In optoelectronic devices like solar cells or light-emitting diodes, the band gap – or energy gap between the top of the valence band and the bottom of the conduction band – determines the photonic performance. One of the ways of controlling that band gap is through strain because deforming a material induces a predictable change in the band gap.
GaAs, which is widely used in optoelectronic devices, is too brittle for such simple strain engineering. But now researchers from the University of Electronic Science and Technology of China, Tsinghua University, and the Institute of Semiconductors in Beijing have found a way around the problem.
By creating very thin ribbons of GaAs, or nanoribbons, the researchers introduce a wave or buckle into the structure that allows manipulation of the band gap [Wang et al., ACS Nano (2016), doi: 10.1021/acsnano.6b03434].
The team led by Xue Feng of Tsinghua University created the wavy nanoribbons by using photolithography to cut thin strips of GaAs grown by metal-organic chemical vapor deposition (MOCVD). The nanoribbons are then transfer-printed onto a pre-stretched soft substrate of the polymer polydimethylsiloxane (PDMS). When the stretched substrate is released, ribbons of undulating GaAs are formed.
The wavy structure creates alternating regions of tension and compression in the nanoribbons. In step with this strain variation, the band gap narrows and widens periodically and continuously along the length ofthe structure. Photoluminescence (PL) measurements provide a direct insight into the band gap. Over a distance of 100 m in a single nanoribbon, the researchers found that the band gap varies by up to ∼1%.
“Our approach can produce continuous strain in the same piece of material, from tension to compression, making its performance unique,” says Feng.
The ability to control the band gap in such a predictable and periodic way within a nanostructure could inspire new designs of optical and optoelectronic devices, suggest the researchers. Because of the overall flexibility of the GaAs nanoribbons, further levels of complexity in the modulation and enhancement of the band gap can be achieved through tension or compression of the soft substrate.
“We are intending to induce more complicated strain into the optoelectronic material,” Feng explains. “[For example] we could divide the whole ribbon into several parts with different band gaps to make separate LED cells. Every cell would have a different emitting wavelength based on its gap - we could achieve a multi-wavelength LED device using a single film.”
John A. Rogers of the University of Illinois at Urbana-Champaign believes that Feng and colleagues have made very interesting use of mechanical buckling in semiconductor nanoribbons to scrutinize the effects of strain on electronic bandgap.
“The work is an interesting combination of nanoscale mechanics and electronic structure, where the unique ‘wavy’ geometry of the materials allows systematic investigation of how strain and intrinsic properties relevant to electronic and optoelectronic performance can be examined at sub-micron length scales,” he says. “The outcomes have relevance to engineering design of both conventional, wafer-based forms of semiconductor devices as well as newer stretchable and flexible technologies.”
This article was originally published in Nano Today (2016), doi:10.1016/j.nantod.2016.08.002