TARGET OF IMAGE PICKUP TUBE

Abstract: A target of image pickup tube, which comprises a transparent substrate (1), a transparent conductive film (2), a p-type photoconductive film (3) made mainly from amorphous Se, an n-type conductive film capable of forming a rectifying contact at the interface with the p-type photoconductive film, using the rectifying contact as a backward bias, characterized in that the p-type photoconductive film (3) contains at least a region (b) containing over 35% to 60% by weight of Te in the film thickness direction and at least a region containing 0.005 to 5% by weight of at least a material capable of forming shallow levels in the amorphous Se in the film thick­ ness direction, has good after-image characteristics even if operated at a high temperature (Fig. 2).

Description: This invention relates to a target of an image pickup tube for television, and more particularly to a target of an image pickup tube capable of reducing the after-image when operated at a high temperature as described in the first part of claim 1. Amorphous selenium (Se) has a photoconductivity and generally also has a p-type conductivity, forming a rectifying contact with an n-type conductive material. Thus, a photodiode type target of image pickup tube can be made from the amorphous Se on the basis of these characteristics. However, the amorphous Se has no sensitivity to the long wavelength of light and it has been a practice to add tellurium (Te) to a region of a Se layer to improve the sensitivity to the long wavelength of light (US-A-3,890,525 and US-A-4,040,985). Furthermore, it has been also proposed to add a specific fluoride to a region of the p-type photoconductive layer to improve a response when an incident light of high intensity is cut off (US-A-4,330,733). Fig. 1. shows one example of a target structure according to the prior art, wherein numeral 1 is a transparent substrate, 2 a transparent conductive film, 3 a p-type photoconductive layer made from Se-As-Te, 4 a p-type photoconductive layer made from Se-As, and 5 a landing layer of scanning electron beam made from porous Sb₂S₃. Te is a component for enhancing the sensitivity to red light, as described above, and arsenic (As) is a component for increasing the viscosity of an amorphous film composed mainly of Se and enhancing the thermal stability. With this structure the target can act as a photodiode type to block the injection of holes and scanning electrons and thus can have such imaging characteristics as less dark current and less lag. In Patent Abstracts of Japan, unexamined applications, E field, vol. 9, no. 69, March 29, 1985, The patent office Japanese government, page 12E 305 Kokai-no JP-A-59-205 135 (Sony), there is disclosed a target of an image pickup tube comprising the features of the precharacterizing part of claim 1. US-A-4 563 611 describes an image pick-up tube target comprising a transparent electrode on a glass substrate and a first layer of a p-type photoconductive film on a very thin n-type conductive film composed of e.g. CeO and forming a rectifying contact with said p-type film, and further a second layer of the p-type photoconductive film of a Se-As-Te system containing 64 ± 4 wt. % of Se, 3 ± 0,5 wt. % As and 33 ± 2 wt. % of Te. The target of image pickup tube according to the prior art can have good imaging characteristics under the normal operating conditions, but still has such a drawback as an increased after-image when operated at a high temperature, because no thorough consideration is given to a higher temperature during the operation of image pickup tubes. An object of the present invention is to provide a target of image pickup tube having an improved photoconductive film made mainly from Se and capable of reducing the after-image of target even if operated at a high temperature. Said object is attained, according to the present invention, by a target of an image pickup tube as claimed in claim 1. In a target of image pickup tube comprising at least a p-type photoconductive film made mainly from amorphous Se and a transparent conductive film and the claimed film of n-type conductive oxide that forms a rectifying contact at the interface with the p-type photoconductive film using the rectifying contact as a reverse bias, the after-image when operated at a high temperature can be reduced in the present invention by using the p-type photoconductive film having a region containing over 35% to 60% by weight of Te in the film thickness direction (which will be hereinafter referred to as region of high Te concentration) and a region containing 0.005 to 5% by weight of at least one material capable of forming shallow levels in the amorphous Se in the film thickness direction. Fig. 2 shows, as one embodiment of the present invention, a profile of component distribution in the part corresponding to the layer 3 of Fig. 1 showing the structure in principle of a target of image pickup tube according to the prior art, where the ratio of components is or will be expressed by weight. In Fig. 2, only the part corresponding to layer 3 of Fig. 1 is shown, but in this embodiment a film of n-type conductive oxide for a rectifying contact is provided between the layers 2 and 3 . In the embodiment of Fig. 2, no Te exists at the position of zero film thickness which corresponds to the interface with the transparent conductive film (region a ) in the film thickness direction, and the concentration of Te steeply rises at the position corresponding to 50 nm (500 Å) and Te is uniformly distributed at the concentration of 45% therefrom over a distance of 60 nm (600 Å) in the film thickness direction (region b ). In the region b , LiF is uniformly distributed at the concentration of 0.8% from the contact point between the regions a and b over a distance of 30 nm (300 Å) in the film thickness direction. As is uniformly distributed at the concentration of 6% throughout the region a and at the concentration of 3% throughout the region b in the film thickness direction. The structure of Te and As distributions is the same as that of Fig. 1 in principle, but the after-image when operated at a high temperature can be reduced without deteriorating the so far available characteristics of the p-type photoconductive film by providing a region of high Te concentration and a region containing LiF capable of forming shallow levels in a p-type photoconductive film in the film thickness direction. In Fig. 2, the region c is an auxiliary sensitizing region made from Se and As, where the concentration of As is 30% at the position in contact between the regions b and c , and is continuously decreased therefrom to 3% over a distance of 30 nm (300 Å) in the film thickness direction. In the embodiment of Fig. 2, Te is uniformly distributed in the film thickness direction, but it is not always necessary that the distribution is uniform. That is, the distribution can have a variation of concentration. For example, the region b can be made from one region containing Te at the concentration of 30% from the position in contact between the regions a and b over a distance of 15 nm (150 Å) in the film thickness direction and another region containing Te at the concentration of 50% from the 30% Te region over a distance of 20 nm (200 Å) in the film thickness direction, or from one region containing Te at the concentration of 40% from the position in contact between the regions a and b over a distance of 15 nm (150 Å) in the film thickness direction and another region containing Te at the concentration of 45% from the 40% Te region over a distance of 20 nm (200 Å) in the film thickness direction. In this embodiment, LiF is used as a material capable of forming shallow levels in the amorphous Se, but the material is not limited to LiF, and can be at least one of fluorides such as LiF, NaF, MgF₂, CaF₂, AlF₃, CrF₃, MnF₂, CoF₂, PbF₂, BaF₂, CeF₃ and TlF, alkali and alkaline earth metals such as Li, Na, K, Cs, Ca, Mg, Ba and Sr, and Tl. It is essential that the p-type photoconductive film has a region containing Te at a concentration of over 35% to 60%, preferably over 35% to 50% in the film thickness direction, and a region containing a material capable of forming shallow levels in the amorphous Se at a concentration of 0.005 to 5% in the film thickness direction. It is preferable that the region containing a material capable of forming shallow levels in the amorphous Se is located within the region containing Te or nearer the light incident side than the region containing Te. Fig. 3 shows the effect of the present invention when targets of image pickup tubes having the photoconductive film shown in Fig. 2 were operated at varied temperatures, i.e. 40°C, 45°C and 50°C, while changing the concentration of Te in the targets. In Fig. 3 a group of curves 101 shows dependence of the after-image level upon the concentration of Te when the targets of image pickup tubes having a photoconductive film of the present invention are operated at various high temperatures, and a group of curves 102 shows dependence of the after-image decay time upon the concentration of Te when targets of image pickup tubes having a photoconductive film of the present invention are operated at various high temperatures. As is obvious from Fig. 3, it is necessary that the concentration of Te is in a range of over 35% to 60% to obtain a practical after-image in a high temperature range of 40° to 50°C. When the after-image decay time in the high temperature range is also taken into account, the concentration of Te is preferably in a range of over 35% to 50%. Fig. 4 shows results of detailed studies on changes in the characteristics with the concentration of Te and that of LiF in the target shown in Fig. 2. In the region (A) with a lower concentration of LiF, the dark current is increased when the target of image pickup tube is operated in the high temperature range, and the target fails to act as a blocking type target of image pickup tube. In the region (B) with a higher concentration of LiF, the after-image is undesirably increased after a high light incidence exceeding the normal light level of incident light, when the target is operated in the high temperature range. In the regions (C) and (D), the after-image is larger when targets of image pickup tubes having such a photoconductive film are operated in the high temperature range, as described before. As is obvious from these results, it is necessary that the material capable of forming shallow levels has a concentration of 0.005 to 5% to attain the effect of the present invention. There have been also already proposed a process for reducing the dark current by providing an auxiliary layer for the rectifying contact, made, for example, from oxides that show n-type conduction, such as CeO₂, etc. between the n-type conductive film and the p-type photoconductive film composed mainly from amorphous Se (e.g. US-A-4,307,319) and a process for making a photoconductive film for the target of image pickup tube, made mainly from amorphous Se, by deposition with a good reproducibility by heating and maintaining the substrate at an appropriate temperature below 60°C during the deposition (Japanese Patent Application No. JP-A-60-114,090). The object of the present invention can be effectively attained since the present invention is combined with these processes. Furthermore, the object of the present invention can be also attained by combining the present invention with a process for decreasing the after-image by strong light or the variation of sensitivity right after the actuation of image pickup tube by adding GaF₃, MoO, In₂O₃, etc. to at least a region of the auxiliary sensitizing layer (US-A-4,463,279, or Japanese Patent Application Kokai (Laid-open) No. 60-245283) without deteriorating the desired effects of the latter process. Fig. 1 is a cross-sectional view of a target of image pickup tube according to the prior art. Fig. 2 is a profile of distribution of component materials that constitute the essential part of a target of image pickup tube according to the present invention. Fig. 3 is a diagram showing the dependence of after-image level and decay time upon the concentration of Te in a photoconductive film of the target of image pickup tube when operated at a high temperature. Fig. 4 is a diagram defining the present invention by the concentration of Te and that of LiF in a photoconductive film of the target of image pickup tube. Fig. 5 is a diagram comparing the after-image characteristics of a target of image pickup tube according to the prior art with that according to the present invention. The present invention will be described in detail below, referring to Examples. Example 1 A transparent conductive film made mainly from tin oxide is formed on a glass substrate, and GeO₂ and CeO₂ are deposited to a thickness of 20 nm (200 Å) each in this order under vacuum of 2.7 x 10⁻⁴ Pa (2 x 10⁻⁶ Torr) as auxiliary layers for rectifying contact. Se and As₂Se₃ are then deposited thereon to a thickness of 20 to 50 nm (200 to 500 Å) from the respective deposition sources as a first layer, where As is uniformly distributed at the concentration of 10% in the film thickness direction. As the former half region of a second layer, Se, As₂Se₃, Te and LiF are deposited to a thickness of 15 nm (150 Å) onto the first layer from the respective deposition sources, where As, Te and LiF are uniformly distributed at the As concentration of 2%, the Te concentration of 30% and the LiF concentration of 8,000 ppm in the film thickness direction. Then, as the latter half region of the second layer, Se, As₂Se₃ and Te are deposited to a thickness of 15 nm (150 Å) onto the former half region from the respective deposition source, where As and Te are uniformly distributed at the As concentration of 2% and the Te concentration of 60%. A third layer made from Se and As is deposited to a thickness of 30 nm (300 Å) onto the second layer as an auxiliary sensitizing layer, where Se and As₂Se₃ are deposited at the same time from the respective deposition sources. By controlling the current to the respective deposition sources, the As concentration of the third layer is adjusted initially from 33% at the beginning of the third layer finally to 2% at the end of the third layer while continuously decreasing the As concentration as the deposition proceeds. Then, Se and As₂Se₃ are deposited onto the third layer from the respective deposition source at the same time as a fourth layer to make the total film thickness 4 µm, where the As is uniformly distributed at the As concentration of 2% in the film thickness direction throughout the fourth layer. Deposition of the first layer up to the fourth layer is carried out under vacuum of 2.7 x 10⁻⁴ Pa (2 x 10⁻⁶ Torr). Sb₂S₃ is deposited to a thickness of 70 nm (700 Å) onto the fourth layer in the atmosphere of argon under 27 Pa (2 x 10⁻¹ Torr) as an auxiliary layer for beam landing. Example 2 A transparent conductive film made mainly from tin oxide is formed on a glass substrate, and then CeO₂ is deposited thereon to a thickness of 30 nm (300 Å) under vacuum of 2.7 x 10⁻⁴ Pa (2 x 10⁻⁶ Torr) as an auxiliary layer for rectifying contact. Se, As₂Se₃ and LiF are deposited thereon to a thickness of 20 nm (200 Å) from the respective deposition sources as a first layer, where As and LiF are uniformly distributed at the As concentration of 10% and the LiF concentration of 8000 ppm in the film thickness direction. Then, Se, As₂Se₃ and Te are deposited to a thickness of 50 to 75 nm (500 to 750 Å) onto the first layer from the respective deposition sources as a second layer, where Te and As are uniformly distributed at the Te concentration of 36% and the As concentration of 2% in the film thickness direction. A third layer is deposited onto the second layer. As the former half region of the third layer, Se and As₂Se₃ are deposited onto the second layer to a thickness of 6 nm (60 Å) from the respective deposition source, where As is uniformly distributed at the As concentration of 25% in the film thickness direction. Then, as a latter half region of the third layer, Se, As₂Se₃ and GaF₃ are deposited thereon to a thickness of 15 nm (150 Å) from the respective deposition sources, where As and GaF₃ are uniformly distributed at the As concentration of 25% and the GaF₃ concentration of 2,500 ppm in the film thickness direction. The former half region and the latter half region of the third layer constitute an auxiliary sensitizing layer together. Then, a fourth layer made from Se and As is deposited thereon to make the entire film thickness 5 µm, where As is uniformly distributed at the As concentration of 2% in the film thickness direction throughout the fourth layer. Deposition of the first layer up to the fourth layer is carried out under vacuum of 2.7 x 10⁻⁴ Pa (2 x 10⁻⁶ Torr). Sb₂S₃ is deposited onto the fourth layer to a thickness of 50 nm (500 Å) in the atmosphere of argon of 40 Pa (3 x 10⁻¹ Torr). Example 3 A transparent conductive film made mainly from indium oxide is formed on a glass substrate, and then CeO₂ is deposited thereon to a thickness of 20 nm (200 Å) under vacuum of 2.7 x 10⁻⁴ Pa (2 x 10⁻⁶ Torr) as an auxiliary layer for rectifying contact. As a former half region of a first layer, Se, As₂Se₃ and CaF₂ are deposited thereon to a thickness of 15 nm (150 Å) from the respective deposition sources, where As and CaF₂ are uniformly distributed at the As concentration of 6% and the CaF₂ concentration of 3,000 ppm in the film thickness direction. Then, as a latter half region of the first layer, Se, As₂Se₃ and CaF₂ are deposited onto the former half region to a thickness of 15 nm (150 Å) from the respective deposition sources, where As and CaF₂ are uniformly distributed at the As concentration of 15% and the CaF₂ concentration of 9,000 ppm in the film thickness direction. The former half region and the latter half region constitute the first layer together. As a former half region of a second layer, Se, As₂Se₃, Te and CaF₂ are deposited onto the first layer to a thickness of 10 to 15 nm (100 to 150 Å) from the respective deposition sources, where Te, As and CaF₂ are uniformly distributed at a Te concentration of 45 to 50%, the As concentration of 2%, and the CaF₂ concentration of 6,000 ppm in the film thickness direction. As a latter half region of the second layer, Se, As₂Se₃ and Te are deposited onto the former half region to a thickness of 10 to 15 nm (100 to 150 Å) from the respective deposition sources, where Te and As are uniformly distributed at a Te concentration of 45 to 50% and the As concentration of 2% in the film thickness direction. Then, a third layer is deposited on the second layer. As a former half region of the third layer, Se, As₂Se₃ and In₂O₃ are deposited onto the second layer to a thickness of 5 nm (50 Å) from the respective deposition sources, where As and In₂O₃ are uniformly distributed at the As concentration of 25% and the In₂O₃ concentration of 500 ppm in the film thickness direction. Furthermore, as a latter half region of the third layer, Se and As₂Se₃ are deposited onto the former half region to a thickness of 30 nm (300 Å) from the respective deposition sources, where by controlling the current to the respective deposition sources the As concentration is adjusted initially from 25% at the beginning of the region finally to 3% at the end of the region while continuously decreasing the As concentration as the deposition proceeds. The former half region and the latter half region of the third layer constitute an auxiliary sensitizing layer. Then, a fourth layer made from Se and As is deposited onto the third layer to make the entire film thickness 4 µm, where As is uniformly distributed at the As concentration of 3% in the film thickness direction throughout the fourth layer. Deposition of the first layer up to the fourth layer is carried out under vacuum of 2.7 x 10⁻⁴ Pa (2 x 10⁻⁶ Torr). Sb₂S₃ is deposited onto the fourth layer to a thickness of 100 nm (1,000 Å) in the atmosphere of argon under 67 Pa (5 x 10⁻¹ Torr). Example 4 A transparent conductive film made mainly from tin oxide is formed on a glass substrate, and then GeO₂ and CeO₂ are deposited thereon to a thickness of 15 nm (150 Å) and a thickness of 20 nm (200 Å), respectively, in this order under vacuum of 2.7 x 10⁻⁴ Pa (2 x 10⁻⁶ Torr) as auxiliary layers for rectifying contact. Then, as a former half region of a first layer, Se, As₂Se₃ and LiF are deposited thereon to a thickness of 20 nm (200 Å) from the respective deposition sources, where As and LiF are uniformly distributed at the As concentration of 5% and the LiF concentration of 2,000 ppm in the film thickness direction. The substrate temperature is kept at 30°C during the deposition of the former half region of the first layer. Then, as a latter half region of the first layer, Se, As₂Se₃ and LiF are deposited on the former half region to a thickness of 10 nm (100 Å) from the respective deposition sources, where As and LiF are uniformly distributed at the As concentration of 18% and the LiF concentration of 8,000 ppm. The substrate temperature is kept at 35°C during the deposition of the latter half region. The former half region and the latter half region constitute the first layer together. As a former half region of a second layer, Se, As₂Se₃, Te and LiF are deposited onto the first layer to a thickness of 15 nm (150 Å) from the respective deposition sources, where As, Te and LiF are uniformly distributed at the As concentration of 2%, the Te concentration of 45%, and the LiF concentration of 6,000 ppm in the film thickness direction. Then, Se, As₂Se₃, Te and LiF are deposited onto the former half region to a thickness of 15 to 20 nm (150 to 200 Å) from the respective deposition sources to form the latter half region of the second layer, where As, Te and LiF are uniformly distributed at the As concentration of 2%, the Te concentration of 50% and the LiF concentration of 6,000 ppm. The substrate temperature is kept at 40°C during the deposition of the second layer. A third layer made from Se and As is deposited onto the second layer to a thickness of 35 nm (350 Å) as an auxiliary sensitizing layer, where Se and As₂Se₃ are deposited from the respective deposition sources at the same time, and by controlling the current to the respectove deposition sources the As concentration is adjusted initially from 30% at the beginning of the third layer finally to 2% at the end of the third layer, while continuously decreasing the concentration as the deposition proceeds. As a fourth layer, Se and As₂Se₃ are deposited onto the third layer from the respective deposition sources at the same time to make the entire film thickness 6 µm, where As is uniformly distributed at the As concentration of 2% throughout the fourth layer. The substrate temperature is kept at 43°C during the deposition of the third and fourth layers. Deposition of the first layer up to the fourth layer is carried out under vacuum of 2.7 x 10⁻⁴ Pa (2 x 10⁻⁶ Torr). Sb₂S₃ is deposited onto the fourth layer to a thickness of 70 nm (700 Å) in the atmosphere of argon under 40 Pa (3 x 10⁻¹ Torr) as an auxiliary layer for beam landing. Example 5 A transparent conductive film made mainly from tin oxide is formed on a glass substrate, and then CeO₂ is deposited thereon to a thickness of 20 nm (200 Å) in vacuum of 2.7 x 10⁻⁴ Pa (2 x 10⁻⁶ Torr) as an auxiliary layer for rectifying contact. Then, Se and As₂Se₃ are deposited thereon to a thickness of 30 nm (300 Å) from the respective deposition sources as a first layer, where As is uniformly distributed at the As concentration of 10% in the film thickness direction. As a second layer, Se, As₂Se₃, Te and LiF are deposited onto the first layer to a thickness of 30 nm (300 Å) from the respective deposition sources, where As, Te and LiF are uniformly distributed at the As concentration of 2%, the Te concentration of 60% and the LiF concentration of 5% in the film thickness direction. Then, a third layer made from Se and As is deposited onto the second layer to a thickness of 30 nm (300 Å) as an auxiliary sensitizing layer, where by controlling the current to the respective deposition sources the As concentration is continuously decreased from 30% to 2% in the film thickness direction in a constant rate. Then, a fourth layer made from Se and As is deposited onto the third layer to make the entire film thickness 4 µm, where As in uniformly distributed at the As concentration of 2% in the film thickness direction throughout the fourth layer. Deposition of the first layer up to the fourth layer is carried out under vacuum of 2.7 x 10⁻⁴ Pa (2 x 10⁻⁶ Torr). Sb₂S₃ is deposited onto the fourth layer to a thickness of 100 nm (1,000 Å) in the atmosphere of argon under 67 Pa (5 x 10⁻¹ Torr). Fig. 5 shows comparison of the after-image characteristics of a target of image pickup tube having the photoconductive film according to the prior art with that according to the present invention, where the after-image after a black-and-white pattern has been picked up for 10 minutes is given, and curve 6 is directed to the after-image characteristics of the target of image pickup tube having a photoconductive film (Te concentration: 30%) according to the prior art, whereas curve 7 is directed to that of the target of image pickup tube having a photoconductive film (Te concentration: 45%) according to the present invention. As is obvious from Fig. 5, the target of image pickup tube having the photoconductive film according to the prior art has a considerably increased after-image when operated at a high temperature, whereas that of the present invention has only a slight increase in the after-image when operated at the high temperature. A target of image pickup tube having a photoconductive film according to the present invention has good after-image characteristics, even if operated at a high temperature, without deteriorating the so far available characteristics.

Claims: A target of an image pickup tube, which comprises a transparent substrate (1), a transparent conductive film (2) and a p-type photoconductive film (3) made mainly from amorphous Se, wherein the photoconductive film (3) has a region containing over 35 % to 60 % by weight of Te in the film thickness direction and a region containing 0.005 % to 5 % by weight of a material capable of forming shallow levels in the amorphous Se in the film thickness direction, characterized in that the target further comprises a film of n-type conductive oxide made mainly from CeO₂ or a stacked layer of n-type conductive oxides made mainly from CeO₂ and GeO₂, respectively, disposed between the conductive film (2) and the photoconductive film (3). A target of an image pickup tube according to claim 1, wherein the material capable of forming shallow levels in the amorphous Se is at least one member selected from fluorides comprising LiF, NaF, MgF₂, CaF₂, AlF₃, CrF₃, MnF₂, CoF₂, PbF₂, BaF₂, CeF₃, and TlF; alkali metals and alkaline earth metals comprising Li, Na, K, Cs, Ca, Mg, Ba and Sr; and Tl. A target of an image pickup tube according to claim 1 or 2, wherein the p-type photoconductive film (3) contains at least a region containing over 35 % to 50 % by weight of Te in the film thickness direction. A target of an image pickup tube according to claim 1, wherein said p-type photoconductive film includes an auxiliary sensitizing region. A target of an image pickup tube according to claim 4, wherein said auxiliary sensitizing region includes at least one substance selected from the group consisting of GaF₃, MoO₃ and In₂O₃. A target of an image pickup tube according to claim 1, wherein the region containing a material capable of forming shallow levels in the amorphous Se is provided within the region containing over 35 % to 60 % by weight of Te. A target of an image pickup tube according to claim 1, wherein the region containing a material capable of forming shallow levels in the amorphous Se is provided closer to the transparent substrate (1) than is the region containing over 35 % to 60 % by weight of Te. A target of an image pickup tube according to claim 1, wherein the p-type photoconductive film (3) includes (1) a first layer formed by deposition of Se and As₂Se₃, (2) a second layer comprising a first sub-layer including As, Se, Te and the material capable of forming shallow levels in the amorphous Se, and a second sublayer consisting essentially of As, Se and over 35 % to 60 % by weight of Te, (3) an auxiliary sensitizing layer, of Se and As, having a decreasing As concentration in the auxiliary sensitizing layer, in the thickness direction, and (4) a layer formed by deposition of Se and As₂Se₃. A target of an image pickup tube according to claim 1, wherein the p-type photoconductive film (3) comprises (1) a first layer including As, Se and the material capable of forming shallow levels in the amorphous Se, (2) a second layer consisting essentially of As, Se and Te, the second layer containing over 35 % to 60 % by weight of Te, (3) an auxiliary sensitizing layer including a first sub-layer formed by depositing Se and As₂Se₃, and a second sub-layer formed by depositing Se, As₂Se₃ and GaF₃, and (4) a fourth layer made from Se and As. A target of an image pickup tube according to claim 1, wherein the p-type photoconductive film (3) includes (1) a first layer comprising first and second sub-layers, each sub-layer including As, Se and the material capable of forming shallow levels in the amorphous Se, (2) a second layer comprising first and second sublayers, the first sub-layer of said second layer including As, Se, Te and the material capable of forming shallow levels in the amorphous Se, the first sub-layer containing over 35 % to 60 % by weight of Te, and the second sub-layer consisting essentially of As, Se and Te, the second sub-layer containing over 35 % to 60 % by weight of Te, (3) an auxiliary sensitizing layer comprising first and second sub-layers, the first sub-layer being formed by depositing Se, As₂Se₃ and In₂O₃, the second sub-layer being formed by depositing Se and As₂Se₃, the As concentration in the second sub-layer of the auxiliary sensitizing layer decreasing in the thickness direction, and (4) a fourth layer formed by depositing Se and As. A target of an image pickup tube according to claim 1, wherein the p-type photoconductive film (3) comprises (1) a first layer comprising first and second sub-layers, each of the first and second sub-layers including As, Se and the material capable of forming shallow levels in the amorphous Se, (2) a second layer comprising first and second sub-layers, each of the first and second sub-layers of the second layer consisting essentially of As, Se, Te and the material capable of forming shallow levels in the amorphous Se, the first and second sub-layers containing over 35 % to 60 % by weight of Te, (3) a third layer made from Se and As, having a decreasing As concentration in the thickness direction of the third layer, and (4) a fourth layer formed by depositing Se and As₂Se₃. A target of an image pickup tube according to claim 1, wherein the p-type photoconductive film (3) comprises (1) a first layer formed by depositing Se and As₂Se₃, (2) a second layer consisting essentially of As, Se, Te, and the material capable of forming shallow levels in the amorphous Se, the second layer containing over 35 % to 60 % by weight of Te, (3) a third layer made from Se and As, wherein the As concentration in the third layer decreases in the thickness direction, and (4) a fourth layer made from Se and As. A target of an image pickup tube according to claim 1, wherein the p-type photoconductive film (3) includes (1) a first layer comprising first and second sub-layers, each sub-layer including As, Se and the material capable of forming shallow levels in the amorphous Se, (2) a second layer comprising first and second sub-layers, the first sub-layer of said second layer including As, Se, Te and the material capable of forming shallow levels in the amorphous Se, the first sub-layer containing over 35 % to 60 % by weight of Te, and the second sub-layer consisting essentially of As, Se and Te, the second sub-layer containing over 35 % to 60 % by weight of Te, (3) a third layer made from Se and As, having decreasing As concentration in the thickness direction of the third layer, and (4) a fourth layer formed by depositing Se and As₂Se₃. A target of an image pickup tube according to claim 8, wherein said first sub-layer of the second layer consists essentially of As, Se, Te and the material capable of forming shallow levels in the amorphous Se. A target of an image pickup tube according to claim 9, wherein the first layer consists essentially of As, Se and the material capable of forming shallow levels in the amorphous Se. A target of an image pickup tube according to claim 10, wherein each sub-layer of the first layer consists essentially of As, Se and the material capable of forming shallow levels in the amorphous Se, and wherein the first sub-layer of the second layer consists essentially of As, Se, Te and the material capable of forming shallow levels in the amorphous Se. A target of an image pickup tube according to claim 11, wherein each of the first and second sub-layers of the first layer consists essentially of As, Se and the material capable of forming shallow levels in the amorphous Se. A target of an image pickup tube according to claim 13, wherein each sub-layer of the first layer consists essentially of As, Se and the material capable of forming shallow levels in the amorphous Se, and wherein the first sub-layer of the second layer consists essentially of As, Se, Te and the material capable of forming shallow levels in the amorphous Se. The target as set forth in claim 1, wherein the photoconductive film is a film deposited under the temperature below 60 °C.

Inventors: AIBA, MASAAKI, GOTO, NAOHIRO, HIRAI, TADAAKI, HIRUMA, EIKYUU, INOUE, EISUKE, KAWAMURA, TATSURO, MAKISHIMA, TATSUO, NONAKA, YASUHIKO, SHIDARA, KEIICHI, SUZUKI, SHIROU, TAKASAKI, YUKIO, TANIOKA, KENKICHI, TSUJI, KAZUTAKA, YAMAMOTO, MASANAO, YAMASHITA, TAKASHI
Applicants: HITACHI, LTD., NIPPON HOSO KYOKAI
Application year: 1987
Main classification: H01J29
IPC codes: H01J29