Technical Insight
Shallow wells bolster green LED emission
Modified active region trims electric fields, increases electron-hole overlap and boosts brightness
Output power increases by almost 30 percent with the addition of a shallow quantum well
Engineersfrom the Chinese Academy of Sciences have increased the efficiency of green LEDs by almost one-third by adding a shallow quantum well to the active region.
This effort will help the quest to close the difference in efficiency between blue and red LEDs and their green cousins, which are plagued by strong polarization fields and a large lattice mismatch. Ultimately, this will make the use of lighting systems based on all three types of LED more attractive. Several groups have already modified the architecture of the green LED to improve its efficiency. “For staggered, graded and chirped multi-quantum wells, the emitting-well-layer is divided into two layers or three layers,” explains lead-author Hongjian Li. “However, the strong indium diffusion during the epi-growth of the InGaN active region makes the precise control of indium composition difficult.”
He and his co-workers take a slightly different approach, which involves maintaining the composition in the emitting layer, and inserting before it a shallow quantum well layer with a far lower indium composition. According to Li, it is far easier to carry out the growth of this structure, and that should make it easier to transfer the growth process to mass production.
Inserting a shallow quantum well has additional benefits: It reduces the strength of the polarization fields within the structure, and it increases the crystal quality of the InGaN emitting layer.
Although this approach might appear to deliver similar results to that of a graded multiple quantum well, Li asserts that there is a fundamental difference between the two structures: “Carriers won’t recombine within the shallow well layer due to the low indium composition, but for graded multi-quantum wells, carriers will recombine within the whole graded well.”
The researchers have investigated the characteristics of their LED architecture with APSYS software produced by Crosslight of Vancouver, Canada. In addition, the team have fabricated and measured the performance of chips based on this design.
Modelling efforts were based on assessing the band structure and carrier distributions of an LED with a shallow In0.10Ga0.9N quantum well just 2 nm-thick before the 3 nm-thick InGaN emitting layer, which has an indium composition of up to 30 percent. Results were compared with a more conventional design, with 3 nm-thick InGaN quantum wells.
Insertion of the shallow quantum wells led to less band-bending in the light-emitting wells, which is claimed to result from alleviation of the electrostatic field within the active region. In addition, the novel design pushed carriers towards the centre of the well and increased electron-hole wavefunction overlap from 18.3 percent to 25.8 percent.
LEDs with dimensions of 256 µm by 300 µm were fabricated by processing 2-inch, sapphire-based epiwafers grown by MOCVD. Heterostructures featured a 30 nm-thick GaN nucleation layer; a 2 µm-thick GaN layer; a 2 µm-thick, silicon-doped GaN layer; a multi-quantum well region; a 20 nm-thick p-type AlGaN electron-blocking layer; and a 200 nm-thick p-GaN layer. The active region for the novel LED contained 12 periods of 2 nm-thick, shallow In0.10Ga0.90N quantum wells, and 3 nm-thick In0.30Ga0.70N emitting layers, sandwiched between 12 nm-thick GaN barriers. A control structure was also formed that had a more conventional active region: 12 periods of 3 nm-thick In0.30Ga0.70N emitting layers interlaced with 12 nm-thick GaN barriers.
The team performed photolu minescence measurements at 85 K and 298 K on both structures. The design with a shallow quantum well had a narrower, stronger photoluminescence peak, which is believed to result from superior crystal quality. No emission can be detected from the shallow quantum well. Driven at 150 mA, the device with the shallow quantum well produced 49.3 mW at an emission wavelength of about 525 nm. In comparison, the output from the control sample was 38.4 mW.
The 29 percent increase in output power is attributed to enhanced radiative efficiency that stems from greater electron-hole overlap.
Li says that the team now plans to transfer the technology that they have developed to a high-volume LED manufacturing process. They will also optimise the design of this green-emitting chip.
H. Li et. al. Appl. Phys. Express
6 052102 (2013)
