Single-Chip LED Emits White Light

Daniel S. Burgess

Scientists in Taiwan have developed a white LED that promises to simplify the manufacture of these devices. Rather than optically stimulating a phosphor or mixing the outputs of two or three LEDs of different wavelengths to generate white light, they have employed silicon- and zinc-doped InGaN/GaN multiple quantum wells.

A research team in Taiwan has developed a single-chip, phosphorless white LED. The room-temperature photoluminescence spectra of the devices grown at different temperatures illustrate the dependence of the output on the indium concentration in the codoped multiple-quantum-well active region. Courtesy of Gou-Chung Chi, National Central University.

Gou-Chung Chi of National Central University in Chung-Li, who developed the device with his colleagues at the university and at National Cheng-Kung University in Taiwan, explained that today's white LEDs face several problems. The emission of those that use wavelength conversion depends on the uniformity of the thickness of the phosphor on the chip and on the composition of the phosphor. Color mixing eases manufacture and boosts output efficiency, but it necessitates more complicated driving circuits, he said.

The researchers grew the LED on a sapphire substrate using standard metallorganic vapor phase epitaxy. The active region of the structure comprises five pairs of 30-Å-thick codoped InGaN quantum-well layers that are separated by GaN barrier layers. The photoluminescence spectra of the devices reveal a blue peak attributable to the quantum wells and a broader, yellow-green peak due to donor-acceptor combination caused by the silicon and zinc dopants.

5 lm/W at 10-mA drive current

Chi noted, however, that the electroluminescence spectra of the LEDs under different injection currents revealed that the short-wavelength peak continues to increase with higher current even after the donor-acceptor pairs have been saturated. The researchers suggest that increasing the concentration of zinc would push the emission spectrum toward the middle of the visible region but that doing so would negatively affect the crystalline quality of the quantum wells. Alternatively, they could shift the center of the emission spectrum by increasing the ratio of indium to gallium in the wells -- for example, by growing the chips at a different temperature.

The efficiency of the new LEDs is another concern, Chi said, adding that the research team hopes to address this by improving the quality of the codoped wells and by increasing the activation efficiency of the zinc. The devices display an output of approximately 5 lm/W at a drive current of 10 mA. In contrast, white LEDs that use phosphors produce 15 lm/W at the same current, he said.