JP2001343347A - Oxide semiconductor and nitrogen oxide sensor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an oxide semiconductor and a nitrogen oxide sensor having a low resistance value while maintaining the original characteristics (sensor sensitivity) of tungsten oxide. SOLUTION: A Pt electrode 32 is formed on the rear face of an Si wafer 2, two Pt electrodes 31 are formed on the front face of the Si wafer 2, and a sensor layer 1 is mounted on the Pt electrodes 31 so as to form the nitrogen oxide sensor. In the sensor layer 1, the tungsten oxide WO3 is mixed with titanium oxide TiO2, whereto any one of the Group 5A or 5B elements of the periodic table is added as impurity for a reduction in resistance, in order to reduce the resistance value for the sensor.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an oxide semiconductor in which a high-resistance tungsten oxide is mixed with a low-resistance titanium oxide by adding an impurity, and a nitrogen oxide for detecting nitrogen oxide using the same. Object sensor.

[0002]

2. Description of the Related Art Nitrogen oxides (NO _x ) are contained in large quantities, for example, in the exhaust gas of a combustion engine and have various effects on the environment. Therefore, despite the detection of the NO _X gas is high demand from society for expensive analytical detector is used, not widespread in still generally. Under these circumstances, the development of inexpensive and compact NO _X sensor is demanded for gas detection in a combustion engine exhaust gas detected or enclosed space.

[0003] Oxide semiconductor gas sensors are commercialized as city gas (methane) sensors and the like, and are highly reliable gas sensors. If applying this to the NO _X sensor, it is expected to small and can realize an inexpensive sensor. for that purpose,
First it is necessary to find a material that is sensitive to NO _X. Table 1
To show the oxide semiconductor material has been studied as a detection material currently NO _X gas sensor for, NO _X gas because NO ₂ and NO is a main component, shows both the sensitivity. Note that sensitivity is defined as a value obtained by dividing the resistance value of an oxide semiconductor in gas by the resistance value of an oxide semiconductor in air.

[Table 1]

[0004]

[0007] The n-type oxide of tungsten oxide WO ₃ as a semiconductor increases the resistance by NO ₂ gas, NO ₂ gas is negatively charged adsorbed on WO ₃ surface, WO ₃ in the This is for trapping the carrier electrons inside. As a result, the sensor resistance during NO ₂ gas adsorption reaches several hundred MΩ, exceeding the control range of the sensor detection circuit. Therefore, if you attempted to lower the resistance of the tungsten oxide, for example, it is conceivable to oxygen deficiency type, or buried oxygen deficiency during heating at the time of NO _X sensing,
Since NO _x adsorption occurs, there is a problem in the stability of the sensor.

[0005] Both tungsten and oxygen are Group 6 elements of the periodic table, and it is difficult to replace the oxygen with a Group 7B element such as chlorine to make them n-type. Further, even if the 7A group such as manganese is replaced with tungsten, it is difficult to obtain a valence electronic state. On the other hand, titanium oxide (TiO ₂ ) can reduce the resistance by adding a group 5A element as an impurity, but has low sensitivity as shown in Table 1. Accordingly, an object of the present invention is to reduce the resistance value remains the sensitivity of the tungsten oxide were maintained, and to realize an NO _X sensor with these characteristics.

[0006]

According to the first aspect of the present invention, there is provided an oxide semiconductor comprising a mixture of tungsten oxide and titanium oxide, comprising vanadium,
At least one of Group 5A elements of the periodic table including niobium and tantalum or elements of the Group 5B of the periodic table including phosphorus, arsenic, antimony, and bismuth is added as an impurity element to have n-type conductivity. .

According to the first aspect of the present invention, the titanium oxide can be mixed in the range of 30 to 70% by weight (the second aspect of the invention), or the amount of the impurity element added to the titanium element is It can be 10 atomic% or less (the invention of claim 3). The oxide semiconductor according to any one of claims 1 to 3 can be used as a nitrogen oxide sensor (invention of claim 4), and the nitrogen oxide sensor of claim 4 has a thin film structure. (Invention of claim 5), and the thin film can be formed on a heater with a diaphragm (invention of claim 6).

[0008]

FIG. 1 is an overall configuration diagram of a gas sensor according to an embodiment of the present invention. First, a Pt heater 32 is provided on the back surface of a Si substrate (Si wafer) 2, and two electrodes 31 also made of Pt are formed on the front surface. A mixed oxide film of tungsten oxide WO ₃ and titanium oxide TiO ₂ as the sensor layer 1 was formed by sputtering using a metal mask over the electrodes 31 on the substrate 2. The thin film was formed by reactive sputtering using a W and Ti alloy as a sputtering target and a mixed gas of argon and oxygen.

Example 1 First, a measure was taken by changing the alloy ratio between WO ₃ and TiO ₂ . FIG. 2 shows the relationship between the resistance at 350 ° C. in air and the sensitivity of the gas sensor when the sensor layer is not doped with anything. The sensor resistance and sensitivity, NO ₂ is 6p
pm. The sensitivity is the ratio of the sensor's resistance R _NO2 in NO ₂ gas R _NO2 to resistance R _air in the atmosphere R _NO2 /
Defined by R _air . From FIG. 2, WO ₃ is 70 wt%.
Below (TiO ₂ is 30 wt% or more) and R _NO2 is 10M
Ω or less. When WO ₃ is 30 wt% or more (TiO ₂ is 70 wt% or less), sensitivity R _NO2 / R _air
Is 2.5 or more, indicating that sufficient sensitivity has been obtained.

Embodiment 2 Here, the amount of impurities added to the sensor layer is adjusted.
For that purpose, for example, it is realized by placing a tip or a bar of an impurity element on a target. When the NO ₂ sensitivity of the gas sensor is examined by changing the amount of addition of the impurities as described above, the result is as shown in FIG. This example is based on the case that 35 atomic% of niobium (Nb) is added to Ti of the sensor layer.
The relationship between the resistance R _air at 0 ° C., the resistance R _{NO2 in} gas, and the sensitivity R _NO2 / R _air is shown. Note that the sensor resistance and sensitivity in gas are values when NO ₂ is 6 ppm.

[0011] As is apparent from FIG. _3, WO 3 is 70
At less than 30 wt% (TiO ₂ is 30 wt% or more), R _NO2 was as low as 1 MΩ or less. When WO ₃ is 30 wt% or more (TiO ₂ is 70 wt% or less), the sensitivity R _NO2 / R
It can be said that _air is 2.5 or more, and sufficient sensitivity is obtained. The same effect can be obtained even if Nb is added to Ti in an amount of 1 atomic% or more.

FIG. 4 shows the relationship between the gas sensor resistances R _air and R _NO2 and the gas sensor sensitivity R _NO2 / R _air when the addition ratio of Nb as an impurity is changed. The horizontal axis is Nb / (Ti + Nb) indicating the addition ratio of Nb, and the unit is atomic%. As is apparent from this figure, the decrease stops at around 10 atomic%, and it can be seen that the further addition of impurities has the opposite effect. Note that the effect of the addition of the impurity element can be obtained by using a Group 5A element of the periodic table such as vanadium (V) or tantalum (Ta) or a Group 5B element of the periodic table such as antimony (Sb) or bismuth (Bi). The same is true.

[0013]

According to the present invention, a low resistance can be obtained by adding at least one element of Group 5A or Group 5B of the periodic table to an oxide semiconductor in which WO ₃ and TiO ₂ are mixed. In particular, there are provided advantages that are suitable for use as a NO ₂ gas sensor.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram showing an embodiment as a gas sensor according to the present invention.

FIG. 2 is a diagram illustrating resistance and gas sensor sensitivity characteristics when an alloy ratio of WO ₃ and TiO ₂ is changed.

FIG. 3 is a graph showing resistance and gas sensor sensitivity characteristics when the amount of impurities added to a sensor layer is adjusted.

FIG. 4 is a graph showing resistance and gas sensor sensitivity characteristics when the addition ratio of Nb as an impurity is changed.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Sensor layer, 2 ... Si substrate (Si wafer), 31,3
2. Pt electrode.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 2G046 AA13 BA01 BA09 BB02 BC04 BE03 DB05 DC14 FE06 FE24 FE27 FE36 FE41 FE44 FE45 FE46 4G048 AA03 AC04 AC08 AD02

Claims (6)

[Claims] 1. An oxide semiconductor in which titanium oxide is mixed with tungsten oxide, wherein at least one of a group 5A element in the periodic table containing vanadium, niobium, and tantalum, or a group 5B element in the periodic table containing phosphorus, arsenic, antimony, and bismuth is used. An oxide semiconductor having n-type conductivity in which one element is added as an impurity element. 2. The oxide semiconductor according to claim 1, wherein the titanium oxide is mixed in a range of 30 to 70% by weight. 3. The oxide semiconductor according to claim 1, wherein the amount of the impurity element is 10 atomic% or less with respect to the titanium element. 4. A nitrogen oxide sensor using the oxide semiconductor according to claim 1. 5. The nitrogen oxide sensor according to claim 4, which has a thin film structure. 6. The nitrogen oxide sensor according to claim 5, wherein the thin film is formed on a heater with a diaphragm.