Double Diode Structures Research Articles

Effect of interface recombination on the efficiency of intracavity double diode structures

In 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 300 K 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.

Electroluminescent Cooling in III–V Intracavity Diodes: Efficiency Bottlenecks

Recent 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.

Towards fully self-consistent optoelectronic simulation of planar devices

Photon recycling plays an important role in various optoelectronic devices, such as thin-film solar cells and intracavity double-diode structures presently studied for electroluminescent cooling. However, a complete description of photon recycling requires developing fully self-consistent simulation tools of photon and electronic charge transport in realistic device structures. In this paper, we describe a possible route towards such simulation tools in planar devices by combining the radiative transfer (RT) equation of photon transport with the drift-diffusion (DD) equations of electron and hole dynamics. We investigate the feasibility of the approach by selected proof-of-principle device studies. The results indicate that combining the RT and DD models not only yields the expected device characteristics, but also produces new insight into the pertinent local emission and absorption properties within the device structures. In particular, the model allows to accurately investigate how emission directivity and saturation affect the coupling coefficient in the double diode structures, showing that the main contribution to the coupling coefficient arises from the reflectivity of the top contact. For completeness, we also discuss how interference affects photon transport in resonant devices by an example calculation using the quantized fluctuational electrodynamics framework, paving way for self-consistent modeling tools that also account for wave-optical effects.

Electroluminescent Cooling in III–V Intracavity Diodes: Practical Requirements

Recent 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.

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 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.

Electroluminescent cooling in intracavity light emitters: modeling and experiments

We 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.