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).
The present project is aimed at pushing a step forward the production protocols of the promising GaAs-bismides, which have many potential applications spanning from fast electronic and spintronics devices to high efficient and clean energy systems.
The possible applications of this innovative material fall into three important focus areas of Horizon2020, namely: secure clean energy; smart cities and digital security. To tackle these social challenges the technology is asked to supply innovative responses and among them ensures low consumption devices, high-speed data flow and high cryptography levels. GaAs_(1-x)Bi_x has been recently indicated as having enormous potentiality for a huge number of advanced applications in the fields of electronics, optoelectronics, nanophotonics, thermoelectricity, photovoltaic, etc. Consequently its development and optimization can open the way to the production of innovative devices able to answer the above challenges.
The light emission from GaAs_(1-x)Bi_x falls into the strategic window of telecom applications and the effective suppression of non-radiative recombination processes which cause high threshold currents in semiconductor lasers hold the promise of its application in advanced high speed communication systems. Moreover, the reported weak temperature dependence of the energy gap of GaAs_(1-x)Bi_x can be exploited to produce laser diodes working at telecom wavelengths, which would be more thermally stable and less susceptible to losses compared to conventional InP-based devices.
The formation of isolated defect states around Bi atoms structures can behave as single photon emitters and more complex structures hosting coupled excitons are potential sources of entangled photons. Fascinating applications of these quantum emitters can be envisaged in the field of communication and quantum cryptography with the value added of an efficient coupling to optical fibers.
In the case of hetero-junction bipolar transistors (HBTs) the smaller band gap of the dilute bismide compared with GaAs, when incorporated into the base of a transistor, will enable lower threshold devices, which in turn can be expected to reduce power consumption [1]. Low power consumption is a critical issue for output amplifiers on portable wireless devices. The band alignment of GaAs_(1-x)Bi_x with GaAs is favorable for HBTs since the conduction band maximum of GaAs_(1-x)Bi_x is expected to be approximately aligned with that of GaAs, presenting no barrier to electron injection into the base while the relatively large valence band offset will tend to block the back-flow of holes into the emitter, increasing the current gain of the transistor. Furthermore the weak reduction in electron mobility with Bi alloying means that the minority carrier transit time in the base will be short, enabling high speed devices. Further, the increase in the energy of the valence band edge would be expected to increase the maximum hole density that can be achieved with p-type doping since the Fermi level does not have to move down as far.
Another potential application of the dilute bismides is as a 0.9-1.1 eV band gap layer in multiple junction solar cells grown on Ge substrates, as an alternative to Ga_(1-x)In_xAs_(1-y)N_y [2].
The development of a reliable material platform based on GaAs-bismide could be the starting point for the design of innovative devices.
1. S. Francoeur, J. F. Klem, and A. Mascarenhas, Phys. Rev. Lett. 93, 067403 (2004).
2. G. Éthier-Majcher, P. St-Jean, G. Boso, A. Tosi, J. F. Klem S. Francoeur, Nat. Commun. 5, 3980 (2014).