JPH04299579A - Photoelectric transducer - Google Patents

Abstract

PURPOSE:To obtain a photoelectric transducer wherein photo current and optical response speed are large, and formation is easy. CONSTITUTION:The title element has the following; an insulating layer 16, a photoconductive semiconductor layer 12 in contact with the layer 16, a first electrode 14, a second electrode 14' which electrodes are in contact with the layer 12, and a third electrode 15 in contact with the layer 16. The forbidden bandwidth of the photoconductive layer 12 on the side of the insulating layer 16 is different from the bandwidth on the side of the first electrode 14 and the second electrode 14' side. Layers 13, 13' are ohmic contact layers.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a photoelectric conversion element used in image processing devices such as facsimile machines, digital copying machines, and image readers.

0002

[Prior Art] Conventionally, thin film semiconductors made of non-single crystal silicon (polysilicon, microcrystalline silicon, and amorphous silicon) have been used for photoelectric conversion elements, and these semiconductors are particularly suitable for large-area devices. It is used. There are two types of photoelectric conversion elements using thin film semiconductors: a primary photocurrent type (photodiode type) and a secondary photocurrent type. The primary photocurrent type is a photoelectric conversion element that extracts electrons and holes generated by incident light and performs photoelectric conversion, but has a problem in that the photocurrent is small. On the other hand, a secondary photocurrent type photoelectric conversion element can obtain a larger photocurrent (secondary photocurrent) than the primary photocurrent type photoelectric conversion element.

FIG. 10 is a schematic diagram of a conventional secondary photocurrent type photoelectric conversion element. In FIG. 10, 11 is an insulating substrate such as glass, 12 is a photoconductive semiconductor layer made of CdS/Se or hydrogenated amorphous silicon (hereinafter abbreviated as "a-Si:H"), etc., and 13 and 13' are Impurity layers 14 and 14' for ohmic contact are electrodes. In such a configuration, if a voltage is applied between the electrodes 14 and 14', when light is incident from the substrate 11 side or the electrodes 14 and 14' side, a large secondary photocurrent flows and photoelectric conversion is performed.

Furthermore, a thin film transistor type photoelectric conversion element has been proposed in which an auxiliary electrode is provided in order to stabilize and improve characteristics (photocurrent, dark current, etc.). FIG. 11 is a schematic configuration diagram of a thin film transistor type photoelectric conversion element provided with an auxiliary electrode. In FIG. 11, the same reference numerals as in FIG. 10 indicate the same elements, 15 is a transparent or opaque gate electrode, and 16 is a gate insulating layer made of SiNX or the like. Note that here, 14 is a drain electrode, 14' is a source electrode, and the three electrodes 14, 1
A voltage is applied to each of 4' and 15.

The linearity r of the photocurrent IP and the dark current Id depending on the light amount when light with an illuminance F is incident between the electrodes 14 and 14' of the secondary photocurrent type photoelectric conversion element shown in FIG.
An example of the gate potential VG dependence of (IP ∝Fr) is
They are shown in FIGS. 12 and 13, respectively. Furthermore, the optical response speed (TON: rise time, TO
An example of the dependence of FF (fall time) on gate potential VG is shown in FIG.

For the purpose of improving the characteristics (the magnitude of photocurrent and the photoresponse speed) of such a secondary photocurrent type photoelectric conversion element, the forbidden band width of the photoconductive semiconductor layer 12 is set to the insulating layer 16 side (the electrode 15 side).
It has been devised to reduce the size from the side) toward the ohmic contact layer 13, 13' side (electrode 14, 14' side).

[0007]

[Problem to be Solved by the Invention] In a thin film transistor type photoelectric conversion element, the forbidden band width of the photoconductive semiconductor layer 12 is changed from the electrode 15 side to the ohmic contact layers 13, 13'.
Conventionally, the composition of a-Si:H is modulated by Ge, etc., which has a small forbidden band width, to decrease the size toward the side.
-SiGeX: forms H, for example, forbidden band width 1.7e
It was made to vary from V to 1.5eV, but a-S
Alloys such as iGeX:H tend to cause defects such as dangling bonds, making it difficult to significantly improve the characteristics (photocurrent magnitude and photoresponse speed) of secondary photocurrent type photoelectric conversion elements and to easily produce them. there were.

The present invention solves the problems of conventional thin film transistor type photoelectric conversion elements, and provides a photoelectric conversion element that has a large photocurrent, a fast photoresponse speed, and can be easily formed. The purpose is to

[0009]

[Means for Solving the Problems] According to the present invention, the above object is achieved by providing an insulating layer, a photoconductive semiconductor layer provided in contact with the insulating layer, and a photoconductive semiconductor layer provided in contact with the photoconductive semiconductor layer. A photoelectric conversion element comprising a photoelectric conversion section having at least a first electrode, a second electrode, and a third electrode provided in contact with the insulating layer, wherein the forbidden band width of the photoconductive semiconductor layer is Provided is a photoelectric conversion element, characterized in that: is narrow on the insulating layer side and wide on the first electrode and second electrode sides.

In the present invention, a non-single crystal consisting of at least silicon and hydrogen can be used as the photoconductive semiconductor layer. At least the photoconductive semiconductor layer can be formed by alternately repeating the step of depositing a non-single crystal layer and the step of irradiating the deposited non-single crystal layer with hydrogen plasma.

[0011]

Embodiments Hereinafter, embodiments of the present invention will be described with reference to the drawings.

(Embodiment 1) FIG. 1 is a schematic diagram of the structure of a photoelectric conversion element of the present invention. In the figure, 11 is a substrate, 16 is an insulating layer, 15 is a third electrode (gate electrode), 14 is a first electrode (drain electrode),
14' is the second electrode (source electrode), 13, 13
' is an ohmic contact layer, and 12 is a photoconductive semiconductor layer.

FIG. 2 is an energy band diagram of the photoconductive semiconductor layer of the photoelectric conversion element of this example. The photoconductive semiconductor layer 12 has a small forbidden band width (optical bandgap) on the third electrode (gate electrode) 15 and gate insulating layer 16 side, and has a small forbidden band width (optical band gap) on the side of the first electrode (drain electrode) 14 and the second electrode. (
The forbidden band width is made larger on the side of the source electrode 14'.

FIG. 3 is a conceptual diagram of a special plasma CVD apparatus used for manufacturing the photoelectric conversion element of this example. Figure 3
, 70 is a reaction chamber, 11 is a substrate on the surface of which each functional layer such as a photoconductive semiconductor layer is formed, 71 is an anode electrode having a heating heater (not shown), 72 is a cathode electrode, and 73 is a high frequency of 13.56 MHz. Power supply, 74 is an exhaust pump, 75 is a SiH4 gas introduction pipe, 76 is an H2 gas (including Ar gas) introduction pipe, 77 is a 2.45 GHz microwave source and microwave applicator, V1, V2
are valves that control SiH4 gas and H2 gas, respectively, and are connected to a computer to precisely control opening and closing times.

The photoelectric conversion element of this example was manufactured as follows: (1) A Cr layer with a thickness of 0.1 μm was deposited on a glass substrate 11 (#7059 manufactured by Corning Inc.) by sputtering. ,
The gate electrode 15 was formed by patterning into a predetermined pattern; (2) This substrate 11 was set in a normal plasma CVD apparatus, and after the substrate temperature was raised to 350°C, SiH4 gas,
Introducing NH3 gas and H2 gas at a predetermined mixing ratio,
iNX: H is deposited to form an insulating layer 16 with a thickness of 3000 Å.
(3) Next, the special plasma CVD shown in FIG.
Using the apparatus, the photoconductive semiconductor layer 12 was deposited according to the following procedure; ■ First, the substrate 11 was set, the inside of the chamber 70 was evacuated to a predetermined pressure with the exhaust pump 74, and at the same time the substrate 11 was heated (not shown). The temperature was raised to 340°C with a heater; Next, the timing of introducing SiH4 gas and H2 gas was adjusted to
Control was performed as shown in FIG. That is, the time t1 for depositing the film
and H2 plasma irradiation time t2 (time tA) was repeated. Time t for depositing the film
1, both valves V1 and V2 were in the open state, and SiH4 gas, Ar gas, and H2 gas were introduced into the reaction chamber. SiH4 gas is 10SCCM
, H2 gas was set at 10 SCCM, and the pressure inside the reaction chamber was adjusted to 0.1 Torr with Ar gas. At this time, the deposition rate was about 3 Å/sec. Further, the film thickness deposited at time t1 was set to 10 to 50 Å. During H2 plasma irradiation time t2, H2 plasma was irradiated with the valve V1 closed and the valve V2 opened. Depending on the H2 plasma irradiation time t2, the film quality of the film deposited at time t1 changes, especially the amount of H contained, and at the same time, as time t2 increases, the forbidden band width (optical band gap) changes to 1. 7eV to 1.58
It changed to eV. FIG. 5 shows an example of the dependence of the hydrogen content and the forbidden band width (optical band gap) on time t2. From the above, it has been found that the forbidden band width (optical band gap) of the photoconductive semiconductor layer 12 can be easily controlled by controlling the time t2. In this example, as the deposition progresses, the time t2 is gradually increased from the insulating layer 16 side to the first electrode 14 and second electrode 14' sides, and the forbidden band width (optical band gap) is set to 1. 7eV to 1.58
eV to form a photoconductive semiconductor layer 12 with a thickness of 6000 Å; (4) This substrate was set in a normal plasma CVD apparatus, and a photoconductive semiconductor layer 12 with a thickness of 1500 Å was formed using SiH4 gas, PH3 gas, and H2 gas. An ohmic contact layer was formed; (5) Finally, a film thickness of 8000 Å was formed by sputtering.
1 layer is formed and patterned together with the ohmic contact layer to form ohmic contact layers 13, 13'.
Then, a drain electrode 14 and a source electrode 14' were formed.

The photocurrent and photoresponse speed of the thin film transistor type photoelectric conversion device produced as described above were measured. As a result, by setting the potential VG of the gate electrode 15 to about -5V, the photocurrent can be made equal to the potential VG of the a-Si:H simple layer 0V, and both the photoresponse speeds TON and TOFF are lower than those of the conventional As shown in Table 1, it was significantly improved compared to the example. As a conventional example, a thin film transistor type photoelectric conversion element with a simple photoconductive semiconductor layer and a forbidden band width of 1.7 eV was used:

[0017]

[Table 1] Here, TON is the time when the photocurrent reaches 90% of the saturation value, and TOFF is the time when the photocurrent reaches 10% of the saturation value.

(Example 2) Using the thin film transistor type photoelectric conversion element, thin film transistor, etc. obtained in Example 1, a one-dimensional complete contact type optical sensor (photoelectric conversion element) was fabricated. Figure 6 shows the circuit. However, here, we will discuss the case of a sensor array having nine thin film transistor type photoelectric conversion sections. In the figure, three thin film transistor type photoelectric conversion units E1 to E9 constitute a sensor array, each of which constitutes one block. Capacitors C1 to C9 and switching transistors T1 to T9 are connected to correspond to each of the thin film transistor type photoelectric conversion units E1 to E9. Further, the individual electrodes of the photoelectric conversion units E1 to E9 are connected to one of the common lines 102 to 104 via switching transistors T1 to T9, respectively. Specifically, the first switching transistors T1, T4, T7 of each block are connected to the common line 1
02, the second switching transistors T2, T5, T8 of each block are connected to the common line 103, and the third switching transistors T3, T6, T9 of each block are connected to the common line 104, respectively. Common line 102-1
04 are switching transistors T10 to T1, respectively.
2 to the amplifier 105.

[0019] Switching transistors ST1 to ST
The gate electrode of 9 is the switching transistor T1~
Like the gate electrode of T9, it is commonly connected for each block, and connected to the parallel output terminal of the shift register 109 for each block. Therefore, shift register 10
According to the shift timing 9, the switching transistors ST1 to ST9 are sequentially turned on for each block.

Further, in FIG. 6, common lines 102 to 104
are grounded via capacitors C10 to C12 and grounded via switching transistors CT1 to CT3, respectively. The capacitance of the capacitors C10 to C12 is set to be sufficiently larger than that of the capacitors C1 to C9. Switching transistor CT1~C
Each gate electrode of T3 is connected in common and connected to terminal 108. That is, by applying a high level to the terminal 108, the switching transistors CT1 to
CT3 is simultaneously turned on and the common lines 102 to 104 are grounded. Further, the thin film transistor type photoelectric conversion sections E1 to E9 have gate electrodes G1 to G9.

FIG. 7 is a partial plan view of a one-dimensional complete contact sensor array created based on the circuit diagram shown in FIG. In the same figure, 111 is a common line 102~
104, etc., a matrix-like wiring section, 112, a thin film transistor type photoelectric conversion section, 113, a charge storage section consisting of capacitors C1 to C9, and 114, switching transistors T1 to T9, which are thin film transistors having the same structure as the photoelectric conversion section. Transfer switch used, 115
116 is a lead line that connects the signal output of the transfer switch 114 to the signal processing IC, and 117 is a transfer switch that is made up of switching transistors ST1 to ST9 and uses thin film transistors having the same structure as the photoelectric conversion section. This is a load capacitor for accumulating and reading signal charges transferred by the switch 114.

FIG. 8 is a sectional view taken along line AA' in FIG. As is clear from FIG. 8, the thin film transistor type photoelectric conversion section 1
12, the charge storage section 113, the transfer switch 114, the discharge switch 115, the matrix wiring section 111, the load capacitor 117, etc. are all formed on the substrate 11 with metal (gate electrode 15 in the photoelectric conversion section), insulating layer ( In the photoelectric conversion section, there is a gate insulating layer 16), a photoconductive semiconductor layer (12 in the photoelectric conversion section), an ohmic contact layer (13, 13' in the photoelectric conversion section), and a metal (electrode 1 in the photoelectric conversion section).
4, 14') are formed in this order.

When various characteristics of the thin film transistor type photoelectric conversion section were examined in the same manner as in Example 1, the characteristics were found to be similar to those in Example 1. Furthermore, the thin film transistors in the drive circuit section showed sufficient characteristics.

FIG. 9 is a cross-sectional view of the photoelectric conversion element of the present invention mounted as a one-dimensional complete contact optical sensor array. In FIG. 9, a wear-resistant layer 1 made of glass or the like is provided on the photoelectric conversion section and the drive circuit section with a protective layer 120 interposed therebetween.
21 is formed, and is used in a configuration in which a light source 122 such as a light emitting diode illuminates the back surface of a transparent substrate 11 such as glass to read a document 123. It goes without saying that the optical sensor array using the photoelectric conversion element of the present invention can also be used as a one-dimensional contact optical sensor array using a 1-magnification imaging lens.

In the above embodiments, SiH4, H2, etc. were used as the thin film forming material, but the material is not limited to these, and materials containing F etc. may also be used.
n is an integer of 2 or more). Further, silicon as used in the present invention includes silicon and other materials containing, for example, fluoride, in addition to materials consisting of at least silicon and hydrogen.

[0026]

As explained above, according to the present invention, since the forbidden band width of the photoconductive semiconductor layer is made different between the insulating layer side and the first electrode and second electrode sides, the photocurrent is large,
A photoelectric conversion element that has a fast photoresponse speed and can be easily formed is provided.

[Brief explanation of the drawing]

FIG. 1 is a configuration diagram of a photoelectric conversion element of the present invention.

FIG. 2 is an energy band diagram of the photoconductive semiconductor layer of the photoelectric conversion element of the present invention.

FIG. 3 is a conceptual diagram of a plasma CVD apparatus used to create the photoelectric conversion element of the present invention.

FIG. 4 is a gas introduction timing diagram in a plasma CVD apparatus in producing the photoelectric conversion element of the present invention.

FIG. 5 is a diagram showing how the amount of hydrogen contained in the photoconductive semiconductor layer and the forbidden band width of the photoconductive semiconductor layer of the photoelectric conversion element of the present invention depend on manufacturing conditions.

FIG. 6 is a circuit diagram of a photoelectric conversion element of the present invention.

FIG. 7 is a partial plan view of a contact sensor array with the circuit of FIG. 6;

FIG. 8 is a sectional view taken along line AA' of the contact sensor array in FIG. 7;

FIG. 9 is an implementation diagram of a contact optical sensor array using the photoelectric conversion element of the present invention.

FIG. 10 is a configuration diagram of a conventional photoelectric conversion element.

FIG. 11 is a configuration diagram of a conventional photoelectric conversion element.

FIG. 12 is a diagram showing characteristics of a photoelectric conversion element.

FIG. 13 is a diagram showing characteristics of a photoelectric conversion element.

FIG. 14 is a diagram showing characteristics of a photoelectric conversion element.

[Explanation of symbols]

11 Substrate 12 Photoconductive semiconductor layer 13, 13' Ohmic contact layer 14
Drain electrode 14' Source electrode 15 Gate electrode 16 Gate insulating layer 70 Reaction chamber 71 Anode electrode 72 Cathode electrode 73 High frequency power source 74 Exhaust pump 75 SiH4 gas introduction tube 76 H2 gas introduction tube 77 Microwave source and microwave applicator E1 to E9 Photoelectric conversion section C1 to C9 Capacitor T1 to T9 Switching transistor 102
~104 Common line ST1~ST9 Switching transistor 1
06,107,109 Shift register C10
~C12 Capacitor 108 Terminal CT1~CT3 Switching transistor T
10~T12 Switching transistor 11
1 Matrix wiring section 112 Photoelectric conversion section 113 Charge storage section 114 Transfer switch 115 Discharge switch 116 Lead line 117 Load capacitor 120 Protective layer 121 Wear-resistant layer 122 Light source 123 Original

Claims (3)

[Claims] 1. An insulating layer, a photoconductive semiconductor layer provided in contact with the insulating layer, a first electrode provided in contact with the photoconductive semiconductor layer, and a second electrode provided in contact with the insulating layer. A photoelectric conversion element comprising a photoelectric conversion section having at least a third electrode, wherein the forbidden band width of the photoconductive semiconductor layer is narrower on the insulating layer side and closer to the first electrode and the second electrode. A photoelectric conversion element characterized by a wide area. 2. The photoelectric conversion element according to claim 1, wherein the photoconductive semiconductor layer is a non-single crystal consisting of at least silicon and hydrogen. 3. At least the photoconductive semiconductor layer is formed by alternately repeating a step of depositing a non-single crystal layer and a step of irradiating the deposited non-single crystal layer with hydrogen plasma. The photoelectric conversion element according to claim 1, characterized in that: