Di-chromatic InGaN based color tuneable monolithic LED with high color rendering index

We demonstrate a phosphor free, dichromatic GaN-based monolithic white LED with vertically stacked green and blue emitting multiple quantum wells. The optimal thickness of GaN barrier layer between green and blue quantum wells used is 8 nm. This device can be tuned over a wide range of correlated color temperature (CCT) to achieve warm white (CCT = 3600 K) to cool white (CCT = 13,000 K) emission by current modulation from 2.3 A/cm2 to 12.9 A/cm2. It is also demonstrated for the first time that a color rendering index (CRI) as high as 67 can be achieved with such a dichromatic source. The observed CCT and CRI tunability is associated with the spectral power evolution due to the pumping-induced carrier redistribution

Correlation between p-GaN growth environment with electrical and optical properties of blue LEDs

Two blue (450 nm) light–emitting diodes (LED), which only differ in top p-GaN layer growth conditions, were comparatively investigated. I-V, C-V, TLM, Electroluminescence (EL) and Photoluminescence (PL) techniques were applied to clarify a correlation between MOCVD carrier gas and internal properties. The A-structure grown in the pure N2 environment demonstrated better parameters than the B-structure grown in the N2/H2 (1:1) gas mixture. The mixed growth atmosphere leaded to an increase of sheet resistances of p-GaN layer. EL and PL measurements confirmed the advantage of the pure N2 utilization, and C(VR) measurement pointed the increase of static charge concentration near the p-GaN interface in the B structure

Scattering Analysis of AlGaN/AlN/GaN Heterostructures with Fe-Doped GaN Buffer

The results of the study of the influence of Fe segregation into the unintentionally doped GaN channel layer in AlGaN/AlN/GaN heterostructures with Fe-doped GaN buffer layer on the electrical properties of two-dimensional electron gas are presented. A set of several samples was grown by metal-organic vapor-phase epitaxy and characterized by the van der Pauw method. The dependence of concentration and mobility of the two-dimensional electron gas on the channel layer thickness was analyzed theoretically by self-consistent solving of 1D Poisson and Schrödinger equations and scattering rate calculations within the momentum relaxation time approximation. It was found that both concentration and mobility decreases were responsible for the increase in the sheet resistance in the structures with a thinner channel layer, with a drop in mobility being not only due to ionized impurity scattering, but also due to a combined effect of weakening of screening, lower carrier energy and change in form-factors on scattering by interface roughness, dislocations and polar optical phonons

Impact of Local Composition on the Emission Spectra of InGaN Quantum-Dot LEDs

A possible solution for the realization of high-efficiency visible light-emitting diodes (LEDs) exploits InGaN-quantum-dot-based active regions. However, the role of local composition fluctuations inside the quantum dots and their effect of the device characteristics have not yet been examined in sufficient detail. Here, we present numerical simulations of a quantum-dot structure restored from an experimental high-resolution transmission electron microscopy image. A single InGaN island with the size of ten nanometers and nonuniform indium content distribution is analyzed. A number of two- and three-dimensional models of the quantum dot are derived from the experimental image by a special numerical algorithm, which enables electromechanical, continuum k→·p→, and empirical tight-binding calculations, including emission spectra prediction. Effectiveness of continuous and atomistic approaches are compared, and the impact of InGaN composition fluctuations on the ground-state electron and hole wave functions and quantum dot emission spectrum is analyzed in detail. Finally, comparison of the predicted spectrum with the experimental one is performed to assess the applicability of various simulation approaches

Epitaxial growth of GaN/AlN/InAlN heterostructures for HEMTs in horizontal MOCVD reactors with different designs

cited By 0International audienceThe epitaxial growth of InAlN layers and GaN/AlN/InAlN heterostructures for HEMTs in growth systems with horizontal reactors of the sizes 1 × 2", 3 × 2", and 6 × 2" is investigated. Studies of the structural properties of the grown InAlN layers and electrophysical parameters of the GaN/AlN/InAlN heterostructures show that the optimal quality of epitaxial growth is attained upon a compromise between the growth conditions for InGaN and AlGaN. A comparison of the epitaxial growth in different reactors shows that optimal conditions are realized in small-scale reactors which make possible the suppression of parasitic reactions in the gas phase. In addition, the size of the reactor should be sufficient to provide highly homogeneous heterostructure parameters over area for the subsequent fabrication of devices. The optimal compositions and thicknesses of the InAlN layer for attaining the highest conductance in GaN/AlN/InAlN transistor heterostructures

Multi-color monolithic III-nitride light-emitting diodes: Factors controlling emission spectra and efficiency

cited By 6International audienceInGaN-based monolithic multi-color light emitting diodes (LEDs) are studied both experimentally and theoretically with the focus on factors controlling their emission spectra and efficiency. A number of LEDs with different designs of spacers separating active regions providing blue and green light emission is examined. Unexpected behavior of the multi-color LED efficiency is explained in terms of a simple balance model, assuming some degradation of materials quality of the green active region grown on top of the blue one. Electrical properties of spacers separating different active regions in the multi-color LED structures are identified as the major factor controlling the contributions of these active regions to the total emission spectrum. Correlations between the type and level of the spacer doping and the emission spectrum are found by simulations. Alternative ways of the spectral control by using polarization doping in the graded-composition InGaN and AlGaN alloys used as the spacers are suggested. Color characteristics of blue/green dual-wavelength LEDs are also measured and discussed, regarding their possible applications

Internal quantum efficiency and tunable colour temperature in monolithic white InGaN/GaN LED

Internal Quantum Efficiency (IQE) of two-colour monolithic white light emitting diode (LED) was measured by temperature dependant electro-luminescence (TDEL) and analysed with modified rate equation based on ABC model. External, internal and injection efficiencies of blue and green quantum wells were analysed separately. Monolithic white LED contained one green InGaN QW and two blue QWs being separated by GaN barrier. This paper reports also the tunable behaviour of correlated colour temperature (CCT) in pulsed operation mode and effect of self-heating on device performance

Effect of the design of the active region of monolithic multi-color LED heterostructures on their spectra and emission efficiency

cited By 2International audienceThe design features of light-emitting-diode heterostructures with a monolithic InGaN/GaN active region containing several InGaN quantum wells (QWs) emitting at different wavelengths, grown by metal-organic chemical vapor deposition, are studied. It is shown that the number of emission bands can be raised to three by increasing the number of deposited InGaN QWs with different indium contents. The emission efficiency decreases by approximately 30% with increasing number of QWs at high currents. The dependences of the optical properties of the heterostructures on the number of QWs and types of barriers between the QWs (GaN layer or InGaN/GaN short-period superlattice) are analyzed. It is demonstrated that the ratio between the intensities of the emission lines widely varies with current flowing through the structure and greatly depends on the type and width of the barriers between the QWs

Formation of three-dimensional islands in the active region of InGaN based light emitting diodes using a growth interruption approach

cited By 4International audienceHere, we develop a technological approach to the formation of three-dimensional island-like structures in the active medium of InGaN/GaN based light emitting diodes with an enhanced efficiency with respect to reference quantum wells emitting at the same wavelengths. The reference structures contain two-dimensional In x Ga1– x N quantum wells with x ≤ 18% immediately overgrown after their formation. The method consists in the application of a growth interruption in N2 or N2–H2 mixed atmospheres at different H2 flows and times after the deposition of In0.18Ga0.82N quantum wells, prior to their overgrowth by a GaN layer. The growth interruptions allow a controlled blue shift of the emission peak position with respect to that of the In0.18Ga0.82N structure. The integrated photoluminescence intensity of the so-formed structures is about 1.5 times higher than that of the reference structures emitting at the same peak wavelengths. Light emitting diode structures subjected to growth interruption exhibit higher external quantum efficiency than the reference structures emitting at the same wavelengths. We demonstrate that the observed phenomenon is related to a better charge carrier confinement within a quantum well due to the transformation of planar InGaN layers into laterally connected flat islands