Deep Ultraviolet Light Emitting Diodes with Emission below 300 nm
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Deep Ultraviolet Light Emitting Diodes with Emission below 300 nm M. Asif Khan Department of Electrical Engineering, University of South Carolina, Columbia SC 29208, USA
ABSTRACT In this paper we will describe the problems in growth and fabrication of deep UV LED devices and the approaches that we have used to grow AlGaN-based multiple quantum well deep UV LED structures and to overcome issues of doping efficiency, cracking, and slow growth rates both for the n- and the p-type layers of the device structures. Several innovations in structure growth, device structure design and fabrication and packaging have led to the fabrication of devices with emission from 250-300 nm and cw-milliwatt powers at pump currents of only 20 mA (Vf ≤ 6 V). Record wall plug efficiencies above 1.5 % are now achievable for devices with emission at 280 nm. Thermal management and a proper device design are not only key factors in achieving these record performance numbers but are also crucial to device reliability. We will also discuss some of our initial research to clarify the factors influencing the lifetime of the deep UV LEDs. In addition to our own work, we will review the results from the excellent research carried out at several other laboratories worldwide.
INTRODUCTION At present, several research groups are developing deep ultraviolet light emission devices. The motivation behind this research is the enormous application potential of these devices to be used in bio-medicine, environmental protection, and public health. In addition to being environmentally friendly, LED-based solid state deep UV sources provide significant advantages in size, operation voltage, emission wavelength tunability and control over their conventional counterparts – namely, the mercury vapor lamps. Fabrication of III-N deep UV LEDs using conventional approaches leads to several major problems. Transparency at the operation wavelengths severely limits the choices of substrates to either sapphire or AlN. In either case one has to resort to heteroepitaxy to deposit the device structures. Deep UV LEDs require active and buffer epilayers of AlxGa1-xN with alloy compositions well over 30%. High Al mole fraction in these layers results in low doping efficiency, cracking, and slow growth rates both for the n- and the p-type layers of the device structures. In this paper we will discuss approaches such as the use of migration enhanced epitaxy and short period superlattices to overcome the above problems and to fabricate deep UV LED devices emitting at 280 nm with record wall plug efficiency. We will also present results of our studies of device reliability performance and discuss factors affecting the LED reliability. It is shown that such important parameters as junction temperature and pump current density strongly affect long-term LED performance, which can be greatly improved by proper device design and packaging approaches. We will also discuss the development of interconnected micropixel array devices and microlenses for potential microsensi
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