Heterostructural Alloys = Better Semiconductors

It’s an area that’s largely understudied: heterostructural alloys, or blends of compounds made from materials that do not share the same atom arrangement. Further research into these alloys could lead to greater materials control — leading to better semiconductors, improved metallic glasses for industrial applications and advances in nanotechnology for pharmaceuticals.

Conventional alloys are isostructural, meaning they consist of compounds with the same crystal structure. “This is a very interesting piece of materials science that represents a somewhat uncharted area and it may be the beginning something quite important,” says Janet Tate, a physicist at Oregon State University. Tate gives an LED as an example of an isostructural alloy. “You have a semiconductor like aluminum gallium arsenide, dope it with a particular material and make it emit light, and change the color of the light by changing the relative concentration of aluminum and gallium,” she explains.

“If two materials have different structures, as you mix them together it’s not so clear which structure will win,” she adds. “The two together want to take different structures, and so this is an extra way of tuning an alloy’s properties, a structural way. The transition between different crystal structures provides an additional degree of control.”

The primarily the NERL’s theoretical work being supported by other collaborators’ experimental work,

As part of a new study — comprised of theoretical work by the National Renewable Energy Laboratory and supported by collaborators’ experimental work — Tate and graduate student Bethany Matthews have combined tin sulfide and calcium sulfide in order to focus on the semiconductor application. “Tin sulfide is a solar cell absorber, and the addition of calcium sulfide changes the structure and therefore the electrical properties necessary for an absorber,” Tate says.

In their study, thin-film synthesis confirmed something that had been predicted theoretically, related to the unstable, or metastable, phases of the alloys. “Many alloys are metastable… if you gave them enough time and temperature, they’d eventually separate,” Tate explains. “The way we make them, with pulsed laser deposition, we allow the unstable structure to form, then suppress the decomposition pathways that would allow them to separate. We don’t give them enough time to equilibrate.”

Such metastable materials are thermodynamically stable as long as they are not subjected to large disturbances. Generally, Tate says, these are understudied.

“When theorists predict properties, they tend to work with materials that are stable,” she notes. “In general the stable compounds are easier to attack. The idea here with heterostructural alloys is that they give us a new handle, a new knob to turn to change and control materials’ properties.”