LEDs grown on the (2021) plane produce tiny shifts in emission wavelength with increasing drive current, thanks to reduced polarization effects.

 

The (2021) plane produces LEDs with astable emission peak.

As the drive current through a green LED increases, its wavelength tends to shift towards the blue by ten nanometres or more, hampering the adoption of this source in red-green-blue displays.

But researchers at the University of California, Santa Barbara, have shown that it is possible to produce a stable green LED by turning to growth on a lesser-known cut of the GaN crystal: the semi-polar (202̅1̅) plane.

Improved wavelength stability on this plane is attributed to balancing of two electric fields. One of them is the built-in electric field resulting from the p-n junction, which is largely offset by the polarization-related electric field that arises from the quantum-confined Stark effect. In the more widely used semi-polar (202̅1) plane, these two electric fields are in the same direction, which explains the shift in wavelength with current density.

Efforts by the West-coast team commenced with the fabrication of blue-green LEDs emitting at about 495 nm, which were formed on both the (202̅1̅) plane and the (202̅1) plane. The former produced a “negligible” shift in wavelength up to a current density of 10,000 A cm-2, while the latter delivered a blue-shift of 15 nm.

One noteworthy feature of these LEDs is the structure of their active region: They feature a single quantum well, rather than multiple wells.

“For semi-polar LEDs, because we have a reduced quantum-confined Stark effect, instead of the multi-quantum wells, we can grow a relatively thick single quantum well, but still have perfect electron and hole recombination,” explains corresponding author Yuji Zhao.

He argues that turning to a single quantum well also leads to benefits in material growth and device design.

What’s more, it promises to lead to a reduction in droop, the decline in LED efficiency as the current through the device is cranked up. “Last year we showed that a wide single-quantum-well structure worked perfectly for blue LEDs in reducing the efficiency droop. This is the major reason that we are trying a single quantum-well for our semi-polar LEDs.”

Efforts have kicked-off with a thin quantum well, because longer-wavelength wells require a far higher indium content than their blue-emitting cousins, and thicker layers degrade material quality.

LEDs formed on the semi-polar plane by the UCSB team had wells just 3 nm wide. These formed part of a MOCVD-grown epitaxial stack featuring a magnesium-doped, 15-nm thick Al0.15Ga0.85N electron-blocking layer and a 60 nm-thick p-type GaN layer. LEDs with a diameter of just 80 µm were fabricated from these epiwafers. Such a small size was chosen, because the team are on a quest to create devices that produce an output that is limited by the injection current rather than chip dimensions.

According to Zhao, droop is the biggest barrier to achieving this: “The most common way for industry to get around this problem is by increasing the chip size to reduce the current density. But this obviously adds a lot of manufacturing cost to LED-based products and is one of the major reasons preventing the wide-spread adoption of LED lighting.”

In contrast, initial results from semi-polar LEDs made by Zhao and his co-workers suggest that these devices are able to reduce efficiency droop, and thus allow the fabrication of emitters with a very small chip size. In addition to improvements in wavelength stability, the (202̅1̅) plane led to sharper spectral emission profiles. Driven with a 1 percent duty cycle at current densities of 1,000 A cm-2 and 10,000 A cm-2, the full-width half maximum (FWHM) of the electroluminescence peak produced by this device was 26.2 nm and 33.4 nm, respectively. In comparison, at the same current densities, the values of FWHM for the (2021) LED were 30.4 nm and 40.5 nm, respectively.

Zhao and his co-workers point out this narrower spectral line width suggests that laser diodes could benefit from growth on the (202̅1̅) plane.

This research team has also produced green emitting, 510 nm LEDs with dimensions of 490 µm by 292 µm.  A chip formed on the (202̅1̅) plane had an output power of 5.8 mW and an external quantum efficiency of 11.9 percent, values that were slightly inferior to its cousin on the (202̅1) plane. The team suggest that the relatively poor performance stems from increased indium in the well, which leads to a higher density of defects.

Plans for the future include improving the efficiency and spectral coverage of LEDs formed on the (202̅1̅) plane.

“We are also performing different material characterisation and physical analysis, to gain a better understanding of these devices,” adds Zhao. “We believe that knowledge gained from this study will ultimately lead us to droop-free LEDs.”

Y. Zhao et. al. Appl. Phys. Expess 6 062102 (2013)