JP5138163B2 - Field effect transistor - Google Patents

Field effect transistor Download PDF

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JP5138163B2
JP5138163B2 JP2005325366A JP2005325366A JP5138163B2 JP 5138163 B2 JP5138163 B2 JP 5138163B2 JP 2005325366 A JP2005325366 A JP 2005325366A JP 2005325366 A JP2005325366 A JP 2005325366A JP 5138163 B2 JP5138163 B2 JP 5138163B2
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JP2006165529A5 (en
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政史 佐野
克己 中川
秀雄 細野
利夫 神谷
研二 野村
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キヤノン株式会社
国立大学法人東京工業大学
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Abstract

<P>PROBLEM TO BE SOLVED: To provide a new amorphous oxide applicable to an active layer of a TFT or the like. <P>SOLUTION: An amorphous oxide is characterized in that it comprises microcrystals, that its composition changes in a layer thickness direction, or that it contains a predetermined material. <P>COPYRIGHT: (C)2006,JPO&amp;NCIPI

Description

  The present invention relates to an amorphous oxide. The present invention also relates to a field effect transistor using the amorphous oxide.

  2. Description of the Related Art In recent years, flat and thin image display devices (Flat Panel Displays: FPD) have been put into practical use due to advances in liquid crystal and electroluminescence (EL) technologies.

  These FPDs are driven by an active matrix circuit of a field effect thin film transistor (TFT) using an amorphous silicon thin film or a polycrystalline silicon thin film provided on a glass substrate as an active layer.

  On the other hand, in order to further reduce the thickness, weight, and breakage resistance of these FPDs, an attempt has been made to use a lightweight and flexible resin substrate instead of a glass substrate.

  However, the manufacture of the transistor using the above-described silicon thin film requires a relatively high temperature thermal process and is generally difficult to form directly on a resin substrate having low heat resistance.

  In view of this, TFTs that can be formed at a low temperature and that use an oxide semiconductor thin film made of, for example, ZnO have been actively developed (Patent Document 1).

On the other hand, TFTs using conventional oxide semiconductor thin films have not obtained sufficient characteristics comparable to TFTs using silicon.
Japanese Patent Laid-Open No. 2003-298062

  An object of the present invention is to provide an amorphous oxide and a field effect transistor that can be used in an active layer of a semiconductor device such as a TFT and become a suitable semiconductor.

The first outline of the present invention is:
An amorphous oxide,
The amorphous oxide includes microcrystals and has an electron carrier concentration of less than 10 18 / cm 3 .
It is characterized by that.

The second gist of the present invention is an amorphous oxide,
The amorphous oxide contains microcrystals and has a tendency to increase electron mobility as the electron carrier concentration increases.
It is characterized by that.

The third essence of the present invention is a field effect transistor,
An active layer having an amorphous oxide containing microcrystals;
A gate electrode provided on the active layer via a gate insulating film;
It is characterized by comprising.

The fourth aspect of the present invention is an amorphous oxide,
Wherein the amorphous oxide is changing in composition to the film thickness direction, and,
The amorphous oxide is
The electron carrier concentration is less than 10 18 / cm 3 ;
It is characterized by that.

The fifth essence of the present invention is a field effect transistor,
An active layer containing an amorphous oxide in composition in the film thickness direction is changed,
A gate electrode provided on the active layer via a gate insulating film;
Comprising
The active layer includes a first region and a second region closer to the gate insulating film than the first region,
The oxygen concentration in the second region is higher than the oxygen concentration in the first region.

The sixth aspect of the present invention is a field effect transistor,
An active layer containing an amorphous oxide having at least one of In or Zn;
A gate electrode provided on the active layer via a gate insulating film;
Comprising
The active layer includes a first region and a second region closer to the gate insulating film than the first region,
The In concentration or Zn concentration in the second region is higher than the In concentration or Zn concentration in the first region.

The seventh aspect of the present invention is:
An amorphous oxide,
The amorphous oxide has a composition changing in the film thickness direction, and
The amorphous oxide is
As the electron carrier concentration increases, the electron mobility tends to increase.
It is characterized by that.

The eighth aspect of the present invention is:
A field effect transistor,
An active layer comprising an amorphous oxide comprising In and Zn;
A gate electrode provided on the active layer via a gate insulating film;
Comprising
The active layer includes a first region and a second region closer to the gate insulating film than the first region,
The In concentration of the second region is higher than the In concentration of the first region, or the Zn concentration of the second region is higher than the Zn concentration of the first region.

The ninth aspect of the present invention is:
An amorphous oxide,
The amorphous oxide has an electron carrier concentration of less than 10 18 / cm 3 , and
It contains one or more elements selected from Li, Na, Mn, Ni, Pd, Cu, Cd, C, N, P, Ti, Zr, V, Ru, Ge, Sn, and F. .

The tenth aspect of the present invention is:
An amorphous oxide,
The amorphous oxide is
As the electron carrier concentration increases, the electron mobility tends to increase, and
It contains at least one element selected from Li, Na, Mn, Ni, Pd, Cu, Cd, C, N, P, Ti, Zr, V, Ru, Ge, Sn, and F.

The eleventh aspect of the present invention is:
A field effect transistor,
It has an amorphous oxide containing at least one element selected from Li, Na, Mn, Ni, Pd, Cu, Cd, C, N, P, Ti, Zr, V, Ru, Ge, Sn, and F An active layer,
A gate electrode provided on the active layer via a gate insulating film;
It is characterized by comprising.

  In the present invention, the amorphous oxide is preferably an oxide containing at least one of In, Zn, and Sn.

  In the present invention, it is preferable that the amorphous oxide is an oxide containing In, Ga, and Zn.

In the present invention,
The amorphous oxide is preferably an oxide containing In, Zn, and Sn, an oxide containing In and Zn, an oxide containing In and Sn, or an oxide containing In It is.

  By the way, when the present inventor examined an oxide semiconductor, it was found that ZnO cannot generally form a stable amorphous phase. Since most ZnO exhibits a polycrystalline phase, carriers are scattered at the interface between the polycrystalline particles, and as a result, it seems that the electron mobility cannot be increased.

  In addition, oxygen defects are easily introduced into ZnO, and a large number of carrier electrons are generated. Therefore, it is difficult to reduce the electrical conductivity. For this reason, it was found that even when the gate voltage of the transistor is not applied, a large current flows between the source terminal and the drain terminal, and the normally-off operation of the TFT cannot be realized. It also seems difficult to increase the on / off ratio of the transistor.

Further, the present inventor has amorphous oxide film Zn x M y In z O ( x + 3y / 2 + 3z / 2) ( in the formula as described in JP 2000-044236, M is Al And at least one element of Ga). This material has an electron carrier concentration of 10 18 / cm 3 or more, and is a suitable material as a simple transparent electrode.

However, it has been found that when an oxide having an electron carrier concentration of 10 18 / cm 3 or more is used for the TFT channel layer, the on / off ratio is not sufficient, which is not suitable for a normally-off type TFT.

That is, in the conventional amorphous oxide film, a film having an electron carrier concentration of less than 10 18 / cm 3 could not be obtained.

The present inventor manufactured a TFT using an amorphous oxide having an electron carrier concentration of less than 10 18 / cm 3 as an active layer of a field effect transistor. As a result, a TFT having desired characteristics was obtained. Discovered that it can be applied to semiconductor devices.

As a result of intensive research and development on InGaO 3 (ZnO) m and film formation conditions of this material, the present inventors have controlled the oxygen atmosphere conditions during film formation to reduce the electron carrier concentration to 10 It has been found that it can be less than 18 / cm 3 .

  Note that, as described above, the description has been given mainly when the amorphous oxide is used as an active layer serving as a channel of the TFT. However, the present invention is limited to the case where the amorphous oxide is used for such an active layer. is not.

  According to the present invention, for example, it is possible to provide an amorphous oxide that is suitably used for a channel layer of a TFT. In addition, a field effect transistor having excellent characteristics can be provided.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

  The first to third aspects of the present invention will be described below in the first to third embodiments.

Thereafter, the amorphous oxide material applicable to the present invention will be described in detail.
Note that in the following embodiments, an In—Ga—Zn—O-based oxide will be mainly described; however, the present invention is not limited to materials having these compositions.
(First Embodiment: Amorphous oxide containing microcrystals)
The invention according to this embodiment relates to an amorphous oxide,
The amorphous oxide is characterized by containing microcrystals.
Here, whether or not microcrystals are contained in the amorphous oxide is determined from, for example, a cross-sectional TEM (transmission electron microscope) photograph of the formed amorphous oxide film.
For example, the amorphous oxide film specifically includes In—Ga—Zn—O, and the composition in the crystalline state is represented by InGaO 3 (ZnO) m (m is a natural number less than 6). The

As used herein, an amorphous oxide is an oxide having an electron carrier concentration of less than 10 18 / cm 3 , or an oxide that tends to increase electron mobility as the electron carrier concentration increases. Etc. Depending on the type of TFT used, it is preferable to form a normally-off type TFT using the amorphous oxide.

Alternatively, it is composed of In—Ga—Zn—Mg—O, and the composition of the crystalline state is represented by InGaO 3 (Zn 1−x Mg × O) m (m is a natural number of less than 6, 0 <x ≦ 1). The

Note that in these amorphous oxide films, it is also preferable that the electron mobility exceeds 1 cm 2 / (V · sec).

When the above film is used for the channel layer, the transistor current is normally off with a gate current of less than 0.1 microamperes when the transistor is off, the transistor has an on / off ratio of more than 10 3 , transparent to visible light, and flexible. It was found that a simple TFT can be produced.

  The transparent film is characterized in that the electron mobility increases as the number of conduction electrons increases. As the substrate on which the transparent film is formed, a glass substrate, a plastic substrate, a plastic film, or the like can be used.

When the transparent oxide film is used for a channel layer, a gate insulating film containing a mixed crystal compound containing at least one of Al 2 O 3 , Y 2 O 3 , or HfO 2 or at least two of these compounds is used. It is also a preferable form to use a transistor.

  In addition, it is also preferable to form a film in an atmosphere containing oxygen gas and under light irradiation without intentionally adding impurity ions for increasing electric resistance.

  The present inventors have found that the semi-insulating oxide amorphous thin film has a unique characteristic that the electron mobility increases as the number of conduction electrons increases. Then, a TFT was formed using the film, and it was found that transistor characteristics such as an on / off ratio, a saturation current in a pinch-off state, and a switch speed were further improved.

When a transparent semi-insulating amorphous oxide thin film is used as a channel layer of a film transistor, the electron mobility is more than 1 cm 2 / (V · sec), preferably more than 5 cm 2 / (V · sec) and the electron carrier concentration is 10 When it is less than 18 / cm 3 , preferably less than 10 16 / cm 3 , the current between the drain and source terminals when off (when no gate voltage is applied) is less than 10 microamperes, preferably 0.1 microamperes. Can be less than. When the thin film is used, when the electron mobility is more than 1 cm 2 / (V · sec), preferably more than 5 cm 2 / (V · sec), the saturation current after pinch-off can be more than 10 microamperes, The on / off ratio can be greater than 10 3 .

  In the TFT, in a pinch-off state, a high voltage is applied to the gate terminal, and high-density electrons exist in the channel. Therefore, according to the present invention, the saturation current value can be further increased by the amount of increase in electron mobility. As a result, almost all transistor characteristics such as an increase in on / off ratio, an increase in saturation current, and an increase in switching speed are improved. In a normal compound, when the number of electrons increases, electron mobility decreases due to collisions between electrons.

  The TFT structure can be used for a staggered (top gate) structure in which a gate insulating film and a gate terminal are sequentially formed on a semiconductor channel layer. Further, it can be used for an inverted stagger (bottom gate) structure in which a gate insulating film and a semiconductor channel layer are formed in order on a gate terminal.

(About film composition)
A transparent amorphous oxide thin film whose composition in the crystalline state is represented by InGaO 3 (ZnO) m (m is a natural number less than 6) is stable in amorphous state up to a high temperature of 800 ° C. or higher when the value of m is less than 6. However, it becomes easier to crystallize as the value of m increases. That is, as the ratio of ZnO to InGaO 3 increases and approaches the ZnO composition, it becomes easier to crystallize.

  Therefore, the value of m is preferably less than 6 for the channel layer of the amorphous TFT. However, it was also found that microcrystals can be formed under light irradiation during film formation even if the value of m is small.

As a film forming method, a vapor phase film forming method is preferably used with a polycrystalline sintered body having an InGaO 3 (ZnO) m composition as a target. Of the vapor deposition methods, sputtering and pulsed laser deposition are suitable. Furthermore, the sputtering method is most suitable from the viewpoint of mass productivity.

However, when the amorphous film is produced under normal conditions, oxygen vacancies mainly occur, and until now, the electron carrier concentration has been less than 10 18 / cm 3 and the electric conductivity has not been able to be 10 S / cm or less. .

  When such a thin film is used, a normally-off transistor cannot be formed.

It is composed of In-Ga-Zn-O prepared by pulse laser deposition using the apparatus shown in FIG. 7, and the composition in the crystalline state is represented by InGaO 3 (ZnO) m (m is a natural number less than 6). The transparent amorphous oxide thin film can reduce the electron carrier concentration to less than 10 18 / cm 3 . For example, the electron carrier concentration can be reduced to less than 10 18 / cm 3 by forming a film in an atmosphere having a high oxygen partial pressure of more than 3.2 Pa.

  In this case, the temperature of the substrate is maintained at substantially room temperature without intentionally heating. Since a plastic film can be used as a substrate, the substrate temperature is preferably kept below 100 ° C.

  The invention according to this embodiment is composed of In-Ga-Zn-O produced by a pulse laser deposition method under light irradiation.

Specifically, it is a transparent amorphous oxide thin film containing microcrystals represented by the composition InGaO 3 (ZnO) m (m is a natural number less than 6) in the crystalline state. Can be configured.

Further, the electron mobility of the thin film can be more than 1 cm 2 / V · second, and the on / off ratio can be increased to more than 10 3 .

  Further, the present invention is composed of In—Ga—Zn—O prepared by a sputter deposition method using argon gas under light irradiation.

For example, the amorphous oxide according to the present invention can be obtained using the apparatus shown in FIG. In this case, the amorphous oxide according to the present invention is obtained by converting a transparent amorphous oxide thin film containing a microcrystal whose composition in the crystalline state is represented by InGaO 3 (ZnO) m (m is a natural number of less than 6) to an oxygen content. It can be obtained by forming a film in an atmosphere having a high pressure exceeding 1 × 10 −2 Pa.

  In this case, the temperature of the substrate is maintained at substantially room temperature without intentionally heating. Since a plastic film can be used as a substrate, the substrate temperature is preferably kept below 100 ° C. By further increasing the oxygen partial pressure, the number of electron carriers can be reduced.

That is, the present invention is composed of In—Ga—Zn—O produced by sputtering deposition under light irradiation.
A transistor having a transparent amorphous oxide thin film containing a microcrystal represented by a composition InGaO 3 (ZnO) m (m is a natural number less than 6) in a crystalline state, normally off, and having an on / off ratio of more than 10 3 Can be configured.

  In a thin film prepared by pulse laser deposition and sputtering under light irradiation, the electron mobility increases as the number of conduction electrons increases.

Similarly, if polycrystalline InGaO 3 (Zn 1-x Mg x O) m (m is a natural number less than 6 and 0 <x ≦ 1) is used as a target, a high-resistance amorphous InGaO even under an oxygen partial pressure of less than 1 Pa. 3 (Zn 1-x Mg x O) m film can be obtained.

  As described above, by controlling the oxygen partial pressure, oxygen defects can be reduced, and as a result, the electron carrier concentration can be decreased without adding specific impurity ions. The amorphous oxide according to the present invention can be obtained by irradiating with light during the production of a thin film produced using FIGS.

  Conditions such as oxygen partial pressure can be produced with a partial pressure in the range described later when the apparatus shown in FIGS. 7 and 8 is used.

  In addition, in the amorphous state including microcrystals, the interface between the microcrystalline grain boundaries is covered with an amorphous structure, so unlike the polycrystalline state such as zinc oxide, essentially particles that trap the movement of electrons and holes. There is no interface. Therefore, an amorphous thin film with high electron mobility can be obtained. Furthermore, since the number of conduction electrons can be reduced without adding specific impurities, there is no scattering due to impurities, and electron mobility can be kept high.

The microcrystal used in the present invention is not limited to a microcrystal having a composition ratio represented by InGaO 3 (ZnO) m (m is a natural number of less than 6).

In the thin film transistor using the transparent film described above, it is preferable that a mixed crystal compound containing at least two of Al 2 O 3 , Y 2 O 3 , HfO 2 , or a compound thereof is used as the gate insulating film. If there is a defect at the interface between the gate insulating thin film and the channel layer thin film, the electron mobility is lowered and the transistor characteristics are hysteresis. Further, the leakage current varies greatly depending on the type of the gate insulating film. For this purpose, it is necessary to select a gate insulating film suitable for the channel layer. If an Al 2 O 3 film is used, leakage current can be reduced. Further, the hysteresis can be reduced by using a Y 2 O 3 film. Further, if a high dielectric constant HfO 2 film is used, the electron mobility can be increased. Further, by using mixed crystals of these films, a TFT with small leakage current and hysteresis and high electron mobility can be formed. In addition, since the gate insulating film formation process and the channel layer formation process can be performed at room temperature, both a staggered structure and an inverted staggered structure can be formed as the TFT structure.

  A thin film transistor (TFT) is a three-terminal element including a gate terminal, a source terminal, and a drain terminal. The TFT uses a semiconductor thin film formed on an insulating substrate such as ceramic, glass, or plastic as a channel layer through which electrons or holes move. A TFT is an active element having a function of switching a current between a source terminal and a drain terminal by applying a voltage to a gate terminal to control a current flowing in a channel layer.

Note that microcrystals included in the amorphous oxide may be formed by light irradiation as described above (specifically, for example, light irradiation using a halogen lamp or ultraviolet irradiation). Of course, other methods that do not rely on light irradiation may be used.
(Second Embodiment: Composition Distribution of Amorphous Oxide)
The amorphous oxide according to this embodiment is
The composition changes in the film thickness direction.

  Here, the composition changes in the film thickness direction means that the amount of oxygen contained in the oxide changes in the film thickness direction, or the elements constituting the oxide change in the middle (that is, It means that the content of the elements constituting the oxide has changed.

  Therefore, when the amorphous oxide is used for an active layer (also referred to as a channel layer) of a field effect transistor, for example, the following configuration is preferable.

Specifically, in a transistor comprising an active layer containing the amorphous oxide and a gate insulating film that forms an interface with the active layer,
The oxygen concentration contained in the amorphous oxide is configured such that the region on the interface side is higher than the region far from the interface.

  In this case, the electrical resistance in the vicinity of the interface in the amorphous oxide layer is increased, and a so-called channel of the transistor is formed in the amorphous oxide from the interface. When the interface is present, such a configuration is preferable from the viewpoint of reducing leakage current.

  That is, when the amorphous oxide is used as an active layer of a transistor, the active layer includes a first region and a second region closer to the gate insulating film side than the first region. The oxygen concentration of the second region is preferably higher than that of the first region.

  The boundary between the two regions does not need to be clear, and the composition ratio may change in a gradient or step shape.

In particular, the electron carrier concentration of the amorphous oxide is preferably less than 10 18 / cm 3 .

Note that the film thickness direction when the film is formed on the substrate is a direction that is not the in-plane direction of the substrate (for example, a direction perpendicular to the in-plane direction of the substrate).
Further, in a transistor including an active layer containing an amorphous oxide having at least one of In or Zn and a gate insulating film that forms an interface with the active layer, the concentration of In or Zn contained in the amorphous oxide It is also a preferred form that the region on the interface side is configured to be higher than the region distant from the interface. In this case, field effect mobility can be increased.

  That is, when the above amorphous oxide is used as an active layer of a transistor, the active layer includes a first region and a second region closer to the gate insulating film side than the first region. It is preferable that at least one of the In and Zn concentrations in the region is higher than the respective concentrations in the first region.

  Of course, it is also preferable to make the In concentration of the second region higher than the In concentration of the first region, or to make the Zn concentration of the second region higher than the Zn concentration of the first region. Furthermore, it is also preferable that the In concentration of the second region is higher than the In concentration of the first region, and the Zn concentration of the second region is higher than the Zn concentration of the first region.

Specifically, the oxide film according to the second aspect of the present invention includes In—Ga—Zn—O, the composition is distributed in the film thickness direction, and the composition in the crystalline state is InGaO 3 (ZnO) m ( m is a natural number less than 6), and the electron carrier concentration is less than 10 18 / cm 3 .

Alternatively, it is configured to include In—Ga—Zn—Mg—O, the composition is distributed in the film thickness direction, and the composition in the crystalline state is InGaO 3 (Zn 1−x Mg × O) m (m is a natural number less than 6, A transparent amorphous oxide film represented by 0 <x ≦ 1) and having an electron carrier concentration of less than 10 18 / cm 3 .

In these films, it is also a preferable mode that the electron mobility exceeds 1 cm 2 / (V · sec).

When the above film is used for the channel layer, the transistor current is normally off with a gate current of less than 0.1 microamperes when the transistor is turned off, the transistor has an on / off ratio of more than 10 4 , transparent to visible light, and flexible. TFT can be made.

  The transparent film is characterized in that the electron mobility increases as the number of conduction electrons increases.

  As the substrate on which the transparent film is formed, a glass substrate, a plastic substrate, a plastic film, or the like can be used.

When the transparent oxide film is used for a channel layer, a gate insulating film containing a mixed crystal compound containing at least one of Al 2 O 3 , Y 2 O 3 , or HfO 2 or at least two of these compounds is used. It is also a preferable form to use a transistor.

  In addition, it is also a preferable mode to form a film in an atmosphere containing oxygen gas without intentionally adding impurity ions for increasing electric resistance.

  The present inventors have found that the semi-insulating oxide amorphous thin film has a unique characteristic that the electron mobility increases as the number of conduction electrons increases. Then, a TFT was formed using the film, and it was found that transistor characteristics such as an on / off ratio, a saturation current in a pinch-off state, and a switch speed were further improved.

When a transparent semi-insulating amorphous oxide thin film is used as a channel layer of a film transistor, the electron mobility is more than 1 cm 2 / (V · sec), preferably more than 5 cm 2 / (V · sec) and the electron carrier concentration is 10 When it is less than 18 / cm 3 , preferably less than 10 16 / cm 3 , the current between the drain and source terminals when off (when no gate voltage is applied) is less than 10 microamperes, preferably 0.1 microamperes. Can be less than. When the thin film is used, when the electron mobility is more than 1 cm 2 / (V · sec), preferably more than 5 cm 2 / (V · sec), the saturation current after pinch-off can be more than 10 microamperes, The on / off ratio can be greater than 10 4 .

  In the TFT, in a pinch-off state, a high voltage is applied to the gate terminal, and high-density electrons exist in the channel. Therefore, according to the present invention, the saturation current value can be further increased by the amount of increase in electron mobility. As a result, almost all transistor characteristics such as an increase in on / off ratio, an increase in saturation current, and an increase in switching speed are improved. In a normal compound, when the number of electrons increases, electron mobility decreases due to collisions between electrons.

  The TFT structure can be a staggered (top gate) structure in which a gate insulating film and a gate terminal are sequentially formed on a semiconductor channel layer. Further, it can be used for an inverted stagger (bottom gate) structure in which a gate insulating film and a semiconductor channel layer are formed in order on a gate terminal.

(About film composition)
A transparent amorphous oxide thin film whose composition in the crystalline state is represented by InGaO 3 (ZnO) m (m is a natural number less than 6) is stable in amorphous state up to a high temperature of 800 ° C. or higher when the value of m is less than 6. However, it becomes easier to crystallize as the value of m increases. That is, as the ratio of ZnO to InGaO 3 increases and approaches the ZnO composition, it becomes easier to crystallize.

  Therefore, the value of m is preferably less than 6 for the channel layer of the amorphous TFT.

In the thin film transistor using the transparent film described above, it is preferable that a mixed crystal compound containing at least two of Al 2 O 3 , Y 2 O 3 , HfO 2 , or a compound thereof is used as the gate insulating film. If there is a defect at the interface between the gate insulating thin film and the channel layer thin film, the electron mobility is lowered and the transistor characteristics are hysteresis. Further, the leakage current varies greatly depending on the type of the gate insulating film. For this purpose, it is necessary to select a gate insulating film suitable for the channel layer. If an Al 2 O 3 film is used, leakage current can be reduced. Further, the hysteresis can be reduced by using a Y 2 O 3 film. Further, if a high dielectric constant HfO 2 film is used, the electron mobility can be increased. Further, by using mixed crystals of these films, a TFT with small leakage current and hysteresis and high electron mobility can be formed. In addition, since the gate insulating film formation process and the channel layer formation process can be performed at room temperature, both a staggered structure and an inverted staggered structure can be formed as the TFT structure.

  A thin film transistor (TFT) is a three-terminal element including a gate terminal, a source terminal, and a drain terminal. A TFT uses a semiconductor thin film formed on an insulating substrate such as ceramics, glass, or plastic as a channel layer through which electrons or holes move. Furthermore, a TFT is an active element having a function of switching a current between a source terminal and a drain terminal by applying a voltage to a gate terminal to control a current flowing in a channel layer.

  As described above, according to the second aspect of the present invention, when the FET is manufactured using the transparent film, the composition in the film thickness direction of the transparent film which is the active layer of the FET is improved.

Specifically, in the pulse laser deposition method, the oxygen partial pressure is changed in the film thickness direction, the oscillation power of the pulse laser is changed, the oscillation frequency is changed, or the target-substrate is changed. The composition is changed in the film thickness direction by changing the distance in the film thickness direction. When the sputtering deposition method is used, the composition change in the film thickness direction is performed by additionally sputtering an In 2 O 3 or ZnO target.

For example, when a film is formed in an oxygen atmosphere, the amount of oxygen contained in the film increases as the distance between the target and the substrate increases. Further, if, for example, a ZnO target is added during film formation, the amount of Zn increases in the film that is additionally formed later.
(Third Embodiment: Amorphous Oxide Containing Additive)
The amorphous oxide according to this embodiment is
As an additive, one or more elements selected from Li, Na, Mn, Ni, Pd, Cu, Cd, C, N, P, Ti, Zr, V, Ru, Ge, Sn, and F are included. It is characterized by.

  Such introduction of the additive into the amorphous oxide is realized by including it in the gas in the film forming apparatus, in the film forming apparatus, or in the target material used in the apparatus. The Of course, after a film made of an amorphous oxide not containing such an additive is produced, the additive may be introduced into the film later as in an example described later.

Note that the electron carrier concentration of the amorphous oxide is preferably less than 10 18 / cm 3 .

Specifically, the amorphous oxide according to the present invention includes In—Ga—Zn—O, and has a transparent composition represented by InGaO 3 (ZnO) m (m is a natural number of less than 6) in the crystalline state. Amorphous oxide. The amorphous oxide according to the present invention includes In—Ga—Zn—Mg—O, and the composition of the crystalline state is InGaO 3 (Zn 1-x Mg x O) m (m is a natural number less than 6). , 0 <x ≦ 1). These oxides further contain one or more elements selected from Li, Na, Mn, Ni, Pd, Cu, Cd, C, N, P and the like as impurities. Thereby, the electron carrier concentration can be reduced.

  In addition, even when the electron carrier concentration is significantly reduced, the reduction of the electron carrier mobility can be suppressed, and the control of the electron carrier concentration becomes easy. Therefore, if the transparent amorphous oxide film is used for the channel layer, it is easy to obtain a TFT panel having high uniformity of characteristics even in a large area.

  In addition, when Li, Na, Mn, Ni, Pd, Cu, Cd, C, N, and P are used as impurities (additives), these impurities are in any one of In, Ga, Zn, and O sites. Substitution may function as an acceptor and reduce the electron carrier density. However, details of the mechanism are unknown.

  In a normal oxide semiconductor, a large amount of oxygen defects are generated because the oxygen concentration cannot be appropriately controlled. In addition, since it is a polycrystal, defects generated at grain boundaries often prevent the electron carrier density from being well controlled even if impurities are introduced.

  In that respect, the transparent amorphous oxide film of the present invention has few oxygen vacancies, is amorphous and does not have a grain boundary, and therefore, the acceptor effect appears clearly.

  When a thin film is formed by increasing the oxygen partial pressure for the purpose of reducing the electron carrier density, the skeleton of the atomic bond changes and the tail level of the conduction band increases, and electrons are trapped at this level and effective. There is a risk of reducing the electron carrier mobility.

  However, by using Li, Na, Mn, Ni, Pd, Cu, Cd, C, N, and P together, the carrier density can be controlled while maintaining the oxygen partial pressure in an appropriate range, so that the electron carrier mobility can be controlled. It seems that the influence can be reduced.

  Therefore, it is easier to improve the in-plane uniformity of the characteristics of the oxide film even in the case of a large-area substrate than when the electron carrier concentration and the electron carrier mobility are controlled only by adjusting the oxygen partial pressure.

  As additives, Ti, Zr, V, Ru, Ge, Sn, and F described later may be used.

  Note that the concentration of impurities necessary for obtaining a desired effect is higher than that of a crystal such as Si, and is about 0.1 to 3 atomic%.

  This is probably because the film is amorphous and the probability that impurity atoms enter the site effective for valence electron control is lower than that of crystals.

The most common method for introducing impurities is to contain the desired impurities in the target to be used. In the case of C, N, or P, a gas such as CH 4 , NO, or PH 3 is used together with oxygen. It can be introduced into the film in an atmosphere. When the impurity is a metal element, the transparent amorphous oxide film can be formed and then brought into contact with a solution or paste containing metal ions. In the case of using a substrate having high heat resistance such as glass, these elements can be included in the substrate, and the substrate can be heated and diffused into the transparent amorphous oxide film during or after the formation of the film. For example, soda glass contains 10-20 atomic% Na and can be used as a Na source.

  FIG. 5 shows a typical TFT element structure, and it is effective to reduce the electron carrier density here in a portion of the channel layer 2 sandwiched between the drain electrode 5 and the source electrode 6. On the other hand, in the portion of the channel layer 2 that contacts the drain electrode 5 and the source electrode 6, the higher the electron carrier density, the better the electrical contact with the electrode, which is advantageous. That is, it is preferable that the impurity concentration is lower in this portion. In such a configuration, after the drain electrode 5 and the source electrode 6 are formed, before the gate insulating film 3 is formed, the channel layer 2 is brought into contact with a solution containing impurities, and the drain electrode 5 and the source electrode 6 are used as impurities as a mask. Can be realized by a method of diffusing.

Further, in the channel layer 2 of FIG. 5, it is difficult to control the electron carrier density by the gate electrode 4 particularly in the portion in contact with the substrate. Therefore, if the electron carrier density in this portion is kept low beforehand, the on / off ratio is increased. To help. Therefore, it is effective to increase the concentration of the impurities particularly at the interface with the substrate. In such a configuration, the channel layer is formed while the concentration of gases such as CH 4 , NO, and PH 3 introduced into the atmosphere is started from an excessive level and gradually decreased, or the Na contained in the substrate is reduced. It can also be realized by a method of diffusing at an appropriate temperature.

  As an additive, the amorphous oxide may contain one or more selected from Ti, Zr, V, Ru, Ge, Sn, and F as impurities.

Thereby, the electron carrier concentration is less than 10 18 / cm 3 , and at the same time, the effect of increasing the electron mobility to 1 cm 2 / (V · second) or more and further to 5 cm 2 / (V · second) or more is expected .

  Even when the field effect mobility is increased, the electron carrier concentration is rarely increased.

  Therefore, if the transparent amorphous oxide film is used for the channel layer, a TFT having a high on / off ratio, a large saturation current at the time of pinch-off, and a high switching speed can be obtained.

  In addition, even if the electron carrier density and electron carrier mobility are controlled only by adjusting the oxygen partial pressure, the influence on the electron carrier density is less, and even in the case of using a large area substrate, the in-plane uniformity of the oxide film characteristics is improved. Easy to increase.

  Although the details of the mechanism are unknown, an oxide formed with a high oxygen partial pressure has a high tail level density below the conduction band, which may reduce mobility.

  It is assumed that by introducing Ti, Zr, V, Ru, Ge, Sn, and F, the electron carrier mobility is increased while maintaining the electron carrier density because it acts on the skeleton of the atomic bond and reduces the tail level. Is done.

  These series of impurities are preferably used in the range of about 0.1 to 3 atomic%, or 0.01 to 1 atomic%.

  Atomic% is the ratio of the number of atoms of each constituent element contained in the oxide. In addition, when it is difficult to measure the amount of oxygen, the above-described range may be the ratio of the number of atoms of each constituent element excluding oxygen.

As a method for introducing the impurity, it is most common to include a target in a target using a desired impurity. In the case of F, a gas such as SF 6 , SiF 4 , or ClF 3 is contained in the atmosphere together with oxygen. It can be introduced into the membrane. When the impurity is a metal element, the transparent amorphous oxide film can be formed and then brought into contact with a solution or paste containing metal ions.

In the typical TFT device structure shown in FIG. 5, a portion that is particularly in contact with the gate insulating film 3 is required to have high mobility. Therefore, it is effective to increase the impurity concentration of the present invention particularly at the interface with the gate insulating film 3. Such a configuration can be realized by a method of starting a gas such as SF 6 , SiF 4 , and ClF 3 introduced into the atmosphere when the channel layer is formed from a low level and gradually increasing the gas.

  In the present invention, as a premise, it is important to control the amount of oxygen (the amount of oxygen vacancies) so that atomic bonds have an appropriate structure.

  In the above description, the oxygen amount of the transparent oxide film is controlled by performing it in an atmosphere containing oxygen at a predetermined concentration during film formation. However, after the film formation, the oxide film is placed in an atmosphere containing oxygen. It is also preferable to control (reduce or increase) the amount of oxygen deficiency by post-treatment.

  In order to effectively control the oxygen deficiency, the temperature in the atmosphere containing oxygen is 0 ° C. or higher and 300 ° C. or lower, preferably 25 ° C. or higher and 250 ° C. or lower, more preferably 100 ° C. or higher and 200 ° C. or lower. Is good.

Needless to say, the film formation may be performed in an atmosphere containing oxygen, and the post-treatment after the film formation may be performed in the atmosphere containing oxygen. If a predetermined electron carrier concentration (less than 10 18 / cm 3 ) can be obtained, oxygen partial pressure control is not performed during film formation, and post-treatment after film formation is performed in an atmosphere containing oxygen. Also good.

Note that the lower limit of the electron carrier concentration in the present invention is, for example, 10 14 / cm 3 or more, although it depends on what kind of element, circuit or device the oxide film obtained is used for.

Hereinafter, the amorphous oxide applicable to the first to third embodiments will be described in detail. In the invention according to the first embodiment, a condition of light irradiation, for example, is added to the amorphous oxide shown below or the manufacturing method thereof. In the second embodiment, the device as described in the examples is devised so that the film composition changes. In the invention according to the third embodiment, in addition to the manufacturing conditions shown below, an impurity addition gas or target material is used, or impurities are added to the amorphous oxide shown below by a predetermined method. Will do.
(Amorphous oxide)
The electron carrier concentration of the amorphous oxide according to the present invention is a value when measured at room temperature. The room temperature is, for example, 25 ° C., specifically, a certain temperature appropriately selected from the range of about 0 ° C. to 40 ° C. Note that the electron carrier concentration of the amorphous oxide according to the present invention does not need to satisfy less than 10 18 / cm 3 in the entire range of 0 ° C. to 40 ° C. For example, a carrier electron density of less than 10 18 / cm 3 may be realized at 25 ° C. Further, when the electron carrier concentration is further reduced to 10 17 / cm 3 or less, more preferably 10 16 / cm 3 or less, a normally-off TFT can be obtained with a high yield.

The term “less than 10 18 / cm 3” is preferably less than 1 × 10 18 / cm 3 , and more preferably less than 1.0 × 10 18 / cm 3 .

  The electron carrier concentration can be measured by Hall effect measurement.

  In the present invention, an amorphous oxide refers to an oxide that exhibits a halo pattern in an X-ray diffraction spectrum and does not exhibit a specific diffraction line.

The lower limit of the electron carrier concentration in the amorphous oxide of the present invention is not particularly limited as long as it can be applied as a TFT channel layer. The lower limit is, for example, 10 12 / cm 3 .

Therefore, in the present invention, the material, composition ratio, production conditions, etc. of the amorphous oxide are controlled as in the examples described later, for example, the electron carrier concentration is 10 12 / cm 3 or more and 10 18 / cm 3. Less than. More preferably, it is in the range of 10 13 / cm 3 or more and 10 17 / cm 3 or less, and more preferably 10 15 / cm 3 or more and 10 16 / cm 3 or less.

As the amorphous oxide, in addition to InZnGa oxide, In oxide, In x Zn 1-x oxide (0.2 ≦ x ≦ 1), In x Sn 1-x oxide (0.8 ≦ x ≦ 1) or In x (Zn, Sn) 1-x oxide (0.15 ≦ x ≦ 1).

Note that an In x (Zn, Sn) 1-x oxide can be described as an In x (Zn y Sn 1-y ) 1-x oxide, and the range of y is 1 to 0.

Note that in the case of an In oxide containing no Zn and Sn, part of In can be substituted with Ga. That is, it is the case of In x Ga 1-x oxide (0 ≦ x ≦ 1).

Hereinafter, an amorphous oxide having an electron carrier concentration of less than 10 18 / cm 3 successfully produced by the present inventors will be described in detail.

The oxide includes In—Ga—Zn—O, the composition in the crystalline state is represented by InGaO 3 (ZnO) m (m is a natural number less than 6), and the electron carrier concentration is 10 18 / cm 3. It is characterized by being less than.

The oxide includes In—Ga—Zn—Mg—O, and the composition of the crystalline state is InGaO 3 (Zn 1−x Mg × O) m (m is a natural number less than 6, 0 <x ≦ 1 The electron carrier concentration is less than 10 18 / cm 3 .

Note that it is also preferable to design a film formed using these oxides so that the electron mobility exceeds 1 cm 2 / (V · sec).

When the above film is used for a channel layer, transistor characteristics with a normally-off gate current of less than 0.1 microampere and an on / off ratio of more than 10 3 can be realized. In addition, a flexible TFT having transparency or translucency with respect to visible light is realized.

  The film is characterized in that the electron mobility increases as the number of conduction electrons increases. As the substrate on which the transparent film is formed, a glass substrate, a resin plastic substrate, a plastic film, or the like can be used.

When the amorphous oxide film is used as a channel layer, a gate insulating layer of a mixed crystal compound containing at least one of Al 2 O 3 , Y 2 O 3 , or HfO 2 or a compound thereof is used. Available for membranes.

  In addition, it is also preferable to form a film in an atmosphere containing oxygen gas without intentionally adding impurity ions for increasing electric resistance to the amorphous oxide.

  The present inventors have found that the semi-insulating oxide amorphous thin film has a unique characteristic that the electron mobility increases as the number of conduction electrons increases. Then, a TFT was formed using the film, and it was found that transistor characteristics such as an on / off ratio, a saturation current in a pinch-off state, and a switch speed were further improved. That is, it has been found that a normally-off type TFT can be realized by using an amorphous oxide.

When an amorphous oxide thin film is used as the channel layer of the film transistor, the electron mobility can exceed 1 cm 2 / (V · sec), preferably 5 cm 2 / (V · sec).

When the electron carrier concentration is less than 10 18 / cm 3 , preferably less than 10 16 / cm 3 , the current between the drain and source terminals when off (when no gate voltage is applied) is less than 10 microamperes, Preferably it can be less than 0.1 microamperes.

When the film is used, when the electron mobility is more than 1 cm 2 / (V · sec), preferably more than 5 cm 2 / (V · sec), the saturation current after pinch-off can be more than 10 microamperes, The on / off ratio can be greater than 10 3 .

  In the TFT, in a pinch-off state, a high voltage is applied to the gate terminal, and high-density electrons exist in the channel.

  Therefore, according to the present invention, the saturation current value can be further increased by the amount of increase in electron mobility. As a result, improvements in transistor characteristics such as an increase in on / off ratio, an increase in saturation current, and an increase in switching speed can be expected.

  In a normal compound, when the number of electrons increases, electron mobility decreases due to collisions between electrons.

  The TFT structure includes a stagger (top gate) structure in which a gate insulating film and a gate terminal are sequentially formed on a semiconductor channel layer, or a reverse structure in which a gate insulating film and a semiconductor channel layer are sequentially formed on a gate terminal. A staggered (bottom gate) structure can be used.

(First film formation method: PLD method)
An amorphous oxide thin film whose composition in the crystalline state is represented by InGaO 3 (ZnO) m (m is a natural number less than 6) is amorphous up to a high temperature of 800 ° C. or higher when the value of m is less than 6. Although the quality state is kept stable, it becomes easier to crystallize as the value of m increases. That is, as the ratio of ZnO to InGaO 3 increases and approaches the ZnO composition, it becomes easier to crystallize.

  Therefore, the value of m is preferably less than 6 for the channel layer of the amorphous TFT.

As a film forming method, a vapor phase film forming method is preferably used with a polycrystalline sintered body having an InGaO 3 (ZnO) m composition as a target. Of the vapor deposition methods, sputtering and pulsed laser deposition are suitable. Furthermore, the sputtering method is most suitable from the viewpoint of mass productivity.

However, when the amorphous film is formed under normal conditions, oxygen vacancies mainly occur, and until now, the electron carrier concentration has been less than 10 18 / cm 3 and the electric conductivity has not been reduced to 10 S / cm or less. It was. When such a film is used, a normally-off transistor cannot be formed.

  The present inventors produced In—Ga—Zn—O produced by a pulse laser deposition method using the apparatus shown in FIG.

  Film formation was performed using a PLD film formation apparatus as shown in FIG.

  In the figure, 701 is RP (rotary pump), 702 is TMP (turbo molecular pump), 703 is a preparation chamber, 704 is an electron gun for RHEED, 705 is a substrate holding means for rotating and moving the substrate up and down, and 706 is This is a laser incident window. 707 is a substrate, 708 is a target, 709 is a radical source, 710 is a gas inlet, and 711 is a target holding means for rotating and moving the target up and down. Reference numeral 712 denotes a bypass line, 713 denotes a main line, 714 denotes a TMP (turbo molecular pump), 715 denotes an RP (rotary pump), 716 denotes a titanium getter pump, and 717 denotes a shutter. In the figure, 718 is an IG (ion vacuum gauge), 719 is a PG (Pirani vacuum gauge), 720 is a BG (Baratron vacuum gauge), and 721 is a growth chamber (chamber).

An In-Ga-Zn-O amorphous oxide semiconductor thin film was deposited on a SiO 2 glass substrate (Corning 1737) by pulsed laser deposition using a KrF excimer laser. As a pre-deposition treatment, the substrate was degreased and cleaned with ultrasonic waves for 5 minutes each using acetone, ethanol, and ultrapure water, and then dried at 100 ° C. in air.

As the polycrystalline target, an InGaO 3 (ZnO) 4 sintered body target (size 20 mmΦ5 mmt) was used. This is because, as a starting material, In 2 O 3 : Ga 2 O 3 : ZnO (each 4N reagent) is wet-mixed (solvent: ethanol), calcined (1000 ° C .: 2 h), dry pulverized, main sintered ( 1550 ° C: 2 hours). The electric conductivity of the target thus prepared was 90 (S / cm).

The film was formed while the ultimate vacuum in the growth chamber was 2 × 10 −6 (Pa) and the oxygen partial pressure during growth was controlled to 6.5 (Pa).

  The partial pressure of oxygen in the chamber 721 is 6.5 Pa, and the substrate temperature is 25 ° C.

The distance between the target 708 and the film formation substrate 707 is 30 (mm), and the power of the KrF excimer laser incident from the incident window 716 is 1.5-3 (mJ / cm 2 / pulse). It is a range. The pulse width was 20 (nsec), the repetition frequency was 10 (Hz), and the irradiation spot diameter was 1 × 1 (mm square).

  Thus, film formation was performed at a film formation rate of 7 (nm / min).

  The thin film obtained was subjected to grazing incidence X-ray diffraction (thin film method, incident angle 0.5 degree) of the thin film, and no clear diffraction peak was observed. Thus, the produced In-Ga-Zn-O thin film Can be said to be amorphous.

  Furthermore, as a result of measuring the X-ray reflectivity and analyzing the pattern, it was found that the mean square roughness (Rrms) of the thin film was about 0.5 nm and the film thickness was about 120 nm. As a result of X-ray fluorescence (XRF) analysis, the metal composition ratio of the thin film was In: Ga: Zn = 0.98: 1.02: 4.

The electrical conductivity was less than about 10-2 S / cm. The electron carrier concentration is estimated to be about 10 16 / cm 3 or less, and the electron mobility is estimated to be about 5 cm 2 / (V · sec).

From the analysis of the light absorption spectrum, the energy band gap of the fabricated amorphous thin film was found to be about 3 eV. From the above, the fabricated In-Ga-Zn-O-based thin film exhibits an amorphous phase close to the composition of crystalline InGaO 3 (ZnO) 4 , has a small oxygen deficiency, and has a low electrical conductivity and is a transparent flat surface. It turned out to be a thin film.

This will be specifically described with reference to FIG. This figure shows a transparent amorphous oxide thin film composed of In-Ga-Zn-O and having a composition expressed by InGaO 3 (ZnO) m (m is a number less than 6) assuming a crystalline state. It is a characteristic view in the case of creating under the same conditions as the example. This characteristic diagram shows the change in the electron carrier concentration of the deposited oxide when the oxygen partial pressure is changed.

By forming a film in an atmosphere where the oxygen partial pressure is higher than 4.5 Pa under the same conditions as in this example, the electron carrier concentration can be reduced to less than 10 18 / cm 3 as shown in FIG. did it. In this case, the temperature of the substrate is maintained at substantially room temperature without intentionally heating. In order to use a flexible plastic film as a substrate, the substrate temperature is preferably kept below 100 ° C.

If the oxygen partial pressure is further increased, the electron carrier concentration can be further reduced. For example, as shown in FIG. 1, in the InGaO 3 (ZnO) 4 thin film formed at a substrate temperature of 25 ° C. and an oxygen partial pressure of 5 Pa, the number of electron carriers could be further reduced to 10 16 / cm 3 .

The obtained thin film had an electron mobility of more than 1 cm 2 / (V · sec) as shown in FIG. However, in the pulse laser vapor deposition method of the present embodiment, when the oxygen partial pressure is set to 6.5 Pa or more, the surface of the deposited film becomes uneven, making it difficult to use it as a TFT channel layer.

Therefore, it is expressed by the composition InGaO 3 (ZnO) m (m is a number of less than 6) in a crystalline state by pulse laser deposition in an atmosphere having an oxygen partial pressure of more than 4.5 Pa, desirably more than 5 Pa and less than 6.5 Pa. A transparent amorphous oxide thin film is prepared. If this transparent amorphous oxide thin film is used, a normally-off transistor can be formed.

Further, the electron mobility of the thin film was obtained to exceed 1 cm 2 / V · second, and the on / off ratio could be increased to more than 10 3 .

  As described above, when an InGaZn oxide film is formed by the PLD method under the conditions shown in this embodiment, the oxygen partial pressure can be controlled to be 4.5 Pa or more and less than 6.5 Pa. desirable.

Note that, in order to realize the electron carrier concentration of less than 10 18 / cm 3 , the electron carrier concentration depends on the oxygen partial pressure conditions, the configuration of the film formation apparatus, the material and composition of the film formation, and the like.

Next, an amorphous oxide was produced under the condition of an oxygen partial pressure of 6.5 Pa in the above apparatus, and a top gate type MISFET element shown in FIG. 5 was produced. Specifically, first, a semi-insulating amorphous InGaO 3 (ZnO) having a thickness of 120 nm used as a channel layer (2) is formed on the glass substrate (1) by the above-described method for producing an amorphous In—Ga—Zn—O thin film. Four films were formed.

Further, an InGaO 3 (ZnO) 4 film and a gold film having a large electric conductivity were stacked in a thickness of 30 nm by a pulse laser deposition method with an oxygen partial pressure in the chamber of less than 1 Pa. And the drain terminal (5) and the source terminal (6) were formed by the photolithographic method and the lift-off method. Finally, a Y 2 O 3 film used as the gate insulating film (3) is formed by electron beam evaporation (thickness: 90 nm, relative dielectric constant: about 15, leakage current density: 10 -3 A when 0.5 MV / cm is applied) / cm 2 ), and a gold film was formed thereon. And the gate terminal (4) was formed by the photolithographic method and the lift-off method.

FIG. 6 shows the current-voltage characteristics of the MISFET element measured at room temperature. As the drain voltage V DS increases, the drain current I DS increases, which indicates that the channel is an n-type semiconductor. This is consistent with the fact that amorphous In-Ga-Zn-O based semiconductors are n-type. I DS shows the behavior of a typical semiconductor transistor that saturates (pinch off) at about V DS = 6 V. When the gain characteristic was examined, the threshold value of the gate voltage V GS when V DS = 4 V was applied was about −0.5 V. When V G = 10 V, a current of I DS = 1.0 × 10 −5 A flowed. This corresponds to the fact that carriers can be induced in the In-Ga-Zn-O amorphous semiconductor thin film of the insulator by the gate bias.

The on / off ratio of the transistor was more than 10 3 . When the field effect mobility was calculated from the output characteristics, a field effect mobility of about 7 cm 2 (Vs) −1 was obtained in the saturation region. A similar measurement was performed by irradiating the fabricated device with visible light, but no change in transistor characteristics was observed.

  According to this embodiment, it is possible to realize a thin film transistor having a channel layer having a low electron carrier concentration, a high electrical resistance, and a high electron mobility.

  The above-described amorphous oxide had excellent characteristics that the electron mobility increased with an increase in the electron carrier concentration and further exhibited degenerate conduction.

  In this embodiment, a thin film transistor is formed on a glass substrate. However, since the film formation itself can be performed at room temperature, a substrate such as a plastic plate or a film can be used.

Further, the amorphous oxide obtained in this example hardly absorbs visible light and can realize a transparent flexible TFT.
(Second film formation method: sputtering method (SP method))
A case where a film is formed by a high-frequency SP method using argon gas as an atmosphere gas will be described.

  The SP method was performed using the apparatus shown in FIG. In the figure, reference numeral 807 denotes a film formation substrate, 808 denotes a target, 805 denotes a substrate holding means with a cooling mechanism, 814 denotes a turbo molecular pump, 815 denotes a rotary pump, and 817 denotes a shutter. Reference numeral 818 denotes an ion vacuum gauge, 819 denotes a Pirani vacuum gauge, 821 denotes a growth chamber (chamber), and 830 denotes a gate valve.

As the film formation substrate 807, a SiO 2 glass substrate (1737 manufactured by Corning) was prepared. As pre-deposition treatment, the substrate was subjected to ultrasonic degreasing and cleaning with acetone, ethanol, and ultrapure water for 5 minutes each and then dried at 100 ° C. in air.

As a target material, a polycrystalline sintered body (size 20 mmΦ5 mmt) having an InGaO 3 (ZnO) 4 composition was used.

In this sintered body, as a starting material, In 2 O 3 : Ga 2 O 3 : ZnO (each 4N reagent) is wet-mixed (solvent: ethanol), calcined (1000 ° C: 2h), dry pulverized, main-fired It was produced after crystallization (1550 ° C: 2h). The electric conductivity of the target 808 was 90 (S / cm) and was in a semi-insulating state.

The ultimate vacuum in the growth chamber 821 is 1 × 10 −4 (Pa), and the total pressure of oxygen gas and argon gas during growth is constant in the range of 4 to 0.1 × 10 −1 (Pa). The value of And the partial pressure ratio of argon gas and oxygen was changed, and the oxygen partial pressure was changed in the range of 10 < -3 > -2 * 10 <-1> (Pa).

  The substrate temperature was room temperature, and the distance between the target 808 and the deposition target substrate 807 was 30 (mm).

  The input power was RF 180 W, and the film formation rate was 10 (nm / min).

  With respect to the obtained film, grazing incidence X-ray diffraction (thin film method, incident angle 0.5 degree) was performed on the film surface, but no clear diffraction peak was detected, and the produced In—Zn—Ga—O-based film was amorphous. It was shown to be a membrane.

  Furthermore, as a result of measuring the X-ray reflectivity and analyzing the pattern, it was found that the mean square roughness (Rrms) of the thin film was about 0.5 nm and the film thickness was about 120 nm. As a result of X-ray fluorescence (XRF) analysis, the metal composition ratio of the thin film was In: Ga: Zn = 0.98: 1.02: 4.

  The oxygen partial pressure of the atmosphere during film formation was changed, and the electrical conductivity of the obtained amorphous oxide film was measured. The result is shown in FIG.

As shown in FIG. 3, the electrical conductivity could be reduced to less than 10 S / cm by forming a film in an atmosphere having a high oxygen partial pressure exceeding 3 × 10 −2 Pa.

  By further increasing the oxygen partial pressure, the number of electron carriers could be reduced.

For example, as shown in FIG. 3, in an InGaO 3 (ZnO) 4 thin film formed at a substrate temperature of 25 ° C. and an oxygen partial pressure of 10 −1 Pa, the electrical conductivity is further reduced to about 10 −10 S / cm. I was able to. In addition, the InGaO 3 (ZnO) 4 thin film formed at an oxygen partial pressure exceeding 10 −1 Pa had an electrical resistance that was too high to measure the electrical conductivity. In this case, although the electron mobility could not be measured, the electron mobility was estimated to be about 1 cm 2 / V · second by extrapolating from the value in the film having a high electron carrier concentration.

That is, it is composed of In—Ga—Zn—O produced by sputter deposition in an argon gas atmosphere with an oxygen partial pressure of over 3 × 10 −2 Pa, preferably over 5 × 10 −1 Pa, and the composition InGaO in the crystalline state. A transparent amorphous oxide thin film represented by 3 (ZnO) m (m is a natural number of less than 6) was prepared. Using this transparent amorphous oxide thin film, a transistor having a normally-off and an on / off ratio exceeding 10 3 could be constructed.

In the case of using the apparatus and materials shown in this embodiment, the oxygen partial pressure during film formation by sputtering is, for example, in the range of 3 × 10 −2 Pa to 5 × 10 −1 Pa. In the thin film formed by the pulse laser deposition method and the sputtering method, as shown in FIG. 2, the electron mobility increases as the number of conduction electrons increases.

  As described above, by controlling the oxygen partial pressure, oxygen defects can be reduced, and as a result, the electron carrier concentration can be reduced. In the amorphous state, unlike the polycrystalline state, there is essentially no particle interface, so that an amorphous thin film with high electron mobility can be obtained.

Even when a polyethylene terephthalate (PET) film having a thickness of 200 μm was used instead of the glass substrate, the obtained InGaO 3 (ZnO) 4 amorphous oxide film showed similar characteristics.

If polycrystalline InGaO 3 (Zn 1-x Mg x O) m (m is a natural number less than 6 and 0 <x ≦ 1) is used as a target, a high-resistance amorphous material even under an oxygen partial pressure of less than 1 Pa. An InGaO 3 (Zn 1-x Mg x O) m film can be obtained.

For example, when a target in which Zn is replaced with 80 at% Mg is used, the electron carrier concentration of the film obtained by the pulse laser deposition method is less than 10 16 / cm 3 in an atmosphere with an oxygen partial pressure of 0.8 Pa. (The electric resistance value is about 10 −2 S / cm).

The electron mobility of such a film is lower than that of the Mg-free film, but the degree is small, and the electron mobility at room temperature is about 5 cm 2 / (V · sec), which is one digit that of amorphous silicon. A large value is shown. When the film is formed under the same conditions, both the electrical conductivity and the electron mobility decrease as the Mg content increases, so the Mg content is preferably more than 20% and less than 85% (x). 0.2 <x <0.85).

In the thin film transistor using the above-described amorphous oxide film, a gate insulating film is preferably formed using a mixed crystal compound containing at least two of Al 2 O 3 , Y 2 O 3 , HfO 2 , or a compound thereof.

If there is a defect at the interface between the gate insulating thin film and the channel layer thin film, the electron mobility is lowered and the transistor characteristics are hysteresis. Further, the leakage current varies greatly depending on the type of the gate insulating film. For this purpose, it is necessary to select a gate insulating film suitable for the channel layer. If an Al 2 O 3 film is used, leakage current can be reduced. Further, the hysteresis can be reduced by using a Y 2 O 3 film. Further, if a high dielectric constant HfO 2 film is used, the electron mobility can be increased. Further, by using mixed crystals of these films, a TFT with small leakage current and hysteresis and high electron mobility can be formed. In addition, since the gate insulating film formation process and the channel layer formation process can be performed at room temperature, both a staggered structure and an inverted staggered structure can be formed as the TFT structure.

  The TFT thus formed is a three-terminal element having a gate terminal, a source terminal, and a drain terminal. In this TFT, a semiconductor thin film formed on an insulating substrate such as ceramics, glass, or plastic is used as a channel layer through which electrons or holes move. Further, the TFT is an active element having a function of switching a current between the source terminal and the drain terminal by applying a voltage to the gate terminal to control a current flowing in the channel layer.

  It is important in the present invention that the desired electron carrier concentration can be achieved by controlling the oxygen deficiency.

  In the above description, the amount of oxygen (oxygen deficiency) in the amorphous oxide film is controlled by performing it in an atmosphere containing oxygen at a predetermined concentration during film formation. However, it is also preferable to control (reduce or increase) the amount of oxygen vacancies after film formation by post-processing the oxide film in an atmosphere containing oxygen.

  In order to effectively control the oxygen deficiency, the temperature in the atmosphere containing oxygen is 0 ° C. or higher and 300 ° C. or lower, preferably 25 ° C. or higher and 250 ° C. or lower, more preferably 100 ° C. or higher and 200 ° C. or lower. Is good.

Needless to say, the film formation may be performed in an atmosphere containing oxygen, and the post-treatment after the film formation may be performed in the atmosphere containing oxygen. If a predetermined electron carrier concentration (less than 10 18 / cm 3 ) can be obtained, oxygen partial pressure control is not performed during film formation, and post-treatment after film formation is performed in an atmosphere containing oxygen. Also good.

Note that the lower limit of the electron carrier concentration in the present invention is, for example, 10 14 / cm 3 or more, although it depends on what kind of element, circuit or device the oxide film obtained is used for.
(Expansion of materials)
Furthermore, as a result of expanding the composition system and researching it, an amorphous oxide composed of an oxide of at least one of Zn, In and Sn, an amorphous material with a low electron carrier concentration and a high electron mobility. It has been found that an oxide film can be produced.

  Further, the present inventors have found that this amorphous oxide film has a unique characteristic that the electron mobility increases as the number of conduction electrons increases.

  A TFT is formed using the film, and a normally-off type TFT excellent in transistor characteristics such as an on / off ratio, a saturation current in a pinch-off state, and a switch speed can be formed.

In the present invention, an oxide having the following characteristics (a) to (h) can be used.
(A) An amorphous oxide having an electron carrier concentration at room temperature of less than 10 18 / cm 3 .
(B) An amorphous oxide characterized by an increase in electron carrier concentration and an increase in electron mobility.

Here, room temperature refers to a temperature of about 0 ° C. to 40 ° C. Amorphous refers to a compound in which only a halo pattern is observed in an X-ray diffraction spectrum and does not show a specific diffraction line. Moreover, the electron mobility here means the electron mobility obtained by Hall effect measurement.
(C) The amorphous oxide described in the above (a) or (b), wherein the electron mobility at room temperature is more than 0.1 cm 2 / V · sec.
(D) The amorphous oxide described in any one of (b) to (c) above showing degenerate conduction. Here, degenerate conduction refers to a state in which the thermal activation energy in the temperature dependence of electrical resistance is 30 meV or less.
(E) The amorphous oxide described in any one of (a) to (d) above, which contains at least one element of Zn, In, and Sn as a constituent component.
(F) To the amorphous oxide described in (e) above, the Group 2 element M2 having an atomic number smaller than Zn (M2 is Mg, Ca), the Group 3 element M3 having an atomic number smaller than In (M3 is B, Among Al, Ga, Y), Sn group 4 element M4 (M4 is Si, Ge, Zr), group 5 element M5 (M5 is V, Nb, Ta) and Lu, W An amorphous oxide film containing at least one element.
(G) the crystal composition in a state that In 1-x M3 x O 3 (Zn 1-y M2 y O) m (0 ≦ x, y ≦ 1, m is 0 or less than 6 natural number) is a compound alone or m The amorphous oxide film according to any one of (a) to (f), which is a mixture of different compounds. M3 is, for example, Ga, and M2 is, for example, Mg.
(h) The amorphous oxide film according to the above (a) to (g) provided on a glass substrate, metal substrate, plastic substrate or plastic film.

  The present invention is (10) a field effect transistor using the amorphous oxide or the amorphous oxide film described above as a channel layer.

Note that a field effect type in which an amorphous oxide film having an electron carrier concentration of less than 10 18 / cm 3 and more than 10 15 / cm 3 is used for a channel layer, and a gate terminal is arranged via a source terminal, a drain terminal, and a gate insulating film. A transistor is formed. When a voltage of about 5 V is applied between the source and drain terminals, the current between the source and drain terminals when no gate voltage is applied can be about 10 −7 ampere.

The electron mobility of the oxide crystal increases as the s orbital overlap of the metal ions increases, and the oxide crystal of Zn, In, Sn having a large atomic number has a value of 0.1 to 200 cm 2 / (V · sec). It has a large electron mobility.

  Further, in the oxide, oxygen and metal ions are ionically bonded.

  Therefore, even in the amorphous state where there is no chemical bond directionality, the structure is random, and the bond direction is non-uniform, the electron mobility should be comparable to the electron mobility in the crystalline state. Is possible.

On the other hand, by substituting Zn, In, and Sn with an element having a small atomic number, the electron mobility is reduced. As a result, the electron mobility of the amorphous oxide according to the present invention is about 0.01 cm 2 / (V · second) to 20 cm 2 / (V · second).

In the case where a channel layer of a transistor is formed using the above oxide, in the transistor, a mixed crystal compound containing at least two of Al 2 O 3 , Y 2 O 3 , HfO 2 , or a compound thereof is used as a gate insulating film. It is preferable.

If there is a defect at the interface between the gate insulating thin film and the channel layer thin film, the electron mobility is lowered and the transistor characteristics are hysteresis. Further, the leakage current varies greatly depending on the type of the gate insulating film. For this purpose, it is necessary to select a gate insulating film suitable for the channel layer. If an Al 2 O 3 film is used, leakage current can be reduced. Further, the hysteresis can be reduced by using a Y 2 O 3 film. Furthermore, if a high dielectric constant HfO 2 film is used, the field effect mobility can be increased. In addition, by using a film made of a mixed crystal of these compounds, a TFT with small leakage current and hysteresis and high field effect mobility can be formed. In addition, since the gate insulating film formation process and the channel layer formation process can be performed at room temperature, both a staggered structure and an inverted staggered structure can be formed as the TFT structure.

The In 2 O 3 oxide film can be formed by a vapor phase method, and an amorphous film can be obtained by adding about 0.1 Pa of moisture to the atmosphere during film formation.

In addition, although it is difficult to obtain an amorphous film of ZnO and SnO 2 , an amorphous film can be obtained by adding In 2 O 3 to about 20 atomic% in the case of ZnO and about 90 atomic% in the case of SnO 2. Can be obtained. In particular, in order to obtain a Sn—In—O-based amorphous film, nitrogen gas may be introduced into the atmosphere at about 0.1 Pa.

  From the group II element M2 (M2 is Mg, Ca) having an atomic number smaller than Zn and the group 3 element M3 (M3 is B, Al, Ga, Y), Sn having an atomic number smaller than In Consists of at least one complex oxide of group 4 element M4 having a small atomic number (M4 is Si, Ge, Zr), group 5 element M5 (M5 is V, Nb, Ta) and Lu, W Can be added.

  Thereby, the amorphous film at room temperature can be further stabilized. Moreover, the composition range in which an amorphous film is obtained can be expanded.

  In particular, the addition of B, Si, and Ge, which has strong covalent bonding, is effective for stabilizing the amorphous phase, and the complex phase composed of ions having a large difference in ionic radius stabilizes the amorphous phase.

  For example, in the case of the In—Zn—O system, it is difficult to obtain an amorphous film that is stable at room temperature unless In is in a composition range of more than about 20 atomic%. With this composition range, a stable amorphous film can be obtained.

In film formation by a vapor phase method, an amorphous oxide film having an electron carrier concentration of less than 10 18 / cm 3 and more than 10 15 / cm 3 can be obtained by controlling the atmosphere.

As a film formation method of the amorphous oxide, it is preferable to use a vapor phase method such as a pulse laser deposition method (PLD method), a sputtering method (SP method), or an electron beam evaporation method. Among the gas phase methods, the PLD method is suitable from the viewpoint of easily controlling the composition of the material system, and the SP method is suitable from the viewpoint of mass productivity. However, the film forming method is not limited to these methods.
(Formation of In-Zn-Ga-O-based amorphous oxide film by PLD method)
An In—Zn—Ga—O amorphous oxide film was deposited on a glass substrate (1737 manufactured by Corning) by a PLD method using a KrF excimer laser. At this time, polycrystalline sintered bodies having InGaO 3 (ZnO) and InGaO 3 (ZnO) 4 compositions were used as targets, respectively.

  As the film forming apparatus, the apparatus described in FIG. 9 described above was used, and the film forming conditions were the same as in the case of using the apparatus.

  The substrate temperature is 25 ° C. With respect to the obtained film, grazing incidence X-ray diffraction (thin film method, incident angle 0.5 degree) was performed on the film surface, and no clear diffraction peak was detected. In—Zn—Ga— produced from two types of targets All of the O-based films were shown to be amorphous films.

  Furthermore, the X-ray reflectivity measurement of the In—Zn—Ga—O-based amorphous oxide film on the glass substrate was performed and the pattern was analyzed. As a result, the mean square roughness (Rrms) of the thin film was about 0.5 nm. The film thickness was found to be about 120 nm.

As a result of X-ray fluorescence (XRF) analysis, the metal composition ratio of a film obtained using a polycrystalline sintered body having an InGaO 3 (ZnO) composition as a target was In: Ga: Zn = 1.1: 1.1: 0. .9. The metal composition ratio of the film obtained using the polycrystalline sintered body having the InGaO (ZnO) 4 composition as a target was In: Ga: Zn = 0.98: 1.02: 4.

The oxygen partial pressure of the atmosphere during film formation was changed, and the electron carrier concentration of the amorphous oxide film obtained using a polycrystalline sintered body having an InGaO 3 (ZnO) 4 composition as a target was measured. The result is shown in FIG. By forming a film in an atmosphere having an oxygen partial pressure of over 4.2 Pa, the electron carrier concentration could be lowered to less than 10 18 / cm 3 . In this case, the temperature of the substrate is maintained at substantially room temperature without intentionally heating. When the oxygen partial pressure was less than 6.5 Pa, the surface of the obtained amorphous oxide film was flat.

When the oxygen partial pressure is 5 Pa, the electron carrier concentration of an amorphous oxide film obtained using a polycrystalline sintered body having an InGaO 3 (ZnO) 4 composition as a target is 10 16 / cm 3 , and the electric conductivity is 10 −2. S / cm. The electron mobility was estimated to be about 5 cm 2 / V · sec. From the analysis of the light absorption spectrum, the band gap energy width of the fabricated amorphous oxide film was found to be about 3 eV.

Increasing the oxygen partial pressure further reduced the electron carrier concentration. As shown in FIG. 1, an In—Zn—Ga—O-based amorphous oxide film formed at a substrate temperature of 25 ° C. and an oxygen partial pressure of 6 Pa has an electron carrier concentration of 8 × 10 15 / cm 3 (electric conduction: about 8 × 10 −3 S / cm). The obtained film was estimated to have an electron mobility exceeding 1 cm 2 / (V · sec). However, in the PLD method, when the oxygen partial pressure is set to 6.5 Pa or more, the surface of the deposited film becomes uneven, making it difficult to use as a TFT channel layer.

Regarding the In—Zn—Ga—O amorphous oxide film formed with different oxygen partial pressures, targeting a polycrystalline sintered body having an InGaO 3 (ZnO) 4 composition, the relationship between the electron carrier concentration and the electron mobility is as follows. Examined. The result is shown in FIG. It is shown that as the electron carrier concentration increases from 10 16 / cm 3 to 10 20 / cm 3 , the electron mobility increases from about 3 cm 2 / (V · sec) to about 11 cm 2 / (V · sec). It was done. Further, with regard InGaO 3 (ZnO) amorphous oxide film obtained as a target, a polycrystalline sintered body having a composition similar trend was observed.

Even when a polyethylene terephthalate (PET) film having a thickness of 200 μm was used instead of the glass substrate, the obtained In—Zn—Ga—O-based amorphous oxide film exhibited similar characteristics.
(Formation of In-Zn-Ga-Mg-O-based amorphous oxide film by PLD method)
Polycrystalline InGaO 3 (Zn 1-x Mg x O) 4 (0 <x ≦ 1) was used as a target, and InGaO 3 (Zn 1-x Mg x O) 4 (0 <x ≦ 1) A film was formed. As the film forming apparatus, the apparatus shown in FIG. 7 was used.

As a film formation substrate, a SiO 2 glass substrate (1737 manufactured by Corning) was prepared. As a pretreatment, the substrate was subjected to ultrasonic degreasing and washing with acetone, ethanol, and ultrapure water for 5 minutes each and then dried at 100 ° C. in air. As a target, an InGa (Zn 1-x Mg x O) 4 (x = 1-0) sintered body (size 20 mmΦ5 mmt) was used.

The target is starting material In 2 O 3 : Ga 2 O 3 : ZnO: MgO (each 4N reagent), wet mixing (solvent: ethanol), calcining (1000 ° C: 2h), dry grinding, main sintering (1550 (C: 2h).

The growth chamber reaching vacuum was 2 × 10 −6 (Pa), and the oxygen partial pressure during growth was 0.8 (Pa). The substrate temperature was room temperature (25 ° C.), and the distance between the target and the deposition target substrate was 30 (mm).

The power of the KrF excimer laser is 1.5 (mJ / cm 2 / pulse), the pulse width is 20 (nsec), the repetition frequency is 10 (Hz), and the irradiation spot diameter is 1 × 1 (mm square) ). The film formation rate was 7 (nm / min).

  The atmosphere is an oxygen partial pressure of 0.8 Pa, and the substrate temperature is 25 ° C. With respect to the obtained film, grazing incidence X-ray diffraction (thin film method, incident angle 0.5 degree) was performed on the film surface, but no clear diffraction peak was detected, and the produced In—Zn—Ga—Mg—O-based film Was shown to be an amorphous film. The surface of the obtained film was flat.

The electric conductivity, electron carrier concentration, and electron mobility x of an In—Zn—Ga—Mg—O-based amorphous oxide film formed in an atmosphere having an oxygen partial pressure of 0.8 Pa using targets with different x values. The value dependency was examined.
The result is shown in FIG. It was shown that when the x value exceeds 0.4, the electron carrier concentration can be made less than 10 18 / cm 3 in the amorphous oxide film formed by the PLD method in an atmosphere having an oxygen partial pressure of 0.8 Pa. Further, in the amorphous oxide film having an x value exceeding 0.4, the electron mobility was more than 1 cm 2 / V · second.

As shown in FIG. 4, when a target in which Zn is replaced with 80 atomic% Mg is used, the electron carrier concentration of the film obtained by the pulse laser deposition method is 10 16 / in an atmosphere with an oxygen partial pressure of 0.8 Pa. It can be less than cm 3 (the electrical resistance is about 10 −2 S / cm). The electron mobility of such a film is lower than that of the Mg-free film, but the degree is small, and the electron mobility at room temperature is about 5 cm 2 / (V · sec), which is one digit that of amorphous silicon. A large value is shown. When the film is formed under the same conditions, both the electrical conductivity and the electron mobility decrease with an increase in the Mg content. Therefore, the Mg content is preferably more than 20 atomic% and less than 85 atomic% ( x is 0.2 <x <0.85), and more preferably 0.5 <x <0.85.

Even when a polyethylene terephthalate (PET) film having a thickness of 200 μm is used instead of the glass substrate, the obtained InGaO 3 (Zn 1-x Mg x O) 4 (0 <x ≦ 1) amorphous oxide film is Showed similar characteristics.
(In 2 O 3 amorphous oxide film deposition by PLD method)
An In 2 O 3 film was formed on a 200 μm thick PET film by using a PLD method using a KrF excimer laser and targeting an In 2 O 3 polycrystalline sintered body.

The apparatus shown in FIG. 7 was used. A SiO 2 glass substrate (1737 manufactured by Corning) was prepared as a film formation substrate.

  As a pretreatment of this substrate, ultrasonic degreasing was performed for 5 minutes each with acetone, ethanol, and ultrapure water, and then dried at 100 ° C. in air.

As a target, an In 2 O 3 sintered body (size 20 mmΦ5 mmt) was used. This was prepared by calcining the starting material In 2 O 3 (4N reagent) through calcining (1000 ° C .: 2 h), dry grinding, and main sintering (1550 ° C .: 2 h).

The growth chamber reaching vacuum was 2 × 10 −6 (Pa), the oxygen partial pressure during growth was 5 (Pa), and the substrate temperature was room temperature.

  The oxygen partial pressure was 5 Pa, the water vapor partial pressure was 0.1 Pa, and 200 W was applied to the oxygen radical generator to generate oxygen radicals.

The distance between the target and the deposition substrate is 40 (mm), the power of the KrF excimer laser is 0.5 (mJ / cm 2 / pulse), the pulse width is 20 (nsec), the repetition frequency is 10 (Hz), The irradiation spot diameter was 1 × 1 (mm square). The film formation rate was 3 (nm / min).

  Regarding the obtained film, grazing incidence X-ray diffraction (thin film method, incident angle 0.5 degree) was performed on the film surface, and no clear diffraction peak was detected, and the produced In-O film was an amorphous film. It has been shown. The film thickness was 80 nm.

The obtained In—O amorphous oxide film had an electron carrier concentration of 5 × 10 17 / cm 3 and an electron mobility of about 7 cm 2 / V · sec.
(Formation of In-Sn-O amorphous oxide film by PLD method)
By using a PLD method using a KrF excimer laser, an (In 0.9 Sn 0.1 ) O 3.1 polycrystalline sintered body is used as a target and an In—Sn—O-based oxide film is formed on a 200 μm-thick PET film. Was deposited.
In particular,
A SiO 2 glass substrate (1737 manufactured by Corning) was prepared as a film formation substrate.
As the substrate pretreatment, ultrasonic degreasing was performed for 5 minutes each using acetone, ethanol, and ultrapure water. Then, it was dried in air at 100 ° C.
As a target, an In 2 O 3 —SnO 2 sintered body (size 20 mmΦ5 mmt) was prepared. As a starting material, In 2 O 3 -SnO 2 (4N reagent) is wet mixed (solvent: ethanol), calcined (1000 ° C: 2h), dry pulverized, and finally sintered (1550 ° C: 2h). can get.

  The substrate temperature is room temperature. The oxygen partial pressure was 5 (Pa), the nitrogen partial pressure was 0.1 (Pa), and 200 W was applied to the oxygen radical generator to generate oxygen radicals.

The distance between the target and the deposition substrate was 30 (mm), the power of the KrF excimer laser was 1.5 (mJ / cm 2 / pulse), and the pulse width was 20 (nsec). The repetition frequency was 10 (Hz), and the irradiation spot diameter was 1 × 1 (mm square). The film formation rate was 6 (nm / min).

  With respect to the obtained film, grazing incidence X-ray diffraction (thin film method, incident angle 0.5 degree) was performed on the film surface, but no clear diffraction peak was detected, and the produced In—Sn—O film was an amorphous film. It was shown that there is.

The obtained In—Sn—O amorphous oxide film had an electron carrier concentration of 8 × 10 17 / cm 3 and an electron mobility of about 5 cm 2 / V · sec. The film thickness was 100 nm.
(Formation of In-Ga-O amorphous oxide film by PLD method)
A SiO 2 glass substrate (1737 manufactured by Corning) was prepared as a film formation substrate.

  As a pretreatment of the substrate, ultrasonic degreasing cleaning was performed for 5 minutes each using acetone, ethanol, and ultrapure water, and then dried at 100 ° C. in air.

As a target, an (In 2 O 3 ) 1-x- (Ga 2 O 3 ) x (X = 0-1) sintered body (size 20 mmΦ5 mmt) was prepared. For example, when x = 0.1, the target is an (In 0.9 Ga 0.1 ) 2 O 3 polycrystalline sintered body.

This consists of starting material: In 2 O 3 -Ga 2 O 2 (4N reagent), wet mixing (solvent: ethanol), calcining (1000 ° C: 2h), dry grinding, main sintering (1550 ° C: 2h) It is obtained through

The growth chamber reaching vacuum was 2 × 10 −6 (Pa), and the oxygen partial pressure during growth was 1 (Pa).

The substrate temperature was room temperature, the distance between the target and the deposition target substrate was 30 (mm), and the power of the KrF excimer laser was 1.5 (mJ / cm 2 / pulse). The pulse width was 20 (nsec), the repetition frequency was 10 (Hz), and the irradiation spot diameter was 1 × 1 (mm square). The film formation rate was 6 (nm / min).

  The substrate temperature is 25 ° C. The oxygen partial pressure was 1 Pa. Regarding the obtained film, grazing incidence X-ray diffraction (thin film method, incident angle 0.5 degree) was performed on the film surface, but no clear diffraction peak was detected, and the produced In—Ga—O film was an amorphous film. It was shown that there is. The film thickness was 120 nm.

The obtained In—Ga—O amorphous oxide film had an electron carrier concentration of 8 × 10 16 / cm 3 and an electron mobility of about 1 cm 2 / V · sec.
(Production of TFT element using In—Zn—Ga—O amorphous oxide film (glass substrate))
Fabrication of TFT Element A top gate TFT element shown in FIG. 5 was fabricated.

First, on a glass substrate (1), a polycrystalline sintered body having an InGaO 3 (ZnO) 4 composition is used as a target, and the above-described PLD apparatus is used under the condition of an oxygen partial pressure of 5 Pa. An O-based amorphous oxide film was prepared. An In-Ga-Zn-O-based amorphous film having a thickness of 120 nm used as the channel layer (2) was formed.

  Further, an In—Ga—Zn—O-based amorphous film and a gold film having a high electric conductivity were stacked by 30 nm by the PLD method with an oxygen partial pressure in the chamber of less than 1 Pa. And the drain terminal (5) and the source terminal (6) were formed by the photolithographic method and the lift-off method.

Finally, a Y 2 O 3 film used as the gate insulating film (3) is formed by electron beam evaporation (thickness: 90 nm, relative dielectric constant: about 15, leakage current density: 10 -3 A when 0.5 MV / cm is applied) / cm 2 ), and a gold film was formed thereon. And the gate terminal (4) was formed by the photolithographic method and the lift-off method. The channel length was 50 μm and the channel width was 200 μm.

FIG. 6 shows the current-voltage characteristics of the TFT element measured at room temperature. As the drain voltage V DS increases, the drain current I DS increases, indicating that the channel is n-type conductive.

This is consistent with the fact that the amorphous In—Ga—Zn—O amorphous oxide film is an n-type conductor. I DS shows the behavior of a typical semiconductor transistor that saturates (pinch off) at about V DS = 6 V. When the gain characteristic was examined, the threshold value of the gate voltage V GS when V DS = 4 V was applied was about −0.5 V.

When V G = 10 V, a current of I DS = 1.0 × 10 −5 A flowed. This corresponds to the fact that carriers can be induced in the insulator In-Ga-Zn-O amorphous oxide film by the gate bias.

The on / off ratio of the transistor was more than 10 3 . When the field effect mobility was calculated from the output characteristics, a field effect mobility of about 7 cm 2 (Vs) −1 was obtained in the saturation region. A similar measurement was performed by irradiating the fabricated device with visible light, but no change in transistor characteristics was observed.

In addition, it can apply as a channel layer of TFT by making the electron carrier density | concentration of an amorphous oxide less than 10 < 18 > / cm < 3 >. The electron carrier concentration is more preferably 10 17 / cm 3 or less, and even more preferably 10 16 / cm 3 or less.
(Production of TFT element using In-Zn-Ga-O-based amorphous oxide film (amorphous substrate))
The top gate type TFT element shown in FIG. 5 was produced. First, an In—Zn—Ga—O-based amorphous oxide film having a thickness of 120 nm used as a channel layer (2) on an polyethylene terephthalate (PET) film (1) by an PLD method in an atmosphere having an oxygen partial pressure of 5 Pa. Formed. At this time, a polycrystalline sintered body having an InGaO 3 (ZnO) composition was used as a target.

Further, an In—Zn—Ga—O-based amorphous oxide film and a gold film having a high electric conductivity were stacked by 30 nm by a PLD method with an oxygen partial pressure in the chamber of less than 1 Pa. And the drain terminal (5) and the source terminal (6) were formed by the photolithographic method and the lift-off method. Finally, a gate insulating film (3) was formed by an electron beam evaporation method, gold was formed thereon, and a gate terminal (4) was formed by a photolithography method and a lift-off method. The channel length was 50 μm and the channel width was 200 μm. Three types of TFTs having the above-described structures using Y 2 O 3 (thickness: 140 nm), Al 2 O 3 (thickness: 130 μm) and HfO 2 (thickness: 140 μm) as gate insulating films were prepared. .

Characteristic Evaluation of TFT Element The current-voltage characteristic measured at room temperature of the TFT formed on the PET film was the same as that shown in FIG. That is, as the drain voltage V DS increases, the drain current I DS increases, indicating that the channel has n-type conduction. This is consistent with the fact that the amorphous In—Ga—Zn—O-based amorphous oxide film is an n-type conductor. I DS shows the behavior of a typical transistor that saturates (pinch off) at around V DS = 6 V. Further, when V g = 0, a current of I DS = 2.0 × 10 −5 A flowed when I ds = 10 −8 A and Vg = 10 V. This corresponds to the fact that electron carriers can be induced in the In-Ga-Zn-O amorphous oxide film of the insulator by the gate bias.

The on / off ratio of the transistor was more than 10 3 . When the field effect mobility was calculated from the output characteristics, a field effect mobility of about 7 cm 2 (Vs) −1 was obtained in the saturation region.

  The device prepared on the PET film was bent with a curvature radius of 30 mm, and the same transistor characteristics were measured, but no change was observed in the transistor characteristics. Further, the same measurement was performed by irradiating visible light, but no change in transistor characteristics was observed.

The TFT using the Al 2 O 3 film as the gate insulating film also showed transistor characteristics similar to those shown in FIG. 6, but when V g = 0, I ds = 10 −8 A, Vg = 10 V Occasionally, a current of I DS = 5.0 × 10 −6 A flowed. On-off ratio of the transistor was 10 greater than 2. When the field effect mobility was calculated from the output characteristics, a field effect mobility of about 2 cm 2 (Vs) −1 was obtained in the saturation region.

The TFT using the HfO 2 film as the gate insulating film also showed similar transistor characteristics to those shown in FIG. 6, but when V g = 0, I ds = 10 −8 A, and Vg = 10 V, A current of I DS = 1.0 × 10 −6 A flowed. On-off ratio of the transistor was 10 greater than 2. Further, when the field effect mobility was calculated from the output characteristics, a field effect mobility of about 10 cm 2 (Vs) −1 was obtained in the saturation region.
(Creation of TFT element using In 2 O 3 amorphous oxide film by PLD method)
The top gate type TFT element shown in FIG. 5 was produced. First, an 80 nm thick In 2 O 3 amorphous oxide film used as a channel layer (2) was formed on a polyethylene terephthalate (PET) film (1) by a PLD method.

Further, an In 2 O 3 amorphous oxide film and a gold film having a high electric conductivity are formed by PLD method by setting the oxygen partial pressure in the chamber to less than 1 Pa and further applying zero voltage to the oxygen radical generator. Each was laminated with 30 nm. And the drain terminal (5) and the source terminal (6) were formed by the photolithographic method and the lift-off method. Finally, a Y 2 O 3 film used as a gate insulating film (3) is formed by an electron beam evaporation method, gold is formed thereon, and a gate terminal (4) is formed by a photolithography method and a lift-off method. Formed.

Characteristic Evaluation of TFT Element A current-voltage characteristic measured at room temperature of a TFT formed on a PET film was measured. As the drain voltage V DS increases, the drain current I DS increases, which indicates that the channel is an n-type semiconductor. This is consistent with the fact that the In—O amorphous oxide film is an n-type conductor. I DS shows the behavior of a typical transistor that saturates (pinch off) at about V DS = 5 V. In addition, when V g = 0V, a current of 2 × 10 −8 A flows, and when V G = 10 V, a current of I DS = 2.0 × 10 −6 A flows. This corresponds to the fact that electron carriers can be induced in the In-O amorphous oxide film of the insulator by the gate bias.

On-off ratio of the transistor was about 10 2. Further, when the field effect mobility was calculated from the output characteristics, a field effect mobility of about 10 cm 2 (Vs) −1 was obtained in the saturation region. The TFT element formed on the glass substrate also showed similar characteristics.

The device prepared on the PET film was bent with a radius of curvature of 30 mm and the same transistor characteristics were measured, but no change was observed in the transistor characteristics.
(Preparation of TFT element using In-Sn-O amorphous oxide film by PLD method)
The top gate type TFT element shown in FIG. 5 was produced. First, an In—Sn—O amorphous oxide film having a thickness of 100 nm used as a channel layer (2) was formed on a polyethylene terephthalate (PET) film (1) by a PLD method. Further, an In-Sn-O amorphous oxide film having a high electrical conductivity and a PLD method are used by setting the partial pressure of oxygen in the chamber to less than 1 Pa, further reducing the voltage applied to the oxygen radical generator to zero. Each gold film was laminated to 30 nm. And the drain terminal (5) and the source terminal (6) were formed by the photolithographic method and the lift-off method. Finally, a Y 2 O 3 film used as a gate insulating film (3) is formed by an electron beam evaporation method, gold is formed thereon, and a gate terminal (4) is formed by a photolithography method and a lift-off method. did.

Characteristic Evaluation of TFT Element A current-voltage characteristic measured at room temperature of a TFT formed on a PET film was measured. As the drain voltage V DS increases, the drain current I DS increases, which indicates that the channel is an n-type semiconductor. This is consistent with the fact that the In—Sn—O-based amorphous oxide film is an n-type conductor. I DS shows the behavior of a typical transistor that saturates (pinch off) at about V DS = 6 V. Further, when V g = 0V, a current of 5 × 10 −8 A flows, and when V G = 10 V, a current of I DS = 5.0 × 10 −5 A flows. This corresponds to the fact that electron carriers could be induced in the insulator In—Sn—O amorphous oxide film by the gate bias.

The on / off ratio of the transistor was about 10 3 . Further, when the field effect mobility was calculated from the output characteristics, a field effect mobility of about 5 cm 2 (Vs) −1 was obtained in the saturation region. The TFT element formed on the glass substrate also showed similar characteristics.

The device prepared on the PET film was bent with a radius of curvature of 30 mm and the same transistor characteristics were measured, but no change was observed in the transistor characteristics.
(Preparation of TFT element using In-Ga-O amorphous oxide film by PLD method)
The top gate type TFT element shown in FIG. 5 was produced. First, an In—Ga—O-based amorphous oxide film having a thickness of 120 nm used as the channel layer (2) was formed on the polyethylene terephthalate (PET) film (1) by the film forming method shown in Example 6. . Further, an In—Ga—O amorphous oxide film having a high electrical conductivity is formed by the PLD method by setting the oxygen partial pressure in the chamber to less than 1 Pa and further applying zero voltage to the oxygen radical generator. And 30 nm thick gold films. And the drain terminal (5) and the source terminal (6) were formed by the photolithographic method and the lift-off method. Finally, a Y 2 O 3 film used as a gate insulating film (3) is formed by an electron beam evaporation method, gold is formed thereon, and a gate terminal (4) is formed by a photolithography method and a lift-off method. did.

Characteristic Evaluation of TFT Element A current-voltage characteristic measured at room temperature of a TFT formed on a PET film was measured. As the drain voltage V DS increases, the drain current I DS increases, which indicates that the channel is an n-type semiconductor. This is consistent with the fact that the In—Ga—O amorphous oxide film is an n-type conductor. I DS shows the behavior of a typical transistor that saturates (pinch off) at about V DS = 6 V. Further, when V g = 0V, a current of 1 × 10 −8 A flows, and when V G = 10 V, a current of I DS = 1.0 × 10 −6 A flows. This corresponds to the fact that electron carriers could be induced in the insulator In-Ga-O amorphous oxide film by the gate bias.

On-off ratio of the transistor was about 10 2. Further, when the field effect mobility was calculated from the output characteristics, a field effect mobility of about 0.8 cm 2 (Vs) −1 was obtained in the saturation region. The TFT element formed on the glass substrate also showed similar characteristics.

  The device prepared on the PET film was bent with a radius of curvature of 30 mm and the same transistor characteristics were measured, but no change was observed in the transistor characteristics.

In addition, it can apply as a channel layer of TFT by making the electron carrier density | concentration of an amorphous oxide less than 10 < 18 > / cm < 3 >. The electron carrier concentration is more preferably 10 17 / cm 3 or less, and even more preferably 10 16 / cm 3 or less.

  Examples of the present invention will be described below.

(Example 1: Preparation of amorphous In-Ga-Zn-O thin film containing microcrystals)
This is performed using the apparatus shown in FIG.

A polycrystalline sintered body having an InGaO 3 (ZnO) 4 composition is used as a target by pulsed laser deposition using a KrF excimer laser.

  An In-Ga-Zn-O-based amorphous oxide semiconductor thin film containing microcrystals is deposited on a glass substrate (Corning 1737).

During film formation, the substrate surface is irradiated with a halogen lamp of 20 mW / cm 2 for light irradiation.

  In addition, confirmation of presence of a microcrystal is confirmed by cross-sectional TEM (transmission electron microscope) observation.

(Preparation of MISFET element)
The top gate type MISFET element shown in FIG. 5 is manufactured.

First, the semi-insulating amorphous InGaO 3 containing microcrystals with a thickness of 30 nm used as the channel layer (2) is prepared by the above-described method for producing an amorphous In—Ga—Zn—O thin film containing microcrystals on the glass substrate (1). A (ZnO) 4 film is formed.

Further, an InGaO 3 (ZnO) 4 film and a gold film having a large electric conductivity were stacked in a thickness of 30 nm by a pulse laser deposition method with an oxygen partial pressure in the chamber of less than 1 Pa. Then, the drain terminal (5) and the source terminal (6) are formed by a photolithography method and a lift-off method.

Finally, a Y 2 O 3 film used as a gate insulating film (3) is formed by electron beam evaporation (thickness: 110 nm, relative dielectric constant: about 15, leakage current density: 10 -3 A when 0.5 MV / cm is applied) / cm 2 ). And gold | metal | money is formed on it and a gate terminal (4) is formed by the photolithographic method and the lift-off method.

  Thus, a field effect transistor is formed.

The on / off ratio of the transistor is greater than 10 4 . Further, when the field effect mobility was calculated from the output characteristics, a field effect mobility of about 7.5 cm 2 (Vs) −1 was obtained in the saturation region. A similar measurement was performed by irradiating the fabricated device with visible light, but no change in transistor characteristics was observed.

In yet an embodiment of the thin film preparation, such as In-Ga-Zn-O containing the microcrystals, when the light irradiation to the substrate, in the range of 0.3mW / cm 2 ~100mW / cm 2 , was more effective . This is more preferable in that the ON / OFF ratio of the transistor can be increased and the field effect mobility can be increased.

Although it depends on the amount of microcrystals in the amorphous oxide film, the presence of microcrystals in the film can be confirmed by observing a specific peak by measurement by X-ray diffraction. I can confirm.
(Example 2: Preparation of amorphous In-Ga-Zn-O thin film having a composition distribution in the film thickness direction)
An In-Ga-Zn-O-based amorphous oxide semiconductor thin film having a composition distribution in the film thickness direction is deposited on a glass substrate (Corning 1737) by a pulsed laser deposition method using a KrF excimer laser. At this time, a polycrystalline sintered body having an InGaO 3 (ZnO) 4 composition was used as a target.

  The deposition is performed while the oxygen partial pressure in the chamber is set within a predetermined range and the distance between the target and the substrate is changed by 5 mm. As the distance between the two becomes longer, the amount of oxygen taken into the film to be formed increases. The substrate temperature is 25 ° C.

  Note that, in the example of manufacturing a thin film having a composition distribution in the film thickness direction, the oxygen partial pressure may be changed in the film thickness direction when the composition distribution is present in the film thickness direction. Alternatively, the composition can be changed in the film thickness direction even if the oscillation power of the pulse laser is changed or the oscillation frequency is changed.

In this way, the leakage current can be reduced, the on / off ratio of the transistor can be increased, or the field effect mobility can be increased.
(Example 3: Preparation of amorphous In-Ga-Zn-O thin film having a composition distribution in the film thickness direction)
A sputter deposition method using argon gas is used.

As targets, 1) a polycrystalline sintered body having an InGaO 3 (ZnO) 4 composition and 2) a sintered body of zinc oxide are prepared.

  Then, an amorphous In—Ga—Zn—O thin film having a composition distribution in the film thickness direction is formed on a glass substrate (Corning 1737).

  First, film formation by sputtering is performed using the target 1) in an atmosphere in which the oxygen partial pressure is set to a predetermined value.

  Next, a film is formed by sputtering using the targets 1) and 2) simultaneously. Thus, an amorphous In—Ga—Zn—O thin film having a composition distribution in the film thickness direction can be manufactured. The substrate temperature is 25 ° C.

  Note that a thin film such as In—Ga—Zn—O having a composition distribution in the film thickness direction can also be manufactured as follows.

For example, when the composition distribution is in the film thickness direction, the In 2 O 3 target is sputtered simultaneously or separately, or the oxygen partial pressure is changed in the film thickness direction, or the sputtering input power is Each target is changed in the film thickness direction.

Particularly in the vicinity of the gate insulating film, the field effect mobility is expected to increase as the In 2 O 3 or ZnO composition increases.
(Example 4: Preparation of amorphous In-Ga-Zn-O (N) thin film)
A method for manufacturing an oxide containing nitrogen (N) as an additive in an amorphous oxide will be described.

Next, an In-Ga-Zn-O-based amorphous oxide semiconductor containing nitrogen as an impurity (In-Ga-Zn-O (N) is abbreviated on the same glass substrate by pulsed laser deposition using a KrF excimer laser. Deposit a thin film. At this time, an InGaO 3 (ZnO) 4 polycrystalline sintered body was used as a target.

Specifically, the oxygen partial pressure in the chamber is 4 Pa, for example, and the nitrogen partial pressure is 1 Pa.
The substrate temperature is 25 ° C.

In addition, when the secondary ion mass spectrometry (SIMS) analysis is performed, the composition ratio of oxygen and nitrogen in the thin film is preferably about 50: 1.
(Example 5: Preparation of amorphous In-Ga-Zn-O (Ti) thin film)
An In-Ga-Zn-O amorphous material is formed on a glass substrate (Corning 1737) using a polycrystalline sintered body with InGaO 3 (ZnO) 4 composition as a target by pulsed laser deposition using a KrF excimer laser. An oxide semiconductor thin film is deposited.

  The obtained In—Ga—Zn—O-based thin film is immersed in an aqueous solution of titanium trichloride kept at 80 ° C.

  Thereafter, it is pulled up and annealed at 300 ° C. in air.

  Thus, Ti can be introduced into the amorphous oxide as an additive.

  In addition, when the concentration of Ti is analyzed from the surface of the thin film by SIMS, it is preferable that the content is about 0.5% at the outermost surface, and the concentration of Ti decreases as the distance from the surface increases.

  The amorphous oxide according to the present invention can be applied to a transistor using the film as a channel layer as a switching element of an LCD or an organic EL display. In addition, a thin film of a semiconductor is formed on a flexible material such as a plastic film, so that it can be widely applied to panels such as flexible displays, IC cards and ID tags.

It is a graph which shows the relationship between the electron carrier density | concentration of the In-Ga-Zn-O type | system | group amorphous film formed into a film by the pulse laser vapor deposition method, and the oxygen partial pressure during film-forming. It is a graph which shows the relationship between the electrical conductivity of the In-Ga-Zn-O type | system | group amorphous film formed into a film by the sputtering method using argon gas, and the oxygen partial pressure during film-forming. It is a graph which shows the relationship between the number of electron carriers and the electron mobility of the In-Ga-Zn-O type amorphous film formed by the pulse laser deposition method. Is a graph showing the electric conductivity, carrier concentration, the change in electron mobility with respect to the value of x of InGaO 3 was deposited by pulsed laser deposition method in an atmosphere of an oxygen partial pressure of 0.8Pa (Zn 1-x Mg x O) . It is a schematic diagram which shows a top gate type MISFET element structure. It is a graph which shows the current-voltage characteristic of a top gate type MISFET element. It is a schematic diagram of the film-forming apparatus in the case of forming into a film by PLD method. It is a schematic diagram of the film-forming apparatus in the case of forming into a film by SP method.

Explanation of symbols

1 Substrate 2 Channel layer 3 Gate insulating film 4 Gate electrode (gate terminal)
5 Drain electrode (drain terminal)
6 Source electrode (source terminal)

Claims (3)

  1. A field effect transistor,
    An active layer,
    A gate electrode provided on the active layer via a gate insulating film;
    A source electrode and a drain electrode provided on both sides of the gate electrode in contact with the active layer ;
    The active layer includes an amorphous oxide containing at least In, Ga, and Zn, the amorphous oxide includes a microcrystal, and the microcrystal has an amorphous structure in which a crystal grain boundary interface is an amorphous structure. and covered with at the electron carrier concentration of the amorphous oxide 10 12 / cm 3 or more, Ri 10 18 / cm 3 less than der further current between the drain and source terminals of the gate voltage is not applied is 10 less than microamperes, field-effect transistor the field effect mobility, characterized in 1cm 2 / (V · sec) ultra der Rukoto.
  2. The field effect transistor according to claim 1, wherein the amorphous oxide is an In—Ga—Zn—O-based oxide.
  3. The field effect transistor according to claim 1, wherein the amorphous oxide is an In—Ga—Zn—Mg—O-based oxide.
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