L. I. Ryabova

Effect of combined Pd and Cu doping on microstructure, electrical and gas sensor properties of nanocrystalline tin dioxide

The effect of combined Pd and Cu doping on microstructure, electrical and gas sensor properties of nanocrystalline tin dioxide was studied. SnO2, SnO2(PdO), SnO2(CuO), and SnO2(PdO+CuO) films thickness of 0.8–1 μm with doping metal content 0.5–1.6 at.% were synthesized by aerosol pyrolysis. An average SnO2 grain size decreased with the addition of both Pd and Cu. The resistance measurements at 77–373 K showed that all types of doping induce resistivity increase accompanied by the appearance of conductivity activation process. Conductivity transients in the presence of CO were studied at 323–523 K. For the samples doped with Pd the sensor response to CO was found to be comparable with the resistivity increment induced by Pd incorporation into SnO2 matrix. To reveal the effect of CO on the conductivity the low temperature resistance was measured for the films in non-equilibrium state reached by cooling down the film exposed to CO at T=523 K. Experimental data proved that CO adsorption may be regarded as a factor neutralizing the Pd doping action on the films conductivity. The catalytic effect of Pd clusters was found in the interaction of SnO2(PdO+CuO) films with CO.

Dopants in nanocrystalline tin dioxide

The review surveys studies aimed at constructing new materials for gas sensors based on nanocrystalline tin dioxide. The influence of doping with various impurities (Pt, Pd, Ru, Rh, Cu, Ni, or Fe) on the composition, microstructure, and electrophysical and sensor properties of nanocrystalline SnO2 was discussed. The conditions for the preparation of powders and thick and thin SnO2 films by the wet chemical method and aerosol pyrolysis of organometallic compounds are reported. The mechanism of interaction of pure and doped nanocrystalline SnO2 with a gas phase was analyzed based on the data from Mossbauer, Auger electron, and X-ray photoelectron spectroscopy and the results of in situ Raman spectroscopy, XANES, and conductivity measurements.

Microstructure and electrophysical properties of SnO2, ZnO and In2O3 nanocrystalline films prepared by reactive magnetron sputtering

The influence of oxygen concentration in the plasma-forming gas on the microstructure, phase composition and electrical conductivity has been investigated for SnO2, ZnO and In2O3 films grown by reactive magnetron sputtering method. The evolution of the oxide microstructure and resistivity under annealing at 400 8C was also studied. The nanocrystallite size remains unaltered for all the investigated films, while the agglomerate size varies significantly depending on the type of oxide and annealing duration. The agglomerate size growth leads to reduction of film resistance and conductivity activation energy. # 2002 Elsevier Science B.V. All rights reserved.

Effect of interdiffusion on electrical and gas sensor properties of CuO/SnO2 heterostructure

The influence of annealing on the electrical and H2S gas sensor properties of p-CuO/n-SnO2 heterostructures has been investigated. The heterostructures were prepared by magnetron sputtering technique with subsequent oxidation. The depth composition was analyzed by the secondary neutral mass-spectrometry (SNMS) method. The Sn and Cu interdiffusion coefficients at 573 K were estimated as 3×10−14 cm2 s−1 and 1×10−15 cm2 s−1, respectively. Resistance behavior and high H2S gas sensitivity are associated with the formation of a transitional layer at CuO–SnO2 interface due to interdiffusion processes.

Copper and nickel doping effect on interaction of SnO2 films with H2S

The pyrosol spraying deposition technique has been used for the synthesis of SnO 2 , SnO 2 –CuO and SnO 2 –NiO polycrystalline films with grain size of 3–10 nm. The composition, microstructure and electrical properties of the films have been investigated by X-ray diffraction, electron probe microanalysis, Auger electron spectroscopy and X-ray photoelectron spectroscopy. The interaction of SnO 2 , SnO 2 –CuO and SnO 2 –NiO polycrystalline films with the reducing gases: H 2 S, C 2 H 5 OH, CO and CH 4 has been investigated by conductance measurements. It has been found that copper and nickel have a significant effect on the sensitivity of SnO 2 films to H 2 S. The model of interaction of SnO 2 films with H 2 S gas and different sensor properties of tin dioxide films doped with copper and nickel are discussed with regard to the position of these metals in the films.

Dynamics of the semiconductor-metal transition induced by infrared illumination in Pb1-xSnx Te(In) alloys

Abstract The photoconductivity and quantum oscillation effects induced by infrared illumination have been investigated in Pb 1- x Sn x Te(In) samples. The impurity monopolar character of the photoeffect is established. It is shown that the time-dependence of the residual photoconductivity is described by a power law, the relaxation time being ≈ 10 4 s at temperatures T ⩽ 20 K.

Experimental study of negative photoconductivity in n-PbTe(Ga) epitaxial films

We report on low-temperature photoconductivity (PC) in n-PbTe(Ga) epitaxial films prepared by the hot-wall technique on -BaF_2 substrates. Variation of the substrate temperature allowed us to change the resistivity of the films from 10^8 down to 10_{-2} Ohm x cm at 4.2 K. The resistivity reduction is associated with a slight excess of Ga concentration, disturbing the Fermi level pinning within the energy gap of n-PbTe(Ga). PC has been measured under continuous and pulse illumination in the temperature range 4.2-300 K. For films of low resistivity, the photoresponse is composed of negative and positive parts. Recombination processes for both effects are characterized by nonexponential kinetics depending on the illumination pulse duration and intensity. Analysis of the PC transient proves that the negative photoconductivity cannot be explained in terms of nonequilibrium charge carriers spatial separation of due to band modulation. Experimental results are interpreted assuming the mixed valence of Ga in lead telluride and the formation of centers with a negative correlation energy. Specifics of the PC process is determined by the energy levels attributed to donor Ga III, acceptor Ga I, and neutral Ga II states with respect to the crystal surrounding. The energy level corresponding to the metastable state Ga II is supposed to occur above the conduction band bottom, providing fast recombination rates for the negative PC. The superposition of negative and positive PC is considered to be dependent on the ratio of the densities of states corresponding to the donor and acceptor impurity centers.

Terahertz photoconductivity of Pb1−xSnxTe(In)

We have analyzed photoconductivity in Pb1−xSnxTe(In) under the action of ∼100 ns long terahertz laser pulses with the wavelength varying from 90 to 280 μm in the temperature range 4.2–300 K. Strong photoresponse has been observed at all laser wavelengths used. Two types of photoresponse have been detected. Positive persistent photoconductivity, which is observed at T<10 K is due to photoexcitation of impurity states, whereas negative nonpersistent photoresponse prevailing at higher temperatures T∼25 K results from free carrier heating. Specific features of photoconductivity mechanisms are discussed.

Problem of impurity states in narrow-gap lead telluride-based semiconductors

A review of recent results of experimental investigations devoted to studying unusual properties of impurity states in doped narrow-gap lead telluride-based semiconductors is presented. These results are analyzed in the framework of existing theoretical concepts.

Band edge motion in quantizing magnetic field and nonequilibrium states in Pb1?x Sn x Te alloys doped with In

Galvanomagnetic and oscillation effects in Pb1−xSnxTe single crystals doped with 0.5 at % In have been studied in magnetic fields up to 60 kOe at temperatures from 4.2 to 30 K under hydrostatic pressure up to 18 kbar. Beyond the ultraquantum magnetic field limit (Huql) for the metallic state of Pb1−xSnxTe(In) alloys, Fermi level pinning by high-density quasilocal states takes place. In a strong fieldH>Huql the equationEF = const is valid instead of the equationn = const which is usual for degenerate semiconductors (EF is the electron or hole Fermi energy, andn is their concentration). This makes it possible to determine the direction of the band edge motion in the Pb1−xSnxTe energy spectrum in a quantizing magnetic field in the direct and inverse spectral regions. It is found that the charge carrier transitions between quasilocal and band states are of anomalously long duration (∼105 sec atT=4.2 K). By the application of a quantizing magnetic field we obtained a nonequilibrium metallic state of the system with a frozen or slowly diminishing Fermi surface. The characteristic time of the transition was found as a function of temperature and pressure. The relaxation kinetics of the nonequilibrium states induced by a quantizing magnetic field and infrared irradiation is discussed.