Ultraviolet optoelectronic devices based on AIGaN alloys grown by molecular beam epitaxy
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Plasmonics, Photonics, and Metamaterials Prospective Article
Ultraviolet optoelectronic devices based on AlGaN alloys grown by molecular beam epitaxy Theodore D. Moustakas, Electrical and Computer Engineering Department, Division of Materials Science and Engineering, Photonics Center, Boston University, Boston, MA 02215, USA Address all correspondence to Theodore D. Moustakas at [email protected] (Received 25 April 2016; accepted 27 July 2016)
Abstract This paper reviews progress in ultraviolet (UV) optoelectronic devices based on AlGaN films and their quantum wells (QWs), grown by plasmaassisted molecular beam epitaxy. A growth mode, leading to band-structure potential fluctuations and resulting in AlGaN multiple QWs with internal quantum efficiency as high as 68%, is discussed. Atomic ordering in these alloys, which is different from that observed in traditional III–V alloys, and its effect on device performance is also addressed. Finally, progress in UV-light-emitting diodes, UV lasers, UV detectors, electroabsorption modulators, and distributed Bragg reflectors is presented.
Introduction The field of AlGaN-based ultraviolet (UV) optoelectronic devices (primarily emitters and photodetectors), has been an active area of research over the past two decades.[1–5] AlGaN alloys are well suited for such devices because their energy gap can be tuned by changing the alloy composition to cover the entire UV spectral region from 210 to 360 nm. This research is motivated by a plethora of potential industrial and medical applications. Such include, for example, free-space non-line-of-sight communications, water/air/food sterilization, surface disinfection, fluorescence or Raman identification of biological/chemical agents, epoxy curing, counterfeit detection, and various diagnostic and therapeutic medical applications. However, despite intense efforts worldwide, the maximum external quantum efficiency (EQE) of fully packaged AlGaN-based deep UV-light-emitting diodes (LEDs), emitting below 300 nm, is only 1%–3%.[5–9] Only recently Shatalov et al. reported a UVLED emitting at 278 nm with EQE of about 10%.[10] This is to be contrasted with InGaN-based violet–blue LEDs, whose EQE is more than 50%.[11] The EQE is defined as the product of the internal quantum efficiency (IQE), the injection efficiency (IE), and the extraction efficiency (EE). Thus, the poor EQE of the deep UV-LEDs may be the result of poor IQE, IE, EE, or a combination of all three factors. The IQE depends sensitively on extended and point defects, which act as non-radiative recombination centers. The IE depends on the ability to dope the n- and p-sides of the junction efficiently and to form Ohmic contacts with low contact resistivity on both sides. The EE depends sensitively on the ability to extract the light from the active region of the device to the free space.
The development of AlGaN-based UV lasers is even at an earlier stage of development. Several groups have reported the development of optically pumped deep UV lasers as well as the demonstration of stimulated emission
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