Deep UV LEDs

Abstract

Summary form only given. III-Nitride semiconductor-based Deep UV (DUV) LEDs are emerging as an enabling technology for diverse military, homeland security, industrial and commercial markets and space exploration. Current technology allows to fabricate AlGaN-based DUV LEDs with wall-plug efficiency (WPE) between 1-2%, which is substantially lower that WPE for visible and near UV LEDs. Significant R&D efforts are under way, including DARPA's “Compact Mid-Ultraviolet Technology” (CMUVT) program, to improve materials quality, device fabrication and light extraction to increase WPE of DUV LEDs by more than order of magnitude. We will present an overview of our latest developments in the development of DUV LED technology with the main focus on novel device designs to increase quantum efficiency and improve UV light extraction and reliability of devices emitting in the range of 230 nm 340 nm. Novel QW design has been implemented to suppress polarization effects and phonon engineering approach to increase electron trapping in the active layer of the devices. Very narrow (<; 3 nm) and deep (total energy band offset >; 0.4 eV) quantum wells were used to suppress Stark effect and increase radiative recombination. Active region of DUV LEDs was embedded inside a deep potential well (larger than the energy of optical phonon) to increase electron-LO phonon scattering and accelerate cooling of hot injected electrons. This allowed us to increase electron capture into the active region without using conventional electron "blocking layer" commonly used in visible LEDs We developed and incorporated UV-transparent p-type cladding layers to reduce optical losses due to strong absorption in the top p-AlGaN cladding layer and p+-AlGaN or p+-GaN contact layers. Combination with new type of ohmic contact reflecting in DUV spectral range allowed us to significantly improve light extraction and increase output power of DUV LEDs by 2 times in the range of 275 nm - 300 nm and 2.5 times in the range from 310 nm to 340 nm. Improved quality of epitaxial layers and device fabrication technology enabled to increase reliability of DUV LEDs and fabricate devices with peak emission wavelength in the range of 270 280 nm with lifetime exceeding 10,000 hours for CW operation. We will also present reliability data for DUV LEDs operating under high current (up to 400 mA) in the pulsed operation mode. We developed new device fabrication technology, primarily to reduce ohmic contact resistance for very high Al-contant DUV LEDs with peak emission wavelengths shorter than 250 run. This resulted in the reduction of the forward bias from >; 20V to less than 8 V, which makes these devices suitable for CW operation. We will also present recent results of space qualification of 250 260 nm DUV LEDs for space applications, which include reliability testing up to 26,000 hours (CW mode), shake and bake, and radiation hardness.