JPH0334666B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a photo sensor used in a wide range of photoelectric conversion devices for image information processing such as facsimile transceivers and character reading devices.

Conventionally, one-dimensional photodiode-type long sensor arrays using crystalline silicon, which have been commonly used, have a limit on the length of the sensor array due to the size of the single crystal that can be manufactured and limitations on processing technology, and the yield is low. It also had some low points. Therefore, when the original to be read has a width of 210 mm, which is equivalent to four versions, it has generally been done to reduce the original image to a sensor array using a lens system and then read the image. This method of using a lens optical system is
Not only does it make it difficult to miniaturize the photodetector, but because the area of each photodetector cannot be large, a large amount of light is required to obtain a sufficient output signal for a sufficient input optical signal, and the reading time becomes long. At present, it is used for low-speed applications and applications that do not require high resolution.

In addition, in order for the designed and manufactured photo sensor to have good signal selection characteristics, it is necessary to efficiently extract the secondary current optical signal current that occurs when information enters the photo sensor's photoreceptor window. It is necessary to ensure ohmic contact between a pair of electrodes sandwiching the photoconductive layer and the photoconductive layer.

It is also required to obtain a sufficiently large secondary current (optical signal current) and sufficiently small dark current and low-illuminance region current compared to the noise generated by the drive circuit for driving the photo sensor.

In order to obtain a sufficiently large optical signal current, it is necessary to apply a relatively large electric field (about 10 ⁴ V/cm or more) to the device, but when an electric field of this strength is applied, the space charge is limited. Superlinear currents such as electric currents and breakdown currents (I∝V ⁿ :n≧
2), dark current and low-light region current increase, reducing the signal-to-noise ratio.

The present invention has been made in view of the above points, and an object of the present invention is to provide a photo sensor that solves the drawbacks of conventional photo sensors.

Another object of the present invention is to provide a photo sensor that can obtain a sufficiently large output signal even with a weak optical signal and can be read at high speed.

Furthermore, it is another object of the present invention to provide a photo sensor that can be easily lengthened.

The photo sensor of the present invention includes: a photoconductive layer made of an amorphous conductive material containing at least one of hydrogen and a halogen element and having silicon as a matrix; a pair of electrode layers provided for the photoconductive layer; A photo sensor comprising a sub-ohmic layer provided between the photoconductive layer and at least one of the pair of electrode layers, wherein the sub-ohmic layer contains at least one of hydrogen and a halogen element and impurities of 100 to 5000 atomic ppm. The layer has a thickness of 25 to 400 Å and is made of an amorphous semiconductor material containing silicon as a matrix, and has an electric field strength of 10 ⁴ V/cm or more and 10 ⁵ V/cm when a voltage is applied to the pair of electrode layers. cm or less, it exhibits a photocurrent-voltage characteristic of I _P ∝V ^N ; 1≦n<2 (I _P is photocurrent, V is applied voltage), and also exhibits a non-ohmic dark field with limited dark current. It is characterized by having current-voltage characteristics.

In this way, by creating a photo sensor structure with a sub-ohmic layer provided in contact with the optically transparent electrode on the side where optical information is input to the photoconductive layer, the signal current can be reduced in the dark or under low illuminance. A light-receiving window that can be used to limit and provide optical information, extract the optical signal current without loss, particularly efficiently extract the secondary current optical signal, and provide a sufficiently high signal-to-noise ratio and resolution. An elongated photo sensor array in which photo sensors of a minute area are arranged can be manufactured very easily.

In addition, in the present invention, a sub-omic layer,
The photoconductive layer and the ohmic layer are made of an amorphous semiconductor (hereinafter referred to as a) containing at least one of hydrogen (hereinafter referred to as H) and halogen atoms (hereinafter referred to as X).
- Since the photo sensor uses a material (referred to as "semiconductor"), an elongated photo sensor that is the same size or longer than the original image to be read, for example, can be manufactured easily and with a high yield. Further, by employing amorphous silicon (hereinafter referred to as a-Si) as the a-semiconductor material, it is possible to extract an efficient optical signal current as an output signal. In particular, since an a-semiconductor photoconductive material containing 1 to 30 atomic % of at least one of H and can be read accurately.

In the present invention, the layer of a-semiconductor material containing at least one of H and Ru.

Therefore, the formed layer can be manufactured uniformly and uniformly over a large area, and has good electrical characteristics.

Furthermore, in the deposition method that utilizes glow discharge energy, the layer thickness can be controlled to the order of several angstroms, so that laminated photo sensors can be mass-produced accurately and reproducibly according to the desired element design. Furthermore, since it is sufficiently easy to manufacture a defect-free elongated photo sensor over a large area with good yield, it is also easy to obtain an elongated photo sensor of the same size.

Alternatively, since a-Si containing at least one of H and X is used as the material forming the element, the photo sensor produced has extremely high light absorption efficiency in the visible light region, has excellent photoconductivity in the visible light region,
Furthermore, since it has excellent heat resistance and abrasion resistance, it can be effectively used as a contact type reading sensor. Furthermore, since the above-mentioned materials are non-polluting, there is almost no need to consider the effects on the human body during the manufacturing process (in the process of use), and they are also excellent in this respect. The photo sensor of the present invention is excellent in its high performance and reliability.

1 to 1 deposited on a predetermined support by the glow discharge energy used in the present invention.
A layer of an a-semiconductor material containing ³⁰ atomic% of H ^or The conductivity type can be easily changed in a predetermined manner by doping. In the present invention, as X, F, cl, Br, etc. are applied.
F is particularly preferred in the present invention.

By laminating layers of such a-semiconductor materials, the photo sensor of the present invention has extremely good characteristics, can be manufactured in a large area with high yield, and has high reliability. It is also excellent in terms of signal amplification factor.

In the present invention, for example, the sub-ohmic layer provided as a layer sandwiched between the electrode on the light irradiation side and the photoconductive layer is a layer that forms a sub-ohmic junction with the electrode and the photoconductive layer. An ohmic contact (sub-ohmic contact) is a junction that does not exhibit clear rectifying characteristics, nor does it exhibit complete ohmic characteristics.
In order to make this characteristic more clear, when the sub-ohmic layer of the present invention is provided between the electrode and the photoconductive layer, and when a conventional ohmic layer or a rectifying layer is used instead of the sub-ohmic layer, The V-I characteristics (voltage-current characteristics) in the case where the voltage-current characteristics are provided are shown in FIGS. 3 to 5, respectively. In each figure, the thickness of the photoconductive layer is approximately 1 μm.

Figure 4 shows the V when a conventional ohmic layer is provided.
−I characteristic, and I _P and _ID are photocurrent and dark current, respectively. The typical V-I characteristic shown in this figure behaves ohmicly up to an electric field strength of approximately 10 ³ V/cm, but rapidly changes to a superlinear region (I∝V ⁿ ; n≧ 2), and space-limited current and pre-knockdown current flow.
This will lower the S/N ratio. Therefore, if a conventional ohmic layer is provided, it will not be possible to obtain satisfactory operation as a photo sensor under an electric field strength of 10 ⁴ V/cm or more, which is required to obtain a sufficiently large photocurrent.

Figure 5 also shows the V-
This is an I characteristic. In this conventional example, a sufficient SN ratio cannot be obtained.

On the other hand, when the sub-ohmic layer of the present invention shown in FIG. ³ is provided, the non-ohmic V-
Realizes L _D characteristics (I _D ∝Vn: n<1) and relatively good ohmic V-I _P characteristics (I _P ∝V ⁿ : 1≦n<2) even at 10 ⁴ V/cm or more. do. By having a sub-ohmic junction in this way, it transitions not only in the normal operating voltage region but also in the superlinear region (I∝V ⁿ :n≧2) where space-limited current and breakdown current occur. The electric field strength is improved by more than one order of magnitude, and the usable voltage range is sufficiently widened. In addition, a photocurrent of sufficient value can be obtained in that voltage range to be extracted as a signal.

In this way, another feature of the V-I characteristic of a photo sensor with a sub-ohmic junction is that the photocurrent I _P is a function of V less than the square of V and greater than the first power, and in the normal operating voltage range. Of course,
It has a wide voltage range up to the region where a relatively large electric field is generated where a space charge limited current or breakdown current occurs, and a photocurrent of sufficient value to be extracted as a signal can be obtained in that voltage range.

The characteristic sub-ohmic layer constituting the photo sensor of the present invention can withstand electric field strengths up to 10 ⁵ V/cm.
It has a non-ohmic current-voltage characteristic that limits the current in the dark or under low illuminance, and achieves a contact state with relatively good ohmic current-voltage characteristics when irradiated with light.

The current-voltage characteristics of this contact state depend on the applied voltage and the amount of light irradiation, and at the applied voltage, as mentioned above, the current-voltage characteristics are in the superlinear region (IV ⁿ , n≧2).
It is recognized that the electric field strength that transitions to is improved by more than one order of magnitude.

Although the understanding of the operating principle of this sub-ohmic layer is currently uncertain, the increase in the electric field distributed to the sub-ohmic layer as the resistance of the photoconductive layer decreases due to light irradiation causes the sub-ohmic layer to The first step is to establish ohmic contact between the photoconductive layer and the photoconductive layer.
It can be considered. In addition, by irradiating light, sub-
It is also highly conceivable that the junction between the ohmic layer and the electrode interface functions as a barrier modulation that facilitates charge injection.

As described above, in the present invention, the above-mentioned sub-ohmic layer is provided between the electrode on the light irradiation side and the photoconductive layer in contact with the electrode, thereby replacing the conventional sub-ohmic layer. Compared to photo sensors in which an ohmic layer is provided in place of the sub-ohmic layer, or a rectifying layer is provided in place of the sub-ohmic layer, the range of application is greatly expanded, and the normally required photocurrent value is 10. A photocurrent of ^-3 Acm ^-2 or more can be stably obtained with good reproducibility with respect to the applied voltage.

When electrons are used as optical signal current carriers, the ohmic layer and sub-ohmic layer contain P or/and
An n-type a-Si layer doped with _As atoms is applied. In the ohmic layer, P is added to the Si host atom.
or/and _As atoms at 1000-10000ppm, 400Å
It is desirable that the layer thickness be greater than or equal to that. On the other hand, in the sub-ohmic layer, the doping amount of the impurity is
A layer thickness of about 25 to 400 Å at 100 to 5000 ppm is desirable. The optimum doping amount and layer pressure in the sub-ohmic layer are highly dependent on the electrodes used, and are ITO (In ₂ O ₃ SNO ₂ = 20:
In oxide transparent electrodes such as 1), the layer thickness is 50 to 200 mm.
Å, doping amount 1000-5000ppm, Au, Pt,
In a metal electrode such as W that is easily compatible with the n ⁺ layer, the layer thickness is 25 to 100 Å and the doping amount is 100 to 1000 ppm. The optimum characteristics of the sub-ohmic layer can be adjusted by changing the thickness and doping amount of the sub-ohmic layer.

The layer made of an amorphous semiconductor material containing 1 to 30 atomic % of hydrogen and/or halogen atoms and having silicon as its matrix and deposited by glow discharge energy used in the present invention can be prepared by the following method. Manufactured by.

A conventional method is to apply RF glow discharge energy or DC glow discharge energy to a gas containing SiH ₄ , SiF ₄ , or SiCl ₄ as its main components, decompose the gas, and deposit it onto a substrate that has been subjected to a predetermined pretreatment. The plasma CVD (Chemical Vapor Deponsition) method is commonly used. In addition, there are other methods for forming an amorphous semiconductor material layer with similar characteristics.
A sputtering method or an in-opplantation method in a gas composition atmosphere containing H ₂ or/and halogen atoms can also be used. A layer of amorphous semiconductor material produced in this way has few levels in the forbidden band (~10 ¹⁰ cm ^-3 ),
The conductivity type and layer conductivity can be easily controlled by impurity doping. In addition, the photo sensor of the present invention is suitable as a material for forming a document reading device, an image pickup device, or a photoelectric exchange device because it has excellent photoconductivity and spectral sensitivity that can be approximated to the visual sensitivity.

The properties of the above amorphous semiconductor material layer are sensitive to discharge power density, substrate temperature, gas pressure, etc.
Carelessly controlled.

In the RF glow discharge method, the discharge power density is 1 W/cm ² or less, preferably 0.1 W/cm ² . Substrate temperature is 100 to 350℃, gas pressure is 0.01
Controlled constant within ~1Torr.

The conduction type is most easily controlled by SiH ₄ , SiF ₄ ,
PH ₃ or AsH ₃ gas is added to gas and gasified silicon compounds such as SiCl ₄ for n-type control.
This can be achieved with good reproducibility by mixing and using a predetermined amount.

Next, the present invention will be specifically explained with reference to the drawings.

FIG. 1 schematically shows the structure of one preferred embodiment of the photo sensor of the present invention.

Figure 1a is a schematic partial plan view, and Figure 1b is a schematic partial plan view.
is a schematic sectional view taken along the dashed-dotted line AB in Figure 1a, and Figure 1c is a schematic cross-sectional view taken along the dashed-dotted line in Figure 1a.
FIG. 3 is a schematic cutaway diagram when cut along XY. 1st
The photo sensor 101 shown in the figure has a structure in which a large number of pixels (only five are shown in the figure) of equal pitch and size are arranged in a horizontal row.

A predetermined number of translucent pixel electrodes 102 are formed on a translucent substrate 104 with a predetermined width and at predetermined intervals, and a portion excluding a portion where a light receiving window 105 is formed on the pixel electrode 102 A light-shielding layer 103 is formed from a non-light-transmitting material.

The light receiving window 105 is the photo sensor 101 to be created.
Determine the pixel size of the image and its resolution. Therefore, when providing the light shielding layer 103 on the pixel electrode 102, it is precisely formed so that the light receiving window 105 of the designed size is formed.

On the pixel electrode 102 provided with the light-shielding layer 103, a sub-ohmic layer 106 is provided in a strip-like manner in the arrangement direction of the pixel electrodes 102 with a predetermined thickness so as to cover the light-receiving window 105.

On the sub-ohmic layer 106, a photoconductive layer 107, an ohmic layer 108, and a common electrode 1 are sequentially formed.
09 is laminated.

In place of the ohmic layer 108, a sub-ohmic layer having characteristics similar to the sub-ohmic layer 106 may be provided.

Next, a detailed description will be given of the production of the photo sensor 101 shown in FIG.

Transparent substrate 1 such as a glass substrate or resin film
ITO (In ₂ O ₃ :SnO ₂ =
20:1) Layer 102 with a thickness of 2000 Å, followed by light shielding layer 1
03 was formed by vacuum evaporation of Cr, and then etched into a pixel pattern.Furthermore, in order to form a light receiving window 103, the light shielding layer in that portion was also removed by etching. The cleaned substrate 104 with the pixel pattern thus formed was loaded into an RF glow discharge device and heated with 10% SiH ₄ gas (H ₂ based).
5SCCM and 1000ppm _PH3 gas ( _H2 base)
With 0.5SCCM flowing and the substrate temperature maintained at 200℃, a glow discharge of 20W RF power was generated.
Connect for 2.5 minutes and connect the sub-ohmic layer 106.
After forming 100 Å, remove the substrate from the device and
The sub-ohmic layer 106 was etched so as to leave only the light-receiving window portion. After thoroughly cleaning and drying the substrate, the above substrate was loaded into the RF glow discharge device again, 10SCCM of 10% SiH ₄ (H ₄ base) was flowed, and glow discharge was performed for 6 hours at a substrate temperature of 200℃ and RF power of 20W. to form a photoconductive layer with a thickness of 1.5μ,
Next, while reducing the SiH ₄ gas from 10 to 5 SCCM, 1000 ppm PH ₃ gas (H ₂ base) was introduced to 8 SCCM to continue glow discharge, and glow discharge was further performed for 30 minutes to reduce the ohmic layer 107 to 800 Å.
After forming a thick layer, the substrate was taken out from the glow discharge device and an Al layer was formed on the entire surface in a metal vapor deposition bath for forming the upper common electrode 109 to a thickness of about 2000 Å. after that,
As shown in FIG. 1c, the photoconductive layer 107, the ohmic layer 108, and the Al layer common electrode 109 were etched in a striped pattern.

In this way, an array-shaped photo sensor was formed. This photo sensor has a resolution of 8 pel/mm and a pixel count of 1792, making it possible to read full lines in the short direction of an A4 sheet. When a DC bias voltage was applied between each pixel electrode and the common electrode and light was irradiated, a photocurrent signal corresponding to the amount of light irradiation could be extracted from each pixel electrode terminal. At this time, the photocurrent value in each pixel was almost constant, indicating that it was an excellent photo sensor with uniform characteristics. Although it is preferable in manufacturing that all silicon-based a-semiconductor layers constituting the device be formed continuously, the photoconductive layer 107 of the present invention is made of an a-semiconductor that is not normally doped. is preferably used, and (a) 10 to 100 ppm of p or doping is applied in order to increase the photocurrent value (the dark current value also increases). (b) Doping with 10 to 100 ppm of B can be carried out in order to reduce the dark current (also reduce the photocurrent).

The photoconductive layer 107 has a layer thickness of 0.3 to 10 microns, preferably 0.5 to 3 microns. The optimum value for the layer thickness is determined by the ease of fabrication (pinholes, extraction time, etc.), the degree of electrolytic spread depending on the distance between pixels (elements) and layer thickness (pixel distance/layer thickness > 5/1), and incident light absorption. It is determined by the degree (absorption coefficient α=10 ⁴ to 10 ⁵ (cm ^-1 ): 400 to 700 nm). On the other hand, the element of the present invention has a structure that extracts secondary current without loss, and its efficiency G is as follows: G=μτE/l where μ: carrier mobility (cm/V・sec) τ: carrier lifetime (sec ) E: Electric field strength of the photoconductive layer 107 (V/cm) l: Electrical thickness of the photoconductive layer 107 (cm) Silicon containing 1 to 30 at.% of hydrogen and/or halogen atoms produced under the conditions described above is The layer of amorphous semiconductor material used as the base is approximately μ=0.1,
τ=10 ⁻⁶ . Therefore, when l = 1 μ (= 10 ^-4 cm), assuming almost perfect ohmic contact during light irradiation, V = 1 V (= 10 ⁴ V/cm), G = 10, V = 10 V.
G = 100 is obtained.

As described above, for a constant layer thickness, the secondary current efficiency G increases as the applied voltage increases, but since it is usually easy to use an applied voltage of about 0.5 to 100 V, the photoconductive layer 107 From this point of view, about 0.3 to 10μ, preferably 0.5 to 3μ, is selected.

When an ohmic layer 108 is used on the side opposite to the light incident side of the photoconductive layer 107, the amount of impurity doped into this layer is selected depending on the metal material of the electrode further laminated on the ohmic layer 108. It will be done.

On the ohmic layer 108, A, Mo, Au,
Common electrode 10 selected from many metal materials such as Ti
9 is formed by means such as a vacuum evaporation method. The laminated pattern thus obtained is formed by an etching process into stripes having approximately the width of the light-receiving window, as shown in FIGS. 1a, b, and c.

FIG. 2 shows another configuration in which the pixels (elements) are separated by a pattern of the upper non-transparent electrode 207, and the light incident electrode 203 is a common transparent electrode. .

The functions of each layer are equivalent to those in FIG. A translucent electrode 203, a sub-ohmic layer 204, and a photoconductive layer 205 are common to each pixel (element), while a non-transparent electrode 206 and an ohmic layer 206 are separated for each pixel. Furthermore, it is preferable to separate all of the silicon-based layers 204, 205, and 206 for each pixel because this further improves the reliability of the device.

The photo sensor fabricated with the structure shown in Figures 1 and 2 can easily achieve G = 10 to 100 (depending on the applied voltage), and the photocurrent under the same light intensity of a photodiode element with the same light receiving area can be easily achieved. It is recognized that the current is significantly higher than the photocurrent, the current is extremely small in the dark and in the low illumination range, and the device has stable operating characteristics with a large S/N ratio.

[Brief explanation of drawings]

FIGS. 1a, b, and c are schematic diagrams for explaining preferred embodiments of the present invention, and FIG.
is a plan view, FIG. 1b is a sectional view taken along the dashed-dotted line AB shown in FIG. 1a, FIG. 1c is a sectional view taken along the dashed-dotted line XY shown in FIG. 1a, and FIG. FIG. 7 is a schematic cross-sectional view for explaining another example embodiment. Third
The figure is a graph for explaining the voltage-current characteristics of the photo sensor according to the present invention, where I _P and _ID mean photocurrent and dark current, respectively. Also, the dotted line is V-
I _P and V- _ID are tangents at the origin of each characteristic curve. Furthermore, FIG. 4 is a graph for explaining the voltage-current characteristics when an ohmic layer is provided instead of the sub-ohmic layer, and FIG. 5 is a graph when a rectifying layer is also provided.

Claims (1)

[Scope of Claims] 1. A photoconductive layer made of an amorphous semiconductor material containing at least one of hydrogen and halogen elements and having silicon as its base material, and a pair of electrode layers provided for the photoconductive layer. , a photo sensor comprising a sub-ohmic layer provided between the photoconductive layer and at least one of the pair of electrode layers, the sub-ohmic layer containing at least one of hydrogen and a calogen element and impurities of 100 to 5000 atomic ppm. A layer with a thickness of 25 ^{to 400} Å made of an amorphous semiconductor material based on silicon containing /cm or less, it exhibits the photocurrent-voltage characteristic I _P ∝V ⁿ ; 1≦n<2 (I _P is photocurrent, V is applied voltage), and it also exhibits a non-ohmic characteristic with limited dark current. A photo sensor characterized by having dark current-voltage characteristics.