Nanopatterned sapphire substrates in deep-UV LEDs: is there an optical benefit?

Abstract

Light emitting diodes (LEDs) in the deep ultra-violet (DUV) offer new perspectives for multiple applications ranging from 3D printing to sterilization. However, insufficient light extraction severely limits their efficiency. Nanostructured sapphire substrates in aluminum nitride based LED devices have recently shown to improve crystal growth properties, while their impact on light extraction has not been fully verified. We present a model for understanding the impact of nanostructures on the light extraction capability of DUV-LEDs. The model assumes an isotropic light source in the semiconductor layer stack and combines rigorously computed scattering matrices with a multilayer solver. We find that the optical benefit of using a nanopatterned as opposed to a planar sapphire substrate to be negligible, if parasitic absorption in the p-side of the LED is dominant. If losses in the p-side are reduced to 20%, then for a wavelength of 265 nm an increase of light extraction efficiency from 7.8% to 25.0% is possible due to nanostructuring. We introduce a concept using a diffuse ('Lambertian') reflector as p-contact, further increasing the light extraction efficiency to 34.2%. The results underline that transparent p-sides and reflective p-contacts in DUV-LEDs are indispensable for enhanced light extraction regardless of the interface texture between semiconductor and sapphire substrate. The optical design guidelines presented in this study will accelerate the development of high-efficiency DUV-LEDs, but the model is also readily applicable to other multilayer opto-electronic nanostructured devices such as photovoltaics or photodetectors.

High‐Temperature Annealing and Patterned AlN/Sapphire Interfaces

“…However, for devices such as UV LEDs, it is advantageous to have a patterned interface between AlN and sapphire to improve the light extraction. [14][15][16] Therefore, the combination of HTA and a patterned AlN/sapphire interface is desirable. Only a few publications so far deal with the combination of both techniques showing that crack-free, smooth AlN surfaces with TDD down to 6.4 Â 10 8 cm À2 can be realized.…”

Light Guide Layer Thickness Optimization for Enhancement of the Light Extraction Efficiency of Ultraviolet Light–Emitting Diodes

“…, who allowed the LED to embed nanoholes in the micropatterned sapphire substrate to improve its photoelectric characteristics [ 16 ]. PhillipManley et al employed a nanopatterned sapphire substrate in deep UV (DUV) LEDs, verifying the effects of such a nanopatterned structure on the LEE of sapphire [ 17 ].…”

Light Guide Layer Thickness Optimization for Enhancement of the Light Extraction Efficiency of Ultraviolet Light–Emitting Diodes

“…The growth of InGaN-based LED mixed patterned sapphire substrates at the microscale and nanoscale was proposed by Wen Cheng Ke et al, who allowed the LED to embed nanoholes in the micropatterned sapphire substrate to improve its photoelectric characteristics [16]. PhillipManley et al employed a nanopatterned sapphire substrate in deep UV (DUV) LEDs, verifying the effects of such a nanopatterned structure on the LEE of sapphire [17].…”