InGaN Active Layer Research Articles

Wet Mesa Etching Process in InGaN-based Light Emitting Diodes

A photoelectrochemical wet mesa etching (WME) process was used to fabricate InGaN-based light emitting diodes (LEDs) as a substitute for the conventional plasma mesa dry etching process. The p-type GaN:Mg layer, InGaN active layer, and n-type GaN:Si layer were etched through a sequential photoelectrochemical oxidation and oxide-removing process to define the mesa region. The higher lateral wet-etching rate (∼3.4 μm/h) of the InGaN active layer was observed to form a wider undercut structure which has 42.7% light output power enhancement compared to a conventional LED fabricated with the plasma dry etching process. The reverse current of a WME-LED was suppressed by avoiding plasma damage during the dry mesa etching process.

Internal Quantum Efficiency of Nitride-based Light-Emitting Diodes

The internal quantum efficiency (IQE) of InGaN-based light-emitting diodes has been studied by means of excitation-power-density- and temperature-dependent photoluminescence spectroscopy. The IQE was evaluated at various excitation photon energies using a wavelength-tunable dye laser system. The IQE obtained under selective excitation of InGaN active layers was higher than that obtained under band-to-band excitation of GaN and AlGaN cladding layers. The enhanced value of IQE under selective excitation reflected the intrinsic optical quality of the InGaN active layer itself.

Characterization of AlGaN/InGaN/AlGaN heterostructure with selective area growth of Te‐doped AlGaN cladding layer grown by mixed‐source HVPE

The selective area growth (SAG) of AlGaN/InGaN/AlGaN light-emitting diodes (LEDs) is performed by mixed-source hydride vapor phase epitaxy (HVPE). The structure is grown on a n-GaN templated (0001) sapphire substrate. The SAG-double heterostructure (DH) is consisted of a Te-doped AlGaN cladding layer, an InGaN active layer, a Mg-doped AlGaN cladding layer, and a Mg-doped GaN capping layer. All of the epitaxial layers of LED structure are grown consecutively with a multi-sliding boat system. Room-temperature electroluminescence (EL) characteristics show an emission peak wavelength of 400 nm with a full width at half-maximum (FWHM) of approximately 0.38 eV (at 20 mA). We find that the mixed-source HVPE method with multi-sliding boat system is possible to be one of the growth methods of III-nitride LEDs. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

InGaN Light-Emitting Diode With Quasi-Quantum-Dot-Shaped Active Layer Using SiCN Interfacial Layer

A quasi-quantum-dot (QQD)-shaped InGaN-GaN multilple-quantum-well light-emitting diode (LED) was achieved using a silicon carbon nitride (SiCN) interfacial layer. QQDs with ~100-nm diameter and ~4-nm height were uniformly formed inside the InGaN active layer due to strain and affinity difference between the InGaN and SiCN layer. The surface morphology and structural properties of QQD-LED were measured with atomic force microscopy, secondary ion mass spectrometry, and X-ray diffraction. Device performance of QQD-LEDs were evaluated and compared with normal LEDs. The QQD-LED showed ~15% higher photoluminescence intensity and ~10% higher optical output power

The influence of quasi-quantum dots on the physical properties of blue InGaN/GaNmultiple quantum wells

The effect of barrier growth temperatures on blue InGaN (2 nm)/GaN (12 nm) multiplequantum wells was studied with samples grown by metallorganic chemical vapourdeposition. It was found that InGaN active layers composed of InGaN quasi-dots of2 ± 0.2 nm in diameter, changing from their homogeneous nature, could beobtained by elevating the barrier growth temperature from 700 to800 °C based on the results of energy-filtered high resolution transmission electron microscopy.These dots may have formed during the ramping process by in situ annealing. Strongpiezoelectric field, ‘S-shape-like’ carrier transition and high internal quantum efficiency of71.3% were observed in the sample with a higher barrier growth temperature closelyrelated to the dot formation. Furthermore, the forward voltage and the lightoutput power at 20 mA of light emitting diodes from the sample with dots were0.3 V lower and 11% higher than that from the homogeneous multiple quantumwells.

자외선 여기용 청색 및 황색 형광체의 발광특성

We have synthesized a Eu 2+ -activated Sr₃MgSi₂O 8 blue phosphor and (Sr,Ba)₂SiO₄ yellow phosphor and prepared white LEDs by combining these phosphors with a InGaN UV LED chip. Three distinct emission bands from the InGaN-based LED and the two phosphors are clearly observed at 405 nm, 460 nm, and at around 560 nm, respectively. The 405 nm emission band is due to a radiative recombination from a InGaN active layer. This blue emission was used as an optical transition of the Sr₃MgSi₂O 8 :Eu blue phosphor and (Sr,Ba)₂SiO₄:Eu yellow phosphor. The 460 nm and 560 nm emission band is ascribed to a radiative recombination of Eu 2+ impurity ions in the Sr₃MgSi₂O 8 and (Sr,Ba)₂SiO₄ host matrix. As a consequence of a preparation of UV White LED lamp using the Sr₃MgSi₂O 8 :Eu blue phosphor and (Sr,Ba)₂SiO₄:Eu yellow phosphor, the highest luminescence efficiency was obtained at the ration of epoxy/two phosphor (1/0.2361). At this time, the CIE chromaticity was CIE x=0.3140, CIE y=0.3201 and CCT (6500 K).

Near-field scanning optical microscopic transient lens for carrier dynamics study in InGaN∕GaN

Time-resolved microscopic transient lens (TR-M-TL) and near-field scanning optical microscopic transient lens (NSOM-TL) were performed to reveal temporal and spatial behavior of carrier dynamics in InGaN∕GaN quantum wells. The carrier and thermal dynamics were observed through the time profile of the TR-M-TL signal. Also, NSOM-photoluminescence and NSOM-TL images were observed at the same time. By comparing these two images, both radiative and nonradiative recombination centers in InGaN active layer were unambiguously discriminated with submicrometer scale. Such nonradiative carrier dynamics has been difficult to observe by conventional techniques in spite of its importance.

Confocal microphotoluminescence of InGaN-based light-emitting diodes

Spatially resolved photoluminescence (PL) of InGaN∕GaN∕AlGaN-based quantum-well-structured light-emitting diodes (LEDs) with a yellow-green light (530nm) and an amber light (600nm) was measured by using confocal microscopy. Submicron-scale spatial inhomogeneities of both PL intensities and spectra were found in confocal micro-PL images. We also found clear correlations between PL intensities and peak wavelength for both LEDs. Such correlations for yellow-green and amber LEDs were different from the reported correlations for blue or green LEDs. This discrepancy should be due to different diffusion, localization, and recombination dynamics of electron-hole pairs generated in InGaN active layers, and should be a very important property for influencing the optical properties of LEDs. In order to explain the results, we proposed a possible carrier dynamics model based on the carrier localization and partial reduction of the quantum confinement Stark effect depending on an indium composition in InGaN active layers. By using this model, we also considered the origin of the reduction of the emission efficiencies with a longer emission wavelength of InGaN LEDs with high indium composition.

Effect of compressive strain relaxation in GaN blue light-emitting diodes with variation of n+-GaN thickness on its device performance

The effects of compressive strain relaxation with increasing n-GaN thickness on device performances of GaN blue light-emitting diodes (LEDs) were investigated. It was found that the compressive strain relaxation in LEDs with thicker n-GaN occurred more considerably, following by the growth of active layer and p-GaN, and generated many stacking faults right beneath the InGaN active layer, which might be related to a decrease of the LED output power. On the contrary, the LED photoluminescence intensity increased surprisingly with n-GaN thickness. It was understood that the compressive strain relaxation enhanced localized states in InGaN wells.

Integration of CdSe quantum dots with GaN optoelectronic materials

Material structures on the nanoscale can enable enhancement of optoelectronic device performance. For example, in the InGaN active layers of MOCVD grown blue light emitting diodes, indium segregation plays a critical role in the interplay between blue luminescent channels and non-radiative recombination centers such as crystal defects. Unfortunately, high efficiency luminescence of InGaN does not extend into the “deep green” spectral region, around the wavelength of peak human eye response. We are investigating whether commercially available luminescent nanostructures such as CdSe quantum dots can be incorporated into III-nitride devices to extend their high-efficiency performance into the “deep green”. Surfactant stabilized CdSe particles in liquid dispersions are drop-cast onto HVPE grown GaN. Physical properties of resultant CdSe surface structures are examined. Luminescence is reported before and after subsequent growth of GaN. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Screening of built‐in electric fields in group III‐nitride laser diodes observed by means of hydrostatic pressure

We investigate screening of polarization induced electric fields in a group III-nitride based laser diode with InGaN active layers. Via hydrostatic pressure dependent electroluminescence measurements and additional photocurrent spectroscopy experiments, it is shown that the introduction of Si doping with a concentration of 1 · 1019 cm–3 in the barriers of the active region leads to adequate screening of polarization induced electric fields. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of threading dislocations on the properties of InGaN-based multiple quantum wells

The relationship of the threading dislocations (TDs) and InGaN-based quantum well (QW) optical characteristics is studied by temperature-dependent photoluminescence, high-resolution X-ray diffraction and atomic force microscopic. It is found that TDs deteriorate the QW interfacial abruptness, and can enhance the localization effect. Temperature-dependent photoluminescence measurements give evidence that the TDs act as nonradiative recombination (NR) centers in InGaN active layer. According to these results, it is inferred that the influence of TD on the optical properties of InGaN-based QW depends on the competition of localization induced by TDs and the NR effect of TDs.

Luminescence Characteristics of Sr[sub 3]MgSi[sub 2]O[sub 8]:Eu Blue Phosphor for Light-Emitting Diodes

We have synthesized a -activated blue phosphor and investigated an attempt to develop blue light-emitting diodes (LEDs) by combining it with an InGaN blue LED chip The InGaN-based LED lamp shows two bands at 405 and 460 nm. The 405 nm emission band is due to a radiative recombination from a InGaN active layer. This 405 nm emission was used as an optical pump for the excitation of the phosphor. The 460 nm emission band is ascribed to a radiative recombination of impurity ions in the host matrix. © 2004 The Electrochemical Society. All rights reserved.

Self-pulsation in an InGaN laser-theory and experiment

Room-temperature operation of self-pulsating InGaN lasers was obtained at a wavelength of 395 nm. The laser structure consists of a multiquantum-well InGaN active layer and a p-type InGaN single-quantum-well saturable absorber. The frequency range of the self-pulsation was from 1.6 to 2.9 GHz. The experimental results were well explained with our theoretical analysis. We found that features of the saturable absorber strongly affect the self-pulsation. Influence of device and material parameters on the laser dynamics was also investigated.

Analysis of self‐pulsation characteristics of InGaN laser diode

Self-pulsation characteristics of InGaN laser diode emitting at 395 nm wavelength are investigated theoretically and experimentally. The laser structure consists of a multi-quantum well InGaN active layer and an InGaN single quantum well saturable absorber. Self-pulsations with the frequency range from 1.6 to 2.9 GHz have been obtained. The results show a good agreement between measured and calculated characteristics of self-pulsation. We also discuss the impact of the saturable absorber on the laser dynamics. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Reduction of Threading Dislocations in InGaN/GaN Double Heterostructure through the Introduction of Low-Temperature GaN Intermediate Layer

The reduction mechanism of threading dislocation at the interface of InGaN/low-temperature GaN (LT-GaN) layers was investigated by atomic force microscopy, transmission electron microscopy and secondary ion mass spectroscopy measurements. Introducing the LT-GaN intermediate layer onto the InGaN active layer not only prevented indium evaporation during the growth of the p-GaN layer but also suppressed the propagation of threading dislocations from InGaN to p-GaN. The propagation of threading dislocations is reduced by the formation of two-dimensional lateral islands, and further defect generation is prevented by the formation of InxGa1-xN alloy due to the relaxation of lattice mismatch between active InGaN and p-GaN.

Recent progress in group-III nitride light-emitting diodes

We review the recent progress of nitride-based light-emitting diodes (LEDs) and discuss the dislocation issue and luminous efficiency. First, candela-class blue LEDs have been developed. InGaN layer was used for nitride LEDs instead of GaN active layer. The quantum-well-structure InGaN active layer dramatically improved the external quantum efficiency. There are a number of threading dislocations in epitaxial layer of nitride-based LEDs. InGaN-LEDs, however, have quite high external quantum efficiency. With regard to this, it is thought that the fluctuation of indium mole fraction is strongly related to the high external quantum efficiency. We also discuss the method to improve the external quantum efficiency of nitride-based LEDs.

Diffusion coefficient of the order parameter in the early stages of the time evolution of InGaN phase separation

We have carried out a study involving a model for the diffusion mechanism of group III atoms in an InGaN alloy semiconductor that uses a cell dynamical system (CDS) approach. In this CDS analysis, we focused on gathering information concerning the In-rich region with a dot-like structure that is formed at an early stage in the time evolution of phase separation of this alloy. A similar dot-like structure is known to appear in the active layer of InGaN-based multiquantum well laser diodes. Thus, we can determine the diffusion coefficient of group III atoms in InGaN by comparing the dot-like structures observed by cross-sectional transmission electron microscopy with those obtained by the CDS numerical approach. In order to perform this comparison quantitatively such that we could use it to calculate the diffusion coefficient, we derived a relationship between the rule of mapping used in CDS, and the discretization of the Cahn–Hilliard–Cook equation, which describes the dynamics of phase separation with a conserved order parameter. The value obtained for the diffusion coefficient is 4×10−16 cm2/s at a growth temperature of 800 °C. From this result, we can conclude that the diffusion of group III atoms arises from atomistic migration on the surface of an InGaN active layer during growth, instead of diffusion within the solid phase.

Current-modulated electroluminescence spectroscopy and its application to InGaN single-quantum-well blue and green light-emitting diodes

Current-modulated electroluminescence spectroscopy was developed to investigate the mechanism of the current-induced spectral blueshift in InGaN single-quantum-well blue and green light-emitting diodes. The evidence was obtained that tail states exist in InGaN active layers, and also the blueshift was found to be dominated by the tail-state filling effect.

In inhomogeneity and emission characteristics of InGaN

Recombination dynamics of spontaneous and stimulated emissions have beenassessed in InGaN-based light emitting diodes (LEDs) and laser diodes (LDs),by employing time-resolved photoluminescence and pump and probe spectroscopy.As for an In0.02Ga0.98N ultraviolet LED, excitons are weaklylocalized by 15 meV at low temperature, but they become almost free at roomtemperature (RT). It was found that addition of a small amount of In resultsin the reduction of nonradiative recombination centres originating from pointdefects. The internal electric field does exist in InGaN active layers, andinduces a large modification of excitonic transitions. However, it alone doesnot explain the feature of spontaneous emission observed in anIn0.3Ga0.7N blue LED such as an anomalous temperature dependence ofpeak energy, almost temperature independence of radiative lifetimes andmobility-edge type behaviour, indicating an important role of excitonlocalization. The lasing mechanism was investigated for In0.1Ga0.9Nnear ultraviolet (390 nm), In0.2Ga0.8N violet-blue (420 nm) andIn0.3Ga0.7N blue (440 nm) LDs. The optical gain was contributed fromthe nearly delocalized states (the lowest quantized levels (LQLs) withinquantum wells) in the violet LD, while it was from highly localized levelswith respect to the LQL by 250 meV for the violet-blue LD, and by 500 meV forthe blue LD. It was found that the photo-generated carriers rapidly (less than1 ps) transferred to the LQL, and then relaxed to the localized tail withinthe timescale of a few ps, giving rise to the optical gain. Such gain spectrawere saturated and other bands appeared in the vicinity of the LQL underhigher photo-excitation.