Apparent metallic behavior at B=0 of a two-dimensional electron system in AlAs

We report the observation of metalliclike behavior at low temperatures and zero magnetic field in two-dimensional (2D) electrons in an AlAs quantum well. At high densities the resistance of the sample decreases with decreasing temperature, but as the density is reduced the behavior changes to insulating, with the resistance increasing as the temperature is decreased. The effect is similar to that observed in 2D electrons in Si metal-oxide-semiconductor field-effect transistors, and in 2D holes in SiGe and GaAs, and points to the generality of this phenomenon.