Plasma-assisted MBE growth enables higher efficiency deep UV LEDs
Mar 03, 2011
Growing the active region of the deep UV-LEDs under a liquid phase growth mode produces AlGaN alloys with a strong band structure potential fluctuations and LEDs with high internal quantum efficiency.
BostonUniversity’s Wide Bandgap Semiconductor Lab claims that it has fabricated the first efficient deep UV-LEDs in the 320 nm to 265 nm range grown by plasma-assisted MBE.
This method of growth was modified to lead to AlGaN alloys with a strong band structure potential fluctuations and LEDs with high internal quantum efficiency (IQE).
Over the last two decades,progress in producing efficient deep UV LEDs has been hindered by the fundamental limitation in material properties of AlGaN. The lack of phase separation in AlGaN means that the band structure prevents potential fluctuations and carrier localisation.
This leads to high sensitivity of the IQE of quantum wells in AlGaN-based LEDs to the density of dislocations and other defects, compared to InGaN-based blue LEDs. The IQE bottleneck in deep UV-LED must be addressed before such devices can be used in high power applications such as municipal and industrial water disinfection.
This breakthrough was accomplished by growing the active region of the deep UV-LEDs under a liquid phase growth mode. The growth proceeds by dissolving the arriving nitrogen and aluminium atoms in the liquid gallium and their incorporation in the growing AlGaN film from the liquid phase.
Thickness variations of the liquid gallium on the top of the growing AlGaN film would normally produce lateral variations in the composition of the AlGaN alloys. This would lead to a band structure promoting potential fluctuations to prevent carriers for migrating and recombining non-radiatively in dislocations or other defects. High IQEs of between 30 percent and 50 percent have been reported by the team since 2009.
The deep UV LEDs structures were grown by plasma-assisted MBE incorporating such high IQE QWs together with a number of novel features such as polarisation enhanced n- and p-type AlGaN carrier injection layers.
300 μm x 300 μm bare-die devices emitting at 273 nm have produced 0.4 mW output power at 25 mA DC drive current and 1.3 mW at 100 mA in the pulsed mode. The EQE peaked at 0.4 percent.
“We identify the problem in the deep UV-LED efficiency being the IQE, where drastic improvement in fundamental material growth technology needs to be obtained compared to incremental gain from chip design optimisation or packaging,” says lead author Yitao Liao.
He explains that he and his co-workers have developed a unique process to manufacture AlGaN-based UV-LEDs with high efficiency on low cost c-plane sapphire. “Our advantages in the IQE will become more evident at the device level after these bare-die LEDs get packaged to improve light extraction and heat sinking.”
Further details of this work will be published in an article by Yitao Liaoet al.,
in Applied Physics Letters 98, 081110 (2011)