JPS61236173A - Amorphous silicon optical sensor - Google Patents

Abstract

PURPOSE:To improve the S/N ratio and to uniformalize the characteristics of elements, by providing a first amorphous silicon layer containing oxygen and carbon atoms to be contacted with a first electrode and adapted to have photoconductivity and a specific resistivity in a specific optical band gap region. CONSTITUTION:An ITO film is formed on a transparent 1 by the vacuum vapor deposition and, successively, a first electrode (discrete electrode) is formed thereon. The structure is then set in a CVD apparatus. A gas mixture of CO2 and SiH4 is introduced in the apparatus so that a first amorphous silicon layer 3 added with oxygen and carbon atoms is formed by the glow discharge decomposition. Subsequently, SiH4 is introduced so that a second amorphous silicon layer 4 is formed by the glow discharge decomposition. Finally, a second electrode (common electrode) 5 is provided. The amorphous silicon layer 3 thus produced has photoconductivity in the optical band gap region of 2.0eV or more and has a resistivity of 10<12>-10<15>OMEGAcm in the dark state. Accordingly, a high S/N ratio can be obtained and each element corresponding to each discrete electrode is allowed to have uniform characteristics.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to an optical sensor using an amorphous silicon film.

(Prior art) Conventional amorphous optical sensors include Cd5-CdSe type,
S e-T e-As s-based and amorphous silicon-based ones are known, but among these, Cd5-CdSe-based ones have a slow photoresponse speed, and There are many problems such as crystallization at low temperatures, making it difficult to obtain high-density, high-speed operation, and good elements. Therefore, amorphous silicon was developed as a new material, and amorphous silicon optical sensors have attracted attention because they have advantages such as being able to fabricate large-area devices with thin films, having a large light absorption coefficient, and having excellent photoconductive properties. ing.

In an optical sensor using amorphous silicon, a set of electrodes is formed in a plane on the same substrate, and an amorphous silicon film is formed across the electrodes. There is a so-called planar type amorphous silicon optical sensor.

A relatively large number of optical sensors with this configuration have been manufactured as a result of the simple manufacturing method, but due to their structure, the optical response speed is slow and it is difficult to use them in high-speed facsimiles. This is because in the planar type, it is necessary to have a wide interval between electrodes of about 10 μm, so it takes time for carriers such as electrons to move. Furthermore, since the electrodes are formed in a plane, it is difficult to increase the density.

In order to eliminate these drawbacks, amorphous silicon was sandwiched between a transparent electrode and another electrode. There is an amorphous silicon optical sensor with a so-called sandwich structure, which has a high density and
Although it has been developed as a device capable of increasing speed, in this sandwich-type configuration, charge is injected from the base electrode when bias voltage is applied, resulting in a large current in the dark, so the current in the dark is low compared to the current in the light irradiation. It has the disadvantage that the (S/N ratio) cannot be made large. Further, since amorphous silicon has a large light absorption coefficient, it is formed with a thin film thickness (usually about 5,000 to 1 μm), which causes many problems such as short circuits in electrodes due to the formation of pinholes.

In order to reduce current in the dark, another thin insulating film is provided between the amorphous silicon and the electrode to prevent charge injection from the electrode, a blocking structure known as MI.
An S-type optical sensor has been proposed, and the insulating film includes, for example, a silicon oxide film, a silicon nitride film, a metal nitride film, etc. 6
For example, Japanese Patent Laid-Open No. 57-106179 discloses that in order to form a thin insulating film, the surface of amorphous silicon is subjected to anodic plasma oxidation, or silane gas containing oxygen is glow discharged to form a thin insulating film on amorphous silicon. A method for forming an oxide film is described, but according to this method, the thickness of the insulating film shifts after about 20 to 40 layers, so it is difficult to form an insulating film with a homogeneous and uniform thickness over the entire surface of a high-density element. It is difficult to form.

Further, when a thin film is normally formed, if the thickness is less than 100 Å, the thin film is formed in an island shape, making it difficult to make the characteristics of the device uniform. When the insulating film is made thicker to make the characteristics of the element uniform.

Although charge injection during dark times can be sufficiently prevented, carriers generated upon light incidence are also blocked, resulting in a decrease in the S/N ratio.

Japanese Unexamined Patent Publication No. 56-26418 describes a method using nitride (transparent, electrically conductive, insulating or semi-insulating) as a thin insulating film. According to this, when an oxide is used, it has a high energy bandgap, so even if it is a thin film, it is difficult for current to pass through itself, making it difficult for carriers to pass through when irradiated with light, and reducing the S/N ratio. Because of this problem, nitrides, which have a smaller energy bandgap than oxides, are used. However, even in this configuration, it is necessary to form the nitride thinly by 50 to 100 layers, and when forming the insulating film after forming the individual electrodes, 1. Usually, the insulating film is formed after forming the individual electrodes by etching. Therefore, it is difficult to form a uniform film on the side surfaces and edges of the electrode, and as in the case of oxides, it is very difficult to make the device characteristics uniform.

Furthermore, JP-A-56-14268 discloses a multilayer structure of amorphous silicon, in which at least one of the amorphous silicon layers contains oxygen atoms and impurities for controlling the valence electron concentration. A photoconductive semiconductor device is disclosed. However, when the optical bandgap is widened by adding oxygen atoms, it exhibits photoconductivity between 1.85 and 1.92 eV, but if it is widened beyond that, it hardly exhibits photoconductivity. When a thin film with an optical band gap of 2.0 eV or more is used to block charge injection in the dark, carriers generated during light irradiation are blocked, making it difficult to obtain a device with a high S/N ratio. .

(Problems to be Solved by the Invention) As described above, the conventional examples each have problems such as slow response speed, low S/N ratio, and difficulty in making device characteristics uniform.

The present invention provides a sandwich-type amorphous silicon photosensor that is suitable for high speed and high density, has a large S/N ratio, and has uniform device characteristics.

(Means for solving the problem) The first amorphous silicon layer has a structure including a multilayer amorphous silicon layer between a first electrode and a second electrode, and the first amorphous silicon layer is in contact with the first electrode. In a region where the silicon layer contains oxygen atoms and carbon atoms and has an optical band gap of 2.0 eV or more,
"~10" Q tx (7) The structure has resistivity and photoconductivity.

(Function) The first amorphous silicon layer has a high resistivity of 10"2 to io" Ωl, so it prevents charge injection from the electrode in the dark, and has photoconductivity, so it During irradiation, the generated carriers are sufficiently passed through. This makes it possible to obtain a very high S/N ratio. Further, compared to an amorphous silicon layer containing only oxygen atoms, an amorphous silicon layer containing oxygen atoms and carbon atoms has improved heat resistance, and a highly durable amorphous silicon photosensor can be obtained.

(Example) Embodiments will be described in detail below based on the drawings.

FIGS. 1 and 2 show an embodiment of the present invention, in which an amorphous silicon photosensor is applied as an image sensor. In FIGS. 1 and 2, 1 is a transparent glass substrate, and 2 is a first electrode, which is made of a transparent conductive film such as ITO or Sno. 3 is a first amorphous silicon layer, 4 is a second amorphous silicon layer, and 5 is a second electrode, which is made of a metal film such as At or Cr.

Next, a specific manufacturing method will be explained. First, 800 ITO films were formed on a transparent glass substrate 1 by vacuum evaporation, and first electrodes (individual electrodes) 2 were formed by photolithography. The individual electrodes had a size of 100μ×100μ, and the distance between the electrodes was 25μ. This substrate was set in a CVD apparatus, a mixed gas of Co3 and SiH was introduced, and 350 first amorphous silicon layers 3 were formed by glow discharge decomposition. At this time, the pressure was 1.0 Torr, and the substrate temperature was 250
℃, SUS mask used.

Next, SiH, is introduced, and a second
An amorphous silicon layer 4 with a thickness of 1.25 μm was formed.

Conditions such as pressure are the same as for the first amorphous silicon layer 3. Finally, as the second electrode (common electrode) 5, a SUS mask is made by vacuum evaporation to a width of 100 μm and a thickness of 50 μm.
00 AM membranes were formed. In addition, the formation of the pattern is
A mask may be used, or a film may be formed on the entire surface and photolithography may be used.

The amorphous silicon optical sensor formed in this way is
A very high S/N ratio (ratio of bright current to dark current at 100 n x illumination was 5×103) was obtained, and each element per individual electrode had uniform characteristics.

FIG. 3 shows Co to add oxygen atoms and carbon atoms.
The optical band gap of the first amorphous silicon layer 3 when using a mixed gas of 2 and SiH4 and the photoconductivity σ when irradiated with AMI (simulated sunlight) 100 W/a#,
b, Dark conductivity σ1 is plotted against the gas flow rate ratio of CO2/S x H4. Here, the horizontal axis is COx/S
The iH4 flow rate ratio is shown, and the vertical axis shows the conductivity and optical bandgap, respectively.

As the Co,/SiH4 gas flow rate ratio increases, the optical bandgap linearly widens. Also, the photoconductivity σ2
．． decreases until the gas flow rate ratio of COz/S i H4 reaches 5, and beyond that the decrease becomes slow and becomes 10-@(Ω
Ca1)-1, but the dark conductivity σ1 is Co,
/SiH4 gas flow rate ratio decreases rapidly up to about 2.5, and even above that the ratio is 1o-12 to 1O-15 (Ω3)-1
Therefore, the first amorphous silicon layer 3 containing oxygen atoms and carbon atoms and having photoconductivity in a region where the optical band gap is 2.0 eV or more has a resistivity in the dark. It shows a very high value of 1012~10LsΩcm,
It can be seen that the amorphous silicon photosensor has a sufficiently high ability to prevent charge injection from the electrode during dark times. In addition, AMI (simulated sunlight) 100mW, /aJ
The resistivity during irradiation showed a low value of 101-101Ω,
Since it has the function of facilitating the passage of carriers generated by light irradiation, it is possible to obtain a very high S/N ratio as an amorphous silicon photosensor.

Figure 4 shows the infrared absorption characteristics of amorphous silicon films doped with oxygen atoms and carbon atoms.
Absorption of 5i-0-5i was observed at 60a++-', indicating that oxygen atoms were contained. Although carbon atoms are not detected by this infrared absorption characteristic, their presence is confirmed by other analyses.

FIG. 5 shows the amount of hydrogen released when an amorphous silicon layer is heat-treated. The horizontal axis shows the heating temperature, and the vertical axis shows the amount of hydrogen released. The solid line indicates when only oxygen atoms are included; The broken line is the case where oxygen atoms and carbon atoms are included. It is recognized that the hydrogen release peak of the amorphous silicon layer containing oxygen atoms and carbon atoms has shifted to the high temperature side, and the heat resistance has improved.

Figure 6 shows an example of the spectral sensitivity characteristics of an amorphous silicon photosensor, where the horizontal axis shows the light wavelength and the vertical axis shows the charging current density (A/mu''). wavelength 45
A very uniform charge current density was obtained over 0-620 nm.

In addition, in those using conventional oxide films and nitride films,
However, it was necessary to use a very thin film of 100 layers, which made it difficult to make the characteristics of the device uniform.

In the present invention, since it has high photoconductivity, the first
The film thickness of the amorphous silicon layer 3 is 200 to 2000.

Since it can be formed thick, preferably from 300 to 700, there is an advantage that the device characteristics can be made uniform.

Note that, in addition to the glow discharge method, a sputtering method can also be applied to form the amorphous silicon layer. For example, to form an amorphous silicon film using the glow discharge method, gaseous substances such as SiH, Si, H, etc. are used as raw materials for silicon.
Preferably, gases such as II, SiF, 5iC1, etc. are used.

It can also be used diluted with hydrogen, helium, argon, or the like. In addition, when forming by sputtering, a silicon wafer is used as a target and sputtered in various gas atmospheres.

The additive gas for including oxygen atoms and carbon atoms in the amorphous silicon layer does not directly react with silicon raw materials such as SiH4, but is decomposed by energy such as glow discharge to generate oxygen atoms and carbon atoms. Oxygen-carbon compounds 1 such as co, co, etc.
Preferred examples include compounds of oxygen, carbon, and hydrogen. Incorporation of oxygen atoms and carbon atoms can be easily carried out by increasing decomposition energy such as glow discharge within a range that does not deteriorate the film quality of the amorphous silicon layer.

In addition, amorphous silicon films usually have n-type properties, but in order to obtain intrinsic or near-intrinsic properties, a trace amount of group III atoms must be mixed in using a gas such as B or H. do it.

As the first electrode, in order to obtain a Schottky junction and to obtain light transmission, for example, a very thin metal thin film such as Pt or Au or a silicide such as PtSi can be used, and as a material for the heterojunction, For example, ITO, SnO□
etc. can be used.

When a transparent conductive film such as ITO or SnO is used as the first electrode, the ITO etc. may be reduced during glow discharge decomposition of SiH4 and its properties may deteriorate;
In the case of a gas mixed with O2 or the like, the presence of oxygen gas during glow discharge suppresses the reduction of ITO and the like and prevents deterioration of characteristics.

As the second electrode, All, NiCr, Cr, M
Metals such as opW, Ag, and 'ri can be used.

Alternatively, it may have a two-layer structure of polycrystalline silicon and microcrystalline silicon.

In the optical sensor having the configuration shown in FIG. 1, by providing a light shielding mask made of Cr or the like on the light incident side, it is possible to obtain a sensor with even higher resolution.

In the example, only the amorphous silicon layer in contact with the first electrode contains oxygen atoms and carbon atoms, but in order to prevent injection of both electron and hole carriers, the amorphous silicon layer in contact with the second electrode contains oxygen atoms and carbon atoms. Oxygen atoms and carbon atoms may also be added to the layer.

Furthermore, in the embodiment, the case where a transparent substrate was used as the substrate was explained, but electrodes are formed with AI, Cr, Ni, etc. on a metal plate such as AJ, SUS, Ni, etc. or an insulating substrate, and these are used as the second electrode. Then, a second amorphous silicon layer, a first amorphous silicon layer, and a transparent electrode such as ITO are formed as a first electrode in order to fabricate an amorphous silicon photosensor. can.

(Effects of the Invention) As explained above, the present invention provides an amorphous silicon photosensor that is suitable for high speed and high density, has a very large S/N ratio, and has uniform element characteristics. can be provided.

[Brief explanation of the drawing]

FIG. 1 is a cross-sectional view of one embodiment of the present invention, FIG. 2 is a plan view of the same, and FIG. 3 is a gas flow rate of CO□/SiH of an amorphous silicon film added with oxygen atoms and carbon atoms. Figure 4 is an infrared absorption characteristic diagram. Figure 5 is a diagram showing the amount of hydrogen released during heat treatment. The figure is a diagram showing spectral sensitivity characteristics. 1... Glass substrate, 2... First electrode, 3...
- First amorphous silicon layer, 4... Second amorphous silicon layer, 5... Second electrode. Patent applicant Rico Co., Ltd. -Figure 1 Figure 2 Figure 3 Q) 2/sL+<+A#Pj Figure 4 Figure 5 Hot crab L friend ('C) λ (nm)

Claims (5)

[Claims] (1) A multilayered amorphous silicon layer is provided between a first electrode and a second electrode, and the first amorphous silicon layer in contact with the first electrode contains oxygen atoms and carbon atoms. , and in the region where the optical band gap is 2.0 eV or more, 10^
An amorphous silicon optical sensor characterized by having a resistivity and photoconductivity of 1^2 to 10^1^5 Ωcm. (2) The amorphous silicon optical sensor according to claim (1), wherein the amorphous silicon layer contains at least one of hydrogen atoms, halogen atoms, and deuterium atoms. (3) The layer thickness of the first amorphous silicon layer is 200 to 2
000 Å
) Amorphous silicon optical sensor described in section 2. (4) The amorphous silicon optical sensor according to claim (1), wherein the first electrode and the first amorphous silicon layer are a Schottky junction. (5) Claim (1) characterized in that the first electrode and the first amorphous silicon layer are a heterojunction.
Amorphous silicon optical sensor described in Section 1.