Silicon germanium grown monolithically to avoid crystal defects

March 16, 2012 — A research team from ETH Zurich, CSEM, the Politecnico di Milano and Universit├á di Milano-Bicocca manufactured defect-free structures of different semiconductors on silicon wafers, using semiconductor manufacturing processes.

The structures are grown monolithically, with silicon wafers pattered via photolithography and etched into a structure with trenches separating elevated regions (see the image). This allows arrays of individual crystals separated by gaps in the tens of nanometers. Trench depth typically exceeds width (a few micrometers). Semiconductor structures are then grown on each elevated area, with minimal separation of neighboring crystals.

Image: Perspective scanning electron micrograph (SEM) of an 8um-tall array showing germanium crystals grown on silicon pillars. SOURCE: ETH Zurich.

The researchers can tailor structure thickness via this method. Crystal defects are eliminated due to this manufacturing method, which also avoids layer bonding technologies. The different coefficients of thermal expansion (CTE) of each element does not induce layer cracking, as seen in other heterogenous integrations with continuous layers.

Defect-free germanium structures were created on top of silicon wafers at a record height of 50┬Ám.

This silicon-germanium manufacturing process could be tailored to other material combinations with similar results.

Applications for the defect-free monolithic structures include energy-resolving X-ray imaging detectors with ultra-high spatial resolution, using thick germanium absorbers; high-efficiency multiple-junction solar cells; and power electronic devices grown onto large silicon wafers.

The Swiss research is funded by the Nano-Tera project "NEXRAY" or Next Generation X-ray Systems.  

The results are presented in the March 1, 2012 issue of Science, "Scaling Hetero-Epitaxy from Layers to Three-Dimensional Crystals."

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