Rhombohedral Super Hetero Epitaxy of Silicon-Germanium (SiGe)

Faster: Has over twice the charge mobility of single-crystal, silicon-based chips


Better performance: Improves performance with new lattice-matched conditions


Superior quality: Produces superior quality, graded, indexed SiGe layers with fewer defects


Low implementation cost: Uses existing silicon fabrication facilities for production


Mass production: Enables wafer-scale mass production through compatibility with conventional SiO² insulators


Versatile: Allows varying material thicknesses including Si/Ge/SiGe quantum wells

High electron mobility transistors and hetero-bipolar transistors used for high-speed computing and wireless communications


Photovoltaic solar cells


Thermoelectric systems


Radiation detectors


Optoelectronic devices with quantum wells

Until now, no alternative materials have offered high charge mobility with compatible insulator materials to replace the widely used silicon, so chipset speed essentially has been limited by the feature size of the chip. Development of faster chipsets has been cost prohibitive without new semiconductor materials that exhibit high carrier mobility as an intrinsic property because new manufacturing fabs, capable of reaching nanometer scales, are necessary for reducing feature size.

How It Works

The traditional alloys (e.g., carbon, silicon, germanium, and tin) cannot be simply deposited on commonly used substrates (e.g., silicon) because the alloys’ crystal lattice constants differ from the substrate structure. Lattice mismatches, which occur when a SiGe alloy is deposited onto silicon, lead to defects including dislocations, cracks, and delamination. Traditional epitaxy technology uses two distinctive mathematical frames to design new alloys, the cubic crystal unit cell and the hexagonal crystal unit cell.

The new epitaxy technique aligns a rhombohedral transformation of SiGe cubic crystals with trigonal crystals such as a sapphire substrate. Because rhombohedral crystal structures are mathematically equivalent to trigonal crystal structures, cubic crystals can be epitaxially grown on trigonal crystals in rhombohedral alignment and vice versa. With this new rhombohedral-trigonal alignment, unprecedented lattice-matching conditions can be found as well. Such conditions reduce defects (e.g., dislocations) and result in improved semiconductor devices. Although trials by others failed because of the lack of a non-destructive evaluation method, this method stabilizes rhombohedral epitaxial growth of SiGe alloys on trigonal sapphire with a new X-ray diffraction characterization.

Conventional silicon-based chipsets have achieved clock frequencies of approximately 144 gigahertz by decreasing micro-circuit line resolution to 45 nm. However, further improvement has been limited because of the material properties. This rhombohedral, lattice-matched, single-crystal SiGe with quantum well structures can increase the charge mobility of SiGe beyond 3,000 cm²/Vs, which at the same resolution offers clocked speeds of up to 308 gigahertz.

Why It Is Better

NASA’s rhombohedrally aligned SiGe is the first of its kind. Ultra-fast semiconductor chipmakers have long awaited the increases in speed possible with the SiGe compound—well beyond that of silicon-based chip technology. Because this technology allows Si/Ge/SiGe quantum wells with faster electron and hole mobilities than bulk single-crystal silicon, it will enable the development of ultra-fast chipsets for numerous applications.

Although several high-charge mobility compound semiconductor materials (e.g., zinc blends and wurzites) are available, they are costly and do not allow the proper insulator materials to make mass production possible. The rhombohedrally aligned SiGe enables narrow gate length in wafer- scale mass production because a conventional silicon oxide insulator can be used. In addition, Si/Ge/SiGe quantum well structures and new lattice-matching conditions on various trigonal substrates are to be explored and utilized further.

Patents

Two U.S. patents have been issued for this technology (Nos. 7,514,726 (link opens new browser window) and 7,341,883 (link opens new browser window)). Four additional patent applications have been filed.

This technology is part of NASA’s Innovative Partnerships Program, which seeks to transfer technology into and out of NASA to benefit the space program and U.S. industry. NASA invites companies to consider licensing this Rhombohedrally Lattice-Matched Silicon-Germanium (SiGe) technology (LAR-16868-1, LAR-16872-1, LAR-17185-1, and LAR-17553-1) for commercial applications.

If you would like more information or want to pursue transfer of this technology, please contact us by e-mail or phone: or (919) 249-0327.

For more information about other technology licensing and partnering opportunities, please visit:

Innovative Partnerships Program Office (link opens new browser window)
NASA’s Langley Research Center