A wider emission band is better for broadband light sources, which are important for spectrum-sliced wavelength division multiplexing, fiber-based data transmission and optical coherence tomography applications. Consequently, scientists have been seeking ways to make semiconductor LEDs broadband sources.Researchers created a quantum-dot LED with a 360-nm-wide output by annealing part of it with a laser. Here the device is under test, with an electrical probe on the right and an optical fiber aligned in front of the ridge on the left. Courtesy of C.K. Chia, Institute of Materials Research and Engineering, Singapore.A group from the Institute of Materials Research and Engineering in Singapore has demonstrated a quantum-dot LED with an emission that stretches from 1284 to 1644 nm — a 360-nm span.Team member C.K. Chia, who also is an adjunct assistant professor of electrical and computer engineering at National University of Singapore, noted that two factors contributed to the broad bandwidth. The first was the use of three unequally thick layers of InAs quantum dots, which the group grew via metallorganic chemical vapor deposition on InP substrates. They capped and separated each layer with 30 nm of InP. On top of the stack they grew an InP cladding and finished with an InGaAs contact layer. The three layers of quantum dots produced a beam of approximately 300-nm bandwidth.They expanded the bandwidth to 360 nm by using a laser to anneal selective areas, using photolithography to pattern and open up an optical access window for the laser. The annealing process was performed after the device manufacturing process traditionally would have been finished. “We are the first to demonstrate bandwidth broadening on fabricated devices. To do this, one needs to design the contact mask with an optical access window for the laser irradiation,” Chia said.They used a continuous-wave Nd:YAG laser operating at 1064 nm to thermally intermix the quantum-dot layers. At that wavelength, only the quantum-dot layers absorbed the incoming laser radiation and, because the laser was not pulsed, no point defects resulted.After the laser treatment, measurements showed that the annealed end of the device blueshifted 315 nm, as compared with the unannealed end. Moreover, the bandwidth had grown, broadening from 295 to 360 nm after laser annealing. In addition, the photoluminescence intensity of the annealed end was 2.5 times that of the unannealed part of the device.Chia noted that the technique is fairly uncomplicated but that the same cannot be said for the implementation. “It is not so straightforward, as the outcomes are sensitive to the use of the laser power density, background temperature and annealing duration. So careful optimization is needed.” He added that they are working to implement this laser postprocessing in InAs/GaAs systems. They also are working on device structures that will improve optical power.Applied Physics Letters, Vol. 90, 061101.