Impact of Photon Recycling on Absorption Losses in Tunnel Junction-Based III-Nitride LEDs

Abstract

Ill-Nitride light emitting diodes have been successfully adopted in visible display and illumination applications, and are very promising for ultra-violet wavelength applications. Tunnel junctions can provide several significant advantages for operation of both visible (for multiple active region LEDs) and UV LEDs (to act as transparent spreading layers). Recent work on tunnel junctions have demonstrated excellent characteristics for both GaN, as well as ultra-wide bandgap AlGaN. Although tunnel junctions (TJs) have been demonstrated with excellent electrical efficiency, and low voltage drop, especially for GaN-based LEDs, the absorption introduced by them has not been well-understood. Design of tunnel junctions for low voltage drop often requires the use of ultra-thin layers of lower bandgap material (such as InGaN) which could have substantial absorption at emission wavelengths. Previously used methods to calculate tunnel junction absorption losses, such as ray tracing and finite-difference time-domain FDTD [1] do not take into account photon recycling in the TJ. In this work we propose a theoretical model that can provide an intuitive understanding of the impact of photon recycling on the optical absorption losses in tunnel junctionbased LEDs. The proposed model is derived using a 1-dimensional N-cascaded GaN LED structure. The top metal contact is considered as a perfect reflector, and the output optical power is collected at the bottom side. GaN layers are assumed to be transparent, while any power absorbed by the TJ is recycled by generating an electron-hole pair (EHP) that is injected back to the active region to recombine there. The efficiency, n , by which the generated EHPs produce new photons in the active region is assumed to be the mathematical product of the IQE and the injection efficiency. The proposed closed form model takes into account the infinitely repeated photon recycling process. The model enables an estimate of the normalized light extraction efficiency, and can be compared with estimates from simple ray tracing model single and cascaded multiple active region TJ LEDs. It is shown that as the number of TJs increases and/or its absorption increases, the recycled power increases. Consequently, while ray tracing models would predict significant absorption losses, taking into account photon recycling shows that the effective loss is significantly lower. The model can also describe the impact of the internal quantum efficiency on the effective light extraction efficiency. For InGaN polarization engineered TJs, a comparison between the two models at various Indium compositions and thicknesses shows that for Ng 94F8q 76 N TJ LED, ray tracing model predicts absorption losses that are significantly higher. For example, for 3 nm 24% In-content tunnel junction, the ray tracing model predicts total absorption loss of 7.95%, whereas the loss including the photon recycling is only 2.94%. In summary, our work shows that absorption effects in tunnel junctions can be significantly overestimated if photon recycling is not taken into account. The work presented enables rational design of tunnel-junction based LEDs with accurate estimation of absorption losses.