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Posted: Oct 15, 2015

Novel nanowire structures lead to white-light and AC-operated LEDs

(Nanowerk Spotlight) Compared to the conventional inefficient incandescent and fluorescent lighting technologies, LED light bulbs can, in principle, operate at an efficiency level of 100%. The current LED lighting technology, however, is not even close to reaching this limit.
This is due to several factors. First, current LED lamps still rely on the use of phosphors to down-convert blue light into green and red light. Associated with this down-conversion process is an energy loss of approximately 30%, or more.
Second, the performance of GaN-based LEDs has been limited by the inefficient current conduction of p-GaN, which typically has a resistance of about 100 times higher than that of n-GaN, leading to poor current spreading, reduced efficiency, and efficiency droop (LEDs operate most efficiently at low currents of tens of milliamps; however,if the current increases, efficiency tails off in a phenomenon known as 'efficiency droop').
Third, unlike conventional light bulbs, LEDs are low-voltage devices and cannot operate on an alternating current voltage. As a consequence, an electrical circuit is required to convert AC power to low-voltage DC power (typically 2-4V). Such a driver adds a significant level of complexity, cost, and efficiency loss to the LED devices and systems.
These above mentioned problems can be by and large solved by employing tunnel junction integration into current nanowire LED structures.
Doing exactly that, researchers at McGill University in Montreal have developed tunnel junction nanowire LEDs that can eliminate the use of resistive p-GaN contact layers, leading to reduced voltage loss and enhanced hole injection. Moreover, by using tunnel junction interconnect, they have demonstrated multiple-active-region (MAR) nanowire LEDs with significantly enhanced light intensity.
The team has published their findings in Nano Letters ("Alternating-Current InGaN/GaN Tunnel Junction Nanowire White-Light Emitting Diodes").
"We have also realized AC operated nanowire LEDs on a silicon platform which operate efficiently in both polarities (positive and negative) of applied voltage," Sharif Sadaf, a PhD student in Zetian Mi's MBE group at McGill, and the paper's first author, tells Nanowerk. "Compared to the current quantum well LEDs, the demonstrated tunnel junction nanowire LED technology enables phosphor-free white emission and reduced efficiency droop. Moreover, it offers extreme flexibility in the operation voltage and can completely eliminate the use of an AC/DC converter required in conventional LED lighting technologies, thereby leading to reduced cost and further enhanced efficiency."
Alternating current tunnel junction dot-in-a-wire LED arrays
Alternating current tunnel junction dot-in-a-wire LED arrays. (a) Two-step selective area growth of p-GaN up and p-GaN down AC nanowire LEDs on Si substrate. p-Up nanowire LED arrays were first grown on the opening areas of SiOx coated Si substrate. Then the SiOx and the nanowires on top were selectively removed using chemical etching. The p-up nanowire LED structures were then covered with SiOx and additional opening areas were created prior to the growth of the p-down nanowire LED structures. Subsequently, the SiOx and the nanowires on top were selectively etched. This leads to the formation of p-up and p-down nanowire LED arrays on the same Si chip. (b,c) Device schematics. (d) SEM (45° tilted) images of as-grown p-GaN up and p-GaN down nanowire LED structures. (e) Optical image of green light emitting nanowire LED arrays on Si under AC biasing conditions. (Reprinted with permission by American Chemical Society) (click on image to enlarge)
p-GaN contact resistance has been a long standing bottleneck in improving the performance of conventional nitride based LEDs. Researchers have therefore been looking for fundamental design modifications.
"Polarization engineered tunnel junctions offer the unique opportunity to eliminate the p-GaN contact resistance problem by replacing resistive p-GaN with n-GaN," explains Sadaf. "Moreover, by employing the tunnel junction scheme, we demonstrated a multiple-active region nanowire LED that can potentially circumvent the 'efficiency droop' problem."
He notes that, in general, stacking multiple quantum wells/dots in planar structures is not a suitable route to realize low current, high voltage operation since it also significantly increases the densities of defects and dislocations.
"Such issues can be fundamentally addressed in tunnel junction nanowire LED structures, as demonstrated in our work" says Sadaf. "Moreover, such MAR tunnel junction nanowire LEDs can be designed to operate in a broad wavelength range, leading to phosphor-free white light emission."
A unique result of this work is the substantially improved light intensity compared to single active region LEDs.
The scientists attribute this striking improvement to carrier regeneration at each tunnel junction and uniform low mobility hole injection in each active region.
This work has the potential to go a long way in solving low power LED application. According to Sadaf, it is feasible to integrate more active regions into the nanowire structures to achieve higher light output power from a single chip.
Furthermore, it is possible to integrate different wavelength color in a single nanowire to obtain white light emission.
More importantly, having demonstrated AC power operation of their devices, this could give lighting devices great flexibility for usage in household applications.
In principle, this concept can be further extended to other semi polar or non polar III-V nanowire based applications. Already, the team is exploring other nanowire based optoelectronics devices such as lasers, UV-LEDs, and photodetectors.
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