Strain tunes quantum-dot LED wavelength
Researchers in China are now able to exert control over a chanced-upon technique that allows individual semiconductor die to emit white light.
The LEDs made by Hong Chen and colleagues at the Chinese Academy of Sciences in Beijing produce white emission thanks to precipitation of indium quantum dots in their InGaN quantum wells.
In a paper published online on March 20 in Applied Physics Letters Chen and colleagues show how to vary the extent of indium precipitation.
The quantum well emits approximately 440 nm light in each device, while the quantum dots emit at 545 nm and above.
Additional indium precipitation increases the size and density of quantum dots produced. This raises the wavelength of the original quantum dot emission peak and adds another at around 495 nm.
The team s original intention was to use an InGaN layer at the bottom of the GaN/InGaN quantum well to collect carriers and consequently enhance light emission.
“Based on the question What will happen if partly relaxed InGaN is used? , white emission was found in an LED wafer with a thick underlayer,” Chen told compoundsemiconductor.net.
Now Chen s team has found that varying the thickness of this underlayer changes the strain in the GaN semiconductor crystal, and in turn the concentration of quantum dots.
Dice with 160 nm, 190 nm and 220 nm underlayers below the quantum well emitted green, yellow-green and white light, respectively.
Electroluminescence spectra of the different LEDs show that the first sample emits light at two wavelengths, while the others emit at three.
The thinnest underlayer retains the highest amount of biaxial strain, with Chen and colleagues deducing that the lattice is about 9.6 percent relaxed. The thickest, by contrast, is 64.4 percent relaxed, while all the LEDs have around 4.4 percent indium in their underlayers.
Using transmission electron microscopy the researchers saw that the strain in the thinner layers prevents precipitation of In-rich quantum dots.
One downside of the approach is that electroluminescent intensity decreases as the thickness of the underlayer increases. Chen suggests that this is because dislocations introduced at a higher level of relaxation act as non-radiative recombination centers.
He concedes that uniformity and reproducibility are issues for the commercial exploitation of this approach to white emission.
“The wavelength reproducibility closely depends on the composition and thickness of the InGaN underlayer, which is sensitive to growth temperature,” Chen commented.
As long as devices emit at the same wavelengths, light emission performance is “well reproducible”, he added.
