An application of synchrotron radiation to study the microstructure of passivation layers on GaAs

Some of the background to the application of GaAs as a field-effect, addressable, potentiometric sensor/stimulator (FAPS) is reviewed and the need for the passivation of this material prior to its use in this application is outlined. (3-mercaptopropyl) trimethoxysilane (MPT) was deposited on a GaAs surface to provide passivation against corrosion by oxygen and to prevent AsO33- escape into the surrounding environment. X-ray reflectivity was used to probe the surface to obtain an electron density depth profile that could then be fitted to a computer model revealing the number of layers and their corresponding thicknesses, densities and roughnesses, respectively. Since GaAs surfaces are highly reactive due to dangling bond formation it was hoped that the thiol (SH) group in the MPT overlayer could effectively stick to the surface and block the thermodynamic path for Ga 2O3 and As2O3 formation via oxidation. However, it was found from the models fitted that all samples had buried oxide layers due to either inadequate cleanliness during preparation or subsequent surface corrosion occurring after sample preparation. Further study is required regarding the effectiveness of thiol overlayers against arsenic and oxygen diffusion and perturbation of the electronic states of GaAs. © Institute of Materials Engineering Australasia Ltd - Materials Forum Volume 27 - Published 2004.