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Electron-emitting Region Yields Brighter LEDs

Adding an electron-emitting region based on the pairing of AlInGaN and InGaN creates brighter, more robust LEDs that are less prone to droop


A team of Chinese researchers is claiming that the insertion of an electron-emitting heterostructure of AlInGaN and InGaN delivers three major benefits to the performance of GaN-based LEDs: Higher efficiency, lower droop, and substantially improved robustness to electrostatic discharge.

Adding the electron-emitter delivered a 65 percent hike in the output power of a blue LED at 20 mA, increases the value of the current associated with maximum external quantum efficiency from 2 mA to 5 mA, and upps the chances that the device will survive after being subjected to a electrostatic discharge of 2 kV from 10 percent to 82 percent.

Although there is a price to pay for all these gains, it’s a very small one, according to corresponding author Yanyan Fang from Huazhong University of Science and Technology: “Since the AlInGaN is a quaternary alloy, it requires careful tailoring of the growth parameters, such as V/III  source flux."

Growing such a structure is relatively simple for Fang and his co-workers, because they have a lot of experience in producing these quaternaries for UV LEDs.

Attempts to improve the performance of visible LEDs with an electron-emitting region can be traced back to the start of the millennium. These structures slow electrons before they enter the multiple-quantum-well region, so that the likelihood that these carriers undergo radiative recombination with holes increases, and the chances that they spill over into the p-type region falls.

The first generation of electron-emitting regions was formed from the pairing of GaN and InGaN. However, the bandstructure of this heterostructure is not ideal for capturing and re-emitting electrons with low enough energies to ensure that they are subsequently trapped in the active region. In addition, the lattice mismatch between GaN and InGaN spawns a high density of misfit dislocations.

To determine the benefits of switching from a GaN/InGaN electron-emitting layer to one made from the pairing of GaN and AlGaInN, the team compared the performance of three different structures: One without an electron-emitting layer, and variants with electron emitting layers that consists of five pairs of undoped 1.7 nm-thick In0.08Ga0.92N, sandwiched between either a 16 nm-thick silicon doped Al0.12In0.04Ga0.84N layer or GaN of identical thickness.

All three LED epistructures begin with a 2 µm-thick GaN layer on sapphire, and are followed with: A 2 µm-thick silicon-doped layer, the electron-emitting structure, an active region featuring twelve In0.21Ga0.79N wells, a p-doped electron-blocking layer, a p-type GaN layer and a heavily doped contact layer. After thinning sapphire to 90 µm, wafers were diced into 300 µm by 300 µm chips.

“The number of quantum wells is the result of optimization, according to output power at 20 milliamps," explains Fang. “We haven't studied the relationship between the number of wells and droop, but I do believe that the appropriate number of multiple quantum wells, with enhanced electron capture rate, helps to reduce droop."

Current-voltage plots of these LEDs showed that the electron-emitting layer reduces forward voltage, probably due to improved spreading of the electrons before they enter the active region. The lowest forward-voltages occurred in devices with an AlGaInN/InGaN electron-emitting region, which has a higher value for conduction band offset. Reverse biasing at 30 V revealed another benefit of the AlGaInN/InGaN electron-emitting structure: A lower leakage current, which is thought to result from a lower dislocation density.

Evidence for reduced leakage currents comes from atomic force microscopy images of incomplete variants of all three samples. Scrutinizing epiwafers that stopped after the growth of the active region revealed that the pit density of the structure with an AlGaInN/InGaN electron-emitting region was 1.9 x 108 cm-2, compared to 2.6 x 108 cm-2 and 3.2 x 108 cm-2 for the sample with an InGaN/GaN emitting region and the control, respectively.

The pits in the structure with the AlGaInN/InGaN electron-emitting region were just 95 nm in diameter, compared to 125 nm for the other two samples. Driven at 20 mA, the LED with the AlGaInN/InGaN electron-emitting region produced 21.8 mW, compared to just 17.3 mW and 13.2 mW for the devices with an InGaN/GaN electron-emitter and a standard structure. The AlGaInN/InGaN electron-emitting region also helped to reduce droop, probably by slowing down electrons and spreading them out, so that they are less likely to spill over into the p-type region at high drive currents.

The chances of damage resulting from electrostatic discharge plummeted with the introduction of the AlGaInN/InGaN electron-emitting region. Subjected to a 5 kV shock, this device had a pass rate of 70 percent, compared to almost complete failure in the other two types of LED.

Benefits of the AlGaInN/InGaN electron-emitting region could soon be extended to other types of device. “We are planning to try to design and insert an electron-emitting layer into UV LEDs, to see if we can improve the efficiency," says Fang.

J. Zhang et al. Appl. Phys. Express 5 112101 (2012)

                                                                                     



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