RU2546849C2 - Semiconductor oxygen sensor - Google Patents

Abstract

FIELD: measurement equipment.

SUBSTANCE: invention relates to the field of gas analysis, in particular, to detecting devices used for registration and measurement of oxygen content. A gas sensor according to the invention comprises a dielectric substrate with the applied layer of the semiconductor material with thickness from 0.07 mcm to 0.2 mcm. Metal electrodes are applied onto the layer. Semiconductor material is polycrystal material of composition Sm_1-xLn_xS, where x changes from 0 to 0.14, and Ln is one of elements: La, Ce, Pr, Nd, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu.

EFFECT: invention makes it possible to make a sensor for measurement of oxygen content having higher sensitivity.

2 cl, 3 dwg, 4 ex

Description

The invention relates to the field of gas analysis, in particular to detection devices used for recording and measuring the content of oxygen microimpurities, and can be used in medicine, in the field of ecology, in the nuclear industry, in the automotive industry.

Semiconductor Gas Sensors [Handbook of sensors and actuators, v.4 ″ Semiconductor sensors in physico-chemical studies ″ (edited by L. Yu. Kupriyanov), 1996, Chapter 1, ″ Physical and chemical basics of the method of semiconductor sensors ″, p.5-24] such as zinc oxide (ZnO), cadmium sulfide (CdS), tin dioxide (SnO _2), may be used as proportional detectors of a number of molecular gases, including oxygen, which allows convert the output electrical signal obtained as a result of adsorption of active particles into a numerical value of the concentrations of the measured components of the gaseous medium. The principle of operation of such resistive sensors is based on the effect of changing the impurity conductivity of semiconductor sulfides and metal oxides as a result of adsorption of molecular gases on their surface.

A known sensor for analyzing the oxygen content in the gas [Vyakhirev D.A., Shushukova A.F. Guide to gas chromatography. M .: Higher school, 1987]. The sensor consists of platinum, tungsten or gold-plated wire spirals placed in the channels of the metal block through which the carrier gas passes.

Structurally, the detector is made in the form of two identical thermostatically controlled reaction cells: measuring and comparative, in which heating elements are placed - wire spirals included in the adjacent measuring circuit of the Winston bridge. The principle of operation of the sensor is based on the fact that a heated body loses heat at a speed depending on the composition of the surrounding gas, so the heat transfer rate can be used to determine the composition of the gas. The heating elements in the comparative and working cells are heated with direct electric current from the battery or from a special stabilized power source. The thermal conductivity of the gas surrounding the heating elements determines the temperature, and therefore the resistance of the heating elements. When pure carrier gas flows through both sensor cells, the temperature of the heating elements is the same. If pure carrier gas flows through the comparative cell, and carrier gas plus the component exiting the chromatographic column flows through the measuring cell, then the temperature and, therefore, the resistance of the heating elements will be different, which violates the balance of the measuring bridge. The output from the sensor is compared with standard heating elements.

However, there is a limitation of the sensitivity of the device to gases (oxygen, argon) due to the close thermal conductivity coefficients. For example, when using such a sensor for oxygen analysis, the accuracy of determining its concentration is small.

The closest technical solution to the invention is a semiconductor gas oxygen sensor [RF Patent RU 2235316, dated June 5, 2003] containing a substrate and a semiconductor layer with metal electrodes deposited on its surface. The semiconductor layer is made in the form of a single crystal plate of indium arsenide.

The principle of operation of such a sensor is based on the relationship between the adsorption-desorption processes occurring on the semiconductor wafer and the change in electrical conductivity caused by them. The operation of the sensor is as follows. The sensor is placed in the test medium. When oxygen is adsorbed, accompanied by the formation of ions and radical ions (O ^- ₂ , O ^- and others), the surface of the semiconductor wafer is charged, respectively, the energy zones bend and, as a result, the concentration of free charge carriers and electrical conductivity change, which is fixed by electrical appliance. The magnitude of the change in electrical conductivity using calibration curves can determine the oxygen content in the test medium.

, где ε - диэлектрическая проницаемость полупроводника. В InAs ε~14. Величина L_d (а значит, и ε) является определяющим фактором для поверхностной чувствительности полупроводникового материала к воздействию газов на его электропроводность и, соответственно, чувствительности датчика [М.Н. Румянцева, Е.А. Макеева, А.М. Гаськов Влияние микроструктуры полупроводниковых сенсорных материалов на хемосорбцию кислорода на их поверхности Рос. хим. ж. (Ж. Рос. хим. об-ва им. Д.И. Менделеева), 2008, т.LII, №2, с.122-129].The disadvantages of this sensor include its relatively low sensitivity to oxygen. It is a consequence of the small value of the Debye length of the screening of the charge on the surface of the material (L _d ). The value of L _{d is} directly proportional

where ε is the dielectric constant of the semiconductor. In InAs ε ~ 14. The value of L _d (and hence ε) is a determining factor for the surface sensitivity of a semiconductor material to the effects of gases on its electrical conductivity and, accordingly, the sensitivity of the sensor [M.N. Rumyantseva, E.A. Makeeva, A.M. Gaskov. Influence of the microstructure of semiconductor sensor materials on the chemisorption of oxygen on their surface. Ros. Chem. g. (J. Ros. Chem. Society named after D.I. Mendeleev), 2008, vol. LII, No. 2, pp. 122-129].

The objective of the invention is to increase the sensitivity of the sensor to oxygen.

The problem is solved by a semiconductor oxygen sensor containing a dielectric substrate and a semiconductor layer with metal electrodes deposited on its surface, in which the semiconductor layer is made with a thickness of from 0.07 μm to 0.2 μm from a material with the composition Sm _1-x Ln _x S, where Ln is one of the following elements: La, Ce, Pr, Nd, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, ax has a value from the interval from 0 to 0.14.

The present invention is based on the relationship between the adsorption-desorption processes occurring in the semiconductor layer of a material of a certain thickness based on samarium sulfide in the presence of oxygen, and the changes in the electrical conductivity of this layer caused by them.

The electrophysical parameters of the semiconductor gas-sensitive layer, which determine the nature of the semiconductor (the conductivity of the main charge carriers, the Fermi level, the depth of the impurity level from the bottom of the conduction band, the population of this level by the main charge carriers, the length of the Debye screening, the Hall constant, and the mobility of the main charge carriers), may be purposefully form in such a way as to most adequately meet the nature of the detected gas molecule (in this case, oxygen but). The parameters of the semiconductor can be changed so that the activation energy of conduction electrons in the semiconductor corresponds to the electron affinity energy in the oxygen molecule. This result can be achieved by changing the value of ε by doping the material. Therefore, semiconductor samarium sulfide and semiconductor materials based on it were chosen as the gas-sensitive layer, since the dielectric constant for the semiconductor modification of samarium sulfide is in the range ε≈18-20, which is 2-2.5 times more than for the oxides used in the known sensors metals. So, for example, for zinc oxide used in oxygen sensors, the dielectric constant lies in the range ε≈8-10. On this basis, the sensitivity of samarium sulfide semiconductor films to oxygen adsorption is significantly increased in comparison with metal oxides. In addition, the main distinguishing feature of these materials in comparison, for example, with metal oxides is their stable radiation thermodynamic characteristics, including electrophysical ones.

Regarding the composition of the working substance, the following can be said. It was revealed that samarium sulfide SmS in substitutional solid solutions at a concentration of impurities of another rare-earth element (from the Ln group) greater than 0.14 undergoes a semiconductor-metal phase transition and goes into the metallic state. It loses the semiconductor properties necessary for the functioning of the sensor, and becomes unsuitable for the manufacture of an oxygen sensor.

Of fundamental importance is the application of thin-film coatings of materials based on SmS of a certain thickness. The thickness of the polycrystalline film (layer) is selected in the range from 0.07 to 0.2 μm, because at thicknesses less than 0.07 μm, the film has the properties of the SmS metal phase [Kaminsky VV, Volodin NM, Zhukova TB, Romanova MV, Sosova GA Electrical properties and structural features of polycrystalline films of samarium monosulfide. FTT, 1991, t.33, v.1, s.187-191] and does not work as a sensor. At thicknesses of more than 0.2 μm, surface conductivity is shunted by the electrical conductivity of the rest of the material (bulk). It should be borne in mind that the thickness of the working part of the surface is Ld, which in the case of SmS is ~ 0.01 μm, but this working surface can only form in the semiconductor phase of SmS.

The inventive sensor is shown in Figure 1.

Figure 1 shows the design of the inventive sensor,

Where:

1 - dielectric substrate;

2 - polycrystalline layer of material based on samarium sulfide (Sm _1-x Ln _x S);

3 - metal electrodes.

Figure 2 shows a graph of the temperature dependence of the change in electrical conductivity (Δσ) of a SmS-based material under the influence of adsorbed oxygen (oxygen pressure Po ₂ = 27.89 Pa).

As follows from Figure 2, at temperatures of 150 ± 10 ° C, the sensor has the highest sensitivity.

Figure 3 presents typical calibration curves: curve 1 - for the sensor based on the proposed semiconductor material Sm _1-x Ln _x S and curve 2 - alogical curve for the sensor prototype.

From a comparison of these dependencies it follows that the inventive sensor allows you to determine the oxygen content with a sensitivity significantly exceeding the sensitivity of the prototype sensor.

The operation of the sensor is as follows.

The sensor is placed in the test medium. When oxygen is adsorbed, accompanied by the formation of ions and radical ions (O ^- ₂ , O ^- and others), the surface of the semiconductor layer is charged and, as a result, the concentration of free charge carriers and its electrical conductivity change. The magnitude of the change in electrical conductivity using calibration curves can determine the oxygen content in the test medium.

Example 1

The application of thin films of materials based on samarium sulfide was carried out on a glass substrate by the explosive method of evaporation in vacuum / L.N. Vasiliev, V.V. Kaminsky, Yu.M. Kurapov, M.V. Romanov, N.V. Sharenkova. FTT 38, 779 (1996). Four films were prepared with the compositions Sm _0.9 Gd _0.1 S, Sm _0.92 Ce _0.08 S, Sm _0.9 La _0.1 S, Sm _0.95 Eu _0.05 S. First, solid solutions with the corresponding compositions were created, from which the mixture was prepared in the form of powders by grinding bulk samples at the ball mill. The method of explosive evaporation was implemented as follows. A mixture was loaded into a vibrating bunker, which gradually spilled out of the bunker onto a tantalum boat heated by passing current through it to a temperature of 2500 ° C. Upon contact with the boat, each individual grain of the charge evaporated (“explosion”) and the vapor was deposited through the mask onto a substrate heated to 450 ° C. Over the semiconductor layer by resistive spraying through masks, nickel contacts 0.3 μm thick were deposited. The deposition operations were carried out in a vacuum of 105 mm RT. Art. The thickness of the semiconductor layers was 0.1-0.15 microns. The thickness of the layers was measured using a domestic MII-4 interferometer. The composition of the semiconductor material layers was monitored by X-ray diffraction analysis on a DRON-2 X-ray diffractometer of OAO NPP Burevestnik. The wires were connected to the obtained resistors using copper wire contacts soldered to the nickel contacts with POS-60 tin-lead solder. The overall dimensions of the resistors were ~ 3 × 3 mm ² with a substrate thickness of 80 μm. The thus obtained structure was subjected to annealing at a temperature of 150 ± 10 ° C for 2-3 hours in a vacuum of 10 ^-5 mmHg. Art. When testing the action of the sensor, the substrate of the structure was placed on a massive copper plate heated by a resistive type electric stove. The temperature of the copper plate and the substrate was measured using a copper-constantan thermocouple embedded in a copper plate so that its junction touches the substrate. Signals from current outputs and thermocouples were fed to two channels of the ADC of a personal computer and were taken during measurements. The semiconductor resistor was maintained at a constant temperature lying in the range from 140 ° C to 160 ° C. An important point when taking the calibration characteristics of the sensor is that the semiconductor resistor was maintained at a constant temperature, lying in the range from 140 ° C to 160 ° C. In order to find the optimum oxygen detection temperature for each sensor, the curves of the dependence of its electrical conductivity on the temperature of the sensor at a given oxygen concentration were taken. The Δσ values were determined as the difference between the electrical conductivity in vacuum and in the presence of oxygen, obtained at the same temperature. The calibration curves obtained for the same sensors were taken. The value of Δσ is presented depending on the partial pressure of oxygen. In this case, various partial oxygen pressures were gradually supplied to the measuring chamber in which the sensor was located, and Δσ measurements were carried out at Т = 140-160 ° C. The measurement results for films with compositions Sm _0.9 Gd _0.1 S, Sm _0.92 Ce _0.08 S, Sm _0.9 La _0.1 S, Sm _0.95 Eu _0.05 S showed at optimal measurement temperatures 150 ° C, 150 ° C, 150 ° C 153 ° C, respectively , the excess sensitivity of the proposed sensors compared with the prototype, with an oxygen pressure of 0.5 Pa, by 13.25%, 14.49%, 17.05% and 19.24%.

As follows from the results, the proposed technical solution allows to increase the sensitivity of the proposed oxygen sensor by 15-20% compared with the prototype.

Example 2

The same as in example 1, but the thickness of the samples with compositions Sm _0.9 Gd _0.1 S, Sm _0.92 Ce _0.08 S, Sm _0.9 La _0.1 S, Sm _0.95 Eu _0.05 S was 0.05 μm (less than the value of 0.07 μm indicated in the formula). The resulting films had metallic electrical properties. In this case, the sensitivity of the obtained oxygen sensors were in the range from 3 to 4.5 Ohm ^{-1 *} m ^-1 , which is 70-80% lower than that of the prototype.

Example 3

The same as in example 1, but the thickness of the samples with compositions Sm _0.9 Gd _0.1 S, Sm _0.92 Ce _0.08 S, Sm _0.9 La _0.1 S, Sm _0.95 Eu _0.05 S was 0.3 μm (more than the value from the interval in the formula) . In this case, films with pronounced semiconductor properties were obtained, but their sensitivity to oxygen was from 5 to 8 Ohm ^{-1 *} m ^-1 , which is 30-60% lower than that of the prototype (due to shunting of surface conductivity by the electrical conductivity of the rest of the material) .

Example 4

In the same manner as in Example 1, samples were prepared with the compositions Sm _0.8 Gd _0.2 S, Sm _0.8 Ce _0.2 S, Sm _0.8 La _0.2 S, Sm _0.8 Eu _0.2 S, beyond the range of x from the claims. The films had metallic electrical properties. The same measurements were taken of the sensitivity of the sensors to oxygen. Their sensitivity ranged from 10 to 12 Ohm ^{-1 *} m ^-1 , which is 15-30% lower than that of the prototype.

Thus, the advantages of the proposed technical solution are shown, consisting in the possibility of creating a semiconductor oxygen sensor having a sensitivity exceeding the sensitivity of the prototype sensor.

The advantages of the proposed oxygen sensor should also include its very small size (~ 1 mm ³ compared with 3 mm ^{3 of} the prototype) and low cost.

Claims (2)

1. A semiconductor oxygen sensor containing a dielectric substrate and a semiconductor layer with metal electrodes deposited on its surface, characterized in that the semiconductor layer is made of a material with the composition Sm _1-x Ln _x S, where Ln is one of the following elements: La, Ce, Pr, Nd, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, ax has a value from the interval from 0 to 0.14. 2. The device according to claim 1, characterized in that the thickness of the semiconductor layer lies in the range from 0.07 μm to 0.2 μm.

Images