Composite semiconductors could boost safety of X-rays

Swiss researchers have found a way to seamlessly integrate two different semiconductor materials for applications including X-ray imaging chips and improved solar cells.

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.

The upshot is that they can create composite layers of almost any thickness, with virtually no defects. One application could be a CMOS X-ray digital imaging chip incorporating a layer of germanium crystals.

‘You really need very thick germanium layers in order to absorb a significant fraction of hard X-rays used in medical applications,’ von Känel said.

‘The problem is that under normal circumstances it is completely out of the question to have germanium grown onto silicon of thicknesses of dozens of microns, because you will inevitably end up with thermal mismatch and you would form lots of cracks.’

Enabling such large thicknesses of germanium with the new method could mean high-resolution X-ray chips that need far lower radiation doses, and so less risk to patients during medical scanning.

Another application where this method could find a use is multi-junction solar cells. These require different semiconductor materials to form junctions tuned to different wavelengths of light, thereby increasing efficiency.

Currently, germanium is used as a base substrate alongside indium gallium phosphide and gallium arsenide for highly efficient solar cells that are used in space applications.

The problem is that these germanium-based cells are delicate and rather expensive and also suffer from thermal mismatch. Using silicon as a base for multi-junction cells with germanium and other layers on top would make for highly efficient cells that are tougher and cheaper.

‘You would end up with a much more stable structure not subject to problems of thermal expansion mismatch, because of course in a satellite there are very large temperature changes, between day and night,’ said von Känel.