Sunday, April 15, 2012

Singapore - Photonics: Getting a fair compensation

Compensation doping can improve the efficiency of silicon optical modulators

Silicon is widely used in electronics devices, such as computer chips and solar cells. It is also becoming the material of choice for making photonic devices that lie at the heart of communications, including light-emitting diodes, photodetectors and optical modulators.

One of the drawbacks of silicon photonic devices is 'insertion loss' — the loss of optical signals when these devices are integrated into the optical network. Silicon optical modulators, for example, may have comparable switching speed and modulation efficiency as optical modulators made of other materials such as lithium niobate, but their insertion loss can be on the double. Improving the optical performance of silicon modulators is highly desirable as these devices are compatible with complementary metal–oxide–semiconductor (CMOS) technology that is widely used in today’s electronic devices.

Xiaoguang Tu and co-workers at the A*STAR Institute of Microelectronics have now demonstrated how to improve silicon modulators by using an appropriate way of doping the silicon with electrons and holes1.

Doping can provide active modulators with extra electrons and holes. The process normally involves implanting acceptor and donor impurities in the main component of the modulator — the silicon waveguide. Unfortunately, the extra carriers produce a reduction in efficiency due to their light absorption. The loss efficiency of silicon modulators is typically 20% worse than that of lithium niobate modulators. There are several possible routes to minimize the loss efficiency, but they all tend to degrade the devices in one way or another. 

Tu and his team overcame the problem using an approach called compensation doping (see image). In this approach, the central area of the silicon waveguide is highly doped as usual, so that the electrons and holes remain on opposite sides of the central plane. Moving away from the centre, however, the doping is reduced so that the total number of carriers and the light they absorb is compensated.

The researchers monitored several characteristics of the devices while varying the profile of the non-compensated region on the cross-section of the modulator. They found that in the best-case scenario, the loss efficiency of silicon modulators was comparable to that of lithium niobate modulators without affecting the modulation efficiency or the shifting speed.

“With these improvements, silicon modulators may become a main competitor of lithium niobate modulators currently on the market,” says Tu. “These modulators may also be the perfect candidate for future integrated photonics and electronics circuits.” Tu and his team are now working towards improving the performance of silicon modulators further by exploring new structure designs and doping profiles.

The A*STAR-affiliated researchers contributing to this research are from the Institute of Microelectronics


1.     Tu, X., Liow, T.-Y., Song, J., Yu, M. & Lo, G. Q. Fabrication of low loss and high speed silicon optical modulator using doping compensation method. Optics Letters 19, 18029–18035 (2012). | article

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