Donor binding energy and doping efficiency in high mobility AlAs quantum wells

Shivaji Dasgupta

Technische Universität München


Shivaji Dasgupta, Matthew Grayson, Max Bichler, Anna Fontcuberta-i-Morral
We present a systematic study of double-sided modulation-doped AlAs quantum wells (QW) on (001) oriented substrates and interpret from the saturation density both the binding energy of the donors and the doping efficiency of the Si dopant. Transport measurements were performed in the dark at liquid He temperatures on L-shaped Hall bars with the arms oriented along the crystallographic axes. Saturation of the dark electron density of the two dimensional electron system was observed for an incrementally doped series of samples, allowing the symmetric electric field above and below the well at saturation to be calibrated. Accounting for the strain induced energy shift of the Xx, Xy valleys, there remains an unexplained energy difference in the X-point conduction band diagram. We attribute this to the binding energy (~30 meV) of hydrogenically bound X-valley Si-donors in the dopant layer, and observe it to be a significant factor in explaining the conductance band diagram for AlAs/AlGaAs heterostructures. This model can also explain the saturation density in published AlAs data from other groups on double-sided doped QWs [1], and we propose that in the dark, the bottom doping layer screens any electric fields due to dilute charge traps in the substrate [2]. Anisotropies in the electron mobility in the two different directions of the L-shaped Hall bar are reported. [1] Lay, et al. Appl. Phys. Lett. 62, 3120 (1993). [2] De Poortere, et al. Phys. Rev. B 67, 153303 (2003).