Thin film transistor

Abstract

To provide a thin film transistor including a hydrogenated indium oxide thin film as an active layer, in which a secular change of a transistor characteristic due to gate voltage application is prevented.SOLUTION: A thin film transistor includes: a substrate; a gate electrode provided on the substrate; a gate insulating film provided on the gate electrode; an active layer provided on the gate insulating film and composed of hydrogenated indium oxide, and a source electrode and a drain electrode disposed with the active layer therebetween. A surface of the active layer is covered with rare earth oxide: Ln2O3 (Ln: rare earth element). The rare earth oxide is selected from the group consisting of yttrium oxide, erbium oxide, gadolinium oxide, and ytterbium oxide, for example.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a thin film transistor for driving pixels in liquid crystal displays and organic electroluminescence displays, and more particularly to a thin film transistor using hydrogenated indium oxide in the active layer. [Background technology]

[0002] Thin-film transistors with transparent oxide semiconductors as active layers are used as pixel driving elements for displays such as large LCD televisions and organic EL televisions. For example, thin-film transistors using amorphous InGaZnO4 as a transparent oxide semiconductor have a field-effect mobility of 10 cm 2 / Vs has been obtained (see Patent Document 1).

[0003] Such displays require even higher resolution and faster operation. The next generation of oxide semiconductor thin-film transistors will have a 50 cm 2 A field effect mobility of more than 140 cm / Vs is required. Thin film transistors with various transparent oxide semiconductors as active layers have been proposed. For example, a thin film transistor with an active layer of hydrogenated indium oxide has a field effect mobility of 140 cm 2 It has been reported that the thermal conductivity is 100 / Vs (see Non-Patent Document 1). [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 4620046 [Non-patent literature]

[0005] [Non-Patent Document 1] Yusaku Magari, Taiki Kataoka, Wenchang Yeh & Mamoru Furuta, "High-mobility hydrogenated polycrystalline In2O3 (In2O3:H) thin-film transistors", Nature Communications 13, 1078 (2022) Summary of the Invention [Problem to be solved by the invention]

[0006] Although thin-film transistors using a hydrogenated indium oxide thin film as an active layer can meet the switching speed requirements for next-generation displays, they have the problem that the transistor characteristics change over time.

[0007] In response to this, the inventors discovered that in a thin-film transistor using hydrogenated indium oxide as the active layer, the surface of the hydrogenated indium oxide thin film is hydrophilic and many water molecules are adsorbed thereon, but when a negative gate electric field is applied to this, the hydroxyl groups adsorbed on the surface of the active layer are released, and the trapped electrons return to the active layer, increasing the electron density in the active layer, causing a change over time. They then discovered that the release of hydroxyl groups can be suppressed by covering the surface of the active layer of the hydrogenated indium oxide thin film with a protective layer having the same crystal structure as indium oxide, which led to the completion of the present invention.

[0008] That is, an object of the present invention is to provide a thin film transistor having an active layer made of a thin film of hydrogenated indium oxide, which prevents deterioration over time of the transistor characteristics caused by application of a gate voltage. [Means for solving the problem]

[0009] One aspect of the present invention is A substrate; a gate electrode provided on a substrate; a gate insulating film provided on the gate electrode; an active layer made of hydrogenated indium oxide provided on the gate insulating film, and a source electrode and a drain electrode disposed on either side of the active layer; Furthermore, the thin film transistor is characterized in that the surface of the active layer is covered with a rare earth oxide: Ln2O3 (Ln: rare earth element). Effect of the Invention

[0010] The thin-film transistor of the present invention has a good field-effect mobility and its transistor characteristics do not change over time by covering the surface of the active layer of indium oxide hydride with a protective layer having the same crystal structure as indium oxide. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a top view of a thin film transistor according to an embodiment of the present invention. [Diagram 2] 2 is a cross-sectional view taken along the line II-II in FIG. [Diagram 3] 1 shows the relationship between the gate voltage Vg and the drain current Id and the field effect mobility μFE of the thin film transistor according to the embodiment of the present invention. [Figure 4] 4 shows the characteristics of the thin film transistor of the embodiment after bias stress is applied. [Diagram 5] 4 shows characteristics of a thin film transistor of a comparative example after application of a bias stress. [Figure 6] FIG. 1 is a schematic diagram of a thin film transistor according to a comparative example. [Figure 7] FIG. 1 is a schematic diagram of a thin film transistor according to an embodiment of the present invention. [Figure 8] This shows the results of a reliability test of a thin film transistor when the material of the protective layer is changed. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] FIG. 1 is a top view of a thin film transistor according to an embodiment of the present invention, generally designated by 100, and FIG. 2 is a cross-sectional view taken along the line II-II in FIG.

[0013] 1 and 2, a thin film transistor 100 includes an alkali-free glass substrate 10. An ITO (Indium Tin Oxide) film 20 is laminated on the glass substrate 10. The ITO film 20 functions as a back gate electrode.

[0014] A gate insulating film 30 is provided on the ITO film 20. The gate insulating film 30 is made of aluminum oxide having a thickness of, for example, 100 nm.

[0015] An active layer 40 is provided on the gate insulating film 30. The active layer 40 is made of, for example, polycrystalline hydrogenated indium oxide (In2O3:H) having a film thickness of 5 nm. Hydrogen in the active layer 40 has the effect of increasing the crystallization temperature, causing explosive grain growth at around 200°C, thereby increasing the field-effect mobility, and the effect of adsorbing as hydroxyl groups on the surface of the In2O3 film, capturing electrons on the surface of the In2O3 film, thereby determining the threshold voltage. For this reason, as shown in Table 1, in order to obtain a good transistor threshold voltage and field-effect mobility, the hydroxyl group concentration (=hydrogen concentration) on the surface of the active layer 40 made of hydrogenated indium oxide should be, for example, 1×10 13 ~1×10 16 pieces / cm 2 and preferably 1×10 14 ~1×10 15 pieces / cm 2 It is.

[0016] Table 1 TIFF2025074676000002.tif83141

[0017] A source electrode 50 and a drain electrode 60 are provided on both ends of the active layer 40. The source electrode 50 and the drain electrode 60 are made of a conductive material, for example, ITO.

[0018] Furthermore, a protective layer 70 is provided so as to cover the entire surface of the active layer 40. The protective layer 70 is made of yttrium oxide having a thickness of, for example, 50 nm. The material of the protective layer 70 is preferably a rare earth oxide Ln2O3 (Ln: rare earth element) which has the same crystal structure as the indium oxide of the active layer 40, and in particular preferably has a C-type rare earth structure. Specifically, yttrium oxide, erbium oxide, gadolinium oxide, and ytterbium oxide are preferred.

[0019] In other words, there must be a degree of lattice matching between the active layer 40 and the protective layer 70 thereon, to the extent that the protective layer 70 can grow epitaxially on the active layer 40. In particular, it is preferable to cover the active layer 40 with a protective layer 70 having the same crystal structure as the active layer 40.

[0020] As shown in Figures 1 and 2, the thin film transistor 100 has a back-gate (inverse staggered) structure in which a gate voltage is applied from the back surface (bottom in Figure 2) of the active layer 40. The active layer 40 made of hydrogenated indium oxide functions as an n-type channel, and the n-type channel is turned on / off by applying a gate voltage to the ITO film 20. In this thin film transistor 100, the field effect mobility is 50 to 150 cm 2 / Vs, and the transistor characteristics do not change even when a negative gate voltage is applied.

[0021] Here, for each layer, a specific material and film thickness are shown, but these are merely examples and are not intended to be limiting.

[0022] Next, a description will be given of a method for manufacturing the thin film transistor 100. In the manufacturing method, first, a commercially available non-alkali glass substrate 10 with an ITO film 20 is prepared. The ITO film 20 may be deposited on the non-alkali glass substrate 10.

[0023] Next, a gate insulating film 30 made of aluminum oxide and having a thickness of 100 nm is formed on the ITO film 20 by atomic layer deposition.

[0024] Next, a 5 nm-thick amorphous hydrogenated indium oxide thin film is deposited on the gate insulating film 30 by pulsed laser deposition (PLD) using a metal mask. Here, the pulsed laser deposition method is used, but the method is not limited to this, and for example, a physical vapor deposition method such as sputtering, an atomic layer deposition method, a chemical method such as a chemical vapor transport method, and a sol-gel method may be used. Hydrogen atoms can be introduced into the indium oxide thin film by supplying hydrogen gas, water vapor, hydrogen radicals, or atomic hydrogen during the deposition of the amorphous indium oxide thin film, or by using moisture remaining in the film formation chamber.

[0025] Next, the amorphous hydrogenated indium oxide thin film is crystallized by annealing in air at 300°C for 30 minutes to form a polycrystalline hydrogenated indium oxide active layer 40. The crystal grain size of the polycrystalline hydrogenated indium oxide is, for example, 50 nm or more, preferably 50 nm to 100 μm, and more preferably 50 nm to 10 μm. If the crystal grain size becomes larger than the channel length of the thin film transistor, no grain boundary exists in the channel region, and the thin film transistor does not turn off. The annealing atmosphere may be air, pseudo air, oxygen, water vapor, nitrogen, or the like. The annealing temperature is selected from, for example, 150 to 450°C, and the annealing time is selected from, for example, 1 to 180 minutes.

[0026] Next, an ITO film having a thickness of 100 nm is formed on the gate insulating film 30 and the active layer 40 through a metal mask, and a source electrode 50 and a drain electrode 60 are formed.

[0027] Next, a protective layer 70 having a thickness of 50 nm is formed so as to cover the surface of the active layer 40. The protective layer 70 is made of a polycrystalline thin film of yttrium oxide having the same crystal structure as the indium oxide that forms the active layer 40. The protective layer 70 preferably covers the entire surface of the active layer 40.

[0028] After the structure of the thin film transistor 100 is fabricated, a post-annealing process is performed. In the post-annealing process, a heat treatment is performed, for example, in the atmosphere at 350° C. for 30 minutes. The atmosphere for the post-annealing process is not limited to the atmosphere, and may be a pseudo atmosphere, oxygen, water vapor, nitrogen, or the like. The annealing temperature is selected, for example, from 150 to 450° C., and the annealing time is selected, for example, from 1 to 180 minutes.

[0029] Through the above steps, a thin film transistor 100 with a back gate (inverse staggered) structure is completed. In this example, the thin film transistor has a channel length of 800 μm, a channel width of 400 μm, and a capacitance of the gate insulating film 30 of 70 nF / cm. 2 The size of the thin film transistor 100 and the capacitance of the gate insulating film are not limited to these. EXAMPLES

[0030] FIG. 3 is a graph showing the relationship between the gate voltage Vg, the drain current Id, and the field effect mobility μ of the thin film transistor 100 according to an embodiment of the present invention. FE The thin film transistor 100 had a channel length of 800 μm, a channel width of 400 μm, and a drain voltage of 5 V. The transistor characteristics were measured at room temperature in air using a semiconductor device analyzer (B1500A).

[0031] As can be seen from FIG. 3, the field-effect mobility μ FE is 95cm at a gate voltage of 20V 2 This value is equal to the 50 cm / Vs required for next-generation oxide semiconductor thin-film transistors. 2 This is significantly higher than / Vs.

[0032] 4 shows the relationship between the gate voltage Vg and the drain current Id of the thin film transistor 100 according to this embodiment, measured after applying a positive bias or a negative bias to the gate electrode for a certain period of time (application of bias stress). (a) shows the result of measurement after applying a positive gate voltage bias stress (PBS: Positive Bias Stress) of +20V for 0 to 10,000 seconds in an air atmosphere at 60°C, and (b) shows the result of measurement after applying a negative gate voltage bias stress (NBS: Negative Bias Stress) of -20V for 0 to 10,000 seconds in the same air atmosphere at 60°C. The application of the gate voltage and the measurement of the drain current Id were performed in a darkroom.

[0033] As can be seen from Figures 4(a) and (b), no change was observed in the gate voltage Vg-drain current Id characteristics even when a positive gate voltage and a negative gate voltage were applied for 10,000 seconds, and the drain current Ig did not flow when the gate voltage Vg was 0 V or less, but a current flowed when a positive gate voltage Vg was applied, resulting in good switching characteristics. Thus, with the thin film transistor 100 according to this embodiment, a stable thin film transistor whose transistor characteristics do not change over time is obtained. Comparative Example

[0034] 5 shows, as a comparative example, the relationship between the gate voltage Vg and the drain current Id measured after applying a positive or negative bias to the gate electrode for a certain period of time (application of bias stress) to a thin film transistor that does not have a protective layer covering the surface of the active layer 40. The structure of the thin film transistor other than the protective layer is the same as that of the thin film transistor 100 of the embodiment.

[0035] In Figure 5, (a) shows the results measured after applying a positive gate voltage (PBS: Positive Bias Stress) of +20V for 0 to 10,000 seconds in an air atmosphere at 60°C, and (b) shows the results measured after applying a negative gate voltage (NBS: Negative Bias Stress) of -20V for 0 to 10,000 seconds in the same air atmosphere at 60°C. The application of the gate voltage and the measurement of the drain current Id were carried out in a darkroom.

[0036] As shown in Figure 5(a), when a positive gate voltage (+20 V) is applied for 0 to 10,000 seconds, the gate voltage Vg-drain current Id curve gradually shifts in the positive direction (to the right in Figure 5) with the application time, and after 10,000 seconds of application, it has shifted in the positive direction by about +2 V (in Figure 5(a) the leftmost curve is 0 seconds, and the rightmost curve is 10,000 seconds).

[0037] On the other hand, as shown in FIG. 5(b), when a negative gate voltage (-20 V) is applied for 0 to 10,000 seconds, the gate voltage Vg-drain current Id curve shifts in the negative direction with the application time, and after 10,000 seconds of application, it has shifted by about -10 V (in FIG. 5(b), the rightmost curve is 0 seconds, and the leftmost curve is 10,000 seconds).

[0038] Thus, in the comparative thin film transistor having no protective layer, the change over time is particularly large when a negative gate voltage is applied, and stable transistor characteristics cannot be obtained, resulting in a thin film transistor with low reliability.

[0039] Next, the cause of the difference in transistor characteristics between the thin film transistors of the comparative example and the embodiment will be described. Figure 6 is a schematic diagram of the thin film transistor of the comparative example, where (a) shows the case where no gate voltage is applied and (b) shows the case where a negative gate voltage is applied. In Figure 6, the same symbols as in Figure 1 indicate the same or corresponding parts.

[0040] 6(a), the active layer 40 of the thin-film transistor 100 is made of indium oxide hydride, and the surface of the indium oxide hydride thin film is hydrophilic, with many water molecules adsorbed to it. These chemically adsorbed water molecules are stabilized by their hydroxyl groups trapping electrons.

[0041] As shown in FIG. 6B, when a negative gate voltage is applied, the gate insulating film 30 is polarized, and the hydroxyl groups adsorbed on the surface are polarized due to the influence of the electrons accumulated on the active layer 40 side. 2OH - → H2O+(1 / 2)O2+2e - The reaction produces water and oxygen, which leave the surface.

[0042] As a result, electrons are left behind in the active layer 40, the electron density increases, the conductivity increases, and the transistor characteristics change (for example, the threshold voltage shifts) as shown in FIG. 5(b).

[0043] 7 is a schematic diagram of a thin film transistor according to an embodiment of the present invention, where (a) shows no gate voltage applied and (b) shows a negative gate voltage applied. In FIG. 7, the surface of the active layer 40 is covered with a protective layer 70 (see FIG. 1), but the protective layer 70 is omitted here for ease of explanation. In FIG. 7, the same reference numerals as in FIG. 1 indicate the same or corresponding parts.

[0044] As in the comparative example, when no gate voltage is applied as shown in (a), water molecules are adsorbed onto the surface of the indium oxide hydride thin film, and the hydroxyl groups trap electrons, stabilizing the thin film.

[0045] Next, as shown in (b), when a negative gate voltage is applied, the gate insulating film 30 is polarized and electrons are accumulated on the active layer 40 side, but since the surface of the active layer 40 is covered with a protective layer 70 (not shown), the hydroxyl groups on the surface of the active layer 40 are prevented from being released. Therefore, unlike the comparative example, electrons are not left behind in the active layer 40, and the electron density does not increase, and the transistor characteristics do not change. As a result, stable transistor characteristics without a shift in the gate current as shown in FIG. 4(b) are obtained.

[0046] In order to prevent the detachment of hydroxyl groups from the surface of the active layer 40, it is preferable to use rare earth oxide Ln2O3 (Ln: rare earth element) which has the same crystal structure as the indium oxide of the active layer 40, for the protective layer 70, and it is particularly preferable to use a C-type rare earth structure. Specifically, it is preferable to use yttrium oxide, erbium oxide, gadolinium oxide, or ytterbium oxide.

[0047] FIG. 8 and Table 2 show the results of a reliability test of a thin-film transistor when the material of the protective layer 70 is changed. In FIG. 8, when HfO2, Al2O3, Y2O3, Er2O3, Gd2O3, Yb2O3, Sm2O3, and Nd2O3 are used as the material of the protective layer 70, (a) shows the result measured after applying a positive gate voltage (PBS: Positive Bias Stress) of +20V for 5000 seconds, and (b) shows the result measured after applying a negative gate voltage (NBS: Negative Bias Stress) of -20V for 5000 seconds. The application of the gate voltage and the measurement of the drain current Id were performed in a dark room. Table 2 summarizes these measurement results.

[0048] Table 2 TIFF2025074676000003.tif66127

[0049] From the results of FIG. 8 and Table 2, it can be seen that the protective layer 70 having the C-type rare earth structure hardly shifts the gate voltage Vg-drain current Id curve after application of a positive gate voltage and a negative gate voltage, and is effective in preventing the transistor characteristics from changing over time. On the other hand, Nd2O3 and Sm2O3 have a rare earth ion radius of In 3+ (0.8Å) or larger (Nd 3+ :0.983Å, Sm 3+ : 0.958 Å), and the crystal structures are A-type rare earth structure and B-type rare earth structure, respectively. For this reason, it is difficult to heteroepitaxially grow the protective layer 70 on the indium oxide thin film that constitutes the active layer 40, and as a result, it is not possible to prevent the elimination of hydroxyl groups, particularly when a negative gate voltage is applied, causing deterioration of the device characteristics.

[0050] Thus, in the thin-film transistor 100 according to the embodiment of the present invention, by covering the surface of the active layer 40 with a protective layer 70 of rare earth oxide Ln2O3 (Ln: rare earth element), which has the same crystal structure as the indium oxide of the active layer 40, it is possible to provide a thin-film transistor 100 having transistor characteristics that do not change over time. [Industrial Applicability]

[0051] INDUSTRIAL APPLICABILITY The present invention can be used for devices for driving pixels of displays, particularly for devices for driving pixels of next-generation displays that require higher definition and higher speed. [Explanation of symbols]

[0052] 10 Glass substrate 20 ITO film 30 Gate insulating film 40 Active layer 50 Source electrode 60 Drain electrode 70 protective layer 100 Thin Film Transistor

Claims

1. A substrate; a gate electrode provided on a substrate; a gate insulating film provided on the gate electrode; an active layer made of hydrogenated indium oxide provided on the gate insulating film, and a source electrode and a drain electrode disposed on either side of the active layer; Furthermore, the surface of the active layer is made of a rare earth oxide: Ln ２ O ３ A thin film transistor characterized in that it is covered with (Ln: rare earth element).

2. 2. The thin film transistor according to claim 1, wherein the rare earth oxide is an oxide having a C-type rare earth structure.

3. 2. The thin film transistor of claim 1, wherein the rare earth oxide is an oxide selected from the group consisting of yttrium oxide, erbium oxide, gadolinium oxide, and ytterbium oxide.

4. The concentration of hydroxyl groups on the surface of the active layer is 1×10 １３ ~1×10 １６ pieces / cm ２ 2. The thin film transistor according to claim 1 ,

5. The concentration of hydroxyl groups on the surface of the active layer is 1×10 １４ ~1×10 １５ pieces / cm ２ 2. The thin film transistor according to claim 1 ,

6. 2. The thin film transistor according to claim 1, wherein the crystal grain size of the hydrogenated indium oxide in the active layer is 50 nm or more.

7. 7. The thin film transistor according to claim 6, wherein the crystal grain size of the hydrogenated indium oxide in the active layer is 50 nm to 100 μm.

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