Researchers develop new ways to shape semiconductors

According to teams from ETH Zurich and Centre Suisse d´Electronique et de Microtechnique (CSEM), the novel structures can be manufactured to virtually any thicknesses on cheap and abundant silicon substrates. Rather than being assembled by current bonding techniques, the structures are grown in one piece – monolithically – by deposition techniques customary in semiconductor processing. This new approach means that crystal defects, normally present in layers of atoms differing in size, are entirely eliminated. It also means disruptive substrate bowing upon temperature changes, caused by unequal thermal expansion coefficients of dissimilar materials, is largely absent, while fatal layer cracking induced by thermal stresses can no longer occur. Instead of continuous layers, the researchers composed structures from space filling arrays of individual crystals separated by narrow gaps of several tens of nanometres. The first step is to pattern a silicon wafer by photolithography and etch it into a structure resembling a chocolate bar. According to the researchers, the depth of the trenches separating the elevated regions typically exceeds their width of a few micrometres. The desired 3D semiconductor structures are then grown onto the substrate pillars under conditions assuring a minimum separation of neighbouring crystals. The method has been perfected for defect free germanium structures on top of silicon wafers, for which a record height of 50µm was achieved. The researchers believe that in the future the new concept will be transferrable to many other material combinations. This ability to synthesise virtually defect free monolithic semiconductor structures paves the way for novel applications in many fields. The research team says one example is given by energy resolving X-ray imaging detectors with ultra high spatial resolution. Here, thick germanium absorbers are highly suited. They might result in far lower doses necessary for X-ray imaging in medical applications. High efficiency multiple junction solar cells for concentrator and space applications are another example for which the new method could offer substantial cost and weight reductions by replacing expensive, fragile and heavy germanium substrates by cheap and light silicon substrates with high mechanical stability. Yet another example is the potential for power electronic devices grown onto large silicon wafers. Researchers from ETH Zurich and CSEM worked in collaboration with colleagues from the Politecnico di Milano and Università di Milano-Bicocca. First results are presented in the 16 March 2012 issue of Science.