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Smoothing process boosts green LED output

The secret to reducing out-diffusion of indium from quantum wells is adding an extra trimethylindium-ammonia step during epiwafer growth.

Taiwanese researchers have developed a method to improve the crystal properties of green LEDs, boosting their devices light output by 43 percent at 20 mA driving current.

Jen-Inn Chyi and his colleagues from the National Central University in Jhongli, Taiwan, made this improvement by using a material growth technique that produces smoother InGaN/GaN interfaces.

The team describes adding a key extra step to MOCVD deposition of a 525 nm-emitting epi structure in a paper published online in Applied Physics Letters on April 25.

Their approach inserts a growth stage in which only trimethylindium and ammonia flow into the reactor after depositing each InGaN quantum well layer.

Analysis performed after multi-quantum well (MQW) growth showed that surface roughness measurements could be halved by using this trimethylindium treatment. The density of V-shape defects "“ the most common defect in indium-rich quantum wells "“ reduced from 7.8 x 108 to 4.7x 108 cm-2 in the same samples.

Chyi s team says that one explanation for the observed smoothing is that adding indium precursor suppresses indium aggregation and InGaN decomposition in favor of InN deposition.

They point out that when green MQWs are made with other growth methods and the deposition process is interrupted, the wavelengths detected in photoluminescence spectra of the sample become bluer with time. This phenomenon is attributed to indium out-diffusion.

In contrast, when the growth was interrupted but kept in a trimethylindium-ammonia atmosphere there was no wavelength shift detected. In fact, luminescent intensity increased and the wavelength peak became sharper as time went on.

The LEDs fabricated using this method maintain an output power advantage over the control devices up to a 70 mA drive current. The Taiwanese group s results show little drop-off in this trend, suggesting that this benefit should extend to even higher currents.

The same method can be used to smooth blue InGaN/GaN LEDs, however because indium content is lower in these devices the improvement is less noticeable.

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