The selective eptiaxy of GaAs multilayer structures through windows in SiO2deposited on (001) oriented semi−insulating GaAs substrates for GaAs coplanar diodes, has been studied. It is shown that excellent flat, smooth, and straight boundaries of windows can be obtained by use of sidewall orientation with [1̄01] or [101]directions. GaAs coplanar diodes were fabricated by this technique. The characteristics of the polycrystalline GaAs grown on SiO2(001)GaAs, SiO2‐(100)Si, and SiO2‐(111)Ge substrates have been examined by TEM, SEM, and x‐ray diffraction. The results obtained show grain size of 1.0 to 3.3 μm can be achieved. The preferred orientation of the grains were [111], [400]and [800] for polycrystalline GaAs SOI on SiO2‐(001)GaAs and [111] for polycrystalline GaAs SOI on SiO2‐(100)Si and SiO2‐(111)Ge.