Electroluminescent cooling (ELC) of light–emitting diodes (LEDs) at high powers is yet to be demonstrated. Earlier studies of photoluminescent cooling (PLC) suggested that temperature strongly affects the light emission efficiency and therefore it is useful to explore the temperature range below room temperature (RT) where ELC might be easier to observe. With that purpose in mind, we electrically characterised three different sized (0.2, 0.5 and 1 mm diameter) double–diode structure (DDS) devices, consisting of a coupled LED and photodiode (PD), at temperatures ranging from 100 K to 325 K to investigate how the temperature affects the efficiency of the structures in practice. We found that, for the studied devices, the coupling quantum efficiency (CQE) as well as the overall efficiency indeed increase when temperature decreases and reach their highest values at temperatures below room temperature.
The cooling of a light emitting diode (LED) by photons carrying out more energy than was used to electrically bias the device, has been predicted decades ago.1, 2 While this effect, known as electroluminescent cooling (ELC), may allow e.g. fabricating thermophotonic heat pumps (THP) providing higher efficiencies than the existing solid state coolers,3 ELC at powers sufficient for practical applications is still not demonstrated. To study high-power ELC we use double diode structures (DDSs), which consist of a double heterojunction (DHJ) LED and a photodiode (PD) grown within a single technological process and, thus, enclosed in a cavity with a homogeneous refractive index.4, 5 The presence of the PD in the structure allows to more directly probe the efficiency of the LED, without the need for light extraction from the system, reducing undesirable losses. Our analysis of experimentally measured I − V curves for both the LED and the PD suggests that the local efficiency of the high-performance LEDs we have fabricated is approximately 110%, exceeding unity over a wide range of injection current densities of up to about 100A/cm2 . At present the efficiency of the full DDS, however, still falls short of unity, not allowing direct evidence of the extraction of thermal energy from the LED. Here we review our previous studies of DDS for high-power EL cooling and discuss in more detail the remaining bottlenecks for demonstrating high-power ELC in the DDS context: the LED surface states, resistive and photodetection losses. In particular we report our first surface passivation measurements. Further optimization therefore mainly involves reducing the influence of the surface states, e.g. using more efficient surface passivation techniques and optimizing the PD. This combined with the optimization of the DDS layer thicknesses and contact metallization schemes is expected to finally allow purely experimental observation of high-power ELC.
The optical and electrical properties of planar optoelectronic devices are well known, but their fully self-consistent modeling has remained a serious challenge. At the same time, the improving device fabrication capabilities and shrinking device sizes make it possible to reach higher efficiencies and develop totally new device applications. Success in this context, however, requires sophisticated device modeling frameworks, such as fully self-consistent models of optical and electrical characteristics. In this article, we explore the predictions provided by the recently introduced interference radiative transfer (IRT) model and apply it to a simplified double-diode structure presently used to study the possibility of electroluminescent cooling. The purpose of this proof-of-principle study is to show that the IRT model is straightforward to implement once one has access to the dyadic Green's functions, and that it produces solutions that satisfy the more general quantized fluctuational electrodynamics framework. We examine the photon numbers, propagating optical intensities and net radiative recombination rates from the IRT model solved by assuming a constant quasi-Fermi level separation in the active region. We find that they behave qualitatively as expected for the chosen device structure. However, the results also exhibit waveoptical characteristics, as e.g. the propagating intensity depends non-monotonously on the propagation angle due to constructive and destructive interferences. Based on the results, the IRT model offers a promising way to self-consistently combine the modeling of photon and charge carrier dynamics, also fully accounting for all interference effects.
In recent years, experimental work has shown that significant luminescence enhancement can be obtained from quantum-well (QW) light-emitting diodes (LEDs) by using metallic grating, which diffracts efficiently optical modes and resonances trapped in these structures and converts surface plasmon (SP) modes into radiative modes. We employ a powerful simulation tool to provide a deep insight into the physics of plasmonic enhancement and present guidelines on how to optimize light-extraction in III-nitride LED structures incorporating an emitting InGaN QW located in the vicinity of a grated silver surface. The model uses first-principle theory, coupling the dyadic Green’s function formalism for solving Maxwell’s equations to fluctuational electrodynamics, and employs a recursive and transparent solution method allowing the fields to be written in a closed form. We demonstrate the significant effect of the type of the periodic grating and layer structure on light-extraction efficiency by simulating various structures with different grating shapes and dimensions. Careful optimization of the grating features shows that the maximum enhancement can reach a factor of around 8 as compared to the flat semiconductor structure and that the plasmonic losses can be significantly reduced.
Recent measurements have generated a need to better understand the physics of hot carriers in III-Nitride (III-N) lightemitting diodes (LEDs) and in particular their relation to the efficiency droop and current transport. In this article we present fully self-consistent bipolar Monte Carlo (MC) simulations of carrier transport for detailed modeling of charge transport in III-N LEDs. The simulations are performed for a prototype LED structure to study the effects of hot holes and to compare predictions given by the bipolar MC model, the previously introduced hybrid Monte Carlo–drift-diffusion (MCDD) model, and the conventional drift-diffusion (DD) model. The predictions given by the bipolar MC model and the MCDD model are observed to be almost equivalent for the studied LED. Therefore our simulations suggest that hot holes do not significantly contribute to the basic operation of multi-quantum well LEDs, at least within the presently simulated range of material parameters. With the added hole transport simulation capabilities and fully self-constistent simulations, the bipolar Monte Carlo model provides a state-of-the-art tool to study the fine details of electron and hole dynamics in realistic LED structures. Further analysis of the results for a variety of LED structures will therefore be very useful in studying and optimizing the efficiency and current transport in next-generation LEDs.
Performance of III-N based solid-state lighting is to a large extent limited by current transport effects that are also expected
to contribute to the efficiency droop in real devices. To enable studying the contributions of electron transport in drooping
more accurately, we develop and study a coupled Monte Carlo–drift-diffusion (MCDD) method to model the details of
electron current transport in III-N optoelectronic devices. In the MCDD method, electron and hole distributions are first
simulated by solving the standard drift-diffusion (DD) equations. The hole density and recombination rate density obtained
from solving the DD equations are used as inputs in the Monte Carlo (MC) simulation of the electron system. The MC
simulation involves solving the Boltzmann transport equation for the electron gas to accurately describe electron transport.
As a hybrid of the DD and MC methods, the MCDD represents a first-order correction for electron transport in III-N LEDs
as compared to DD, predicting a significant hot electron population in the simulated multi-quantum well (MQW) LED
device at strong injection.
The unique properties of surface plasmons (SPs) are expected to provide a great improvement of light extraction in light-emitting diodes (LEDs). Surface plasmon modes are characterized by a high local density of states, and if scattered by gratings, significantly high emission enhancement is achievable. We investigate the physical role of SPs in improving light extraction from GaN quantum-well (QW) light-emitting structures incorporating metallic grating, by using first-principle theory based on Maxwell's equations and fluctuational electrodynamics. We demonstrate how careful nano-engineering, specifically by choosing the right nano-grating period, can reduce absorption losses and provide optimal enhancement; in the investigated test geometries, light extraction is increased by a factor of four, with the plasmonic losses being reduced from ~ 90% to below ~ 60% thanks to the metallic grating. While the results confirm a strong enhancement and reduction in the plasmonic losses, the overall losses still represent a significant obstacle for plasmonic-enhanced emission. With further optimization of the structure, the grating shapes and the materials, a much larger enhancement and lower losses are expected to be possible.
It has recently been shown that multiphoton absorption in cavities containing an emitter and a nonlinear mirror or a two photon absorber can be used to create antibunched photons (i.e. nonclassical light). We investigate the generation of nonclassical photon states using nonlinear laser cavities where the excitation has been modified so that it consists of short current pulses. The light fields in the studied setups are ideally formed of superpositions of zero photon and one photon states. Our goal is to study and develop single photon sources which are needed e.g. in quantum information processing and quantum computing, and fundamental quantum optical experiments. We investigate the effect of exciting the photon emitter with time dependent current pulses to provide single-photon-on-demand sources. We maximize the probability of the single photon state by optimizing the strengths of linear losses, nonlinear absorption, photon emission, and the length of the current injection pulse into the amplifier. Furthermore, we analyze the output photon statistics and waiting times using Monte Carlo simulations. This type of a setup is technologically attractive since it potentially provides room temperature realization of photon antibunching with essentially standard optoelectronic materials and processing techniques.
Surface plasmons (SPs) have recently gained substantial attention due to their sub-wavelength localization and strong interactions in the near-field. Their unique properties are expected to be essential for the next-generation photonic nanodevices, for instance, to improve light extraction in light-emitting diodes (LEDs). We discuss and develop a rigorous and transparent method to model luminescence enhancement and absorption in grated multilayer structures. The method is based on Green's functions, obtained as a perturbative solution to Maxwell's equations, and the fluctuational electrodynamics description of the structures. The model provides an analytical alternative to numerical methods such as finite-element methods and gives insight beyond the numerical solutions, offering a direct means of studying emission and luminescence from the periodic structures. The model is applied to answer key fundamental questions regarding luminescence enhancement, absorption and reflection in realistic
plasmonic GaN light-emitting diode (LED) structures. Two aspects are considered in particular: (1) modeling the reflectometry measurements of grated LED structures to explain and map the interference patterns observed experimentally by our collaborators, and (2) modeling the enhancement in plasmonic structures where the emission takes place in quantum wells in the vicinity of the metallic grating. The results clearly reveal e.g. the SP-related luminescence enhancement in InGaN quantum well structures incorporating periodic silver grating.