“…11 For bulk GaAs layers grown with arsine, we obtained mobilities of 160 000 cm 2 /V s at 77 K, comparable to the best to date at 77 K. For 2DEG studies, the basic epitaxial structure consisted of 1 m undoped GaAs, an undoped Al 0.28 Ga 0.72 As spacer of varying thickness, a ϳ4ϫ10 12 cm Ϫ2 Si delta-doping layer, 625 Å undoped Al 0.28 Ga 0.72 As, and a 50 Å GaAs cap. 11 For bulk GaAs layers grown with arsine, we obtained mobilities of 160 000 cm 2 /V s at 77 K, comparable to the best to date at 77 K. For 2DEG studies, the basic epitaxial structure consisted of 1 m undoped GaAs, an undoped Al 0.28 Ga 0.72 As spacer of varying thickness, a ϳ4ϫ10 12 cm Ϫ2 Si delta-doping layer, 625 Å undoped Al 0.28 Ga 0.72 As, and a 50 Å GaAs cap.…”