Browsing by Author "Radevici, Ivan"
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Item Back-Contacted Carrier Injection for Scalable GaN Light Emitters(WILEY-V C H VERLAG GMBH, 2022-01) Kim, Iurii; Kauppinen, Christoffer; Radevici, Ivan; Kivisaari, Pyry; Oksanen, Jani; Department of Electronics and Nanoengineering; Department of Neuroscience and Biomedical Engineering; Markku Sopanen GroupIt has recently been proposed that back-contacted III-V light-emitting diodes (LEDs) could offer improved current spreading as compared to conventional mesa or double side contacted structures. This has inspired also experimental efforts to realize such structures, but fabrication methods for them have not yet been fully established. Herein, the use of unintentionally doped and partially carrier-selective contacts (SC) is studied to realize back-contacted indium gallium nitride (InGaN) LEDs. The sharp electroluminescence peak at 439 nm from the multiquantum well stack demonstrates that the approach allows fabricating back-contacted InGaN LEDs without intentionally doped n-GaN layers and without inflicting damage in the active region, often observed in alternative approaches relying on lateral doping and the use of high energy particles during fabrication. The samples are fabricated on a finger configuration with several finger widths between 1 and 20 mu m. It is observed that the emission spreads most uniformly throughout the structure for fingers with the width of 5 mu m. As shown by the simulations, with improved contact resistances, the structures reported herein could enable fabricating back-contacted LEDs with unity injection efficiency and improved current spreading, offering a path toward large-area LEDs without contact shading even in materials where n-doping is elusive.Item Back-Contacted GaInP/GaAs LED Structures by Ex-Situ Dopant Redistribution(IEEE, 2023-10-01) Myllynen, Antti; Shahahmadi, Seyed Ahmad; Radevici, Ivan; Oksanen, Jani; Department of Neuroscience and Biomedical EngineeringCompound semiconductor devices utilizing interdigitated back-contact (IBC) designs with a uniform active region (AR) can enable a new generation of optoelectronic devices with eliminated contact shading, reduced resistive losses, and minimal current crowding. However, appropriate lateral doping techniques for such devices are not yet established. This work demonstrates selective-area diffusion doping from an epitaxially grown dopant source layer enabling the fabrication of GaAs-based light-emitting diodes (LEDs) utilizing diffusion-driven charge transport (DDCT) and the IBC design. The effects of doping and device dimensions are analyzed by comparing current-voltage characteristics and electroluminescence (EL) of laterally doped DDCT structures and control structures with several characteristic finger widths between 15 and 300 μm. Additional simulations confirm that the DDCT structure enables effective carrier injection into a buried AR outside the p-n junction. A current density of 1.25 A/cm2 was measured for the fabricated DDCT-LED with 15-μm wide fingers at a moderate bias voltage of 1.3 V. The light emission from the DDCT-LEDs shows clear signs of lateral current injection, improved current spreading, and a tenfold increase in EL, when compared to control structures specifically designed to validate the presence of diffusion doping. These results indicate that diffusion doping can enable the means to fabricate DDCT structures with effective carrier injection into a uniform AR.Item Chemovoltaic effect for renewable liquid and vapor fuels on semiconductor surfaces(Wiley-VCH Verlag, 2024-03-08) Alizadeh, Mahdi; Radevici, Ivan; Li, Shengyang; Oksanen, Jani; Department of Neuroscience and Biomedical EngineeringThe chemovoltaic effect – generation of electronic excitation by exergonic redox reactions – has been observed on metallic surfaces of Schottky junctions and is proving to be pivotal in explaining in detail the momentum conservation relations of chemically active collisions. As shown in this work, it can hold keys for direct chemical energy harvesting by semiconductor solar cells. To study the possibilities of chemovoltaic energy conversion by semiconductors, we have modeled and designed an ‘electrolyte-free fuel cell’ formed by a GaAs diode that can host electrochemical fuel oxidation and oxidant reduction reactions on its conduction and valence bands and as a result convert renewable chemical energy (as well as light) into electricity. The experimental results show that exposing the surface of a suitably designed solar cell to methanol liquid or vapor in the presence of oxygen or hydrogen peroxide leads to the generation of electrical power.Item Effect of interface recombination on the efficiency of intracavity double diode structures(SPRINGER, 2019-06-01) Sadi, Toufik; Radevici, Ivan; Kivisaari, Pyry; Casado, Alberto; Oksanen, Jani; Department of Neuroscience and Biomedical EngineeringIn the past ten years, there has been significant progress in solid-state optical refrigeration, causing a renewed interest in the possibility of electroluminescent cooling (ELC) in light emitting diodes (LEDs). More recently, our work on III-As based intracavity double diode structures (DDSs) indicates that the threshold for ELC can be reached, in practice, at high powers and 300K if certain non-radiative recombination mechanisms and photodetector (PD) losses are minimized. The studied DDSs consist of a LED, incorporating a high-quality GaAs active layer, optically coupled to a GaAs p-n homojunction PD. Both the LED and PD are integrated in a single device, offering a unique environment for studying ELC. In this paper, we provide a brief overview of the DDS characteristics and investigate the impact of non-radiative interface recombination on the LED, showing how the choice of the barrier layer materials can suppress this effect. We use experimental characterization techniques to calibrate numerical simulations, coupling the drift-diffusion model for charge transport to a photon transport model. To explore the interface effects, we compare DDSs with either GaInP/GaAs or AlGaAs/GaAs double heterojunctions. The results suggest that GaInP barriers allow interface recombination suppression that is sufficient to reach internal cooling in the LED.Item Efficient Fully-Coupled Electro-Optical Simulation Framework for Large-Area Planar Device(2019-07-01) Sadi, Toufik; Casado, Alberto; Radevici, Ivan; Kivisaari, Pyry; Oksanen, Jani; Department of Neuroscience and Biomedical Engineering; Hinzer, Karin; Piprek, JoachimOngoing progress in optoelectronic devices necessitates computational tools that self-consistently account for both electronic charge carrier and photon dynamics and interactions. In this paper, we introduce an efficient simulation framework, using the concepts of nonlinear transmission lines, to study fully-coupled charge and photon transport in planar devices. Within the developed framework, the drift-diffusion equations for charge transport are self-consistently coupled with the radiative transfer equation for photon transport and a separate lateral transport model, to obtain a realistic picture of the electro-optical device behaviour. The model allows the detailed study of large-area devices with full access to the wavelength and angle dependent features. It also accounts for photon recycling, providing deeper insight into the complex nature of optical energy transfer and losses in planar multi-layer structures. The efficiency of the framework is illustrated by applying it to study intracavity diode structures, which are intended for exploring high-power electroluminescent cooling in III-V light-emitting diodes.Item Electro-Optical Coupling in Double Diode Structures(American Physical Society, 2023-06) Anttu, Nicklas; Dagytė, Vilgailė; Behaghel, Benoît; Radevici, Ivan; Sadi, Toufik; Kivisaari, Pyry; Oksanen, Jani; Department of Neuroscience and Biomedical EngineeringAlternative types of artificial cooling techniques are of large interest for multiple applications. Here, we develop a framework for studying the role of electro-optical coupling in the analysis of solid-state refrigerators based on electroluminescent cooling (ELC) by combining device measurements with optical simulations. The studied device consists of a light-emitting diode (LED) epitaxially connected to a photodetector (PD) in a double-diode structure (DDS). Previous results of the DDS have indicated that the LED side already operates at conditions corresponding to ELC, but Ohmic losses and imperfect photodetection of the LED light in the PD have prevented observing the effect directly. Here, to break down the detection losses of the DDS, we report on the electro-optical response of the LED and the PD in detail, as well as the role of the spectral coupling from the LED to the PD. We present a detailed framework for combining measurements and simulations of the DDS to gain quantitative insight of the electro-optical response of the LED and PD, as well as the coupling between them, including the analysis of effects that are not directly accessible by standard measurements. The developed approach allows identifying the different photodetection loss mechanisms from the current-voltage and electroluminescence measurements and thereby gives guidance for designs toward a direct demonstration of ELC at practically relevant cooling powers. Somewhat surprisingly, the results show that an imperfect spectral absorption efficiency of the PD, in addition to its below unity quantum efficiency, are together required to explain the previously observed low photodetection efficiency of the DDS even for several microns thick PD structures. In comparison, the LED top mirror introduces only a minuscule drop in photodetection efficiency. Put in plain numbers, our analysis reveals that in the current DDS designs, there is headroom by 14% in the spectral matching between the LED and the PD, 5% in the charge collection efficiency of the PD, and 4% in the efficiency at which photons emitted from the LED reach the PD.Item Electroluminescent Cooling in III-V Intracavity Diodes: Efficiency Bottlenecks(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2019-06-01) Sadi, Toufik; Radevici, Ivan; Kivisaari, Pyry; Oksanen, Jani; Department of Neuroscience and Biomedical EngineeringRecent advances in the photoluminescent cooling of doped glasses provoke the question of whether similar progress is possible in electroluminescent cooling (ELC), and if so, what are the conditions for observing it at high powers. Here, we establish a simulation framework for III-V intracavity double-diode structures (DDSs) intended for studying ELC and introduce and analyze the most relevant figures of merit for the recently measured devices exhibiting the highest reported quantum efficiency of 70%. In essence, the DDSs optically couple a GaInP/GaAs double heterojunction light-emitting diode (LED) and a GaAs p-n homojunction photodetector (PD), integrated as a single device. The modeling framework couples the drift-diffusion charge transport model with a photon transport model and uses our recent experimental measurements for validation and the extraction of important material parameters. Results show that the model can accurately describe the experimental behavior over many orders of magnitude and suggest that the internal efficiency of the LED already exceeds the cooling threshold. Directly observing cooling in the presently studied devices, however, is still hindered by bottlenecks arising from the surface recombination at the LED walls and recombination losses in the PD.Item Electroluminescent Cooling in III-V Intracavity Diodes: Practical Requirements(2018-12) Sadi, Toufik; Radevici, Ivan; Kivisaari, Pyry; Oksanen, Jani; Department of Neuroscience and Biomedical EngineeringRecent studies of electroluminescent cooling (ELC) in III-V structures demonstrate the need to better understand the factors affecting the efficiency of light emission and energy transport in light-emitting diodes (LEDs). In this paper, we establish the physical and operational requirements for reaching the efficiencies needed for observing ELC in the III-V intracavity double-diode structures at high powers. The experimentally validated modeling framework used in this paper, coupling the drift-diffusion charge transport model with a photon transport model, indicates that the bulk properties of the III-V materials are already sufficient for ELC. Furthermore, the results suggest that the bulk power conversion efficiency of the LED in the devices, which allowed the experimentally measured record high coupling quantum efficiency of 70%, already exceeds 115%. However, as shown here, direct observation of ELC by electrical measurements still requires a combination of a more efficient suppression of the nonradiative surface recombination at the LED walls and the reduction of the detection losses in the photodetector of the intracavity structures.Item Electroluminescent cooling in intracavity light emitters(2018-01-01) Sadi, Toufik; Kivisaari, Pyry; Tiira, Jonna; Radevici, Ivan; Haggren, Tuomas; Oksanen, Jani; Department of Neuroscience and Biomedical EngineeringWe develop a coupled electronic charge and photon transport simulation model to allow for deeper analysis of our recent experimental studies of intracavity double diode structures (DDSs). The studied structures consist of optically coupled AlGaAs/GaAs double heterojunction light emitting diode (LED) and GaAs p–n-homojunction photodiode (PD) structure, integrated as a single semiconductor device. The drift–diffusion formalism for charge transport and an optical model, coupling the LED and the PD, are self-consistently applied to complement our experimental work on the evaluation of the efficiency of these DDSs. This is to understand better their suitability for electroluminescent cooling (ELC) demonstration, and shed further light on electroluminescence and optical energy transfer in the structures. The presented results emphasize the adverse effect of non-radiative recombination on device efficiency, which is the main obstacle for achieving ELC in III-V semiconductors.Item Electroluminescent cooling in intracavity light emitters: modeling and experiments(Springer Nature, 2017) Sadi, Toufik; Kivisaari, Pyry; Tiira, Jonna; Radevici, Ivan; Haggren, Tuomas; Oksanen, Jani; Neurotieteen ja lääketieteellisen tekniikan laitos; Department of Neuroscience and Biomedical Engineering; Engineered Nanosystems; Perustieteiden korkeakoulu; School of ScienceWe develop a coupled electronic charge and photon transport simulation model to allow for deeper analysis of our recent experimental studies of intracavity double diode structures (DDSs). The studied structures consist of optically coupled AlGaAs/GaAs double heterojunction light emitting diode (LED) and GaAs p–n-homojunction photodiode (PD) structure, integrated as a single semiconductor device. The drift–diffusion formalism for charge transport and an optical model, coupling the LED and the PD, are self-consistently applied to complement our experimental work on the evaluation of the efficiency of these DDSs. This is to understand better their suitability for electroluminescent cooling (ELC) demonstration, and shed further light on electroluminescence and optical energy transfer in the structures. The presented results emphasize the adverse effect of non-radiative recombination on device efficiency, which is the main obstacle for achieving ELC in III-V semiconductors.Item Electroluminescent cooling using double diode structures(2018-12-07) Sadi, Toufik; Radevici, Ivan; Kivisaari, Pyry; Casado, Alberto; Oksanen, Jani; Department of Neuroscience and Biomedical Engineering; Piprek, Joachim; Djurisic, Aleksandra B.The progress in optical cooling in recent years is resulting in a renewed interest in electroluminescent (EL) cooling using conventional III-V semiconductor light emitting diodes (LEDs). In this work, we address the limiting factors for observing EL cooling in III-As intracavity double diode structures (DDSs), at high powers at and close to 300K, by using a combination of experimental characterization and physical device models. The studied DDSs incorporate optically-coupled III-As LED and p-n homojunction photodiode (PD) structures, integrated in a single device and providing a favourable environment for EL cooling observation. We employ a modelling framework coupling the drift-diffusion charge transport model to a photon transport model calibrated using measurements on real devices at different temperatures. Results suggest that the bulk properties of the III-V materials are already sufficient for EL cooling.Item Electron Injection in Metal Assisted Chemical Etching as a Fundamental Mechanism for Electroless Electricity Generation(AMERICAN CHEMICAL SOCIETY, 2022-06-23) Li, Shengyang; Chen, Kexun; Vähänissi, Ville; Radevici, Ivan; Savin, Hele; Oksanen, Jani; Department of Neuroscience and Biomedical Engineering; Department of Electronics and Nanoengineering; Hele Savin GroupMetal-assisted chemical etching (MACE) is a widely applied process for fabricating Si nanostructures. As an electroless process, it does not require a counter electrode, and it is usually considered that only holes in the Si valence band contribute to the process. In this work, a charge carrier collecting p-n junction structure coated with silver nanoparticles is used to demonstrate that also electrons in the conduction band play a fundamental role in MACE, and enable an electroless chemical energy conversion process that was not previously reported. The studied structures generate electricity at a power density of 0.43 mW/cm2 during MACE. This necessitates reformulating the microscopic electrochemical description of the Si-metal-oxidant nanosystems to separately account for electron and hole injections into the conduction and valence band of Si. Our work provides new insight into the fundamentals of MACE and demonstrates a radically new route to chemical energy conversion by solar cell-inspired devices.Item Impact of etching process and surface recombination on the performance of GaAs light-emitting diodes(2021-08-24) Liu, Cang; Radevici, Ivan; Kemian tekniikan korkeakoulu; Franssila, SamiThe quantum efficiency performance of GaAs-based light emitting diodes (LEDs) can be visibly degraded by its surface recombination. One of the detrimental influencing factors is the presence of surface defects. Especially when an LED has a high surface-to-volume ratio, the injection of carriers is strongly is weakened by the surface defects on sidewalls. There are two etching techniques to define the mesa of LED. Wet etching has equivalent vertical and horizontal etch rates, forming visible undercutting and undulating wall profiles. Dry etching, referring to reactive ion etching (RIE) used in the thesis, provides more regular profiles but may cause harmful plasma damage. Here, two GaAs/AlGaAs/InGaP double heterojunction (DHJ) LEDs were processed by wet and dry etching respectively. They were characterized with current-voltage (IV) measurements and scanning electron microscope (SEM) to compare the electrical properties and surface topography with each other. The results confirm that the surface recombination is not negligible for small size LEDs. Dry etching generates a higher surface current associated with non-radiative recombination. Particularly, the calculated perimeter current density J02P increases by 142%. J02 contributes to the total LED current at low bias (<1.0 V). According to the previous studies, higher J02 means higher leakage current on the surface and lower internal quantum efficiency (IQE). It is demonstrated by SEM that wet etching produced visible lateral etching on the InGaP layers. Dry etching formed more uniform sidewalls but generated more rough fissures. The devices are required to be exposed to plasma, producing more unwanted surface defects. Therefore, although dry etching can avoid lateral etching, a higher leakage current was observed due to the larger surface area exposed to plasma. Wet etching is more advantageous than dry etching for GaAs-based LEDs in a small size (e.g., 250 micrometer).Item Improving the Efficiency of GaInP/GaAs Light Emitters Using Surface Passivation(IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2020-09) Tripathi, Tripurari S.; Radevici, Ivan; Dagyte, Vilgaile; Sadi, Toufik; Oksanen, Jani; Department of Neuroscience and Biomedical EngineeringThe quantum efficiency and reliability of III-V semiconductor-based light emitters can be significantly degraded by nonradiative recombination resulting from the presence of harmful surface states at the mesa sidewalls of the devices. Specifically, surface states are expected to substantially affect the possibility to observe electroluminescent cooling (ELC) in III-As light-emitting diodes (LEDs). Here, we confirm the existence of significant surface currents by exploring the effect of ammonium polysulfide [(NH4)(2)S-x] surface passivation on a GaInP/GaAs heterojunction double diode structure (DDS) designed to study the feasibility of ELC. The DDS consists of an LED and a photodiode (PD) within a single device structure, enabling easy monitoring of the photon-mediated thermal energy transport between the LED and the PD, and eliminating challenges associated with light extraction. Our results show that the surface passivation can improve the internal quantum efficiency of the LEDs by more than 10% points under optimal bias conditions.Item Influence of photo-generated carriers on current spreading in double diode structures for electroluminescent cooling(IOP Publishing, 2018) Radevici, Ivan; Tiira, Jonna; Sadi, Toufik; Oksanen, Jani; Neurotieteen ja lääketieteellisen tekniikan laitos; Department of Neuroscience and Biomedical Engineering; Engineered Nanosystems Group; Engineered Nanosystems Group; Perustieteiden korkeakoulu; School of ScienceCurrent crowding close to electrical contacts is a common challenge in all optoelectronic devices containing thin current spreading layers (CSLs). We analyze the effects of current spreading on the operation of the so-called double diode structure (DDS), consisting of a light emitting diode (LED) and a photodiode (PD) fabricated within the same epitaxial growth process, and providing an attractive platform for studying electroluminescent (EL) cooling under high bias conditions. We show that current spreading in the common n-type layer between the LED and the PD can be dramatically improved by the strong optical coupling between the diodes, as the coupling enables a photo-generated current through the PD. This reduces the current in the DDS CSL and enables studying EL cooling using structures that are not limited by the conventional light extraction challenges encountered in normal LEDs. The current spreading in the structures is studied using optical imaging techniques, electrical measurements, simulations, as well as simple equivalent circuit models developed for this purpose. The improved current spreading leads further to a mutual dependence with the coupling efficiency, which is expected to facilitate the process of optimizing the DDS. We also report a new improved value of 63% for the DDS coupling quantum efficiency (CQE).Item Influence of photo-generated carriers on current spreading in double diode structures for electroluminescent cooling(2018-03-29) Radevici, Ivan; Tiira, Jonna; Sadi, Toufik; Oksanen, Jani; Department of Neuroscience and Biomedical EngineeringCurrent crowding close to electrical contacts is a common challenge in all optoelectronic devices containing thin current spreading layers (CSLs). We analyze the effects of current spreading on the operation of the so-called double diode structure (DDS), consisting of a light emitting diode (LED) and a photodiode (PD) fabricated within the same epitaxial growth process, and providing an attractive platform for studying electroluminescent (EL) cooling under high bias conditions. We show that current spreading in the common n-type layer between the LED and the PD can be dramatically improved by the strong optical coupling between the diodes, as the coupling enables a photo-generated current through the PD. This reduces the current in the DDS CSL and enables the study of EL cooling using structures that are not limited by the conventional light extraction challenges encountered in normal LEDs. The current spreading in the structures is studied using optical imaging techniques, electrical measurements, simulations, as well as simple equivalent circuit models developed for this purpose. The improved current spreading leads further to a mutual dependence with the coupling efficiency, which is expected to facilitate the process of optimizing the DDS. We also report a new improved value of 63% for the DDS coupling quantum efficiency.Item Intracavity double diode structures with GaInP barrier layers for thermophotonic cooling(SPIE, 2017) Tiira, Jonna; Radevici, Ivan; Haggren, Tuomas; Hakkarainen, Teemu; Kivisaari, Pyry; Lyytikäinen, Jari; Aho, Arto; Tukiainen, Antti; Guina, Mircea; Oksanen, Jani; Neurotieteen ja lääketieteellisen tekniikan laitos; Department of Neuroscience and Biomedical Engineering; Perustieteiden korkeakoulu; School of ScienceOptical cooling of semiconductors has recently been demonstrated both for optically pumped CdS nanobelts and for electrically injected GaInAsSb LEDs at very low powers. To enable cooling at larger power and to understand and overcome the main obstacles in optical cooling of conventional semiconductor structures, we study thermophotonic (TPX) heat transport in cavity coupled light emitters. Our structures consist of a double heterojunction (DHJ) LED with a GaAs active layer and a corresponding DHJ or a p-n-homojunction photodiode, enclosed within a single semiconductor cavity to eliminate the light extraction challenges. Our presently studied double diode structures (DDS) use GaInP barriers around the GaAs active layer instead of the AlGaAs barriers used in our previous structures. We characterize our updated double diode structures by four point probe IV- measurements and measure how the material modifications affect the recombination parameters and coupling quantum efficiencies in the structures. The coupling quantum efficiency of the new devices with InGaP barrier layers is found to be approximately 10 % larger than for the structures with AlGaAs barriers at the point of maximum efficiency.Item Lock-in thermography approach for imaging the efficiency of light emitters and optical coolers(SPIE, 2017) Radevici, Ivan; Tiira, Jonna; Oksanen, Jani; Neurotieteen ja lääketieteellisen tekniikan laitos; Department of Neuroscience and Biomedical Engineering; Perustieteiden korkeakoulu; School of ScienceDeveloping optical cooling technologies requires access to reliable efficiency measurement techniques and ability to detect spatial variations in the efficiency and light emission of the devices. We investigate the possibility to combine the calorimetric efficiency measurement principles with lock-in thermography (LIT) and conventional luminescence microscopy to enable spatially resolved measurement of the efficiency, current spreading and local device heating of double diode structures (DDS) serving as test vessels for developing thermophotonic cooling devices. Our approach enables spatially resolved characterization and localization of the losses of the double diode structures as well as other light emitting semiconductor devices. In particular, the approach may allow directly observing effects like current crowding and surface recombination on the light emission and heating of the DDS devices.Item Measuring efficiency of light-emitting diodes with integrating sphere and thermal measurements(2024-05-31) Silvennoinen, Oskari; Radevici, Ivan; Perustieteiden korkeakoulu; Halme, JanneImproving the efficiency of light emitting diodes (LEDs) is important from both technical and economical points of view. In order to improve the efficiency of LEDs, it should be well defined and accurately measured. Wall-plug efficiencies (defined as the ratio of the output optical power to the input electrical power) of modern LEDs have approached 80%. It is also predicted that LEDs may reach efficiencies above unity, when their ability to efficiently use thermal energy for photon emission is fully harnessed. As relatively small measuring errors become more significant when approaching the efficiencies near 100%, the measurement of the high efficiencies becomes more challenging. In this bachelor’s thesis, two different methods to measure the efficiency of an LED are compared through theory and practical measurements. The first method is the classical method, which includes measuring the output optical power with an integrating sphere and a spectrometer. This approach works well, but calibrating it for high accuracy is challenging. To simplify the measurement procedure for LEDs that do not directly fit the input port of the integrating sphere, a modified measurement technique, referred to as hemisphere efficiency measurement, was also tested and compared with the standard integrating sphere measurement. Additionally, the thesis studies a novel method to estimate the efficiency of LEDs. Originally developed for photoluminescence systems, the method is based on the energy balance of the LED: all input power injected into the LED will result in either optical power or heat. The heat produced by the LED is measured together with the optical output power, and energy conservation requirements are used to calibrate the measurement. The results indicate that the classical method with integrating sphere works as supposed, but is challenging to calibrate. It is also possible to use integrating hemisphere, but it complicates the calibration. The method based on energy balance needs further development, but the preliminary results are promising, and the method also produces the temperature measurement that can directly indicate crossing the boundary of 100% efficiency of the LED.Item Observation of local electroluminescent cooling and identifying the remaining challenges(SPIE, 2019-01-01) Radevici, Ivan; Sadi, Toufik; Tripurari, Tripathi; Tiira, Jonna; Ranta, Sanna; Tukiainen, Antti; Guina, Mircea; Oksanen, Jani; Department of Neuroscience and Biomedical Engineering; Seletskiy, Denis V.; Epstein, Richard I.; Sheik-Bahae, Mansoor; Tampere UniversityThe 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/cm 2 . 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.
