J. Mizsei

How can sensitive and selective semiconductor gas sensors be made

In this paper the theoretical and practical background of the realization of highly sensitive and selective semiconductor gas sensors is discussed. This discussion is based on the literature and some earlier published experimental results. The most important items are the quality of the semiconductor surface, the role of additives (catalytically active materials), the size of crystals or the layer thickness compared to the Debye length of the semiconductor, the quality of the interface between semiconductor crystals and the interface between the semiconductor and the grain of doping material (potential barriers). All of the listed factors depend on the technology; some of them depend on the temperature, as well as the characteristics of the realized sensor or sensor system.

Activating technology of SnO2 layers by metal particles from ultrathin metal films

Abstract The selectivity and sensitivity of gas sensitive semiconductor materials can be improved by doping the resistor material with catalytically active dopants. Recent work deals with the activating technology of SnO 2 sensors by metal particles originating from a sputtered ultrathin metal layer. Ultrathin metal films are sputtered on top of the SnO 2 layer (0.5 W/cm 2 r.f. power density in argon at 0.1 Pa pressure, 0–60 s sputtering time, resulting in 0–8 nm layer thickness). The sputtered samples were heated up to 600 K in the atmosphere until the end of agglomeration of the ultrathin metal film. The sheet resistivity of the Pd-SnO 2 layer is 10–100 Ω before agglomeration, and 10 6 –10 8 Ω after the heat treatment. Before the heat treatment the sheet resistivity is determined by the continuous Pd layer only, after the agglomeration process the resistivity is controlled by the SnO 2 layer and the metal-semiconductor Schottky barriers. SIMS and XPS results clearly show the decrease in surface coverage of the activator after agglomeration, and the presence of small amounts of oxidized metal phase on the surface. The agglomerated Pd layer is the best activator for H 2 sensors: the sensitivity increases by three to four orders of magnitude after this procedure, compared to the sensitivity of the simple SnO 2 layer. Ultrathin metal films from other materials (Ag, Pt, Au) behave similarly to the Pd layers. An agglomerated silver layer is a very good activator for H 2 S. Gold and platinum are also effective for CO and H 2 , respectively.

Simultaneous response of work function and resistivity of some SnO2-based samples to H2 and H2S

Abstract The simultaneous response of the work function and resistivity to H 2 and H 2 S of some pure and doped SnO 2 samples in the form of thick films is measured in humid laboratory air. The gas-sensitive layer of thick-film samples is screen printed on alumina substrates containing gold electrodes on a finger-like structure. Palladium and silver are used as catalytic activators in the SnO 2 . The work function change is measured with a vibrating capacitor (Kelvin probe) automatically compensated by a feedback system and with graphite as the reference electrode. Silver doping has a large effect on the response to H 2 S at temperatures around 150°C, but no effect is found in the case of exposure to H 2 at these temperatures. Palladium doping has an effect only in the case of H 2 exposure, the effect being strongest at lower temperatures around 150°C. The resistance changes give much higher values for the calculated work function changes as compared to the measured ones in the case of H 2 S exposure at 420 and 620 K, whereas in the case of H 2 exposure, the measured one are higher and a linear dependence between the calculated and measured changes of work function is obtained.

Resistivity and work function measurements on Pd-doped SnO2 sensor surface

Abstract Thin films of SnO 2 were prepared by the radio frequency sputtering technique. Ultrathin metal films were sputtered as activator on the top of an SnO 2 layer. The sheet resistivity of the layer and the thermoelectric power change have been measured with a Keithley electrometer and the change in surface work function has been measured by a vibrating capacitor probe. Results were plotted as a function of H 2 partial pressure of ambient air (adsorption isotherms). Based on these results, the theory of operation can be summarized: adsorbed gas gives rise to a change of the surface work function, and consequently the charge carrier concentration changes in the underlying semiconducting film.

H2S monitoring as an air pollutant with silver-doped SnO2 thin-film sensors

Abstract An important application of gas sensors is for the measurement of air pollutants, concentrations of which are usually in the low ppb range. To obtain good sensitivity and selectivity to certain polluting gases with semiconductor gas sensors is not very straightforward at the moment. Some Ag-doped SnO 2 thin-film sensors have been used in this study to monitor the H 2 S concentration as a polluting agent, both in laboratory air and in a city atmosphere (Oulu, Finland). A sensor array construction consisting of four sensors has been installed at a city air-pollution monitoring station where the H 2 S and SO 2 concentrations are being simultaneously recorded by commercial analysing equipment based on Coulometric titration together with the signals from the semiconductor gas-sensor array. The preliminary results support the conclusion that the Ag-doped thin-film sensors running in the constant-temperature mode at around 150 °C may be useful for monitoring H 2 S concentrations present as a pollutant in city air.

STRUCTURAL STUDIES OF SPUTTERED NOBLE METAL CATALYSTS ON OXIDE SURFACES

Abstract Catalytically active noble metal additives are used in semiconductor gas sensors as supported catalysts in order to increase both the selectivity and the sensitivity of the sensors. This study considers the formation of nanoparticles from ultra-thin noble metal deposits on oxide surfaces during heating of the sputtered metal deposits. Ag, Au, Pd and Pt layers of various thicknesses were sputtered on SiO 2 surfaces and on thin-film surfaces of semiconducting SnO 2 . The ultra-thin Ag, Au and Pt layers, contiguous after the sputtering, become discontinuous during the heating and form discrete metal nanoparticles. Because of the oxidation, the behaviour of Pd layers is very different from that of Ag, Au and Pt layers during heating. The structure of the metal layers was studied by atomic force microscopy and X-ray diffraction before and after the heat treatment. In addition, resistivity measurements were used to monitor the structural phenomena during the heat treatment. The agglomerated layer does not have the high conductivity of the contiguous film. A sudden conductivity decrease by many orders of magnitude was found at relatively low temperatures during the heat treatment. The conductivity decrease occurred within a narrow temperature range.

H2-induced surface and interface potentials on Pd-activated SnO2 sensor films

Abstract The surface and interface potentials of an SnO 2 gas-sensor film have been investigated by a vibrating capacitor probe, resistivity measurements, the MOS QV method and thermoelectric power measurements. All results are converted into potential changes, and plotted as a function of the H 2 partial pressure in the ambient air. The maximum value of the potential change on the surface is about 0.5–0.6 V. Potential changes calculated from the resistivity measurements are smaller, as well as the potential changes at the SnO 2 substrate interface. Based on the results, a simple energy-band model and the charge-carrier concentrations are discussed.

Fermi-level pinning and passivation on the oxide-covered and bare silicon surfaces and interfaces

Abstract The properties of the bare (oxide-free) and oxide-covered silicon have been discussed in details in the literature. Positive charge is usually assumed at the insulator–semiconductor interface, thus depletion or inversion layer develops in the case of p-type, and accumulation in the case of n-type semiconductor. However, the semiconductor (Si) covered by an ultra-thin tunnelable insulator (native SiO2) layer has some peculiarities, i.e. connection between surface charge and the interface charge carrier density. Theoretical considerations and vibrating capacitor experiments show that the surface charge tends to shift the surface nearer to the intrinsic condition in the case of the ultra-thin insulator-covered semiconductor surfaces.

In situ AFM, XRD and Resistivity Studies of the Agglomeration of Sputtered Silver Nanolayers

In situ atomic force microscopy (AFM), X-ray diffraction (XRD) and resistivity studies have been made for RF cathode-sputtered silver nanolayers on different oxide surfaces during heating between room temperature and 400°C. Our earlier AFM and resistivity measurements revealed the agglomeration of layers during heating. The present in situ AFM, XRD and resistivity measurements show a sudden rapid agglomeration of the sputtered ultra-thin silver nanolayers during heating at specific temperatures, which depend on the layer thickness. When the AFM picturing was initiated from a layer surface at a temperature slightly below the specific agglomeration temperature of the layer, the AFM tip excited the surface starting the agglomeration in the area under picturing. This tip-assisted agglomeration phenomenon made it possible to restrict the area of agglomeration and to produce sub-micron structures in the silver nanolayer by AFM.

Structural transformations of ultra-thin sputtered Pd activator layers on glass and SnO2 surfaces

Abstract Catalytically active metal surfaces are used in semiconductor gas sensors, and also in calorimetric gas sensors, as metal-cluster deposits to increase the selectivity and sensitivity, and to reduce the response and recovery times. In this paper, we have studied the properties of ultra-thin sputtered palladium layers on insulator and oxide–semiconductor surfaces during annealing up to 500°C. Pd layers were sputtered on glass and tin-dioxide thin-film surfaces. Atomic force microscopy (AFM), X-ray diffraction (XRD), vibrating capacitor (Kelvin probe), and resistance measurements were used to study the sputtered Pd layers. The results help us to understand the phenomena during the activation process of Pd catalyst (amorphous–crystalline transition, oxidisation, reduction, agglomeration) as well as to develop a more effective and reproducible technology for the activation of sputtered catalyst layers in semiconductor gas sensors.