GOI for Fast Transistors, Photodetectors

Germanium on insulator (GOI or GeOI) is an interesting material for two reasons: It can be used for photodetectors (germanium absorbs 850 nm wavelength light about 70× more efficiently than does silicon), and also to make high-speed transistors. Germanium-based materials allow electrons to flow faster through the material, potentially speeding transistor switching by 3-4× over silicon.

Because of bulk silicon's scaling limitations — at and beyond the 45 nm chip generation — many chipmakers are evaluating engineered GOI-type substrates to enhance device performance. GOI has also been used to create high-speed photodetectors (running at 30 GHz), bringing on-chip optical interconnects closer to reality.

The fundamental speed advantage of germanium over silicon has been known for decades; however, the unstable nature of germanium oxide made MOS devices unfeasible. Today, next-generation high-k dielectric deposition technologies, coupled with these new GOI substrates, offer device manufacturers the enhanced mobility of germanium without MOS gate oxide issues.

GOI was in the news last year when Soitec, IMEC and Umicore announced a joint development effort, and Silicon Genesis announced that it was shipping GOI wafers. Earlier this year, Applied Materials and Soitec announced a strategic agreement to jointly develop advanced GOI and other related critical germanium-based processes. Germanium films will be grown with Applied's Centura RP Epi system, then cleaved and bonded onto another wafer using Soitec's SmartCut process.

1. At 850 nm wavelength, germanium absorbs light about 70× more efficiently than silicon, yet is compatible with silicon process technology. (Source: IBM)

More recently, IBM announced that it has developed a high-speed photodetector based on a newly developed GOI technology that is compatible with silicon CMOS processing. "This is a major step toward overcoming the biggest bottleneck in system performance, the interconnection between chips," said T.C. Chen, vice president of science and technology at IBM Research. "Optoelectronic components such as these GOI photodetectors will be essential for future high-performance computing systems."

2. Thin layers of germanium on SOI enable 4 µm spacing between the surface electrodes. (Source: IBM)

The new devices use GOI technology to create a photodetector that has an optical frequency response of nearly 30 GHz, making it, in principle, suitable for detecting signals at speeds of >50 Gb/sec.

The new detector is compatible with standard microchip technology because the germanium can be placed selectively in the regions where the photodetectors reside. This compatibility opens the door for making optoelectronic circuits on the same chips as microprocessors and other electronic components. "The way we created this germanium on insulator is we grew germanium directly on very thin SOI," explained Steve Koester, one of the IBM researchers involved with the project. "This allows us to get essentially GOI material by this direct growth of germanium."

Koester said that, to make efficient detectors, the germanium didn't need to be very thick; on the order of 400 nm. This allows the lateral dimensions of the device to be very small. The spacing between the surface electrodes is, at its smallest point, 4 µm. "This is what allows us to make the devices so fast, essentially because we have such a thin absorbing layer, we can put the electrodes very close together and collect the light very fast and also very efficiently," Koester said.

"The reason we made the devices like this is to make them as compatible as possible with silicon technology," Koester added. "With these devices, you don't have to make an entire substrate of GOI. You can hypothetically put the germanium only in the areas that you need the germanium."

IBM presented details on the work at the recent Device Research Conference in Notre Dame, Ind.

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