Modern light-emitting diodes (LEDs) and lasers are based on a physical effect known as super-injection. For decades this effect has been believed to occur only in semiconductor hetero-structures (structures composed of two or more semiconductor materials). But now researchers from the Moscow Institute of Physics and Technology have found super-injection to be possible in homo-structures too (made of a single material). This opens up entirely new prospects for the development of light sources. The research was reported in the journal Semiconductor Science and Technology.
Semiconductor light sources, such as lasers and LEDs, are at the core of modern technology. They enable laser printers and high-speed internet. But a mere 60 years ago, no one would imagine semiconductors being used as materials for bright light sources. The problem was that to generate light, such devices require electrons and holes — the free charge carriers in any semiconductor — to recombine. The higher the concentration of electrons and holes, the more often they recombine, making the light source brighter. However, for a long time, no semiconductor device could be manufactured to provide a sufficiently high concentration of both electrons and holes.
The solution was found in the 1960s by Zhores Alferov and Herbert Kroemer. They proposed to use hetero-structures, or “sandwich” structures, consisting of two or more complementary semiconductors instead of just one. If one places a semiconductor between two semiconductors with wider band-gaps and applies a forward bias voltage, the concentration of electrons and holes in the middle layer can reach values that are orders of magnitude higher than those in the outer layers. This effect, known as super-injection, underlies modern semiconductor lasers and LEDs. Its discovery earned Alferov and Kroemer the Nobel Prize in physics in 2000.
However, two arbitrary semiconductors cannot make a viable heterostructure. The semiconductors need to have the same period of the crystal lattice. Otherwise, the number of defects at the interface between the two materials will be too high, and no light will be generated. In a way, this would be similar to trying to screw a nut on a bolt whose thread pitch does not match that of the nut. Since homo-structures are composed of just one material, one part of the device is a natural extension of the other. Although homo-structures are easier to fabricate, it was believed that they could not support super-injection and therefore are not a viable basis for practical light sources.
But physicists Igor Khramtsov and Dmitry Fedyanin from the Moscow Institute of Physics and Technology have now made a discovery that drastically changes the perspective on how light-emitting devices can be designed. They have found that it is possible to achieve super-injection with just one material. What is more, most of the known semiconductors can be used. According to Dr. Dmitry Fedyanin, in the case of silicon and germanium, super-injection requires cryogenic temperatures, and this casts doubt on the utility of the effect. But in diamond or gallium nitride, strong super-injection can occur even at room temperature.
This means that the effect can be used to create mass market devices. As mentioned in the published paper, super-injection can produce electron concentrations in a diamond diode that are 10,000 times higher than those previously believed to be ultimately possible. As a result, diamond can serve as the basis for ultraviolet LEDs thousands of times brighter than what the most optimistic theoretical calculations predicted. Surprisingly, the effect of super-injection in diamond is 50 to 100 times stronger than that used in most mass market semiconductor LEDs and lasers based on hetero-structures.
The physicists emphasized that super-injection should be possible in a wide range of semiconductors, from conventional wide-bandgap semiconductors to novel two-dimensional materials. This opens up new prospects for designing highly efficient blue, violet, ultraviolet, and white LEDs, as well as light sources for optical wireless communication (Li-Fi), new types of lasers, transmitters for the quantum internet, and optical devices for early disease diagnostics.
The study was supported by the Russian Science Foundation.
Published Paper: Super-injection in Diamond Homo-junction P-I-N Diodes by I.A. Khramtsov and D.Yu. Fedyanin