QW LEDs Research Articles

Electroluminescence study of InGaN/GaN QW based p-i-n and inverted p-i-n junction based short-wavelength LED device using laser MBE technique

In the present work, the Laser Molecular Beam Epitaxy (Laser MBE) technique has been used for the fabrication of InGaN/GaN quantum well LEDs. A comparative study was performed for analysing the performance of LEDs with two different device structures i.e conventional (p-GaN/QW/i-GaN/n-GaN/Substrate) and inverted (n-GaN/QW/i-GaN/p-GaN/Substrate). The fabricated QW based Inverted p-i-n junction and conventional (p-i-n) LEDs exhibited non-linear rectifying current-voltage characteristics with a forward threshold voltage of about 0.7 V and 0.5 V, respectively. Room temperature electroluminescence was observed with increasing injection current in both conventional and inverted junction LED, where LEDs with 5 QW periods showed carrier localization effect and higher external quantum efficiency (EQE) at fixed injection current. The emission in fabricated QW LEDs was found to exhibit a spectral blue shift with increasing injection current. Convention LEDs (topmost layer: p-type GaN) exhibited higher luminous efficiency (i.e., 2.83 lm/W) than the Inverted LEDs (top most layer: n-type GaN) luminous efficiency (i.e., 1.56 lm/W). The efficiency droop observed is 14% in the conventional and 8% in the inverted LED device, respectively. This study focuses on the demonstration of room temperature electroluminescence in InGaN/GaN based LEDs grown using Laser MBE technique. The successful utilization of Laser MBE technique for growth of InGaN/GaN quantum wells can be a revolutionary step towards the establishment of a reliable, cost-effective, and ammonia-free process for fabrication of LED.

Performance improvement of green QW LEDs, for the different doping in the barriers, using InAlN interlayer and strain compensated AlGaN interlayer at the InGaN/GaN interface

Abstract Significant improvement of green emission using different doping in the barriers has been studied when the In0.2Al0.8N interlayer (IL) or Al0.8Ga0.2N IL is used at the interface of In0.26Ga0.74N/GaN QW LEDs. With the increment of doping in the barriers the transition energy(TE) and the square of the overlap of electron and hole wave functions ( M e h 2 ) is significantly increased. We have considered the maximum barrier doping 1 × 1019 cm−3, beyond that, it may reduce the optical properties. It is seen that the maximum M e h 2 is obtained when the Al0.8Ga0.2N IL is used. Furthermore, we have compared the TE and M e h 2 with current, considering the ILs and without ILs. The best result is obtained when Al0.8Ga0.2N IL is used and it is more than 2 times higher than without ILs of LEDs at higher current. We have seen that the radiative recombination rate of QW LEDs with Al0.8Ga0.2N IL is more than 5 times higher than without ILs at the higher current. Due to significant improvement of the radiative recombination rate, the internal quantum efficiency of the devices has also increased significantly for different ILs. The J-V characteristics also shown, the diode cut-in voltage is increased when the ILs are used.

Room temperature electroluminescence from Laser MBE grown Gallium nitride LEDs

Gallium Nitride (GaN) based inverted p-i-n junction and InGaN/GaN quantum well LEDs have been demonstrated using Laser Molecular Beam Epitaxy technique. The fabricated p-i-n junction and QW LEDs exhibited rectifying I-V characteristics with a forward threshold voltage of about 3.4 V and 4.5 V, respectively. The electrically injected LEDs showed room temperature electroluminescence, wherein inverted p-i-n junction LED exhibited single peaked UV emission around 365 nm wavelength and the InGaN/GaN QW LED demonstrated both UV (368 nm) and visible (430 nm) spectral components. The emission in QW LED was found to exhibit a spectral blue shift with increasing injection current.

Modeling and simulation of GaN based QW LED for UV emission

In this paper, a simulation approach has been adopted to study GaN based Multi Quantum Well Ultra Violet Light Emitting Diodes (MQWUV-LED). Under simulation, a 6 × 6 Kohn-Luttinger Hamiltonian has been solved to determine the wavefunctions, probability density of the charge carriers (for finding the localization of the charge carriers) and their discrete energy levels in the valence bands of the LED heterostructure. The normalized optical gain, luminous power and power spectral density for the device have been simulated. The well and barrier width effect on the luminous power and temperature effect on emission wavelength corresponding to maximum gain have also been studied. The outcome of the simulation suggests that the generated light has spectral wavelength peak in the UV range at 349 nm. The optical gain of the quantum wells near the n-side of the heterostructure is found slightly lower due to the poor mobility and injection of holes towards n-side.

A multispectral linear optical sweep source, based on tunable InGaN/GaN QW LEDs

Applications of multispectral sources are numerous. The previous forms of optical sources are continuously being replaced by efficient LEDs with an emphasis on III-nitrides, since it covers a very wide optical range. In this paper, we are proposing a multispectral linear optical sweep source, based on tunable InGaN/GaN QW LEDs. For linearization, a λ verses current curve of tunable InGaN/GaN QW LED, as obtained from our well established method of solution of Schrödinger and Poisson equations is considered as a model. The entire process of linearization is outlined in a simple form, for clear understanding. The linear sweep is so designed that it can be stopped at any λ,. i.e. color. Changing of the range of the spectral sweep through changes in the device parameters is pointed out. Procedures, necessary constituents and results are presented with suitable discussions. The multispectral ramp should be of much use for a wide range of applications.

Inclusion of Indium, with doping in the barriers of InxGa1-xN/InyGa1-yN quantum wells reveals striking modifications of the emission properties with current for better operation of LEDs

Although that the continuous tunability of InGaN/GaN QW LEDs, carries the promise of a significant impact in optoelectronics, the reduction of the square of the overlap of electron and hole wave functions (Meh2) in InGaN/GaN QW LEDs, under certain conditions, is a sizable problem, difficult to overcome. Theoretical investigations have been carried out on the incorporation of Indium (In) in the GaN barrier layers, with an aim of increasing the overlap of electron and hole wave functions. Rigorous studies through the self consistent solution of Schrödinger and Poison equations expose some new and striking results. With suitable doping, the inclusion of In in the barriers can increase Meh2 to more than two times that of a conventional InGaN/GaN QW LED. In in the barrier along with doping may be suitably utilized to tailor the transition energy and Meh2 with current density, as desired. The transition energy and the Meh2 may be made to have a positive or a negative slope with current density or they may be made fairly constant. This paper will outline the theoretical details, computational methodologies, the parameters used, and the striking new results with suitable depictions and discussions. These new information ought to be interesting for current optoelectronics.

Advantages of InGaN/InGaN quantum well light emitting diodes: Better electron-hole overlap and stable output

Shift of the emission spectrum and the efficiency loss with the operating current are the major challenges in the conventional InGaN/GaN QW LEDs. Several approaches are being tried by different research groups to overcome this hurdle, including, introduction of In in the barriers for which a rigorous study seemed essential. In this paper, our comprehensive and systematic studies on InGaN/InGaN quantum well light emitting diodes reveal that the overlap of electron and hole wave functions can be increased even at low operating currents by the introduction of In in the barriers. The changes of output emission for different In content in the barrier and various doping at different currents are reported, which show that the shift of the energy can be minimized for suitable choice of parameters. The emission and the overlap of electron and hole wave functions have a significant dependence on the doping in the barrier layers which may be used advantageously. Computations have been performed through the self-consistent solutions of Schrödinger and Poisson equations.

Analysis of defect-related inhomogeneous electroluminescence in InGaN/GaN QW LEDs

The inhomogeneous electroluminescence (EL) of InGaN/GaN quantum well light emitting diode structures was investigated in this study. Electroluminescence hyperspectral images showed that inhomogeneities in the form of bright spots exhibited spectrally blue-shifted and broadened emission. Scanning electron microscopy combined with cathodoluminescence (SEM-CL) was used to identify hexagonal pits at the centre of approximately 20% of these features. Scanning transmission electron microscopy imaging with energy dispersive X-ray spectroscopy (STEM-EDX) indicated there may be p-doped AlGaN within the active region caused by the presence of the pit. Weak beam dark-field TEM (WBDF-TEM) revealed the presence of bundles of dislocations associated with the pit, suggesting the surface features which cause the inhomogeneous EL may occur at coalescence boundaries, supported by trends in the number of features observed across the wafer.

(Invited) Spatially Resolved Study of the EQE Droop in InGaN QW LEDs: Interplay of Point Defects, Extended Defects, and Carrier Localization

Spatially-resolved electroluminescence (as a function of injection current) and spatially-resolved photoluminescence (as a function of excitation power) are used to investigate the external quantum efficiency (EQE) droop in InGaN QWs based blue LEDs. The spatially-resolved EL mapping allows us to study two most commonly observed but distinctly different droop behaviors on a single device, minimizing the ambiguity trying to compare independently fabricated devices. We conclude that the EQE ~ current curve reflects the interplay of three effects: nonradiative recombination through point defects, carrier localization due to either In composition or well width fluctuation, and nonradiative recombination of the extended defects, which is in fact common to various optoelectronic devices.[1] By comparing EL and PL mapping data, we show that individual singular sites exhibiting either particularly strong or weak emission in PL do not usually play any significant and direct role in the EQE droop. We introduce a two-level model that can capture the basic physical processes that dictate the EQE ~ current dependence and describe the whole operating range of the device from 0.01 – 100 A/cm2.[1] Y. Lin, Y. Zhang, Z. Q. Liu, L. Q. Su, J. H. Zhang, T. B. Wei, and Z. Chen, Appl. Phys. Lett. 101, 252103 (2012).In collaboration with Yue Lin, Zhiqiang Liu, Liqin Su, Jihong Zhang, Tongbo Wei, and Zhong Chen.

(Invited) Spatially Resolved Study of the EQE Droop in InGaN QW LEDs: Interplay of Point Defects, Extended Defects, and Carrier Localization

We offer a concise overview for the mechanisms on the external quantum efficiency (EQE) droop in InGaN QW LEDs. We believe that non-radiative recombination due to extended defects and the deficiency in the electron blocking layer are most likely two primary mechanisms responsible for the droop. We perform spatially resolved electroluminescence (EL) measurements with varying injection current on an InGaN QW LED. The analyses of the EL mapping data reveal a correlation between carrier delocalization and EQE droop. In addition, we observe that at very low current, there is virtually no correlation between the local EL intensity and peak energy, reflecting very low carrier mobility; whereas with increasing current, an anti-correlation between the intensity and energy is eventually established, indicating substantial changes in the carrier mobility and diffusion length with increasing current. The results suggest the material inhomogeneity occur in different scales.

Efficiency-Droop Suppression by Using Large-Bandgap AlGaInN Thin Barrier Layers in InGaN Quantum-Well Light-Emitting Diodes

The electrical and optical characteristics of InGaN quantum-well light-emitting diodes with large-bandgap AlGaInN thin barriers were analyzed with the consideration of carrier transport effect for efficiency droop suppression. The lattice-matched AlGaInN quaternary alloys with different compositions, thicknesses, and positions were employed as thin barrier layers (1-2 nm) surrounding the InGaN QW in LED structures. The increased effective barrier heights of AlGaInN thin barrier led to suppression of carrier leakage as compared to conventional InGaN QW LEDs with GaN barrier only. The current work provides a comprehensive simulation taking into consideration the carrier transport in self-consistent manner, and the finding indicated the use of thin layers of AlGaInN or AlInN barriers as sufficient for suppressing the droop in InGaN-based QW LEDs. The efficiency of InGaN QW LED with the insertion of lattice-matched Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.82</sub> In <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.18</sub> N thin barrier layers showed the least droop phenomenon at high current density among the investigated LEDs. The thickness study indicated that a thin layer (<; 2 nm) of large-bandgap material in the barrier region was sufficient for efficiency droop suppression.

Well width dependent luminescence characteristics of UV‐violet emitting GaInN QW LED structures

AbstractTemperature and excitation power dependent photoluminescence spectroscopy on GaInN UV‐violet LED structures allows to separate the influence of the quantum confined stark effect (QCSE) from carrier localization in In‐induced band tail states as well as to determine the internal quantum efficiency (IQE). A LED series with different quantum well widths (1‐3.7 nm) was grown by MOVPE on sapphire and low defect density (LDD) GaN templates. The photoluminescence data reveal, that the QCSE cannot be neglected even for low In‐content (&lt;10%) ultra violet LED structures. From temperature and laser power density dependent photoluminescence, the internal QE is evaluated. The data shows, that upon growth on LDD substrates, the internal QE is considerably increased for short wavelength (375‐400 nm) LEDs. (© 2008 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Time‐ and bias‐dependent effects in ZnCd(Mg)Se blue and red quantum well light emitting diodes studied by cathodoluminescence

AbstractLuminescence variations in MBE‐grown blue and red ZnCd(Mg)Se/InP QW LEDs have been studied by time‐resolved and bias‐dependent cathodoluminescence (CL) spectroscopy. CL from the red LED reversibly increased under forward bias and decreased under negative bias during several complete biasing cycles. CL from blue LEDs was bias dependent only during the first few cycles. The irreversible effects were QW CL intensity decreases after electron bombardment for the red LED and QW CL intensity enhancements seen as bright regions in CL images and dark regions in SE micrographs for blue LEDs. For the red LED, the bombarded regions were dark in CL images and had a dark dot and halo contrast in SE micrographs. The enhancements of CL intensity in the blue LEDs may result from buildup of internal fields due to persistent charge trapping in preexisting or electron bombardment created defects. (© 2004 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Cathodoluminescence, Electroluminescence, and Degradation of ZnCdSe Quantum Well Light Emitting Diodes

AbstractWe have studied luminescence and degradation of a ZnCdMgSe/ZnCdSe/ZnCdMgSe QW LED grown by MBE and lattice matched to its InP substrate. Results have been obtained from time-resolved and bias-dependent cathodoluminescence spectroscopy and from electroluminescence spectroscopy. Reversible and irreversible variations in luminescence intensity were observed during bias cycling and electron bombardment. Cathodoluminescence from the QW LED was modeled by calculating current generation and transport, bias-dependent energy levels, envelope wave functions, overlap integrals, and carrier escape rates. Results of these calculations agree qualitatively, but not quantitatively, with bias-dependent intensity and wavelength changes observed in the experiments.

1-15 MeV proton and alpha particle radiation effects on GaAs quantum well light emitting diodes [and QWIPs

Permanent radiation damage was investigated in GaAs quantum well light emitting diodes (QW LEDs) due to protons and alpha particles in the 1-15 MeV energy range. At room temperature, these devices under forward bias emit infrared radiation at 980 mm. Current-voltage (I-V) and light emission characteristics of these devices were studied as a function of fluence and energy of protons and alpha radiations. For a given particle, the radiation damage in these devices increased with increase in fluence at a given energy, and decreased with increase in energy of the radiation at a fixed fluence, for both protons and alpha particles. Further, the alpha particles were observed to be far more damaging than the protons. These results agree qualitatively with the principal concepts of dependence of permanent displacement damage on nonionizing energy loss (NIEL) in the device. They are however found to deviate from the widely accepted concepts of damage dependence on fluence and particle energy. Based on the decrease in damage with increase in energy observed in QW LEDs in the present work and the results of earlier workers on energy dependence of damage in GaAs LEDs, it is concluded that these QW LEDs are more radiation hard than the modern GaAs LEDs reported recently. Further, there is less device to device variability of radiation damage between the QW LEDs than in the double heterojunction LEDs. 3 MeV proton radiation effects on device characteristics of quantum well infrared photodetectors operating in the short wavelength infrared region are also reported.

Monolithic integration of a GaAs MESFET with a resonant cavity LED using a buried oxide layer

A new method to monolithically integrate a GaAs MESFET and a resonant cavity InGaAs QW LED is demonstrated. Current confinement in these two dissimilar devices is accomplished using a buried insulating layer formed by the thermal oxidation of AlAs. Fabrication and device performance under dc bias as well as modulation results are briefly described.

16% external quantum efficiency from planar microcavityLEDsat 940 nm by precise matching of cavity wavelength

High efficiency substrate emitting microcavity InGaAs/(Al)GaAs 3 QW LEDs are reported. The use of regrowth for cavity resonance tuning and its effect on device performance, are demonstrated. The best results obtained include external quantum efficiencies of 16%. At 5 mA, 1 mW of optical power is delivered with an intensity of 280 µW/sr.

6% external quantum efficiency from InGaAs/(Al)GaAssingle quantum well planar microcavity LEDs

High efficiency substrate emitting microcavity InGaAs/(Al)GaAs single QW LEDs are reported. The influence of the reflectivity of the bottom GaAs/AlAs DBR and cavity dimensions have been investigated. The best results obtained include peak external quantum efficiencies of 6.2%, and an intensity of 210 µW/steradian at 10 mA.

Spontaneous lifetime and quantum efficiency in light emitting diodes affected by a close metal mirror

The spontaneous lifetime and quantum efficiencies of dipoles placed close to a metal mirror are calculated. The importance of these effects is demonstrated in the recent experimental work on InGaAs-GaAs quantum-well light-emitting diodes (QW LEDs) in which the modulations rates were varied from 0.8 GHz to 1.4 GHz through the placement of a QW next to a Ag mirror (D.G. Deppe et al., 1990). Classical energy transfer theory is used to treat the spontaneous radiation of a QW in a semiconductor system. The spontaneous radiative lifetime and internal quantum efficiency are strongly affected by the location of the QW, which confines the dipoles, by the orientation of the dipole in the well, by the metal reflector material, and by the metal mirror thickness. In general, the internal quantum efficiency decreases when the dipole is closer to the metal, because of the nonradiative energy transfer from the dipole to the absorptive metal. The overall effect of closely spaced metal mirrors on the measurable external quantum efficiency, which is important for fast light emitting diode design, is presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Monolithic integration of an InGaAs/GaAs/AlGaAs strained layer SQW LED and GaAs MESFET using epitaxial lift-off

The successful realisation of a fully electrically interconnected optical and electronic device using the epitaxial lift-off technique is reported. The OEIC consists of a high output power InGaAs/GaAs/AlGaAs strained QW LED and a GaAS MESFET driver. The surface emitting LED shows an external quantum efficiency of 1.7%. The output/input ratio of the LED-FET combination was 54 μWV−1 sr−1.