Amorphous Oxide And Thin Film Transistor

Abstract

The present invention relates to an amorphous oxide and a thin film transistor using the amorphous oxide. In particular, the present invention provides an amorphous oxide having an electron carrier concentration less than 1018 / cm3, and a thin film transistor using such an amorphous oxide. In a thin film transistor having a source electrode 6, a drain electrode 5, a gate electrode 4, a gate insulating film 3, and a channel layer 2, an amorphous oxide having an electron carrier concentration less than 1018 / cm3 is used in the channel layer 2.

Description

TECHNICAL FIELD [0001] The present invention relates to amorphous oxides and thin film transistors. BACKGROUND ART [0002] A thin film transistor (TFT) is a three-terminal element having a gate terminal, a source terminal, and a drain terminal. It is an active element in which a semiconductor thin film deposited on a substrate is used as a channel layer for transportation of electrons or holes and a voltage is applied to the gate terminal to control the current flowing in the channel layer and switch the current between the source terminal and the drain terminal. Currently, the most widely used TFTs are metal-insulator-semiconductor field effect transistors (MIS-FETs) in which the channel layer is composed of a polysilicon or amorphous silicon film. [0003] Recently, development of TFTs in which ZnO-based transparent conductive oxide polycrystalline thin films are used as the channel layers has been actively pursued (Patent Document 1). These thin films can be formed at low temperatur...

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Benefits of technology

[0030] The elements M2, M3, and M4 having atomic numbers smaller than those of Zn, In, and Sn, respectively, have higher ionicity than Zn, In and Sn; thus, generation of oxygen defects is less frequent, and the electron carrier concentration can be decreased. Although Lu has a larger atomic number than Ga, the ion radius is small and the ionicity is high, thereby achieving the same functions as those of M3. M5, which is ionized at a valency of 5, strongly bonds to oxygen and rarely causes oxygen defects. Tungsten (W), which is ionized at a valency of 6, strongly bonds to oxygen and rarely causes oxygen defects.

[0036] Addition of the above-described elements will increase the stability of the amorphous film and expands the composition range that can give an amorphous film. In particular, addition of highly covalent B, Si, or Ge is effective for stabilization of the amorphous phase, and a compound oxide composed of ions with largely different ion radii can stabilize the amorphous phase. For example, in the In—Zn—O system, a stable amorphous film is rarely obtained at room temperature unless the range of In content is more than about 20 at %. However, by adding Mg in an equivalent amount to In, a stable amorphous film can be obtained at an In content of more than about 15 at %.

[0045] Use of an Al2O3 film can decrease the leak current. Use of an Y2O3 film can reduce the hysteresis. Use of a high dielectric constant HfO2 film can increase the field effect mobility. By using a film composed of a mixed crystal of these compounds, a TFT having small leak current and hysteresis and large field effect mobility can be produced. the process for forming the gate insulating film and the process for forming the channel layer can be conducted at room temperature; thus, a TFT of a staggered or inverted staggered structure can be formed. (Transistor)

[0048] The electron mobility of the oxide crystals increases as the overlap of the s orbits of the metal ion increases. The oxide crystals of Zn, In, and Sn having large atomic numbers exhibit high electron mobility of 0.1 to 200 cm2 / (V·sec). Since ionic bonds are formed between oxygen and metal ions in an oxide, electron mobility substantially comparable to that in a crystallized state can be exhibited in an amorphous state in which there is no directionality of chemical bonding, the structure is random, and the directions of the bonding are nonuniform. In contrast, by replacing Zn, In, and Sn each with an element having a smaller atomic number, the electron mobility can be decreased. Thus, by using the amorphous oxide described above, the electron mobility can be controlled within the range of about 0.01 cm2 / (V·sec) to 20 cm2 / (V·sec).

[0050] Once a voltage is applied to the gate terminal, electrons are injected into the amorphous oxide channel layer, and current flows between the source and drain terminals, thereby allowing the part between the source and drain terminals to enter an ON state. According to the amorphous oxide film of the present invention, since the electron mobility increases with the electron carrier concentration, the current that flows when the transistor is turned ON can be further increased. In other words, the saturation current and the on / off ratio can be further increased. When the amorphous oxide film having high electron mobility is used as the channel layer of a TFT, the saturation current can be increased and the switching rate of the TFT can be increased, thereby achieving high-speed operation.

[0086] As described above, by controlling the oxygen partial pressure, oxygen defects can be reduced, and therefore the electron carrier concentration can be reduced. Unlike in the polycrystalline state, in the amorphous state, there is essentially no grain interface; therefore, an amorphous thin film with high electron mobility can be obtained. Note that when a polyethylene terephthalate (PET) film having a thickness of 200 μm was used instead of the glass substrate, the resulting InGaO3(ZnO)4 amorphous oxide thin film exhibited similar characteristics. Example 4 Formation of In—Zn—Ga—Mg—O Amorphous Oxide Film by PLD Method

Problems solved by technology

However, known ZnO rarely forms a stable amorphous phase at room temperature and mostly exhibits polycrystalline phase; therefore, the electron mobility cannot be increased because of the diffusion at the interfaces of polycrystalline grains.

Moreover, ZnO tends to contain oxygen defects and a large number of carrier electrons, and it is thus difficult to decrease the electrical conductivity.

Therefore, it has been difficult to increase the on / off ratio of the transistors.

Although this is sufficient for regular transparent electrodes, the film cannot be easily applied to a channel layer of a TFT.

This is because it has been found that a TFT having a channel layer composed of this amorphous oxide film does not exhibit a sufficient on / off ratio and is thus unsuitable for TFT of a normally off type.

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