JPH05291610A - Optoelectric transducer - Google Patents

Abstract

PURPOSE:To provide a title device which can increase quantum efficiency above 1 and suppress the electric resistance of forward characteristics at low level. CONSTITUTION:A glass substrate 10 is overlaid with a 2000Angstrom chromium layer 12 serving as the lower electrode, a 300Angstrom p-type a-Si: H layer 14, a 6000Angstrom thickness i-type a-Si: YH layer 16, a 50Angstrom i-type a-SiC layer 18, a 300Angstrom n-type a-SiC layer 20, and an ITO film 22 to constitute a pin diode. Alternately, an n-type muc-SiC layer 24 can be formed by using an ECRCVD device instead of the a-SiC layer 20. Because of high electric resistance, the i-type a-SiC layer 18 generates avalanche effect by concentration of electric fields, so that quantum efficiency can be increased equivalently above 1. The band gap of the n-type a-SiC layer 20 and that of the muc-SiC layer 24 are almost equal to that of the i-type a-SiC layer 18, so that resistance in a forward direction can be suppressed at low level.

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 a contact type image sensor or the like.

[0002]

2. Description of the Related Art Diodes made of hydrogenated amorphous silicon (a-Si: H) used in a contact type image sensor include a photodiode type and a photodiode type. In the photoconductor type diode, as shown in FIG. 6, for example, an i-type or n-type a-Si: H film 62 is formed on a substrate 60, and two electrodes 64 and 66 are planarly formed on the film 62. Made by forming. The two electrodes and the a-Si: H film are connected by ohmic contact.

FIG. 7 is a band diagram of the photodiode of the photoconductor type shown in FIG. As described above, since the portions in contact with the electrodes on both sides of the a-Si: H layer 62 are ohmic contacts, electrons can freely move between the a-Si: H layer 62 and the electrodes. Therefore, when light is incident and a pair of electron and hole is generated to move the electron to the plus side and the hole to the minus side, the electron is injected from the minus electrode into the a-Si: H layer 62. Therefore, more current flows, and the quantum efficiency is equivalently larger than 1. Therefore, the photoconductor type diode is characterized by high photosensitivity.

However, since these carriers are injected from both electrodes, the current continues to flow for a time longer than the time it takes for the electrons to move inside the a-Si: H layer 62. Therefore, the photodiode of the photoconductor type has a very slow response speed of about several milliseconds, which is not suitable for high-speed operation. Due to the above features, the photodiode of the photoconductor type is often used for low-priced facsimiles. That is, it is difficult to achieve high functionality due to the slow response speed, but the circuit configuration is simplified due to the high photosensitivity.

FIG. 8 is a schematic cross-sectional view of a photodiode of a photodiode type, in which a chromium layer 70 to be a lower electrode is formed on a substrate 68, on which a p-type semiconductor layer 72, an i-type semiconductor layer 74 and an n-type semiconductor layer 74 are formed. Semiconductor layer 76, and Indium Tin Oxi
It has a structure in which de (ITO) films 78 are laminated. Each semiconductor layer is made of, for example, a-Si: H. A photodiode of a photodiode type applies a reverse bias to both ends and carries out photoelectric conversion by causing a current to flow by electrons and holes generated by the incidence of light.

A band diagram of this photodiode type diode is shown in FIG. Since the photodiode of the photodiode type has a high energy barrier at both ends of the band gap, carriers are not injected from the electrodes. Therefore, the quantum efficiency usually does not become 1 or more, and therefore the photosensitivity is low. However, since there is no carrier injection, the response speed corresponds to the time for the carriers to pass through the semiconductor region, so a high response speed of about several microseconds can be obtained. Due to such a high response speed, the photodiode type diode is often used for a relatively expensive facsimile contact image sensor or the like. However, since the photosensitivity is low, it is necessary to devise a circuit such as providing an amplifier circuit.

There is a phototransistor as another photoelectric conversion element. FIG. 10 is a band diagram of an npn type phototransistor. In this case, when light is incident and electrons and holes are generated, holes are stored in the p layer portion. Then, the n-type connected to the minus and the p-layer where holes are stored are further forward biased, so that electrons are injected from the n-layer connected to the negative, and thus the light is emitted. Sensitivity increases. However, the injection of such a carrier has a drawback that the response speed becomes substantially the same as that of the photoconductor type and the operation speed is also slow.

In order to realize a photodiode having a high response speed and high photosensitivity, a high voltage is applied to a photodiode type diode to produce an effect such as avalanche multiplication, and equivalently while maintaining a high speed operation. Attempts have been made to increase the quantum efficiency to 1 or more. As one of them, for example, as described in Japanese Patent Laid-Open No. 3-66178, in a pin diode in which an i layer is composed of a-Si: H, an a-SiN film is formed between the p layer and the n layer. There is something to do. Since this a-SiN film has a high electric resistance, when a voltage is applied, the electric field concentrates on this portion and a steep gradient of the electric field is generated, so that the electrons are accelerated and have high kinetic energy. As a result, the avalanche effect is generated, the carriers are multiplied, and as a result, the photosensitivity is increased. There is also an attempt to generate avalanche multiplication using a superlattice composed of a-SiC and a-Si: H (Shin-Cheng Two et al., IEEE Transactions on Electron.
Devices, vol.35 No.8, p1279-1283, 1988). Since these diodes can multiply carriers without injection of electrons, there is an advantage that photosensitivity can be improved while maintaining a high response speed.

[0009]

As an image sensor capable of reading an image at high speed, each pixel is formed by a set of a photodiode and a blocking diode in which each pixel is connected to back-to-back or front-to-front.
There is a contact type image sensor in which a large number of pixels are connected in a matrix and these are sequentially driven for each predetermined block to read an image. Such a contact image sensor is described in, for example, Japanese Patent Application No. 2-83258.

In such a contact image sensor, a reverse bias is applied to the photodiode when accumulating the charges generated by the incident light, but a forward bias is applied to the blocking diode when the accumulated charges are taken out so that a forward current flows. Shed. However, a- as described above
Although a SiN film or a superlattice composed of a-SiC and a-Si is excellent in improving photosensitivity, a
-SiN is formed between a-Sipin diodes,
When a superlattice of a-SiC and a-Si is formed, the band gap difference between a-Si and a-SiN and a-SiC in the n-layer or p-layer is large, so that charges are injected from the p-layer and the n-layer during forward bias. Therefore, there is a problem that the forward characteristic is deteriorated. Therefore, conventionally, an a-SiN film or a
Providing a superlattice layer composed of -SiC and a-Si on a diode forming a pixel of a matrix-driven contact image sensor is not an appropriate method, and thus it is difficult to improve photosensitivity.

The present invention has been made based on the above circumstances, and even when it is used in a contact type image sensor having the photodiode and the blocking diode, it is possible to improve the photosensitivity while maintaining a fast response speed. The purpose is to provide a conversion element.

[0012]

The invention according to claim 1 for achieving the above object comprises a lower electrode, a p-type semiconductor layer, an i-type semiconductor layer, an n-type semiconductor layer, and a transparent electrode from the substrate side. In the photoelectric conversion element stacked in this order, the film thickness between the i-type semiconductor layer and the n-type semiconductor layer is 20 Å or more and 100 or more.
It is characterized in that an i-type hydrogenated amorphous silicon carbon (a-SiC) layer having a thickness of Å or less is formed and the n-type semiconductor layer is formed of n-type a-SiC.

According to a second aspect of the present invention, there is provided a photoelectric conversion element in which a lower electrode, a p-type semiconductor layer, an i-type semiconductor layer, an n-type semiconductor layer, and a transparent electrode are laminated in this order from the substrate side. An i-type a-SiC layer having a film thickness of 20 Å or more and 100 Å or less is formed between the n-type semiconductor layer and the n-type semiconductor layer, and the n-type semiconductor layer includes n-type amorphous silicon carbon containing microcrystals ( μc-SiC).

Further, the invention according to claim 3 is a photoelectric conversion element in which a lower electrode, an n-type semiconductor layer, an i-type semiconductor layer, a p-type semiconductor layer, and a transparent electrode are laminated in this order from the substrate side, wherein the i An i-type a-SiC layer having a film thickness of 20 Å or more and 100 Å or less is formed between the p-type semiconductor layer and the p-type semiconductor layer, and the p-type semiconductor layer is formed as a p-type a-SiC layer.
It is characterized by being formed more.

According to a fourth aspect of the present invention, there is provided a photoelectric conversion element in which a lower electrode, an n-type semiconductor layer, an i-type semiconductor layer, a p-type semiconductor layer, and a transparent electrode are laminated in this order from the substrate side. An i-type a-SiC layer having a film thickness of 20Å or more and 100Å or less is formed between the p-type semiconductor layer and the p-type semiconductor layer, and the p-type semiconductor layer is formed of μc-SiC. It is a thing.

[0016]

The invention according to claim 1 has the following features.
By forming the i-type a-SiC layer having a thickness of Å or more and 100 Å or less, when the electric field is applied, the electric field is concentrated in this part due to the high electric resistance of this part, and a steep electric field is formed. Therefore, the carriers generated by the incident light have an avalanche multiplication effect and equivalently have a quantum efficiency of 1
As described above, the photosensitivity is increased. In addition, since the n-type semiconductor layer is formed of n-type a-SiC, i-type a-S
The energy band gaps of the iC layer and the n-type a-SiC layer are almost equal to each other, and charges are favorably injected from the n-type a-SiC to the i-type a-SiC at the time of forward bias, so that the resistance value of the forward characteristic is kept low. be able to.

According to a second aspect of the present invention, by the above configuration,
By forming an i-type a-SiC layer of 20 Å or more and 100 Å or less, the high electric resistance of this portion
When an electric field is applied, the electric field concentrates on this portion to form a steep electric field. Therefore, the carriers generated by the incident light have an avalanche multiplication effect, equivalently having a quantum efficiency of 1 or more and increasing photosensitivity. In addition, since the n-type semiconductor layer is formed of n-type μc-SiC, the energy band gap of the n-type μc-SiC layer is almost equal to that of the i-type a-SiC layer, and the n-type μc-type is increased during forward bias.
Since the charge is favorably injected from -SiC to i-type a-SiC, the resistance value of the forward characteristic can be suppressed to be low.
Moreover, since the μc-SiC has a smaller electric resistance than the a-SiC, the forward characteristic can be further improved.

In the invention according to claim 3, the order of forming the layers of the diode is reverse to that of the invention according to claim 1, and in the invention of claim 4, the invention of claim 2 is formed with the layers of the diode. The order is reversed. Therefore, claim 3
In the invention described in claim 1, the conductivity type of each semiconductor layer corresponding to the invention of claim 1 is changed, and in the invention described in claim 4, the conductivity type of each semiconductor layer corresponding to the invention of claim 2 is changed. It is a thing. Therefore, the invention according to claim 3 has the same effect as the invention according to claim 1 and the invention according to claim 4 has the same effect as the invention according to claim 2.

When the avalanche effect occurs due to the steep gradient of the electric field, it is considered that, in addition to the avalanche effect, some multiplication due to electron injection actually occurs. However, since the avalanche effect is considered to be dominant, the term avalanche effect will be simply used in the description below.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A photoelectric conversion element which is an embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows the first of the present invention.
2 is a sectional view showing the structure of a pin diode which is an embodiment, FIG. 2 is a sectional view showing the structure of a pin diode which is a second embodiment of the present invention, and FIG. 3A is a sectional view showing the structure of the first and second embodiments. FIG. 3B is a band diagram showing an outline of a band structure when a reverse bias is applied to the pin diode, FIG. 3B is a band diagram when a forward bias is applied, and FIG.
The graph which shows the reverse direction characteristic of the pin diode of an Example,
FIG. 5 is a graph showing the forward characteristics of the pin diodes of the first and second embodiments.

First, referring to FIG. 1, the pin of the first embodiment will be described.
A procedure for manufacturing the diode will be described. First, a chrome layer 12 to be a lower electrode is deposited on the glass substrate 10 by an electron beam vapor deposition apparatus to a thickness of 2000Å. Next, ordinary high frequency plasma (RF plasma) CV
P-type hydrogenated amorphous silicon (a
Deposit a layer 14 of Si: H) to a thickness of 300Å.
The film forming conditions at this time are as follows: the substrate temperature is 250 ° C., and the mixing ratio of SiH ₄ and B ₂ H ₆ used as a process gas is 1 :.
0.001, gas pressure is 0.3 Torr, and RF output is 30 watts. Then, an i-type a-Si: H layer 16 having a thickness of 6000Å is deposited thereon by the same RF plasma CVD apparatus. The film forming conditions at this time are that the substrate temperature is 250 ° C. and SiH ₄ is 100 as the process gas.
%, Gas pressure is 0.05 Torr, and RF output is 30 watts.

Next, on this layer 16, RF plasma C
An i-type hydrogenated amorphous silicon carbon (a-SiC) layer 18 is deposited to a thickness of 50Å by a VD device. The film forming condition of the a-SiC layer 18 is that the substrate temperature is 2
50 ° C., process gases C ₂ H ₄ , SiH ₄ and H ₂
Mixing ratio is 1: 2: 100, gas pressure is 0.3 Torr, RF
The output is 100 watts. Furthermore, on this layer 18,
Similarly, an a-SiC layer 20 as an n-type semiconductor layer is laminated to a thickness of 300Å by an RF plasma CVD apparatus. The film forming conditions are as follows: substrate temperature is 250 ° C., SiH ₄ , PH ₃ , CH _4, and H ₂ used as process gas are mixed at a ratio of 1: 0.01: 2: 20, gas pressure is 0.3 Torr, and RF output is It is 30 watts. Finally, ITO on this layer 20
The film 22 is formed to form the pin diode of FIG.

Next, referring to FIG. 2, the pin of the second embodiment will be described.
A procedure for manufacturing the diode will be described. Chrome layer 12 serving as lower electrode, p-type a-Si: H layer 14, i-type a
-Si: H layer 16, i-type a-SiC layer 18, and ITO
22 is formed by a procedure similar to that of the pin diode of the first embodiment shown in FIG. The difference from the pin diode of FIG. 1 is that instead of the n-type a-SiC layer 20 of FIG. 1, an n-type SiC (μc-Si) containing microcrystals is formed by using an electron cyclotron resonance CVD (ECRCVD) device.
C) The point of forming the layer 24. This film forming condition is that the substrate temperature is 300 ° C., SiH ₄ and PH ₃ which are process gases are used.
And the mixing ratio of CH ₄ and H ₂ is 1: 0.01: 2: 200,
The gas pressure is 0.005 Torr and the output is 300 watts.

Next, the pin of the first and second embodiments
The operating characteristics of the diode will be described. i type aS
The electric resistance of iC is approximately 10 ⁻¹² to 10 ⁻¹³ S / cm, which is considerably higher than that of a-Si: H. Therefore, i
By providing a thin a-SiC layer 18 on the type semiconductor layer, when a voltage is applied, the i-type a-type a layer is formed as shown in FIG.
A steep electric field is generated in the SiC layer 18. Therefore, when the electrons generated by the incident light pass through this part,
Accelerated by this steep electric field, a large number of electron-hole pairs are generated by the avalanche effect. As a result,
Equivalently, the quantum efficiency becomes larger than 1, and the photosensitivity of the photodiode can be increased. Moreover, since the carriers are multiplied without injection of the carriers from the electrodes, the response speed is kept high.

However, if the p-type semiconductor layer on the i-type a-SiC layer 18 is an a-Si: H layer similar to the conventional one, the electric resistance in the forward direction becomes high, and the forward characteristic deteriorates. There is. The main reason for this is that the energy band gaps of the two differ. That is, a-Si:
The band gap of H is about 1.7 eV, while a-
The band gap of SiC is about 2.0 eV, and due to the difference in energy band gap,
It is difficult for the electrons of the n layer made of a-Si: H to enter the a-SiC layer 20.

Therefore, in the first embodiment of the present invention, phosphorus-doped a-S is used as the n-type semiconductor layer as described above.
The iC layer 20 is used. Since the energy band gap of a-SiC is about 2.0 eV as described above, Fig. 3 (b)
As shown in, the energy band gaps of the i-type a-SiC layer 18 and the n-type a-SiC layer 20 are almost equal, and therefore electrons can easily enter from the n-layer to the i-layer. As a result, the forward resistance can be reduced.

By the way, a-SiC has an electric resistance of about 10 ^-5 S / cm even if it is doped to be n-type. Therefore,
In the second embodiment, the μc-SiC layer 24 is used as the n-type semiconductor layer in order to further reduce the electric resistance. μ
The electric resistance of c-SiC is about 10 ^-1 S / cm, and a
-It has much lower electrical resistance than SiC. As a result pin
It is possible to further improve the forward characteristic of the diode.

In the graph of FIG. 4, the curve a is p in the first embodiment.
The in-diode, the curve b corresponds to the reverse characteristic of the pin diode of the second embodiment, and the curve c corresponds to the reverse characteristics of the conventional a-Si: H pin diode. As is apparent from this figure, both the first and second embodiments have a larger current value in the opposite direction than the conventional pin diode, and therefore the photosensitivity is improved. From this figure, it may seem like a slight increase, but it is understood that the increase in the current value is about several times, considering that the vertical axis is a logarithmic scale. Therefore, for example, when the contact type image sensor consisting of 8 sensors per 1 mm is to be 16 per 1 mm, in order to keep the sensitivity of the sensors the same, the photosensitivity of each photodiode must be quadrupled. However, the pin diodes of the first and second embodiments can fully meet this requirement.

In the graph of FIG. 5, the curve a is p in the first embodiment.
The in-diode, the curve b corresponds to the forward characteristic of the pin diode of the second embodiment, and the curve c corresponds to the forward characteristic of the conventional pin diode made of a-Si: H. As can be seen from this figure, in the high voltage portion, the current is smaller in both the first and second embodiments than in the conventional pin diode. However, the voltage region important for the forward characteristic of the photodiode used as the contact image sensor is about 0.5 V, and it can be seen that there is almost no difference from any curve in this portion.

Summing up the above points, the pi of the first embodiment is
The n-diode can improve the reverse characteristic as compared with the conventional one while maintaining the sufficient forward characteristic, and thus can have higher photosensitivity than the conventional pin diode. Further, the pin diode of the second embodiment has further improved forward characteristics and reverse characteristics as compared with the first embodiment, and can further improve the photosensitivity. Moreover, in both cases, since the photocurrent is doubled by the avalanche effect or the like, the response speed is maintained at a high speed.

[0031]

As described above, according to the present invention, it is possible to improve the photosensitivity of the backward characteristic by equivalently increasing the quantum efficiency while keeping the electric resistance of the forward characteristic low. Therefore, for example, it is possible to provide a photoelectric conversion element capable of increasing the response speed while maintaining the current sensitivity even if the number of pixels per unit length of the contact image sensor is increased.

[Brief description of drawings]

FIG. 1 is a sectional view showing the structure of a pin diode according to a first embodiment of the present invention.

FIG. 2 is a sectional view showing a structure of a pin diode according to a second embodiment of the present invention.

FIG. 3A is a band diagram showing an outline of a band structure when a reverse bias is applied to the pin diodes of the first embodiment and the second embodiment, and FIG. 3B is a case where a forward bias is applied. FIG.

FIG. 4 is a graph showing the reverse direction characteristics of the pin diodes of the first and second examples.

FIG. 5 is a graph showing the forward characteristics of the pin diodes of the first and second embodiments.

FIG. 6 is a cross-sectional view of a conventional photoconductor type diode.

FIG. 7 is a band diagram when a forward bias is applied to the diode shown in FIG.

FIG. 8 is a sectional view of a conventional photodiode type diode.

9 is a band diagram when a reverse bias is applied to the diode shown in FIG.

FIG. 10 is a band diagram when a bias voltage is applied to the phototransistor.

[Explanation of symbols]

10 glass substrate 12 lower electrode of chromium 14 p-type hydrogenated amorphous silicon layer (a-S)
i: H) 16 i-type hydrogenated amorphous silicon layer 18 i-type hydrogenated amorphous silicon carbon layer (a-SiC) 20 n-type hydrogenated amorphous silicon carbon layer 22 Indium Tin Oxide (ITO) film 24 n-type containing microcrystals Silicon carbon layer (μc-
SiC)

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[Procedure amendment]

[Submission date] July 9, 1992

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Front page continuation (72) Inventor Koichi Kitamura 1618 Ida, Nakahara-ku, Kawasaki-shi, Kanagawa Nippon Steel Corporation Advanced Technology Research Laboratories

Claims (4)

[Claims] 1. A lower electrode, a p-type semiconductor layer, and i from the substrate side.
In a photoelectric conversion element in which an n-type semiconductor layer, an n-type semiconductor layer, and a transparent electrode are laminated in this order, a film thickness is 20 between the i-type semiconductor layer and the n-type semiconductor layer.
A photoelectric conversion element, wherein an i-type hydrogenated amorphous silicon carbon layer having a thickness of Å or more and 100 Å or less is formed, and the n-type semiconductor layer is formed of n-type amorphous silicon carbon. 2. A lower electrode, a p-type semiconductor layer, and i from the substrate side.
In a photoelectric conversion element in which an n-type semiconductor layer, an n-type semiconductor layer, and a transparent electrode are laminated in this order, a film thickness is 20 between the i-type semiconductor layer and the n-type semiconductor layer.
A photoelectric conversion element, characterized in that an i-type hydrogenated amorphous silicon carbon layer of Å or more and 100 Å or less is formed, and the n-type semiconductor layer is formed of n-type amorphous silicon carbon containing microcrystals. 3. A lower electrode, an n-type semiconductor layer, and i from the substrate side.
In a photoelectric conversion element in which a p-type semiconductor layer, a p-type semiconductor layer, and a transparent electrode are laminated in this order, the film thickness is 20 between the i-type semiconductor layer and the p-type semiconductor layer.
A photoelectric conversion element, characterized in that an i-type hydrogenated amorphous silicon carbon layer of Å or more and 100 Å or less is formed, and the p-type semiconductor layer is formed of p-type amorphous silicon carbon. 4. A lower electrode, an n-type semiconductor layer, and i from the substrate side.
In a photoelectric conversion element in which a p-type semiconductor layer, a p-type semiconductor layer, and a transparent electrode are laminated in this order, the film thickness is 20 between the i-type semiconductor layer and the p-type semiconductor layer.
A photoelectric conversion element, characterized in that an i-type hydrogenated amorphous silicon carbon layer of Å or more and 100 Å or less is formed, and the p-type semiconductor layer is formed of p-type amorphous silicon carbon containing microcrystals.