Multilayer AlN boosts deep-UV LED power
by Andy Extance
Japanese researchers have produced deep-UV LEDs using low-pressure MOCVD, with a device design that s delivered big increases in output power and external quantum efficiency (EQE).
A collaboration led by Hideki Hirayama at the Riken research institute in Wako, Japan, reported UV LEDs with emission wavelengths between 231 and 261 nm in the 13 August issue of Applied Physics Letters.
This range of devices, which were developed with a team at nearby Saitama University, has now expanded to include a 227.5 nm LED, according to a September 6 report on Nikkei s Tech-on website.
In the APL paper, maximum output power and EQE steadily diminished from 1.65 mW and 0.23 percent respectively for a 261 nm LED to 0.0011 mW and 0.0004 percent for a 237 nm LED.
However, after publication of the paper the team has obtained remarkable improvements, Hirayama told compoundsemiconductor.net, allowing a maximum output power of 0.15 mW for the 227.5 nm device. By comparison the deepest-UV LED to date, a 210 nm device reported by NTT, recorded 0.02 ?W maximum output power.
To achieve this, the Riken/Saitama team optimized the key multilayer AlN buffer layer in the LED, significantly reducing threading dislocation density (TDD).
“Also, we optimized the thickness and barrier height of the electron blocking layer so as to increase the electron injection efficiency,” Hirayama said. “We also optimized quantum well thickness, in order to reduce the piezoelectric field and to obtain a high internal quantum efficiency.”
Power and balance
The biggest single advance in obtaining useful output power at low wavelengths came in balancing the amount of AlGaN used in the device, according to Hirayama.
He and the Saitama team use a technique similar to the migration-enhanced MOCVD (ME-MOCVD) process developed by Asif Khan, and exploited commercially by Sensor Electronic Technology.
“Khan s ME-MOCVD obtains low TDD by using high crystal quality layers with an alternating supply of group III and V gas flow,” explained Hirayama.
ME-MOCVD normally uses superlattice AlGaN/AlN layers as a buffer to control crystal quality. This has delivered record power and efficiency for sub-300 nm LEDs, in the form of a 280 nm, 5.2 mW, 0.94 percent EQE device.
However, Hirayama says that the absorption properties of AlGaN make its extensive use unsuitable for deep-UV LEDs with emission below 240 nm.
His group instead obtains a high-quality, atomically flat AlN buffer layer using “ammonia pulsed multilayer AlN growth”. In this method, a flow of ammonia is pulsed into the reactor at the same time as a continuous trimethylaluminum flow. This enhances the migration of these precursors to the growing surface in a similar manner to ME-MOCVD.
Otherwise Riken and Saitama use an AlGaN template and quantum well structure to form the basis for higher power devices in a method close to Khan s.
According to Hirayama, Riken is currently looking at commercializing 260-270 nm LEDs with Japanese partners, but have not yet found suitable applications for the new, lower wavelengths.
Author
Andy Extance is a reporter at compoundsemiconductor.net.
