technology LEDs
yellow, orange and red emission (see Figure 3 a). And by combining dots of different colours in nanowires, we have formed high-performance, phosphor-free white LEDs on a silicon platform. These devices combine a strong white-light output with highly stable emission. This high level of performance over a wide range of operating conditions is seen in pulsed bias measurements of relative external quantum efficiency with injection current (see Figure 3b). There is no degradation in room-temperature device efficiency up to injection current densities of 2.2 kA cm-2
.
We estimate that our internal quantum efficiency is about 60 percent, using an approach that is essentially based on the well-known ‘ABC’ model (the basis of this model is that carriers in an LED undergo one of three processes: Shockley-Reed Hall recombination, a non- radiative process that is proportional to the carrier density; radiative recombination, which is proportional to the square of the carrier density; or other higher order carrier loss processes, such as Auger recombination that depends on the cube of the carrier density). Values of the internal quantum efficiency extracted from our model agree with those obtained from optical pumping and electrical injection measurements.
Our dot-based devices set a new benchmark for internal quantum efficiency for any class of LED operating in the green, red, and entire visible spectral range. What’s more, the light emission characteristics are incredibly stable over a wide current range (from 333 A cm-2 1100 A cm-2
to ) (see Figure 3 c)
Another great attribute of our novel LEDs is their absence of droop over a very wide operating range (see Figure 4). Simulations of the internal quantum efficiency, using what is essentially an ABC model, reveal that the third-order non-radiative carrier recombination coefficient is of the order of 10-34
cm6
nearly four decades smaller than the commonly reported Auger coefficients in GaN-based quantum-well LEDs.
This extremely small value should not raise any eyebrows, given the absence of efficiency droop. What’s more, it provides unambiguous evidence that Auger recombination plays a negligible role on the performance of InGaN/GaN dot-in-a-wire LEDs operating in the entire visible spectral range.
Figure 4 Relative external quantum efficiency (EQE) of a dot-in-a-wire white LED measured at different injection currents in the temperature range of 6 K to 440
K.The simulated internal quantum efficiency (IQE) (solid curve) using the ABCmodel is also shown for comparison.
Our technology fundamentally addresses some of the major bottlenecks for the growth of phosphor-free solid-state lighting, such as low quantum efficiency and efficiency droop. Though still in its infancy, this remarkable dot-in-a-wire LED technology is already showing enormous potential for applications in future lighting and full-color displays.
s-1 –
What’s more, it provides an extremely powerful, unprecedented approach for controlling the LED emission properties at the wafer level, which can significantly reduce manufacturing cost and improve device yield.
To advance the promise of these LEDs, we are now focusing on methods to transfer nanowire devices to transparent substrates, a step that will lead to high external quantum efficiency and effective thermal management. In addition, we are undertaking a detailed investigation of device reliability.
© 2012 Angel Business Communications. Permission required.
Further reading H. P. T. Nguyen et. al. Nano Lett. 12 1317 (2012) H. P. T. Nguyen et. al., IEEE Photonics Technol. Lett. 24 321 (2012) H. P. T. Nguyen et. al. Nano Lett. 11 1919 (2011) H. P. T. Nguyen et. al. Nanotechnology 22 445202 (2011) Y. L. Chang et. al. Appl. Phys. Lett. 96 013106 (2010) W. Guo et. al. Nano Lett. 10 3355 (2010)
April / May 2012
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