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review  research Polarity ups


UV power Polarity control layer spurs deep UV LEDs to record-breaking external quantum efficiencies


A JAPANESE TEAM has raised the bar for the output of ultraviolet LEDs operating in the short-wave band. Such devices, which have wavelengths below 280 nm, could serve many applications, including air and water purification, surface disinfection, UV curing and medical phototherapy.


Driven in continuous-wave mode at 60 mA, the 266 nm UV LEDs produced by engineers from UV Craftory, Meijo University and Nagoya University deliver an output power of 5.3 mW at an external quantum efficiency (EQE) of 1.9 percent. When the emission wavelength is increased to 278 nm, an output power of 8.4 mW at an EQE of 3.4 percent is realised at the same drive current.


According to lead-author Myunghee Kim from UV Craftory, these LEDs set a new benchmark for performance in this spectral range – up until now, state-of-the-art results were an output of 2.7 mW at 273 nm for an LED driven continuously at 700 mA. This equates to an EQE of just 0.04 percent.


Kim says that the key to the team’s success was the polarity-controlled layer beneath the lateral epitaxial overgrowth. This enabled growth of AlN films with very low defect densities.


UV LED fabrication began by etching 2 µm-wide stripes with a 10 µm period into sapphire with inductively coupled, plasma- assisted reactive ions. MOCVD with a low III-V ratio formed 0.5 µm-thick AlN films with Al polarity, before the additional growth of 8 µm of AlN created a crack-free, smooth surface, indicative of good coalescence.


Cross-sectional microscopy of this structure reveals that defects plague AlN that is close to the interface with sapphire. These defects are composed of inversion domains and threading dislocations, which terminate within 400 nm of the interface and do not propagate through the film. According to plan-view, transmission electron microscopy images, the threading dislocation on the terrace and groove are 4x 107


cm-2 and 3.1 x 107 cm-2 , respectively. October 2011 www.compoundsemiconductor.net 49


LED structures featuring five period multi- quantum wells and a magnesium-doped, aluminium-rich AlGaN electron blocking layer were deposited on this AlN platform. This enabled fabrication of 266 nm and 278 nm, square-shaped LEDs with 800 µm edges. Driven at 20 mA, the forward voltages for the 266 nm and 278 nm LEDs were 6.11 V and 5.98 V, respectively. Under these operating conditions both LEDs showed relatively little degradation during the first


1000 hours.


Kim hopes to continue to study defects in UV epilayers: “It is still a challenge to grow high-quality, III-nitride films on sapphire due to various defects, such as threading dislocations, stacking faults, voids and inversion domains.”


M. Kim et. al. Appl. Phys. Express 4 092102 (2011)


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