28 June 2012
Compared with conventional planar LED heterostructures, the use of nanowires offers several extraordinary advantages, including drastically reduced dislocation densities and polarization fields, enhanced light output efficiency due to the large surface-to-volume ratios and compatibility with low cost, large-area silicon substrates. However, the quantum efficiency of currently reported nanowire LEDs generally exhibits a very slow rise with injection current. To find out more, researchers at McGill University in Canada have examined the emission characteristics in detail and attempted to identify the fundamental carrier loss mechanisms.
In their work, the scientists have developed unique InGaN/GaN dot-in-a-wire nanoscale heterostructures with light emission characteristics that can be well controlled by varying the sizes and/or compositions of the dots. The team has demonstrated phosphor-free white LEDs on low cost, large area silicon substrates that can exhibit record internal quantum efficiency (~58%) across the entire visible spectral range. Furthermore, the group has identified that electron leakage out of the active region of the device, rather than Auger recombination, is primarily responsible for efficiency degradation at high injection levels in such nanowire devices.
By incorporating an AlGaN electron blocking layer between the quantum dot active region and p-GaN, the researchers demonstrated phosphor-free white LEDs that can exhibit virtually zero efficiency droop for operating temperatures up to 150 °C and under injection currents well over 1000 A/cm2. The resulting InGaN/GaN dot-in-a-wire LEDs also display highly stable emission characteristics. However, compared with conventional quantum well LEDs, nanowire devices generally exhibit a very slow rise in the quantum efficiency with increasing current. For example, while the peak efficiency of quantum well LEDs is generally measured in the range of ~10 to 20 A/cm2, the efficiency of nanowire LEDs reaches their maximum values at significantly higher current densities (>100 A/cm2).
Measurement and simulation
Through detailed temperature-dependent electroluminescence measurements and simulation studies, the scientists concluded that that Shockley-Read-Hall (SRH) recombination on the lateral surfaces of the wire, due to the presence of surface states and defects and large surface-to-volume ratios, is largely responsible for the slow rise in the quantum efficiency. For example, the SRH recombination can account for nearly 40% of the total carrier recombination at an injection current of ~100 A/cm2 at room temperature.
The team believes that this work has unambiguously identified the various carrier loss mechanisms that can limit the performance of the emerging nanowire LEDs, and the results provide critical insight to further the development of practical nanowire-based nanophotonic devices.
Futher details can be found in the journal Nanotechnology.