Marc Bendahan

Development of an ammonia gas sensor

Copper (I) bromide (CuBr) thin films for ammonia detection are prepared by two different techniques: (1) magnetron sputtering and (2) chemical or electrochemical oxidation of copper in presence of bromine ions. The microstructure of the films is related to these processes. The electrical properties of these films are studied and the temperature dependence of the bulk conductivity is in agreement with previous work on polycrystalline CuBr. After a preliminary strong exposure to ammonia gas, ammonia adsorption on films leads to a reversible decrease of conductivity. This effect is used to design a selective and sensitive ammonia gas sensor operating at ambient temperature. The sensitivity and the response time are related to the film preparation.

NiTi shape memory alloy thin films: composition control using optical emission spectroscopy

One of the major barriers in the development of applications using NiTi shape memory alloy films is the difficult control over their chemical composition. In fact, the transformation temperatures of NiTi are very sensitive to the alloy composition. In this paper we show that optical emission spectroscopy can be used to monitor the composition of NiTi films during sputter deposition. The parameter controlling the composition of the films is the product of sputtering gas pressure and target-to-substrate distance. The influence of this parameter is discussed in terms of thermalisation of the sputtered atoms. We find a linear relationship between the ratio of optical emission intensity of Ni and Ti atoms and the Ni concentration in the films.

Control composition study of sputtered NiTi shape memory alloy film

Since the transition temperature of the shape memory effect in NixTi1−x films depends on the composition of the layer, it is indispensable to have an accurate control on their composition. We are now able to control in situ the composition of r.f. sputtered NiTi films in the whole domain of shape memory effect (0.476 < x < 0.531), by varying the product of sputtering gas pressure P and target-to-substrate distance d. Pressure measurements are found to give a good sensitivity for the control of Ti-rich layers, while the optical emission spectroscopy of the sputtering plasma gives better sensitivity for Ni-rich layers.

NiTi thin films as a gate of M.O.S. capacity sensors

Abstract In this study, a NiTi-shape memory alloy thin film is used as a gate in a M.O.S.-type structure. The shape memory effect involves a thermally induced phase transformation between a low temperature ductile phase and a high temperature high strength phase. The NiTi gate is deposited on a SiO 2 /Si substrate using RF sputtering deposition method. We have studied the evolution of C ( V ) characteristics as a function of temperature in order to provide evidence of phase transition within the NiTi layer, and also to investigate the effect of such a transition on the electrical properties of the structure. A rapid change in the C ( V , T ) curves which is a direct consequence of the modification in the NiTi structure is observed and discussed. The possibility of realizing a sensor of current to be integrated in microelectronic process and power integrated circuits is envisaged.

Preparation of thin films of copper(I) bromide by r.f. sputtering: morphology and electrical properties

Abstract The preparation of thin films of copper(I) bromide (CuBr) on glass, silicon and copper substrates by r.f. sputtering is described. Polycrystalline films with the γ-CuBr structure and randomly oriented grains in the micrometer range were obtained. Exponential current–voltage characteristics were interpreted by a p-type semiconductivity of CuBr thin films and related to the copper deficiency of the material.

Electrical properties of thin-films of the mixed ionic-electronic conductor CuBr: influence of electrode metals and gaseous ammonia

Abstract The electrical properties of sputtered thin-films of CuBr are measured using gold or copper electrodes and in presence of ammonia. The I–V characteristics are strongly dependent on the choice of the electrode metal: an ohmic response is observed with copper electrodes, which act reversibly for ion exchange, but exponential dependencies are measured with blocking gold contacts. An increase of the film resistance is observed in presence of ammonia: the response time is very short and the signal is reversible.

Impedance analysis of CuBr films for ammonia gas detection

CuBr films prepared by magnetron sputtering can be used to design a selective and sensitive ammonia microsensor, operating at ambient temperature. Impedance spectra of the films under different gas atmospheres can be fitted by a simple equivalent circuit. The roughness of the electrode/solid electrolyte interface leads to a distribution of the electrode resistance and capacitance values. The evolution of the bulk resistance of CuBr can be attributed to sorption of ammonia molecules, which is also investigated by Fourier-transform infrared (FTIR) spectroscopy.

Mixed ionic–electronic conducting thin-films of CuBr: a new active component for gas sensors?

Abstract Thin-films of mixed ionic–electronic conducting CuBr were prepared by r.f. sputtering. Microstructure and electrical properties of the films depend on the nature of the substrate, e.g., silicate glass, nitrided silicon wafers and copper metal. Current–voltage characteristics are discussed with respect to defect chemistry and semiconductor properties of CuBr.

Electrical properties and sensor characteristics for NH3 gas of sputtered CuBr films

Abstract We show that sputtered CuBr films can be used to detect the presence of NH3 gas. The increase of electrical resistance is very fast with a time constant of only a few seconds. The possible sensor mechanisms are discussed. Cross effects for various gases are investigated; only H2S degrades irreversibly the film. Ageing experiments show a t1/2 dependence of the film resistance.

New type of Schottky barriers using NiTi shape memory alloy films

Abstract With the aim of studying the feasibility of sensors using NiTi shape memory alloy films, we have realised NiTi/Si(n)/Si(n+) structures by sputter depositing NiTi contacts on Si substrates. The NiTi sputtering was performed at low substrate temperature (423 K) to avoid the formation of a silicide interfacial layer. Current–voltage (I(V)) measurements on these structures show a rectifying behaviour. The barrier height and ideality factor were determined from these measurements. The influence of the martensitic phase transformation in the NiTi electrode on the electrical properties of the diodes was investigated by recording the I(V) characteristics at various temperatures. This study shows an unusual current decrease with increasing temperature above 353 K. The mechanisms responsible for this phenomenon are discussed.