RESEARCH REVIEW
Better electron blocking boosts UV efficiency
Electron-blocking layers formed from AlN and Al0.7 efficiency by almost an order of magnitude.
A TEAM OF SCIENTISTS from Germany have improved ultraviolet LED efficiency by introducing superior electron-blocking structures into the devices. By replacing the conventional Al0.7
Ga0.3 Ga0.3 N electron-
blocking layer with the pairing of an AlN layer with a thickness greater than 3 nm and an Al0.7
N layer, the team has
increased the efficiency of its 290 nm LEDs by a factor in excess of eight.
This hike in efficiency will aid the development of brighter ultraviolet LEDs, which are promising sources for water purification, curing and phototherapy. Today, these ultraviolet emitters have an external quantum efficiency of typically just a few percent, due to: poor carrier confinement, which can be addressed with a superior electron-blocking structure; a high defect density; and unsatisfactory carrier injection.
Efforts by the German team – a partnership between researchers at the Technical University of Berlin and the Leibniz Institute for High Frequency Technology – kicked-off with a series of simulations of different LED structures. Their calculations employed a one- dimensional drift-diffusion model.
Team member Micheal Kneissl, who has positions at both universities, admits that it is hard to know how accurate the model is. That’s because its accuracy depends on the values that are used for material characteristics, such as effective mass, carrier mobility, band offset and magnesium acceptor ionisation energy.
“In the end, the simulations provide us with guidance for the optimisation of the heterostructure design, which has to be confirmed by experiment.” Hindsight, however, shows a strong correlation between the experimental results and the simulations, which offered a great insight in how to improve ultraviolet LED performance.
Experimental work involved the fabrication of two series of structures:
Ga0.3 N improve
power nearly tripled when aluminium content in the 4 nm-thick AlGaN layer in the first series of samples increased from 80 percent to 100 percent. Meanwhile, the second experiment showed that if the AlN layer was at least 3 nm-thick, output power was 8.5 times higher than if it were not there at all.
The optimum 290 nm LED, which had an electron-blocking heterostructure comprising 3 nm-thick AlN and 25 nm- thick Al0.7
Ga0.3
On-wafer measurements reveal that by optimising the electron-blocking layer, ultraviolet LED’s external quantum efficiency can hit 0.4 percent, roughly double that of the best devices reported by other groups
The first featured an electron-blocking heterostructure that combined a 4 nm- thick layer of Alx
Ga1-x Ga0.3 an AlN/Al0.7 N (with values for
x of 0.8, 0.9 and 1.0) and a 25 nm-thick Al0.7
N electron-blocking
heterostructure with AlN layers 0 nm to 8 nm thick.
These electron-blocking regions had a strong influence on the performance of LEDs, which were grown by MOCVD on AlN/sapphire templates, and contained: a short-period superlattice based on the pairing of 130 layers of 0.8 nm-thick AlN and 0.8 nm-thick GaN; a 600 nm-thick Al0.5
Ga0.5 N layer; a silicon-doped,
4.1 µm-thick contact layer; an active region with three Al0.35
Ga0.65 surrounded by Al0.46 Ga0.54 N wells N barriers with
identical thickness; an electron-blocking region; a short-period superlattice based on the pairing of 30 layers of magnesium- doped Al0.3
Ga0.7 N and Al0.35 Ga0.65 N,
both 2.5 nm-thick; and a 20 nm-thick magnesium-doped GaN cap.
To activate the magnesium, epiwafers were annealed in nitrogen gas. Devices were then formed that featured 150 µm by 150 µm palladium-based p-contacts, which had an indium n-contact deposited on the cleaved edge of the wafer. Output
The team’s new electron-blocking structure should benefit shorter wavelength LEDs even more. “This is the focus of our current work, where we aim at LEDs with a wavelength below 265 nm,” says Kneissl. “We already have very encouraging results from our first series of experiments in this spectral region.”
<Ref.> T. Kolbe et. al. Appl. Phys. Lett. T. Kolbe et. al.
Appl. Phys. Lett. 103 031109 (2013)
N layer; and the second featured Ga0.3
N, had a maximum output
of 1.1 mW and a peak external quantum efficiency of 0.4 percent. The latter figure is roughly twice that of the best value for other LEDs emitting at the same wavelength.
“We should note that a number of other research groups have reported even higher external quantum efficiencies for LEDs working in the adjacent UV-A or UV-C spectral ranges,” points out Kneissl. According to him, to assess the performance of all these LEDs, it is essential to consider whether the measurements are on-wafer, or the chips have been packaged.
“External quantum efficiency results that we have reported are for on-wafer measurements,” says Kneissl. “The extraction efficiency is quite low – just 6 to 9 percent of the total ultraviolet light generated within the LED is coupled out.” However, this extraction efficiency could hit 50 percent with an optimal chip and package design.
August / September 2013
www.compoundsemiconductor.net 59
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