Molecular beam epitaxy and characterisation of GaAsBi for photovoltaic applications

• Main Author: Richards, Robert D.
• Other Authors: David, John P. R. ; Roberts, John S.
• Published: University of Sheffield 2014
• Subjects: 621.3
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.632576

GaAsBi is a promising candidate material for a 1 eV junction for multi-junction photovoltaics, as well as having many other potential applications in areas such as telecommunications and spintronics. The growth of GaAsBi has proven problematic due to the large size and low electronegativity of the Bi atom, and this has hindered its development. In this thesis, the growth and material characterisation of GaAsBi is presented. A systematic series of bulk GaAsBi samples were grown by molecular beam epitaxy to investigate the effects of growth temperature, As flux and As species on the bismuth content and optical quality of the samples. Two growth regimes became apparent: a temperature limited regime in which the Bi content is limited by the miscibility of GaAs and GaBi, and a Bi flux limited regime in which the Bi incorporation coefficient approaches unity. The production of good quality GaAsBi was shown to require near a near stoichiometric Ga:As atomic flux ratio. The dependence of Bi content on As species was explained by considering the results of Foxon and Joyce, which show the necessary desorption of 50 % of the incident As4 flux during GaAs growth. The optical quality of GaAsBi was shown to have no dependence on the As species used during growth. Using the expertise gained from the growth of bulk GaAsBi, a series of GaAsBi/GaAs multiple quantum well p-i-n diodes was grown and characterised. Preliminary results showed that good quality structures were grown with photoluminescence peaks at around 1050 nm. The samples containing a large number of quantum wells showed signs of strain relaxation and a redshift and attenuation of their photoluminescence spectra. Calculations of the effects of strain relaxation and loss of quantum confinement on the photoluminescence emission wavelength, suggest that both factors contribute to the observed redshift. The onset of strain relaxation in these samples appeared to occur at a similar average strain to InGaAs/GaAs samples reported in the literature. These results suggest that GaAsBi could provide a competitive alternative to InGaAs for high efficiency multi-junction photovoltaics.