Doped InGaN barriers attack LED droop
Replacing undoped GaN barriers in blue LEDs with p-type InGaN can significantly delay the onset of droop, the fall in device efficiency at higher drive currents.
That s according to Hadis Morkoç s group at Virginia Commonwealth University, whose findings are scheduled for publication in the September 22 issue of Applied Physics Letters.
These researchers determined that 420 nm LEDs containing In0.01Ga0.99N barriers with 5x1017 cm-3 of magnesium doping hit a maximum external quantum efficiency at 900 A cm-2, which equates to 9 A for a 1 mm x 1 mm chip. In comparison, conventional devices peaked at 35 Acm-2.
The team also studied the cause of droop - a hot topic for LED researchers (see related stories) - with electrical injection and photoluminescence experiments.
According to them, droop is ultimately caused by electron leakage. This stems from poor hole transport into the multi-quantum well active region.
One possible remedy would be to embed a p-type quantum well into the p-type region. However, this would probably lead to very low quantum efficiency, because magnesium is a luminance “killer”.
So the researchers have just doped the barriers. This provides another way of getting holes into the quantum wells.
Testing various samples
The team made several samples, which showed a typical internal quantum efficiency of 15 percent.
The LEDs were grown by low-pressure MOCVD on GaN-on-sapphire templates with a defect density of 2x108 cm-2. Six 2 nm-thick undoped In0.20Ga0.80N wells featured in the chips, which were separated by 12 nm barriers.
External quantum efficiency measurements didn't just reveal that the magnesium-doped InGaN barriers were best at delaying the onset of droop. They also quantified the effect of switching from undoped GaN barriers to undoped InGaN barriers, which pushed peak efficiency from 35 Acm-2 to 220 Acm-2.
Photoluminescence experiments were carried out with a Ti:Sapphire laser that delivered 100 fs pulses at 385 nm. This excitation wavelength is below GaN's bandgap.
No droop was observed in the undoped-barrier samples at excitation densities of up to 0.34 kWcm-2. This equates to an electrical injection rate that is four orders of magnitude higher than that used to drive any of these LEDs.
When the LED chipmaker Philips Lumileds carried out similar experiments they observed droop. This led them to dismiss carrier leakage as the cause of the decline in LED efficiency at high current densities, and claim that droop is caused by Auger recombination.
Morkoç s group still has to investigate device reliability. “One thing that I am worrying about is the lifetime, especially under high injection current,” explains Jinqiao Xie. If magnesium migrates into the wells, this will immediately degrade device performance.
Xie says that the team is developing longer and shorter wavelength LEDs, and is hoping to integrate the superior structure into a VCSEL. Somewhat surprisingly, the university is not planning to file any patents relating to this technology.
