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

Abstract

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).

Thermophotonic cooling in GaAs based light emitters

“…2(c). 14,15 The shoulder of the LED's J -V curve observed from experimental data is fully reproducible with the EC in Fig. 2(c).…”

“…The g CQE ¼ 0.70 reported here exceeds the CQE records (g CQE ¼ 0.63) we have reported previously 14,15 and also slightly exceeds the highest EQE g EQE ¼ 0.68 reported for GaAs based LEDs in the literature. 3 While directly comparing the EQE of established GaAs LEDs and our devices' CQE is not straightforward, we nevertheless point out that they both provide a lower limit for the IQE; thus, our result increases the directly measured lower bound for the IQE of GaAs based LEDs.…”

Observation of local electroluminescent cooling and identifying the remaining challenges

“…In this paper, we generalize and summarize our previous results 4,5,13,14 on analyzing the performance of the DDS from the point of view of high-power EL cooling. Additionally, here we separately analyze the LED itself…”

“…The contact to the GaAs was made through openings in the silicon nitride layer and varied from 5 to 100 % of the top LED area. 5 A three point scheme with a common grounded cathode for the LED and PD was used for the analysis of the electrical characteristics of the DDS. The used setup allows biasing the LED with a voltage U 1 and the simultaneous measurement of both the LED current I 1 and PD current I 2 , while the PD is under a short circuit condition.…”

“…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.…”