Ricerc@Sapienza

The search for semiconducting materials with improved optical properties relies on the possibility to manipulate the band structure by using quantum confinement, strain effects, and by the addition of diluted amounts of impurity elements such as Bi.
The different strain conditions strongly impact the Bi incorporation in the GaAs matrix and the luminescence properties of the sample. A striking improvement of the photoluminescence (PL) with a strongly increased radiative efficiency when GaAsBi is grown under tensile strain has been found [1], together with a higher redshift with respect to GaAsBi grown compressively on GaAs.
Different Bi-Ga bonding configurations due to Bi complexes [2] formed by one, two or three fcc Bi nearest neighbors to the same Ga atom, or by Bi-Ga-Bi-Ga-Bi chains along given crystal directions can occur in the grown GaAsBi layer. The compressive-grown and tensile-grown samples show a different ratio in the relative X-Ray Photoemission Spectroscopy (XPS) peak intensity of the components related to dilute Bi and Bi-Ga complexes. This result gives a direct evidence of the presence of different Bi configurations in the tensile/compressive grown GaAsBi films.
In this project we intend, by means of XPS and Raman spectroscopy resolved both in energy and space, to investigate the structural properties of the material at the micro and nanoscale.
Finally by means of Extended X-ray Absorption Fine Structure the local structure around the Bi atoms in GaAsBi layers will be investigated. This will allows to find a correlation between the exact atomic structure around the Bi centers and the PL properties as a function of the strain field. The deep understanding of this interrelation opens the possibility to find specific conditions of strain and Bi concentration able to stabilize specific Bi structures emitting in a narrow selected PL band.
[1] E. Tisbi, E. Placidi et al, Phys. Rev. Appl. (in press)
[2] P. Laukkanen, et al. Appl. Surf. Sci. 396, 688 (2017).