Toshiba Hones UV Chips For White LEDs
Toshiba claims to have produced the most efficient sapphire substrate UV-LED chips ever, to support an unconventional approach to replacing incandescent lamps.
In an Applied Physics Express paper published online on September 26, the Japanese conglomerate described flip-chip 383 nm LEDs with an external quantum efficiency of 36 percent. Output power under 20 mA, 3.5 V operation was 23 mW, and the Toshiba team estimated that internal quantum efficiency (IQE) could be as high as 72 percent.
“To the best of our knowledge this is the highest [IQE] reported for UV-LEDs formed on sapphire substrates," wrote Yasuo Ohba and his colleagues. “These values are thought comparable to those for visible LEDs, considering no special techniques such as surface patterning are used to enhance light extraction."
The team used a high-temperature MOCVD-grown AlN buffer on the sapphire substrate to lay the foundations for subsequent high-quality GaN growth. AlN-buffer test wafers were made using conditions similar to those Toshiba had previously used for blue-violet laser diodes, but changing the growth temperature for depositing the active layer.
Photoluminescence studies on wafers produced using Ohba s technique suggested that LEDs with peak emission wavelengths above 380 nm would be free from significant non-radiative recombination caused by threading dislocations. As a result, the team expected high quantum efficiencies for those chips in comparison to standard LED structures, where efficiency decreases drastically below 400 nm.
Ohba showed this to be the case by producing final 400 µm x 400 µm LED devices using a flip-chip approach, and testing their electroluminescent performance. The measured output power of the 383 nm LEDs increased linearly with current, reaching 68 mW at 60 mA.
UV LEDs are important to Toshiba in order to exploit europium-based phosphors that convert UV light to a red emission. Ohba says that this phosphor is “a key material for realizing both high luminous efficacy and high color rendering."
However, the europium phosphor is not well matched to visible wavelength LEDs. To enable its use, according to the Toshiba team: “development of UV-LEDs is very important for realizing white LEDs of low color-temperature warm light to replace conventional incandescent lamps."