research review
Carrier manipulation combats droop Grading quantum barriers cuts LED droop
CALCULATIONS by engineers from National Chiao Tung University, Taiwan, have shown that alterations to the composition of barriers can improve GaN LED performance. By modifying hole transport, it is possible to design devices that are more efficient and less prone to droop, the decline in LED efficiency as current is cranked up.
The team looked at three different LED structures with APSYS software developed by Crosslight of Burnaby, Canada. All these LED designs are formed on 100 µm- thick sapphire and have: A 4 µm-thick n-type GaN layer: an active region comprising six 2.5 nm-thick In0.15
Ga0.85 Ga0.85 N
quantum wells interspersed with 10 nm- thick GaN barriers; a 20 nm-thick, p-doped Al0.15
N electron-blocking layer; and a 200 nm-thick, p-doped cap.
Differences between the structures relate to variations in barrier design. One device has all its barriers graded in indium composition from 5 to 0 percent along the growth direction; another has just the fifth barrier graded; and the third structure has grading in the fourth and fifth barriers. It is
possible to realise these doping profiles in real devices. “We’ve demonstrated similar structure experimentally,” says corresponding author Hao-Chung Kuo, who has reported the results in a paper in the journal Applied Physics Letters (Appl. Phys. Lett. 99 171106).
LED modelling – employing a Shockley- Read-Hall recombination time of 1 ns, an Auger recombination coefficient in the quantum wells of 10-31
cm6 s-1 and device
dimensions of 300 µm by 300 µm – revealed that doping of just the fifth barrier wrought the greatest improvements.
Efforts involved modelling electron and hole distributions at current densities of 40 mA cm-2
and 200 mA cm-2 . In
conventional LEDs, holes accumulate in the well nearest the p-region. This is not the case in any of the three types of device with graded barriers, where holes were distributed slightly more uniformly, with significant populations in the fourth, fifth and sixth wells. Electron distributions were modified by the presence of of holes, with most negative carriers found in the well nearest the p-type region.
Calculations included those for light output power and external quantum efficiency (EQE) for all three types of LED with graded barriers. Those with two or more graded barriers are best at combatting droop – the efficiency at 200 mA cm-2
is
only down by 6 percent or less compared to the peak efficiency.
However, these designs have a fatal flaw: Their EQEs are inferior to that of a conventional LED, because excessive improvements in hole doping don’t translate to higher device efficiency. In comparison, although grading just the fifth barrier leads to a small improvement in hole transport, it delivers a 42 percent hike in EQE over the standard LED at 200 mA cm-2
(droop, measured by the same criteria as before, is 10 percent).
The team is now hoping to demonstrate the benefits and pitfalls of graded barriers in real devices. “We have been cooperating with Epistar for a while,” says Kuo. “We’ll work together on this project.”
C. –H. Wang et. al. Appl. Phys. Express 5 042101 (2012)
Optogenetics for GaN LEDs and CMOS sensors Imaging the brain of a mouse with arrays of 470 nm LEDs and silicon pixels.
RESEARCHERS at Nara Institute of Science and Technology, Japan, claim that they have built the first integrated optical neural stimulation and observation device incorporating an LED and a CMOS image sensor. This device could aid researchers in the field of optogenetics, which involves the use of light to alter the behaviour of cells.
It is possible to build a similar system with an avalanche photodiode array rather than a CMOS sensor, which is an approach that has been adopted by engineers at the University of Strathclyde, UK. According to lead-author Takashi Tokuda from Nara Institute of Science and Technology, one of the advantages of the avalanche photodiode array is that it can deliver high-speed detection, which is essential for time- resolved fluorescence measurements. But he adds that this type of detector is unsuitable for on-chip imaging of biological cells and
tissues, because each of the photodiodes has dimensions of the order of 10 µm.
“The resolution of a conventional CMOS image sensor can be as small as 1-2 µm ,” says Tokuda, who admits that he and his co-workers are still to shrink their pixels to such small dimensions.
The team builds its neural interface device by flip-chip bonding an LED-on-sapphire array to a CMOS image sensor. Thanks to very low levels of absorption of visible light in the sapphire and nitride layers of the LED, it is possible to place samples, such as a slice of brain, on the backside of the substrate. The neural interface device is formed by combining an array of 470 nm LEDs with a 128 by 268 array of detector pixels, each 15 µm by 7.5 µm. This has been used to image a slice of brain taken from a mouse.
This work is still in its infancy, and Tokuda admits that there is much to do before he and his co-workers will start to acquire high-quality images. In order to realise such images, an on-chip filter is needed to distinguish between emission resulting from fluorescence and light originating from scattering of the excitation source. In addition, detector sensitivity must be improved so that it is possible to measure very small changes in intensity, and the instrument needs to provide a higher spatial resolution, which will require reductions to pixel sizes and the distance between cell and target.
Tokuda and his co-workers will try to tackle many of these issues. Their goals for the future include shrinking the size of their LEDs and improving image performance.
T. Tokuda et al. Electronics Lett. 48 312 (2012)
April / May 2012
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