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UBC Theses and Dissertations

Molecular beam epitaxy growth technology and properties of GaAsBi alloys Lewis, Ryan B.


In this thesis, molecular beam epitaxy (MBE) technology and the MBE growth of GaAsBi are investigated. MBE is a non-equilibrium technique whereby precisely controlled molecular beams are deposited onto a heated substrate at temperatures much lower than for equilibrium growth techniques. A novel closed-cycle cooling setup is implemented to replace liquid nitrogen (LN₂) cooling of the MBE cryo-shroud. The temperature dependence of cryopanel pumping is explored, and GaAs and AlGaAs layers grown using the new cooling setup and with LN₂ cooling of the shroud are characterized. Strong AlGaAs photoluminescence and low impurity concentrations indicate closed-cycle cooling is a promising cost-saving technique for MBE. The relatively unexplored III-V-Bi family of alloys is an exciting frontier of III-V semiconductor alloy exploration. The GaAsBi alloy exhibits many novel properties, including an unparalleled bandgap reduction per change in the size of the crystal lattice, presenting a wide range of potential device applications. A systematic study of the dependence of Bi incorporation on MBE growth conditions is presented. Bi incorporation is found to rapidly increase as the As₂:Ga flux ratio is lowered to 0.5 and saturate for lower flux ratios. This indicates Bi incorporation is sensitive to the surface stoichiometry. A GaAsBi growth model is proposed where Bi from a wetting layer incorporates on surface sites which are terminated by Ga. Low growth temperatures are required as the weak Bi-Ga incorporation bond can be broken thermally, ejecting Bi back to the wetting layer. GaAsBi layers with up to 21.8% Bi, record Bi-content, were grown at temperatures as low as 200C. These layers have up to 2.6% mismatch from the GaAs substrates and show unusually large critical thicknesses for relaxation, a result of the low growth temperature. Optical absorption measurements on pseudomorphic GaAsBi layers with up to 18.7% Bi show the bandgap decreases strongly with increasing Bi-content, reaching 0.5 eV at 18.7% Bi. Si-doped n-GaAsBi layers with up to 4% Bi show the concentration of acceptor states increases rapidly with increasing Bi-content. The acceptor concentration is equal to that of closed Bi3 clusters, suggesting they are the source of deep acceptor states in GaAsBi.

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