JPH03185763A - Photoelectric conversion device - Google Patents

Abstract

PURPOSE:To lessen a photoelectric conversion device in after image caused by that it is formed of amorphous material by a method wherein a capacitor larger than a photodiode in electric capacity is connected to the photodiode of amorphous material in parallel. CONSTITUTION:A photodiode 1 of amorphous material is connected in parallel to a capacitor 2 formed of material lowe than the amorphous material concerned in capture level density. In this case, charge Q induced by optical pumping can be separately stored in the capacitance Cp of the photodiode 1 comparatively larger than the capacitor 2 in capture level density and the capacitance Ca of the capacitor 2 of material small in capture level density. By this setup, an after-image phenomenon can effectively be reduced.

Description

Detailed Description of the Invention [Industrial Application Field] The present invention relates to a photoelectric conversion device, and in particular to a charge storage type device that uses a photodiode formed of an amorphous material as a photoelectric conversion element and has improved noise characteristics. The present invention relates to a photoelectric conversion device.

[Prior Art] In recent years, photoelectric conversion elements using amorphous materials have been researched and developed. Known amorphous materials used in photoelectric conversion elements include amorphous silicon (hereinafter referred to as a-5i), amorphous selenium, amorphous germanium, and the like. Among them, a-St has excellent optical properties such as a large light absorption coefficient and spectral sensitivity close to human visual sensitivity, the ability to create a low-temperature process, and the ability to form a film over a large area. Therefore, it is attracting particular attention as an amorphous material for photoelectric conversion elements.

Furthermore, photodiodes are conventionally known as photoelectric conversion elements. Photodiodes have the advantage of having a simple structure and being suitable for microfabrication. Conventional photodiodes are known as pn-type photodiodes, pin-type photodiodes, avalanche photodiodes, etc., depending on their layer configurations and material combinations. It is possible to configure various configurations, and these are also being actively researched.

A photodiode is a type of photoelectric conversion element that converts an optical signal into an electrical signal by utilizing the fact that when a bias voltage is applied in the reverse direction, the reverse current changes in proportion to the amount of light irradiation.
Although it is a seed, it also has the property of being a capacitor that accumulates photoelectric charges inside when it is irradiated with an optical signal. A photodiode that utilizes this capacitive property is generally called a charge storage type photodiode. Charge storage type photodiodes produce high output and high S/N by accumulating photo-excited charges for a certain period of time and then outputting them.
It has been widely used as a conventional photoelectric conversion element because it can obtain a high ratio.

FIG. 4 is an equivalent circuit diagram showing a photoelectric conversion section of a photoelectric conversion device using a charge storage type photodiode as a photoelectric conversion element. In the figure, 1 is a charge storage type photodiode. The output circuit is a circuit for converting a photocurrent generated by irradiating the photodiode 1 with an optical signal into an electrical signal and outputting the electrical signal. In addition, a reset circuit is a circuit for erasing the charge that remains after transferring the charge accumulated in the photodiode l, that is, a circuit that forcibly sets the output terminal of the photodiode to a fixed potential. It is a circuit. This reset circuit is made of, for example, a bipolar transistor or an MIS transistor.

In such a photoelectric conversion device, the photodiode alternately performs an accumulation operation in which one or both of electron-hole pairs generated by being irradiated with light and a reset operation in which the charge is reset and returned to the initial state. Let's do it.

The causes of noise in such conventional light beam conversion devices include the wiring capacitance of the signal line and the electrode-to-substrate capacitance when an MIS transistor is used for the reset switch, which makes it difficult to read out the noise charge remaining in the line. There are two types of noise: noise caused by the opening and closing of the reset switch, and noise caused by the electric charge generated by light incident on areas other than the photodiode's light receiving part getting mixed into the signal line, that is, noise caused by the smear phenomenon. Solutions were implemented depending on the cause.

[Problems to be Solved by the Invention] However, according to the findings of the present inventors, in a photodiode formed of an amorphous material, in addition to the noise caused by the above-mentioned causes, the following causes are also caused. It turned out that this was occurring. In other words, an amorphous material has a defect level density (capture level density) due to the bonding state of its atoms, and therefore a photodiode made of an amorphous material has a
In addition to the original junction capacitance of the diode, charges due to photoexcitation are also accumulated in the trap level. However, 1. The charges accumulated in the trap level cannot be completely reset by a short reset operation.

If the charge remaining after being transferred as an output by the photodiode cannot be completely reset, the charge remaining after the reset is added to the next accumulation operation, resulting in a so-called afterimage.

As an example, light enters from the 9th layer side of a pn type a-3i photodiode as shown in Fig. 5(a), and during storage operation, p
Consider a configuration in which the layers are in a floating state. In FIG. 5(a), 510 is a substrate, 511 is an ITO electrode, and 512 is a P-type a-
St, 513 is an n-type a-3t electrode, and 514 is a Cr electrode. FIG. 5(b) is a band potential diagram showing the electron energy of a-St, and shows that tail levels, defect level density, etc. conceptually exist in the band gap. In the 9 layers, not only holes due to photoexcitation are accumulated, but also some of the electrons generated at the same time are accumulated.

A similar phenomenon occurs on the n-layer side, but the relationship between electrons and holes is reversed. Also, since light enters from the p-layer side, n
The number of electrons and holes excited on the layer side is smaller than on the p-layer side.

If a reset operation is performed after accumulation, the electrode positions of the 9th layer are forcibly returned to a thermal equilibrium state with a fixed potential.
), the electrons in the conduction band and the tail of the conduction band 5
01, the holes 502 in the valence band and the tail of the valence band and the electrons in the shallow trap levels are erased.

In addition, a certain lifetime is required for an electron captured in a defect level (capture unit) to return to the conduction band from this level and be erased, and this lifetime is longer than the time for normal carrier movement. Because it is extremely slow, a photodiode made of a-St has an additional slow time constant component that is not present in a photodiode made of crystalline silicon. In other words, if the reset time is short, a −3
Since the electrons 503 in the capture level in i are transferred to the next accumulation operation without being fully reset, the electrons captured in this capture level contribute as signal charges during the next accumulation operation. , causing an afterimage.

An object of the present invention is to reduce such afterimages caused by formation of an amorphous material.

[Means for Solving the Problems] A photoelectric conversion device of the present invention includes a photodiode formed of an amorphous material, an insulating material having a lower trap level density than the amorphous material, and a photodiode formed of an amorphous material. The photodiode is characterized in that it includes at least a capacitor having a capacitance equal to or larger than that of the photodiode, and the photodiode and the capacitor are connected in parallel.

[Function] According to the present invention, since the photodiode and the capacitor are connected in parallel, it is possible to reduce the afterimage described above.

The reason for this will be explained in detail below.

In order to reduce the afterimage, it is sufficient to reduce the amount of charge accumulated in the trap level. To this end, the present invention combines a photodiode formed of an amorphous material and a non-crystalline material, as shown in FIG. A capacitor made of a material with a lower capture unit density than a crystalline material is connected in parallel. In FIG. 1, 1 indicates a photosensor and 2 indicates a capacitor. By this method, the charge Q generated by photoexcitation is
It is possible to divide the trap level density into the photodiode capacitor C2, which contains a relatively higher density than the capacitor, and the capacitor C1, which uses a material with a lower trap level density, for storage. In this case, the charge Q accumulated in C2 is expressed as follows, which means that the amount of afterimage is reduced by 1/(as +c,) by adding C1. FIG. 2 is a diagram showing the relationship between the amount of afterimage and C, /C. According to the 3g2 diagram, for example, if C1 has the same capacity as C, the afterimage will be C1.
When there is no! /2, indicating that when Ca has twice the capacity of C, the afterimage will be 173 when C1 is not present. As mentioned above, photodiodes using amorphous silicon have significantly more afterimages than photodiodes using crystalline silicon, so reducing the amount of afterimages to less than half is an effective way to improve photosensor performance. The afterimage reduction effect according to the present invention is sufficiently effective when C1 is sufficiently larger than C, that is, when C,>C.

Note that by forming the photodiode and the MIS transistor on the same substrate and arranging them in a direction perpendicular to the substrate, it is possible to reduce the above-mentioned afterimage without increasing the device area.

[Example] As an example of the present invention, pi is used as a photodiode.
A case of a line sensor using an n-type photodiode and an MO3 type switching transistor as a reset switch in a reset circuit will be described.

FIGS. 3(a) to 3(e) are schematic cross-sectional views showing the manufacturing process of the portion forming the pin diode and MO3 type switching transistor of the line sensor of this example, and FIG. 3(e) is This is a completed drawing.

Hereinafter, the manufacturing process of the line sensor according to this embodiment will be explained using FIGS. 3(a) to 3(e).

■S, O, as a semiconductor layer 302 on the substrate 301
Polycrystalline silicon with large grain size was deposited as disclosed in Japanese Patent No. 2-73630.

The reason why polycrystalline silicon with a large grain size is deposited as the semiconductor layer 302 is to create a transistor with good performance in a large-area line sensor. Note that the semiconductor layer 30
2 is a P-type semiconductor, and during deposition, 5i2HzCItx is used as a source gas and H2 is used as a carrier gas.
, HCl2 was used as an etching gas, and B2H8 was used as a doping gas.

Note that as the semiconductor layer 302, a single crystal material or an amorphous material can be used. Single crystal materials include those recrystallized by laser annealing, S, and S on O2.
A flattened single crystal Si grown by a single crystal growth method seeded with an iH4 pattern can be used, and amorphous materials include a-3T and a-5iGe.
, a-5iC, a-3ICGe%a-5e, etc. can be used.

(2) Pattern low-resistance polycrystalline silicon electrodes 303 (a) and 303 (b) containing P in desired regions on the semiconductor layer 302, and then diffuse P by thermal diffusion to form n
A mold semiconductor region 302(a) was formed.

Through the above steps, the switching transistor region and the lower electrode 303 (a) of the capacitor were formed (Fig. 3(a)).
a)).

■In order to form the gate oxide film and the capacitor insulating film, as a material with lower trap level density than a-3i,
A SiO film 304 was deposited to a thickness of 1000λ using the CVD method (FIG. 3(b)).

(2) An A42 layer 305, which serves as the upper electrode of the capacitor and the lower electrode of the photodiode, was created by sputtering.

(2) A contact pole 306 was opened at Sin@304 between the capacitor region and the switching transistor region (FIG. 3(C)).

■N-type amorphous silicon layer 307 (
a) for 100 people, i-type amorphous silicon layer 307 (b) for 8000 people, and p-type amorphous silicon layer 307 (c) for 2 people.
00λ, respectively, to create a pin photodiode.

■S, o, layer 30B was formed as an insulating layer on the side surface of the photodiode on the switching transistor side (third
Figure (d)).

(2) An ITO layer 309 was formed on the p-type amorphous silicon layer 307 by sputtering as the upper transparent electrode of the photodiode.

[Phase] Finally, a low resistance polycrystalline silicon electrode 310 that forms the gate part of the switching transistor was created (
Figure 3(e)).

In the line sensor manufactured in this way, the contact hole 306 is connected to the lower electrode 303 (a
) and the upper transparent conductor 1i309 of the photodiode, and the amorphous silicon pin photodiode and the capacitor using S, O2 as the insulating material were arranged perpendicularly to the substrate surface and connected in parallel. The photodiode section and the capacitor section have the same area. Therefore, the capacitance per unit area of the photodiode section is −13,2[
pF/c11 Capacitance per unit area of capacitor section 3,'l x 8.85x 10-"1000 people = 34.5 [pF/cm'1, and afterimage is 13.2/(13,2◆34.5 )-1
It decreases to 73.6.

Note that the film thickness of each layer shown in the above description of the manufacturing process is an example, and can be arbitrarily changed depending on the material and purpose of the sensor.

Furthermore, in this example, the photodiode and capacitor were arranged perpendicularly to the substrate surface in order to prevent an increase in the area of the element. If there were no such restrictions, it would also be possible to arrange them laterally.

Furthermore, although the switching transistor is formed on an insulating film in this embodiment, it may be formed in a generally used bulk crystal.

If these are combined and laminated, ie, a switch transistor in the bulk, a capacitor thereon, and a photodiode thereon, higher density will be further promoted.

The electrode 305 that applies a fixed voltage to the capacitor is in common with the electrode that applies a fixed voltage to the photodiode in this embodiment, but even if they are formed separately, the effects of the present invention are not affected in any way. .

Although the insulating film 304 used to form the capacitor is the same as the transistor 302 in this embodiment, the effect of the present invention is not affected even if the two are formed separately. The insulating film is not limited to 5i02 as long as it has a low capture unit density, but it can also be made of Si.
N.

5iON or the like, and any insulating film with a relatively high dielectric constant may be used.

[Effects of the Invention] As explained above, according to the present invention, the afterimage phenomenon can be effectively reduced.

Furthermore, since the capacitance in which charge can be stored increases, assuming that the reverse bias voltage applied to the photodiode is fixed, this also has the effect of expanding the dynamo's main range toward the high illumination side by the increased capacitance.

[Brief explanation of drawings]

Fig. 1 is a circuit diagram showing an equivalent circuit of the photoelectric conversion device of the present invention, Fig. 2 is a graph showing the size of the additional capacitance and the improvement rate of afterimage according to the present invention, and Figs. A schematic cross-sectional view for explaining the process of a photoelectric conversion section of a photo sensor as an embodiment of the invention, FIG. 4 is a circuit diagram showing an equivalent circuit of a conventional photoelectric conversion device, and FIGS. 5(a) and (b) ) are a schematic side view of a conventional pn type a-3i photodiode and a carrier energy distribution diagram after reset, respectively. 1... Photodiode made of amorphous material, 2...
・Capture N1! Capacity using materials with low potential density, 501.
. . . Electrons erased by reset, 502 . . . Holes erased by reset, 503 . . . Electrons in the trap level density that remained after being reset. Figure Co/Cp Figure 3 Figure Figure Figure (a) Figure (b) Carrier energy density

Claims (5)

[Claims] (1) A photodiode formed of an amorphous material, and an insulating material having a lower trap level density than the amorphous material, and having the same or larger electric capacity than the photodiode. A photoelectric conversion device comprising at least a capacitor having a photodiode and a capacitor, the photodiode and the capacitor being connected in parallel. (2) The photoelectric conversion device according to claim 1, wherein amorphous silicon is used as the amorphous material. (3) The photoelectric conversion device according to claim 1 or 2, wherein the photodiode has a pin structure. (4) The photoelectric conversion device according to any one of claims 1 to 3, wherein the photodiode and the capacitor are formed on the same substrate. (5) The photoelectric conversion device according to claim 4, wherein the photodiode and the capacitor are arranged in a direction perpendicular to the surface of the base.