Lumileds challenges quantum well wisdom
The drop-off in luminous efficiency at high injection currents, often known as droop, is a major challenge to the commercial success of high-power LEDs. One approach to circumvent this problem is to distribute charge carriers between multiple quantum wells, but according to Philips Lumileds such efforts may be futile.
“Conventional wisdom would assume that a large number of quantum wells (QWs) lead to a smaller carrier density per QW, enabling efficient carrier recombinations,” researchers from the company write.
But in their Applied Physics Letters paper, published online on February 4, the Lumileds team shows that the performance of InGaN/GaN LEDs with varying numbers of QWs hardly differs.
“No matter how many QWs are grown, only the QW nearest the p[-doped] layer emits light under electrical pumping, which can limit the performances of high-power devices.”
The researchers, led by Mike Krames, found that the variation in luminous efficiency with injection current was identical for two, four and six QW LEDs having an otherwise uniform structure. The peak emission wavelength for all three devices was 445 nm.
Lumileds used angle- and wavelength-resolved far-field pattern measurements to determine which quantum wells emitted light. These patterns were again nearly identical for each device.
“In all cases the results are consistent with emission from the p-side QW alone and never suggest that more than 20 percent of the emission may come from the next QW,” the paper says.
Because of these observations, Lumileds suggests a move from multi-QW LEDs to other design approaches, which will make them more efficient at higher currents.
One such alternative, a double heterostructure that ensures uniform carrier spreading and hence lower current density, has already been put forward by Lumileds itself (see related story).
