Green InGaN Research Articles

Investigation of green InGaN light-emitting diodes with asymmetric AlGaN composition-graded barriers and without an electron blocking layer

In this study, a green InGaN light-emitting diode with asymmetric AlGaN composition-graded barriers and without the use of an AlGaN electron blocking layer is presented to possess markedly enhanced optical and electrical performance. The simulation results show that the output power is increased by 10.0% and 33.2%, which corresponds to an increment of 7% and 29.4% in internal quantum efficiency, at 100 mA when the conventional GaN barriers are replaced by the asymmetric AlGaN composition-graded barriers and the commonly used AlGaN electron blocking layer is removed. The simulation results suggest that the improved device performance is due mainly to the markedly enhanced injection of holes into the active region.

Study of defects and lifetime of green InGaN laser diodes

AbstractInGaN quantum wells for direct green lasers with indium concentrations of about 30% are studied in respect to defect structure and influence of defects on device lifetime. A new test structure enables us to determine strain relaxation in In‐rich layers by X‐ray diffraction even at typical well thicknesses of few nanometers. We show that there is a high risk to generate screw and edge dislocations in the In‐rich quantum wells in non‐optimized structures as verified by transition electron microscopy. Such defects strongly influence the device lifetimes of green lasers. Non‐optimized devices have a drop of output power on a time scale of 100 h. The threshold current increases proportional to the square root of time. Additionally, a linear correlation of slope efficiency versus one over laser threshold current is observed. The degradation process is explained as reduced carrier injection efficiencies related to the generation of additional charged defects in the active layers. Optimized active layers already enable stable operation over a period of more than 1000 h at 50 mW constant power at 517 nm emission wavelength with an extrapolated increase of operation current by <30% within this time.

It's not easy being green: Strategies for all‐nitrides, all‐colour solid state lighting

AbstractThe design strategy presently employed to obtain ‘white’ light from semiconductors combines the emission of an InGaN blue or UV light‐emitting diode (LED) with that of one or more yellow‐orange phosphors. While commercially successful, this approach achieves good colour rendering only by increasing the number and spectral range of the phosphors used; compared to the alternative of combining ‘true’ red, green and blue (RGB) sources, it is intrinsically inefficient. The two major roadblocks to the RGB approach are 1. the green gap in the internal quantum efficiency (IQE) of LEDs; 2. the diode droop in the efficiency of LEDs at higher current densities. The physical origin of these effects, in the case of III‐nitrides, is generally thought to be a combination of Quantum Confined Stark Effect (QCSE) and Auger Effect (AE). These effects respectively reduce the electron–hole wave‐ function overlap of In‐rich InGaN quantum wells (QW), and provide a non‐radiative shunt for electron–hole recombination, particularly at higher excitation densities. SORBET, a novel band gap engineering strategy based upon quantum well intermixing (QWIM), offers solutions to both of the roadblocks mentioned above. In this introduction to SORBET, its great potential is tested and confirmed by the results of simulations of green InGaN diodes performed using the TiberCAD device modelling suite, which calculates the macroscopic properties of real‐world optoelectronic and electronic devices in a multiscale formalism. An alternative approach to the realisation of RGB GaN‐based LEDs through doping of an active layer by rare earth (RE) ions will also be briefly described. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Electrical, spectral and optical performance of yellow–green and amber micro-pixelated InGaN light-emitting diodes

Micro-pixelated InGaN LED arrays operating at 560 and 600 nm, respectively, are demonstrated for what the authors believe to be the first time. Such devices offer applications in areas including bioinstrumentation, visible light communications and optoelectronic tweezers. The devices reported are based on new epitaxial structures, retaining conventional (0 0 0 1) orientation, but incorporating electron reservoir layers which enhance the efficiency of radiative combination in the active regions. A measured output optical power density up to 8 W cm−2 (4.4 W cm−2) has been achieved from a representative pixel of the yellow–green (amber) LED array, substantially higher than that from conventional broad-area reference LEDs fabricated from the same wafer material. Furthermore, these micro-LEDs can sustain a high current density, up to 4.5 kA cm−2, before thermal rollover. A significant blueshift of the emission wavelength with increasing injection current is observed, however. This blueshift saturates at 45 nm (50 nm) for the yellow–green (amber) LED array, and numerical simulations have been used to gain insight into the responsible mechanisms in this microstructured format of device. In the relatively low-current-density regime (<3.5 kA cm−2) the blueshift is attributable to both the screening of the piezoelectric field by the injected carriers and the band-filling effect, whereas in the high-current regime, it is mainly due to band-filling. Further development of the epitaxial wafer material is expected to improve the current-dependent spectral stability.

Effect of stimulated phase separation on properties of blue, green and monolithic white LEDs

AbstractDifferent methods of stimulation of phase separation in an InGaN QWs by technological methods and by design of structure were investigated. Effect of admixing of hydrogen during growth interruptions (GIs) after deposition of the InGaN QWs on their structural and optical properties and properties of InGaN‐based LEDs was investigated. Effect of growth pressure on the phase separation was investigated and formation of separate InGaN islands at increase in growth pressure was revealed. It was shown that the phase separation is stumulated in composite InAlN/GaN/InGaN heterostructures and formation of well isolated InGaN islands was observed. Effect of the phase separation on the properties of the blue and deep green LEDs was investigated and strong changes in the spectral position and current dependence of the quantum efficiency were revealed. It was shown that formation of the island due to the phase separation allows control position and width of the emission line and maximum in current dependence of the quantum efficiency. Monolithic white LEDs are containing in active region blue and green InGaN QWs grown with applying of the GIs and emitting in spectral range from 440 nm to 560 nm were studied. Monolithic white LEDs having optimal design of active region demonstrate CCT in the range of 9000‐12000 K and maximal external quantum efficiency up to 14 lm/W. (© 2012 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

True-green (11-22) plane optically pumped laser with cleaved m-plane facets

We present a detailed experimental study of optical property of green InGaN quantum wells and optically pumped lasers, with cleaved m-plane facets and a lasing wavelength in the range of 520-530 nm, grown on semipolar (11-22) planes. Taking advantage of low transparency carrier density of the lowest-energy valence band and overcoming low differential gain by minimizing the optical loss of the laser structure, we demonstrated a low threshold pumping power of 120 mW.

Approaches for high internal quantum efficiency green InGaN light-emitting diodes with large overlap quantum wells

Optimization of internal quantum efficiency (IQE) for InGaN quantum wells (QWs) light-emitting diodes (LEDs) is investigated. Staggered InGaN QWs with large electron-hole wavefunction overlap and improved radiative recombination rate are investigated for nitride LEDs application. The effect of interface abruptness in staggered InGaN QWs on radiative recombination rate is studied. Studies show that the less interface abruptness between the InGaN sub-layers will not affect the performance of the staggered InGaN QWs detrimentally. The growths of linearly-shaped staggered InGaN QWs by employing graded growth temperature grading are presented. The effect of current injection efficiency on IQE of InGaN QWs LEDs and other approaches to reduce dislocation in InGaN QWs LEDs are also discussed. The optimization of both radiative efficiency and current injection efficiency in InGaN QWs LEDs are required for achieving high IQE devices emitting in the green spectral regime and longer.

Investigation of long wavelength green InGaN lasers on c‐plane GaN up to 529 nm continuous wave operation

AbstractWe present analysis of the experimentally determined gain coefficients g0 for green InGaN lasers with pulsed wavelengths from 495 to 526 nm. Variable facet coating as well as variable cavity length method are used to deduce the dependency of g0 on wavelength. From the experimental data we found that better laser performance at shorter wavelengths correlates with higher material gain. Therefore, lasers at 504 nm reach wall plug efficiencies (WPE) up to 5.3% at 50 mW cw output power, while lasers with 525 and 529 nm show a WPE up to 3 and 2.3%, respectively. The WPE for longer wavelengths is limited due to thermal rollover in cw operation.

InGaN/GaN short‐period superlattices: synthesis, properties, applications

AbstractInGaN/GaN short‐period superlattices were fabricated using method based on cycle conversion of surface InGaN layer to GaN by applying of growth interruptions in hydrogen atmosphere. SPSLs having total thickness from 12 nm to 120 nm were grown and investigated by combination of optical methods, XRD and high resolution transmission electron microscopy (HRTEM) with geometric phase analysis. Blue, deep green (540‐560 nm) and monolithic white LEDs (containing in active region green and blue InGaN QWs separated by the SPSL or GaN barrier) were grown and investigated. Deep green LEDs have maximal external quantum efficiency of 8‐16% in the range of 560‐540 nm correspondingly. Monolithic white LED structures with CCT in the range of 5000‐6000 K have maximal external quantum efficiency of more than 6%. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The impact of trimethylindium treatment time during growth interruption on the carrier dynamics of InGaN/GaN multiple quantum wells

Solid-state lighting through light emitting diodes (LEDs) is considered the next generation white-lighting. Because green light affects the quality of white light, significant improvement of the luminescence efficiency of green InGaN LEDs are crucial. In this study, the effects of trimethylindium (TMIn) treatment time during growth interruption on the emission and carrier dynamic characteristics of InGaN/GaN multiple quantum wells with green emission were investigated. TMIn treatment during growth interruption suppresses InGaN decomposition and indium aggregation such that more homogeneous indium composition, higher effective potential level, higher energy (localized) states, stronger photoluminescence (PL) intensity, and an apparent S-shaped variation of the temperature-dependent PL peak position were observed. In addition, as the treatment time increases, the decay time and its variation both become smaller. Because indium composition within the InGaN quantum wells is more homogeneous the longer the treatment time, weaker carrier transport and carrier-localized effects lead to a shorter decay time and better recombination efficiency. The research results provide important information to optimize the performance of green and white LEDs.

Interdependency of surface morphology and wavelength fluctuations of indium‐rich InGaN/GaN quantum wells

AbstractThe origin of indium fluctuations in indium‐rich quantum wells (QWs) is of high interest for direct green laser diodes. We present the correlation of morphological features such as macrosteps investigated by AFM measurements and wavelength fluctuations seen in cathodoluminescence images of InGaN/GaN QWs grown by metal organic vapor phase epitaxy. We observe an opposed wavelength shift of 5 nm in the vicinity of macrosteps for the investigated UV and green InGaN QW samples. We present a growth model taking adsorption, desorption, and migration processes into account to explain this difference via a temperature dependent change in indium incorporation in the vicinity of the observed macrostep edges.

Future of group-III nitride semiconductor green laser diodes [Invited

Some research groups have realized green laser diodes (LDs) using InGaN-based nitride semiconductors. However, the performances of their devices, in particular wall-plug efficiency, were still one digit lower than those of established red and blue semiconductor LDs. To achieve high-performance green LDs and apply them to LD displays or micro-projectors, the field should focus on improving the internal quantum efficiency (IQE) of green spontaneous emission from InGaN quantum wells (QWs). Green InGaN QWs have completely different features than blue InGaN QWs in terms of growth and concomitant material quality. The exploration of 100% IQE-quality InGaN materials for green emission continues.

Pros and cons of green InGaN laser on c‐plane GaN

AbstractThe challenges of green InGaN lasers are discussed concerning material quality as a function of InGaN composition, quantum well design and piezoelectrical fields. Investigations of polar quantum well designs and comparison with simulated non‐polar structures demonstrate that the quality of the indium rich layers is more important than the influence of interface charges. A high risk of dark spots at high In concentrations of 26–33% is observed. Small changes of about 2% of In significant reduce or increase the quantity and size of dark luminescence areas. Polar designs are a trade‐off between low indium concentrations of 4 nm wide quantum wells and high overlap of electrons and holes in 2 nm narrow designs. Furthermore, our single quantum wells have less non‐radiative defects than indium rich multi‐quantum well structures. Optimized active layer designs and the material qualities enable us to get green InGaN lasers on c‐plane substrates for cw operation at 515–524 nm and wall plug efficiencies of 3.9–2.3%. Slope efficiency of 0.3–0.4 W/A allows up to now highest optical output power of 50 mW.

Gain Anisotropy Analysis in Green Semipolar InGaN Quantum Wells with Inhomogeneous Broadening

Polarization switching phenomena in semipolar (1122)-oriented InGaN quantum wells (QWs) were theoretically investigated by a newly formulated model considering the effect of In compositional fluctuation. The theoretical model successfully reproduced the reported polarization switching phenomena for both the emission wavelength and the excitation density in blue-green (1122) QWs, and this showed the importance of inhomogeneous broadening effects to understand polarization properties of semipolar quantum wells. Then, the model was applied for pure-green (1122) QWs, and we predicted that optical polarization was kept in the [1123] direction up to the carrier density high enough to create population inversion in such long-wavelength QWs. These results support the possibility for semipolar-oriented pure green InGaN laser diodes with cleaved facet cavity mirrors.

True green InGaN laser diodes

AbstractWe present true green InGaN ridge waveguide (RWG) laser diodes (LDs) at 520 nm on c‐plane GaN substrates in pulse operation at room temperature. Defect reduction in the In‐rich quantum wells by improving growth conditions of the epitaxial layers is the key parameter to demonstrate laser operation at this wavelength. Carrier lifetime measurements in combination with electroluminescence (EL) data and simulations of competing recombination processes below laser threshold confirm that the reduction of defects in the light emitting InGaN quantum wells is essential to realize true green nitride‐based LDs with low laser threshold of 125 mA and good slope efficiency of 220 mW/A for optical output power level up to 50 mW. Investigation of the lateral far field of 520 nm RWG LDs shows a perfect beam quality. The measured vertical far field shows a substrate mode due to leakage into the transparent GaN substrate supported by waveguide simulations with an additional internal loss of less than 1 cm−1. Mounted InGaN‐based ridge LDs at 500 nm demonstrate high optical output power of 35 mW in cw operation with good wall plug efficiency of 3.5% at room temperature.

Waveguide design of green InGaN laser diodes

AbstractIn this paper we investigate the waveguiding (WG) of direct green InGaN laser diodes grown on c‐plane GaN substrates. The problem of parasitic modes emerges due to the reduced refractive index difference between the GaN waveguide and AlGaN cladding layers for green compared to blue emitting laser diodes. We discuss several approaches to avoid substrate modes. We investigate different materials and designs for optimized WG of green InGaN laser diodes using a 1D transfer matrix simulation tool.

Improved InGaN LED Efficiency using Photonic Crystal Patterning and Surface Plasmon Enhanced Emission

Surface plasmon mediated emission can improve the efficiency of light emitting diodes (LEDs) by coupling excited carriers to surface plasmon modes. By combining surface plasmon effects with photonic crystal patterning it is possible to enhance the internal quantum efficiency as well as the light extraction efficiency of blue and green InGaN LEDs. Time-resolved and time-integrated photoluminescence measurements are used to show that surface plasmons can enhance the radiative decay rate as well as the overall total luminescence for a properly designed sample. Our design for a practical, electrically injected InGaN surface plasmon LED will be presented.

Behaviors of Emission Wavelength Shift in AlInGaN-Based Green Laser Diodes

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> InGaN quantum-well (QW) green laser diodes (LDs) with an emission wavelength of 483.7 nm were characterized by controlling the injection pulsewidth. The emission wavelength of LDs showed a large blueshift ( <formula formulatype="inline"><tex Notation="TeX">$≫ {\hbox {20}}\ \hbox{nm}$</tex></formula>) of spontaneous emission peak with increasing injection current below the threshold current. The huge blueshift was ascribed to the deep In localization states and the strong piezoelectric field in the green InGaN QW structure with higher In contents than conventional violet/blue InGaN QWs. However, the lasing wavelength of LDs was slightly redshifted by increasing the injection pulsewidth due to the thermal heating effects. </para>

Phosphor-free white light-emitting diode with laterally distributed multiple quantum wells

A phosphor-free white light-emitting diode (LED) was fabricated with laterally distributed blue and green InGaN∕GaN multiple quantum wells (MQWs) grown by a selective area growth method. Photoluminescence and electroluminescence (EL) spectra of the LED showed emission peaks corresponding to the individual blue and green MQWs. The integrated EL intensity ratio of green to blue emission varied from 2.5 to 6.5 with the injection current below 300mA, but remained constant at high injection currents above 300mA. The stability of the emission color at high currents is attributed to parallel carrier injection into both MQWs.

Auger recombination in InGaN measured by photoluminescence

The Auger recombination coefficient in quasi-bulk InxGa1−xN (x∼9%–15%) layers grown on GaN (0001) is measured by a photoluminescence technique. The samples vary in InN composition, thickness, and threading dislocation density. Throughout this sample set, the measured Auger coefficient ranges from 1.4×10−30to2.0×10−30cm6s−1. The authors argue that an Auger coefficient of this magnitude, combined with the high carrier densities reached in blue and green InGaN∕GaN (0001) quantum well light-emitting diodes (LEDs), is the reason why the maximum external quantum efficiency in these devices is observed at very low current densities. Thus, Auger recombination is the primary nonradiative path for carriers at typical LED operating currents and is the reason behind the drop in efficiency with increasing current even under room-temperature (short-pulsed, low-duty-factor) injection conditions.