Thin-film semiconductor sensors for hyperthermal oxygen atoms

In Earth orbit, atomic oxygen (AO) erodes and degrades many spacecraft materials. Several sensors have been employed to make in situ measurements of these oxygen atom fluxes, yet none has proved suitable for general application to microsatellites. We have investigated the potential of thin-film zinc oxide (ZnO) AO sensors. It is known that single crystal and thin-film semiconductor sensors may be used to measure fluxes of low-energy (thermal) oxygen atoms. This work extends the principle to the measurement of hyperthermal (high energy) oxygen atoms in a ground-based simulation facility. It is found that, upon exposure to AO, the conductance of the ZnO sensors decreases. Moreover, the rate of sensor conductance change is proportional to the flux of oxygen atoms. Two sensors, which were manufactured simultaneously, demonstrated very similar responses when exposed concurrently to hyperthermal AO. Further experiments showed that the sensors were not affected by fluxes of molecular oxygen, but were influenced by ultraviolet (UV) radiation. A ZnO film covered with silica — to prevent the action of AO — was used to examine the influence of UV, which was shown to cause a small, permanent change of the conductance of the semiconductor