Y. Komem

The effect of grain size on the sensitivity of nanocrystalline metal-oxide gas sensors

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 line...

Sensing behavior of TiO2 thin films exposed to air at low temperatures

The sensing behavior of thin TiO2 films exposed to air pulses was studied by means of electrical conductivity measurements. The TiO2 rutile films, with a grain size of ∼20–30 nm, were deposited on oxidized Si substrates by means of reactive sputtering and subsequently vacuum annealed (reduced) at 400°C. The films were exposed to air pulses at temperatures between 100°C and 325°C and their electrical conductivity was measured with embedded interdigital Au electrodes. When air was introduced into the system, the conductivity decreased and vice-versa. The conductivity changes during identical pulses were the same and reproducible, though the conductivity underwent some drift. The kinetics of the conductivity response during a pulse of air followed a logarithmic law, typical of Elovich–Roginsky chemisorption kinetics, during the first stage of exposure and a modified parabolic law, typical of oxidation kinetics, afterwards. A two-stage model is proposed to describe the response kinetics which involves first surface processes where charge transfer to oxygen adsorbates dominates followed by oxidation of the pre-reduced nonstoichiometric TiO2−x film by diffusion of oxygen.

Interfacial reactions between Ni films and GaAs

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: (1011)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 Ni2Ga...

Electronic and transport properties of reduced and oxidized nanocrystalline TiO2 films

Electronic properties of reduced (vacuum-annealed) and oxidized (air-annealed) TiO2 films were investigated by in situ conductivity and current–voltage measurements as a function of the ambient oxygen pressure and temperature, and by ex situ surface photovoltage spectroscopy. The films were quite conductive in the reduced state but their resistance drastically increased upon exposure to air at 350 °C. In addition, the surface potential barrier was found to be much larger for the oxidized versus the reduced films. This behavior may be attributed to the formation of surface and grain boundary barriers due to electron trapping at interface states associated with chemisorbed oxygen species.

The threshold current density and incubation time to electromigration in gold films

Abstract The electromigration in thin gold films was studied by means of direct drift velocity measurements. A threshold in the electromigration drift velocity was found, and its value is inversely proportional to the gold stripe length. Electron-beam-deposited films, in contrast to sputtered ones, also demonstrated the presence of an incubation time for the cathode displacement. The electromigration activation energy was found to be 0.67–0.72 eV and the effective charge -0.3 to -186. In addition, alloyed Au-10 vol.% Mo specimens were examined. These films showed a much slower mass transport from the anode to the cathode—the opposite direction to that of non-alloyed gold films.

Numerical computation of chemisorption isotherms for device modeling of semiconductor gas sensors

Abstract A computational method for numerical calculations of adsorption isotherms for both non-dissociative and dissociative chemisorption of gases on semiconductors is presented. The method enables calculating the equilibrium coverage of chemisorbed species and the chemisorption-induced potential barrier as a function of the ambient gas pressure, temperature, doping level, and the characteristic properties of the semiconductor/gas interaction. The computational method is applied for simulating the depletive chemisorption of oxygen on n-type SnO 2 . For both non-dissociative and dissociative chemisorption it is found that the chemisorption-induced potential barrier is proportional to the logarithm of the ambient oxygen pressure. This logarithmic relationship is important for device modeling of SnO 2 -based oxygen sensors since the sensor response, i.e. the change in the electrical conductivity, is related to the chemisorption-induced surface or intergranular potential barrier. The origin of this logarithmic relationship is attributed to the equilibration of the electrochemical potentials of chemisorbed oxygen adions and free oxygen molecules in the ambient gas phase.

Initial crystallization stage of amorphous germanium films

The incubation time for the crystallization of amorphous Ge (a‐Ge) films was studied as a function of temperature between 150 and 500 °C by means of both in situ transmission electron microscopy and Raman scattering spectroscopy. The temperature dependence of the incubation time for free‐sustained a‐Ge films follows an Arrhenius curve with an overall (nucleation+growth) crystallization process activation energy of 2.0 eV. In the case where the a‐Ge films were on Si3N4 substrates, an earlier stage of the crystallization was observed (nucleation), having an activation energy of 1.3 eV. In addition, it was found that a thin metallic layer of Al or Au, deposited on the a‐Ge films, induces a very fast crystallization in the mode of dendritic growth, as reflected by a low activation energy (0.9 eV) for the incubation time temperature dependence.

Stress relaxation in thin aluminium films

Abstract The stress relaxation phenomenon in thin aluminium films deposited onto silicon strips was studied by the cantilever beam technique using X-ray diffraction. At low temperatures the stress decreases linearly as a function of temperature at a rate of −2.17 × 10 7 dyn cm −2 K −1 , in good agreement with the elasticity theory. At high temperatures stress relaxation was observed on heating when the aluminium films could not sustain the compressive thermal stresses created by the difference in thermal expansion between the silicon substrate and the film. Stress relaxation was also found on cooling, for tensile thermal stresses. The activation energy for the compressive stress relaxation was estimated to be 0.43 eV and for the tensile stress relaxation 0.23 eV.

Surface photovoltage spectroscopy study of reduced and oxidized nanocrystalline TiO2 films

Abstract Nanocrystalline TiO 2 films used for gas sensors have been studied by means of surface photovoltage spectroscopy and other analytical tools to investigate the oxygen chemisorption effect on the electrical properties of the films. The results show that the surface (and intergranular interface) band bending increases with oxygen exposure due to electron trapping at midgap states induced by chemisorption. The surface electronic structure is revealed by the measurements, allowing determination of the sensing mechanism of these important films. In addition, a photoinduced chemisorption of oxygen at room temperature is observed. This has important implications for low-temperature gas sensors.

Quantitative evaluation of chemisorption processes on semiconductors

This article presents a method for numerical computation of the degree of coverage of chemisorbates and the resultant surface band bending as a function of the ambient gas pressure, temperature, and semiconductor doping level. This method enables quantitative evaluation of the effect of chemisorption on the electronic properties of semiconductor surfaces, such as the work function and surface conductivity, which is of great importance for many applications such as solid- state chemical sensors and electro-optical devices. The method is applied for simulating the chemisorption behavior of oxygen on n-type CdS, a process that has been investigated extensively due to its impact on the photoconductive properties of CdS photodetectors. The simulation demonstrates that the chemisorption of adions saturates when the Fermi level becomes aligned with the chemisorption-induced surface states, limiting their coverage to a small fraction of a monolayer. The degree of coverage of chemisorbed adions is proportional to ...