DISPLAY DEVICE

A highly reliable display device is provided. The display device including a light-emitting element and an insulating layer placed to cover the light-emitting element and the light-emitting element includes a first conductive layer, an EL layer over the first conductive layer, and a second conductive layer over the EL layer and the insulating layer includes a first layer, a second layer over the first layer, and a third layer over the second layer and the first layer has a function of capturing or fixing at least one of water and oxygen, the second layer has a function of inhibiting diffusion of at least one of water and oxygen, and the third layer has a higher concentration of carbon than at least one of the first layer and the second layer.

TECHNICAL FIELD

One embodiment of the present invention relates to a display device and a display module. One embodiment of the present invention relates to a fabrication method of the display device.

Note that one embodiment of the present invention is not limited to the above technical field. Examples of the technical field of one embodiment of the present invention disclosed in this specification and the like include a semiconductor device, a display device, a light-emitting apparatus, a power storage device, a storage device, an electronic device, a lighting device, an input device, an input/output device, a driving method thereof, and a fabrication method thereof. Note that in this specification and the like, a semiconductor device refers to every device that can function by utilizing semiconductor characteristics.

BACKGROUND ART

In recent years, higher-resolution display panels have been required. For example, devices for virtual reality (VR), augmented reality (AR), substitutional reality (SR), or mixed reality (MR) are given as devices requiring high-resolution display panels and have been actively developed in recent years.

Examples of display devices that can be used for a display panel include, typically, a liquid crystal display device, a light-emitting apparatus including a light-emitting element such as an organic EL (Electro Luminescence) element or a light-emitting diode (LED), and electronic paper performing display by an electrophoretic method or the like.

For example, the basic structure of an organic EL element is a structure in which a layer containing a light-emitting organic compound is provided between a pair of electrodes. By applying voltage to this element, light emission can be obtained from the light-emitting organic compound. A display device using such an organic EL element does not need a backlight that is necessary for a liquid crystal display device and the like; thus, a thin, lightweight, high-contrast, and low-power display device can be achieved. Patent Document 1, for example, discloses an example of a display device using an organic EL element.

REFERENCE

SUMMARY OF THE INVENTION

Problems to be Solved by the Invention

For example, in the above-described devices for VR, AR, SR, or MR that are wearable, a lens for focus adjustment needs to be provided between eyes and the display panels. Since part of the screen is enlarged by the lens, low resolution of the display panels might cause a problem of weak sense of reality and immersion.

The display panel is also required to have high color reproducibility. In particular, when display panels which have high color reproducibility are used in the above-described devices for VR, AR, SR, or MR, display with colors which are close to those of actual objects can be performed and sense of reality and immersion can be enhanced.

An object of one embodiment of the present invention is to provide a display device with extremely high resolution. Another object of one embodiment of the present invention is to provide a highly reliable display device. Another object of one embodiment of the present invention is to provide a display device in which high color reproducibility is achieved. Another object of one embodiment of the present invention is to provide a high-luminance display device. Another object of one embodiment of the present invention is to provide a fabrication method of the above-described display device.

Note that the description of these objects does not preclude the existence of other objects. One embodiment of the present invention does not need to achieve all these objects. Note that objects other than these can be derived from the description of the specification, the drawings, the claims, and the like.

Means for Solving the Problems

One embodiment of the present invention is a display device including a light-emitting element and an insulating layer placed to cover the light-emitting element; the light-emitting element includes a first conductive layer, an EL layer over the first conductive layer, and a second conductive layer over the EL layer; the insulating layer includes a first layer, a second layer over the first layer, and a third layer over the second layer; the first layer has a function of capturing or fixing at least one of water and oxygen; the second layer has a function of inhibiting diffusion of at least one of water and oxygen; and the third layer has a higher concentration of carbon than at least one of the first layer and the second layer.

Another embodiment of the present invention is a display device including a transistor over a substrate, a first insulating layer over the transistor, a plug placed to be embedded in the first insulating layer, a light-emitting element over the first insulating layer, and a second insulating layer placed to cover the light-emitting element; the light-emitting element includes a first conductive layer, an EL layer over the first conductive layer, and a second conductive layer over the EL layer; the plug electrically connects one of a source and a drain of the transistor and the first conductive layer; the second insulating layer includes a first layer, a second layer over the first layer, and a third layer over the second layer; the first layer has a function of capturing or fixing at least one of water and oxygen, the second layer has a function of inhibiting diffusion of at least one of water and oxygen, and the third layer has a higher concentration of carbon than at least one of the first layer and the second layer.

In the above structure, the third insulating layer may be included between the first insulating layer and the light-emitting element, and the third insulating layer may have a function of inhibiting diffusion of at least one of water and oxygen. In the above structure, the third insulating layer preferably contains nitrogen and silicon. In the above structure, the third insulating layer may be in contact with the second insulating layer in a region not overlapping with the light-emitting element.

In the above structure, the substrate may be a silicon substrate and the transistor may contain silicon in a channel formation region. In the above structure, an oxide semiconductor film may be provided over the substrate and the transistor may include an oxide semiconductor film in a channel formation region.

In the above structure, the first layer is preferably in contact with a side surface of the EL layer. In the above structure, the first layer is preferably deposited by a sputtering method. In the above structure, the first layer preferably contains oxygen and aluminum. In the above structure, the first layer may contain oxygen and hafnium.

In the above structure, the second layer is preferably deposited by a sputtering method. In the above structure, the second layer preferably contains nitrogen and silicon.

In the above structure, the third layer is preferably deposited by an ALD method. In the above structure, the third layer may have a higher concentration of hydrogen than at least one of the first layer and the second layer. In the above structure, the third layer may have a lower density than at least one of the first layer and the second layer. In the above structure, the third layer may contain oxygen and aluminum.

In the above structure, the side surface of the EL layer may be positioned inward from a side surface of the first conductive layer. In the above structure, the EL layer may cover the side surface of the first conductive layer. In the above structure, an insulator may be provided between the EL layer and the first conductive layer, the insulator may include an opening over the first conductive layer, and the EL layer may be in contact with the first conductive layer in the opening.

Effect of the Invention

According to one embodiment of the present invention, a display device with extremely high resolution can be provided. A highly reliable display device can be provided. A display device in which high color reproducibility is achieved can be provided. A high-luminance display device can be provided. A fabrication method of the above-described display device can be provided.

Note that the description of these effects does not preclude the existence of other effects. One embodiment of the present invention does not need to have all these effects. Note that other effects can be derived from the description of the specification, the drawings, and the claims.

MODE FOR CARRYING OUT THE INVENTION

In each drawing described in this specification, the size, the layer thickness, or the region of each component is exaggerated for clarity in some cases. Therefore, they are not limited to the illustrated scale.

In this specification and the like, the ordinal numbers such as “first” and “second” are used in order to avoid confusion among components and do not limit the number.

In this embodiment, a display device of one embodiment of the present invention and a fabrication method of the display device will be described.

The display device of one embodiment of the present invention includes a light-emitting element (also referred to as a light-emitting device) which emits light of different colors. The light-emitting element includes a lower electrode, an upper electrode, and a light-emitting layer (also referred to as a layer containing a light-emitting compound) therebetween. As the light-emitting element, an electroluminescent element such as an organic EL element or an inorganic EL element is preferably used. Alternatively, a light-emitting diode (LED) may be used.

As the EL element, an OLED (Organic Light Emitting Diode), a QLED (Quantum-dot Light Emitting Diode), or the like can be used. Examples of the light-emitting compound (also referred to as a light-emitting substance) contained in the EL element include a substance that emits fluorescent light (a fluorescent material), a substance that emits phosphorescent light (a phosphorescent material), an inorganic compound (e.g., a quantum dot material), and a substance that exhibits thermally activated delayed fluorescence (a thermally activated delayed fluorescence (TADF) material).

As the light-emitting substance, a substance that exhibits an emission color of blue, purple, bluish purple, green, yellowish green, yellow, orange, red, or the like is appropriately used. A substance that emits near-infrared light may be used.

The light-emitting layer may contain one or more kinds of compounds (a host material and an assist material) in addition to the light-emitting substance (a guest material). As the host material and the assist material, one or more kinds of substances having a larger energy gap than the light-emitting substance (the guest material) can be selected and used. As the host material and the assist material, compounds which form an exciplex are preferably used in combination. In order to form an exciplex efficiently, it is particularly preferable to combine a compound which easily accepts holes (a hole-transport material) and a compound which easily accepts electrons (an electron-transport material).

Either a low molecular compound or a high molecular compound can be used for the light-emitting element and an inorganic compound (such as a quantum dot material) may also be contained in the light-emitting element.

In the display device of one embodiment of the present invention, the light-emitting elements of different colors can be separately formed with extremely high accuracy. Thus, a display device with higher resolution than a conventional display device can be achieved. For example, the display device preferably has extremely high resolution in which pixels including one or more light-emitting elements are arranged with a resolution greater than or equal to 2000 ppi, preferably greater than or equal to 3000 ppi, further preferably greater than or equal to 5000 ppi, still further preferably greater than or equal to 6000 ppi, and less than or equal to 20000 ppi or less than or equal to 30000 ppi.

More specific structure examples and a fabrication method example will be described below with reference to drawings.

Structure Example 1

FIG.1Ais a schematic cross-sectional view illustrating a display device of one embodiment of the present invention.FIG.1Bis an enlarged view of a region A illustrated inFIG.1Awhich is sandwiched between the light-emitting elements120. A display device100includes a light-emitting element120R, a light-emitting element120G, and a light-emitting element120B. The light-emitting element120R is a light-emitting element which emits red light, the light-emitting element120G is a light-emitting element which emits green light, and the light-emitting element120B is a light-emitting element which emits blue light.

In the following description common to the light-emitting element120R, the light-emitting element120G, and the light-emitting element120B, the alphabets added to the reference numerals are omitted and the term “light-emitting element120” is used in the description in some cases. An EL layer115R, an EL layer115G, and an EL layer115B which are described later are also described by the term “EL layer115” in some cases. The EL layer115R is included in the light-emitting element120R. Similarly, the EL layer115G is included in the light-emitting element120G and the EL layer115B is included in the light-emitting element120B. A conductive layer114R, a conductive layer114G, and a conductive layer114B which are described later are also described by the term “conductive layer114” in some cases. The conductive layer114R is included in the light-emitting element120R. Similarly, the conductive layer114G is included in the light-emitting element120G and the conductive layer114B is included in the light-emitting element120B.

The light-emitting element120includes a conductive layer111which functions as a lower electrode, the EL layer115, and a conductive layer116which functions as an upper electrode. The conductive layer111has a reflective property with respect to visible light. The conductive layer116has a transmissive property and a reflective property with respect to visible light. Alternatively, the conductive layer116has a semi-transmissive and semi-reflective property with respect to visible light in some cases. The EL layer115contains a light-emitting compound. The EL layer115includes at least a light-emitting layer included in the light-emitting element120.

As the light-emitting element120, the electroluminescent element having a function of emitting light in accordance with current flowing into the EL layer115when a potential difference is applied between the conductive layer111and the conductive layer116can be used. In particular, an organic EL element using a light-emitting organic compound is preferably used for the EL layer115. In addition, the light-emitting element120is preferably an element emitting white light, which has two or more peaks in the visible light region of the emission spectrum.

The top surface of the conductive layer111has a reflective property with respect to visible light.

The display device100includes a substrate101including a semiconductor circuit and the light-emitting element120over the substrate101. The display device100illustrated inFIG.1Aincludes an insulating layer121over the substrate101, the light-emitting element120over the insulating layer121, and an insulating layer124placed to cover the light-emitting element120. The insulating layer124is preferably in contact with the top surface and a side surface of the conductive layer116, a side surface of the EL layer115, and a side surface of the conductive layer111. The insulating layer124is in contact with the insulating layer121in a region not overlapping with the light-emitting element120in some cases.

A circuit board including a transistor, a wiring, and the like can be used as the substrate101. Note that in the case in which a passive matrix method or a segment method can be employed, an insulating substrate such as a glass substrate can be used as the substrate101. The substrate101is a substrate provided with a circuit for driving each light-emitting element (also referred to as a pixel circuit). The substrate101may be provided with a semiconductor circuit functioning as a driver circuit for driving the pixel circuit. A semiconductor element included in the pixel circuit or the semiconductor circuit may be formed using a semiconductor substrate such as a silicon substrate or using an oxide semiconductor film. More specific structure examples of the substrate101will be described later.

In the display device100inFIG.1A, the substrate101and the conductive layer111of the light-emitting element120are electrically connected to each other through a plug131. The plug131is formed to be embedded in an opening provided in the insulating layer121. The conductive layer111is formed over the insulating layer121. The conductive layer111is provided over the plug131. The conductive layer111and the plug131are electrically connected to each other. The conductive layer111is preferably in contact with the top surface of the plug131. The conductive layer111may be in contact with the top surface of the insulating layer121.

The insulating layer124preferably functions as a barrier insulating film against at least one of water and oxygen. The insulating layer124further preferably functions as a barrier insulating film against hydrogen, substance to which hydrogen is bonded (e.g., water (H2O)), oxygen, chlorine, and the like. Moreover, the insulating layer124preferably includes a layer having a function of inhibiting diffusion of at least one of water and oxygen. The insulating layer124further preferably includes a layer having a function of inhibiting diffusion of hydrogen, substance to which hydrogen is bonded (e.g., water (H2O)), oxygen, chlorine, and the like. Moreover, the insulating layer124preferably includes a layer having a function of capturing or fixing (also referred to as gettering) at least one of water and oxygen. The insulating layer124further preferably includes a layer having a function of capturing or fixing hydrogen, substance to which hydrogen is bonded (e.g., water (H2O)), oxygen, chlorine, and the like.

Note that in this specification, a barrier insulating film refers to an insulating film having a barrier property. In this specification, a barrier property means a function of inhibiting diffusion of a targeted substance (also referred to as having low permeability). Alternatively, a barrier property means a function of capturing or fixing a targeted substance.

Here, as illustrated inFIG.1B, the insulating layer124preferably includes a layer124a, a layer124bover the layer124a, and a layer124cover the layer124b. Note that the layer124ahas a function of capturing or fixing at least one of water and oxygen. The layer124bhas a function of inhibiting diffusion of at least one of water and oxygen. The layer124cis a layer with favorable coverage.

The layer124ais placed at least between the layer124band the EL layer115. The layer124ais preferably in contact with the top surface and the side surface of the conductive layer116, the side surface of the EL layer115, and the a side surface of the conductive layer111. The layer124ais in contact with the insulating layer121in a region sandwiched between the conductive layers111in some cases. The layer124bis preferably provided to cover the layer124aand in contact with the top surface of the layer124a.

When the layer124aand the layer124bare provided in the above manner, the layer124acan capture or fix impurities such as oxygen or water in the light-emitting element120in a region covered with the layer124b, whereby the amount of impurities included in the light-emitting element120can be reduced. In particular, since the layer124ais provided in contact with the side surface of the EL layer115to which impurities are easily attached in processing, an unintended layer including the impurities can be inhibited from being formed on the side surface of the EL layer115. Furthermore, the layer124bcan prevent impurities such as oxygen or water from diffusing from above the insulating layer124to the light-emitting element120and prevent an increase in the amount of impurities in the light-emitting element.

The layer124cis preferably provided to cover the layer124band in contact with the top surface of the layer124b. The layer124cis preferably deposited by an atomic layer deposition (ALD) method that achieves favorable coverage.

Here, the layer124bhas unevenness that reflects the shape of the formation surface, that is, step shapes of the conductive layer111, the EL layer115, and the conductive layer116. Thus, disconnection might be formed in the layer124b. However, since the layer124cwith favorable coverage is provided as described above, even when disconnection is formed in the layer124b, the disconnection can be filled by the layer124c. Thus, the layer124bcan maintain the function of inhibiting diffusion of impurities such as water or oxygen.

In the above manner, one embodiment of the present invention can reduce impurities such as water and oxygen in the light-emitting element and prevent deterioration of the light-emitting element, and thus a highly reliable display device can be provided.

Alternatively, as illustrated inFIG.1C, an insulating layer122may be provided between the insulating layer121and each of the conductive layer111and the insulating layer124. The insulating layer122as well as the insulating layer124preferably functions as a barrier insulating film against at least one of water and oxygen. As the insulating layer122, it is preferable to use either one or both of an insulating layer having a function similar to that of the layer124aand an insulating layer having a function similar to that of the layer124b. For example, as the insulating layer122, a stacked layer including an insulating layer having a function similar to that of the layer124band an insulating layer thereover having a function similar to that of the layer124acan be used.

The insulating layer122functioning as a barrier insulating film against water, oxygen, or the like is provided below the light-emitting element120, whereby impurities such as water or oxygen included in the interlayer insulating film and the semiconductor circuit such as the pixel circuit which are provided below the light-emitting element120can be inhibited from diffusing into the light-emitting element120. Thus, deterioration of the light-emitting element120can be prevented.

When an oxide semiconductor is provided in the above semiconductor circuit, impurities such as water or hydrogen included in the light-emitting element and the interlayer insulating film over the light-emitting element can be inhibited from diffusing into the oxide semiconductor. Thus, a decrease in electrical characteristics and reliability of an element including the oxide semiconductor can be prevented.

In the structure illustrated inFIG.1C, the insulating layer124is preferably in contact with the insulating layer122in a region not overlapping with the conductive layer111. Here, the insulating layer124is in contact with the top surface and the side surface of the conductive layer116, the side surface of the EL layer115, and the side surface of the conductive layer111. Thus, the light-emitting element120is surrounded by the insulating layer124and the insulating layer122. When the layer124ais provided in a region surrounded by the insulating layer124and the insulating layer122, impurities such as water or oxygen in the light-emitting element can be more efficiently captured by or fixed in the layer124a.

In the display device100illustrated inFIG.1AtoFIG.1C, each of the EL layer115and the conductive layer116is separated between adjacent light-emitting elements of different colors. Hence, leakage current flowing through the EL layer115between adjacent light-emitting elements of different colors can be prevented. Thus, light emission due to the leakage current can be suppressed and display with a high contrast can be achieved. Furthermore, since the EL layer115can be formed using a highly conductive material even when the resolution is increased, the range of choices for materials can be widened; thus, an improvement in efficiency, a reduction in power consumption, and an increase in reliability can be easily achieved.

Note that in the display device100, the EL layer115and the conductive layer116are preferably processed so that they each are continuous without separation between pixels having the same color. For example, the EL layer115and the conductive layer116can be processed into a stripe shape. Thus, the conductive layers116of all the light-emitting elements can be supplied with a predetermined potential without being in a floating state.

The EL layer115and the conductive layer116may be patterned into an island shape by deposition using a shadow mask such as a metal mask or an FMM (a fine metal mask), it is particularly preferable to use a processing method without using a metal mask or an FMM. As such a processing method, typically, a photolithography method can be used. Alternatively, formation method such as a nanoimprinting method or a sandblasting method can be used. Note that in this specification and the like, a device formed using a metal mask or an FMM is sometimes referred to as a device having an MM (metal mask) structure. In this specification and the like, a device formed without using a metal mask or an FMM is sometimes referred to as a device having an MML (metal maskless) structure.

Thus, a device having an MML structure can have an extremely minute pattern, and therefore can have improved resolution and aperture ratio compared to a device having an MM structure.

As illustrated inFIG.1AorFIG.1C, end portions of the EL layer115may be substantially aligned with end portions of the conductive layer111. End portions of the conductive layer116may be substantially aligned with the end portions of the conductive layer111. One end portion of the EL layer115may be positioned outward from the conductive layer111and the other may be substantially aligned with the end portion of the conductive layer111. One end portion of the conductive layer116may be positioned outward from the conductive layer111and the other may be substantially aligned with the end portion of the conductive layer111. As illustrated inFIG.2A, the end portions of the EL layer115may be positioned to be inward from the end portions of the conductive layer111in a cross section of the display device100.

The conductive layer116may be placed at least not to be short-circuited with the conductive layer111. For example, as illustrated inFIG.2B, the end portions of the EL layer115may be positioned to be outward from the end portions of the conductive layer111in the cross section of the display device100. The end portions of the conductive layer111are covered with the end portions of the EL layer115. The end portions of the EL layer115are located outward from the end portions of the conductive layer111, whereby a short circuit between the conductive layer111and the conductive layer116can be inhibited. As illustrated inFIG.2B, the end portions of the conductive layer116may be positioned to be outward from the end portions of the conductive layer111in the cross section of the display device100.

As illustrated inFIG.2C, an insulator117covering the end portions of the conductive layer116may be provided. The insulator117can also be referred to as a bank, a partition, a barrier, an embankment, or the like. The insulator117is provided to expose the top surface of the conductive layer111. When the insulator117is provided, a short circuit between the conductive layer111and the conductive layer116can be inhibited.

Note that althoughFIG.2AtoFIG.2Cillustrate the structure in which the insulating layer124is provided as inFIG.1A, the structure is not limited to this and a structure similar to that illustrated inFIG.1Cmay be employed.

As a light-emitting element that can be used as the light-emitting element120, a self-luminous element can be used, and an element whose luminance is controlled by current or voltage is included in its category. For example, an LED, an organic EL element, or an inorganic EL element can be used. In particular, an organic EL element is preferably used.

The light-emitting element has a top-emission structure, a bottom-emission structure, a dual-emission structure, or the like. A conductive film that transmits visible light is used as the electrode through which light is extracted. A conductive film that reflects visible light is preferably used as the electrode through which light is not extracted.

In one embodiment of the present invention, in particular, a light-emitting element with a top-emission structure in which light is emitted toward the side opposite to the formation surface side or a dual-emission structure in which light is emitted toward both the formation surface side and the side opposite to the formation surface side can be favorably used.

The EL layer115includes at least a light-emitting layer. In addition to the light-emitting layer, the EL layer115may further include layers containing a substance with a high hole-injection property, a substance with a high hole-transport property, a hole-blocking material, a substance with a high electron-transport property, a substance with a high electron-injection property, a substance with a bipolar property (a substance with a high electron-transport property and hole-transport property), and the like.

Either a low molecular compound or a high molecular compound can be used for the EL layer115and an inorganic compound may also be contained. Layers included in the EL layer115can each be formed by a method such as an evaporation method (including a vacuum evaporation method), a transfer method, a printing method, an inkjet method, or a coating method.

When a voltage higher than the threshold voltage of the light-emitting element120is applied between a cathode and an anode, holes are injected to the EL layer115from the anode side and electrons are injected to the EL layer115from the cathode side. The injected electrons and holes are recombined in the EL layer115and a light-emitting substance contained in the EL layer115emits light.

Here, an EL layer115used for the light-emitting element120B is referred to as an EL layer115B, an EL layer115used for the light-emitting element120G is referred to as an EL layer115G, an EL layer115used for the light-emitting element120R is referred to as an EL layer115R. The EL layer115B contains a light-emitting substance emitting B (blue) light. The EL layer115G contains a light-emitting substance emitting G (green) light. The EL layer115R contains a light-emitting substance emitting R (red) light. Such a structure in which emission colors (here, blue (B), green (G), and red (R)) are separately patterned for each of the light-emitting elements is referred to as a SBS (Side By Side) structure in some cases. With such a structure, a display device with lower power consumption than a display device with a structure in which a white light-emitting element is colored by a coloring layer can be provided.

Note that the above-described light-emitting layer and layers containing a substance with a high hole-injection property, a substance with a high hole-transport property, a substance with a high electron-transport property, a substance with a high electron-injection property, a substance with a bipolar property, and the like may include an inorganic compound such as a quantum dot or a high molecular compound (e.g., an oligomer, a dendrimer, and a polymer). For example, when used for the light-emitting layer, the quantum dots can function as a light-emitting material.

As the quantum dot material, a colloidal quantum dot material, an alloyed quantum dot material, a core-shell quantum dot material, a core quantum dot material, or the like can be used. The material containing elements belonging to Group 12 and Group 16, elements belonging to Group 13 and Group 15, or elements belonging to Group 14 and Group 16, may be used. Alternatively, the quantum dot material containing an element such as cadmium, selenium, zinc, sulfur, phosphorus, indium, tellurium, lead, gallium, arsenic, or aluminum may be used.

The conductive film that can be used for the conductive layer116or the like and transmits visible light can be formed using, for example, indium oxide, indium tin oxide, indium zinc oxide, zinc oxide, or zinc oxide to which gallium is added. Alternatively, a metal material such as gold, silver, platinum, magnesium, nickel, tungsten, chromium, molybdenum, iron, cobalt, copper, palladium, or titanium; an alloy containing any of these metal materials; a nitride of any of these metal materials (e.g., titanium nitride); or the like formed thin enough to have a light-transmitting property can be used. Furthermore, a stacked-layer film of the above materials can be used for a conductive layer. For example, a stacked-layer film of indium tin oxide and an alloy of silver and magnesium is preferably used, in which case conductivity can be increased. Alternatively, graphene or the like may be used.

The conductive film having a semi-transmissive and semi-reflective property which can be used for the conductive layer116preferably has a reflectance with respect to visible light (e.g., the reflectance with respect to light having a predetermined wavelength within the range of 400 nm to 700 nm) of higher than or equal to 20% and lower than or equal to 80%, preferably higher than or equal to 40% and lower than or equal to 70%. The conductive film having reflectivity preferably has a reflectance with respect to visible light of higher than or equal to 40% and lower than or equal to 100%, further preferably higher than or equal to 70% and lower than or equal to 100%. The conductive film having a light-transmitting property preferably has a reflectance with respect to visible light of higher than or equal to 0% and lower than or equal to 40%, further preferably higher than or equal to 0% and lower than or equal to 30%.

The conductive film that reflects visible light is preferably used in a portion of the conductive layer111that is located on the EL layer115side. For the conductive layer111, for example, a metal material such as aluminum, gold, platinum, silver, nickel, tungsten, chromium, molybdenum, iron, cobalt, copper, or palladium or an alloy containing any of these metal materials can be used. Copper is preferably used because of its high reflectivity of visible light. In addition, aluminum is preferably used because an aluminum electrode is easily etched, so that processing of the electrode is easy, and aluminum has high reflectivity of visible light and near-infrared light. Lanthanum, neodymium, germanium, or the like may be added to the above metal material or alloy. Alternatively, an alloy (an aluminum alloy) containing aluminum and titanium, nickel, or neodymium may be used. Alternatively, an alloy containing silver and copper, palladium, or magnesium may be used. An alloy containing silver and copper is preferable because of its high heat resistance.

The conductive layer111may have a structure in which a conductive metal oxide film is stacked over the conductive film that reflects visible light. With such a structure, oxidization and corrosion of the conductive film reflecting visible light can be inhibited. For example, when a metal film or a metal oxide film is stacked in contact with an aluminum film or an aluminum alloy film, oxidization can be inhibited. Examples of materials for the metal film or the metal oxide film include titanium and titanium oxide. Alternatively, the above conductive film that transmits visible light and a film containing a metal material may be stacked. For example, a stacked-layer film of silver and indium tin oxide or a stacked-layer film of an alloy of silver and magnesium and indium tin oxide can be used.

In the conductive layer111, as illustrated inFIG.3A, a conductive layer111amay be provided as a lower layer, and a conductive layer111bmay be provided over the conductive layer111aas an upper conductive layer. When such a structure is employed, a conductive film that reflects visible light is preferably used as the conductive layer111b. The reflectance of the conductive layer111amay be lower than that of the conductive layer111b. A material with high conductivity may be used for the conductive layer111a. A material having a high processing property may be used for the conductive layer111a.

For the conductive layer111b, the above-described material and structure which can be used for the conductive layer111are preferably employed.

For the conductive layer111a, for example, a metal material such as gold, silver, platinum, magnesium, nickel, tungsten, chromium, molybdenum, iron, cobalt, copper, palladium, titanium, yttrium, zirconium, or tantalum; an alloy containing any of these metal materials; or a nitride of any of these metal materials (e.g., titanium nitride) can be used.

When aluminum is used as the conductive layer111or the conductive layer111b, the thickness of aluminum is preferably greater than or equal to 40 nm, further preferably greater than or equal to 70 nm, whereby the reflectivity of visible light or the like can be sufficiently increased. When silver is used as the conductive layer111or the conductive layer111b, the thickness of silver is preferably greater than or equal to 70 nm, further preferably greater than or equal to 100 nm, whereby the reflectivity of visible light or the like can be sufficiently increased.

As an example, tungsten can be used for the conductive layer111aand aluminum or an aluminum alloy can be used for the conductive layer111b. The conductive layer111bmay have a structure in which titanium oxide is provided in contact with an upper portion of aluminum or an aluminum alloy. Alternatively, the conductive layer111bmay have a structure in which titanium is provided in contact with the upper portion of aluminum or the aluminum alloy, and titanium oxide is provided in contact with the upper portion of the titanium.

Alternatively, both the conductive layer111aand the conductive layer111bcan be formed using the materials and structures selected from those that can be used for the conductive layer111.

The conductive layer111may be a stacked film including three or more layers.

Note that althoughFIG.3Aillustrates the structure in which the insulating layer124is provided as inFIG.1A, one embodiment of the present invention is not limited thereto. In the structure illustrated inFIG.3A, the insulating layer122may be further provided as in the structure illustrated inFIG.1C.

Examples of a material that can be used for the plug131include metals such as aluminum, titanium, chromium, nickel, copper, yttrium, zirconium, molybdenum, gold, silver, platinum, magnesium, iron, cobalt, palladium, tantalum, or tungsten; an alloy containing any of these metal materials; and nitride of any of these metal materials. For the plug131, a film containing any of these materials can be used in a single layer or as a stacked-layer structure. For example, a single-layer structure of an aluminum film containing silicon, a two-layer structure in which an aluminum film is stacked over a titanium film, a two-layer structure in which an aluminum film is stacked over a tungsten film, a two-layer structure in which a copper film is stacked over a copper-magnesium-aluminum alloy film, a two-layer structure in which a copper film is stacked over a titanium film, a two-layer structure in which a copper film is stacked over a tungsten film, a three-layer structure in which an aluminum film or a copper film is stacked over a titanium film or a titanium nitride film and a titanium film or a titanium nitride film is formed thereover, and a three-layer structure in which an aluminum film or a copper film is stacked over a molybdenum film or a molybdenum nitride film and a molybdenum film or a molybdenum nitride film is formed thereover can be given. Note that an oxide such as indium oxide, tin oxide, or zinc oxide may be used. Copper containing manganese is preferably used because it increases controllability of a shape by etching.

The electrodes included in the light-emitting element may each be formed by an evaporation method or a sputtering method. Alternatively, a discharging method such as an inkjet method, a printing method such as a screen printing method, or a plating method may be used for the formation.

For the insulating layer124, an oxide, a nitride, an oxynitride, or a nitride oxide which contains at least one of aluminum, hafnium, magnesium, gallium, indium, zinc, and silicon can be used. Alternatively, the stacked film of any of these materials can be used. For example, aluminum oxide, hafnium oxide, hafnium aluminate, magnesium oxide, gallium oxide, indium gallium zinc oxide, silicon oxide, silicon nitride, silicon oxynitride, or silicon nitride oxide can be used.

For the layer124ahaving a function of capturing or fixing impurities such as water or oxygen, for example, a metal oxide deposited by a sputtering method, such as aluminum oxide (AlOx: x is a given number greater than 0) or hafnium oxide (HfOy: y is a given number greater than 0), is preferably used. When aluminum oxide is used for the layer124a, the layer124ais an insulator containing at least oxygen and aluminum. When hafnium oxide is used for the layer124a, the layer124ais an insulator containing at least oxygen and hafnium.

The layer124apreferably contains a large amount of oxygen vacancies. A large number of dangling bonds are formed in a metal oxide containing such a large amount of oxygen vacancies in some cases, and the metal oxide has a property of capturing or fixing impurities such as water or oxygen in the dangling bonds in some cases. When such a metal oxide containing a large amount of oxygen vacancies is used, impurities such as water or oxygen can be captured or fixed in the layer124a. In particular, impurities such as water or oxygen attached to the side surface of the EL layer115are preferably captured or fixed.

The layer124ais preferably deposited by a sputtering method. When the layer124ais deposited by a sputtering method, the deposition can be performed without using impurities such as water in a deposition gas, whereby an increase in a concentration of impurities such as water in the layer124aand the light-emitting element120can be inhibited. When the layer124ais deposited by a sputtering method, it is preferable that the amount of oxygen in the deposition gas be decreased or oxygen be not contained in the deposition gas. Thus, an increase in the amount of oxygen in the layer124aand the light-emitting element120can be inhibited. Moreover, a large amount of oxygen vacancies can be contained in the deposited layer124a.

A metal oxide having an amorphous structure may be used for the layer124a. Note that a crystal region may be formed in a part of the layer124a. The layer124amay have a multilayer structure in which a layer with an amorphous structure and a layer including a crystal region are stacked. For example, the layer124acan have a stacked-layer structure in which a layer including a crystal region, typically, a layer with a polycrystalline structure, is provided over a layer with an amorphous structure.

As the layer124bhaving a function of inhibiting diffusion of impurities such as water or oxygen, for example, a silicon nitride (SiNx: x is a given number greater than 0) deposited by a sputtering method or the like is preferably used. In that case, the layer124bis an insulator containing at least nitrogen and silicon. It is preferable that the layer124bhave a film thickness of greater than or equal to 10 nm, for example, about greater than or equal to 20 nm and less than or equal to 100 nm, or for example, about greater than or equal to 20 nm and less than or equal to 50 nm over the conductive layer116so as to inhibit diffusion of impurities such as water or oxygen.

The layer124bis preferably deposited by a sputtering method. When the layer124bis deposited by a sputtering method, a metal nitride can be deposited without using impurities such as water or oxygen as a deposition gas, whereby an increase in a concentration of impurities such as water or oxygen in the layer124b, the layer124a, and the light-emitting element120can be inhibited.

For the layer124cwith favorable coverage, the insulating material that can be used for the above-described insulating layer124can be used. For example, aluminum oxide or hafnium oxide deposited by an ALD method is preferably used. When a metal oxide is deposited by an ALD method, H2O, O3, or the like is used as an oxidizer; however, the layer124band the layer124aare formed before the layer124cis deposited; thus, the amount of impurities such as water or oxygen entering the EL layer115and the like can be decreased.

Note that a precursor used in an ALD method sometimes contains impurities such as hydrogen or carbon. Thus, a film provided by an ALD method contains impurities such as hydrogen or carbon in a larger amount than a film provided by another deposition method in some cases. Thus, the layer124chas a higher concentration of carbon than at least one of the layer124aand the layer124bwhich are deposited by a sputtering method in some cases. Furthermore, the layer124chas a higher concentration of hydrogen than at least one of the layer124aand the layer124bwhich are deposited by a sputtering method in some cases. Note that impurities such as hydrogen or carbon can be quantified by energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), or the like.

The density of films deposited by an ALD method tends to be lower than that of films formed by a sputtering method in some cases. Thus, the density of the layer124cis lower than that of at least one of the layer124aand the layer124bdeposited by a sputtering method in some cases. Note that the density can be measured using an X-ray reflectometry (XRR) analysis or the like.

Examples of an ALD method include a thermal ALD method, in which a precursor and a reactant react with each other only by thermal energy, and a PEALD (Plasma Enhanced ALD) method, in which a reactant excited by plasma is used. If the deposition can be performed at such a temperature as not to deteriorate the EL layer115(e.g. about higher than or equal to room temperature and lower than or equal to 100° C.), any ALD method can be used for the deposition.

Similarly, any of the insulating materials that can be used for the layer124aor the layer124bmay be used for the insulating layer122. Furthermore, the insulating layer122may also have a stacked-layer structure. For example, when the insulating layer122has a two-layer structure, a structure in which an aluminum oxide film deposited by a sputtering method is provided over a silicon nitride film deposited by a sputtering method can be employed.

It is preferable that the insulating layer121function as an interlayer insulating film and have a low permittivity. When a material with a low permittivity is used for an interlayer film, the parasitic capacitance generated between wirings can be reduced. For the insulating layer121, for example, silicon oxide, silicon oxynitride, silicon oxide to which fluorine is added, silicon oxide to which carbon is added, silicon oxide to which carbon and nitrogen are added, or porous silicon oxide is appropriately used. In this case, in the insulating layer121, a depressed portion is formed on the surface where the conductive layer111is not provided, in some cases. For example, in an etching process of forming the conductive layer111, the insulating layer121is etched, whereby the depressed portion is formed.

In addition, the insulator117illustrated inFIG.2Cmay be formed using any of insulating materials that can be used for the insulating layer121.

Although in the structure illustrated inFIG.1C, the insulating layer122is in contact with the conductive layer111and the insulating layer124, a structure in which the insulating layer125is provided between the insulating layer122and each of the conductive layer111and the insulating layer124may be employed as illustrated inFIG.3B, for example. As the insulating layer125, an insulating material that can be used as the insulating layer121can be used. In this case, in the insulating layer125, a depressed portion is formed on the surface where the conductive layer111is not provided, in some cases. For example, in an etching process of forming the conductive layer111, the insulating layer125is etched, whereby the depressed portion is formed.

As illustrated inFIG.4A, a light-emitting substance emitting white light may be employed as the EL layer115included in the light-emitting element120. In that case, a coloring layer overlapping with the light-emitting element120may be provided as described later. When a light-emitting substance emitting white light is employed as the EL layer115, the EL layer115preferably contains two or more kinds of light-emitting substances. White emission can be obtained by selecting two or more light-emitting substances so that the light-emitting substances emit light of complementary colors, for example. For example, it is preferable to contain two or more light-emitting substances emitting light of R (red), G (green), B (blue), Y (yellow),0(orange), and the like or light-emitting substances emitting light containing two or more spectral components of R, G, and B. A light-emitting element whose emission spectrum has two or more peaks in the wavelength range of a visible light region (e.g., 350 nm to 750 nm) is preferably employed. An emission spectrum of a material having a peak in a yellow wavelength range preferably has spectral components also in green and red wavelength ranges.

The EL layer115can have a structure in which a light-emitting layer containing a light-emitting material emitting light of one color and a light-emitting layer containing a light-emitting material emitting light of another color are stacked. For example, a plurality of light-emitting layers in the EL layer115may be stacked in contact with each other or may be stacked with a region including no light-emitting material therebetween. For example, between a fluorescent layer and a phosphorescent layer, a region that contains the same material as the fluorescent layer or the phosphorescent layer (for example, a host material or an assist material) and including no light-emitting material may be provided. This facilitates the fabrication of the light-emitting element and reduces the drive voltage.

The light-emitting element120may be a single element including one EL layer or a tandem element in which a plurality of EL layers are stacked with a charge-generation layer therebetween.

As illustrated inFIG.4A, the EL layer115may be provided in common for the light-emitting elements120. InFIG.4A, the continuous EL layer115is provided to cover the conductive layer111of each light-emitting element120. As illustrated inFIG.4A, the conductive layer116may be provided in common for the light-emitting element120R, the light-emitting element120G, and the light-emitting element120B. The conductive layer116functions as, for example, an electrode to which a common potential is applied. It is preferable to provide the conductive layer116in common because the fabrication steps of the light-emitting element120can be reduced.

In the light-emitting element120, a conductive layer114(which refers to the conductive layer114B, the conductive layer114G, and the conductive layer114R) may be provided between the conductive layer111and the EL layer115as illustrated inFIG.4B. The conductive layer114has a function of transmitting visible light.

As the conductive layer114, a conductive film having a property of transmitting visible light described above can be used. As the conductive layer114, a film which is thin enough to transmit visible light formed using a conductive film which reflects visible light described above can be used. In addition, with the stacked-layer structure of the conductive film and the conductive film transmitting visible light described above, the conductivity and the mechanical strength can be increased.

As illustrated inFIG.4B, the conductive layer114is placed between the conductive layer111and the EL layer115. The conductive layer114is positioned over the conductive layer111. Here, the EL layer115is preferably provided to cover the end portions of the conductive layer114.

As illustrated inFIG.4B, the conductive layers114included in the light-emitting elements120preferably have different thicknesses from each other. Among the three conductive layers114, the thickness of the conductive layer114B is the smallest and the thickness of the conductive layer114R is the largest. Here, the distance between the top surface of the conductive layer111and the bottom surface of the conductive layer116(i.e., the interface between the conductive layer116and the EL layer115) is the largest in the light-emitting element120R and the smallest in the light-emitting element120B among the light-emitting elements. In each light-emitting element, by changing the distance between the top surface of the conductive layer111and the bottom surface of the conductive layer116, optical distance (optical path length) of each light-emitting element can be changed.

Among the three light-emitting elements, the light-emitting element120R has the longest optical path length, and thus emits light R that is the intensified light with the longest wavelength. In contrast, the light-emitting element120B has the shortest optical path length, and thus emits light B that is the intensified light with the shortest wavelength. The light-emitting element120G emits light G that is the intensified light with the intermediate wavelength. For example, the light R is the intensified red light, the light G is the intensified green light, and the light B is the intensified blue light.

With such a structure, the EL layer included in the light-emitting elements120need not be separately formed for the light-emitting elements of different colors; thus, color display with high color reproducibility can be performed using elements with the same structure. In addition, the light-emitting element120can be arranged extremely densely. For example, a display device having a resolution exceeding 5000 ppi can be achieved.

In each light-emitting element, the optical distance between the surface of the conductive layer111reflecting visible light and the conductive layer116having a semi-transmissive and a semi-reflective property with respect to visible light is preferably adjusted to mλ/2 (m is a natural number; however, m is not 0) or in the vicinity thereof with respect to wavelength λ whose intensity is desired to be increased.

Note that the above-described optical distance depends on a product of the physical distance between the reflective surface of the conductive layer111and the reflective surface of the conductive layer116having a semi-transmissive and a semi-reflective property and the refractive index of a layer provided therebetween, and thus is difficult to adjust exactly. Thus, it is preferable to adjust the optical distance on the assumption that the surface of the conductive layer111and the surface of the conductive layer116having a semi-transmissive and a semi-reflective property are each the reflective surface.

As described later, by providing the coloring layer which overlaps with the light-emitting element120, color purity of light from the light-emitting element can be increased.

Note that the light-emitting layer120may have a structure in which a plurality of EL layers are stacked. For example, the EL layer115may have a structure in which the EL layer115B containing a light-emitting substance emitting blue light, the EL layer115G containing a light-emitting substance emitting green light, and the EL layer115R containing a light-emitting substance emitting red light are stacked. The EL layers may each include an electron-injection layer, an electron-transport layer, a charge-generation layer, a hole-transport layer, and a hole-injection layer, for example, in addition to a layer containing a light-emitting compound. Note that a charge-generation layer may be provided between the EL layer115B and the EL layer115G. In addition, a charge-generation layer may be provided between the EL layer115G and the EL layer115R.

<Structure Examples of EL Layer>

An EL layer115included in the light-emitting element120can be formed of a plurality of layers such as a layer4420, a light-emitting layer4411, and a layer4430, as illustrated inFIG.17A. The layer4420can include, for example, a layer containing a substance with a high electron-injection property (an electron-injection layer) and a layer containing a substance with a high electron-transport property (an electron-transport layer). The light-emitting layer4411contains a light-emitting compound, for example. The layer4430can include, for example, a layer containing a substance with a high hole-injection property (a hole-injection layer) and a layer containing a substance with a high hole-transport property (a hole-transport layer).

The structure including the layer4420, the light-emitting layer4411, and the layer4430, which are provided between a pair of electrodes, can serve as a single light-emitting unit, and the structure inFIG.17Ais referred to as a single structure in this specification.

FIG.17Bis a modification example of the EL layer115included in the light-emitting element120illustrated inFIG.17A. Specifically, the light-emitting element120illustrated inFIG.17Bincludes a layer4430-1over the conductive layer111, a layer4430-2over the layer4430-1, the light-emitting layer4411over the layer4430-2, a layer4420-1over the light-emitting layer4411, a layer4420-2over the layer4420-1, and the conductive layer116over the layer4420-2. For example, when the conductive layer111is an anode and the conductive layer116is a cathode, the layer4430-1functions as a hole-injection layer, the layer4430-2functions as a hole-transport layer, the layer4420-1functions as an electron-transport layer, and the layer4420-2functions as an electron-injection layer. Alternatively, when the conductive layer111is a cathode and the conductive layer116is an anode, the layer4430-1functions as an electron-injection layer, the layer4430-2functions as an electron-transport layer, the layer4420-1functions as a hole-transport layer, and the layer4420-2functions as a hole-injection layer. With such a layer structure, carriers can be efficiently injected to the light-emitting layer4411, and the efficiency of the recombination of carriers in the light-emitting layer4411can be increased.

Note that the structure in which a plurality of light-emitting layers (light-emitting layers4411,4412, and4413) are provided between the layer4420and the layer4430as illustrated inFIG.17Cis a variation of the single structure.

The structure in which a plurality of light-emitting units (EL layers115aand115b) are connected in series with an intermediate layer (charge-generation layer)4440therebetween as illustrated inFIG.17Dis referred to as a tandem structure in this specification. Note that in this specification and the like, the structure illustrated inFIG.17Dis referred to as a tandem structure; however, without being limited to this, a tandem structure may be referred to as a stack structure, for example. The tandem structure enables a light-emitting element capable of high-luminance light emission.

Note that also inFIG.17CandFIG.17D, the layer4420and the layer4430may each have a stacked-layer structure of two or more layers as illustrated inFIG.17B.

The emission color of the light-emitting element can be red, green, blue, cyan, magenta, yellow, white, or the like depending on the material included in the EL layer115. Furthermore, the color purity can be further increased when the light-emitting element has a microcavity structure.

The light-emitting element that emits white light preferably contains two or more kinds of light-emitting substances in the light-emitting layer. To obtain white light emission, two or more light-emitting substances may be selected so that their emission colors are complementary.

The light-emitting layer preferably contains two or more light-emitting substances that emit light of R (red), G (green), B (blue), Y (yellow),0(orange), and the like. Alternatively, the light-emitting layer preferably contains two or more light-emitting substances that each emit light containing two or more spectral components of R, G, and B.

A specific structure example of the light-emitting element will be described below.

The light-emitting element includes at least the light-emitting layer. The light-emitting element may further include, as a layer other than the light-emitting layer, a layer containing a substance with a high hole-injection property, a substance with a high hole-transport property, a hole-blocking material, a substance with a high electron-transport property, an electron-blocking material, a substance with a high electron-injection property, a substance with a bipolar property (a substance with a high electron-transport property and a high hole-transport property), or the like.

Either a low molecular compound or a high molecular compound can be used for the light-emitting device, and an inorganic compound may also be included. Each of the layers included in the light-emitting device can be formed by an evaporation method (including a vacuum evaporation method), a transfer method, a printing method, an inkjet method, a coating method, or the like.

For example, the light-emitting device can include one or more of a hole-injection layer, a hole-transport layer, a hole-blocking layer, an electron-blocking layer, an electron-transport layer, and an electron-injection layer.

The hole-injection layer is a layer injecting holes from an anode to a hole-transport layer, and a layer containing a material with a high hole-injection property. Examples of the material with a high hole-injection property include an aromatic amine compound and a composite material containing a hole-transport material and an acceptor material (electron-accepting material).

The hole-transport layer is a layer transporting holes, which are injected from an anode by a hole-injection layer, to a light-emitting layer. The hole-transport layer is a layer containing a hole-transport material. As the hole-transport material, a substance having a hole mobility greater than or equal to 1×10−6cm2/Vs is preferable. Note that other substances can also be used as long as they have a property of transporting more holes than electrons. As the hole-transport material, materials having a high hole-transport property, such as a n-electron rich heteroaromatic compound (e.g., a carbazole derivative, a thiophene derivative, and a furan derivative) and an aromatic amine (a compound having an aromatic amine skeleton), are preferable.

The electron-transport layer is a layer that transports electrons, which are injected from a cathode by an electron-injection layer, to a light-emitting layer. The electron-transport layer is a layer containing an electron-transport material. As the electron-transport material, a substance having an electron mobility greater than or equal to 1×10−6cm2/Vs is preferable. Note that other substances can also be used as long as they have a property of transporting more electrons than holes. As the electron-transport material, it is possible to use a material having a high electron-transport property, such as a metal complex having a quinoline skeleton, a metal complex having a benzoquinoline skeleton, a metal complex having an oxazole skeleton, a metal complex having a thiazole skeleton, an oxadiazole derivative, a triazole derivative, an imidazole derivative, an oxazole derivative, a thiazole derivative, a phenanthroline derivative, a quinoline derivative having a quinoline ligand, a benzoquinoline derivative, a quinoxaline derivative, a dibenzoquinoxaline derivative, a pyridine derivative, a bipyridine derivative, a pyrimidine derivative, or a π-electron deficient heteroaromatic compound such as a nitrogen-containing heteroaromatic compound.

The electron-injection layer is a layer injecting electrons from a cathode to the electron-transport layer, and a layer containing a material with a high electron-injection property. As the material with a high electron-injection property, an alkali metal, an alkaline earth metal, or a compound thereof can be used. As the material with a high electron-injection property, a composite material containing an electron-transport material and a donor material (electron-donating material) can also be used.

Alternatively, as the above-described electron-injection layer, an electron-transport material may be used. For example, a compound having an unshared electron pair and having an electron deficient heteroaromatic ring skeleton can be used as the material having an electron-transport property. Specifically, a compound with at least one of a pyridine ring, a diazine ring (a pyrimidine ring, a pyrazine ring, and a pyridazine ring), and a triazine ring can be used.

Note that the lowest unoccupied molecular orbital (LUMO) of the organic compound having an unshared electron pair is preferably greater than or equal to −3.6 eV and less than or equal to −2.3 eV. In general, the highest occupied molecular orbital (HOMO) level and the LUMO level can be estimated by CV (cyclic voltammetry), photoelectron spectroscopy, optical absorption spectroscopy, inverse photoelectron spectroscopy, or the like.

The light-emitting layer is a layer containing a light-emitting substance. The light-emitting layer can contain one or more kinds of light-emitting substances. As the light-emitting substance, a substance that exhibits an emission color of blue, purple, bluish purple, green, yellowish green, yellow, orange, red, or the like is appropriately used. As the light-emitting substance, a substance that emits near-infrared light can also be used.

Examples of the light-emitting substance include a fluorescent material, a phosphorescent material, a TADF material, and a quantum dot material.

Examples of the fluorescent material include a pyrene derivative, an anthracene derivative, a triphenylene derivative, a fluorene derivative, a carbazole derivative, a dibenzothiophene derivative, a dibenzofuran derivative, a dibenzoquinoxaline derivative, a quinoxaline derivative, a pyridine derivative, a pyrimidine derivative, a phenanthrene derivative, and a naphthalene derivative.

Examples of the phosphorescent material include an organometallic complex (particularly an iridium complex) having a 4H-triazole skeleton, a 1H-triazole skeleton, an imidazole skeleton, a pyrimidine skeleton, a pyrazine skeleton, or a pyridine skeleton; an organometallic complex (particularly an iridium complex) having a phenylpyridine derivative including an electron-withdrawing group as a ligand; a platinum complex; and a rare earth metal complex.

The light-emitting layer may contain one or more kinds of organic compounds (e.g., a host material and an assist material) in addition to the light-emitting substance (a guest material). As one or more kinds of organic compounds, one or both of the hole-transport material and the electron-transport material can be used. Alternatively, as one or more kinds of organic compounds, a bipolar material or a TADF material may be used.

The light-emitting layer preferably includes, for example, a phosphorescent material and a combination of a hole-transport material and an electron-transport material that easily forms an exciplex. With such a structure, light emission can be efficiently obtained by ExTET (Exciplex—Triplet Energy Transfer), which is energy transfer from an exciplex to a light-emitting substance (a phosphorescent material). When a combination of materials is selected so as to form an exciplex that exhibits light emission whose wavelength overlaps with the wavelength of a lowest-energy-side absorption band of the light-emitting substance, energy can be smoothly transferred and light emission can be efficiently obtained. With this structure, high efficiency, low-voltage driving, and a long lifetime of the light-emitting device can be achieved at the same time.

An example of a fabrication method of the display device of one embodiment of the present invention will be described with reference to drawings.

Note that thin films included in the display device (insulating films, semiconductor films, conductive films, and the like) can be formed by a sputtering method, a chemical vapor deposition (CVD) method, a vacuum evaporation method, a pulsed laser deposition (PLD) method, an ALD method, or the like. Examples of the CVD method include a plasma-enhanced chemical vapor deposition (PECVD: Plasma Enhanced CVD) method and a thermal CVD method. As an example of the thermal CVD method, a metal organic chemical vapor deposition (MOCVD: Metal Organic CVD) method can be given.

Alternatively, thin films included in the display device (insulating films, semiconductor films, conductive films, or the like) can be formed by a method such as spin coating, dipping, spray coating, ink jetting, dispensing, screen printing, offset printing, a doctor knife, a slit coater, a roll coater, a curtain coater, and a knife coater.

When the thin films included in the display device are processed, a photolithography method or the like can be used for the processing. Besides, a nanoimprinting method, a sandblasting method, a lift-off method, or the like may be used for the processing of the thin films. Island-shaped thin films may be directly formed by a deposition method using a blocking mask such as a metal mask.

There are two typical examples of a photolithography method. In one of the methods, a resist mask is formed over a thin film that is to be processed, the thin film is processed by etching or the like, and the resist mask is removed. In the other method, after a photosensitive thin film is formed, exposure and development are performed, so that the thin film is processed into a desired shape.

As the light used for exposure in the photolithography method, for example, an i-line (with a wavelength of 365 nm), a g-line (with a wavelength of 436 nm), an h-line (with a wavelength of 405 nm), or combined light of any of them can be used. Besides, ultraviolet light, KrF laser light, ArF laser light, or the like can be used. Exposure may be performed by a liquid immersion exposure technique. Furthermore, as the light used for the exposure, extreme ultra-violet (EUV) light or X-rays may be used. Furthermore, instead of the light used for the exposure, an electron beam can also be used. It is preferable to use extreme ultra-violet light, X-rays, or an electron beam because extremely minute processing can be performed. Note that in the case of performing exposure by scanning of a beam such as an electron beam, a photomask need not be used.

For etching of the thin film, a dry etching method, a wet etching method, a sandblasting method, or the like can be used.

Examples of a fabrication method of the display device illustrated inFIG.1Bwill be described with reference toFIG.5AtoFIG.5DandFIG.6AtoFIG.6D. By using the fabrication method illustrated inFIG.5AtoFIG.5DandFIG.6AtoFIG.6D, the processing of the EL layer115and the conductive layer116can be performed without using a metal mask.

As the substrate101, a substrate having at least heat resistance high enough to withstand later heat treatment can be used. When an insulating substrate is used as the substrate101, a glass substrate, a quartz substrate, a sapphire substrate, a ceramic substrate or the like can be used. Alternatively, a single crystal semiconductor substrate or a polycrystalline semiconductor substrate using silicon or silicon carbide as a material, a compound semiconductor substrate of silicon germanium or the like, a semiconductor substrate such as an SOI substrate, or the like can be used.

As the substrate101, it is particularly preferable to use a semiconductor substrate or an insulating substrate over which a semiconductor circuit including a semiconductor element such as a transistor is formed. Such a semiconductor element may be formed using a semiconductor substrate such as a single crystal silicon substrate or may be formed using an oxide semiconductor film. The semiconductor circuit preferably forms, for example, a pixel circuit, a gate line driver circuit (a gate driver), or a source line driver circuit (a source driver). In addition to the above, an arithmetic circuit, a memory circuit, or the like may be formed.

In this embodiment, a substrate including at least a pixel circuit is used as the substrate101.

The insulating layer121is deposited over the substrate101(seeFIG.5A). The insulating layer121can be appropriately formed using any of the above insulating materials and any of the above deposition methods.

Note that when the structure illustrated inFIG.1Cis formed, the insulating layer122may be deposited over the insulating layer121. The insulating layer122can be appropriately formed using any of the above insulating materials and any of the above deposition methods. Here, when the insulating layer122is formed using a material with a low etching rate, the insulating layer122can function as an etching stopper at the time of forming the conductive layer111, the EL layer115, and the conductive layer116.

An opening reaching the substrate101is formed in the insulating layer121in a position where the plug131is to be formed. The opening is preferably an opening reaching an electrode or a wiring provided in the substrate101. Then, a conductive film is formed to fill the opening and planarization treatment is performed to expose the top surface of the insulating layer121. Thus, the plug131embedded in the insulating layer121can be formed (seeFIG.5A).

A conductive film is formed over the insulating layer121and the plug131. The conductive film is processed into an island shape to form the conductive layer111(seeFIG.5B). The conductive layer111is electrically connected to the plug131. Here, in a region of the insulating layer121, which does not overlap with the conductive layer111, a depressed portion is formed in some cases.

Subsequently, the EL layer115Bf and the conductive layer116fof the light-emitting element120B are deposited in this order over the conductive layer111and the insulating layer121. Next, a pattern using a resist RES1is formed over the conductive layer116f(seeFIG.5C). Here, the EL layer115Bf is a layer to be the EL layer115B in a later step. The conductive layer116fis a layer to be the conductive layer116in a later step. In addition, the EL layer115Gf and the EL layer115Rf which are formed later and the EL layer115Bf are collectively referred to as an EL layer115fin some cases.

The EL layer115fincludes at least a layer containing a light-emitting compound. In addition to this, a structure in which an electron-injection layer, an electron-transport layer, a charge-generation layer, a hole-transport layer, and a hole-injection layer are stacked may be employed. The EL layer115fcan be formed by, for example, a liquid phase method such as an evaporation method or an inkjet method.

The conductive layer116fis formed to have a transmissive property and a reflective property with respect to visible light. For example, a metal film or an alloy film which is thin enough to transmit visible light can be used. Alternatively, a conductive film (e.g., a metal oxide film) having a light-transmitting property may be stacked over such a film.

Subsequently, after etching is performed using the resist RES1as a mask and the conductive layer116and the EL layer115B are formed in this order, the resist RES1is removed (seeFIG.5D).

Here, when the EL layer115Bf is etched, chlorine or the like used in an etching gas is attached to the side surface of the EL layer115B in some cases. In addition, by the removal of the resist RES1after the conductive layer116and the EL layer115B are formed or by an exposure of the conductive layer116and the EL layer115B to the air, impurities such as water or oxygen are adsorbed on the side surface of the EL layer115B in some cases. The same applies to the EL layer115G and the EL layer115R, which are described later.

Subsequently, the EL layer115Gf and the conductive layer116fof the light-emitting element120G are deposited in this order over the conductive layer111, the insulating layer121, and the conductive layer116of the light-emitting element120B. Next, a pattern using a resist RES2is formed over the conductive layer116f(seeFIG.6A). Here, the EL layer115Gf is a layer to be the EL layer115G in a later step.

Next, after etching is performed using the resist RES2as a mask and the conductive layer116and the EL layer115G are formed in this order, the resist RES2is removed.

Subsequently, the EL layer115Rf and the conductive layer116fof the light-emitting element120R are deposited in this order over the conductive layer111, the insulating layer121, the conductive layer116of the light-emitting element120B, and the conductive layer116of the light-emitting element120G. Next, a pattern using a resist RES3is formed over the conductive layer116f(seeFIG.6B). Here, the EL layer115Rf is a layer to be the EL layer115R in a later step.

Next, after etching is performed using the resist RES3as a mask and the conductive layer116and the EL layer115R are formed in this order, the resist RES3is removed (seeFIG.6C).

Note that although the EL layer115and the conductive layer116are formed after the conductive layer111is formed in this embodiment, the present invention is not limited thereto. For example, a process in which a layer to be the conductive layer111, the EL layer115f, and the conductive layer116fare deposited in this order, and then the layers are collectively processed into an island shape to form the conductive layer111, the EL layer115, and the conductive layer116can also be employed.

In this embodiment, the EL layer115fand the conductive layer116fare successively deposited in each light-emitting element120; however, the present invention is not limited thereto. The conductive layer116may be formed after the formation of the EL layer115only, by a method similar to that illustrated inFIG.5CtoFIG.6C. In this case, the conductive layer116can be processed to be continuous without being separated between the light-emitting element120B, the light-emitting element120G, and the light-emitting element120R.

Alternatively, after the formation of a portion of the EL layer115in a method similar to that illustrated inFIG.5CtoFIG.6C, the other portions of the EL layer115and the conductive layer116may be formed. For example, the electron injection layer of the EL layer115and the conductive layer116may be formed later. In this case, the electron injection layer of the EL layer115and the conductive layer116can be processed to be continuous without being separated between the light-emitting element120B, the light-emitting element120G, and the light-emitting element120R.

Here, when a resist is directly formed over the EL layer115f, a solvent of the resist might deteriorate the EL layer115f. Therefore, it is preferable that an inorganic film functioning as a sacrifice layer be provided between the EL layer115fand the resist so that the resist is not in direct contact with the EL layer115f. For example, when the EL layer115has a structure illustrated inFIG.17B, an inorganic film functioning as a sacrifice layer may be provided over the layer4420-1functioning as an electron-transport layer, a resist may be provided thereover, and then the layer4430-1, the layer4430-2, the light-emitting layer4411, and the layer4420-1may be etched.

Next, the insulating layer124is deposited over the insulating layer121and the conductive layer116(seeFIG.6D). The insulating layer124can be appropriately formed using any of the above insulating materials and any of the above deposition methods. Note that a deposition temperature of the insulating layer124is preferably in a range in which the EL layer115does not deteriorate; for example, in the range of room temperature to 100° C., inclusive.

Here, a step of forming the insulating layer124usingFIG.7AtoFIG.7Cwhich are enlarged views of a region A inFIG.6Dis described.

First, the layer124ais deposited to cover the insulating layer121, the conductive layer111, the EL layer115, and the conductive layer116(seeFIG.7A). The layer124acan be appropriately formed using any of the above insulating materials and any of the above deposition methods. For example, aluminum oxide may be deposited by a sputtering method. Here, it is preferable that the amount of oxygen in the deposition gas be decreased or oxygen be not contained in the deposition gas. Thus, oxygen vacancies in the layer124aare increased, and the function of capturing or fixing impurities such as water or oxygen in the layer124acan be further enhanced.

Next, the layer124bis deposited to cover the layer124a(seeFIG.7B). The layer124bcan be appropriately formed using any of the above insulating materials and any of the above deposition methods. For example, silicon nitride may be deposited using a sputtering method.

Next, the layer124cis deposited to cover the layer124b(seeFIG.7C). The layer124ccan be appropriately formed using any of the above insulating materials and any of the above deposition methods. For example, aluminum oxide may be deposited by an ALD method.

As described above, in the etching step, impurities such as water or oxygen or impurities such as chlorine used in the etching may be attached to the light-emitting element120, especially, the side surfaces of the EL layer115B, the EL layer115G, and the EL layer115R. In that case, by providing the insulating layer124as described above, these impurities can be captured by or fixed in the layer124a. Thus, the formation of an unintended layer on the side surface of the EL layer115by the impurities is prevented, whereby the light-emitting element120can have improved reliability.

Through the above steps, the display device100including the light-emitting element120R, the light-emitting element120G, and the light-emitting element120B can be fabricated.

Structure Example 2

An example of the display device including a transistor will be described below.

Structure Example 2-1

FIG.8Ais a schematic cross-sectional view of a display device200A.

The display device200A includes a substrate201, the light-emitting element120R, the light-emitting element120G, the light-emitting element120B, a capacitor240, a transistor210, and the like.

A stacked structure from the substrate201to the capacitor240corresponds to the substrate101in the above-described structure example 1.

The transistor210is a transistor whose channel formation region is formed in the substrate201. As the substrate201, a semiconductor substrate such as a single crystal silicon substrate can be used, for example. The transistor210includes part of the substrate201, a conductive layer211, a low-resistance region212, an insulating layer213, and an insulating layer214. The conductive layer211functions as a gate electrode. The insulating layer213is positioned between the substrate201and the conductive layer211and functions as a gate insulating layer. The low-resistance region212is a region where the substrate201is doped with an impurity, and functions as one of a source and a drain. The insulating layer214is provided to cover a side surface of the conductive layer211and functions as an insulating layer.

In addition, an element isolation layer215is provided between two adjacent transistors210to be embedded in the substrate201.

Furthermore, an insulating layer261is provided to cover the transistor210, and the capacitor240is provided over the insulating layer261.

The capacitor240includes a conductive layer241, a conductive layer242, and an insulating layer243positioned therebetween. The conductive layer241functions as one electrode of the capacitor240, the conductive layer242functions as the other electrode of the capacitor240, and the insulating layer243functions as a dielectric of the capacitor240.

The conductive layer241is provided over the insulating layer261and is electrically connected to one of the source and the drain of the transistor210through a plug271embedded in the insulating layer261. The insulating layer243is provided to cover the conductive layer241. The conductive layer242is provided in a region overlapping with the conductive layer241with the insulating layer243therebetween.

The insulating layer121is provided to cover the capacitor240, and the light-emitting element120R, the light-emitting element120G, the light-emitting element120B, and the like are provided over the insulating layer121. Here, an example of the structure is illustrated inFIG.1Aas the structure of the light-emitting element120R, the light-emitting element120G, the light-emitting element120B, and the like; however, the structure is not limited thereto and a variety of structures described above can be employed.

In the display device200A, the insulating layer124, an insulating layer162, and an insulating layer163are provided in this order to cover the conductive layer116of the light-emitting element120. These three insulating layers each function as a protective layer which inhibits diffusion of impurities such as water into the light-emitting element120. As the insulating layer163, it is preferable to use an inorganic insulating film with low moisture permeability, such as a silicon oxide film, a silicon nitride film, or an aluminum oxide film. As the insulating layer162, an organic insulating film having a high light-transmitting property can be used. Using an organic insulating film as the insulating layer162can reduce the influence of uneven shape below the insulating layer162, so that the formation surface of the insulating layer163can be a smooth surface. Accordingly, a defect such as a pinhole is unlikely to be generated in the insulating layer163, leading to higher moisture permeability of the protective layer. Note that the structure of the protective layer covering the light-emitting element120is not limited thereto, and a single layer or a two-layer structure may be employed or a stacked-layer structure of four or more layers may be employed.

Providing the insulating layer124can inhibit diffusion of impurities such as water or oxygen into the light-emitting element120as described in the above structure examples.

The display device200A includes the substrate202on the viewing side. The substrate202and the substrate201are bonded to each other with an adhesive layer164which has a light-transmitting property. As the substrate202, a substrate having a light-transmitting property such as a glass substrate, a quartz substrate, a sapphire substrate, or a plastic substrate can be used.

In the case of using light-emitting elements which emit white light as illustrated inFIG.4AandFIG.4B, the coloring layer165R, the coloring layer165G, and the coloring layer165B are preferably provided as illustrated inFIG.8B. A coloring layer165R overlapping with the light-emitting element120R, a coloring layer165G overlapping with the light-emitting element120G, and a coloring layer165B overlapping with the light-emitting element120B are provided over the insulating layer163. For example, the coloring layer165R transmits red light, the coloring layer165G transmits green light, and the coloring layer165B transmits blue light. This can increase the color purity of light from each of the light-emitting elements, so that a display device with higher display quality can be achieved. Furthermore, forming the coloring layers over the insulating layer163makes it easy to align the light-emitting units with the respective coloring layers as compared with the case, which is described later, where the coloring layers are formed over a substrate202, so that a display device with extremely high resolution can be achieved. Note that the structure is not limited to the above, and the coloring layer165R, the coloring layer165G, and the coloring layer165B may be provided even when the light-emitting elements are divided into elements which emit red light, green light, and blue light.

With such a structure, a display device with extremely high resolution and high display quality can be achieved.

Structure Example 2-2

FIG.9is a schematic cross-sectional view of a display device200B. The display device200B is different from the display device200A mainly in a transistor structure.

A transistor220is a transistor in which a metal oxide (also referred to as an oxide semiconductor) is used in a semiconductor layer where a channel is formed.

The transistor220includes a semiconductor layer221, an insulating layer223, a conductive layer224, a pair of conductive layers225, an insulating layer226, and a conductive layer227.

As the substrate201over which the transistor220is provided, the above-described insulating substrate or semiconductor substrate can be used.

An insulating layer232is provided over the substrate201. The insulating layer232functions as a barrier layer that prevents diffusion of impurities such as water or hydrogen from the substrate201into the transistor220and release of oxygen from the semiconductor layer221to the insulating layer232side. As the insulating layer232, for example, a film in which hydrogen or oxygen, such as an aluminum oxide film, a hafnium oxide film, and a silicon nitride film, is unlikely to be diffused than in a silicon oxide film can be used.

The conductive layer227is provided over the insulating layer232, and the insulating layer226is provided to cover the conductive layer227. The conductive layer227functions as the first gate electrode of the transistor220, and part of the insulating layer226functions as the first gate insulating layer. For at least part of the insulating layer226that is in contact with the semiconductor layer221, an oxide insulating film such as a silicon oxide film is preferably used. The top surface of the insulating layer226is preferably planarized.

The semiconductor layer221is provided over the insulating layer226. The semiconductor layer221preferably includes a film of a metal oxide exhibiting semiconductor characteristics (also referred to as an oxide semiconductor). The material that can be favorably used for the semiconductor layer221is described later in detail.

The pair of conductive layers225is provided over and in contact with the semiconductor layer221, and functions as a source electrode and a drain electrode.

An insulating layer228is provided to cover the top surface and a side surface of the pair of conductive layers225, a side surface of the semiconductor layer221, and the like, and an insulating layer261bis provided over the insulating layer228. The insulating layer228functions as a barrier layer that prevents diffusion of impurities such as water or hydrogen from the insulating layer261bor the like into the semiconductor layer221and release of oxygen from the semiconductor layer221. As the insulating layer228, an insulating film similar to the insulating layer232can be used.

An opening reaching the semiconductor layer221is provided in the insulating layer228and the insulating layer261b. The insulating layer223that is in contact with side surfaces of the insulating layer261b, the insulating layer228, and the conductive layer225and the top surface of the semiconductor layer221; and the conductive layer224are embedded in the opening. The conductive layer224functions as the second gate electrode and the insulating layer223functions as the second gate insulating layer.

The top surface of the conductive layer224, the top surface of the insulating layer223, and the top surface of the insulating layer261bare planarized so that they are substantially level with each other, and an insulating layer229and an insulating layer261aare provided to cover these layers.

The insulating layer261aand the insulating layer261bfunction as an interlayer insulating layer. The insulating layer229functions as a barrier layer that prevents diffusion of impurities such as water or hydrogen from the insulating layer261aor the like into the transistor220. As the insulating layer229, an insulating film similar to the insulating layer228and the insulating layer232can be used.

A plug271electrically connected to one of the pair of conductive layers225is provided to be embedded in the insulating layer261a, an insulating layer229, and an insulating layer261b. Here, the plug271preferably includes a conductive layer271acovering side surfaces of the opening in the insulating layer261a, the insulating layer261b, the insulating layer229, and the insulating layer228and part of the top surface of the conductive layer225; and a conductive layer271bin contact with the top surface of the conductive layer271a. In this case, a conductive material in which hydrogen and oxygen are unlikely to be diffused is preferably used for the conductive layer271a.

Providing the insulating layer122or the insulating layer124can inhibit diffusion of impurities such as water or hydrogen into the transistor220as described in the above embodiments. Thus, the transistor220can have improved electrical characteristics and reliability.

Structure Example 2-3

FIG.10is a schematic cross-sectional view of a display device200C. The display device200C has a structure in which the transistor210whose channel is formed in the substrate201and the transistor220including a metal oxide in a semiconductor layer where its channel is formed are stacked.

The insulating layer261is provided to cover the transistor210and the conductive layer251is provided over the insulating layer261. In addition, an insulating layer262is provided to cover the conductive layer251and a conductive layer252is provided over the insulating layer262. The conductive layer251and the conductive layer252each function as wirings. An insulating layer263and the insulating layer232are provided to cover the conductive layer252, and the transistor220is provided over the insulating layer232. An insulating layer265is provided to cover the transistor220, and the capacitor240is provided over the insulating layer265. The capacitor240and the transistor220are electrically connected to each other through a plug274.

The transistor220can be used as a transistor included in a pixel circuit. The transistor210can also be used as a transistor included in a pixel circuit or a transistor included in a driver circuit (a gate line driver circuit or a source line driver circuit) for driving the pixel circuit. The transistor210and the transistor220can also be used as transistors included in a variety of circuits such as an arithmetic circuit or a memory circuit.

With such a structure, not only the pixel circuit but also the driver circuit or the like can be formed directly under the light-emitting unit; thus, the display device can be downsized as compared with the case where the driver circuit is provided around a display region.

Structure Example 2-4

FIG.11is a schematic cross-sectional view of a display device200D. The display device200D is different from the display device200C mainly in that two transistors using an oxide semiconductor are stacked.

The display device200D includes a transistor230between the transistor210and the transistor220. The transistor230has a structure similar to that of the transistor220except that the first gate electrode is not included. Note that the transistor230may have a structure including the first gate electrode.

The insulating layer263and an insulating layer231are provided to cover the conductive layer252, and the transistor230is provided over the insulating layer231. The transistor230and the conductive layer252are electrically connected to each other through a plug273, a conductive layer253, and a plug272. An insulating layer264and the insulating layer232are provided to cover the conductive layer253, and the transistor220is provided over the insulating layer232.

The transistor220functions as, for example, a transistor for controlling current flowing through the light-emitting element120. The transistor230functions as a selection transistor for controlling the selection state of a pixel. The transistor210functions as a transistor included in a driver circuit for driving a pixel, for example.

When three or more layers in which a transistor is formed are stacked in this manner, the area occupied by the pixel can be further reduced and a high-resolution display device can be achieved.

Components such as a transistor that can be employed in the display device will be described below.

The transistor includes a conductive layer functioning as a gate electrode, a semiconductor layer, a conductive layer functioning as a source electrode, a conductive layer functioning as a drain electrode, and an insulating layer functioning as a gate insulating layer.

Note that there is no particular limitation on the structure of the transistor included in the display device of one embodiment of the present invention. For example, a planar transistor, a staggered transistor, or an inverted staggered transistor may be used. In addition, the transistor structure may be either a top-gate structure or a bottom-gate structure. Gate electrodes may be provided above and below a channel.

In particular, a transistor in which a metal oxide film is used for a semiconductor layer where a channel is formed will be described below.

As a semiconductor material used for the transistors, a metal oxide whose energy gap is greater than or equal to 2 eV, preferably greater than or equal to 2.5 eV, further preferably greater than or equal to 3 eV can be used. A typical example thereof is a metal oxide containing indium, and for example, a CAC-OS described later or the like can be used.

A transistor in which a metal oxide having a wider band gap and a lower carrier density than silicon is used has a low off-state current; thus, charges accumulated in a capacitor that is connected in series with the transistor can be held for a long time.

The semiconductor layer can be, for example, a film represented by an In-M-Zn-based oxide that contains indium, zinc, and M (M is a metal such as aluminum, titanium, gallium, germanium, yttrium, zirconium, lanthanum, cerium, tin, neodymium, or hafnium).

When the metal oxide contained in the semiconductor layer is an In-M-Zn-based oxide, it is preferable that the atomic ratio of metal elements of a sputtering target used for depositing the In-M-Zn oxide satisfy In ≥M and Zn≥M. The atomic ratio of metal elements of such a sputtering target is preferably In:M:Zn=1:1:1, In:M:Zn=1:1:1.2, In:M:Zn=3:1:2, In:M:Zn=4:2:3, In:M:Zn=4:2:4.1, In:M:Zn=5:1:6, In:M:Zn=5:1:7, In:M:Zn=5:1:8, or the like. Note that the atomic ratio in the deposited semiconductor layer varies from the atomic ratio of metal elements contained in the sputtering target in a range of ±40%.

A metal oxide film with a low carrier density is used as the semiconductor layer. For example, for the semiconductor layer, a metal oxide whose carrier density is lower than or equal to 1×1017/cm3, preferably lower than or equal to 1×1015/cm3, further preferably lower than or equal to 1×1013/cm3, still further preferably lower than or equal to 1×1011/cm3, even further preferably lower than 1×1010/cm3, and higher than or equal to 1×10−9/cm3can be used. Such a metal oxide is referred to as a highly purified intrinsic or substantially highly purified intrinsic metal oxide. The oxide semiconductor has a low density of defect states and can be regarded as a metal oxide having stable characteristics.

Note that the composition is not limited to those, and an oxide semiconductor having an appropriate composition may be used depending on required semiconductor characteristics and electrical characteristics (field-effect mobility, threshold voltage, or the like) of the transistor. In addition, to obtain the required semiconductor characteristics of the transistor, it is preferable that the carrier density, impurity concentration, defect density, atomic ratio between a metal element and oxygen, an interatomic distance, a density, and the like of the semiconductor layer be set to be appropriate.

When silicon or carbon, which is one of Group 14 elements, is contained in the metal oxide contained in the semiconductor layer, oxygen vacancies are increased in the semiconductor layer, and the semiconductor layer becomes n-type. Thus, the concentration (concentration obtained by secondary ion mass spectrometry) of silicon or carbon in the semiconductor layer is set lower than or equal to 2×1018atoms/cm3, preferably lower than or equal to 2×1017atoms/cm3.

Alkali metal and alkaline earth metal might generate carriers when bonded to a metal oxide, in which case the off-state current of the transistor might be increased. Thus, the concentration of alkali metal or alkaline earth metal in the semiconductor layer that is obtained by secondary ion mass spectrometry is set to lower than or equal to 1×1018atoms/cm3, preferably lower than or equal to 2×1016atoms/cm3.

When nitrogen is contained in the metal oxide contained in the semiconductor layer, electrons serving as carriers are generated and the carrier density increases, so that the semiconductor layer easily becomes n-type. As a result, a transistor using a metal oxide that contains nitrogen easily have normally-on characteristics. Accordingly, the nitrogen concentration in the semiconductor layer that is obtained by secondary ion mass spectrometry is preferably set to lower than or equal to 5×1018atoms/cm3.

Hydrogen contained in the oxide semiconductor reacts with oxygen bonded to a metal atom to be water, and thus forms an oxygen vacancy in some cases. Entry of hydrogen into the oxygen vacancy generates an electron serving as a carrier in some cases. Furthermore, bonding of part of hydrogen to oxygen bonded to a metal atom generates an electron serving as a carrier in some cases. Thus, a transistor including an oxide semiconductor containing hydrogen tends to have normally-on characteristics. Accordingly, hydrogen in the channel formation region of the oxide semiconductor is preferably reduced as much as possible. Specifically, the hydrogen concentration in the channel formation region of the oxide semiconductor that is obtained by secondary ion mass spectrometry is set lower than 1×1020atoms/cm3, preferably lower than 5×1019atoms/cm3, further preferably lower than 1×1019atoms/cm3, still further preferably lower than 5×1018atoms/cm3, even further preferably lower than 1×1018atoms/cm3.

When an oxide semiconductor with sufficiently reduced impurities is used for a channel formation region in a transistor, stable electrical characteristics and reliability can be given.

Oxide semiconductors are classified into a single crystal oxide semiconductor and a non-single-crystal oxide semiconductor. Examples of the non-single-crystal oxide semiconductor include a CAAC-OS (c-axis-aligned crystalline oxide semiconductor), a polycrystalline oxide semiconductor, an nc-OS (nanocrystalline oxide semiconductor), an amorphous-like oxide semiconductor (a-like OS), and an amorphous oxide semiconductor.

In addition, a CAC-OS (cloud-aligned composite oxide semiconductor) may be used for a semiconductor layer of a transistor disclosed in one embodiment of the present invention.

Note that the above-mentioned non-single-crystal oxide semiconductor can be suitably used for a semiconductor layer of a transistor disclosed in one embodiment of the present invention. In addition, as the non-single-crystal oxide semiconductor, the nc-OS or the CAAC-OS can be suitably used.

Note that in one embodiment of the present invention, a CAC-OS is preferably used for a semiconductor layer of a transistor. The use of the CAC-OS allows the transistor to have high electrical characteristics or high reliability.

The semiconductor layer may be a mixed film including two or more kinds of a region of a CAAC-OS, a region of a polycrystalline oxide semiconductor, a region of an nc-OS, a region of an amorphous-like oxide semiconductor, and a region of an amorphous oxide semiconductor. The mixed film has, for example, a single-layer structure or a stacked-layer structure including two or more kinds of regions selected from the above regions in some cases.

The composition of a CAC-OS that can be used in a transistor disclosed in one embodiment of the present invention will be described below.

The CAC-OS is, for example, a composition of a material in which elements that contained in a metal oxide are unevenly distributed to have a size of greater than or equal to 0.5 nm and less than or equal to 10 nm, preferably greater than or equal to 1 nm and less than or equal to 2 nm, or a similar size. Note that in the following description, a state in which one or more metal elements are unevenly distributed and regions including the metal element(s) are mixed to have a size of greater than or equal to 0.5 nm and less than or equal to 10 nm, preferably greater than or equal to 1 nm and less than or equal to 2 nm, or a similar size in a metal oxide is referred to as a mosaic pattern or a patch-like pattern.

Note that the metal oxide preferably contains at least indium. In particular, indium and zinc are preferably contained. Moreover, in addition to these, one kind or a plurality of kinds selected from aluminum, gallium, yttrium, copper, vanadium, beryllium, boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the like may be contained.

For example, a CAC-OS in an In—Ga—Zn oxide (an In—Ga—Zn oxide in the CAC-OS may be particularly referred to as CAC-IGZO) has a composition in which materials are separated into indium oxide (hereinafter, InOX1(X1 is a real number greater than 0)) or indium zinc oxide (hereinafter, InX2ZnY2OZ2(X2, Y2, and Z2 are real numbers greater than 0)) and gallium oxide (hereinafter, GaOX3(X3 is a real number greater than 0)) or gallium zinc oxide (hereinafter, GaX4ZnY4OZ4(X4, Y4, and Z4 are real numbers greater than 0)), so that a mosaic pattern is formed, and mosaic-like InOX1or InX2ZnY2OZ2is evenly distributed in the film (which is hereinafter also referred to as cloud-like).

That is, the CAC-OS is a composite metal oxide having a composition in which a region where GaOX3is a main component and a region where InX2ZnY2OZ2or InOX1is a main component are mixed. Note that in this specification, for example, when the atomic ratio of In to an element M in the first region is larger than the atomic ratio of In to the element M in a second region, the first region is regarded as having a higher In concentration than the second region.

Note that IGZO is a common name and sometimes refers to one compound formed of In, Ga, Zn, and O. A typical example is a crystalline compound represented by InGaO3(ZnO)m1(m1 is a natural number; however, m1 is not 0) or In(1+x0)Ga(1−x0)O3(ZnO)m0(−1≤x0≤1; m0 is a given number).

The crystalline compound has a single crystal structure, a polycrystalline structure, or a CAAC structure. Note that the CAAC structure is a crystal structure in which a plurality of IGZO nanocrystals have c-axis alignment and are connected in an a-b plane without alignment.

Meanwhile, the CAC-OS relates to the material composition of a metal oxide. In the material composition of a CAC-OS containing In, Ga, Zn, and O, some regions that contain Ga as a main component and are observed as nanoparticles and some regions that contain In as a main component and are observed as nanoparticles are each randomly dispersed in a mosaic pattern. Therefore, the crystal structure is a secondary element for the CAC-OS.

Note that the CAC-OS is regarded as not including a stacked-layer structure of two or more kinds of films with different compositions. For example, a two-layer structure of a film containing In as a main component and a film containing Ga as a main component is not included. Note that a clear boundary between the region where GaOX3is a main component and the region where InX2ZnY2OZ2or InOX1is a main component cannot be observed in some cases.

Note that when one kind or a plurality of kinds selected from aluminum, yttrium, copper, vanadium, beryllium, boron, silicon, titanium, iron, nickel, germanium, zirconium, molybdenum, lanthanum, cerium, neodymium, hafnium, tantalum, tungsten, magnesium, and the like are contained instead of gallium, the CAC-OS refers to a composition in which some regions that contain the metal element(s) as a main component and are observed as nanoparticles and some regions that contain In as a main component and are observed as nanoparticles are each randomly dispersed in a mosaic pattern.

The CAC-OS is characterized in that no clear peak is observed at the time of measurement using θ/2θ scan by an Out-of-plane method, which is one of the X-ray diffraction (XRD) measurement methods. That is, it is found from the X-ray diffraction measurement that no alignment in the a-b plane direction and the c-axis direction is observed in the measured region.

In an electron diffraction pattern of the CAC-OS which is obtained by irradiation with an electron beam with a probe diameter of 1 nm (also referred to as a nanometer-sized electron beam), a ring-like region with high luminance and a plurality of bright spots in the ring-like region are observed. Thus, the electron diffraction pattern indicates that the crystal structure of the CAC-OS includes an nc (nano-crystal) structure with no alignment in a plan-view direction and a cross-sectional direction.

Moreover, for example, it can be confirmed by EDX mapping obtained using energy dispersive X-ray spectroscopy (EDX) that the CAC-OS in the In—Ga—Zn oxide has a composition in which regions where GaOX3is a main component and regions where InX2ZnY2OZ2or InOX1is a main component are unevenly distributed and mixed.

The CAC-OS has a composition different from that of an IGZO compound in which metal elements are evenly distributed, and has characteristics different from those of the IGZO compound. That is, the CAC-OS has a composition in which regions where GaOX3or the like is a main component and regions where InX2ZnY2OZ2or InOX1is a main component are phase-separated from each other, and the regions containing the respective elements as the main components form a mosaic pattern.

Here, a region where InX2ZnY2OZ2or InOX1is a main component is a region whose conductivity is higher than that of a region where GaOX3or the like is a main component. In other words, when carriers flow through regions where InX2ZnY2OZ2or InOX1is a main component, the conductivity of a metal oxide is exhibited. Accordingly, when the regions where InX2ZnY2OZ2or InOX1is a main component are distributed in a metal oxide like a cloud, high field-effect mobility (μ) can be achieved.

In contrast, a region where GaOX3or the like is a main component is a region whose insulating property is higher than that of a region where InX2ZnY2OZ2or InOX1is a main component. In other words, when regions where GaOX3or the like is a main component are distributed in a metal oxide, leakage current can be inhibited and favorable switching operation can be achieved.

Accordingly, when the CAC-OS is used for a semiconductor element, the insulating property derived from GaOX3or the like and the conductivity derived from InX2ZnY2OZ2or InOX1complement each other, whereby a high on-state current (Ion) and high field-effect mobility (μ) can be achieved.

A semiconductor element using the CAC-OS has high reliability. Thus, the CAC-OS is suitably used in a variety of semiconductor devices typified by a display.

Since a transistor including the CAC-OS in a semiconductor layer has high field-effect mobility and high drive capability, the use of the transistor in a driver circuit, a typical example of which is a scan line driver circuit that generates a gate signal, can provide a display device with a narrow bezel width (also referred to a narrow bezel). Furthermore, with the use of the transistor in a signal line driver circuit that is included in a display device (particularly in a demultiplexer connected to an output terminal of a shift register included in a signal line driver circuit), a display device to which a small number of wirings are connected can be provided.

Furthermore, unlike a transistor including low-temperature polysilicon, the transistor including a CAC-OS in the semiconductor layer does not need a laser crystallization step. Thus, the manufacturing cost of a display device can be reduced even when the display device is formed using a large area substrate. In addition, the transistor including a CAC-OS in the semiconductor layer is preferably used for a driver circuit and a display portion in a large display device having high resolution such as ultra-high definition (“4K definition”, “4K2K”, and “4K”) or super high definition (“8K definition”, “8K4K”, and “8K”), in which case writing can be performed in a short time and display defects can be reduced.

Alternatively, silicon may be used for a semiconductor in which a channel of a transistor is formed. Although amorphous silicon may be used as silicon, silicon having crystallinity is particularly preferably used. For example, microcrystalline silicon, polycrystalline silicon, or single crystal silicon is preferably used. In particular, polycrystalline silicon can be formed at a temperature lower than that for single crystal silicon and has higher field-effect mobility and higher reliability than amorphous silicon.

As materials that can be used for conductive layers such as a variety of wirings and electrodes included in a display device in addition to a gate, a source, and a drain of a transistor, metals such as aluminum, titanium, chromium, nickel, copper, yttrium, zirconium, molybdenum, silver, tantalum, and tungsten, or an alloy containing any of these metals as its main component can be given. A single layer or stacked-layer structure including a film containing any of these materials can be used. For example, a single-layer structure of an aluminum film containing silicon, a two-layer structure in which an aluminum film is stacked over a titanium film, a two-layer structure in which an aluminum film is stacked over a tungsten film, a two-layer structure in which a copper film is stacked over a copper-magnesium-aluminum alloy film, a two-layer structure in which a copper film is stacked over a titanium film, a two-layer structure in which a copper film is stacked over a tungsten film, a three-layer structure in which an aluminum film or a copper film is stacked over a titanium film or a titanium nitride film and a titanium film or a titanium nitride film is formed thereover, and a three-layer structure in which an aluminum film or a copper film is stacked over a molybdenum film or a molybdenum nitride film and a molybdenum film or a molybdenum nitride film is formed thereover can be given. Note that an oxide such as indium oxide, tin oxide, or zinc oxide may be used. Copper containing manganese is preferably used because it increases controllability of a shape by etching.

Examples of an insulating material that can be used for the insulating layers include a resin such as an acrylic resin or an epoxy resin, a resin having a siloxane bond, such as silicone, and an inorganic insulating material such as silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, or aluminum oxide.

Note that in this specification, an oxynitride refers to a material that contains more oxygen than nitrogen in its composition, and a nitride oxide refers to a material that contains more nitrogen than oxygen in its composition. For example, when silicon oxynitride is described, it refers to a material that contains more oxygen than nitrogen in its composition; when silicon nitride oxide is described, it refers to a material that contains more nitrogen than oxygen in its composition.

The light-emitting element is preferably provided between a pair of insulating films with low water permeability. In that case, impurities such as water can be inhibited from entering the light-emitting element, and thus a decrease in device reliability can be inhibited.

Examples of the insulating film with low water permeability include a film containing nitrogen and silicon, such as a silicon nitride film and a silicon nitride oxide film, and a film containing nitrogen and aluminum, such as an aluminum nitride film. Alternatively, a silicon oxide film, a silicon oxynitride film, an aluminum oxide film, or the like may be used.

For example, the moisture vapor transmission rate of the insulating film with low water permeability is lower than or equal to 1×10−5[g/(m2·day)], preferably lower than or equal to 1×10−6[g/(m2·day)], further preferably lower than or equal to 1×10−7[g/(m2·day)], still further preferably lower than or equal to 1×10−8[g/(m2·day)].

[Structure Example of Display Module]

A structure example of a display module including the display device of one embodiment of the present invention will be described below.

FIG.12Ais a schematic perspective view of a display module280. The display module280includes a display device200and an FPC290. Any of the display devices (the display device200A to the display device200D) described in Structure Example 2 can be used as the display device200.

The display module280includes the substrate201and the substrate202. A display portion281is also included on the substrate202side. The display portion281is a region of the display module280where an image is displayed and is a region where light emitted from pixels provided in a pixel portion284described later can be seen. The display module280may include a source driver IC290b.

FIG.12Billustrates a perspective view schematically illustrating a structure on the substrate201side. The substrate201has a structure in which a circuit portion282, a pixel circuit portion283over the circuit portion282, and the pixel portion284over the pixel circuit portion283are stacked. In addition, a terminal portion285for connection to the FPC290is included in a portion not overlapping with the pixel portion284over the substrate201. The terminal portion285and the circuit portion282are electrically connected to each other through a wiring portion286formed of a plurality of wirings.

The pixel portion284includes a plurality of pixels284aarranged periodically. An enlarged view of one pixel284ais illustrated on the right side ofFIG.12B. The pixel284aincludes the light-emitting element120R, the light-emitting element120G, and the light-emitting element120B.

The pixel circuit portion283includes a plurality of pixel circuits283aarranged periodically. A plurality of pixels284aand a plurality of pixel circuits283amay be arranged in a stripe pattern as illustrated inFIG.12B. Note that the plurality of pixels284aand the plurality of pixel circuits283amay be arranged in a delta pattern without limitation to the stripe pattern.

One pixel circuit283ais a circuit that controls light emission of three light-emitting elements included in one pixel284a. One pixel circuit283amay be provided with three circuits for controlling light emission of respective light-emitting elements. For example, the pixel circuit283afor one light-emitting element can include at least one selection transistor, one current control transistor (driving transistor), and a capacitor. In this case, a gate signal is input to a gate of the selection transistor and a source signal is input to one of a source and a drain thereof. With such a structure, an active-matrix display device is achieved.

The circuit portion282includes a circuit for driving the pixel circuits283ain the pixel circuit portion283. For example, a gate line driver circuit and a source line driver circuit are preferably included. In addition, an arithmetic circuit, a memory circuit, a power supply circuit, or the like may be included.

The FPC290functions as a wiring for supplying a video signal, a power supply potential, or the like to the circuit portion282from the outside. In addition, an IC may be mounted on the FPC290.

The display module280can have a structure in which the pixel circuit portion283, the circuit portion282, or the like are stacked below the pixel portion284; thus, the aperture ratio (the effective display area ratio) of the display portion281can be significantly high. For example, the aperture ratio of the display portion281can be greater than or equal to 40% and less than 100%, preferably greater than or equal to 50% and less than or equal to 95%, and further preferably greater than or equal to 60% and less than or equal to 95%. Furthermore, the pixels284acan be arranged extremely densely and thus the display portion281can have extremely high resolution. For example, the pixels284aare preferably arranged in the display portion281with a resolution greater than or equal to 2000 ppi, preferably greater than or equal to 3000 ppi, further preferably greater than or equal to 5000 ppi, still further preferably greater than or equal to 6000 ppi, and less than or equal to 20000 ppi or less than or equal to 30000 ppi.

Such a display module280has extremely high resolution, and thus can be suitably used for a device for VR such as a head-mounted display or a glasses-type device for AR. For example, even in the case of a structure in which the display portion of the display module280is seen through a lens, pixels of the extremely-high-resolution display portion281included in the display module280are prevented from being seen when the display portion is enlarged by the lens, so that display providing a high sense of immersion can be performed. Without limitation to the above, the display module280can also be suitably used for an electronic device including a relatively small display portion. For example, the display module280can be suitably used for a display portion of a wearable electronic device such as a wrist watch.

At least part of this embodiment can be appropriately implemented in combination with the other embodiments described in this specification.

In this embodiment, a display device of one embodiment of the present invention will be described with reference toFIG.13.

A display device illustrated inFIG.13Aincludes a pixel portion502, a driver circuit portion504, protection circuits506, and a terminal portion507. Note that the display device of one embodiment of the present invention may have a structure in which the protection circuits506are not provided.

The pixel portion502includes a plurality of pixel circuits501arranged in X rows and Y columns (X and Y each independently represent a natural number of 2 or more). Each pixel circuit501includes a circuit each of which drives a display element.

The driver circuit portion504includes driver circuits such as a gate driver504athat outputs a scanning signal to gate lines GL_1to GL_X and a source driver504bthat supplies a data signal to data lines DL_1to DL_Y. The gate driver504aincludes at least a shift register. The source driver504bis formed using a plurality of analog switches, for example. Alternatively, the source driver504bmay be formed using a shift register or the like.

The terminal portion507refers to a portion provided with terminals for inputting power, control signals, image signals, and the like to the display device from external circuits.

The protection circuit506is a circuit that, when a potential out of a certain range is applied to a wiring to which the protection circuit506is connected, establishes continuity between the wiring and another wiring. The protection circuit506illustrated inFIG.13Ais connected to a variety of wirings such as the gate lines GL that are wirings between the gate driver504aand the pixel circuits501and the data lines DL that are wirings between the source driver504band the pixel circuits501, for example.

The gate driver504aand the source driver504bmay be provided over the same substrate as the pixel portion502, or a substrate where a gate driver circuit or a source driver circuit is separately formed (e.g., a driver circuit board formed using a single crystal semiconductor or a polycrystalline semiconductor) may be mounted on the substrate by COG or TAB (Tape Automated Bonding).

In particular, the gate driver504aand the source driver504bare preferably placed below the pixel portion502.

The plurality of pixel circuits501illustrated inFIG.13Acan have a configuration illustrated inFIG.13B, for example.

The pixel circuit501illustrated inFIG.13Bincludes transistors552and554, a capacitor562, and a light-emitting element572. A data line DL n, a gate line GL_m, a potential supply line VL_a, a power supply line VL_b, and the like are connected to the pixel circuit501.

Note that a high power supply potential VDD is supplied to one of the potential supply line VL_a and the potential supply line VL_b, and a low power supply potential VSS is supplied to the other. Current flowing through the light-emitting element572is controlled in accordance with a potential applied to a gate of the transistor554, whereby the luminance of light emitted from the light-emitting element572is controlled.

At least part of this embodiment can be appropriately implemented in combination with the other embodiments described in this specification.

A pixel circuit including a memory for correcting gray levels displayed by pixels that can be used in a display device of one embodiment of the present invention and the display device including the pixel circuit will be described below.

FIG.14Ais a circuit diagram of a pixel circuit400. The pixel circuit400includes a transistor M1, a transistor M2, a capacitor C1, and a circuit401. A wiring S1, a wiring S2, a wiring G1, and a wiring G2are connected to the pixel circuit400.

In the transistor M1, a gate is connected to the wiring G1, one of a source and a drain is connected to the wiring S1, and the other is connected to one electrode of the capacitor C1. In the transistor M2, a gate is connected to the wiring G2, one of a source and a drain is connected to the wiring S2, and the other is connected to the other electrode of the capacitor C1and the circuit401.

The circuit401is a circuit including at least one display element. Any of a variety of elements can be used as the display element, and typically, a light-emitting element such as an organic EL element or an LED element can be used. In addition, a liquid crystal element, a MEMS (Micro Electro Mechanical Systems) element, or the like can also be used.

A node connecting the transistor M1and the capacitor C1is denoted as a node N1, and a node connecting the transistor M2and the circuit401is denoted as a node N2.

In the pixel circuit400, the potential of the node N1can be retained when the transistor M1is turned off. The potential of the node N2can be retained when the transistor M2is turned off. When a predetermined potential is written to the node N1through the transistor M1with the transistor M2being in an off state, the potential of the node N2can be changed in accordance with displacement of the potential of the node N1owing to capacitive coupling through the capacitor C1.

Here, the transistor using an oxide semiconductor, which is described in Embodiment 1, can be used as one or both of the transistor M1and the transistor M2. Accordingly, owing to an extremely low off-state current, the potentials of the node N1and the node N2can be retained for a long time. Note that when the period in which the potential of each node is retained is short (specifically, the case where the frame frequency is higher than or equal to 30 Hz, for example), a transistor using a semiconductor such as silicon may be used.

Next, an example of a method for operating the pixel circuit400is described with reference toFIG.14B.FIG.14Bis a timing chart of the operation of the pixel circuit400. Note that for simplification of description, the influence of various kinds of resistance such as wiring resistance, parasitic capacitance of a transistor, a wiring, or the like, the threshold voltage of the transistor, and the like is not taken into account here.

In the operation shown inFIG.14B, one frame period is divided into a period T1and a period T2. The period T1is a period in which a potential is written to the node N2, and the period T2is a period in which a potential is written to the node N1.

In the period T1, a potential for turning on the transistor is applied to both the wiring G1and the wiring G2. In addition, a potential Vrefthat is a fixed potential is supplied to the wiring S1, and the first data potential Vwis supplied to the wiring S2.

The potential Vrefis supplied from the wiring S1to the node N1through the transistor M1. The first data potential Vwis supplied to the node N2from the wiring S2through the transistor M2. Accordingly, a potential difference Vw-Vrefis retained in the capacitor C1.

Next, in the period T2, a potential for turning on the transistor M1is supplied to the wiring G1, and a potential for turning off the transistor M2is supplied to the wiring G2. The second data potential Vdatais supplied to the wiring S1. The wiring S2may be supplied with a predetermined constant potential or brought into a floating state.

The second data potential Vdatais supplied to the node N1from the wiring S1through the transistor M1. At this time, capacitive coupling due to the capacitor C1changes the potential of the node N2in accordance with the second data potential Vdataby a potential dV. That is, a potential that is the sum of the first data potential V, and the potential dV is input to the circuit401. Note that although the potential dV is shown as a positive value inFIG.14B, the dV may be a negative value. That is, the second data potential Vdatamay be lower than the potential Vref.

Here, the potential dV is roughly determined by the capacitance of the capacitor C1and the capacitance of the circuit401. When the capacitance of the capacitor C1is sufficiently larger than the capacitance of the circuit401, the potential dV is a potential close to the second data potential Vdata.

In the above manner, the pixel circuit400can generate a potential to be supplied to the circuit401including the display element, by combining two kinds of data signals; hence, a gray level can be corrected in the pixel circuit400.

The pixel circuit400can also generate a potential exceeding the maximum potential that can be supplied to the wiring S1and the wiring S2. For example, in the case of using a light-emitting element, high-dynamic range (HDR) display or the like can be performed. In the case of using a liquid crystal element, overdriving or the like can be achieved.

Application Example

A pixel circuit400EL illustrated inFIG.14Cincludes a circuit401EL. The circuit401EL includes a light-emitting element EL, a transistor M3, and a capacitor C2.

In the transistor M3, a gate is connected to the node N2and one electrode of the capacitor C2, one of a source and a drain is connected to a wiring supplying a potential VH, and the other is connected to one electrode of the light-emitting element EL. The other electrode of the capacitor C2is connected to a wiring supplying a potential Vcom. The other electrode of the light-emitting element EL is connected to a wiring supplying a potential VL.

The transistor M3has a function of controlling current to be supplied to the light-emitting element EL. The capacitor C2functions as a storage capacitor. The capacitor C2can be omitted when not needed.

Note that although the structure in which the anode side of the light-emitting element EL is connected to the transistor M3is described here, the transistor M3may be connected to the cathode side. In that case, the values of the potential VH and the potential VL can be appropriately changed.

In the pixel circuit400EL, a large amount of current can flow through the light-emitting element EL when a high potential is applied to the gate of the transistor M3, which enables HDR display, for example. Moreover, a variation in the electrical characteristics of the transistor M3or the light-emitting element EL can be corrected by supply of a correction signal to the wiring S1or the wiring S2.

Note that the configuration is not limited to the circuit shown inFIG.14C, and a configuration to which a transistor, a capacitor, or the like is further added may be employed.

At least part of this embodiment can be appropriately implemented in combination with the other embodiments described in this specification.

In this embodiment, structure examples of an electronic device for which the display device of one embodiment of the present invention is used will be described.

The display device and the display module of one embodiment of the present invention can be applied to a display portion of an electronic device or the like having a display function. Examples of such an electronic device include a digital camera, a digital video camera, a digital photo frame, a mobile phone, a portable game machine, a portable information terminal, and an audio reproducing device, in addition to electronic devices with a relatively large screen, such as a television device, a laptop personal computer, a monitor device, digital signage, a pachinko machine, and a game machine.

In particular, the display device and the display module of one embodiment of the present invention can have a high resolution, and thus can be suitably used for an electronic device having a relatively small display portion. Examples of the electronic device include a watch-type or bracelet-type information terminal device (wearable device); and a wearable device worn on a head, such as a device for VR such as a head mounted display and a glasses-type device for AR.

FIG.15Ais a perspective view of an electronic device700that is of a glasses type. The electronic device700includes a pair of display panels701, a pair of housings702, a pair of optical members703, a pair of temples704, and the like.

The electronic device700can project an image displayed on the display panel701onto a display region706of the optical member703. Since the optical members703have a light-transmitting property, a user can see images displayed on the display regions706, which are superimposed on transmission images seen through the optical members703. Thus, the electronic device700is an electronic device capable of AR display.

One housing702is provided with a camera705capable of taking an image of what lies in front thereof. Although not illustrated, one of the housings702is provided with a wireless receiver or a connector to which a cable can be connected, whereby a video signal or the like can be supplied to the housing702. Furthermore, when the housing702is provided with an acceleration sensor such as a gyroscope sensor, the orientation of the user's head can be detected and an image corresponding to the orientation can be displayed on the display region706. Moreover, the housing702is preferably provided with a battery and charging can be performed with or without a wire.

Next, a method for projecting an image on the display region706of the electronic device700is described with reference toFIG.15B. The display panel701, a lens711, and a reflective plate712are provided in the housing702. A reflective surface713functioning as a half mirror is provided in a portion corresponding to the display region706of the optical member703.

Light715emitted from the display panel701passes through the lens711and is reflected by the reflective plate712to the optical member703side. In the optical member703, the light715is fully reflected repeatedly by end surfaces of the optical member703and reaches the reflective surface713, whereby an image is projected on the reflective surface713. Accordingly, the user can see both the light715reflected by the reflective surface713and transmitted light716transmitted through the optical member703(including the reflective surface713).

FIG.15illustrates an example in which the reflective plate712and the reflective surface713each have a curved surface. This can increase optical design flexibility and reduce the thickness of the optical member703, compared to the case where they have flat surfaces. Note that the reflective plate712and the reflective surface713may be flat.

The reflective plate712can use a component having a mirror surface, and preferably has high reflectivity. As the reflective surface713, a half mirror utilizing reflection of a metal film may be used, but the use of prism utilizing total reflection or the like can increase the transmittance of the transmitted light716.

Here, the housing702preferably includes a mechanism for adjusting the distance or angle between the lens711and the display panel701. This enables focus adjustment and zooming in/out of image. One or both of the lens711and the display panel701are preferably configured to be movable in the optical-axis direction, for example.

The housing702preferably includes a mechanism capable of adjusting the angle of the reflective plate712. The position of the display region706where images are displayed can be changed by changing the angle of the reflective plate712. Thus, the display region706can be placed at the most appropriate position in accordance with the position of the user's eye.

The display device or the display module of one embodiment of the present invention can be used for the display panel701. Thus, the electronic device700can perform display with extremely high resolution.

FIG.16AandFIG.16Billustrate perspective views of an electronic device750that is of a goggle-type.FIG.16Ais a perspective view illustrating the front surface, the top surface, and the left side surface of the electronic device750, andFIG.16Bis a perspective view illustrating the back surface, the bottom surface, and the right side surface of the electronic device750.

The electronic device750includes a pair of display panels751, a housing752, a pair of temples754, a cushion755, a pair of lenses756, and the like. The pair of display panels751is positioned to be seen through the lenses756inside the housing752.

The electronic device750is an electronic device for VR. A user wearing the electronic device750can see an image displayed on the display panel751through the lens756. Furthermore, when the pair of display panels751displays different images, three-dimensional display using parallax can be performed.

An input terminal757and an output terminal758are provided on the back side of the housing752. To the input terminal757, a cable for supplying a video signal from a video output device or the like, power for charging a battery provided in the housing752, or the like can be connected. The output terminal758can function as, for example, an audio output terminal to which earphones, headphones, or the like can be connected. Note that when audio data can be output by wireless communication or sound is output from an external video output device, the audio output terminal is not necessarily provided.

In addition, the housing752preferably includes a mechanism by which the left and right positions of the lens756and the display panel751can be adjusted to the optimal positions in accordance with the position of the user's eye. In addition, the housing752preferably includes a mechanism for adjusting focus by changing the distance between the lens756and the display panel751.

The display device or the display module of one embodiment of the present invention can be used for the display panel751. Thus, the electronic device750can perform display with extremely high resolution. This enables a user to feel high sense of immersion.

The cushion755is a portion in contact with the user's face (forehead, cheek, or the like). The cushion755is in close contact with the user's face, so that light leakage can be prevented, which increases the sense of immersion. A soft material is preferably used for the cushion755so that the cushion755is in close contact with the face of the user wearing the electronic device750. For example, a material such as rubber, silicone rubber, urethane, or sponge can be used. Furthermore, when a sponge or the like whose surface is covered with cloth, leather (natural leather or synthetic leather), or the like is used, a gap is unlikely to be generated between the user's face and the cushion755, whereby light leakage can be suitably prevented. Furthermore, using such a material is preferable because it has a soft texture and the user does not feel cold when wearing the device in a cold season, for example. The member in contact with user's skin, such as the cushion755or the temple754, is preferably detachable because cleaning or replacement can be easily performed.

At least part of this embodiment can be appropriately implemented in combination with the other embodiments described in this specification.

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