A doping series of AlAs (001) quantum wells with Si δ-modulation doping on both sides reveals different dark and post-illumination saturation densities, as well as temperature dependent photoconductivity. The lower dark two-dimensional electron density saturation is explained assuming deep binding energy of ∆DK = 65.2 meV for Si-donors in the dark. Persistent photoconductivity (PPC) is observed upon illumination, with higher saturation density indicating shallow post-illumination donor binding energy. The photoconductivity is thermally activated, with 4 K illumination requiring post-illumination annealing to T = 30 K to saturate the PPC. Dark and post-illumination doping efficiencies are reported.PACS numbers: 73.20.b,73.21.fg,73.50.Pz,73.43.f,71.18.+y Two dimensional electron systems (2DESs) in aluminum arsenide (AlAs) quantum wells (QWs) are interesting for their valley degeneracy and heavy mass 1,2,3 . The valley index quantum number acts as an extra pseudospin degree of freedom, and the heavy mass allows interactions to play a larger role at a given density 4 . Recently progress has also been made in fabricating and characterizing one-dimensional AlAs nanostructures 5,6,7 . Although improvements in high mobility AlAs 2DES structures have been reported 8,9 , many important material parameters such as the donor binding energy and doping efficiency have been obscured by substrate charge effects 10 . Since these parameters are instrumental in designing and optimizing heterostructures, we have performed a systematic study on double-sided-doped quantum wells which screen away unwelcome substrate effects. In the process, we also identify a thermally activated persistent photoconductivity (PPC) not previously reported.AlAs is an indirect band gap III-V semiconductor with three degenerate conduction band valleys at the X-points of the Brillouin zone edge. In (001) growth, the biaxial strain between AlAs and Al x Ga 1−x As (x = 0.45) decreases the energy of the two in-plane valleys such that for wide wells W > 55Å, 11,12 only these two valleys are degenerately occupied. In AlAs, the longitudinal and transverse electron masses are anisotropic, m l = 1.1 m e and m t = 0.2 m e , respectively 3 , and the effective Landé g-factor g * = 2 (Ref. 11).Free electrons in AlAs/AlGaAs heterostructures come from two different sources: intentional Si-dopant and un-

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.

We studied a doping series of (110)-oriented AlAs quantum wells (QWs) and observed transport evidence of single anisotropic-mass valley occupancy for the electrons in a 150Å wide QW. Our calculations of strain and quantum confinement for these samples predict single anisotropic-mass valley occupancy for well widths W greater than 53Å. Below this, double-valley occupation is predicted such that the longitudinal mass axes are collinear. We observed mobility anisotropy in the electronic transport along the crystallographic directions in the ratio of 2.8, attributed to the mass anisotropy as well as anisotropic scattering of the electrons in the X-valley of AlAs.