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Carnegie Team Tunes GaAs Bandgap Under Pressure

Zincblende and wurtzite structures show badgap differences

New work from a team led by the Carnegie Institution for Science's Alexander Goncharov has shown that the band gap of GaAs can be fine-tuned by putting it under high pressure.

Previous research on cubic  'zincblende' crystalline GaAs nanowires has already proved that the band gap widens under pressure, but Goncharov's team focused instead on nanowires of a less-common form, the hexagonal wurtzite structure. 

The researchers subjected wurtzite GaAs to up to about 227,000 times normal atmospheric pressure (23 gigapascals) in diamond anvil cells. They explored the pressure effects using two different incident lasers of wavelengths of 532nm and 488nm. 

They discovered that the band gap that the electrons need to leap across in the wurtzite nanowires also widened, although not as much as in the case of the zincblende crystal nanowires. Significantly, they discovered that around 207,000 times normal atmospheric pressure (21 gigapascals), the wurtzite GaAs nanowires underwent a structural change that induced a new phase, the so-called orthorhombic one, which may possibly have metallic electronic properties.

Above is a microphotograph of 'wurzite' GaAs nanowire in a diamond anvil cell high pressure cavity kept at 99,000 times normal atmospheric pressure (10 gigapascals); the blue spot is from the 488nm laser spot. Photo is credited to Wei Zhou.

"The similarity in behavior when subjected to high pressure, but resulting in significant differences in the size of the band gap, between the two crystalline structures of GaAs suggests that both types of GaAs structures could theoretically be incorporated into a single device, or even a single nanowire, and realise much more complex and useful electronic functions through interactions across the phases," Goncharov said. "We believe these findings will stimulate further research into GaAs for both basic scientific and practical purposes."

The work is outlined in the paper 'Vibrational, electronic and structural properties of wurtzite GaAs nanowires under hydrostatic pressure'  by Wei Zhou et al is published in Nature Scientific Reports. The research team includes Wei Zhou, Xiao-Jia Chen, Xin-Hua Li and Yu-Qi Wang of the Chinese Academy of Sciences and Jian-Bo Zhang of South China University of Technology.

Scientific Reports 4, Article number: 6472 doi:10.1038/srep06472

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