N. K. Maksimova

Effect of Pt, Pd, Au additives on the surface and in the bulk of tin dioxide thin films on the electrical and gas-sensitive properties

The microstructure and properties of thin (∼100 nm) SnO2 films with noble metals Pt, Pd, Au additives, grown by dc magnetron deposition are studied. It is shown that the introduction of additives into the bulk and the deposition of dispersed catalysts on the semiconductor surface make it possible to control the sensor parameters in pure air and upon exposure to reduction (CO, H2, CH4) and oxidation (NO2) gases. Possible mechanisms for the effect of Pt, Pd, Au on the bulk and surface properties of tin dioxide are discussed. The technological conditions for film growth, which provide the selective detection of low concentrations (10–100 ppm) of CO and H2, below-explosive concentrations (0.5–2.5 vol %) of methane, and trace concentrations (0.05–5 ppm) of NO2 are determined.

Effect of gold on the properties of nitrogen dioxide sensors based on thin WO3 films

The microstructure and properties of gold-doped WO3 (WO3:Au) thin films before and after deposition of dispersed Au layers have been studied. It is shown that the γ-WO2.72 phase arises in WO3:Au layers, which leads to a significant increase in the film conductivity. Deposition of a dispersed gold layer results in an increase in the sensor response to NO2 by several times. The concentration dependences and the dynamics of sensor responses to nitrogen dioxide are described by the analytical expressions derived under the assumption that WO3 films contain grains connected by conducting bridges. An analysis of the experimental data using these expressions made it possible to determine the activation energies of NO2 adsorption and desorption and the adsorption heat.

The microstructure and properties of thin WO3 films modified by gold

Direct current magnetron sputtering of a metallic W + Au target or high-frequency magnetron sputtering of WO3 + Au oxide targets was used to prepare thin (about 100 nm) nanocrystalline WO3 films with the addition of gold (disperse layers of catalytic gold were additionally deposited on the surface of films). The composition and micromorphology of the surface of films and the electrical and gas sensitive characteristics of nitrogen dioxide sensors were studied to determine the mechanism of the influence of gold on the properties of WO3 films. The films were shown to contain the β-WO3 orthorhombic and γ-WO2.72 monoclinic phases and gold particles. The presence of the nonstoichiometric γ-WO2.72 phase was shown to increase the concentration of oxygen vacancies in films and decrease the resistance of sensors to 1–2 MΩ. Gold nanoparticles 9–15 nm in size segregated on the surface of semiconductor crystallites and increased the response of sensors to NO2. The conclusion was drawn that deposited catalytic gold layers increased the response to traces of nitrogen dioxide.

Defects in Schottky barrier structures

ConclusionThe progress achieved in recent years in study of the element and phase composition, as well as the energy spectrum of electron states on semiconductor boundaries with metals and dielectrics, has shown the complexity and multifaceted nature of the physicochemical reactions-occurring on these boundaries. In the present review we have attemped to consider the question of the physical interrelationship between the phenomena of interdiffusion of elements and formation of intermetallic compounds, and the electrophysical properties of metalsemiconductor contacts. Analysis of the results of theoretical and experimental studies permits the conclusion that an important role in both potential barrier formation and charge carrier transmission is played by structural defects developed during these reactions in the contact region of the semiconductor.After chemical processing and deposition of metallic coatings stoichiometric composition defects are formed. Vacancies, antistructural defects, and defect complexes form surface electron states which together with intrinsic states ensure stabilization of the Fermi level on semiconductor boundaries with metals.In many cases structural defects are the vehicle by which physicoehemical interactions in thin film metal-semiconductor contacts affect the electrical parameters of devices and their stability. The initial stage of degradation, as a rule, is the appearance of excess currents in the forward branch of the CVC at temperatures of 150-77°K. Study of the physical nature of excess currents shows that their appearance is related to generation of structural defects during thermal processing, or mechanical or electrical testing of specimens. The defects create a system of inhomogeneously distributed deep centers in the space-charge region. The most probable mechanism for charge carrier transmission with participation of deep levels is resonant tunneling.Thus, to solve problems related to increasing reliability of Schottky barrier devices special attention should be given to conditions under which defects develop in the active region of the semiconductor diode and methods for eliminating these defects. Creation of conditions during device preparation favorable to formation of stable intermetallic compounds in the transition layer, maintenance of stoichiometric composition in the semiconductor, and inhibition of diffusion of electrically active metals, together with selection of barrier producing materials and protective coatings providing minimum mechanical stress in the contact will insure development of Schottky barrier devices having electrical parameters which are stable over a wide temperature range.

Interphase interactions in a Pd-GaAs system and their effect on electrical properties of schottky-barrier structures I. effect of heat treatment on characteristics of GaAs-Pd/Ni contacts

The electrical characteristics of surface-barrier GaAs-Pd/Ni structures and the physicochemical interaction processes at the metal-semiconductor boundary were comprehensively investigated in relation to heat treatment in various atmospheres. X-Ray structural analysis showed that in the investigated system metallurgical reactions begin at 300–350‡C: Unstable intermediate phases (presumably Pd2Ga) are formed. At 400–550‡C all the palladium is converted to the bound state and the intermetallic compound PdGa is formed. The phase changes have no significant effect on the properties of diode structures fired in a hydrogen atmosphere. Heating in vacuum leads to degradation of the contact parameters at 300‡C or more. This effect is attributed to penetration of oxygen to the interface during formation of the intermediate phases.

Effect of humidity on the properties of NO2 sensors based on thin WO3 and SnO2 films modified with gold

The effect of humidity on the electrical and gas-sensitivity properties of NO2 sensors based on thin (∼100 nm) nanocrystalline films of tungsten trioxide and tin dioxide with additions of gold in the bulk and on the surface (Au/WO3:Au, Au/SnO2:Sb,Au) is investigated. It is found that the conductivity of these films in pure air increases only slightly as absolute humidity A rises from 2 to 16 g/m3, the responses for nitrogen dioxide in the concentration range of 0.45–10 ppm do not depend on humidity, and the response time is shortened as A rises. Analysis of experimental data using a model based on the assumed microcrystal availability in WO3 and SnO2 thin films connected by conductivity bridges shows that the values of the heat of adsorption ΔEN and activation energies of adsorption EaN and desorption EdN of ions NO2− on the films surface modified gold, do not depend on the humidity.

Study of sensing properties to CH/sub 4/ of Pt/SnO/sub 2/:Sb thin film gas sensor in pulsing mode

The resistance-time profiles of Pt/SnO/sub 2/:Sb thin films sensor of methane during cyclic variation of the working temperature have been studied. The time-period of one cycle was 10-12 min: 2 sec heating up to 400-570/spl deg/C and 8-10 sec down-heating to 80-150/spl deg/C. It was demonstrated the electrical and sensing properties of sensors as function of pulsing temperatures, methane concentration and humidity. It was shown that the using pulsing mode decreases the effect of humidity on the response to methane but does not provide the perfect stabilization of sensors parameters at humidity and environment temperatures variation.

Solid-state recrystallization processes in Ni-GaAs and Pd-GaAs structures

Electron microscopy, reflection electron diffraction, and x-ray diffraction analysis are used to investigate solid-state recrystallization processes in the Ni-GaAs and Pd-GaAs structures at room temperature and during heat treatment in a hydrogen atmosphere. Contacts were produced by electrodeposition of the metal (Ni, Pd) onto the (111) A surface of a GaAs ingot. It is shown that physical and chemical reactions at Ni-GaAs and Pd-GaAs contacts occur even at room temperature, with the formation of chemical bonds between the metal and both gallium and arsenic. The phases formed at the contacts upon annealing in a hydrogen atmosphere correspond to those expected from the phase diagrams, and solid-state recrystallization occurs under the strong orienting influence of the substrate.

Physicochemical interactions in thin film Pb-GaAs structures upon thermal annealing

Morphology and elemental and phase composition of Pb-GaAs contacts prepared by electroprecipitation of the metal and annealed in a hydrogen atmosphere are studied. It is shown that the interaction of the lead with the gallium arsenide occurs with participation of a liquid phase which appears at 100–300°C due to dissociation of the semiconductor and gallium diffusion into the metallic coating. The absence of complex ions of the PbnGam(Asm)+ type in the secondary ion mass spectra of the contacts analyzed indicates that chemical interaction of the lead with gallium and arsenic does not occur.

Doping of solid solutions in liquid epitaxy. III. Tellurium in AlxGa1−xSb

A calculation is made of the dependences of the coefficient of the tellurium distribution KTe and the electron concentration n in AlxGa1−xSb on the composition of the solid solution and the growth temperature. It is shown that KTe must decrease with increasing x. The dependence of KTe on the temperature is also determined by x. Experimental results were obtained on AlxGa1−x Sb (0 ⩽ x ⩽ 0.74) films grown by liquid epitaxy. The electron concentration in the films was measured through the thermoelectric power and the capacitance-voltage characteristics of Schottky barriers. Satisfactory agreement with the results of the calculation was obtained.