Strain tunes quantum-dot LED wavelength

Researchers in China are now able to exert control over a chanced-upon technique that allows individual semiconductor die to emit white light.

The LEDs made by Hong Chen and colleagues at the Chinese Academy of Sciences in Beijing produce white emission thanks to precipitation of indium quantum dots in their InGaN quantum wells.

In a paper published online on March 20 in Applied Physics Letters Chen and colleagues show how to vary the extent of indium precipitation.

Multiple Emission

The quantum well emits approximately 440 nm light in each device, while the quantum dots emit at 545 nm and above.

Additional indium precipitation increases the size and density of quantum dots produced. This raises the wavelength of the original quantum dot emission peak and adds another at around 495 nm.

The team’s original intention was to use an InGaN layer at the bottom of the GaN/InGaN quantum well to collect carriers and consequently enhance light emission.

“Based on the question ’What will happen if partly relaxed InGaN is used?’, white emission was found in an LED wafer with a thick underlayer,” Chen told compoundsemiconductor.net.

Dice with 160 nm, 190 nm and 220 nm underlayers below the quantum well emitted green, yellow-green and white light, respectively.

Electroluminescence spectra of the different LEDs show that the first sample emits light at two wavelengths, while the others emit at three.

The thinnest underlayer retains the highest amount of biaxial strain, with Chen and colleagues deducing that the lattice is about 9.6 percent relaxed. The thickest, by contrast, is 64.4 percent relaxed, while all the LEDs have around 4.4 percent indium in their underlayers.

Indium Precipitation

Using transmission electron microscopy the researchers saw that the strain in the thinner layers prevents precipitation of In-rich quantum dots.

One downside of the approach is that electroluminescent intensity decreases as the thickness of the underlayer increases. Chen suggests that this is because dislocations introduced at a higher level of relaxation act as non-radiative recombination centers.

He concedes that uniformity and reproducibility are issues for the commercial exploitation of this approach to white emission.

“The wavelength reproducibility closely depends on the composition and thickness of the InGaN underlayer, which is sensitive to growth temperature,” Chen commented.

As long as devices emit at the same wavelengths, light emission performance is “well reproducible”, he added.

Dec 10, 2007

Dots deliver phosphor-free white light

White LEDs featuring indium-rich quantum dots, rather than a yellow-emitting phosphor, have been built by Hong Chen’s team from the Chinese Academy of Sciences.

"Our work provides a novel approach to casting off the limitations of a [down-converting] phosphor," said Chen. According to him, phosphor-free devices promise to deliver longer lifetimes and higher output efficiencies than conventional LED designs.

The researchers fabricated their 300 µm × 300 µm LED chips by low-pressure MOCVD growth on sapphire substrates. A 3 µm thick buffer was grown, followed by 220 nm of InGaN, a four-period active region comprising 3 nm InGaN quantum wells and 14 nm GaN barriers, and a p-type region.

Transmission electron microscopy revealed spinodial decomposition of InGaN in the quantum wells. This phase separation – enhanced by the InGaN underlayer – leads to indium-rich quantum dots with a diameter of 3–4 nm and a density of 10¹² cm^–2.

At LED drive currents of less than 5 mA, yellow emission from the dots dominates the device’s output. However, blue emission kicks in at higher currents and the ratio of blue-to-yellow emission intensity is almost constant between 20 and 60 mA.

This leads to a stable white light output over this current range, which makes the chip suitable for LED lighting applications. In fact, Chen says that the device can overcome the unwanted color change that plagues many phosphor-converted white LEDs when the drive current is changed.

If quantum-dot LEDs were to replace the light bulb, then their ratio of blue-to-yellow emission intensity would have to be maintained at higher drive currents. However, this should not be a major obstacle, because the current density through the team’s chip at 60 mA is almost identical to that of a 1 × 1 mm power chip operating at 700 mA.

One downside of the phosphor-free device is its efficacy, which is lower than 10 lm/W. The researchers are planning to develop new technology to increase quantum efficiency.

"We also need to investigate how to control the ratio of blue and yellow light intensities, and see whether yellow emission can be shifted to longer wavelengths," explained Chen. If this is possible, it would improve the device’s color-rendering index.

• Journal reference
X H Wang 2007 Appl. Phys. Lett. 91 161912.