, the effective potential barrier height of 1.3 times the classical conduction band offset was obtained in the MQB structure.