JP6955145B2 - Transistor and its manufacturing method - Google Patents

Description

The present invention relates to a transistor and a method for manufacturing the same.

In recent years, the development of electronic devices using graphene has been actively carried out, and various applications are being sought. It is known that an electronic device using graphene is greatly affected by the outside world such as an operating atmosphere due to the large specific surface area of graphene. Recently, the application of graphene to chemical sensors has been studied by utilizing this property in reverse. A typical graphene sensor has a back gate type transistor structure using graphene for the channel. This graphene sensor detects gas from a current change caused by electron exchange (charge transfer) between graphene and gas molecules, which occurs when gas molecules are attached to and detached from the surface of graphene. It has been reported that this graphene sensor can detect ammonia and nitrogen dioxide. On the other hand, the change in electrical conductivity with respect to ppm level ammonia is small, and the rate of change is as low as several percent.

A graphene sensor using graphene as a gate has also been proposed. However, the types of gas that can be detected by this graphene sensor are limited.

International Publication No. 2017/002854

Nature Materials, 6, 652 (2007) Materials Science and Engineering C, 31, 1405 (2011) Sensors and Actuators B, 155, 451 (2011)

An object of the present invention is to provide a transistor that can be used for detecting various gases and a method for manufacturing the same.

In one aspect of the transistor, the semiconductor layer, the graphene film provided above the semiconductor layer, and SnS ₂ , polyaniline, polypyrrole, poly (3,) which modifies the surface of the graphene film and is in contact with at least a part of the gas. A modified portion containing 4-ethylenedioxythiophene), phthalocyanine, or pentacene, a barrier film between the semiconductor layer and the graphene film, and a portion below the barrier film on the surface of the semiconductor layer. Includes two electrodes provided so as to sandwich them. The graphene film refers to a film composed of one or more unit layers of graphene.

One aspect of the gas sensor includes the above transistors.

In one aspect of the method for manufacturing a transistor, a graphene film is provided above the semiconductor layer, the surface of the graphene film is modified, and SnS ₂ , polyaniline, polypyrrole, and poly (3,4-ethylenedioxy), which are at least partially in contact with a gas. A modified portion containing thiofene), phthalocyanine, or pentacene is provided to form a barrier film between the semiconductor layer and the graphene film, and a lower portion of the barrier film is sandwiched between the surfaces of the semiconductor layer. Two electrodes are formed.

According to the above-mentioned transistor and the like, since an appropriate graphene film, a modified portion, a barrier film and a semiconductor layer are included, it can be used for detecting various gases.

It is sectional drawing which shows the structure of the gas sensor of a reference example. It is a figure which shows the usage method of the gas sensor of a reference example. It is a figure which shows the relationship between the change of a flat band voltage and the change of a drain current. It is sectional drawing which shows the structure of the gas sensor which concerns on 1st Embodiment. It is a figure which shows the function of the modification part. It is a figure which shows the band structure before or after the formaldehyde molecule is adsorbed. It is sectional drawing which shows the manufacturing method of the gas sensor which concerns on 2nd Embodiment in process order. Following FIG. 7A, it is sectional drawing which shows the manufacturing method of a gas sensor in the order of a process.

First, a reference example of the gas sensor will be described. FIG. 1 is a cross-sectional view showing the structure of a gas sensor of a reference example.

As shown in FIG. 1, the gas sensor 100 of the reference example includes a p-type layer 101, an insulating film 104 on the p-type layer 101, and a graphene film 105 on the insulating film 104. The gas sensor 100 includes an n-type layer 102 and an n-type layer 103 on the surface of the p-type layer 101. The n-type layer 102 and the n-type layer 103 sandwich the insulating film 104 and the graphene film 105 in a plan view. The gas sensor 100 includes a gate electrode 106 on the graphene film 105, a source electrode 107 on the n-type layer 102, and a drain electrode 108 on the n-type layer 103.

Next, a method of using the gas sensor 100 will be described. FIG. 2 is a diagram showing how to use the gas sensor of the reference example.

As shown in FIG. 2A, the gas sensor 100 is used, for example, by connecting a current monitoring device 111 that detects a current (drain current) flowing between the source electrode 107 and the drain electrode 108. .. The source electrode 107 is grounded, and a bias voltage V _bias is applied to the gate electrode 106 by the bias power supply 112. The current monitoring device 111 may include, for example, various power supplies, an amplifier circuit, a sampling circuit, an analog-to-digital (AD) converter, a data processing computer, and the like.

FIG. 2B shows a band diagram of the graphene film 105, the insulating film 104, and the p-type layer 101. Between the work function phi _s work function phi _g and p-type layer 101 of the graphene film 105, the following relationship holds with flat band voltage V _FB.
V _FB = φ _{g −} φ _s

When the graphene membrane 105 adsorbs the ammonia molecule 113, which is a test molecule, the ammonia molecule 113 acts as a donor to the graphene membrane 105, and the graphene membrane 105 is doped in an n-type. As a result, the work function of the graphene film 105 changes, and the flat band voltage also changes. Assuming that the amount of change in the work function of the graphene film 105 is Δφ _g and the amount of change in the flat band voltage is ΔV _FB , the following relationship holds.
ΔV _FB = Δφ _g

When the flat-band voltage is changed, as shown in FIG. 3, the drain current at the same bias point V _bias is changed from I _d1 to I _d2 by [Delta] I _d. By detecting the amount of change [Delta] I _d by the current monitoring device 111 may graphene film 105 to identify the number of ammonia molecules 113 adsorbed, it is possible to identify the concentration of ammonia from this number. Variation [Delta] I _d is an example of a physical quantity.

Since the amount of change [Delta] I _d is dependent on the transconductance, for example by using a voltage of subthreshold region as the bias voltage V _bias, the drain current changes exponentially. Therefore, even a small amount of change in the work function of the graphene layer 105 may be a variation [Delta] I _d of the drain current large. For example, it is possible to the amount of change in the work function is obtained an order of magnitude of the change amount in the case of 60 mV, i.e. a 1000% as ΔI _d / I _d1. Further, for example _{, when a voltage in the on region is used as the bias voltage V bias} , the absolute change in the drain current can be set near the maximum current of the field effect transistor. According to such an embodiment, ppb level ammonia can be easily detected.

However, such a method of use is effective in detecting molecules that act as donors or acceptors for graphene membrane 105 such as ammonia, but can detect molecules that do not act as donors or acceptors for graphene membrane 105. Can not. The present inventor has made diligent studies to enable the detection of a wider variety of molecules. As a result, it was found that by imparting a specific modifying group to the surface of graphene, it becomes possible to detect molecules that cannot interact with graphene itself.

Hereinafter, embodiments will be specifically described with reference to the accompanying drawings.

(First Embodiment)
The first embodiment will be described. FIG. 4 is a cross-sectional view showing the structure of the gas sensor according to the first embodiment.

As shown in FIG. 4, the gas sensor 200 according to the first embodiment includes a p-type layer 101, an insulating film 104 on the p-type layer 101, and a graphene film 105 on the insulating film 104. The gas sensor 100 includes a modified portion 109 that modifies the surface of the graphene film 105 and is in contact with gas at least in part, and an n-type layer 102 and an n-type layer 103 on the surface of the p-type layer 101. The n-type layer 102 and the n-type layer 103 sandwich the insulating film 104 and the graphene film 105 in a plan view. The gas sensor 100 includes a gate electrode 106 on the graphene film 105, a source electrode 107 on the n-type layer 102, and a drain electrode 108 on the n-type layer 103. As described above, the gas sensor 200 includes an n-type MOSFET structure having a graphene film 105 at the gate. The p-type layer 101 is an example of a semiconductor layer, and the insulating film 104 is an example of a barrier membrane.

Next, the function of the modification unit 109 will be described. Here, it is assumed that a semiconductor is used for the modifying portion 109, but an insulator or a metal may be used for the modifying portion 109, and the material of the modifying portion 109 may be an inorganic substance or an organic substance. FIG. 5 is a diagram showing the function of the modifying unit 109.

When the gas molecule 110 is adsorbed on the surface of the modified portion 109, as shown in FIG. 5 (a), the HOMO (highest occupied molecular orbital) of the gas molecule 110 is charged to the graphene film 105 via the conduction band of the modified portion 109. Movement of electrons) occurs, or as shown in FIG. 5 (b), charges (electrons) move from the graphene film 105 to the LUMO (minimum occupied molecular orbital) of the gas molecule 110 via the conduction band of the modified portion 109. .. In this way, the work function of the graphene film 105 changes. That is, even the gas molecule 110 that cannot interact with the graphene film 105 itself can change the work function of the graphene film 105 through the modification portion 109. The change in the work function of the graphene film 105 is detected as a change in the drain current of the field effect transistor (FET) by the same principle as the gas sensor 100 of the reference example.

Therefore, when the specific gas molecule 110 is adsorbed on the modified portion 109 in the first embodiment, the work function of the graphene film 105 changes significantly even if the amount is small. Therefore, by detecting the amount of this change, the amount of gas molecules 110 adsorbed on the graphene film 105 can be detected with high accuracy.

The presence or absence of charge transfer and the amount of transfer also depend on the presence or absence of interaction between the modified portion 109 and the gas molecule 110. Specifically, the presence / absence and amount of charge transfer include the adsorption energy of the gas molecule 110 with respect to the surface of the modified portion 109, the difference in electronegativity between the modified portion 109 and the gas molecule 110, and graphene. It depends on the relative band alignment of the material (relative energy position of the electronic state).

For example, when _{an oxide film or fine particles such as SnO 2} and ZnO are used for the modified portion 109, the gas sensor 200 is suitable for detecting reducing gases such as _{H 2} , CO, H ₂ S and SO _2. .. For example, the gas molecules 110 in the H ₂ is adsorbed on the surface of the modified portion ^{_{109, H 2 + O - →}} H 2 O + e - occurs in the reaction, Figure 5 (a), the emitted electrons graphene film 105 The Fermi level of the graphene film 105 rises. That is, the graphene film 105 is doped in an n-type. As a result, the work function of the graphene film 105 decreases and the drain current changes.

When polyaniline is used in the modifier 109, the gas sensor 200 is suitable for detecting alcohols such as methanol and ethanol. When the gas molecule 110 of alcohol is adsorbed on the modified portion 109, as shown in FIG. 5A, electrons move from the modified portion 109 to the graphene film 105, and the Fermi level of the graphene film 105 rises. That is, the graphene film 105 is doped in an n-type. As a result, the work function of the graphene film 105 decreases and the drain current changes.

For example, when _{an oxide film or fine particles such as SnO 2} and ZnO are used for the modified portion 109, the gas sensor 200 is also suitable for detecting an oxidizing gas such as _{O 2.} For example, the gas molecules 110 of the O ₂ is adsorbed on the surface of the modified portion ^{_{109, O 2 + e - →}} 2O - for reaction, 5 (b), the gas molecules 110 from the electronic modifications 109 Move to. Therefore, electrons move from the graphene film 105 to the modified portion 109, and the Fermi level of the graphene film 105 drops. That is, the graphene film 105 is doped in a p-type. As a result, the work function of the graphene film 105 increases and the drain current changes.

When polypyrrole is used in the modifier 109, the gas sensor 200 is suitable for detecting alcohols such as methanol and ethanol. When the gas molecule 110 of alcohol is adsorbed on the modified portion 109, electrons move from the graphene film 105 to the modified portion 109, and the Fermi level of the graphene film 105 is lowered, as shown in FIG. 5 (b). That is, the graphene film 105 is doped in a p-type. As a result, the work function of the graphene film 105 increases and the drain current changes.

For example, when tin disulfide (SnS ₂ ) is used in the modifier 109, the gas sensor 200 is suitable for detecting formaldehyde (HCHO). When the formaldehyde gas molecule 110 is adsorbed on the modified portion 109, electrons move from the modified portion 109 to the graphene film 105, and the Fermi level of the graphene film 105 rises, as shown in FIG. 5 (a). That is, the graphene film 105 is doped in an n-type. As a result, the work function of the graphene film 105 decreases and the drain current changes.

When the present inventor calculated the presence or absence of charge transfer when tin disulfide was placed on graphene by simulation, it was found that electrons were transferred from graphene to tin disulfide. In addition, when a stable structure model was calculated when formaldehyde molecules were adsorbed on the surface of tin disulfide placed on graphene, the distance between graphene and tin disulfide was 3.7 Å, and tin disulfide and formaldehyde were found. The distance between the molecules was 2.8 Å, suggesting that the formaldehyde molecules were physically adsorbed on the surface of tin disulfide. The present inventor calculated the band structure before and after the formaldehyde molecule was adsorbed for this model. The result is shown in FIG. FIG. 6A shows the band structure after adsorption, and FIG. 6B shows the band structure before adsorption. Similar to graphene, tin disulfide is a layered substance, and its thickness can be changed layer by layer. The number of layers is not limited, but a single layer is assumed here.

As shown in FIG. 6, the work function of graphene was reduced by 10 meV due to the adsorption of formaldehyde. This means that some of the electrons transferred to tin disulfide were transferred to graphene again by the adsorption of formaldehyde molecules, that is, graphene was doped into n-type. When the bias voltage V _bias is set in the sub-threshold region and the amount of change in drain current (sub-threshold swing) per unit voltage of the FET is 60 mV / decade, the maximum amount of change in drain current due to adsorption of formaldehyde molecules is 17%. It was suggested that the degree could be obtained. Thus, tin disulfide interacts strongly with formaldehyde. Therefore, by using tin disulfide for the modifying portion 109, the gas sensor 200 can be used as a formaldehyde sensor.

(Second Embodiment)
Next, the second embodiment will be described. The second embodiment relates to the method for manufacturing the gas sensor 200 according to the first embodiment. 7A to 7B are cross-sectional views showing the manufacturing method according to the second embodiment in the order of processes.

First, as shown in FIG. 7A (a), the n-type layer 102 and the n-type layer 103 are formed on the surface of the p-type layer 101. For example, the p-type layer 101 can be formed by ion implantation of p-type impurities on the surface of a silicon substrate, and the n-type layer 102 and n-type layer 103 can be formed by ion implantation of n-type impurities on the surface of the p-type layer 101. Can be formed by Next, the insulating film 104 is formed on the p-type layer 101, the n-type layer 102, and the n-type layer 103. The insulating film 104 can be formed, for example, by thermal oxidation of the surfaces of the p-type layer 101, the n-type layer 102, and the n-type layer 103.

Then, as shown in FIG. 7A (b), the graphene film 105 is provided on the insulating film 104. The graphene film 105 can be provided, for example, by growing and transferring onto a growth substrate.

Subsequently, as shown in FIG. 7A (c), the graphene film 105 is patterned. The graphene film 105 can be patterned by, for example, a photolithography technique and an etching technique. Examples of the etching technique include reactive ion etching (RIE) using oxygen plasma.

Next, as shown in FIG. 7B (d), the insulating film 104 is patterned to expose at least a part of the n-type layer 102 and at least a part of the n-type layer 103. The insulating film 104 can be patterned by, for example, a photolithography technique and an etching technique.

Then, as shown in FIG. 7B (e), the gate electrode 106 is formed on the graphene film 105, the source electrode 107 is formed on the n-type layer 102, and the drain electrode 108 is formed on the n-type layer 103. In the formation of the gate electrode 106, the source electrode 107, and the drain electrode 108, for example, a mask is formed to expose the area where they are to be formed, a metal film is formed by a vacuum vapor deposition method, and the mask is formed together with the metal film on the mask. Remove. That is, the gate electrode 106, the source electrode 107, and the drain electrode 108 can be formed by the lift-off method. In the formation of the metal film, for example, a Ti film having a thickness of 5 nm is formed, and an Au film having a thickness of 200 nm is formed on the Ti film.

Subsequently, as shown in FIG. 7B (f), the modification portion 109 is provided on the graphene film 105. If the modified portion 109 is a layered substance, for example, the modified portion 109 can be provided by growth and transfer onto a growth substrate. The method of providing the modified portion 109 on the graphene film 105 is not limited, and examples thereof include a vapor deposition method, a sputtering method, a CVD method, a dip method, a spin coating method, and a drop casting method. The vapor deposition, sputtering and CVD methods are examples of dry processes, in which the modified portion 109 is deposited directly on the graphene film 105. The dip method, spin coating method, and drop casting method are examples of wet processes, and in these, a solution in which the material of the modified portion 109 is dispersed or dissolved in a solvent is used.

In this way, the gas sensor 200 according to the first embodiment can be manufactured.

The material of the modifying portion 109 is not limited to the above example. For example, SnO ₂ , Pb, SnS ₂ , poly (3,4-ethylenedioxythiophen), phthalocyanine and pentacene can be used as materials for the modification section 109. SnO ₂ is suitable for detecting _{NO X} and CO _X , Pb is suitable for detecting _{H 2 , and SnS 2} is suitable for detecting formaldehyde and acetone. Poly (3,4-ethylenedioxythiophen) is suitable for detecting DNA, phthalocyanine is suitable for detecting iodine and bromine, and pentacene is suitable for detecting 1-pentanol. Au, Ag, Pd, TiO ₂ and propylene may be used as the material of the modified portion 109.

The number of graphene layers contained in the graphene film 105 is not particularly limited. The thickness of the modified portion 109 is not particularly limited, but the thickness of the modified portion 109 is preferably about one molecular layer. This is to facilitate the movement of electrons between the gas molecule and the graphene film 105.

Examples of the material of the semiconductor layer include Group IV semiconductors, Group III-V compound semiconductors, Group II-VI compound semiconductors, oxide semiconductors, organic semiconductors, metal chalcogenide semiconductors, layered material semiconductors, semiconductor carbon nanotubes, and graphene nanoribbons. .. Examples of Group IV semiconductors include single crystal Si and polycrystalline Si. Examples of III-V compound semiconductors include arsenide semiconductors such as GaAs and nitride semiconductors such as GaN. CdTe is exemplified as a group II-VI compound semiconductor. ZnO is exemplified as an oxide semiconductor. Pentacene is exemplified as an organic semiconductor. _{MoS 2} is exemplified as a metal chalcogenide semiconductor. Black phosphorus is exemplified as a layered material semiconductor. The conductive type of the semiconductor layer may be opposite. That is, an n-type layer may be used instead of the p-type layer 101, a p-type layer may be used instead of the n-type layer 102, and a p-type layer may be used instead of the n-type layer 103.

Examples of the material of the insulating film 104 include silicon oxide, silicon nitride, silicon oxynitride, germanium oxide, high dielectric constant insulator and layered insulator. Examples of high dielectric constant insulators include aluminum, titanium, tantalum or hafnium, or insulators containing any combination thereof, such as aluminum oxide and hafnium oxide. Examples of the layered insulator include hexagonal boron nitride (BN) and hexagonal carbon boron nitride (BCN), which is a mixed crystal of boron nitride and graphite.

The barrier film is not limited to the insulating film. For example, when a compound semiconductor is used for the semiconductor layer, a group III-V compound semiconductor or a group II-VI compound semiconductor having a wider band gap than the compound semiconductor of the semiconductor layer can be used as the barrier film. For example, as a combination of materials for the semiconductor layer and the barrier membrane, a combination of GaAs and AlGaAs, a combination of InGaAs and InAlAs, and a combination of GaN and AlGaN are exemplified.

A protective film covering the gate electrode 106, the source electrode 107, and the drain electrode 108 may be provided so that the modified portion 109 is exposed.

Hereinafter, various aspects of the present invention will be collectively described as appendices.

(Appendix 1)
With the semiconductor layer
The graphene film provided above the semiconductor layer and
A modified portion that modifies the surface of the graphene film and is in contact with at least a part of the gas,
A barrier membrane between the semiconductor layer and the graphene film,
Two electrodes provided on the surface of the semiconductor layer so as to sandwich a lower portion of the barrier membrane,
A transistor characterized by having.

(Appendix 2)
The transistor according to Appendix 1, wherein the graphene film is in direct contact with the barrier membrane.

(Appendix 3)
The transistor according to Appendix 1 or 2, wherein the modified portion contains SnS ₂ , SnO _{2, ZnO, polyaniline or polypyrrole.}

(Appendix 4)
The material of the semiconductor layer shall be a group IV semiconductor, a group III-V compound semiconductor, a group II-VI compound semiconductor, an oxide semiconductor, an organic semiconductor, a metal chalcogenide semiconductor, a layered material semiconductor, a semiconductor carbon nanotube or a graphene nanoribbon. The transistor according to any one of Supplementary Provisions 1 to 3, characterized in that.

(Appendix 5)
Item 3. The transistor described.

(Appendix 6)
A gas sensor comprising the transistor according to any one of Supplementary note 1 to 5.

(Appendix 7)
The process of providing a graphene film above the semiconductor layer and
A step of modifying the surface of the graphene film and providing a modified portion in which at least a part of the graphene film is in contact with a gas.
A step of forming a barrier film between the semiconductor layer and the graphene film,
A step of forming two electrodes on the surface of the semiconductor layer with a lower portion of the barrier membrane sandwiched between them.
A method for manufacturing a transistor, which comprises.

(Appendix 8)
The method for manufacturing a transistor according to Appendix 7, wherein the graphene film is in direct contact with the barrier membrane.

(Appendix 9)
The method for manufacturing a transistor according to Appendix 7 or 8, wherein the modified portion contains SnS ₂ , SnO _{2, ZnO, polyaniline or polypyrrole.}

(Appendix 10)
The material of the semiconductor layer shall be a group IV semiconductor, a group III-V compound semiconductor, a group II-VI compound semiconductor, an oxide semiconductor, an organic semiconductor, a metal chalcogenide semiconductor, a layered material semiconductor, a semiconductor carbon nanotube or a graphene nanoribbon. The method for manufacturing a transistor according to any one of Supplementary Provisions 7 to 9, wherein the method is characterized by the above-mentioned method.

(Appendix 11)
Item 1. The method for manufacturing a transistor according to the description.

100, 200: Gas sensor 101: p-type layer 102, 103: n-type layer 104: Insulating film 105: Graphene film 106: Gate electrode 107: Source electrode 108: Drain electrode 109: Modified part

Claims (5)

With the semiconductor layer
The graphene film provided above the semiconductor layer and
A modified portion containing SnS ₂ , polyaniline, polypyrrole, poly (3,4-ethylenedioxythiophene), phthalocyanine, or pentacene that modifies the surface of the graphene membrane and is in contact with gas at least in part.
A barrier membrane between the semiconductor layer and the graphene film,
Two electrodes provided on the surface of the semiconductor layer so as to sandwich a lower portion of the barrier membrane,
A transistor characterized by having. The transistor according to claim 1, wherein the graphene film is in direct contact with the barrier membrane. A gas sensor comprising the transistor according to claim 1 or 2. The process of providing a graphene film above the semiconductor layer and
A step of modifying the surface of the graphene film and providing a modified portion containing SnS ₂ , polyaniline, polypyrrole, poly (3,4-ethylenedioxythiophene), phthalocyanine, or pentacene which is in contact with gas at least in part.
A step of forming a barrier film between the semiconductor layer and the graphene film,
A step of forming two electrodes on the surface of the semiconductor layer with a lower portion of the barrier membrane sandwiched between them.
A method for manufacturing a transistor, which comprises. The method for manufacturing a transistor according to claim 4 , wherein the graphene film is in direct contact with the barrier membrane.

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