As the constant push for smaller devices has given rise to nanoscale semiconductor structures, two competing features — quantum confinement and lattice strain — take a larger role in shaping the properties of these structures. Understanding the interplay of these two phenomena, however, remains difficult at such a small scale. New work using dynamic compression experiments investigates the details of how the two factors determine the change in the optical efficiency of a semiconductor due to electronic structure changes.
By subjecting gallium arsenide (GaAs) quantum wells, a prototype quantum structure, to shock and ramp wave compression, Grivickas et al. discovered that additional interactions within the quantum wells suppress the real-space type-II bandgap direct-to-indirect transition (DIT), leading to larger than expected photoluminescence signal.
While under uniaxial strain, generated through impact experiments, the authors exposed 3.4-nanometer-thick GaAs crystals to undergo type-II DIT at 1.7 gigapascals. While such pressures affected photoluminescence efficiency, significant signal levels were recorded up until 2.6 gigapascals, at which point type-I DIT occurred. Their results show that under certain conditions semiconductor nanostructures can maintain direct band-gap properties even under high stresses.
The finding is important for future semiconductor devices with reduced dimensions. Current bulk devices withstand less than 1 percent of strain, typically occurring at interfaces between materials, but nanostructures are predicted to withstand nearly 10 times that much.
In the article, the authors point to the possibility that the “engineering of large anisotropic strains can be used to improve efficiency of optoelectronic devices beyond the real-space DIT limits.” They also point out it is important that future research efforts investigate other direct bandgap materials beyond the prototypical GaAs for the presence of similar phenomena.
Source: “Direct-to-indirect electronic state transition in dynamically compressed GaAs quantum wells,” by P. Grivickas, J. F. Geisz, and Y. M. Gupta, Applied Physics Letters (2018). The article can be accessed at https://doi.org/10.1063/1.5038723.