Semiconductor Light-emitting Diodes Research Articles

Heterostructures for Optoelectronics: History and Modern Trends

Semiconductor revolution of the 20th century determined not only technological, but also social development of the modern society. The precursors of modern semiconductor electronics were Oleg Losev's discoveries of “crystadine” and “light-emitting diode (LED)” nearly 100 years ago. Creation of the transistor and semiconductor laser and LED based on the homo p-n structure became the most decisive step. Semiconductor optoelectronics was born with the creation of GaAs and GaAsP lasers and “LED.” The possibility to control the type and level of conductivity and injection in p-n junctions was the seed from which semiconductor electronics developed. Heterostructures-“man-made crystals”-solved a more general problem: the necessity to control electron and photon fluxes in crystals. The development of physics and technology for semiconductor heterostructures resulted in drastic changes in our everyday life. It is hardly possible to imagine our recent life without double-heterostructure (DH) laser-based telecommunication systems, without efficient DH LEDs, heterostructure bipolar transistors, and high electron mobility transistors (HEMTs) for high-frequency applications. DH lasers were introduced in every household with CD players. Heterostructure solar cells have been widely used for space and terrestrial applications. The first proposals for heterostructures in semiconductor devices were made as early as in the 1950s, but the most important experimental results were received at the end of the 1960s when the first ideal AlGaAs heterostructures and low threshold room-temperature lasers, efficient heterostructure LEDs, and solar cells were created. Later semiconductor heterostructure became laboratories of low-dimension electron gases. Molecular-beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) technologies were the basis for the development of new structures, including superlattices, and for the industrial production of a large family of heterostructure semiconductor devices. Quantum dot (QD)-based optoelectronics as a modern trend in optoelectronics is the subject of a more detailed consideration.

Coherent vertical beaming using Bragg mirrors for high-efficiency GaN light-emitting diodes

We propose a dielectric Bragg mirror that utilizes coherent coupling with multiple quantum wells (MQWs) to significantly enhance light extraction from GaN light-emitting diode (LED). Full vectorial electromagnetic simulation showed that, under constructive interference conditions, the Bragg mirror consisting of two dielectric (SiO(2)/TiO(2)) stacks and a silver layer led to >30% enhancement in light extraction, as compared to a single silver mirror. Such significant enhancement by a pre-designed Bragg/metal mirror was ascribed to the vertically oriented radiation pattern and reduced plasmonic metal loss. In addition, the gap distance between the MQWs and a Bragg mirror at which the constructive interference takes place could be controlled by modulating the thickness of the first low-refractive-index layer. Moreover, a two-dimensional periodic pattern was incorporated into an upper GaN layer with the designed Bragg mirror and it was shown that a lattice constant of ~800 nm was optimal for light extraction. We believe that tailoring the radiation profile of light emitters by coherent coupling with designed high-reflectivity mirrors will be a promising route to overcome the efficiency limit of current semiconductor LED devices.

Direct Measurement of Auger Electrons Emitted from a Semiconductor Light-Emitting Diode under Electrical Injection: Identification of the Dominant Mechanism for Efficiency Droop

We report on the unambiguous detection of Auger electrons by electron emission spectroscopy from a cesiated InGaN/GaN light-emitting diode under electrical injection. Electron emission spectra were measured as a function of the current injected in the device. The appearance of high energy electron peaks simultaneously with an observed drop in electroluminescence efficiency shows that hot carriers are being generated in the active region (InGaN quantum wells) by an Auger process. A linear correlation was measured between the high energy emitted electron current and the "droop current"--the missing component of the injected current for light emission. We conclude that the droop phenomenon in GaN light-emitting diodes originates from the excitation of Auger processes.

Novel Thermoelectric Modules for Cooling Powerful LEDs: Experimental Results

We present the results of an experimental study of a cooling system based on a novel thermoelectric module specifically designed for thermal management of high-power light-emitting diodes (LEDs). The Seoul Semiconductor LED W724C0 device was chosen for experimental validation of the efficiency of the proposed cooling unit. Two cooling systems with identical heat sinks were tested for comparison: a state-of-the-art one based on an insulated metal substrate-printed circuit board (IMS-PCB), and a system with thermoelectric cooling. The obtained results show that use of thermoelectrics results in a considerable reduction of the LED operating temperature, providing increased light output and greatly increased LED lifetime.

LED lighting of automobiles and tractors

The article considers the possibilities of application of high-brightness semiconductor light-emitting diodes in lighting systems of automobiles and tractors. There are shown the main parameters of LEDs and circuit diagrams of their connection.

A BRIEF SURVEY ON BASIC PROPERTIES OF THIN FILMS FOR DEVICE APPLICATION

Thin film materials are the key elements of continued technological advances made in the fields of optoelectronic, photonic and magnetic devices. Thin film studies have directly or indirectly advanced many new areas of research in solid state physics and chemistry which are based on phenomena uniquely characteristic of the thickness, geometry and structure of the film. The processing of materials into thin films allows easy integration into various types of devices. Thin films are extremely thermally stable and reasonably hard, but they are fragile. On the other hand organic materials have reasonable thermal stability and are tough, but are soft. Thin film mechanical properties can be measured by tensile testing of freestanding films and by the micro beam cantilever deflection technique, but the easiest way is by means of nanoindentation. Optical experiments provide a good way of examining the properties of semiconductors. Particularly measuring the absorption coefficient for various energies gives information about the band gaps of the material. Thin film materials have been used in semiconductor devices, wireless communications, telecommunications, integrated circuits, rectifiers, transistors, solar cells, light-emitting diodes, photoconductors and light crystal displays, lithography, micro- electromechanical systems (MEMS) and multifunctional emerging coatings, as well as other emerging cutting technologies.

Creation of light-emitting diode optical beacons for spacecraft

A successful development of the semiconductor light-emitting diode (LED) technology made it possible to design efficient high-power light emission sources with small weight-and-size parameters. The technical characteristics of these LEDs allow one to use them as optical beacons on spacecraft. The application of LED optical beacons jointly with other equipment is intended to improve navigation support of space programs.

Enhancement of Light Extraction Efficiency of GaN Light Emitting Diodes Using Nanoscale Surface Corrugation

In this study, we have investigated highly efficient nanoscale surface corrugated light emitting diodes (LEDs) for the enhancement of light extraction efficiency (LEE) of nitride semiconductor LEDs. Nanoscale indium tin oxide (ITO) surface corrugations are fabricated by using the conformal nanoimprint technique; it was possible to observe an enhancement of LEE for the ITO surface corrugated LEDs. By incorporating this novel method, we determined that the total output power of the surface corrugated LEDs were enhanced by 45.6% for patterned sapphire substrate LEDs and by 41.9% for flat c-plane substrate LEDs. The enhancement of LEE through nanoscale surface corrugations was studied using 3-dimensional Finite Different Time Domain (FDTD) calculation. From the FDTD calculations, we were able to separate the light extraction from the top and bottom sides of device. This process revealed that light extraction from the top and bottom sides of a device strongly depends on the substrate and the surface corrugation. We found that enhanced LEE could be understood through the mechanism of enhanced light transmission due to refractive index matching and the increase of light scattering from the corrugated surface. LEE calculations for the encapsulated LEDs devices also revealed that low LEE enhancement is expected after encapsulation due to the reduction of the refractive index contrast.

3-D Simulation of Current Spreading in Semiconductor Light-Emitting and Laser Diodes Using Nonlinear Boundary Method

We describe and demonstrate the use of the nonlinear boundary method for 3-D simulation of semiconductor light-emitting and laser diodes. This method takes advantage of a dominant nonlinear behavior of the geometrically thin active layer in practical optoelectronic devices. This allows one to reduce the modeling only to the active layer coupled to the optical cavity. The reduction is achieved by expressing the nonuniform current injection from bulk regions into the layer using a surface integral, and does not involve any approximations. Using a device meshed with tetrahedral Delaunay meshes, we compare the performance of the boundary method against the more common method based on volume discretization, and show its advantages and limitations.

Superfocusing of mutimode semiconductor lasers and light-emitting diodes

The problem of focusing multimode radiation of high-power semiconductor lasers and light-emitting diodes (LEDs) has been studied. In these sources, low spatial quality of the output beam determines theoretical limit of the focal spot size (one to two orders of magnitude exceeding the diffraction limit), thus restricting the possibility of increasing power density and creating optical field gradients that are necessary in many practical applications. In order to overcome this limitation, we have developed a method of superfocusing of multimode radiation with the aid of interference. It is shown that, using this method, the focal spot size of high-power semiconductor lasers and LEDs can be reduced to a level unachievable by means of traditional focusing. An approach to exceed the theoretical limit of power density for focusing of radiation with high propagation parameter M2 is proposed.

Thermoelectrically Pumped Light-Emitting Diodes Operating above Unity Efficiency

A heated semiconductor light-emitting diode at low forward bias voltage V<k(B)T/q is shown to use electrical work to pump heat from the lattice to the photon field. Here the rates of both radiative and nonradiative recombination have contributions at linear order in V. As a result the device's wall-plug (i.e., power conversion) efficiency is inversely proportional to its output power and diverges as V approaches zero. Experiments directly confirm for the first time that this behavior continues beyond the conventional limit of unity electrical-to-optical power conversion efficiency.

Ultraviolet goes solid-state

Semiconductor light-emitting diodes may soon replace mercury lamps as the ultraviolet source of choice in a wide range of applications. Researchers around the world are now racing to increase the efficiency and output power of such ultraviolet solid-state devices.

Inactivation of Bacterial Viruses in Water Using Deep Ultraviolet Semiconductor Light-Emitting Diode

Disinfection of bacterial viruses MS2, Qβ, and φX174 in water was successfully carried out using deep ultraviolet light-emitting diodes (DUV-LEDs) operated at 280 nm and 255 nm. It was shown that t...

Recent developments in handheld and portable optosensing—A review

Recent developments in portable and handheld opto-chemical analytical instrumentation over the last decade (2000–2010) are reviewed. First, the characteristics of typical portable/handheld instrumentation are discussed from different points of view: in situ operation, low energy consumption, ease of use, and self-contained devices. These advancements have improved or hastened improvements in the development of miniaturized optoelectronic and optical components, mainly solid-state devices such as different types of semiconductor lasers, light-emitting diodes, and photodiodes. A brief review of advances in these components is also presented. The numerous examples of portable instrumentation presented have been classified according to direct-recognition and reagent-based sensing, and within these, by absorption and emission-based systems. The conclusion discusses some key trends and future perspectives for this technology.

Estimating the Junction Temperature of InGaN and AlGaInP Light-Emitting Diodes

The junction temperature of light-emitting diodes (LEDs) directly and significantly affects LEDs performances. Thus, accurate measurement and precise estimation of LEDs junction temperature become extremely important. In this study, we analyze the physical foundation of temperature-dependent electrical characteristics and develop a new scheme to directly express the dependence of junction temperature on injected current for InGaN and AlGaInP LEDs. From a more general viewpoint, our scheme for the estimation of junction temperature is primarily based on the LEDs external properties, and therefore can be applied to other kinds of III–V compound-based semiconductor LEDs.

Annealing Effects on Sensitivity of Atomic-Layer-Deposited SrTiO3-Based Oxygen Sensors

Semiconductive dielectric SrTiO3-based thin films are promising candidates for in situ monitoring of trace levels of oxygen in semiconductor or organic light-emitting diode (LED) display manufacturing processes. SrTiO3-based thin films, which are deposited by atomic-layer deposition (ALD), exhibit high sensitivity to oxygen at room temperature; however, the sensitivity can be affected by SrO-based surface segregation. In this study, we will show that annealing temperature and time are the key parameters for decreasing the coverage of oxygen-insensitive SrO-based surface segregation.

An entangled-light-emitting diode

An optical quantum computer, powerful enough to solve problems so far intractable using conventional digital logic, requires a large number of entangled photons. At present, entangled-light sources are optically driven with lasers, which are impractical for quantum computing owing to the bulk and complexity of the optics required for large-scale applications. Parametric down-conversion is the most widely used source of entangled light, and has been used to implement non-destructive quantum logic gates. However, these sources are Poissonian and probabilistically emit zero or multiple entangled photon pairs in most cycles, fundamentally limiting the success probability of quantum computational operations. These complications can be overcome by using an electrically driven on-demand source of entangled photon pairs, but so far such a source has not been produced. Here we report the realization of an electrically driven source of entangled photon pairs, consisting of a quantum dot embedded in a semiconductor light-emitting diode (LED) structure. We show that the device emits entangled photon pairs under d.c. and a.c. injection, the latter achieving an entanglement fidelity of up to 0.82. Entangled light with such high fidelity is sufficient for application in quantum relays, in core components of quantum computing such as teleportation, and in entanglement swapping. The a.c. operation of the entangled-light-emitting diode (ELED) indicates its potential function as an on-demand source without the need for a complicated laser driving system; consequently, the ELED is at present the best source on which to base future scalable quantum information applications.

Lasing in opal saturated with Rodamin-6G organic dye

It was shown that low-threshold lasing can be implemented by exciting photoluminescence in opal filled with Rodamin-6G dye using semiconductor light-emitting diodes.

Intelligent Non-Conventional Applications of LEDs

Needless to say, an LED (light-emitting diode) is a semiconductor light source and LEDs are used as indicators and display elements in various devices. During the past few decades, the light intensities of LEDs have increased a great deal; therefore, white LEDs in particular are increasingly used for lighting. For more than thirty years, the author has been doing research on applications of LEDs other than such conventional utilizations. His applications are novel and original. Furthermore, they are extensively applied in the fields of engineering, science, horticulture, agriculture, medicine, fishery and various LED commodities. In this paper the author will introduce his pioneering research on his LED applications.

Polarization-Induced Hole Doping in Wide–Band-Gap Uniaxial Semiconductor Heterostructures

Impurity-based p-type doping in wide-band-gap semiconductors is inefficient at room temperature for applications such as lasers because the positive-charge carriers (holes) have a large thermal activation energy. We demonstrate high-efficiency p-type doping by ionizing acceptor dopants using the built-in electronic polarization in bulk uniaxial semiconductor crystals. Because the mobile hole gases are field-ionized, they are robust to thermal freezeout effects and lead to major improvements in p-type electrical conductivity. The new doping technique results in improved optical emission efficiency in prototype ultraviolet light-emitting-diode structures. Polarization-induced doping provides an attractive solution to both p- and n-type doping problems in wide-band-gap semiconductors and offers an unconventional path for the development of solid-state deep-ultraviolet optoelectronic devices and wide-band-gap bipolar electronic devices of the future.