Taking the strain

US research focuses on technique that could lead to semiconductors being formed in any shape. Stuart Nathan reports.

A manufacturing technique could lead to semiconductors being formed in any shape, according to materials scientists at the University of Madison-Wisconsin. The technique can also enhance the conductive properties of the materials, opening up an array of novel applications.

Graduate Michelle Roberts has been working on the technique, which focuses on managing the strain within a semiconducting material as it is formed. Strain is an important factor for semiconductors, which can be both useful and awkward.

When silicon is placed under strain it conducts electricity faster, which can be used to enhance the processing speed of electronics. However, when semiconductors are built up from layers of alloys, the different sizes of atoms can create strain within the crystalline structure which leads to defects having the opposite effect: they scatter electrons through the crystal and impede conduction.

Roberts has devised a method for making a semiconducting membrane from layers of silicon and a silicon- germanium alloy which, although only tens of nanometres thick, has the same properties as a solid wafer of silicon. He explained that these membranes are flexible, and can be made in a variety of shapes, including flat, tubular and curved, depending on the thicknesses of the layers.

The membrane has three layers, with a silicon-germanium layer sandwiched between two silicon layers. These are formed on top of a solid layer of silicon dioxide, which in turn sits on a silicon-on-insulator substrate. When immersed in hydrofluoric acid, the silicon dioxide dissolves away, leaving the layered semiconducting membrane to float free.

Germanium atoms are larger than silicon and prefer to sit in a larger lattice; silicon-germanium alloys are therefore under strain, as the germanium tries to push the silicon into a smaller space. ‘When we remove the membrane, the silicon-germanium is no longer trying to fight the substrate, which is like a big rock holding it from below,’ explained Max Lagally, Roberts’ supervisor. ‘Instead, it’s just fighting the two thin silicon layers, so the silicon-germanium expands, taking the silicon with it.’

The alloy layer expands and stretches the silicon, but the strain produced by this stretching is predictable – it depends on the thickness of the layers, which can be determined during manufacture. Moreover, the silicon layers tend to trap electrons, which is a desirable property for electronics design.

The extra degrees of freedom for the semiconductor opens up new applications, said Lagally. ‘We’re no longer held to the rigid rock of material; we can now transfer the membranes to anything we want. So, there are some really novel things we can do.’

These might include faster transistors and crystals that can guide and steer waves of light, using the enhanced conduction speed of the membrane; sensors for detecting toxins in the atmosphere and for studying the processes inside cells; and flexible electronic devices to be incorporated into fabrics or polymer sheets.

Unfortunately, the step in the process where the membrane is released from the solution renders it unsuitable for bulk manufacturing. ‘What we’ve done is a first demonstration,’ said physics professor Mark Erikkson. ‘But now that we’ve shown the underlying principles are sound, we can take the next steps.’