Realizing single-chip white light-emitting diodes without phosphors

Light-emitting diodes occupy an important position in solid-state lighting projects. In the next 5-10 years, they will gradually replace traditional lighting fixtures and become a new type of energy-saving and environmentally friendly light source. Compared with traditional light sources (incandescent lamps, fluorescent lamps, halogen lamps, etc.), light-emitting diode light sources have many advantages, such as long life, small size, low power consumption, low environmental pollution, high electro-optical conversion efficiency, good applicability and safe use. With the development of GaN-based III-V compound technology and the realization of blue light LEDs , people can already obtain three-primary color light-emitting diodes that realize white light. Usually, the methods for obtaining white light-emitting diodes are: 1) blue light-emitting diodes + yellow phosphors; 2) multi-chip combination, that is, combining the three tube cores of red, green and blue together; 3) photon circulation to achieve white light emission; 4) quantum well method of growing different emission wavelengths on the same substrate. However, the above-mentioned methods of obtaining white light are technically complex, high in production cost, and there are many difficulties that need to be overcome. Therefore, people have been committed to realizing a single-chip white light emitting device that can avoid these problems , and theoretically predict the feasibility of such a device. However, despite the increasing maturity and commercialization of GaN-based blue and green light-emitting diode technology, achieving single-chip white light emission has become a dream of scientists. In 2006, Chen Hong's research group from the Institute of Physics in Taiwan used the stress modulation layer of InGaN to achieve stress modulation and control of InGaN/GaN multi-quantum wells, and successfully developed a single-chip white light-emitting device. This method does not require phosphors or complex control circuits, and the preparation process is similar to that of ordinary light-emitting diodes. Under conventional injection current (20mA-60mA), the color rendering index of white light is almost unchanged. Figure 1 shows the change in luminous color under different injection currents.

Figure 2. Transmission electron microscope cross-section of the InGaN/GaN active region of a light-emitting diode

Electroluminescence spectrum studies show that the LED emits yellow light at low current. As the current increases to more than 20mA, the blue light intensity gradually increases, and the emitted light gradually transitions from yellowish light to white light. The transmission electron microscope cross-sectional image shows that a large number of In-rich quantum dots are formed in the InGaN quantum well. At low injection current, carriers are first captured by In-rich quantum dots to emit yellow light. As the current increases, the quantum well region outside the quantum dots begins to capture carriers, and emits blue light after radiative recombination. The blue light and yellow light mix to produce white light.