Multiple Quantum Wells Light-emitting Diodes Research Articles

Effect of SiN Treatment on Optical Properties of In x Ga1−x N/GaN MQW Blue LEDs

In this paper, Si-doped GaN template layer and nitride-based multiple quantum well (MQW) light-emitting diodes (LEDs) with conventional GaN buffer layer and GaN buffer layer using SiN treatment were elaborated by metalorganic vapor phase epitaxy (MOVPE). On both kinds of structures, five InxGa1−xN/GaN quantum wells were deposited simultaneously in identical growth conditions. GaN template layer defects that influence on the growth of InxGa1−xN/GaN MQW LEDs were systematically studied by means of scanning electron microscopy, high-resolution x-ray diffraction, temperature-dependant photoluminescence measurement, and electroluminescence. It is shown that optical properties of InxGa1−xN/GaN MQWs depend on the defect density of elaborated templates. Thereafter, we report an enhancement of the emission of blue MQW LEDs using SiN treatment, compared to the MQW LED emissions deposited on a conventional GaN buffer layer.

Properties of a CdZnO/ZnO multiple quantum-well light-emitting diode

A CdZnO/ZnO multiple quantum-well light-emitting diode (LED) structure was successfully grown by using plasma-assisted molecular beam epitaxy on a p-GaN template that had been grown by using metal-organic chemical-vapor deposition on a c-sapphire substrate. The properties of the sample were characterized by using high-resolution X-ray diffraction, transmission electron microscopy, and temperature-dependent photoluminescence measurements. The light output performance of the CdZnO/ZnO QW LED device was also investigated in detail by using I-V and electroluminescence spectral measurements. The characterization showed that our CdZnO/ZnO QW LED structure had good crystalline quality and weaker carrier localization. Owing to the heterojunction structure, the I-V curve indicated that the LED device had a higher turn-on voltage and series resistance. The EL measurement demonstrated that for our LED device’s optoelectronic characteristic, the carrier-screening effect played the dominant role in the emission-energy blue-shift mechanism, and the broadening of the emission energy width was mainly ascribed to the band-filling effect. Without a special heat sinking, the L-I curve exhibited slight efficiency droop after 30 mA.

Efficiency Enhancement of InGaN MQW LED Using Compositionally Step Graded InGaN Barrier on SiC Substrate

In this paper, we designed and simulated a silicon carbide (SiC) technology, InGaN multiple quantum well (MQW) light-emitting diode (LED) using a compositionally step graded (CSG) InGaN barrier in the active region. The optical power, internal quantum efficiency (IQE), and carrier distribution profile in the device are theoretically studied by Sentaurus TCAD simulation. The CSG InGaN barrier LED shows substantial improvement in IQE when comparing the structures with conventional GaN barrier or InGaN barrier. This improvement is mainly due to the enhanced hole transportation/injection and modified band bending by polarization effect. The results show an excellent agreement with the experimental data. Moreover, the lattice-matched SiC substrate technology increases the radiative recombination rate in the InGaN MQW LEDs. The optical output and peak IQE obtained is 62 mW and 76.5%, respectively, for the proposed LED emitting at 450 nm wavelength at an injection current of 200 mA. This enhanced performance of the SiC technology makes it a good alternative for sapphire substrate in commercial lighting applications.

A modified ABC model in InGaN MQW LED using compositionally step graded Alternating Barrier for efficiency improvement

In this paper, Multiple Quantum Well (MQW) Light-Emitting Diodes (LEDs) with compositionally step graded (CSG) Alternating Barriers (AB) of InGaN-AlGaN with p-doped GaN barrier is designed and analysed. The improved crystal structure and modified band bending in the device enhances the carrier confinement and diminishes the polarization-related efficiency reduction. Furthermore, the good crystalline quality increases the hole injection and transportation; this significantly improves the radiative recombination rate and reduces the non-radiative recombination as well as carrier leakage out of the active region. Simulation result show mitigated efficiency droop of 3% and light output power of 1500 mW at the injection current of 500 mA. A modified ABC model is also developed to model the carrier leakage mechanism at high injection current density. In the model, total carrier leakage currents from the active region due to thermionic emission and electron overflow at high injection current are considered. Also, the obtained result of the modelled conventional LED shows a good fit with experimental data. Moreover, the SiC substrate technology in the design is attributed with improved crystal structure, reduced polarization effect and thermal conductivity, which improve the optical performance of the device.

Overshoot effects of electron on efficiency droop in InGaN/GaN MQW light-emitting diodes

To evaluate electron leakage in InGaN/GaN multiple quantum well (MQW) light emitting diodes (LEDs), analytic models of ballistic and quasi-ballistic transport are developed. With this model, the impact of critical variables effecting electron leakage, including the electron blocking layer (EBL), structure of multiple quantum wells (MQWs), polarization field, and temperature are explored. The simulated results based on this model shed light on previously reported experimental observations and provide basic criteria for suppressing electron leakage, advancing the design of InGaN/GaN LEDs.

Current density dependence of transition energy in blue InGaN/GaN MQW LEDs

AbstractIn this paper we report on the transition energy in InGaN/GaN multiple quantum well (MQW) light emitting diodes (LEDs) under various injection current density from 2 A/cm2 to 200 A/cm2. Various effects including strain, quantum confined Stark effect (QCSE), screening effect by carriers, bandgap renormalization, Stokes‐like shift, band‐filling effect, and quantum levels in triangular quantum wells, are considered quantitatively and analyzed comprehensively. By comparing these effects altogether, we found that when the In‐content in quantum wells is fixed, the transition energy is mainly determined by QCSE and quantum level energy, between which QCSE overweighs the other. The transition energy shift with current density is also mainly governed by the screening effect of QCSE, with detailed competition between band tailing and filling. Additional effect, the coupling between adjacent quantum wells, is investigated in this paper. The coupling between quantum wells with various barrier thicknesses is compared. By calculating the wavefunctions self‐consistently, it is found that for InGaN/GaN MQWs with 5 nm quantum barriers (QBs), the coupling of electron wavefunctions leads to about 2 meV difference in transition energy. While for the MQWs with 3 nm QBs, the influence of electron wave‐function overlap on transition energy is 12.9 meV, which is more significant than that of 5 nm QB case. However, for hole wavefunctions, the coupling effect is too small to be considered, which is mainly due to the much larger effective mass. (© 2015 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Room temperature spin injection into (110) GaAs quantum wells using Fe/x-AlOx contacts in the regime of current density comparable to laser oscillation

We investigate the electrical spin injection into (110) GaAs single quantum wells (SQWs) and multiple quantum wells (MQWs) using light-emitting diodes (LEDs) having Fe/crystalline-AlOx (x-AlOx) tunnel barrier contacts. A degree of circular polarization (Pc) of 5.0% is obtained for the SQW LED at 4 K with the current density of 1 kA/cm2 which is comparable to that for the laser oscillation in vertical-cavity surface-emitting lasers (VCSELs). On the basis of electron spin relaxation time and carrier lifetime in the (110) GaAs SQW measured by time-dependent photoluminescence and the value of Pc = 5.0%, the degree of spin polarization of initially injected electrons (P0) in the SQW is estimated to be 6.6% at 4 K. By using the MQW LED having a much stronger electroluminescence, a Pc value of 2.6% is obtained at room temperature (RT) with the current density of 1.5 kA/cm2. The temperature and current density dependences of Pc are found to be weak in both the SQW and MQW LEDs. The estimated P0 of 9.3% at RT suggests that the Fe/x-AlOx contacts can be used for the RT electrical spin injection for spin-controlled VCSELs.

Hole injection from the sidewall of V-shaped pits into c-plane multiple quantum wells in InGaN light emitting diodes

The role which the V-shaped pits (V-pits) play in InGaN/GaN multiple quantum well (MQW) light emitting diodes (LEDs) has been proposed to enable the formation of sidewall MQWs, whose higher bandgap than that of the c-plane MQWs is considered to act as an energy barrier to prevent carriers from reaching the dislocations. Here, with increasing proportion of current flowing via the V-pits, the emission of the c-plane MQWs broadens across the short-wavelength band and shows a blueshift successively. This phenomenon is attributed to hole injection from the sidewall of V-pits into the c-plane MQWs, which is a new discovery in the injection mechanism of InGaN/GaN MQW LEDs.

The difference in efficiency droop behaviors of two InGaN/GaN multiple-quantum-well green light-emitting diodes with modified structural parameters

Two different sets of structural parameters are used to fabricate InGaN/GaN multiple quantum well (MQW) green light-emitting diodes (LEDs) which have nearly the same light emission wavelength. It is found that, compared with the thin-well but high-In-content MQW LED, the efficiency droop of InGaN LED with the thick-well but low-In-content MQWs is less significant. Such reduction of droop may be attributed to the less electron overflowing, larger volume of active region and weaker delocalization effect, induced by the thicker well layers and lower In content in the latter.

White emission from non-planar InGaN/GaN MQW LEDs grown on GaN template with truncated hexagonal pyramids.

Non-planar InGaN/GaN multiple quantum well (MQW) structures are grown on a GaN template with truncated hexagonal pyramids (THPs) featuring c-plane and r-plane surfaces. The THP array is formed by the regrowth of the GaN layer on a selective-area Si-implanted GaN template. Transmission electron microscopy shows that the InGaN/GaN epitaxial layers regrown on the THPs exhibit different growth rates and indium compositions of the InGaN layer between the c-plane and r-plane surfaces. Consequently, InGaN/GaN MQW light-emitting diodes grown on the GaN THP array emit multiple wavelengths approaching near white light.

Effect of Si doping in barriers of InGaN/GaN multiple quantum wells on the performance of green light-emitting diodes

Green InGaN/GaN multiple quantum wells (MQWs) light-emitting diodes (LEDs) are of high In composition in well layers, a condition which results in large lattice mismatch between InGaN well layers and GaN barrier layers and hence boosts up the quantum-confined Stark effect (QCSE). Screening of the polarization electric fields within MQWs can effectively improve the internal quantum efficiency (IQE). This work investigates the effect of Si doping in barrier layers on the performance of green InGaN/GaN MQWs LEDs. It is revealed that a suitable doping density of Si (3.4 × 1016 cm−3) in barrier layers can promote the interfacial abruptness between the barrier and the well layers, reduce the In-composition fluctuation, and effectively screen the polarization electric fields. At the same time, suitable Si doping in barrier layers improves the lateral current spreading, alleviating the current crowding effect which is responsible for local heating and efficiency droop. However, overdoping of Si in barrier layers causes a series of negative effects, such as deteriorated interfacial quality and large current leakage. An effective approach for improving the performance of green LEDs is hence proposed.

Research on emission transition mechanisms of InGaN/GaN multiple quantum well light-emitting diodes using low-frequency current noise

In this paper, we measure the emission transition mechanisms in InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LED) using low-frequency current noise from 0.1 to 10 mA. According to the characteristics of the low-frequency current noise and the emission mechanisms of InGaN/GaN LEDs, we study the relationships between low-frequency current noise and current flows through the LEDs. Conclusions indicate that the low-frequency current noise is increased with the increasing current from 0.1 to 10 mA. With a lower current (I10 mA) it is the 1/f noise that dominates in LEDs, so there exists an emission transition mechanism in InGaN/GaN MQW LEDs between 0.1 and 10 mA, showing that the mechanism of the carrier recombination changes from non-radiative recombination to a stable fluctuation of carrier numbers. Conclusions of this paper provide an effective method to characterize the emission transition mechanisms, optimize the design of LED so as to improve the quantum efficiency for InGaN/GaN MQW LEDs.

Efficient charge carrier injection into sub-250 nm AlGaN multiple quantum well light emitting diodes

The design and Mg-doping profile of AlN/Al0.7Ga0.3N electron blocking heterostructures (EBH) for AlGaN multiple quantum well (MQW) light emitting diodes (LEDs) emitting below 250 nm was investigated. By inserting an AlN electron blocking layer (EBL) into the EBH, we were able to increase the quantum well emission power and significantly reduce long wavelength parasitic luminescence. Furthermore, electron leakage was suppressed by optimizing the thickness of the AlN EBL while still maintaining sufficient hole injection. Ultraviolet (UV)-C LEDs with very low parasitic luminescence (7% of total emission power) and external quantum efficiencies of 0.19% at 246 nm have been realized. This concept was applied to AlGaN MQW LEDs emitting between 235 nm and 263 nm with external quantum efficiencies ranging from 0.002% to 0.93%. After processing, we were able to demonstrate an UV-C LED emitting at 234 nm with 14.5 μW integrated optical output power and an external quantum efficiency of 0.012% at 18.2 A/cm2.

Effect of O Impurity on the Properties of InGaN/GaN Multiple Quantum Well and Light Emitting Diode Structures

InGaN/GaN multiple quantum well (MQW) and light emitting diode (LED) structures with different GaN barrier growth temperatures have been grown by metalorganic chemical vapor deposition (MOCVD). Atomic Force Microscopy (AFM)-Conductive AFM (CAFM) analysis has been performed to study the submicron scale structural and electrical properties of the LED structures. The effect of high O impurity concentration, introduced during the low temperature barrier layer growth, on the MQW LED structure has been explored. It is observed that (VGa-ON)2- point defects formed in the MQW layer serve as deep-level traps and lead to defect-assisted tunneling. Increasing barrier growth temperature decreases O impurity incorporation and thus improves the device performance.

Spectral competition of chirped dual-wavelength emission in monolithic InGaN multiple-quantum well light-emitting diodes

In this work, spectral competition of chirped dual-wavelength emission in monolithic InGaN multiple-quantum well (MQW) light-emitting diodes (LEDs) is studied numerically. In addition to the crystalline quality that is generally desired for good LED performance, the simulation results show that effective suppression of piezoelectric polarization is another key issue toward the realization of efficient monolithic dual-wavelength emission in InGaN MQW LEDs. With appropriately tailored configuration of MQW active region, the detrimental effects of hetero-interfaces and polarization charges are diminished and thereby balanced dual-wavelength emission can be achieved.

Effects of ${\hbox{H}}_{{2}}$ in GaN Barrier Spacer Layer of InGaN/GaN Multiple Quantum-Well Light-Emitting Diodes

We demonstrate the optoelectrical characteristics of thick well short-period InGaN/GaN multiple-quantum-well (MQW) light-emitting diodes (LEDs) with H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> in GaN barrier spacer layer. Introducing ramped H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> in the GaN barrier spacer layer creates a wide range of severe well thickness variation randomly distributed in the thick InGaN well. The thickness-fluctuated InGaN well would effectively increase the carrier concentration in the region of the thick InGaN well region during the current injection. Moreover, the ramped H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> in GaN barrier spacer layer would improve the interface and crystal quality of thick well short-period InGaN/GaN MQWs LEDs. Therefore, compared with traditional long-period InGaN/GaN MQW LEDs, thick well short-period InGaN/GaN MQW LEDs with fluctuated InGaN well thickness enhance output power (25.6% at 20 mA) and improve efficiency droop from 55.0% to 36.7%.

Simulation of InGaN/GaN light-emitting diodes with a non-local quantum well transport model

Blue InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) are simulated by the APSYS software with a non-local quantum well transport model which is used to describe the phenomenon that carriers can fly over the quantum wells directly. The simulation results based on this model are in good agreement with the experiment and show its significant influence on the output power, carrier transport, peak wavelength and current crowding effect of the InGaN/GaN MQW LEDs, indicating that the non-local quantum well transport plays an important role in these devices.

Performance of InGaN/GaN MQW LEDs Using Ga-Doped ZnO TCLs Prepared by ALD

Heavily Ga-doped ZnO (n±-GZO) films prepared by atomic layer deposition were used as transparent conducting layers (TCLs) on InGaN/GaN multiple quantum well (MQW) light emitting diodes (LEDs). It was found that N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -annealed n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -GZO-coated InGaN/GaN MQW LEDs exhibited reduced forward voltage and enhanced light extraction under certain conditions. A forward voltage of 3.1 V at 20 mA was achieved for a 400°C N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -annealed n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -GZO-coated InGaN/GaN MQW LED with a specific contact resistance of the n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -GZO on p-GaN contact being 4.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> Ω cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Compared to the same InGaN/GaN MQW LED structure with a commercial grade indium tin oxide (ITO) TCL, the 400 °C N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -annealed n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -GZO-coated InGaN/GaN MQW LED shows an increment of light output power by 15% at 20 mA. It is believed that the enhanced light extraction of the n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -GZO-coated InGaN/GaN MQW LED is a consequence of a higher refractive index of n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -GZO than that of ITO.

Influence of polarity on carrier transport in semipolar (2021¯) and (202¯1) multiple-quantum-well light-emitting diodes

We investigate the influence of polarity on carrier transport in single-quantum-well and multiple-quantum-well (MQW) light-emitting diodes (LEDs) grown on the semipolar (202¯1) and (2021¯) orientations of free-standing GaN. For semipolar MQW LEDs with the opposite polarity to conventional Ga-polar c-plane LEDs, the polarization-related electric field in the QWs results in an additional energy barrier for carriers to escape the QWs. We show that semipolar (2021¯) MQW LEDs with the same polarity to Ga-polar c-plane LEDs have a more uniform carrier distribution and lower forward voltage than (202¯1) MQW LEDs.

Enhancement of light-emission efficiency of ultraviolet InGaN/GaN multiple quantum well light emitting diode with InGaN underlying layer

Two ultraviolet InGaN/GaN light emitting diodes (LEDs) with and without InGaN underlying layer beneath the multiple quantum wells (MQWs) were grown by metal-organic vapor phase epitaxy. Based on the photoluminescence excitation measurements, it was found that the Stokes shift of the sample with a 10-nm-thick In0.1Ga0.9N underlying layer was about 64 meV, which was smaller than that of the reference sample without InGaN underlying layer, indicating a reduced quantum-confined Stark effect (QCSE) due to the decrease of the piezoelectric polarization field in the MQWs. In addition, by fitting the photon energy dependence of carrier lifetime values, the radiative recombination lifetime of the sample with and without InGaN underlying layer were obtained about 1.22 and 1.58 ns at 10 K, respectively. The shorter carrier lifetime also confirmed that the QCSE in the MQWs was weakened after inserting the InGaN underlying layer. In addition, although the depth of carrier localization in the sample with InGaN underlying layer became smaller, the nonradiative recombination centers (NRCs) inside it decreased, and thus suppressed the nonradiative recombination process significantly according to the electroluminescence measurement results. Compared to the reference sample, the efficiency droop behavior was delayed in the sample with InGaN underlying layer and the droop effect was also effectively alleviated. Therefore, the enhanced light-emission efficiency of ultraviolet InGaN/GaN MQW LEDs could be attributed to the decrease of QCSE and NRCs.