Heterostructure Light-emitting Diodes Research Articles

P-ZnO/n-ZnMgO Nanoparticle-Based Heterojunction UV Light-Emitting Diodes.

Heterojunction light-emitting diodes (LEDs), based on p-type ZnO and n-type ZnMgO nanoparticles, have been demonstrated. ZnMgO nanoparticles were prepared by the thermal diffusion of Mg onto ZnO nanoparticles. p-ZnO/GZO homostructure LEDs and p-ZnO/n-ZnMgO/GZO heterostructure LEDs have been fabricated using ZnO and ZnMgO nanoparticles. By comparing the characteristic results of these diodes, it can be seen that LEDs with the p-ZnO/n-ZnMgO/GZO structure showed better I-V characteristics with a lower current density leakage than those with the p-ZnO/GZO LED structure. Moreover, the emission intensity was improved by adding the ZnMgO NP layer to the LEDs. These results show that the ZnMgO NP layer acts as a hetero-barrier layer that suppresses the diffusion of holes into the n-type layer and confines holes to the p-type layer.

Optical transitions in long-wavelength light-emitting diode heterostructures based on InAsSb

Optical transitions in long-wavelength light-emitting diode heterostructures based on InAsSb

Стимулированное излучение в гетероструктурах InAs/InAsSb/InAsSbP с асимметричным электронным ограничением

The electroluminescent characteristics of the InAs/InAs1−ySby /InAsSbP asymmetric light-emitting diode heterostructures with high InSb mole fraction in the active region (y > 0.09) in the temperature range 4.2−300 K have been studied. Stimulated emission was achieved in the wavelength range 4.1−4.2 μm at low temperatures (T < 30 K). It was found that the electroluminescence spectra were formed as a result of the superposition of contributions from different channels of radiative recombination of charge carriers near the type II heterointerface. The effect of the quality of the type II InAsSb/InAsSbP heterojunction on the radiative interface transitions with an increase in the content of InSb in the ternary solid solution is considered.

Stimulated emission in the InAs/InAsSb/InAsSbP heterostructures with asymmetric electronic confinement

The electroluminescent characteristics of the InAs/InAs1-ySby/InAsSbP asymmetric light-emitting diode heterostructures with high InSb mole fraction in the active region (y>0.09) in the temperature range 4.2-300 K have been studied. Stimulated emission was achieved in the wavelength range 4.1-4.2 μm at low temperatures (T<30 K). It was found that the electroluminescence spectra were formed as a result of the superposition of contributions from different channels of radiative recombination of charge carriers near the type II heterointerface. The effect of the quality of the type II InAsSb/InAsSbP heterojunction on the radiative interface transitions with an increase in the content of InSb in the ternary solid solution is considered. Keywords: heterojunctions, InAs, antimonides, electroluminescence, light-emitting diodes.

MXene‐GaN van der Waals Heterostructures for High‐Speed Self‐Driven Photodetectors and Light‐Emitting Diodes

AbstractDue to their excellent electrical conductivity, high transmittance, and adjustable work function, 2D transition‐metal carbides and nitrides have shown great promise in optoelectronic applications, especially in MXene‐semiconductor devices. In this work, Ti3C2TX/(n/p)‐GaN van der Waals heterostructures are fabricated and studied. The Ti3C2TX/(n/p)‐GaN Schottky junctions are confirmed by ultraviolet photoelectron spectroscopy (UPS) with a work function ≈4.2 eV of Ti3C2TX. Based on the Ti3C2TX/(n/p)‐GaN Schottky junctions, high‐speed photodetectors and stable orange light emitting diodes (LEDs) are fabricated. The Ti3C2TX/n‐GaN heterostructure photodetector shows a short rise time (60 ms) and decay time (20 ms), a high responsivity (44.3 mA W−1) and on/off ratio (≈11300) under a light source of 365 nm wavelength and 96.9 µW cm−2 power density. And the Ti3C2TX/p‐GaN heterostructure LED remains a stable orange light emission under bias voltage from 4 to 22 V. The chromaticity coordinates and color temperature of EL spectrum under 22 V are further calculated to be 0.4541, 0.4432, and 2953 K, respectively. The authors believe that this work provides fundamental insight into the applications of MXene in optoelectronic devices.

Suppressing the Temperature Dependence of the Wavelength in Heterostructures with a Staggered Type-II InAsSb/InAsSbP Heterojunction

A study of the electroluminescence of asymmetric InAs/InAs1 – ySby/InAsSbP light-emitting diode heterostructures with a molar fraction of InSb in the ternary solid solution in the active region y = 0.15 and y = 0.16 in the temperature range 4.2–300 K is reported. It is found on the basis of experimental data that a staggered type-II heterojunction is formed at the InAs1 – ySby/InAs0.41Sb0.28P0.40 heterointerface. It is shown that interfacial radiative transitions at the type-II heterointerface make a dominant contribution in the temperature range of 4.2–180 K. This enables minimization of the temperature dependence of the operating wavelength of a light-emitting diode.

Enhanced luminescence of quasi-double heterostructured violet light-emitting diodes based on n-Mg0.027Zn0.973O QD and p-Cu1-xZnxI thin films fabricated via simple thermal diffusion and iodination processes

The advent of GaN and its alloys has revolutionized the field of lighting. However, owing to the associated high-cost materials and vacuum-based processes, alternatives to GaN are being researched. Among recently emerging light-emitting diode (LED) materials, γ-phase cuprous iodide (γ-CuI) exhibits remarkable potential because of its wide direct bandgap (2.95 eV) and high exciton binding energy (62 meV). In this study, we fabricated violet LEDs composed of n-Mg0.027Zn0.973O quantum dots and p-Cu1-xZnxI thin films. To enhance the device performance, a quasi-double heterostructure, which is a slightly modified double heterostructure, was designed. Simple thermal diffusion of Zn and Cu iodination processes were used to realize this structure, and the LED fabricated in this manner exhibited approximately 10-fold stronger luminescence than the LED with the pristine p-CuI thin film.

Study of current-voltage characteristics of InAsSb-based LED heterostructures in 4.2 - 300 K temperature range

Current-voltage characteristics of light-emitting diode heterostructures based on InAsSb epitaxial films and a multi quantum-well structure were studied in the temperature range T = 4.2–300 K. It is shown that transport through the heterostructures is governed by the diffusion and recombination mechanisms at temperatures close to 300 K. The tunnelling effect appears in the temperature range 4.2–77 K. The presence of quantum wells in the active layer results in increased diode leakage currents due to tunnelling.

Cryogenic characteristics of GaAs-based near-infrared light emitting diodes

High performance infrared light emitting diodes (LEDs) have drawn wide attention because of their significant applications in biomedicine, agriculture, aquaculture, night vision security systems and scientific researches. Due to the rapid development of infrared up-conversion devices, especially in the terahertz region, the studies on the GaAs-based infrared LEDs operating at low temperature (<20 K) are quite needed. In this paper, two representative GaAs-based LEDs (GaAs/AlGaAs double heterostructure LED (DH-LED) and GaAs (InGaAs)/AlGaAs quantum well infrared LED (QW-LED)) are fabricated and characterized with electroluminescence (EL) spectroscopy and electroluminescence efficiency (ELE) at different temperatures. The ELE of the QW-LED is about 70% higher at low temperatures (<150 K) but lower at high temperatures (>150 K) than that of the DH-LED. A developed rate equation method is used to analyze the ELE superiority of the QW-LED. Finally, the surface EL uniformity of the two LEDs is studied by using the charge coupled devices imaging. This study provides a guidance to choose and design LED structures for different operation temperatures.

COLOR AND SPECTRAL CHARACTERISTICS OF WHITE LIGHT EMITTING DIODES AND THEIR VARIATION DURING AGING

The relation between numerical values of photometric characteristics (total luminous flux TLF, correlated color temperature CCT, color rendering index CRI) of white light emitting diodes (LED) and the variation of the spectral shape of their radiation during aging has been investigated. All the measurements were made on internationally adopted test methods, taking into account environmental conditions, electrical parameters and evaluated measurement uncertainty. Every piece of test and measurement equipment has actual verification or calibration with traceability to national and international references. It was demonstrated that in the luminescence spectra consisting of the “blue” band around 450 nm originating from the semiconductor heterostructure, and the broad “yellow” band from luminophor, the last band is nonelementary and consists of at least two bands: the “green” one around 530 nm and the “orange” one around 580 nm. The most unstable “green” band has the highest impact on photometric characteristics. As a consequence, further investigation should be performed on how instability of elementary bands and its quantity will link not only with photometric characteristics, but with production conditions and material properties of LED heterostructure and luminophor itself. In particular, for improvement of the color stability of white LED, the parameters of luminophor forming the “green” band should be stabilized. А unified method for accelerated testing of LED products and method for long-time lifetime prediction shall be developed, taking into account not only depreciation of TLF, but also shift of other photometric and spectral characteristics of white LED.

Investigation of Gainp Quantum Dots in the AlGaInP-Based Light Emitting Diodes

Recently, AlGaInP-based light emitting diodes (LEDs) have experienced an impressive evolution in both device performance and market volume. However, development of new applications is required in order to realize their full potential in areas such as use as a light source for auto focusing in digital cameras, special illumination for particular functions in agriculture, and in full color displays. To enlarge their utility in these applications, it is necessary to fabricate and understand a new structure capable of emitting longer wavelengths of around 700 nm. In particular, AlGaInP heterostructure LEDs are lattice-matched with respect to the GaAs substrate, which limits the emitting spectrum to around 650 nm at the longer peak wavelength side. To fabricate an LED structure capable of emitting at a 700 nm peak wavelength, the composition (x) of GaxIn1-xP material in the active layer requires a compressive strain of larger than 1 %. This large lattice mismatch, however, causes significant problems in terms of both growth and device properties due to the formation of defects. To overcome these problems, it is necessary to relieve the well strain via the formation of islands, referred to as a Stranski-Krastanow (S-K) growth mode, in order to prevent the generation of dislocations [1,2]. However, in AlGaInP-based LEDs emitting at a 700 nm peak wavelength, the effects of well strain on the epitaxial growth and the realization of device performance has yet to be extensively studied. In this study, we investigate the behaviors of morphological and optical characteristics on the composition of Ga0.33In0.67P material and demonstrate the performance of a device emitting at around 700 nm using quantum dot (QD)-based LEDs. Figure 1

Investigation into the Distribution of Built-in Electric Fields in LED Heterostructures with Multiple GaN/InGaN Quantum Wells by Electroreflectance Spectroscopy

Electric-field nonuniformity in the active region of a LED (light-emitting diode) heterostructure based on five identical GaN/InGaN quantum wells is investigated by the electroreflectance method. The energies of band-to-band transitions in the quantum wells and barriers are determined from the analysis of electroreflectance spectra using the Kramers–Kronig transforms. The procedure for estimating the electric-field strength in separate quantum wells of the active region by the position of spectral lines is proposed. It is found that the energies of the main transition in quantum wells of the active region with zero bias of the p–n junction differ by a magnitude on the order of 140 meV, which corresponds to a difference in the electric-field strengths of 0.78 MV/cm. It is shown that the nonuniformity of the electric field in the active region depends on the p–n junction bias.

Raman determination of carrier concentration in ZnO-based heterostructure light-emitting diodes.

Understanding of the carrier concentration properties of current spreading layers in LED devices is important although difficult to achieve. Here, we present a solution to determine the carrier concentration for current spreading layers in ZnO-based LEDs, based on Raman spectroscopy. Raman spectra and lineshape fitting indicate a hole concentration below 1×1018 cm-3 in the p-type region and an electron concentration of 1.21×1019 cm-3 in n-type. The results from Raman spectroscopy are further qualitatively confirmed by the electroluminescence spectrum and device simulation, which demonstrates its possible application in carrier concentration assessment in multilayered structures.

Achieving high-resolution pressure mapping via flexible GaN/ ZnO nanowire LEDs array by piezo-phototronic effect

Simulating human tactile sensing through high-resolution electronics is a significant and challenging topic in artificial intelligence. A flexible p-GaN film/n-ZnO nanowire light-emitting diode (LED)-based pressure sensor array, with the merits of flexibility, high resolution, good transparency, fast response, stability, and lightweight, is fabricated through laser lift-off (LLO) process and hydrothermal growth of ZnO nanowires directly on a flexible GaN film, which is used for acquiring the two-dimensional (2D) pressure distribution mapping by reading the illumination intensities from all LED pixels parallelly. The intensity of each pixel composed by a GaN/ZnO nanowire heterostructure LED can be enhanced by the local compressive strain based on piezo-phototronic effect. A high spatial resolution of 2.6 μm and a fast response time of 180 ms were obtained, and also the sensor arrays can still function well after 4000 bending circles. The promising flexible LEDs-based pressure sensor array gains some technological innovations in the field of tactile sensing, with potential applications in smart skin, biomedicine, optical MEMS, and touchpad technology.

High-efficiency perovskite–polymer bulk heterostructure light-emitting diodes

Perovskite-based optoelectronic devices have gained significant attention due to their remarkable performance and low processing cost, particularly for solar cells. However, for perovskite light-emitting diodes (LEDs), non-radiative charge carrier recombination has limited electroluminescence (EL) efficiency. Here we demonstrate perovskite-polymer bulk heterostructure LEDs exhibiting record-high external quantum efficiencies (EQEs) exceeding 20%, and an EL half-life of 46 hours under continuous operation. This performance is achieved with an emissive layer comprising quasi-2D and 3D perovskites and an insulating polymer. Transient optical spectroscopy reveals that photogenerated excitations at the quasi-2D perovskite component migrate to lower-energy sites within 1 ps. The dominant component of the photoluminescence (PL) is primarily bimolecular and is characteristic of the 3D regions. From PL quantum efficiency and transient kinetics of the emissive layer with/without charge-transport contacts, we find non-radiative recombination pathways to be effectively eliminated. Light outcoupling from planar LEDs, as used in OLED displays, generally limits EQE to 20-30%, and we model our reported EL efficiency of over 20% in the forward direction to indicate the internal quantum efficiency (IQE) to be close to 100%. Together with the low drive voltages needed to achieve useful photon fluxes (2-3 V for 0.1-1 mA/cm2), these results establish that perovskite-based LEDs have significant potential for light-emission applications.

Mitigating LED Nonlinearity to Enhance Visible Light Communications

This paper addresses the nonlinear memory effects in the response of typical illumination light emitting diodes (LEDs), in order to enhance the performance of visible light communication (VLC) systems. These LEDs have a limited bandwidth of only several MHz. To reflect the physical mechanisms in the quantum well, we describe the LED transient response by a nonlinear dynamic differential equation. Three different mechanisms of the nonlinearity are relevant in the double hetero-structure LEDs, which result in dynamic nonlinearities, that is, a mixture of nonlinearities and memory effects. Hitherto, generic pre-distorter and non-linear equalizers have been studied for the LEDs. Yet this paper shows that recombination rates of photon generation can be translated into an equivalent discrete-time circuit that can be inverted. This allows us to develop a new pre-distorter with a simpler and more efficient structure than previously studied and overly generic approaches. The novel pre-distorter along with a parameter estimation can effectively overcome LED nonlinearity for high-speed VLC with amplitude-based single carrier modulations, including ON–OFF keying and pulse amplitude modulation-4 systems, and with the multi-carrier orthogonal frequency-division multiplexing. We report experimentally obtained eye-diagrams, first to justify our choice for the LED model on which our nonlinear pre-distorter have been based, and second to verify the effectiveness in enhancing the VLC link performance to the extent predicted by our model.

Electroluminescent refrigeration by ultra-efficient GaAs light-emitting diodes

Electroluminescence—the conversion of electrons to photons in a light-emitting diode (LED)—can be used as a mechanism for refrigeration, provided that the LED has an exceptionally high quantum efficiency. We investigate the practical limits of present optoelectronic technology for cooling applications by optimizing a GaAs/GaInP double heterostructure LED. We develop a model of the design based on the physics of detailed balance and the methods of statistical ray optics, and predict an external luminescence efficiency of ηext = 97.7% at 263 K. To enhance the cooling coefficient of performance, we pair the refrigerated LED with a photovoltaic cell, which partially recovers the emitted optical energy as electricity. For applications near room temperature and moderate power densities (1.0–10 mW/cm2), we project that an electroluminescent refrigerator can operate with up to 1.7× the coefficient of performance of thermoelectric coolers with ZT = 1, using the material quality in existing GaAs devices. We also predict superior cooling efficiency for cryogenic applications relative to both thermoelectric and laser cooling. Large improvements to these results are possible with optoelectronic devices that asymptotically approach unity luminescence efficiency.

Hydrothermal growth and luminescent properties of nonpolar a-plane [formula omitted] ZnCdO films for light-emitting diodes

Nonpolar a-plane ZnCdO films have been obtained on a-plane GaN using a simple low-cost hydrothermal growth method at the low temperature of 80 °C. The morphological, structural, optical, and electrical properties of a-plane ZnCdO films with various Cd contents have been investigated and compared. The photoluminescence peak of the a-plane Zn0.957Cd 0.043O film, was observed to be centered at 429 nm at 25 °C. We demonstrated a heterostructure light-emitting diode (LED) using nonpolar n-type Zn0.957Cd0.043O/p-type GaN films. The rectifying behavior of the current-voltage characteristics was observed with a turn-on voltage of 5 V. The electroluminescence of the LED showed emission peaks including 430 nm, which indicates the near-band-edge emission of a-plane Zn0.957Cd0.043O at 25 °C.

Thermally evaporated hybrid perovskite for hetero-structured green light-emitting diodes

Thermal evaporation of green-light emitting perovskite (MaPbBr3) films is reported. Morphological studies show that a soft thermal treatment is needed to induce an outstanding crystal growth and film organization. Hetero-structured light-emitting diodes, embedding as-deposited and annealed MAPbBr3 films as active layers, are fabricated and their performances are compared, highlighting that the perovskite evolution is strongly dependent on the growing substrate, too.

Si nanotubes as an efficient electrode material for ZnO-based hetero-structure LEDs

Silicon in the forms of nanotube (NT), nanowire (NW), and bulk were used as the p-type electrode for zinc-oxide NWs on Si heterojunction light emitting diodes (LED). The LEDs were fabricated by direct contact between p-type Si and n-type ZnO NWs. The results show that using Si NTs leads to a significant improvement in their LED performance including considerably lower turn-on voltage, smaller series resistance, higher rectification ratio, and stronger light intensity. Using this electrode, an excellent rectifying diode behavior was obtained with a turn-on voltage of 0.6 V and a rectification ratio of 15 at 5 V. This study proposes Si NTs as a promising electrode material for the fabrication of ZnO-based heterojunction LEDs.