Experimental and theoretical determination of the transport properties of n-AlxGa1-xSb/GaSb

Hanks, Laura and Hayne, Manus (2020) Experimental and theoretical determination of the transport properties of n-AlxGa1-xSb/GaSb. PhD thesis, Lancaster University.

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Abstract

The work in this thesis reports on the design, growth, fabrication, measurement and analysis of n-GaSb/Al0.2Ga0.8Sb heterostructures. Initially a growth study was completed in order to minimise the native n-type GaSb defects (usually of p ~ × 1017cm−3 ) and to calibrate the n-type doping (achieved using Te). This growth study then informed simulations of the band structure and the transport properties via Schrödinger-Poisson and transport lifetime modelling, allowing an investigation for the design of a high mobility n-GaSb/Al0.2Ga0.8Sb structure, to ensure correct confinement. The optimum designs from the simulation study were then grown and fabricated into 8-pad geometry non-gated Hall bars. The metallisation recipe researched further as initial recipes were deemed unsuitable. The resulting devices were then measured as a function of magnetic field and temperature to obtain the transport properties (mobility and carrier density).It has been shown that an optimum set of growth conditions (Tg= 475 °C, V/III=1.3) reduces the p-type native defects resulting in a hole concentration of 5 × 1016 cm−3 at room temperature. Simulations of AlGaSb/GaSb heterostructures were completed, investigating the conduction band and the effect of the p-type defects. It was found that doping below the well is necessary for confinement in these structures. Both square and triangular quantum wells were grown on semi-insulating GaAs substrates with an interfacial misfit array to prevent strain. These samples were fabricated where the metallisation recipe Pd/In/Pd/Au was used, allowing cold temperature measurements. Measurements of the transport properties across a temperature range of (3-300) K at lowfield (< 3 T) resulted in a peak mobility of 9030 cm2 /Vs, where background impurity scattering was shown to be the limiting scattering rate. High field measurements of a doped heterojunction resulted in Shubnikov-de Haas oscillations visible up to 100 K. As the first transport measurements of confined n-GaSb/AlGaSb, these results inform the direction of optimisation for high mobility devices in this material.