Photoelectric conversion element, display device, electronic device, and method for manufacturing photoelectric conversion element

A photoelectric conversion element includes a first conductive layer over a substrate; a first insulating layer covering the first conductive layer; a first semiconductor layer over the first insulating layer; a second conductive layer formed over the first semiconductor layer; an impurity semiconductor layer over the second semiconductor layer; a second conductive layer over the impurity semiconductor layer; a second insulating layer covering the first semiconductor layer and the second conductive layer; and a light-transmitting third conductive layer over the second insulating layer. A first opening and a second opening are formed in the second insulating layer. In the first opening, the first semiconductor layer is connected to the third conductive layer. In the second opening, the first conductive layer is connected to the third conductive layer. In the first opening, a light-receiving portion surrounded by an electrode formed of the second conductive layer is provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to photoelectric conversion elements, display devices including the photoelectric conversion elements, and electronic devices. Further, the present invention relates to manufacturing methods thereof.

2. Description of the Related Art

In recent years, techniques relating to flat panel displays (in particular, liquid crystal display devices and light-emitting display devices (including EL display devices)) have been remarkably progressed. In the case of a flat panel display, providing a photoelectric conversion element over a substrate over which a pixel transistor is provided has been examined (for example, see Patent Document 1).

REFERENCE

SUMMARY OF THE INVENTION

Expected to be placed in a variety of locations in the future, flat panel displays need to have durability under severe usage environment, for example, in the case of being used outdoors. Further, it is anticipated that non-touchscreen display devices would also include photoelectric conversion elements as sensors that determine usage environment. Note that even such a photoelectric conversion element is provided over a substrate different from a substrate over which a pixel transistor of a display device is provided, in many cases. Even in the case where a photoelectric conversion element is provided over the same substrate as a pixel transistor, the photoelectric conversion element is manufactured through a process different from that of the pixel transistor in many cases.

In view of the above, an object of one embodiment of the present invention is to provide a photoelectric conversion element which has high durability and is provided over the same substrate as a pixel transistor. Another object of one embodiment of the present invention is to provide a method for manufacturing a photoelectric conversion element which can be manufactured through a manufacturing process not significantly different from that of a pixel transistor.

One embodiment of the present invention is a Schottky diode-type photoelectric conversion element which has a structure similar to that of a transistor including a semiconductor layer including a channel formation region and which is provided with a light-receiving portion where a light-transmitting conductive layer is formed over an exposed portion of the semiconductor layer.

Another embodiment of the present invention is a display device provided with, over one substrate, a transistor including a semiconductor layer including a channel formation region and a photoelectric conversion element which has a structure similar to that of the transistor and is provided with a light-receiving portion where a light-transmitting conductive layer is formed over an exposed portion of the semiconductor layer.

Another embodiment of the present invention is an electronic device including the display device for a display portion.

Another embodiment of the present invention is a method for manufacturing the photoelectric conversion element. The method for manufacturing the photoelectric conversion element is similar to a method for manufacturing a pixel transistor of a display device. A difference between these methods is that an opening is formed in a portion of an insulating layer, which overlaps with a semiconductor layer, to expose the semiconductor layer.

By applying one embodiment of the present invention to a photoelectric conversion element, the photoelectric conversion element can have high durability and can be provided over the same substrate as a transistor.

Further, by applying one embodiment of the present invention to a display device, the display device can be provided with, over one substrate, a pixel transistor and a photoelectric conversion element having high durability.

Further, by applying one embodiment of the present invention to an electronic device, the electronic device can be provided with, over one substrate, a pixel transistor of a display portion and a photoelectric conversion element having high durability.

By applying one embodiment of the present invention to a method for manufacturing a photoelectric conversion element, a photoelectric conversion element can be manufactured over the same substrate as a pixel transistor, through a manufacturing process which is not significantly different from that of the pixel transistor.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the present invention is not limited to the following description and it is easily understood by those skilled in the art that the mode and details can be variously changed without departing from the scope and spirit of the present invention. Thus, the present invention should not be construed as being limited to the description of the embodiments below.

Note that in the following description, regions indicated by the same hatching pattern in drawings are basically the same layer.

In this embodiment, a photoelectric conversion element according to one embodiment of the present invention will be described.

FIGS. 1A and 1Billustrate a photoelectric conversion element according to one embodiment of the present invention. The photoelectric conversion element inFIGS. 1A and 1Bincludes a first conductive layer102provided over a substrate100; a first insulating layer104provided so as to cover the first conductive layer102; a first semiconductor layer106provided over the first insulating layer104; second conductive layer108provided over the first semiconductor layer106and apart from each other; an impurity semiconductor layer110provided over the second semiconductor layer108; a second conductive layer112provided over the impurity semiconductor layer110; a second insulating layer114provided so as to cover at least the first semiconductor layer106and the second conductive layer112; and a third conductive layer116provided over the second insulating layer114. The second insulating layer114has a first opening150and a second opening152. The first opening150overlaps with a first wiring154formed of the first conductive layer102, and the second opening152overlaps with a second wiring156formed of the first conductive layer102. In the first opening150, the first semiconductor layer106and the third conductive layer116are connected to each other, and in the second opening152, the first conductive layer102and the third conductive layer116are connected to each other. The third conductive layer116including a light-transmitting conductive material is formed in the first opening150, whereby a light-receiving portion is provided in the first opening150. The light-receiving portion is surrounded rectangularly by the second conductive layer112. Note that a third wiring158is formed of the second conductive layer112.

Note that in the photoelectric conversion element seen in the top view ofFIG. 1A, the light-receiving portion is surrounded rectangularly by a portion formed of the second conductive layer112; however, one embodiment of the present invention is not limited thereto. The light-receiving portion may be surrounded circularly by the portion formed of the second conductive layer112.

The first semiconductor layer106includes a semiconductor material having high carrier mobility (such as crystalline semiconductor typified by a microcrystalline semiconductor), and the second semiconductor layer108include a semiconductor material having carrier mobility lower than that the semiconductor material of the first semiconductor layer106.

Here, operation of the photoelectric conversion element inFIGS. 1A and 1Bwill be described. In the photoelectric conversion element inFIGS. 1A and 1B, carriers (holes and electrons) are generated due to a photoelectric effect caused when the light-receiving portion formed in the first opening150receives light. Here, to the impurity semiconductor layer110, an impurity element imparting n-type conductivity is added as an impurity element imparting one conductivity type.

The light-receiving portion formed in the first opening150is provided with a bottom electrode connected to the first wiring154formed of the first conductive layer102and a top electrode overlapping with the lower electrode and formed of the third conductive layer116. Here, the potential Vbottomof the bottom electrode may be either higher than or lower than the potential Vtopof the top electrode; however, Vbottomis preferably higher than Vtop. This is because dry etching is preferably performed to form the first opening150, and the first semiconductor layer106in the light-receiving portion is damaged by plasma due to the thy etching, which inhibits the flow of the carriers. Note that this does not apply to the case where plasma damage is not caused in formation of the first opening150.

Further, the potential of the top electrode Vtopis preferably lower than the potential Vlineof the third wiring158formed of the second conductive layer112. This is to make electrons generated due to the photoelectric effect flow toward the third wiring158side.

In the case where the above relation is satisfied and Vbottomis higher than Vtop, electrons generated when light is received are attracted to the bottom electrode, whereas holes are attracted to the top electrode. Electrons excessively generated in the vicinity of the bottom electrode flow toward the third wiring158side, and holes attracted to the top electrode are recombined with some of electrons generated in the vicinity of the top electrode when light is received.

In the case where the above relation is satisfied and Vtopis higher than Vbottom, electrons generated when light is received are attracted to the top electrode, whereas holes are attracted to the bottom electrode. Holes excessively generated in the vicinity of the bottom electrode are recombined with some of electrons generated in the vicinity of the top electrode, and electrons in the vicinity of the top electrode flow toward the third wiring158side.

A photocurrent may be determined by detecting a change in the potential of the third wiring158. For example, one electrode of a capacitor held at a certain potential may be electrically connected to the third wiring158to detect a change in the potential of the capacitor.

In a photoelectric conversion element according to one embodiment of the present invention, a light-receiving portion is not necessarily surrounded by the second conductive layer112when seen in the top view.FIGS. 2A and 2B,FIGS. 3A and 3B, andFIGS. 4A and 4Billustrate examples in each of which a light-receiving portion is not surrounded by the second conductive layer112.

FIGS. 2A and 2Billustrate a photoelectric conversion element according to one embodiment of the present invention. The photoelectric conversion element in FIGS.2A and2B has, over the substrate100, a layered structure similar to that of the photoelectric conversion element inFIGS. 1A and 1B. The second insulating layer114has a first opening160and a second opening162. The first opening160overlaps with a first wiring164formed of the first conductive layer102, and the second opening162overlaps with a second wiring166formed of the first conductive layer102. In the first opening160, the first semiconductor layer106and the third conductive layer116are connected to each other, and in the second opening162, the first conductive layer102and the third conductive layer116are connected to each other. The third conductive layer116including a light-transmitting conductive material is formed in the first opening160, whereby a light-receiving portion is provided in the first opening160. Note that a third wiring168and a fourth wiring169are formed of the second conductive layer112, and the light-receiving portion is sandwiched between the third wiring168and the fourth wiring169.

FIGS. 3A and 3Billustrate a photoelectric conversion element according to one embodiment of the present invention. The photoelectric conversion element inFIGS. 3A and 3Bhas, over the substrate100, a layered structure similar to that of the photoelectric conversion element inFIGS. 1A and 1B. The second insulating layer114has a first opening170and a second opening172. The first opening170overlaps with a first wiring174formed of the first conductive layer102, and the second opening172overlaps with a second wiring176formed of the first conductive layer102. In the first opening170, the first semiconductor layer106and the third conductive layer116are connected to each other, and in the second opening172, the first conductive layer102and the third conductive layer116are connected to each other. The third conductive layer116including a light-transmitting conductive material is formed in the first opening170, whereby a light-receiving portion is provided in the first opening170. Note that a third wiring178is formed of the second conductive layer112. It can be said that the photoelectric conversion element inFIGS. 3A and 3Bis not provided with the fourth wiring169inFIGS. 2A and 2B. Such a structure is preferable because the area of the light-receiving portion can be large.

FIGS. 4A and 4Billustrate a photoelectric conversion element according to one embodiment of the present invention. The photoelectric conversion element inFIGS. 4A and 4Bhas, over the substrate100, a layered structure similar to that of the photoelectric conversion element inFIGS. 1A and 1B. The second insulating layer114has a first opening180and a second opening182. The first opening180overlaps with a first wiring184formed of the first conductive layer102, and the second opening182overlaps with a second wiring186formed of the first conductive layer102. In the first opening180, the first semiconductor layer106and the third conductive layer116are connected to each other, and in the second opening182, the first conductive layer102and the third conductive layer116are connected to each other. The third conductive layer116including a light-transmitting conductive material is formed in the first opening180, whereby a light-receiving portion is provided in the first opening180. Note that a third wiring188is formed of the second conductive layer112. The first opening180is provided so that it partly surrounds an electrode formed of the second conductive layer112from three directions. Such a structure is preferable because the area of a surface of the electrode to which a photocurrent flows can be large.

In a photoelectric conversion element according to one embodiment of the present invention, one of wirings formed of the first conductive layer102in the above embodiment may be formed of the second conductive layer112.

FIGS. 5A and 5Billustrate a photoelectric conversion element according to one embodiment of the present invention. The photoelectric conversion element inFIGS. 5A and 5Bhas, over the substrate100, a layered structure similar to that of the photoelectric conversion element inFIGS. 1A and 1B. The second insulating layer114has a first opening190and a second opening192. The first opening190overlaps with a first wiring194formed of the first conductive layer102, and the second opening192overlaps with a second wiring196formed of the second conductive layer112. In the first opening190, the first semiconductor layer106and the third conductive layer116are connected to each other, and in the second opening192, the second conductive layer112and the third conductive layer116are connected to each other. The third conductive layer116including a light-transmitting conductive material is formed in the first opening190, whereby a light-receiving portion is provided in the first opening190. The light-receiving portion is surrounded rectangularly by the second conductive layer112. Note that a third wiring198is formed of the second conductive layer112.

The structure inFIGS. 5A and 5Bis effective particularly in the case where the conductivity of a conductive material of the first conductive layer102is lower than that of a conductive material of the second conductive layer112.

Note that inFIGS. 1A and 1BtoFIGS. 5A and 5B, wirings given different potentials are provided; however, one embodiment of the present invention is not limited thereto.

For example, inFIGS. 1A and 1B, the second wiring156and the first wiring154are not necessarily independent wirings. A register200may be provided between the first wiring154and the second wiring156so that a potential difference is generated between the first wiring154and the second wiring156(FIG. 6A).

Here, when the resistance of the resistor200is R200and the current flowing to the resistor200is I200, between the potential VHof a high-potential-side wiring (the first wiring154) and the potential VLof a low-potential-side wiring (the second wiring156), VH−VL=I200R200is satisfied. Thus, by setting the resistance R200of the resistor200and the current I200flowing to the resistor200to be constant, a potential difference between the high-potential-side wiring (the first wiring154) and the low-potential-side wiring (the second wiring156) can be constant.

Alternatively, inFIGS. 1A and 1B, the first wiring154and the second wiring156may have a potential difference by providing a resistor202therebetween, instead of being independent form each other (FIG. 6B).

Here, when the resistance of the resistor202is R202and the current flowing to the resistor200is I202, between the potential VHof a high-potential-side wiring (the second wiring156) and the potential VLof a low-potential-side wiring (the first wiring154), VH−VL=I202R202is satisfied. Thus, by setting the resistance R202of the resistor202and the current I202flowing to the resistor202to be constant, a potential difference between the high-potential-side wiring (the second wiring156) and the low-potential-side wiring (the first wiring154) can be constant.

For the resistor200or the resistor202, a highly resistant wiring210formed of the third conductive layer116can be used in the case where the conductivity of a material of the third conductive layer116is low, for example (FIGS. 7A and 7B). Here, the first wiring154is connected to the highly resistant wiring210in a third opening212, and the second wiring156is connected to the highly resistant wiring210in a fourth opening214.

When the conductivity of a material of the third conductive layer116is not sufficiently low, a highly resistant wiring218formed of the third conductive layer116is preferably provided so that the length of the highly resistant wiring218is long (FIG. 8A). When the length of the highly resistant wiring218is long, a sufficient potential difference can be generated between the first wiring154and the second wiring156, which is preferable.

For the resistor200or the resistor202, for example, a transistor can be used (FIG. 8B).FIG. 8Billustrates a structure where a transistor216is provided between the first wiring154and the second wiring156. As illustrated inFIG. 8B, when the transistor216is used as a resistor, the channel length is preferably large and the channel width is preferably small. Further, the transistor and the photoelectric conversion element preferably have the same layered structures. Here, “the same layered structures” between a plurality of elements means that the shapes thereof seen in top views or cross-sectional views may be different from each other, but the stacking order of layers included in the layered structures is the same and the thicknesses of corresponding layers in the layered structures are equivalent to each other. That is to say, the plurality of elements has structures supposed to be manufactured through the same process.

Note that a photoelectric conversion element according to one embodiment of the present invention is not limited to the structures illustrated inFIGS. 1A and 1BtoFIGS. 8A and 8B.FIGS. 9A to 9Care cross-sectional views of photoelectric conversion elements according to other embodiments of the present invention.

For example, as illustrated inFIG. 9A, the first semiconductor layer and the third conductive layer may be in contact with each other without providing the second semiconductor layer.

Alternatively, as illustrated inFIG. 9B, the second semiconductor layer and the third conductive layer may be in contact with each other.

Note that the second semiconductor layer described above with reference to the drawing includes a microcrystalline semiconductor region that extend from the first semiconductor layer; however, one embodiment of the present invention is not limited thereto. A structure illustrated inFIG. 9Cmay be employed in which the microcrystalline semiconductor region does not extend in a conical or pyramidal shape from the first semiconductor layer, and the second semiconductor layer and the third conductive layer are in contact with each other.

Note that in the case where the photoelectric conversion element functions as an infrared sensor and the structure inFIG. 9Bor9C is employed, visible light is preferably removed from light incident on a light-receiving portion. This is because in the structures ofFIGS. 9B and 9C, a portion overlapping with the light-receiving portion includes an amorphous semiconductor which has high sensitivity to visible light.

Note that by removing visible light from light incident on the light-receiving portion, the photoelectric conversion element can be used as an infrared sensor. Here, to remove visible light from light incident on the light-receiving portion, a filter that transmits light within an infrared range is needed. Thus, for example, all color filters of red, blue, and green may be provided so as to overlap with the light-receiving portion. Only in the case of outdoor application, it is preferable to suppress photodegradation by using a filter that transmits only light with a wavelength of approximately 950 nm because light with a wavelength of approximately 950 nm of solar spectrum at ground level is weak.

In this embodiment, the photoelectric conversion element according to one embodiment of the present invention is described above. With the structure described in this embodiment, the photoelectric conversion element can be obtained. Particularly in the case where a crystalline semiconductor (preferably, microcrystalline semiconductor) layer is used as the first semiconductor layer, even when the photoelectric conversion element is exposed to light with high intensity, photodegradation can be suppressed.

The photoelectric conversion element described in Embodiment 1 can be manufactured over the same substrate as a pixel of a display device through the same process as a pixel transistor. That is because the photoelectric conversion element described in Embodiment 1 has a shape obtained by modifying the shape of the pixel transistor of the display device.

FIG. 10Aillustrates a pixel transistor which can be formed over the same substrate as the photoelectric conversion element inFIG. 1B. The pixel transistor inFIG. 10Ais different from the photoelectric conversion element inFIG. 1Bin that the pixel transistor inFIG. 10Adoes not have a light-receiving portion where the first semiconductor layer and the third conductive layer are in contact with each other, and a pixel electrode formed of the third conductive layer is provided in contact with one of a source and a drain of the pixel transistor.

FIG. 10Billustrates a pixel transistor which can be formed over the same substrate as the photoelectric conversion element inFIG. 9A. The pixel transistor inFIG. 10Bis different from the photoelectric conversion element inFIG. 9Ain that the pixel transistor inFIG. 10Bdoes not have a light-receiving portion where the first semiconductor layer and the third conductive layer are in contact with each other, and a pixel electrode formed of the third conductive layer is provided in contact with one of a source and a drain of the pixel transistor.

FIG. 10Cillustrates a pixel transistor which can be formed over the same substrate as the photoelectric conversion element inFIG. 9B. The pixel transistor inFIG. 10Cis different from the photoelectric conversion element inFIG. 9Bin that the pixel transistor inFIG. 10Cdoes not have a light-receiving portion where the second semiconductor layer and the third conductive layer are in contact with each other, and a pixel electrode formed of the third conductive layer is provided in contact with one of a source and a drain of the pixel transistor.

FIG. 10Dillustrates a pixel transistor which can be formed over the same substrate as the photoelectric conversion element inFIG. 9C. The pixel transistor inFIG. 10Dis different from the photoelectric conversion element inFIG. 9Cin that the pixel transistor inFIG. 10Ddoes not have a light-receiving portion where the second semiconductor layer and the third conductive layer are in contact with each other, and a pixel electrode formed of the third conductive layer is provided in contact with one of a source and a drain of the pixel transistor.

Note that in the transistors illustrated inFIGS. 10A to 10D, the third conductive layer is provided so as to overlap with a portion of the first semiconductor layer, which is to be a channel formation region, and the second insulating layer is provided between the first semiconductor layer and the third conductive layer. That is, a second gate electrode is formed of the third conductive layer and the second gate electrode functions as a back gate. In particular, the transistors having back gates inFIGS. 10A and 10Bare preferable because the field effect mobility and the on-state current can be significantly increased as compared to the case where a back gate is not provided. Note that one embodiment of the present invention is not limited thereto, and the second gate electrode functioning as a back gate is not necessarily provided.

Note that the transistors illustrated inFIGS. 10A,10C, and10D each includes the second semiconductor layer which enables reduction in off-state current.

Therefore, the transistor inFIG. 10Acan have a higher on/off ratio and more excellent switching characteristics than any other transistor. Thus, when the transistor inFIG. 10Ais applied to a pixel transistor of a display device, the display device can have a higher contrast ratio. Based on the above, it can be said that the structure inFIG. 10Ais the most preferred embodiment among the structures inFIGS. 10A to 10D.

In this embodiment, a method for manufacturing the photoelectric conversion element of Embodiment 1 and the pixel transistor of the display device which are formed over the same substrate through the same process will be described. Note that in the following description, the same layer as the layer described in Embodiment 1, or the like, is denoted by a common reference numeral.

First, the first conductive layer102is formed over the substrate100, and the first insulating layer104is formed so as to cover the first conductive layer104(FIG. 11A).

The substrate100is an insulating substrate. A glass substrate or a quartz substrate can be used as the substrate100, for example. A glass substrate is used in this embodiment. In the case where the substrate100is mother glass, the substrate may have any of the sizes from the first generation (e.g., 320 mm×400 mm) to the tenth generation (e.g., 2950 mm×3400 mm); however, the substrate is not limited thereto.

The first conductive layer102may be formed in such a manner that a conductive film (e.g., a metal film or a semiconductor film to which an impurity element imparting one conductivity type is added) is formed, a resist mask is formed over the conductive film, and etching is performed using the resist mask. Alternatively, an ink-jet method may be used. Note that the conductive film to be the first conductive layer102may be formed to have either a single-layer structure or a layered structure including a plurality of layers. Here, the conductive film is formed to have a three-layer structure in which an Al layer is sandwiched between Ti layers, for example. Note that the first conductive layer102forms at least a scan line and a gate electrode.

The first insulating layer104may be formed using an insulating material (e.g., silicon nitride, silicon nitride oxide, silicon oxynitride, or silicon oxide). Note that the first insulating layer104may be formed to have either a single-layer structure or a layered structure including a plurality of layers. Here, the first insulating layer104is formed to have two-layer structure in which a silicon oxynitride layer is stacked over a silicon nitride layer, for example. Note that the first insulating layer104forms at least a gate insulating layer.

“Silicon nitride oxide” contains oxygen and nitrogen so that the nitrogen content is higher than the oxygen content, and in the case where measurements are performed using Rutherford backscattering spectrometry (RBS) and hydrogen forward scattering (HFS), preferably contains oxygen, nitrogen, silicon, and hydrogen at 5 at. % to 30 at. %, 20 at. % to 55 at. %, 25 at. % to 35 at. %, and 10 at. % to 30 at. %, respectively.

“Silicon oxynitride” contains oxygen and nitrogen so that the oxygen content is higher than the nitrogen content, and in the case where measurements are performed using RBS and HFS, preferably contains oxygen, nitrogen, silicon, and hydrogen at 50 at. % to 70 at. %, 0.5 at. % to 15 at. %, 25 at. % to 35 at. %, and 0.1 at. % to 10 at. %, respectively.

Note that percentages of nitrogen, oxygen, silicon, and hydrogen fall within the ranges given above, where the total number of atoms contained in the silicon oxynitride or the silicon nitride oxide is defined as 100 at. %.

Next, over the first insulating layer104, a first semiconductor film300to be the first semiconductor layer106, a second semiconductor film302to be the second semiconductor layer108, and an impurity semiconductor film304to be the impurity semiconductor layer110are formed (FIG. 11B).

The first semiconductor film300is preferably formed using a semiconductor material having high carrier mobility. As the semiconductor material having high carrier mobility, a crystalline semiconductor can be given, for example. As the crystalline semiconductor, a microcrystalline semiconductor can be given, for example. Here, a microcrystalline semiconductor is a semiconductor having an intermediate structure between an amorphous structure and a crystalline structure (including a single crystal structure and a polycrystalline structure). A microcrystalline semiconductor is a semiconductor having a third state that is stable in terms of free energy and is a crystalline semiconductor having short-range order and lattice distortion, in which columnar or needle-like crystals having a grain size of 2 nm or more and 200 nm or less, preferably 10 nm or more and 80 nm or less, more preferably 20 nm or more and 50 nm or less have grown in a direction of the normal to the substrate surface. Thus, there is a case where crystal grain boundaries are formed at the interface of the columnar or needle-like crystal grains. Note that the diameter of the grain here means the maximum diameter of the crystal grain in a plane parallel to the substrate surface. Further, the crystal grain includes an amorphous semiconductor region and a crystallite which is a minute crystal that can be regarded as a single crystal. The crystal grain may include a twin crystal.

Microcrystalline silicon which is one of microcrystalline semiconductors has a peak of Raman spectrum which is shifted to a lower wave number than 520 cm−1that represents single crystal silicon. That is, the peak of the Raman spectrum of the microcrystalline silicon exists between 520 cm−1which represents single crystal silicon and 480 cm−1which represents amorphous silicon. Further, the microcrystalline silicon contains hydrogen or halogen of at least 1 at. % or more in order to terminate a dangling bond. Furthermore, the microcrystalline silicon contains a rare gas element such as He, Ar, Kr, or Ne to further promote lattice distortion, so that stability is increased and a favorable microcrystalline semiconductor can be obtained.

Moreover, when the concentration of oxygen and nitrogen contained in the first semiconductor film300(measured by secondary ion mass spectrometry) is less than 1×1018cm−3, the crystallinity of the first semiconductor film300can be increased.

The second semiconductor film302is preferably formed using a semiconductor material having low carrier mobility in order to serve as a buffer layer, and preferably includes an amorphous semiconductor and a minute semiconductor crystal grain. In addition, the second semiconductor film302has lower energy at the Urbach edge, which is measured by a constant photocurrent (CPM) method or photoluminescence spectrometry, and a smaller quantity of absorption spectra of defects, as compared to a conventional amorphous semiconductor film. That is, as compared to the conventional amorphous semiconductor film, such a semiconductor film is a well-ordered semiconductor film which has few defects and a steep tail slope of a level at a band edge (a mobility edge) in the valence band. Note that such a semiconductor film is referred to as a “film containing an amorphous semiconductor” or a “layer containing an amorphous semiconductor” in this specification.

The second semiconductor film302is preferably “a film containing an amorphous semiconductor”, “a film containing an amorphous semiconductor” which contains halogen, or “a film containing an amorphous semiconductor” which contains nitrogen, most preferably “a film containing an amorphous semiconductor” which contains an NH group or an NH2group. Note that one embodiment of the present invention is not limited thereto.

A region (also referred to as an interface region) of the second semiconductor film302which is closer to the first semiconductor film300includes microcrystalline semiconductor regions which extend in conical or pyramidal shapes from the first semiconductor film300and a “film containing an amorphous semiconductor” which is similar to the second semiconductor film302.

When the second semiconductor film302is formed using a “film containing an amorphous semiconductor”, a “film containing an amorphous semiconductor” which contains halogen, a “film containing an amorphous semiconductor” which contains nitrogen, or a “film containing an amorphous semiconductor” which contains an NH group or an NH2group, for example, the off-state current of a transistor can be reduced. Further, since the interface region has the conical or pyramidal microcrystalline semiconductor regions, resistance in the vertical direction (the film thickness direction), that is, resistance between the second semiconductor film302and a source region or a drain region formed of the impurity semiconductor film304, can be lowered. Thus, the on-state current of the transistor can be increased. That is to say, as compared to the case of using the conventional amorphous semiconductor, the off-state current can be sufficiently reduced and reduction in on-state current can be suppressed; thus, switching characteristics of the transistor can be improved.

Note that as the first semiconductor layer106is thinner in the completed transistor, the on-state current is decreased. As the first semiconductor layer106is thicker in the completed transistor, the off-state current is increased because a contact area between the first semiconductor layer106and the second conductive layer112is increased. Therefore, to increase the on/off ratio, it is preferable to form the first semiconductor film300to be the first semiconductor layer106to have a large thickness and further to perform insulation treatment to make side surfaces of a thin film layered body306including the first semiconductor layer106have an insulating property.

A large portion of the above microcrystalline semiconductor region preferably includes a crystal grain having a conical or pyramidal shape whose top gets narrower from the first insulating layer104toward the second semiconductor film302. Alternatively, the large portion of the above microcrystalline semiconductor region may include a crystal grain having a conical or pyramidal shape whose top gets wider from the first insulating layer104toward the second semiconductor film302.

When the microcrystalline semiconductor region includes a crystal grain having a conical or pyramidal shape whose top gets narrower from the first insulating layer104toward the second semiconductor film302in the above interface region, the proportion of the microcrystalline semiconductor region on the first semiconductor film300side is higher than that on the second semiconductor film302side. The microcrystalline semiconductor region grows from a surface of the first semiconductor film300in the film thickness direction. When the flow rate of hydrogen with respect to that of silane in a source gas is low (that is, the dilution ratio is low) or the concentration of a source gas containing nitrogen is high, crystal growth of the microcrystalline semiconductor region is suppressed, and thus, a crystal grain comes to have a conical or pyramidal shape, and a large portion of the deposited semiconductor is amorphous.

The above interface region preferably contains nitrogen, in particular, an NH group or an NH2group. This is because defects are reduced and carriers flow easily when nitrogen, in particular, an NH group or an NH2group is bonded with dangling bonds of silicon atoms at an interface of crystal included in the microcrystalline semiconductor region or at an interface between the microcrystalline semiconductor region and the amorphous semiconductor region. Accordingly, by setting the concentration of nitrogen, preferably, an NH group or an NH2group to 1×1020cm−3to 1×1021cm−3, the dangling bonds of silicon atoms can be easily cross-linked with nitrogen, preferably an NH group or an NH2group, so that carriers can flow easily. As a result, a bond which promotes the carrier transfer is formed at a crystal grain boundary or a defect, whereby the carrier mobility of the interface region is increased. Therefore, the field effect mobility of the transistor is improved.

Further, when the concentration of oxygen in the interface region is reduced, defects at the interface between the microcrystalline semiconductor region and the amorphous semiconductor region or the interface between crystal grains can be reduced, so that bonds which inhibit carrier transfer can be reduced.

Here, when the distance from the interface of the first insulating layer104and the first semiconductor film300to the edge of a step portion of the second semiconductor film302is greater than or equal to 3 nm and less than or equal to 80 nm, preferably greater than or equal to 5 nm and less than or equal to 30 nm, the off-state current of the transistor can be effectively reduced.

The impurity semiconductor film304is formed using a semiconductor to which an impurity element imparting one conductivity type is added. When the transistor is an n-channel transistor, phosphorus (P) or arsenic (As) is used as the impurity element imparting one conductivity type, for example. Meanwhile, when the transistor is a p-channel transistor, for example, boron (B) is used as the impurity element imparting one conductivity type, for example. Note that it is preferable that the transistor be an n-channel transistor. Therefore, for example, silicon to which phosphorus (P) is added is used here. The impurity semiconductor film304may be formed using an amorphous semiconductor or a crystalline semiconductor such as a microcrystalline semiconductor.

When the impurity semiconductor film304is formed using an amorphous semiconductor, the flow rate of a dilution gas is greater than or equal to that of a deposition gas and less than or equal to 10 times that of the deposition gas, preferably greater than or equal to that of the deposition gas and less than or equal to 5 times that of the deposition gas. On the other hand, when the impurity semiconductor film304is formed using a crystalline semiconductor, the flow rate of the dilution gas is greater than or equal to 10 times that of a deposition gas and less than or equal to 2000 times that of the deposition gas, preferably greater than or equal to 50 times that of the deposition gas and less than or equal to 200 times that of the deposition gas.

Next, a resist mask is formed over the impurity semiconductor film304, and the first semiconductor film300, the second semiconductor film302, and the impurity semiconductor film304are etched using the resist mask, so that the thin film layered body306is formed. Then, a conductive film308is formed over the first insulating layer104and the thin film layered body306(FIG. 11C).

The conductive film308may be formed using a conductive material (e.g., metal or a semiconductor to which an impurity element imparting one conductivity type is added) in a manner similar to that of the first conductive layer102. Note that the conductive film308may have a single-layer structure or a layered structure including plural layers. Here, a three-layer structure in which an Al layer is sandwiched between Ti layers is employed, for example.

Note that it is preferable to perform insulation treatment to make the side surfaces of the thin film layered body306have an insulating property. That is because the off-state current increases when the first semiconductor layer106and the second conductive layer112of the completed transistor are in contact with each other. Here, for the insulation, the side surfaces of the thin film layered body306may be exposed to nitrogen plasma. Alternatively, the insulation may be performed as follows: an insulating film is formed while the side surfaces of the thin film layered body306are exposed, and the insulating film is etched in the direction perpendicular to a surface of the substrate100by an etching method with high anisotropy, so that side wall insulating layers are formed in contact with the side surfaces of the thin film layered body306.

Then, a resist mask is formed over the conductive film308, and the conductive film308is etched using the resist mask, whereby the second conductive layer112is formed. Further, in the above step, an upper portion of the thin film layered body306may also be etched so that the first semiconductor layer106, the second semiconductor layer108, and the impurity semiconductor layer110are formed. Alternatively, after removal of the resist mask, etching may be performed using the second conductive layer112as a mask so that the first semiconductor layer106, the second semiconductor layer108, and the impurity semiconductor layer110are formed. After that, an insulating film310is formed so as to cover these layers (FIG. 11D). Note that the second conductive layer112forms at least a signal line, and source and drain electrodes.

Note that in the following description, a method for manufacturing a photoelectric conversion element and a method for manufacturing a transistor will be described with reference to respective drawings. That is to say,FIGS. 12A to 12Cillustrates a method for manufacturing a photoelectric conversion element, andFIGS. 13A to 13Cillustrates a method for manufacturing a transistor.

Next, a plurality of openings is formed in the insulating film310, whereby the insulating layer114is formed. The plurality of openings is formed in such a manner that a resist mask is formed over the insulating film310and etching is performed using the resist mask. Preferably, the first opening150(FIGS. 1A and 1BandFIG. 12A) is formed first, and then, the second opening152(FIG. 1A) and a pixel opening314(FIG. 13A) are formed. Note that one embodiment of the present invention is not limited thereto, and the first opening150, the second opening152, and the pixel opening314may be formed through different processes. Alternatively, the second opening152may be formed first, and then, the first opening150and the pixel opening314may be formed. Still alternatively, the pixel opening314may be formed first, and then the first opening150and the second opening152may be formed. Further alternatively, the first opening150, the second opening152, and the pixel opening314may be formed through the same etching process. In a later step, in the pixel opening314, a pixel electrode formed of the third conductive layer116is formed in contact with one of a source and a drain of a pixel transistor (FIG. 12AandFIG. 13A).

Then, a conductive film312to be the third conductive layer116is formed over the second insulating layer114in which the first opening150, the second opening152, and the pixel opening314are formed (FIG. 12BandFIG. 13B). Since the third conductive layer116forms a pixel electrode connected to the pixel transistor and needs to transmit light in a light-receiving portion, the conductive film312is formed using a light-transmitting material.

The conductive film312can be formed using a conductive composition including a conductive macromolecule (also referred to as a conductive polymer) having a light-transmitting property. It is preferable that the conductive film312formed using the conductive composition have a sheet resistance of less than or equal to 10000 Ω/square and a light transmittance of greater than or equal to 70% at a wavelength of 550 nm. Further, the resistivity of the conductive macromolecule included in the conductive composition is preferably less than or equal to 0.1 Ω·cm.

As the conductive macromolecule, a so-called π-electron conjugated conductive polymer can be used. For example, polyaniline or a derivative thereof, polypyrrole or a derivative thereof, polythiophene or a derivative thereof, and a copolymer of two or more of aniline, pyrrole, and thiophene or a derivative thereof can be given.

The conductive film312can be formed using, for example, an indium oxide containing a tungsten oxide, an indium zinc oxide containing a tungsten oxide, an indium oxide containing a titanium oxide, an indium tin oxide containing a titanium oxide, an indium tin oxide (hereinafter referred to as ITO), an indium zinc oxide, an indium tin oxide to which a silicon oxide is added, or the like.

The conductive film312may be formed by processing a film which is formed using any of the above materials by a photolithography method.

Then, a resist mask is formed over the conductive film312, and the conductive film312is etched using the resist mask, whereby the third conductive layer116is formed (FIG. 12CandFIG. 13C).

In a manner described above, a photoelectric conversion element (FIG. 12C) and a transistor (FIG. 13C) can be provided over the substrate100.

Although not illustrated, an insulating layer formed using an organic resin by a spin coating method or the like may be provided between the second insulating layer114and the third conductive layer116.

As described in Embodiment 2, a photoelectric conversion element according to one embodiment of the present invention can be provided over the same substrate as a pixel transistor of a display device.

In this embodiment, one embodiment of a display device including a photoelectric conversion element according to one embodiment of the present invention will be described with reference to a drawing.

Note that a display element included in a display device according to one embodiment of the present invention is not particularly limited and may be a liquid crystal element, a light-emitting element, an electrophoretic element, or the like. In the case where a display device is a liquid crystal display device provided with a liquid crystal element, it is preferably a field-sequential liquid crystal display device because power consumption can be reduced.

In a display device according to this embodiment, a signal line driver circuit and a scan line driver circuit may be formed over a different substrate (e.g., a semiconductor substrate or an SOI substrate) and then connected to a pixel portion or may be formed over the same substrate as a pixel circuit.

Note that there is no particular limitation on a connecting method in the case of using a different substrate, and a COG method, a wire bonding method, a TAB method, or the like can be used. Further, a connection position is not particularly limited as long as electrical connection is possible. Note also that a controller, a CPU, a memory, and the like may be formed separately and connected to the pixel circuit.

Any of the above can be employed as long as a photoelectric conversion element and a pixel transistor are provided over the same substrate.

FIG. 14is a block diagram of a display device. The display device inFIG. 14includes, over the substrate100, a pixel portion400including a plurality of pixels each provided with a display element, a scan line driver circuit402which selects a pixel, and a signal line driver circuit403which controls input of a video signal to the selected pixel.

The signal line driver circuit403inFIG. 14includes a shift register404and an analog switch405. A clock signal (CLK) and a start pulse signal (SP) are input to the shift register404. When the clock signal (CLK) and the start pulse signal (SP) are input, a timing signal is generated in the shift register404and input to the analog switch405.

Note that a video signal is supplied to the analog switch405. The analog switch405samples the video signal in accordance with the input timing signal and supplies the sampled signal to a signal line of the next stage.

Note that the display device is not limited to the structure illustrated inFIG. 14. That is, the signal line driver circuit is not limited to a structure including only a shift register and an analog switch. In addition to the shift register and the analog switch, another circuit such as a buffer, a level shifter, or a source follower may be provided. Note that the shift register and the analog switch are not necessarily provided. For example, another circuit such as a decoder circuit by which a signal line can be selected may be used instead of the shift register, or a latch or the like may be used instead of the analog switch.

The scan line driver circuit402inFIG. 14includes a shift register406and a buffer407. The scan line driver circuit402may include a level shifter. In the scan line driver circuit402, when the clock signal (CLK) and the start pulse signal (SP) are input to the shift register406, a selection signal is generated. The generated selection signal is buffered and amplified by the buffer407, and the buffered and amplified signal is supplied to a corresponding scan line. Gates of pixel transistors of one line are connected to the scan line. Further, since the pixel transistors of one line need to be turned on at the same time in the operation, the buffer407which can supply a large amount of current is used.

In a full-color display device, when video signals corresponding to R (red), G (green), and B (blue) are sequentially sampled and supplied to corresponding signal lines, the number of terminals for connecting the shift register404and the analog switch405corresponds to approximately ⅓ of the number of terminals for connecting the analog switch405and the signal line of the pixel portion400. Thus, in comparison to the case where the analog switch405and the pixel portion400are formed over different substrates, the number of terminals used for connection to a substrate which is separately provided can be reduced when the analog switch405and the pixel portion400are formed over one substrate. Accordingly, occurrence probability of bad connection can be suppressed and yield can be improved.

Note that although the scan line driver circuit402inFIG. 14includes the shift register406and the buffer407, one embodiment of the present invention is not limited thereto. The scan line driver circuit402may be formed using only the shift register406.

Note that the structures of the signal line driver circuit and the scan line driver circuit are not limited to the structure illustrated inFIG. 14, which are merely one embodiment of the display device.

A photoelectric conversion element according to one embodiment of the present invention may be provided over any portion of the substrate100over which the display device described in this embodiment is provided. For example, the photoelectric conversion element may be provided for every several pixels of some of pixels in the pixel portion400. Alternatively, the photoelectric conversion element may be provided so as to be included in the scan line driver circuit402over the substrate100. Still alternatively, the photoelectric conversion element may be provided so as to be included in the signal line driver circuit403. Further alternatively, the photoelectric conversion element according to one embodiment of the present invention may be provided in a region except for the pixel portion400, the scan line driver circuit402, and the signal line driver circuit403.

Note that in the case where the photoelectric conversion element is provided for every several pixels of some of the pixels in the pixel portion400, the display device can be a touchscreen utilizing the element. In that case, a touchscreen method is preferably a frustrated total internal reflection method (FTIR method) or an infrared block method.

A FTIR method refers to a method in which infrared light and an acrylic panel are used, and when infrared light delivered to side surfaces of the acrylic panel is totally reflected due to the relation between the refractive indices of the acrylic panel and the air and then diffuse reflection occurs instead of the total internal reflection in a contact area of a surface of the acrylic panel, the diffusely-reflected infrared light is detected. On the other hand, an infrared block method refers to a method in which when infrared light delivered from the backlight side is blocked in a contact area of a surface of a display panel, a portion shielded from the infrared light is detected.

Note that the size of the photoelectric conversion element provided over the substrate100may be equivalent to that of the pixel transistor but is preferably larger than that of the pixel transistor.

The photoelectric conversion element described in Embodiment 1 and the display device described in Embodiment 3 can be used in electronic devices. Examples of electronic devices are a television set (also referred to as a television or a television receiver), a monitor of a computer, electronic paper, a camera such as a digital camera or a digital video camera, a digital photo frame, a mobile phone handset (also referred to as a mobile phone or a mobile phone device), a portable game console, a portable information terminal, an audio reproducing device, a large-sized game machine such as a pachinko machine, and the like.

FIG. 15Aillustrates an example of an e-book reader. The e-book reader illustrated inFIG. 15Aincludes a housing500and a housing501. The housing500and the housing501are combined with a hinge504so that the e-book reader can be opened and closed. With such a structure, the e-book reader can be handled like a paper book.

A display portion502and a display portion503are incorporated in the housing500and the housing501, respectively. The display portion502and the display portion503may display one image or different images. In the case where the display portion502and the display portion503display different images, for example, a display portion on the right side (the display portion502inFIG. 15A) can display text and a display portion on the left side (the display portion503inFIG. 15A) can display graphics. The display device described in Embodiment 3 can be applied to the display portions502and503.

InFIG. 15A, the housing500is provided with a power input terminal505, an operation key506, a speaker507, and the like. The operation key506may have, for example, a function of turning pages. Note that a keyboard, a pointing device, and the like may be provided on the surface of the housing, on which the display portion of the housing is provided. Further, an external connection terminal (an earphone terminal, a USB terminal, a terminal that can be connected to various cables such as a USB cable, or the like), a recording medium insertion portion, and the like may be provided on the back surface or the side surface of the housing. Note that the e-book reader inFIG. 15Amay further have a structure with which data can be transmitted and received wirelessly.

FIG. 15Billustrates an example of a digital photo frame. In the digital photo frame illustrated inFIG. 15B, a display portion512is incorporated in a housing511. The display device described in Embodiment 3 can be applied to the display portion512.

Note that the digital photo frame illustrated inFIG. 15Bmay be provided with an operation portion, an external connection terminal (a USB terminal, a terminal that can be connected to various cables such as a USB cable, or the like), a recording medium insertion portion, and the like. Although they may be provided on the surface on which the display portion is provided, it is preferable to provide them on the side surface or the back surface for the design of the digital photo frame. For example, a memory storing data of an image shot by a digital camera is inserted in the recording medium insertion portion of the digital photo frame, whereby the image data can be transferred and displayed on the display portion512. The digital photo frame inFIG. 15Bmay further have a structure with which data can be transmitted and received wirelessly.

FIG. 15Cillustrates an example of a television set. In the television set illustrated inFIG. 15C, a display portion522is incorporated in a housing521. The housing521is supported by a stand523. The display device described in Embodiment 3 can be applied to the display portion522.

The television set illustrated inFIG. 15Ccan be operated by an operation switch of the housing521or a separate remote controller. Channels and volume can be controlled by an operation key of the remote controller so that an image displayed on the display portion522can be controlled. Further, the remote controller may be provided with a display portion for displaying data output from the remote controller.

Note that the television set illustrated inFIG. 15Cis provided with a receiver, a modem, and the like. With the receiver, a general television broadcast can be received. Further, when the television set is connected to a communication network by wired or wireless connection via the modem, one-way (from a transmitter to a receiver) or two-way (between a transmitter and a receiver or between receivers) data communication can be performed.

Note that an electronic device including a photoelectric conversion element according to one embodiment of the present invention is preferably an electronic signboard constantly placed outside. This is because, as described in Embodiment 1, in the case of using a crystalline semiconductor (preferably, microcrystalline semiconductor) layer as a first semiconductor layer of a photoelectric conversion element according to one embodiment of the present invention, photodegradation can be suppressed even when the photoelectric conversion element is exposed to light with high intensity.

FIG. 16Aillustrates an advertisement panel550in a vehicle such as a train. Further, the advertisement panel550in a vehicle may transmit and receive data wirelessly. When a photoelectric conversion element according to one embodiment of the present invention is used for the advertisement panel550in a vehicle, the advertisement panel550in a vehicle can withstand severe usage environment.

FIG. 16Billustrates an electronic signboard560formed using electronic paper. The electronic signboard may transmit and receive data wirelessly. When a photoelectric conversion element according to one embodiment of the present invention is used for the electronic signboard560, the electronic signboard560can withstand severe usage environment. In an electronic signboard such as that inFIG. 16B, light with high intensity is emitted from a backlight; therefore, particularly in the case of such an electronic signboard, it is possible to make good use of merits of the photoelectric conversion element according to one embodiment of the present invention, which enables suppression of photodegradation.

As described above, the photoelectric conversion element described in Embodiment 1 and the display device described in Embodiment 3 can be used in electronic devices.

This application is based on Japanese Patent Application serial no. 2010-139737 filed with the Japan Patent Office on Jun. 18, 2010, the entire contents of which are hereby incorporated by reference.