We report systematic growth optimization of high Al-content AlGaAs, AlAs, and associated modulation-doped quantum well (QW) heterostructures on on-axis and misoriented GaAs (111)B by molecular beam epitaxy. Growth temperatures TG > 690 °C and low As4 fluxes close to group III-rich growth significantly suppress twin defects in high-Al content AlGaAs on on-axis GaAs (111)B, as quantified by atomic force and transmission electron microscopy as well as x-ray diffraction. Mirror-smooth and defect-free AlAs with pronounced step-flow morphology was further achieved by growth on 2° misoriented GaAs (111)B toward [01¯1] and [21¯1¯] orientations. Successful fabrication of modulation-doped AlAs QW structures on these misoriented substrates yielded record electron mobilities (at 1.15 K) in excess of 13 000 cm2/Vs at sheet carrier densities of 5 × 1011 cm−2.

This paper describes a complete analytical formalism for calculating electron subband energy and degeneracy in strained multi-valley quantum wells grown along any orientation with explicit results for AlAs quantum wells. In analogy to the spin index, the valley degree of freedom is justified as a pseudospin index due to the vanishing intervalley exchange integral. A standardized coordinate transformation matrix is defined to transform between the conventional-cubic-cell basis and the quantum well transport basis whereby effective mass tensors, valley vectors, strain matrices, anisotropic strain ratios, piezoelectric fields, and scattering vectors are all defined in their respective bases. The specific cases of (001) (411)-oriented QWs and we define and solve for a shear-to-biaxial strain ratio. The notation is generalized to address non-Miller-indexed planes so that miscut substrates can also be treated, and the treatment can be adapted to other multi-valley biaxially strained systems. To help classify anisotropic intervalley scattering, a valley scattering primitive unit cell is defined in momentum space which allows one to distinguish purely in-plane momentum scattering events from those that require an out-of-plane momentum component.

No abstract

No abstract