U.S. patent number 3,800,194 [Application Number 05/347,663] was granted by the patent office on 1974-03-26 for photoconductive target of an image tube.
This patent grant is currently assigned to Hitachi, Ltd., Nippon Hoso Kyokai. Invention is credited to Naohiro Goto, Tadaaki Hirai, Kiyohisa Inao, Eiichi Maruyama.
United States Patent |
3,800,194 |
Maruyama , et al. |
March 26, 1974 |
PHOTOCONDUCTIVE TARGET OF AN IMAGE TUBE
Abstract
A target for an image pickup tube comprising a
light-transmitting conductive layer supported on a
light-transmitting substrate and a photoconductive layer with
rectifying contact at least at one of its sides; in which the
photoconductive layer has a portion 1,000 A or thicker which
comprises a multiplicity of thin films of two or more different
materials with different photoconductive characteristics, each
having a thickness of not more than 100 A, the thin films of the
different materials being laid alternately one on another.
Inventors: |
Maruyama; Eiichi (Kodaira,
JA), Hirai; Tadaaki (Koganei, JA), Inao;
Kiyohisa (Hachioji, JA), Goto; Naohiro (Machida,
JA) |
Assignee: |
Hitachi, Ltd. (Tokyo,
JA)
Nippon Hoso Kyokai (Tokyo, JA)
|
Family
ID: |
12411953 |
Appl.
No.: |
05/347,663 |
Filed: |
April 4, 1973 |
Foreign Application Priority Data
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|
|
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Apr 7, 1972 [JA] |
|
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47-34359 |
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Current U.S.
Class: |
338/15; 257/917;
250/214LA; 313/366 |
Current CPC
Class: |
H01L
31/00 (20130101); H01J 29/45 (20130101); Y10S
257/917 (20130101) |
Current International
Class: |
H01J
29/10 (20060101); H01L 31/00 (20060101); H01J
29/45 (20060101); H01l 015/00 () |
Field of
Search: |
;317/235N,235AC,241,234S,235NA ;313/66 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Edlow; Martin H.
Attorney, Agent or Firm: Craig and Antonelli
Claims
What we claim is:
1. A photo-conductive target for an image pickup tube comprising a
light-transmitting substrate, a light-transmitting conductive layer
deposited on said light-transmitting substrate and a
photo-conductive layer deposited on said light-transmitting
conductive layer, said photo-conductive layer containing selenium
and including a portion not less than 1,000 A thick, said portion
of said photo-conductive layer consisting of a multiplicity of thin
films of at least two different materials with different
photo-conductive characteristics, each having the thickness of 100
A or less, said thin films being laid alternately one on
another.
2. A photo-conductive target for an image pickup tube according to
claim 1, further comprising one semiconductor layer selected from
the group consisting of ZnS, CdS, ZnSe, CdSe, CdTe and a mixture
thereof deposited on said photo-conductive layer, a rectifying
contact being formed by said semiconductor layer and said
photo-conductive layer.
3. A photo-conductive target for an image pickup tube according to
claim 1, in which said light-transmitting layer and said
photo-conductive layer constitute a rectifying contact.
4. A photo-conductive target for an image pickup tube according to
claim 2, in which an insulating thin film with the thickness from
10 A to 1,000 A and comprising one selected from the group
consisting of ZnS, PbF.sub.2, MnF.sub.2, CaF.sub.2, MgF.sub.2,
Al.sub.2 O.sub.3, SiO and As.sub.2 S.sub.3 is provided adjacent to
that side of said photo-conductive layer which has said rectifying
contact.
5. A photo-conductive target for an image pickup tube according to
claim 3, in which an insulating thin film with the thickness from
10 A to 1,000 A and comprising one selected from the group
consisting of ZnS, PbF.sub.2, MnF.sub.2, CaF.sub.2, MgF.sub.2,
Al.sub.2 O.sub.3, SiO and As.sub.2 S.sub.3 is provided adjacent to
that side of said photo-conductive layer which has said rectifying
contact.
6. A photo-conductive target for an image pickup tube according to
claim 1, in which the surface of said photo-conductive layer to be
scanned by an electron beam is covered with one selected from the
group consisting of a vacuum-evaporated Sb.sub.2 S.sub.3 film, a
porous Sb.sub.2 S.sub.3 film, a combination of a vacuum-evaporated
Sb.sub.2 S.sub.3 and a porous Sb.sub.2 S.sub.3 film, and a porous
As.sub.2 Se.sub.3 film deposited on a vacuum-evaporated Sb.sub.2
S.sub.3 film.
Description
The present invention relates to a target of an image pickup tube
or more in particular to a target of an image pickup tube of
vidicon type utilizing the rectifying contact of a photoconductive
semiconductor.
The materials of the target for the image pickup tube now in
commercial use include photoconductive semiconductors such as
Sb.sub.2 S.sub.3, PbO and Si. A target of Sb.sub.2 S.sub.3 is
characterized by an ohmic contact, while those made of PbO or Si
generally have a rectifying contact or is of a PN junction type.
The target with the rectifying contact or PN junction has many
advantages including the fact that they allow less dark current, is
higher in sensitivity and greater in response speed than the target
with an ohmic contact. The manufacture of an image pickup tube with
a PN junction of single crystal such as a vidicon with Si target
requires highly complex processes, and also it is very difficult to
remove an imperfection of a picture attributable to the defects of
a crystal bulk or unsatisfactory processes. A target of a thin film
with a rectifying contact, by comparison, is manufactured by
comparatively simple processes, but it is not an easy matter to
achieve a rectifying contact which is used successfully for a
vidicon target, the known materials suitable for such a purpose
being limited to PbO and few others including selenium and its
compounds.
Further, in the conventional co-evaporation method of forming an
evaporated thin film of a compound semiconductor with a desirable
sensitive spectral region by controlling its composition, a
plurality of different component elements are deposited
simultaneously from a plurality of sources of evaporation. In such
a method, however, the plurality of sources are located at
different positions relative to the substrate and therefore the
composition of the resulting layer on the substrate is not
necessarily uniform at every point on the substrate.
For the above mentioned reasons, it is difficult in the
conventional methods to obtain a thin film with a rectifying
contact by evaporation which fully meet, all the requirements of an
image pickup tube including a superior dark current characteristic,
spectral sensitivity characteristic and after image.
The inventors have discovered that the alternate deposition of a
multiplicity of thin films of different substances one on another
makes it possible to obtain a photoconductive layer with a
photoconductive characteristic quite similar to that of a material
resulting from the uniform mixing of such substances. Generally,
when a plurality of thin films with different photoconductive
characteristics are laid one on another, the resulting
characteristic is the sum of the different photoconductive
characteristics involved. If, however, the thickness of each
component thin film is sufficiently small and the laminated
structure consists of a multiplicity of such thin films deposited
alternately one on another, the resulting photoconductive
characteristic is intermediate with respect to those of the
component thin films.
Accordingly, it is an object of the present invention to obviate
the disadvantages of the conventional target of an image pickup
tube and to provide a novel target of an image pickup tube which is
capable of controlling the spectral sensitivity characteristic of
the image pickup tube considerably without adversely affecting its
characteristics of dark current and after image.
In order to achieve the above mentioned object, the target of the
image pickup tube according to the present invention comprises a
light-transmitting substrate, a light-transmitting conductive layer
and a photo-conductive layer deposited on the light-transmitting
conductive layer and including a part 1,000 A or thicker consisting
of at least two thin films of different photo-conductive
characteristics alternately laid one on another, each of the thin
films being 100 A or thinner said photo-conductive layer having at
least one rectifying contact on its surface.
The above and other objects, features and advantages will be made
apparent by the detailed description taken in conjunction with the
accompanying drawings, in which:
FIG. 1 is a diagram showing a section of a target for the image
pickup tube according to the present invention;
FIG. 2 is a diagram showing the spectral sensitivity of a
conventional photo-conductive double layer consisting of Se and
As.sub.2 S.sub.3 ;
FIG. 3 is a diagram showing the spectral sensitivity of a layer
consisting of Se and As.sub.2 S.sub.3 used for the target for the
image pickup tube according to the present invention; and
FIGS. 4 to 6 are diagrams showing sections of embodiments of the
present invention.
EMBODIMENT 1
An embodiment of the invention is illustrated in FIG. 1, in which
reference numeral 1 shows a glass substrate, numeral 2 a
light-transmitting conductive layer, and numeral 3 a
photo-conductive layer. The photo-conductive layer 3 of about 3
microns thick is made by depositing by evaporation a multiplicity
of selnium films each about 20 A and As.sub.2 Se.sub.3 films each
about 7 A alternately one on another in a vacuum of 5 .times.
10.sup.-.sup.6 Torr. A rectifying contact is formed between the
light-transmitting conductive layer 2 and the photoconductive layer
3. A photo-conductive layer 3 consisting of only a selenium film
has a superior rectifying contact but has the disadvantage of
insufficient sensitivity to light of long wavelength. When the
photo-conductive layer 3 is formed of only As.sub.2 Se.sub.3, by
contrast, a high sensitivity to red light is obtained but it is
impossible to form a superior rectifying contact.
In case the photo-conductive layer 3 were made of comparatively
thick Se and As.sub.2 Se.sub.3 films of, say, 5,000 A each, two
peaks of spectral sensitivity corresponding to those of the
individual component films of Se and As.sub.2 Se.sub.3 respectively
would be observed as shown in FIG. 2. The photo-conductive layer 3
according to the present invention, by contrast, which comprises a
multiplicity of comparatively thin Se and As.sub.2 Se.sub.3
alternate films of, say, 50 A in thickness each having its peak of
spectral sensitivity intermediate with respect to the two
hypothetical peaks which otherwise might occur separately for the
two component film elements. In the latter case, it is considered
that the photo-conductive layer 3 has similar characteristics as if
Se and As.sub.2 Se.sub.3 are mixed uniformly. Such similarity is
achieved only when the thickness of each film of Se and As.sub.2
Se.sub.3 is approximately 100 A or less.
Further, it is possible by employing the construction of the
photo-conductive layer according to this embodiment to achieve a
photo-conductive layer superior in dark current characteristic,
after image characteristic and sensitivity to long wavelength light
as shown in Table 1 without adversely affecting the rectifying
characteristic.
TABLE 1 -- Characteristics of Target for Image Pickup Tube
According to Embodiment 1
dark target current wavelength construc- at target at peak lag
(after after tion voltage spectral 3 fields) image of 50 V
sensitivity selenium only 1 nA 400 m.mu. 6% long As.sub.2 Se.sub.3
only 20 nA 580 m.mu. 50% very long this in- vention 1 nA 520 m.mu.
6% short
Furthermore, although in the above embodiment the different types
of film may be employed, more than three different types of films
may be deposited one on another to obtain the photo-conductive
layer according to the invention by using as many evaporation
sources. Also, it is possible to achieve any desired mixing ratio
by varying the thickness of each film accordingly.
For the practical purpose of controlling the spectral sensitivity
of the photo-conductive layer the above mentioned multi-layer
construction is not necessarily required all over its total
thickness. Most of the energy of light entering the
photo-conductive layer is absorbed at its portion within the depth
of several thousands of A from the surface thereof. Therefore, the
portion of the layer as thick as 1,000 A or more from the surface
has a controlling effect upon the spectral sensitivity of the
photo-conductive layer.
If the photo-conductive layer with the above mentioned construction
is to be used for a target of the image pickup tube, its dark
current characteristic, after image characteristic, lag
characteristic as well as the spectral sensitivity are required to
be maintained at a satisfactory level for the image pickup tube. It
is already mentioned that a desirable construction for the target
of an image pickup tube is of the rectifying contact type. Since
the characteristic of the target of the rectifying contact type
largely depends upon the shape of the rectifying barrier, the
portion of the photo-conductive multi-layer for controlling the
spectral sensitivity must be so selected as not to adversely affect
the other characteristics of the photo-conductive layer. To achieve
maximum utilization of incident light, it is recommended that the
multi-layer portion of the photo-conductive layer be located as
near to the plane of incidence as possible so far as the effect of
the rectifying barrier is not reduced.
The photo-conductive layer used as a target for an image pickup
tube generally has the thickness of 2 to 20 microns, and it is
possible, by providing a portion of the photo-conductive layer as
thick as 1,000 A for controlling the spectral sensitivity of the
photo-conductive layer, to limit the function of such portion to
the controlling of the spectral sensitivity without any substantial
effect upon the lag, after image and dark current characteristics
of the photo-conductive layer.
EMBODIMENT 2
Referring to FIG. 4 showing another embodiment of the invention,
reference numeral 4 denotes a glass substrate, numeral 5 a
light-transmitting conductive layer, and numeral 6 a
photo-conductive layer approximately 2 microns thick consisting of
a multiplicity of selenium films each about 5 A thick and CdSe
films each about 20 A thick alternately deposited one on another.
Reference numeral 7 shows a CdTe film with a thickness of
approximately 1,000 A. In the embodiment under consideration, a
rectifying contact is interposed between the photo-conductive layer
6 and the CdTe film 7. A photo-conductive layer consisting of only
selenium films develops no sufficient sensitivity to light of long
wavelengths, while one with only CdSe films does not satisfy the
requirements for rectifying contact. The employment of a
multi-layer as represented by the photo-conductive layer 6
consisting of alternately deposited multiplicity of selenium and
CdSe films makes it possible to obtain a target superior both in
rectifying contact and in sensitivity to long wavelength as is
apparent from Table 2.
TABLE 2 -- Characteristics of Target for Image Pickup Tube
According to Embodiment 2
dark target current wavelength construc- at target at peak lag
(after after tion voltage spectral 3 fields) image of 50 V
sensitivity selenium only 1 nA 400 m.mu. 6% long CdSe only 35 nA
650 m.mu. 30% long this in- vention 2 nA 600 m.mu. 15% short
In this embodiment, CdTe may be replaced by ZnS, CdS, ZnSe, CdSe or
a mixture of any ones of them which has a property similar to that
of CdTe.
EMBODIMENT 3
A third embodiment of the invention is shown in FIG. 5. In the
figure, reference numeral 8 shows a glass substrate, numeral 9 a
light-transmitting conductive layer, and numeral 10 a film of
MnF.sub.2 200 A thick. The interposition of the insulating material
MnF.sub.2 between the light-transmitting conductive layer 9 and the
selenium film 11 permits the reverse breakdown voltage of the
rectifying contact between the light-transmitting conductive layer
9 and the selenium film 11 to be increased. The purpose of
increasing the reverse breakdown voltage of the rectifying contact
is also achieved by the interposition of PbF.sub.2, CaF.sub.2,
MgF.sub.2, Al.sub.2 O.sub.3, SiO, ZnS, As.sub.2 S.sub.3, or the
like insulating material instead of MnF.sub.2. Incidentally, the
thickness of the insulating film may be in the range from 10 to
1,000 A.
The selenium film 11 which is approximately 3 microns in thickness
has a central portion 12 approximately 1,000 A thick including a
multiplicity of selenium films each about 20 A and tellurium films
each about 12 A alternately laid one on another. There is provided
on the selenium film 11 an Sb.sub.2 S.sub.3 film 13 about 10,000 A
thick to improve the landing of the scanning electron beams.
Although the Sb.sub.2 S.sub.3 film 13 may be deposited by
evaporation in a vacuum of 1 .times. 10.sup.-.sup.5 Torr. or
thereabouts, a porous Sb.sub.2 S.sub.3 film deposited by
evaporation in an argon gas of about 5 .times. 10.sup.-.sup.2 Torr.
is more effective for the purpose of effective landing of electron
beams. However, since an Sb.sub.2 S.sub.3 film which is porous
through its whole thickness is too high in resistance, resulting in
inferior characteristics, it is desirable that a porous film of
Sb.sub.2 S.sub.3 or As.sub.2 Se.sub.3 be laid on a solid film of
Sb.sub.2 S.sub.3 deposited by vacuum evaporation to obtain an
integrated Sb.sub.2 S.sub.3 film. In the embodiment under
consideration, the central portion 12 in the form of a multi-layer
is such that an improved sensitivity to red light is obtained as
shown in Table 3 without adversely affecting the rectifying contact
formed of the light-transmitting conductive layer 9, insulating
film 10 and the selenium film 11.
TABLE 3 -- Characteristics of Target for Image Pickup Tube
According to Embodiment 3
dark target current wavelength construc- at target at peak lag
(after after tion voltage spectral 3 fields) image of 50 V
sensitivity selenium only 1 nA 400 m.mu. 6% long Te only large this
in- vention 1 nA 600 m.mu. 7% short
EMBODIMENT 4
A fourth embodiment of the invention is illustrated in FIG. 6.
Reference numeral 14 shows a glass substrate, numeral 15 a
light-transmitting conductive layer, numeral 16 a ZnSe film
approximately 500 A thick, and numeral 17 a photo-conductive layer
about 3 microns thick which comprises a multiplicity of selenium
films each about 40 A and arsenic films each about 5 A alternately
laid one on another. The superior rectifying contact between the
ZnSe film 16 and photo-conductive layer 17 is also achieved by
using a ZnS or CdSe film of the same thickness in place of the ZnSe
film. In this embodiment, too, an insulating film may be inserted
on the side of the rectifying contact of the photo-conductive layer
in order to improve the breakdown voltage of the rectifying contact
in the reverse direction as in the preceding embodiment. The
photo-conductive layer 17 is provided with a central multi-layer
portion 18 about 2,000 A thick comprising a multiplicity of
selenium films each about 40 A, arsenic films each about 5 A and
tellurium films each about 30 A which are deposited alternately one
on another.
An Sb.sub.2 S.sub.3 film 19 about 1,000 A thick is provided on the
photo-conductive layer 17 to improve the landing of the scanning
electron beams. According to the present embodiment, the presence
of the tellurium films in the layer 18 permits the sensitivity to
red light to be improved as shown in Table 4 without adversely
affecting the advantage of the rectifying contact.
TABLE 4 -- Characteristics of Target for Image Pickup Tube
According to Embodiment 4
dark target current wavelength construc- at target at peak lag
(after after tion voltage spectral 3 fields) image of 50 V
sensitivity selenium only 1 nA 400 m.mu. 6% long arsenic only large
this in- vention 1 nA 580 m.mu. 8% short
It will be understood from the above explanation that, in a target
for an image pickup tube with a rectifying contact, the provision
of a multi-layer consisting of a multiplicity of thin films of two
or more different photo-conductive types alternately deposited one
on another makes it possible to control more widely the spectral
sensitivity of the target without adversely affecting the other
characteristics thereof. For this reason, the present invention is
applied with great advantages to the construction of a target for a
color television image pickup tube or the like whose requrements
for spectral sensitivity are very severe.
* * * * *