Composite semiconductors could boost safety of X-rays

The resulting composite semiconductors could ultimately lead to X-ray medical detectors that work with much lower doses of radiation.

As Dr Hans von Känel of the Swiss Federal Institute of Technology Zurich explained to The Engineer, combining different semiconductors is desirable for many applications.

‘People always like to start with cheap, abundant silicon wafers but silicon itself is very limited in terms of physical properties; for example, the ability to form optical components. Silicon cannot be used for a laser,’ von Känel said.

Composite semiconductors have been developed previously but they rely on rather crude ‘bump-bonding’ techniques that create hybrid materials rather than true composites. The main problem with this arises when the material is subject to temperature changes; the different thermal expansion coefficients of the materials create layer cracking.

Känel and colleagues claim to have circumvented this thermal stress problem by growing layers in one piece, monolithically.

In a first step a silicon wafer is patterned by photolithography and subsequently etched. The depth of the trenches separating the elevated regions typically exceeds their width of a few micrometres. The desired three-dimensional semiconductor structures are then grown onto the substrate pillars under conditions assuring a minimum separation of neighbouring crystals.