The effect of grain size on the sensitivity of chemoresistive nanocrystalline metal-oxide gas sensors was evaluated by calculating the effective carrier concentration as a function of the surface state density for a typical sensing material, SnO2, with different grain sizes between 5 and 80 nm. This involved numerical computation of the charge balance equation (the electroneutrality condition) using approximated analytical solutions of Poisson’s equation for small spherical crystallites. The calculations demonstrate a steep decrease in the carrier concentration when the surface state density reaches a critical value that corresponds to a condition of fully depleted grains, namely, when nearly all the electrons are trapped at the surface. Assuming that the variations in the surface state density are induced by surface interactions with ambient gas molecules, these calculations enable us to simulate the response curves of nanocrystalline gas sensors. The simulations show that the conductivity increases linearly with decreasing trapped charge densities, and that the sensitivity to the gas-induced variations in the trapped charge density is proportional to 1/D, where D is the average grain size.

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The metallurgical examination of solid-state reaction between nickel thin films and single-crystal GaAs substrates and the resultant electrical properties of the contacts are reported. Annealing at 100–300 °C in forming gas led to formation of a metastable hexagonal phase Ni2GaAs which was stabilized due to its epitaxial growth on (001) and (111) GaAs substrates, as follows: (101̄1)Ni2GaAs∥(001)GaAs and (0001)Ni2GaAs∥(111)GaAs. Nickel atoms were found to be the dominant diffusing species during the ternary phase growth. Ni2GaAs is stable on (111)GaAs up to at least 600 °C, compared to 350 °C on (001)GaAs. The larger stability on (111) is explained by the better epitaxial match found in this case. Reaction on (001)GaAs in the temperature range of 350–550 °C resulted in decomposition of Ni2GaAs by NiAs precipitation. After annealing at 600 °C the reacted film was composed of two phases: NiGa and NiAs. The electrical properties of the contacts were correlated to the phase interfacing the substrate. The Ni2GaAs formed rectifying contact with a barrier height of 0.84 eV, whereas when NiGa and NiAs were at the interface an ohmic behavior was observed.

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