Data are presented showing that the native oxide that can be formed on high Al composition Al(x)Ga(1-x)As (x greater-than-or-similar-to 0.7) confining layers on Al(y)Ga(1-y)As-Al(z)Ga(1-z)As (y > z) superlattices or quantum well heterostructures serves as an effective mask against impurity diffusion (Zn or Si), and thus against impurity-induced layer disordering. The high quality native oxide is produced by the conversion of high Al composition Al(x)Ga(1-x)As (x greater-than-or-similar-to 0.7) confining layers, which can be grown on a variety of heterostructures, via H2O vapor oxidation (greater-than-or-similar-to 400-degrees-C) in an N2 carrier gas.