EP0114652B1

EP0114652B1 - Image pickup tube target

Info

Publication number: EP0114652B1
Application number: EP19840100470
Authority: EP
Inventors: Eisuke Inoue; Yasuhiko Nonaka; Masanao Yamamoto
Original assignee: Hitachi Ltd
Current assignee: Hitachi Ltd
Priority date: 1983-01-19
Filing date: 1984-01-18
Publication date: 1987-09-09
Also published as: JPS59132541A; JPH0522328B2; DE3466130D1; EP0114652A2; EP0114652A3

Description

Background of the invention

I. Field of the invention

The present invention relates to an image pick up tube target and, more particularly, to a structure of a Se-As-Te type chalcogen photoconductive film (Saticon (trade mark) film) with improved sticking characteristics.

II. Description of the prior art

Fig. 1 shows a section of a main part of a conventional image pickup tube target and a concentration distribution of selenium, arsenic and tellurium as major constituents of a photoconductive film.
Referring to Fig. 1, a transparent conductive film 2 containing Sn0₂ or ln ₂0₃ as a major constituent is formed on the rear major surface of a disc-like glass substrate 1. A very thin N-type transparent Ce0₂ conductive film 3 serving as a blocking layer is formed on the rear major surface of the transparent conductive film 2. A P-type photoconductive film 4 comprising a P-type Se-As-Te amorphous semiconductor film is formed on the rear major surface of the N-type transparent conductive film 3. A P-type Sb₂S₃ photoconductive film 5 serving as a beam landing layer is formed on the rear major surface of the P-type photoconductive film 4. The P-type photoconductive film 4 consists of first, second and third P-type photoconductive layers 4a, 4b and 4c. The first P-type photoconductive layer 4a comprises a P-type Se-As amorphous semiconductor film having an Se concentration of 97 to 98 wt% and an As concentration of 3 to 12 wt% and is formed on the rear major surface of the N-type transparent conductive film 3 to have a thickness of 30 to 60 nm. The second P-type photoconductive layer 4b comprises a P-type Se-As-Te amorphous semiconductor film having an Se concentration of about 67 wt%, an As concentration of 3 wt%, and a Te concentration of about 30 wt% and is formed on the rear major surface of the first P-type photoconductive layer 4a to have a thickness of about 60 nm. The third P-type photoconductive layer 4c is formed on the rear major surface of the second P-type photoconductive layer 4b such that a total thickness of the multilayer film 4 is set to be about 3900 nm, for example. The third P-type photoconductive layer 4c comprises a P-type Se-As amorphous semiconductor film wherein in the Se-As concentration distribution, the As concentration continuously changes from 20 to 30 wt% to 3±2 wt% over a thickness of 45±20 nm which starts from the interface between the second and third P-type photoconductive layers 4b and 4c. The 3±2 wt% As concentration remains unchanged as the thickness increases. The P-type photoconductive film 5 is formed on the rear major surface of the multilayer film 4. A light beam 6 is incident on the front major surface of the glass substrate 1, and a scanning electron beam 7 is supplied to the P-type photoconductive film 5.
In the image pickup tube target having the structure described above, the gradient As concentration layer as part of the third P-type photoconductive layer 4c serves as a carrier extraction layer for effectively and stably extracting carriers generated in the Te layer of the second P-type photoconductive film 4b. The gradient As concentration layer also serves to prevent the Te layer from being diffused, thereby preventing degradation of the voltage-photocurrent characteristic (V-I characteristic) forming part of evaluation criterion. The uniform 3±2 wt% As concentration layer contiguous to the gradient As concentration layer serves as a capacitive layer for storage of the carriers. The P-type photoconductive layer 4c including the gradient As concentration layer and the uniform As concentration layer is the most important layer to determine quality of the electrical characteristics of the target in use.
However, when a highly luminous object or a still object is picked up with an image pickup tube having the above-mentioned target, a so-called sticking phenomenon occurs wherein a previous image sticks to the present image. The occurrence of this phenomenon is mainly dependent on the P-type photoconductive film 4.
Especially, when a Saticon (trade mark) film is used, the sticking phenomenon largely depends on the content of highly concentrated arsenic in the third P-type photoconductive layer 4c. With the content of arsenic decreased, the sticking can be suppressed but sufficient extraction of the carriers from the Te layer cannot be sustained. As a result, a practically sufficient signal current cannot be obtained.
EP-A-0 067 015 describes an image pickup tube target in which the multilayer photoconductive film has a first amorphous semiconductor layer mainly made of Se and As, a second amorphous semiconductor layer mainly made of Se, Te and As, a third amorphous semiconductor layer mainly made of Se, As and on the fourth layer side doped with In₂0₃, and a fourth amorphous semiconductor layer mainly made of Se and As with an average concentration of As being lower than that in the third layer. So a layer doped with a dopant (in₂O₃) having negative space charges in selenium is formed as a sublayer of the third amorphous semiconductor layer in contact with the fourth amorphous semiconductor layer and not in contact with the second amorphous semiconductor layer.

Summary of the invention

It is, therefore, an object of the present invention to provide an image pickup tube target capable of suppressing the sticking phenomenon by optimizing the doped layer position and compensating for a decrease in carrier extraction effect due to a decrease in arsenic content of the high arsenic concentration layer.
In order to achieve the above object of the present invention, there is provided an image pickup tube target comprising: a transparent substrate; a first transparent conductive film formed on a rear major surface of said transparent substrate; a second transparent conductive film serving as a blocking layer and formed on a rear major surface of said first transparent conductive film; a multilayer photoconductive film which has a first amorphous semiconductor layer mainly made of Se and As, a second amorphous semiconductor layer mainly made of Se, Te, and As, a third amorphous semiconductor layer mainly made of Se and As, and a fourth amorphous semiconductor layer mainly made of Se and As with an average concentration of As being lower than that in the third layer, in the order named, including a sublayer doped with a dopant having negative space charges in selenium in contact with the fourth amorphous semiconductor layer and not in contact with the second amorphous semiconductor layer, which multilayer photoconductive film is formed on a rear major surface of said second transparent conductive film to have a predetermined thickness; and a single photoconductive film formed on a rear major surface of said fourth amorphous semiconductor layer of said multilayer photoconductive film; said sublayer being formed across the interface between said third and fourth amorphous semiconductor layers of said multilayer photoconductive film; and said interface being defined as a boundary where the arsenic concentration is decreased by about 90% of the arsenic concentration difference between said third and fourth amorphous semiconductor layers.
The dependent claims set out particular embodiments of the invention.

Brief description of the drawings

Fig. 1 is a diagram for explaining a conventional image pickup tube target;
Fig. 2 is a diagram for explaining an image pickup tube target according to an embodiment of the present invention;
Fig. 3 is a graphical representation useful in explaining effects of the present invention;
Figs. 4a to 4c are diagrams showing various positions of the indium-doped layer; and
Fig. 5 is a diagram showing an actual contour of the interface between the third and fourth amorphous semiconductor layers.

Detailed description of the preferred embodiments

Fig. 2 shows a section of a main part of an image pickup tube target according to an embodiment of the present invention, and a diagram of a Se-As-Te concentration distribution. In Figs. 1 and 2, the same reference numerals are used to denote the same parts, a detailed description of which will be omitted.
Referring to Fig. 2, a third P-type photoconductive layer 4d as a carrier extraction layer is formed on the rear major surface of a second P-type photoconductive layer 4b as a carrier generating layer to have a thickness of 12±6 nm. The layer 4d comprises a P-type Se-As amorphous semiconductor film having a concentration distribution such that the Se concentration is 75±5 wt% and the As concentration is 25±5 wt%. A fourth P-type photoconductive layer 4e serving as a capacitive layer is formed on the rear major surface of the third P-type photoconductive layer 4d to have a thickness such that a multilayer film 4' has a total thickness of, for example, about 3900 nm. The fourth P-type photoconductive layer 4e comprises a P-type Se-As amorphous semiconductor film having a concentration such that the Se concentration is 97±2 wt% and the As concentration is 3±2 wt%. In₂0₃ having negative space charges in selenium is doped to a thickness of 3 to 30 nm at a concentration of 100 to 3,000 wtppm across an interface between the third and fourth P-type photoconductive layers 4d and 4e, so that an In₂0₃-doped layer 4f is formed not to contact the Te layer of the second P-type photoconductive layer 4b. Although the layer 4f is illustrated as doped with ln ₂0₃, other dopants such as Mo0₂ or a mixture of In₂0₃ and Mo0₂ which have negative space charges in selenium may also be used.
In the P-type multilayer film 4' having the structure described above, the arsenic content (concentrationxthickness) in the third P-type photoconductive layer 4d as the carrier extraction layer is decreased to 1/3 to 1/6 of that of the conventional image pickup tube target. In addition, the doped layer 4f of In_z0₃, Mo0₂ or mixture thereof which has negative space charges in selenium and which has carrier extraction capability is formed across the interface between the third and fourth P-type photoconductive layers 4d and 4e, so that the carrier extraction efficiency is greatly improved and the sticking phenomenon can be greatly decreased. In this case, when the arsenic content of the carrier extraction layer is decreased to about 1/6 or lower, the effect for preventing tellurium from being diffused from the second P-type photoconductive layer 4b is impaired. Therefore, the arsenic content in a rectangular concentration distribution cannot be decreased to about 1/6 or lower.
Fig. 3 shows the relation between the position of the indium-doped layer 4f and the sticking contrast for various In₂0₃ doping contents (wtppm . nm) as defined by concentration y (wtppm)xthickness x (nm). Curves 1 to 4 correspond to doping contents of 90000 wtppm - nm (3000 wtppmx30 nm), 15000 wtppm . nm (1000 wtppmx15 nm), 8000 wtppm · nm (750 wtppmx12 nm) and 300 wtppm - nm (100 wtppmx3 nm), respectively. Points P1, P2 and P3 correspond to positions of the indium-doped layer as shown in Figs. 4a, 4b and 4c, respectively. It will then be appreciated that when the indium-doped layer is formed across the interface X, preferably, substantially centered to the interface X, all the characteristic curves 1 to 4 representative of 100 to 3000 wtppm ln ₂0₃ doping concentrations and 3 to 30 nm indium-doped layer thicknesses fall in an allowable sticking contrast range as hatched.
In the foregoing embodiment, the ln ₂0₃ layer is formed in a thickness region having as a center the interface or boundary X where the arsenic content (25±5 wt%) of the third P-type photoconductive layer 4d is abruptly decreased to the arsenic content (3±2 wt%) of the fourth P-type photoconductive layer 4e. In effect, however, the arsenic content gradually decreases as shown at solid line or dotted line in Fig. 5. In this case, the In ₂0₃ layer may be formed in a thickness region having as a center a point where the arsenic concentration of the third P-type photoconductive layer 4d is decreased to 10% of an arsenic concentration difference between the third and fourth P-type photoconductive layers 4d and 4e. This point is also defined as interface or boundary X in this application. Further, the total thickness of P-type photoconductive layer 4' is not limited to 3900 nm and the effects of the present invention can be attained irrespective of the total thickness. For example, the total thickness may be 5900 nm with the photoconductive layer 5 ending at 6000 nm.
In a modification of the embodiment described above, In_z0₃, Mo0₂ or a mixture thereof is doped in a highlight sticking prevention Saticon Film (Japanese Patent Application No. 55-157084) doped with Li-F across an interface between the first and second P- type photoconductive layers 4a and 4b as shown at dotted line in Fig. 2, to form a doped layer 4f. In this modification, the sticking phenomenon can be prevented in the same manner as in the above embodiment. In addition, the sticking phenomenon which results from picking up of a highly luminous object can also be suppressed.
As has been described, according to the image pickup tube target of the present invention, the sticking phenomenon due to highly luminous incident light can be greatly decreased to obtain a high-quality image.
The present invention is not limited to the particular embodiments described above, and various changes and modifications may be made within the spirit and scope of the present invention.

Claims

1. An image pickup tube target comprising:

a transparent substrate (1);

a first transparent conductive film (2) formed on a rear major surface of said transparent substrate (1);

a second transparent conductive film (3) serving as a blocking layer and formed on a rear major surface of said first transparent conductive film (2);

a multilayer photoconductive film (4') which has a first amorphous semiconductor layer (4a) mainly made of Se and As, a second amorphous semiconductor layer (4b) mainly made of Se, Te, and As, a third amorphous semiconductor layer (4d) mainly made of Se and As, and a fourth amorphous semiconductor layer (4e) mainly made of Se and As with an average concentration of As being lower than that in the third layer (4d), in the order named, including a sublayer (4f) doped with a dopant having negative space charges in selenium in contact with the fourth amorphous semiconductor layer (4e) and not in contact with the second amorphous semiconductor layer (4b), which multilayer photoconductive film (4') is formed on a rear major surface of said second transparent conductive film (3) to have a predetermined thickness; and

a single photoconductive film (5) formed on a rear major surface of said fourth amorphous semiconductor layer (4e) of said multilayer photoconductive film (4'), characterized in that

said sublayer (4f) is formed across the interface between said third and fourth amorphous semiconductor layers (4d, 4e) of said multilayer photoconductive film (4') and that said interface is defined as a boundary where the arsenic concentration is decreased by about 90% of the arsenic concentration difference between said third and fourth amorphous semiconductor layers (4d, 4e).

2. An image pickup tube target according to claim 1 wherein the dopant is In₂0₃.

3. An image pickup tube target according to claim 1 wherein dopant is Mo0₂.

4. An image pickup tube target according to claim 1 wherein dopant is a mixture of In₂0₃ and Mo0₂.

5. An image pickup tube target according to one of claims 1 to 4, wherein said doped sublayer (4f) is centered to said interface.

6. An image pickup tube target according to one of claims 1 to 5, wherein said doped sublayer is formed such that indium oxide is doped to a concentration of 100 to 3,000 wtppm to a thickness falling within a range of 3 to 30 nm.

7. An image pickup tube target according to one of claims 1 to 6 further comprising an Li-F sublayer formed across an interface between said first and second amorphous semiconductor layers.