Development of Milliwatt Power AlGaN-based Deep UV-LEDs by Plasma-assisted Molecular Beam Epitaxy
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1202-I10-01
Development of Milliwatt Power AlGaN-based Deep UV-LEDs by Plasma-assisted Molecular Beam Epitaxy Yitao Liao, Christos Thomidis, Anirban Bhattacharyya, Chen-kai Kao, Adam Moldawer, Wei Zhang and Theodore D. Moustakas Department of Electrical and Computer Engineering and Photonics Center, Boston University, Boston, MA 02215, U.S.A.
ABSTRACT In this paper, we report the development of AlGaN-based deep ultraviolet LEDs by rf plasma-assisted molecular beam epitaxy (MBE) emitting between 277 and 300 nm. Some of these devices were evaluated after fabrication at bare-die and some at wafer-level configurations. Devices with total optical output of 1.3 mW at injection current of 200 mA were produced, with maximum external quantum efficiency (EQE) of 0.16%. These performance values are equivalent to those reported for deep UV-LEDs grown by the Metalorganic chemical vapor deposition (MOCVD) method and measured at bare-die configuration. In parallel, we have evaluated the internal quantum efficiency (IQE) of AlGaN quantum wells, and found that such wells emitting at 250 nm have an IQE of 50%. From the analysis of these data, we concluded that the efficiency of deep UV LEDs is not limited by the IQE but by the light extraction efficiency, injection efficiency or a combination of both. INTRODUCTION Deep ultraviolet light emitting diodes (UV-LEDs) are expected to find a number of applications such as water purification, epoxy curing, non-line-of-sight communications, fluorescence identification of biological/chemical agents and surface sterilization. However, despite the intense effort, deep UV-LEDs based on AlGaN alloys have been reported to be highly inefficient. Specifically, at wavelength below 300 nm, the reported maximum external quantum efficiency (EQE) is typically equal or less than 1% [1-9]. This result can be accounted for by low internal quantum efficiency (IQE), current injection efficiency, light extraction efficiency, or a combination of all three factors. In the majority of the previous reports the deep UV-LEDs were produced by the MOCVD method. Only limited reports exist on producing such LED structures by the MBE method [10-14]. Contrary to the earlier reports that AlGaN alloys have a relatively poor IQE [15], our group has demonstrated recently that AlGaN multiple quantum wells (MQWs) produced by plasma-assisted MBE have IQE as high as 50% at 250 nm [16]. In this paper, we report the development of AlGaN-based deep UV-LEDs by rf plasmaassisted molecular beam epitaxy emitting in the wavelength range of 277 and 300 nm. Devices with total optical output of 1.3 mW at injection current of 200 mA were produced, with maximum external quantum efficiency (EQE) of 0.16%.
EXPERIMENTAL DETAILS The deep UV-LED structures were grown by plasma-assisted molecular beam epitaxy on to (0001) sapphire substrates in a Varian Gen II MBE system. Nitrogen is activated by an Applied Epi Uni-bulb rf plasma source. The Ga and Al metals as well as Si and Mg dopants were supplied from standard effusion cells. The epilayer st
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