Methods and structures includes providing a substrate, forming a prelayer over a substrate, forming a barrier layer over the prelayer, and forming a channel layer over the barrier layer. Forming the prelayer may include growing the prelayer at a graded temperature. Forming the barrier layer is such that the barrier layer may include GaAs or InGaAs. Forming the channel layer is such that the channel layer may include InAs or an Sb-based heterostructure. Thereby structures are formed.