Emission, kinetic and magnetic phenomena in rare-earth and transition metal doped ZnSe single crystals

In this work emission, optical, electrical and magnetic properties of the d- and f- elements doped zinc selenide crystals were investigated within a wide temperature range. Doping was performed in various technological processes: during the growth by chemical vapor transport method; by thermal diffusion from the Bi or Zn melt. Concentration of the doping impurity in the crystals was controlled by amount of the dopant in the source material or by its concentration in the doping media. Special interest in the work was paid to the influence of the different concentrations of Cr and Yb impurities on ZnSe crystals’ properties, correlations between observed effects and similarities with the Ni, Mn and Gd dopants are analysed. Possibility of formation of the excitons bound to the doping d-ions was shown. In contrast to this, it was observed that f-elements do not bound excitons, but prevent formation of excitons bound to some uncontrolled impurities. A mechanism of Cr doping impurity interaction with background impurities and zinc selenide structural defects was proposed based on experimental data. An assumption about resonant energy transfer between double charged chromium ions and complexes based on crystals’ vacancy defects was made. A correlation between emission and magnetic properties of the d- ions doped samples was established. Based on this correlation a mechanism explaining the concentration quench of the emission was proposed. It was found that f-ions bind electrically active shallow and deep donor and acceptor states of background impurity to electrically neutral complexes. This may be observed as “purification” of ZnSe crystals by doping with the rare-earth elements, resulting i tendency of the properties of f-ion doped crystals to the properties of intrinsic crystals, but with smaller concentration of uncontrolled native and impurity defects. A possible interpretation of this effect was proposed. It was shown that selenium substituting impurities decrease efficiency of the Yb doping. Based on this experimental results an attempt to determine ytterbium ion surroundings in the crystal lattice was made. It was shown that co-doping of zinc selenide crystals with the d- and f- ions leads to the combination of the impurities influence on the material’s properties. On the basis of obtained data an interaction mechanism of the d- and f-elements co-dopants was proposed. Guided by the model of the ytterbium ion incorporation in the selenide sublattice of the ZnSe crystals, an assumption about stabilization of single charged chromium ions in the zinc sublattice crystal nodes, by means of formation of the local charge compensating clusters, was made.Tässä työssä tutkittiin d- ja f-alkuaineilla doopattujen ZnSe-kiteiden emissio-, optisia,sähköisiä ja magneettisia ominaisuuksia laajalla lämpötila-alueella. Dooppaus tehtiin eri tekniikoilla: kasvatuksen aikana kemiallisesessa kaasufaasikasvatuksessa ja termisellä diffuusiolla Bi- tai Zn-nesteestä. Dooppauksen määrä kontrolloitiin säätämällä dopantin määrää lähtöaineissa tai sen konsentraatiolla dooppausnesteessä. Erityisesti tutkittiin Cr- ja Yb-atomien konsentraation vaikutusta ZnSe-kiteiden ominaisuuksiin sekä korrelaatioita havaittujen ilmiöiden välillä ja yhtäläisyyksiä Ni-, Mn- ja Gd-ioneilla doopattuihin kiteisiin. Työssä osoitettiin, että d-ionien sitomien eksitonien syntyminen on mahdollista. Sen sijaan f-ionit eivät sido eksitoneja, vaan estävät niiden synnyn kontrolloimattomien epäpuhtausionien ympäristöön. Pohjautuen kokeelliseen dataan ehdotettiin, että resonoiva energian siirto Cr2+-ionien ja kiteessä olevien vakanssien välillä selittää Cr-dopantin vuorovaikutuksen kontrolloimattomien epäpuhtausatomien kanssa. Työssä havaittiin myös korrelaatio magneettisten ja emissio-ominaisuuksien välillä d-atomeilla doopatuissa näytteissä ja tähän perustuen ehdotettiin mekanismia, joka selittää emission häviämisen dopantin konsentraation kasvaessa. f-ionien havaittiin sitoutuvan kontrolloimattomien epäipuhtausatomien muodostamiin sähköisesti aktiivisiin mataliin ja syviin donori- ja akseptoritiloihin ja yhdessä ne muodostavat sähköisesti neutraaleja komplekseja. Tämän havaittiin näyttävän kiteen “puhdistumisena”, kun sitä doopataan harvinaisilla maametalleilla: kiteen ominaisuudet ovat yhä lähempänä täysin puhtaan ZnSe:n ominaisuuksia. Tälle ilmiölle esitettiin syntymekanismi. Työssä osoitettiin myös, että seleenin korvaavat epäpuhtaudet vähentävät Yb-dooppauksen tehoa. Tähän perustuen yritettiin määrittää Yb-ionien ympäristöä kidehilassa. Työssä osoitettiin myös, että ZnSe-kiteiden dooppaaminen samanaikaisesti d- ja f-atomeilla johtaa dopanttien yhteisvaikutukseen kiteiden ominaisuuksissa. Kokeiden tuloksena ehdotettiin vuorovaikutusmekanismia dopanttien välille, jossa Yb-ionit korvaavat Se-ioneita Se-alihilassa ja samalla stabiloivat Cr+-ioneita Zn-alihilassa muodostamalla paikallisia varauksen kompensoivia klustereita.Siirretty Doriast

Lock-in thermography approach for imaging the efficiency of light emitters and optical coolers

Developing 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.Peer reviewe

Influence of photo-generated carriers on current spreading in double diode structures for electroluminescent cooling

Current 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).Peer reviewe

Intracavity double diode structures with GaInP barrier layers for thermophotonic cooling

Optical 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.Peer reviewe

Enhanced Photoluminescence in Acetylene-Treated ZnO Nanorods

Zinc oxide (ZnO) nanorods were manufactured using the aqueous chemical growth (ACG) method, and the effect of thermal acetylene treatment on their morphology, chemical composition, and optical properties was investigated. Changes in the elemental content of the treated rods were found to be different than in previous reports, possibly due to the different defect concentrations in the samples, highlighting the importance of synthesis method selection for the process. Acetylene treatment resulted in a significant improvement of the ultraviolet photoluminescence of the rods. The greatest increase in emission intensity was recorded on ZnO rods treated at the temperature of 825 degrees C. The findings imply that the changes brought on by the treatment are limited to the surface of the ZnO rods.Peer reviewe

Thermophotonic cooling with light-emitting diodes

| openaire: EC/H2020/638173/EU//iTPXThe currently ubiquitous light-emitting diodes (LEDs) have revolutionized the lighting industry. Contrary to common belief, however, LEDs are much more than just simple electricity-to-light converters. They are solid-state thermodynamic machines, theoretically capable of continuous and near-reversible energy conversion between electrical, thermal and optical energy. For over 50 years, the possibility of exploiting LEDs as efficient solid-state coolers has remained largely out of reach due to the high-material-quality requirements and commercial focus on light emission. Recent promising advances in electroluminescent cooling by LEDs, however, suggest that the remaining challenges in the area may be surmountable and practical cooling could be feasible. This Perspective discusses recent achievements in electroluminescent cooling, outlining the expected promise, the remaining challenges and their potential solutions.Peer reviewe

Temperature dependence of thermophotonic energy transfer in intracavity structures

| openaire: EC/H2020/638173/EU//iTPXElectroluminescent 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.Peer reviewe

Prospects and requirements for thermophotonic waste heat energy harvesting

| openaire: EC/H2020/638173/EU//iTPX | openaire: EC/H2020/951976/EU//TPX-PowerThermophotovoltaic (TPV) power generators offer great possibilities for thermal energy conversion when thermal sources with temperatures nearing or exceeding 1000 K are available. While the power density of conventional TPV systems is generally determined by Planck's law in the far field, their fundamental performance is known to be dramatically affected by near field effects between the thermal emitter and the photovoltaic cell. Another potentially disruptive enhancement to the performance may be reached by transforming the thermal emitter to exploit electroluminescence. Taking advantage of an electroluminescent emitter as the source of radiation fundamentally alters the thermodynamics of the system. This allows boosting the achievable power densities by orders of magnitude, and also provides access to electroluminescent coolers, thermophotonic (TPX) heat pumps, and TPX power generation devices that can outperform both TPV and thermoelectric heat engines, especially at the low-grade waste heat (LGWH) temperature range (300–500 K) containing in total the majority of recoverable energy. In reality, functional TPX devices are yet to be demonstrated experimentally, due to several material and design bottlenecks. Here, we discuss the thermodynamics, ideal characteristics and advantages of TPX heat engines, and quantify how non-idealities such as non-radiative recombination, optical and resistive losses affect their performance. Our results suggest that, at LGWH temperatures, TPX heat engines start to outperform the best TPV systems when reaching quantum efficiencies of the order of 90%; beyond this threshold, TPX systems become increasingly efficient and powerful.Peer reviewe

Chemovoltaic effect for renewable liquid and vapor fuels on semiconductor surfaces

Publisher Copyright: © 2024 The Authors. ChemSusChem published by Wiley-VCH GmbH.The 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.Peer reviewe