Double-heterostructure Light-emitting Diodes Research Articles

(Invited) Emerging Opto-Electronics and Quantum-Photonics Based on Ultra-Low Temperature Epitaxy of Group-IV Nanolayers

Germanium has already successfully entered Si technology through electronics and photonics applications [1,2]. However, due to the inherent lattice mismatch between Ge and Si, high-crystalline SiGe epitaxial layers of a sufficient thickness could only be grown for relatively low Ge concentration (<40%). At higher concentrations, the epitaxial SiGe layers relax the strain through either plastic or elastic relaxation. Thus, for about 2% (and 4%) strain between substrate and epilayer, only layer thicknesses of < 2nm (<0.5 nm) can be grown before elastic relaxation leads to quantum dot formation. In principle, such epitaxial Ge-rich QDs on Si(001) substrates can be successfully used to enhance the light emission properties of SiGe material [3-7]. However, planar Ge-rich two-dimensional layers would be preferred for efficient large-scale integration and reliable addressing of the nanostructures.Here, we demonstrate that ultra-low temperature (ULT) growth, carried out in a temperature window between 100°C and 350°C, is the key to extensive epilayer supersaturation, leading to layer thicknesses that are about one order of magnitude larger than what can be achieved by conventional epitaxy [8]. Thereby, we highlight that molecular beam epitaxy is the suitable choice for ULT growth since, using chemical vapor deposition methods for Si deposition, the ULT range cannot be reached due to the lack of precursor decomposition. However, we also stress that the chamber conditions during the growth need to be excellent in order to limit detrimental point defect formation during epitaxy.If these preconditions are met, ULT growth can lead to up to now unattainable layer structures. These, in turn, can be used, e.g., for the first available double heterostructure (DHS) light-emitting diodes in the group-IV system. We demonstrate that these LEDs, emitting in the telecom band, work efficiently up to room temperature and above. Light emission above room temperature is eventually limited by minority carrier injection.Additionally, novel electronic device concepts such as reconfigurable field effect transistors (RFETs) can be realized using several nanometer-thick Ge layers grown directly on silicon on insulator substrates. We argue that these devices have distinct advantages in performance and integration possibilities as compared to conventional nanoelectronics devices based on Ge-rich material, for which typically vertically grown nanowires are employed [9,10].

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.

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.

Theoretical analysis of performance enhancement in GeSn/SiGeSn light-emitting diode enabled by Si3N4 liner stressor technique.

We comprehensively investigate the energy band diagrams, carrier distribution, spontaneous emission rate rsp, and the internal quantum efficiency ηIQE in the lattice-matched GeSn/SiGeSn double heterostructure light-emitting diode (LED) wrapped in a Si3N4 liner stressor. The large tensile strain introduced into the device by the expansion of the Si3N4 liner is characterized by numerical simulation. A lower Sn composition required for the indirect to direct bandgap transition and a higher ratio of the electron occupation probability in the Γ conduction valley are achieved in the tensile strained GeSn/SiGeSn LED in comparison with the relaxed device. Analytical calculation shows that the tensile strained LED wrapped in the Si3N4 liner stressor exhibits the improved rsp and ηIQE compared to the relaxed device. rsp and ηIQE also can be enhanced by increasing Sn composition, carrier injection density, and n-type doping concentration in the GeSn active layer.

Numerical modeling of an InAsSb/InAsSbP double heterojunction light emitting diode for mid-infrared (2–5 μm) applications

The numerical model of a Mid-infrared (MIR) P+–InAs0.48Sb0.22P0.30/n0–InAs0.89Sb0.11/N−–InAs0.48Sb0.22P0.30 double heterostructure light emitting diode (DH-LED) has been reported in this paper. The governing equation of the structure has been solved numerically under the condition of high injection using fourth order Runge-Kutta method employing the shooting algorithm. The numerical model takes into account all dominant radiative and non-radiative recombination processes, surface recombination velocity and self-absorption in the active layer of DH-LED. The DH-LED has been simulated for mid-infrared applications by considering modulation bandwidth and its dependence on active layer width, doping concentration, and injected carrier density. The effect of surface recombination velocity on the quantum efficiency, peak light intensity for different values of active layer width and doping concentration has been estimated numerically. The rise time of the structure has been evaluated by considering transient response for a step current of 50mA. The output power of DH-LED has been computed as a function of bias current and compared/ contrasted with the reported analytical and experimental results.

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.

The effect of stair case electron injector design on electron overflow in InGaN light emitting diodes

Effect of two-layer (In0.04Ga0.96N and In0.08Ga0.92N) staircase electron injector (SEI) on quantum efficiency of light-emitting-diodes (LEDs) in the context of active regions composed of single and quad 3 nm double heterostructures (DHs) is reported. The experiments were augmented with the first order model calculations of electron overflow percentile. Increasing the two-layer SEI thickness from 4 + 4 nm up to 20 + 20 nm substantially reduced, if not totally eliminated, the electron overflow in single DH LEDs at low injections without degrading the material quality evidenced by the high optical efficiency observed at 15 K and room temperature. The improvement in quad 3 nm DH LEDs with increasing SEI thickness is not so pronounced as the influence of SEI is less for thicker active regions, which in and of themselves necessarily thermalize the carriers.

Effect of Trapezoidal-Shaped Well on Efficiency Droop in InGaN-Based Double-Heterostructure Light-Emitting Diodes

We investigated the effects of different well shapes on the external quantum efficiency (EQE) and the efficiency droop in wide-well InGaN/GaN double-heterostructure light-emitting diodes. For forward current densities in the measurement range of greater than 135 A/cm2, the device featuring a trapezoidal well exhibited improved EQEs and alleviative efficiency droop, relative to those of the device featuring a rectangular well. The decreased Auger loss has been proposed as the main reason for the greater maximum efficiency that occurred at high current density (&gt;50 A/cm2). For the devices incorporating trapezoidal and rectangular wells, the EQEs at 200 A/cm2decreased by 14 and 40%, respectively, from their maximum values, resulting in the EQE at a current density of 200 A/cm2of the device featuring a trapezoidal well being 17.5% greater than that featuring a rectangular well. These results suggest that, in addition to the decreased Auger loss, the alleviation in efficiency droop at higher current densities might be due to higher internal quantum efficiency resulted from the improved carrier injection efficiency of the trapezoidal well.

Electroluminescence of ZnO-based semiconductor heterostructures

Using pulsed laser deposition, we have grown n-ZnO/p-GaN, n-ZnO/i-ZnO/p-GaN and n-ZnO/n-Mg0.2Zn0.8O/i-Cd0.2Zn0.8O/p-GaN light-emitting diode (LED) heterostructures with peak emission wavelengths of 495, 382 and 465 nm and threshold current densities (used in electroluminescence measurements) of 1.35, 2, and 0.48 A cm-2, respectively. Because of the spatial carrier confinement, the n-ZnO/n-Mg0.2Zn0.8O/i-Cd0.2Zn0.8O/p-GaN double heterostructure LED offers a higher electroluminescence intensity and lower electroluminescence threshold in comparison with the n-ZnO/p-GaN and n-ZnO/i-ZnO/p-GaN LEDs.

Effect of an asymmetry AlGaN barrier on efficiency droop in wide-well InGaN double-heterostructure light-emitting diodes

External-quantum-efficiency (EQE) and efficiency droop in wide-well InGaN double-heterostructure light-emitting diodes have been investigated. It was found that the insertion of an AlGaN barrier between the n-type GaN layer and the InGaN well resulted in higher peak EQE and reduced efficiency droop at a higher injection level. EQE was improved by 5.7% and 25.8% over that of a sample without an AlGaN barrier at a current density of 104.3 A/cm2 and 521 A/cm2, respectively. It is suggested that the mechanism is attributed to an electron decelerating effect that enlarges the effective active region.

Hot electron effects on efficiency degradation in InGaN light emitting diodes and designs to mitigate them

Hot electrons and the associated ballistic and quasiballistic transport, heretofore neglected endemically, across the active regions of InGaN light emitting diodes (LEDs) have been incorporated into a first order simple model which explains the experimental observations of electron spillover and the efficiency degradation at high injection levels. The model is in good agreement with experiments wherein an adjustable barrier hot electron stopper, commonly called the electron blocking layer (EBL), is incorporated. The model is also in agreement with experiments wherein the electrons are cooled, eliminating hot electrons, inside a staircase electron injector (SEI) prior to their injection into the active region. Thermionic emission from the active region, even if one uses an uncharacteristically high junction temperature of 1000 K, fails to account for the carrier spillover and the experimental observations in our laboratory in samples with varying EBL barrier heights. The model has been successfully applied to both m-plane (lacking polarization induced electric field) and c-plane (with polarization induced field) InGaN double heterostructure (DH) LEDs with a 6 nm active region featuring a variable barrier hot electron stopper, and a SEI, and the various combinations thereof. The choice of DH LEDs stems from our desire to keep the sample structure simple as well as the model calculations. In this paper, the theoretical and experimental data along with their comparison followed by an insightful discussion are given. The model and the approaches to eliminate carrier spillover proposed here for InGaN LEDs are also applicable to GaN-based laser diodes.

Enhanced Room-Temperature 1.6 µm Electroluminescence from Si-Based Double-Heterostructure Light-Emitting Diodes Using Iron Disilicide

We have fabricated Si/β-FeSi2/Si (SFS) double-heterostructure (DH) light-emitting diodes (LEDs) on Si(111) substrates with β-FeSi2 thickness ranging from 80 nm to 1 µm, and Si0.7Ge0.3/β-FeSi2/Si0.7Ge0.3(SGFSG) DH LEDs with a 200-nm-thick β-FeSi2 layer using lattice-matched Si0.7Ge0.3 layers by molecular-beam epitaxy. The electroluminescence (EL) peaked at an emission wavelength of approximately 1.6 µm at room temperature. As the thickness of the β-FeSi2 layer was increased in the SFS DH LEDs, the emission power of EL increased for a given current density J. EL with an emission power of over 0.4 mW and an external quantum efficiency of approximately 0.1% was achieved for the SFS DH LED with a 1-µm-thick β-FeSi2 layer. The smallest J value necessary for EL output, which is approximately 1 A/cm2, was achieved for the SGFSG DH LEDs.

Droop in InGaN light-emitting diodes: A differential carrier lifetime analysis

To investigate the variation in internal quantum efficiency in InGaN structures, we measure the differential carrier lifetime of an InGaN/GaN double-heterostructure light-emitting diode under varying electroluminescence injection conditions. By coupling this measurement to an internal quantum efficiency measurement, we determine the carrier density and the radiative and nonradiative contributions to the lifetime without making any assumptions on recombination processes. We find that droop is caused by a shortening of the nonradiative lifetime with current. The observed shortening of both radiative and nonradiative lifetimes with current is found to be in excellent agreement with an ABC model including phase-space filling.

The challenge of unity wall plug efficiency: The effects of internal heating on the efficiency of light emitting diodes

We develop a self-consistent model to describe the internal heating of high power light emitting diodes (LEDs) and use this model to simulate the operation of GaAs–AlGaAs double heterostructure LEDs. We account for the heating by nonradiative recombination processes in the simulations and solve self-consistently the steady state junction temperature. Based on the simulation results, we discuss the plausibility of unity conversion efficiency in LEDs and also the mechanisms underlying the efficiency droop. We show that the rise in the junction temperature limits the light output available from LEDs and further degrades the efficiency of operation at high operating currents. In addition to high power applications we study the optimal operating point and discuss the methods to increase the efficiency of LEDs toward the thermodynamical limits.

P-Si/β-FeSi2/n-Si double-heterostructure light-emitting diodes achieving 1.6 μm electroluminescence of 0.4 mW at room temperature

Electroluminescence at an emission power of over 0.4 mW is achieved at an emission wavelength of 1.6 μm using a p-Si/β-FeSi2/n-Si double-heterostructure light-emitting diode. This emission power is obtained at room temperature under current injection of 460 mA, corresponding to an external quantum efficiency of approximately 0.1%. Photoluminescence and time-resolved photoluminescence measurements for devices with different thicknesses of β-FeSi2 indicate that radiative recombination rate increased as the thickness of the β-FeSi2 active layer is increased.

Blue-emitting InGaN–GaN double-heterostructure light-emitting diodes reaching maximum quantum efficiency above 200A∕cm2

Auger recombination is determined to be the limiting factor for quantum efficiency for InGaN–GaN (0001) light-emitting diodes (LEDs) at high current density. High-power double-heterostructure (DH) LEDs are grown by metal-organic chemical vapor deposition. By increasing the active layer thickness, DH LEDs can reach a maximum in quantum efficiency at current densities above 200A∕cm2. Encapsulated thin-film flip-chip DH LEDs with peak wavelength of 432nm have an external quantum efficiency of 40% and output power of 2.3W at 2A.

Measurement of the thermal characteristics of packaged double-heterostructure light emitting diodes for space applications using spontaneous optical spectrum properties

In this paper, the thermal characteristics of packaged infrared double-heterostructure light emitting diode (DH-LED), used in space applications, are measured under conditions that reproduce space environments. The characterisation uses spontaneous optical spectrum characteristics, current–voltage curves and optical power measured under a primary vacuum (<10 −2 Torr) at temperatures between −30 and 100 °C. The investigations have been specifically oriented toward the extraction of junction temperature in the steady-state regime and junction-to-case thermal resistance. A specific model based on semiconductor theory for electrical transport has been used to calculate the shape of the spontaneous emission spectrum between the band-gap energy and higher energies and its change versus temperature. A linear relation between the junction temperature and the dissipated power has been found for various case temperatures appropriately controlled in a LN 2 cryostat. These results confirm that thermal behavior of DH-LEDs depends on both environment temperature and dissipated power level in the active zone and that the junction-to-case thermal resistance is not constant over a large range of temperatures, diminishing at higher currents as already reported by recent papers on high brightness DH-LED. Finally, this study could represent a practical non-destructive method providing qualitative information about variations of junction temperature and junction-to-case thermal resistance taking into account an industrial qualification framework approach based on electroluminescence analysis, frequently measured by manufacturers or end-users.

Application of microsize light-emitting diode structure for monolithic optoelectronic integrated circuits

A Si/III-V-N alloys/Si structure was grown on a Si substrate by solid-source molecular beam epitaxy (SSMBE) with an rf plasma nitrogen source and electron-beam (EB) evaporator. A two-dimensional (2D) growth mode was maintained during the growth of all layers. High-resolution X-ray diffraction (HRXRD) revealed that the structure had a small lattice mismatch to the Si substrate. InGaPN/GaPN double-heterostructure (DH) light-emitting diodes (LEDs) were fabricated on Si/III-V-N alloys/Si structure. The various sized LEDs were fabricated to put into the MOSFET for monolithic optoelectronic integrated circuits (OEIC). The luminescence properties of LEDs were evaluated by electroluminescence (EL). A double emission peak from all LED samples was observed at about 642 nm and 695 nm at room temperature (RT). As injection current increased, the emission peak wavelength changed from the peak wavelength of the InGaPN layer to that of the GaPN layer, likely due to carrier overflow of the active layer. A simplified fabrication process for the microsize LED of the unit circuit was proposed. The LEDs with emission areas from 5 x 5 μm 2 to 20 x 20 μm 2 were fabricated. The LED with an emission area of 5 x 5 μm 2 can be applied to an optical device of a monolithic OEIC.

Investigation of current injection in β-FeSi 2/Si double-heterostructures light-emitting diodes by molecular beam epitaxy

The p-Si/ p-β-FeSi 2/ p-Si/ n-Si light-emitting diode (LED) was fabricated by molecular beam epitaxy (MBE) on Si(111) substrates. Compared to our previous p-Si/ p-β-FeSi 2/ n-Si double heterostructures (DH) LED, the turn-on voltage in the current–voltage ( I– V) characteristics increased by approximately 0.2 V, meaning that defect densities at around the p– n junction were reduced. However, Si-related EL (1.05 eV) became dominant unexpectedly, and thus EL of β-FeSi 2 was suppressed accordingly. The origin of the luminescence is considered to be transitions via defect levels (1.05 eV) being due probably to Fe-B complex.