V-defects boost green LEDs efficiency
Researchers from the University of California, Santa Barbara (UCSB), have shown that V-defects and an optimised AlGaN cap enhance the efficiency of green III-N LEDs. When the emitters produced by the West-coast team also feature distributed Bragg reflectors (DBRs) to increase light extraction, external quantum efficiency peaks at 44 percent, and wall plug efficiency hits 37 percent.
Producing green III-N LEDs with a high efficiency is a well-known challenge, due to the poor material quality that results from a low temperature required for the growth of indium-rich InGaN quantum wells, as well as polarisation-related effects. When stretching to longer emission wavelengths, increases in the quantum-confined Stark effect drag down radiative recombination rates and enhance polarisation-induced barriers, which hamper carrier injection and lead to a higher forward voltage.
Helping address the low efficiency in green LEDs are V-defects. These hexagonal pyramid-shaped depressions, containing six semipolar sidewalls that typically form at the apex of threading dislocations during kinetical limited low-temperature growth, provide an avenue for efficient carrier injection into the quantum wells.
According to the team, it is possible to ‘properly engineer’ the V-defect density on a patterned sapphire substrate through the generation of pure-edge dislocation half-loops – they are realised by growing a preparation layer with a high silane or disilane flow at low temperature. When a low-temperature GaN layer is deposited afterwards, this opens the half-loop threading dislocation arms into V-defects.
Fabrication of team’s LEDs began by loading a patterned GaN-on-sapphire template into an MOCVD chamber and depositing a 600 nm-thick n-GaN buffer at 1220 °C, followed by a 60 nm-thick half-loop formation layer at 800 °C, grown using a disilane flow of 9.9 sccm. It is said that heavy silicon-doping is needed to form the edge-dislocation half-loops that will be opened into V-defects.
During growth at the relatively low temperature of 800 °C, existing threading dislocations open into V-defects. To ensure a uniform V-defect size, the team grew a 60 nm-thick n-GaN layer at 1000 °C to planarize over the open V-defects, before adding another 60 nm-thick n-GaN layer at 800 °C to open these defects.
The addition of a 25 period superlattice, formed from alternating layers of 3.5 nm-thick In0.05Ga0.95N and 3.5 nm-thick GaN, enlarged the V-defects. On this V-defect template the team added an active region, grown under nitrogen to promote V-defect propagation, that contains 8 periods with a: 0.5 nm-thick GaN pre-layer, designed to recover surfaced morphology; followed by a 3.3 nm-thick In0.15Ga0.85N quantum well and a 2.3 nm-thick AlGaN cap, both grown at 810 °C; and a quantum barrier, with 6 nm-thick GaN grown at 850 °C, and then 3 nm-thick GaN grown at 900 °C.
Growth of the green LEDs concluded with the addition of magnesium-doped GaN at 960°C. A 60 nm-thick p-GaN layer planarized the open V-defects, before the addition of a 12 nm-thick contact layer. Samples were then annealed in air at 630°C for 15 minutes.
Device measurements revealed that optimising the growth of the 2.3 nm-thick AlGaN cap – realised by increasing the flow rate for the gallium precursor and reducing that for ammonia – increases the peak external quantum efficiency and wall plug efficiency from 29 percent and 35 percent, to 34 percent and 41 percent, respectively. Even higher values of 37 percent and 44 percent, respectively, come from combining the optimised AlGaN cap with the DBR.
Inspecting the active region with cross-sectional transmission electron microscopy revealed that as well as V-defects, there are trench defects in the active region, originating at the interface between the quantum wells and the caps. These trench defects consist of a basal-plane stacking fault, terminated by a stacking mis-match boundary that acts as a non-radiative centre.
When evaluating their device, the team points out that the quality of the capping layer plays a role in efficiency enhancement. It is noted that last year a team from China reported peak values of 65 percent external quantum efficiency, and 60 percent wall plug efficiency, for LEDs produced with an aluminium treatment step immediately after quantum well growth.
For the UCSB devices, there is a sizeable difference between values for external quantum efficiency and wall plug efficiency. Motivated by this observation, the team is planning to investigate the impact of AlGaN caps on lateral injection through V-defects. The researchers hope that by using the growth conditions they have developed, they will improve the quality of their low-temperature capping layers, and ultimately increase the efficiency of their long-wavelength LEDs.
Reference
A. Quevedo et al. Appl. Phys. Lett 129 053301 (2026)
