JPS62299083A - Thin-film transistor - Google Patents

Abstract

PURPOSE:To prevent a current from being unnecessarily outputted in response to stray light by a method wherein an amorphous semiconductor film, which is a two-film layer of amorphous silicon hydride and amorphous silicon carbide, is employed in a thin-film transistor. CONSTITUTION:A thin-film transistor activation layer is constituted of a two- film layer of an amorphous silicon hydride thin film 4a and amorphous silicon carbide thin film 4b. In such a design, the effective optical gap falls at the middle between the large optical gap of amorphous silicon carbide and the small optical gap of amorphous silicon hydride. Exposure to stray light triggers off but a very little photoelectric current, which reduces the possibility of a current being outputted in response to such light and protects the S/N ratio from lowering. Accordingly, when a thin film transistor of this design is employed to drive a liquid crystal display unit capable of tonal display, the tone is protected from undesired changes attributable to stray light and therefore the tonal display characteristic of the unit is improved.

Description

[Detailed Description of the Invention] [Summary] This is an improvement of a thin film transistor. In particular, it eliminates the effects of stray light, and the S/N ratio does not decrease even when used in an environment where stray light exists, and when this thin film transistor is used to drive a liquid crystal display device, it This improvement improves the gradation display characteristics by preventing the gradation from changing.

The gist of the present invention is to provide a double layer of amorphous silicon carbide 1' [with a large optical gap and low photoconductivity and a thin film of hydrogenated amorphous silicon with a small localized level density, preferably a plurality of layers. The active layer consists of a layered structure, and this significantly reduces the photoconductive properties of the active layer, especially in the visible light and infrared light regions, and this thin film transistor is particularly useful for driving liquid crystal display equipment. In this case, there is no drawback that the voltage applied to the liquid crystal display device changes due to the photocurrent due to the photoconductive properties of its own driving thin film transistor, and therefore there is no drawback that unexpected fluctuations occur in the display of one gray level. I tried to avoid it.

[Industrial application field]

The present invention relates to improvements in thin film transistors.

In particular, this thin film transistor eliminates the negative effects caused by stray light, and even when used in an environment where stray light exists, there is no drawback that the S/N ratio decreases due to the flow of photocurrent due to its photoconductive properties, and this thin film transistor can be used in liquid crystal displays. The present invention relates to an improvement that prevents deterioration of gradation display characteristics due to stray light when used for driving a device.

[Conventional technology]

Thin film transistors used to drive liquid crystal display devices are classified into staggered type and inverted staggered type, but here is an example of an inverted staggered type thin film transistor! 7
In Figure 0 shown in Figure 58, 1 is an amorphous substrate such as a glass substrate, 2 is a gate electrode made of a molybdenum film, a chrome film, etc., and 3 is a gate insulating film made of a silicon dioxide film, a silicon nitride film, etc. 4 is an active layer made of an amorphous semiconductor film such as a hydrogenated amorphous silicon film, and 5 is a contact film made of a low resistance amorphous semiconductor film such as a hydrogenated amorphous silicon film with a high impurity concentration. 6 is a source electrode-drain electrode made of a metal film such as titanium or chromium.

[Problem that the invention seeks to solve]

By the way, since amorphous semiconductors such as hydrogenated amorphous silicon have high photoconductivity, they have the disadvantage that an unexpected output current flows in response to stray light, resulting in a decrease in the S/N ratio.

This drawback becomes an extremely serious drawback when the thin film transistor is used to drive a device that emits light by itself, such as a liquid crystal display device. If the liquid crystal display device, etc. displays gradations, the voltage applied to the liquid crystal display device, etc. changes based on an unexpected change in the output current that flows in response to stray light, resulting in an unexpected gradation. This is because one gradation display cannot be performed accurately.

An object of the present invention is to eliminate this drawback, and to provide a thin film transistor that does not cause unexpected output current to flow in response to stray light.

(l!'!!Means for solving the problem) The means taken by the present invention to achieve the above object is to use an amorphous semiconductor film 4 such as hydrogenated amorphous silicon to serve as an active layer. A gate electrode 2 is provided on one surface with a gate insulating film 3 interposed therebetween, and a source/drain electrode 6 is provided on the other surface of the amorphous semiconductor film 4 without facing the gate electrode 2. As the amorphous semiconductor film 4 of the thin film transistor, a double layer of thin 1194a of hydrogenated amorphous silicon with a small local level density and thin fiAb of amorphous silicon carbide with a large optical gap and low photoconductivity is used. It's about doing.

The thin film transistor according to the present invention can be used as either an inverted staggered type or a staggered type.

fj of hydrogenated amorphous silicon according to the gist of the present invention
@ 4 a and amorphous silicon carbide thin 1194
Advantageously, the thicknesses of b are each 50A or less.

The double layer according to the subject matter of the invention is very advantageously used as the laminate 4c.

Q of hydrogenated amorphous silicon according to the gist of the present invention
M4a and amorphous silicon carbide thin 11514
The double layer with b is advantageously used as a laminate.

The thickness of the double layer of hydrogenated amorphous silicon thin film 4a and carbide amorphous silicon thin film 4b according to the subject matter of the invention is advantageously less than 100 Å.

[Effect]

Optical gap of amorphous silicon carbide (roughly corresponds to the forbidden band width) and leading? ! The i-rate, that is, the η ratio that defines the photocurrent changes in accordance with the carbon content, as shown by A and B in FIG. 4, respectively. That is,
The former increases with carbon content, and the latter decreases with carbon content.

Therefore, using amorphous silicon carbide as the active layer can limit the photocurrent generated in response to stray light. However, since amorphous silicon carbide has many localized levels, it cannot be used as a material for the active layer of a thin film transistor.

Therefore, in the present invention, the active layer is constituted by a double layer of a thin film of amorphous silicon carbide and a thin film of hydrogenated amorphous silicon, particularly a superlattice as a laminate thereof.

A double layer of a thin film of amorphous silicon carbide and a thin film of hydrogenated amorphous silicon, especially a superlattice as a stack of the double layers, has an apparent forbidden band width, that is, an optical gap between the two, and moreover, This is because the adverse effects of unavoidable localized levels on silicon are reduced.

According to the experimental results, the thickness of the thin film of amorphous silicon carbide and the I! of hydrogenated amorphous silicon are different. When the film thickness of the I film is set to 50, and the forbidden band width of the latter is selected to be 2, QeV (the forbidden band width of the former is 1.85 eV), a thin film of amorphous silicon carbide and a thin film of hydrogenated amorphous silicon are selected. The effective forbidden band width of the double layer is 1.9 eV, and the adverse effects of localized levels that are inevitable in amorphous silicon carbide are reduced.

〔Example〕

Hereinafter, a thin film transistor according to an embodiment of the present invention will be further described with reference to the drawings.

Refer to Fig. 2. A film of molybdenum, chromium, nichrome, titanium, etc. is formed to a thickness of 500 to 00 mm on an amorphous insulating substrate 1 such as a glass plate using sputtering or vacuum evaporation. Then, the gate electrode 2 is formed by patterning this using a lithography method.

Refer to FIG. 3. Using a high frequency glow discharge decomposition method, a thin film of silicon oxide, silicon nitride, or silicon oxynitride is deposited to a thickness of 3.5 mm.
000, and this is used as the gate insulating film 3.

The active layer 4 is formed without breaking the vacuum.

This process consists of repeating the following two steps.

In the first step, hydrogenated 7morphosine recon pf
Form J 4 a. In this process, monosilane gas is used, and the total gas flow rate is 20-2503CCM.
(standard cubic centimeter 7 minutes), the substrate temperature was 200 to 300°C, and the gas pressure was 0.2 to 5 T.
orr and the RF power is 5 to 50 W to perform the high frequency glow discharge decomposition method. As a result, the optical gap (
Forbidden band width), S~1. ? A hydrogenated amorphous silicon film 4a of eV is formed.

In the second step, amorphous silicon carbide 1lI2
Form 4b. In this process, a mixed gas of methane and monosilane with a methane content of 0.1 to 0.8 is used (the volume ratio of methane to the remaining mixed gas of methane and monosilane is 0.1 to 0.8). As in the first step, the total gas flow rate was 20-250 SCCM (standard cubic centimeter 77 minutes), the substrate temperature was 200-300°C, and the gas pressure was 0.2-300°C. 5 Torr and RF power is 5 to 5
The high frequency glow discharge decomposition method is carried out at 0W. As a result, the optical gap (forbidden band width) is 1.7 to 2.3 eV
Amorphous silicon carbide l]i4b is formed.

By the way, as described later, hydrogenated amorphous silicon
Con l194a and amorphous silicon carbide g4b,
The thickness is preferably 50 Å or less, and the thickness of these double layers is preferably in the range of 10 to 100 nm. This double layer is a type of superlattice, and by making it a double layer, it is possible to This is because the purpose is to change the forbidden band width.

A plurality of these double layers are laminated to form a laminate 4c.

After forming a resist mask (not shown) in the area facing the gate electrode 2 using a lithography method, see FIG. After forming an amorphous silicon film and a film of titanium, chromium, molybdenum, nichrome, aluminum, or a combination thereof, the above resist mask (not shown) and the phosphorus-doped hydrogenated amorphous silicon film and titanium formed thereon are formed. , chromium, molybdenum, nichrome, aluminum, or a combination thereof to form a source electrode/drain electrode contact film 5 and a source/drain electrode 6, and finally device isolation is performed.

At this time, it is necessary that the source electrode/drain electrode contact film 5 and the source electrode/drain electrode 6 completely cover the side walls of the active layer 4. This is to prevent stray light from entering from the side.

A thin film transistor manufactured using the above process (from a double layer of hydrogenated amorphous silicon with an optical gap of 1.65 eV and a thickness of 10 mm and a thin film of carbon amorphous silicon with an optical gap of 2 OeV and a thickness of 10 mm) A gate voltage of 5 V (off state) and a source/drain voltage of 2 V are applied to a thin film transistor whose active layer is a stacked structure with an effective optical gap of 1.3. 1012 photons/C/sec), the source φ drain Tt
The relationship between the wavelengths of C-stream and incident light was measured and the results are shown in FIG.

As is clear from the figure, there was an improvement of about two orders of magnitude in the visible light region, and especially in the infrared light region, the distance was not measurable.

The active layer of the thin film transistor according to the present invention is constituted by a double layer of a thin film of hydrogenated amorphous silicon and a thin film of amorphous silicon carbide, particularly a superlattice as a laminate thereof. An important factor is the thickness of each layer that forms the double layer with the amorphous silicon thin film. Therefore,
Under the condition that both film thicknesses are the same, we manufactured prototypes with different film thicknesses and measured the relationship between light ttf and film thickness using an applied voltage of 100V and white irradiation light of 251K. In Figure 0 shown in Figure 6.

C is the in-plane direction current, and D is the optical 7Ii current.

As is clear from the figure, when each film thickness is 50 Å or less,
A fully satisfactory result can be obtained.

Refer to FIG. 7 The present invention can also be applied to a staggered type as shown in the figure.

〔Effect of the invention〕

As explained above, since the active layer of the thin film transistor according to the present invention is a double layer of a thin film of hydrogenated amorphous silicon and a thin film of amorphous silicon carbide, the effective forbidden band width, that is, the effective optical gap is the forbidden band width. In other words, it is said to be an intermediate value between that of amorphous silicon carbide, which has a large optical gap, and that of hydrogenated amorphous silicon, which has a small forbidden band width, or optical gap, and almost no photoconductive current flows even when receiving stray light. Even when used in an environment, output current (on current) rarely flows in response to stray light, and the S/N ratio does not decrease.

Therefore, when this thin film transistor is used to drive a liquid crystal display device that displays gradation, the gradation does not change due to stray light, and the gradation display characteristics are improved.

[Brief explanation of drawings]

FIG. 1 is a cross-sectional view of an inverted staggered thin film transistor according to an embodiment of the present invention. 2 and 3 are process diagrams for manufacturing an inverted staggered thin film transistor according to an embodiment of the present invention. FIG. 4 is a graph showing the relationship between the optical gap and η end of amorphous silicon carbide and the carbon content value. FIG. 5 is a graph showing the relationship between the optical gap and the carbon content value of amorphous silicon carbide. FIG. 7 is a graph showing the relationship between the source/drain current that flows in response to stray light and the wavelength of stray light. FIG. 6 is a graph showing the relationship between the photocurrent and the film thickness of a double layer of a thin film of hydrogenated amorphous silicon and a thin film of amorphous silicon carbide according to the gist of the present invention. FIG. 2 is a cross-sectional view of a staggered thin film transistor according to an embodiment (corresponding to claim 3). FIG. 8 is a cross-sectional view of an inverted staggered thin film transistor according to the prior art. 1... Amorphous substrate (glass substrate), 2e... Gate electrode, 3... Gate insulating film. 3a・9 Silicon oxynitride thin film. 3'b... Thin film of silicon nitride. 3C...Silicon oxynitride thin 1113a and silicon nitride thin P! Laminated body with A3b. 3d・9 Silicon nitride thin film. 4... Active layer made of an amorphous semiconductor film such as a hydrogenated amorphous silicon film, 5... Contact film, 6... Source electrode/drain electrode. 1 Process diagram Figure 3 * Invention Figure 1 Prior art Figure 8 Process diagram Figure 2 0 0.5 1. O x L shoulder content, ashing, 7 sushi line a support 1 f, rough 0 must H-ε..., l'4 IJmi. 24 projection length (nm) source tor 4 zot ji 屹ga 1 (4 I ■ 1,000, 2 cho = 5
[-Goseki Atsushi (in) Kyudenshi Nine Lights Puss 4 Kansho No. 6

Claims (1)

[Claims] [1] A gate electrode (2) is provided on one surface of the amorphous semiconductor film (4) via a gate insulating film (3), and a gate electrode (2) is provided on one surface of the amorphous semiconductor film (4). In a thin film transistor in which a source electrode and a drain electrode (6) are provided on the other surface without facing the gate electrode (2), the amorphous semiconductor film (4) is a hydrogenated amorphous silicon thin film (4a). ) and a thin film (4b) of amorphous silicon carbide. [2] The hydrogenated amorphous silicon thin film (4a)
2. The thin film transistor according to claim 1, wherein a plurality of double layers of amorphous silicon carbide and an amorphous silicon carbide thin film (4b) are laminated to form a laminate (4c). [3] The thin film transistor has the gate electrode (2)
3. The thin film transistor according to claim 1, wherein said thin film transistor is an inverted staggered thin film transistor formed on an amorphous substrate (1). [4] The thin film transistor is a staggered thin film transistor in which the source electrode/drain electrode (6) is formed on an amorphous substrate (1), or The thin film transistor according to item 2. [5] The hydrogenated amorphous silicon thin film (4a)
The thickness of the amorphous silicon carbide thin film (4b)
The thickness of each of claims 1, 2, and 3 is 50 Å or less, or
The thin film transistor according to item 4. [6] The hydrogenated amorphous silicon thin film (4a)
Claims 1, 2, 3, 4, or 4, characterized in that the thickness of the double layer of amorphous silicon carbide and the amorphous silicon carbide thin film (4b) is 100 Å or less. The thin film transistor according to item 5.