We report on novel Ir-Al/n-GaAs Schottky contact systems based on sequentially evaporated Ir-Al multilayers which enable us to control the barrier height at the interface with annealing temperature. Auger depth profiling and Rutherford backscattering methods were applied to analyze the Ir-Al composition and the interfacial reaction stability. An increase of the barrier height with annealing temperature has been indicated for all the Schottky contact systems. Barrier heights as high as 0.95 V were measured for annealed Schottky diodes with the first aluminum or very thin iridium interfacial layer. A model of the barrier height enhancement based on a solid phase epitaxy of a graded AlxGa1-xAs layer at the interface at elevated annealing temperatures was considered to explain the electrical properties of the contacts. The model contributions to the formation of a metal/AlxGa1-xAs/GaAs heterojunction diodes with required barrier height and thermal stability are discussed.