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New technique raises efficiency of Si emitters (Silicon Update)

Researchers are using techniques from the photovoltaics industry to improve the efficiency of silicon-based light emitters and bring their integration into silicon microelectronics a step closer.
Silicon Update - New technique raises efficiency of Si emitters In a perfect world, silicon would be as wonderful at emitting light as it is at enabling an incredibly wide range of ICs. The ability to produce on-chip light could be used to transport data into and out of circuitry, eliminating the area-intensive problems and parasitics of metal I/Os that are becoming a real bottleneck in the advance of many circuit applications. But as we well know, the indirect bandgap of Si makes it a poor emitter of light. The material requires more than simply an electron-hole pair to recombine; it also needs help from phonons. Many III-V materials do not require phonons, which allows them to dominate light-emission applications in the form of light-emitting diodes and lasers. Decades of research Because of the potential payoff of an efficient light-emitter in Si, extensive research has been done in this area for decades. This has ranged from basic concepts such as merged epitaxial processes (e.g. GaAs on Si growth), to more esoteric approaches such as the use of porous Si, amorphous Si, complex strained Si-based structures and SiGe/Si superlattices. While all of these approaches have been able to produce some level of optical emission, efficiencies have been incredibly poor, at best reaching 0.010.1%. None of these techniques has attained or even suggested a route that might lead to an emission technique that approaches the capabilities of III-V materials. Despite the great challenge of using silicon as an emission source, this material has been used for many optical applications, in particular solar cells. State-of-the-art Si-based solar cells exhibit conversion efficiencies of 20%, while the very best III-V solar cells (cascaded cells with a wide range of bandgaps) exhibit a 30% conversion efficiency, representing only a 50% improvement over Si. In contrast, the ability of Si to emit light is typically three to four orders of magnitude lower than that obtained by III-V material devices. This discrepancy begs the question is it possible to learn something from the fabrication of Si solar cells, which can crudely be viewed as light emitters operating in reverse bias? Researchers from the University of New South Wales (UNSW), Australia, have asked this question and found that knowledge gained from Si-based solar cells can be applied to the manufacture of high-efficiency Si light emitters. As it turns out, Si does not really absorb light very well, especially if you want that absorption to generate an electron-hole pair and not simply heat up the material through the generation of phonons. A great deal of the light passing into a Si solar cell passes through the active and drift regions, through the substrate, and is collected in the backside metallization regions where it generates no power. To combat this limitation, state-of-the-art Si solar cells have textured front and back surfaces. The absorbed light makes many passes back and forth through the device, so more photons are captured and, with the aid of phonons, generate electron-hole pairs within a diffusion length of the device s electrical junction. A general rule of thumb for a material or device is that the better it is at absorbing light, the better it is at emitting it. Turning an absorber into an emitter The light emitters made by the UNSW researchers rely on the normally weak one- and two-phonon assisted sub-bandgap light emission that takes place in Si. The texturing method in Si photovoltaics to increase absorbance uses what are referred to as small internal escape cones. If the direction of the light entering a semiconductor surface with a refractive index n can have its path randomized, only a small fraction of that light can escape back out the surface. This fraction is proportionalto 1/2n2. As a result, for weak absorption, where phonon assistance is needed to generate electron-hole pairs, absorption improves by a factor of 2n2, or about 25 times. Textured surfaces can also be used in solar cells based on III-V semiconductors. However, because electron-hole pairs are generated through direct energy transitions (no phonon assistance is required), the overall enhancement is not so great since conversion takes place before photons can typically escape the III-V material. In a high-efficiency silicon emitter, inverted pyramid structures enhance effective absorption, in the same way that photovoltaic cells use escape cones (see ). Backside reflectors ensure that photons are reflected back to the surface then back into the material at random angles from the pyramid surface. In addition to improving absorption, the researchers took great care with these devices to reduce non-radiative recombination. They accomplished this by using long-lifetime float-zone Si material, passivating all exposed surfaces with high-quality thermal oxides, keeping metal contact areas small, and reducing heavy doping (where free carrier absorption can occur) and keeping it localized to contact regions. By enhancing absorption and reducing non-radiative recombination, the device shown in the figure emits light with an approximate efficiency of 1%. This represents an improvement of two orders of magnitude on efficiency results from other Si-emitter techniques. While this is still one order of magnitude lower than that obtained by III-V materials, the efficiency should be improved still further by increasing the thickness of the diode, reducing the doping concentrations and improving the reflectivity of the back surface. Looking to the future In fact, this approach may mean that poor electrical-to-photonic efficiency is no longer the limiting issue associated with these devices. The next challenge arises because the material properties are so good that the long lifetimes of carriers make for slow response times. The integration of high-speed modulators is therefore necessary to sweep out long-lived carriers rapidly in order to permit high data rates. These issues aside, it appears that Si can no longer be referred to as a material that lacks the characteristics to make it an adequate emitter of light. Further reading M Green et al. 2001 Efficient Light-Emitting Diodes Nature 412 805
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