Growth and characterisation of low-dimensional semiconductor structures

• Main Author: Atkinson, P.
• Published: University of Cambridge 2003
• Subjects: 537.622
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.596215

Molecular Beam Epitaxy (MBE) is a growth technique which allows a high level of control over the composition of single crystal epilayers deposited on a suitable single-crystal substrate. MBE growth of GaAs/AlGaAs heterostructures allows abrupt, defect-free interfaces to be fabricated, which, combined with modulation doping, provides a means of creating high mobility two-dimensional electron gases (2DEGs) at the heterointerface. This thesis is primarily concerned with the control and assessment of the level of disorder in the heterostructure. Two types of disorder over which the grower has little <i>direct</i> control have been considered. These are the interface morphology and the unintentional background impurities incorporated in the epilayer. The low temperature 2DEG mobility over a range of carrier densities is measured to quantify the level of disorder. The reliability and reproducibility of this measurement is discussed here in some detail. In order to compare different structures illumination should be used to vary the carrier density rather than a surface gate due to variability introduced by the device processing. However, illumination above the GaAs band-gap alters the depletion field which is shown to affect the scattering probability. This must be taken into account when determining scattering parameters. The effect of the anisotropic interface morphology on the 2DEG mobility is investigated. There is a large difference in mobility between the [110] and [?10] current flow directions, especially for high mobility samples. Interface roughness is also assessed by atomic force microscopy and low temperature photoluminescence. Reduction of the growth temperature from 585°C to 450°C increases the interface roughness, but reduces the anisotropy. Addition of atomic hydrogen over this temperature range did not result in any improvement in morphology. In a relatively low mobility regime the mobility is shown to be limited by background impurities in the undoped Al_xGa_l-xAs spacer, evaluated as 1.5x x 10¹⁶cm^-3.