Photocathode assembly of vacuum photoelectronic device with a semi-transparent photocathode based on nitride gallium compounds

Abstract

A photocathode assembly of a vacuum photoelectronic device with a semi-transparent photocathode that consists of an input window in the form of a disk made from sapphire, layers of heteroepitaxial structure of gallium nitride compounds as a semi-transparent photocathode grown on the inner surface of the input window, and an element for connecting the input window with a vacuum photoelectronic device housing, which is vacuum-tight fixed on the outer surface of the input window at its periphery. The element for connecting of the input window with the vacuum photoelectronic device housing is made of a bimetal, in which a layer that is not in contact with the outer surface of the input window consists of a material with a temperature coefficient of linear expansion that differs from the temperature coefficient of linear expansion of sapphire by no more than 10% in the temperature range from 20 C. to 200 C.

Claims

1. A photocathode assembly of a vacuum photoelectronic device with a semi-transparent photocathode, said photocathode assembly comprising an input window made in the form of a sapphire disk, layers of a heteroepitaxial structure of gallium nitride compounds as the semi-transparent photocathode, said layers being grown on an inner surface of the input window, and an element for coupling the input window with a housing of the vacuum photoelectronic device, said element being vacuum-tightly attached to an outer surface of the input window at its periphery, wherein the element for coupling the input window with the housing of the vacuum photoelectronic device is made of a bimetal in which a layer being not in contact with the outer surface of the input window consists of a material with having a linear thermal expansion coefficient different from the linear thermal expansion coefficient of sapphire by not more than 10% in the temperature range from 20 C. to 200 C.

2. The photocathode assembly of a vacuum photoelectronic device with a semi-transparent photocathode according to claim 1, wherein kovar is used as the material having a linear thermal expansion coefficient different from the linear thermal expansion coefficient of sapphire by not more than 10% in the temperature range from 20 C. to 200 C.

3. The photocathode assembly of a vacuum photoelectronic device with a semi-transparent photocathode according to claim 1, wherein the layers of the heteroepitaxial structure of gallium nitride compounds include a GaN compound.

4. The photocathode assembly of a vacuum photoelectronic device with a semi-transparent photocathode according to claim 1, wherein the layers of the heteroepitaxial structure of gallium nitride compounds include an AlGaN compound.

5. The photocathode assembly of a vacuum photoelectronic device with a semi-transparent photocathode according to claim 1, wherein the element for coupling the input window with the housing of the vacuum photoelectronic device is made in the form of a rotation figure having a profile of predetermined shape.

6. The photocathode assembly of a vacuum photoelectronic device with a semi-transparent photocathode according to claim 1, wherein a thickness of the sapphire disk is from 0.4 mm to 0.7 mm.

Description

(1) FIG. 1 shows the photocathode assembly of the vacuum photoelectronic device known from the article by I. Mizuno, T. Nihashi, T. Nagai, M. Niigaki, Y. Shimizu, K. Shimano, K. Katoh, T. Ihara, K. Okano, M. Matsumoto, M. Tachino, Development of UV image intensifier tube with GaN photocathode, Proc. of SPIE Vol. 6945, 2008.

(2) FIG. 2 shows the claimed photocathode assembly of a vacuum photoelectronic device with a semi-transparent photocathode based on gallium nitride compounds.

(3) The claimed photocathode assembly of a vacuum photoelectronic device with a semi-transparent photocathode comprises (FIG. 2) an input window 6, layers 7 of a heteroepitaxial structure of gallium nitride compounds as the semi-transparent photocathode, and an element 8 for coupling the input window 6 with a housing of the vacuum photoelectronic device (not shown in Fig.). The input window 6 is shaped as a disk (this is not shown in Fig.) made of sapphire, wherein the layers 7 of the heteroepitaxial structure of gallium nitride compounds are grown on an inner surface of the input window 6, and the element 8 for coupling the input window 6 with the housing of the vacuum photoelectronic device is vacuum-tightly attached to an outer surface of the input window 6 at its periphery. The element 8 for coupling the input window 6 with the housing of the vacuum photoelectronic device is made of a bimetal in which a layer (not shown in Fig.) that is not in contact with the outer surface of the input window 6 consists of a material having a linear thermal expansion coefficient different from the linear thermal expansion coefficient of sapphire by not more than 10% in the temperature range from 20 C. to 200 C.

(4) The claimed technical solution of the photocathode assembly of the vacuum photoelectronic device with the semi-transparent photocathode is implemented as follows. A semi-transparent photocathode of the photocathode assembly of the vacuum photoelectronic device is manufactured, for which purpose layers 7 of a heteroepitaxial structure of gallium nitride compounds are grown on a sapphire disk. Here, a diameter of the sapphire disk is chosen to be corresponding to one of the standard photocathode diameters which can be in particular of 18 mm or more. A thickness of the sapphire disk can be from 0.4 mm to 0.7 mm. The layers 7 of the heteroepitaxial structure of gallium nitride compounds can include GaN and/or AlGaN compounds, in particular as an active layer of the heteroepitaxial structure. The heterostructure of gallium nitride compounds is epitaxially grown by one of known methods. For example, an organometallic vapor phase epitaxy (OMVPE) method or a molecular-beam epitaxy (MBE) method is used for the epitaxial growth of GaN and AlGaN compounds. The sapphire disk used as a substrate for the layers 7 of the heteroepitaxial structure of gallium nitride compounds which are thus grown thereon and which form the semi-transparent photocathode is simultaneously used as the input window 6 of the photocathode assembly of the vacuum photoelectronic device. Here, a surface of the input window 6 on which the layers 7 of the heteroepitaxial structure of the gallium nitride compounds are grown is defined as its inner surface which is configured to be placed during the manufacture of the vacuum photoelectronic device within the internal volume of the vacuum PED housing. Another, free surface of the input window 6 is defined as its outer surface which is configured for vacuum-tight attachment thereto of the element 8 for coupling the input window 6 with the housing of the vacuum photoelectronic device during the manufacture of the photocathode assembly of the vacuum photoelectronic device. The element 8 for coupling the input window 6 with the housing of the vacuum photoelectronic device is manufactured by means of that layers of a bimetal are formed as a rotation figure having a profile of predetermined shape by one of known methods for manufacturing bimetallic parts. Here, a material having a linear thermal expansion coefficient different from the linear thermal expansion coefficient of sapphire by not more than 10% in the temperature range from 20 C. to 200 C. is used for the bimetal layer which is not in contact with the outer surface of the input window 6 in the finished photocathode assembly. For example, kovar which is an alloy based on nickel (Ni) in the amount of 29%, cobalt (Co) in the amount of 17%, and iron (Fe) in the balance amount, and has a linear thermal expansion coefficient value which is (46-52)10.sup.7 K.sup.1 (or an average value of 4910.sup.7 K.sup.1) in the temperature range from 20 C. to 200 C. is used as said material. For the bimetal layer by which the element 8 for coupling the input window 6 with the housing of the vacuum photoelectronic device is attached to the outer surface of the input window 6 in the finished photocathode assembly, a material is chosen that ensures its vacuum-tight bonding to sapphire which the disk of the input window 6 is made of. For example, titanium is used as this material. The element 8 for coupling the input window 6 with the housing of the vacuum photoelectronic device can be manufactured, for example, by thermal-compression bonding to each other of two blanks of parts made in the form of rotation figures having profiles of predetermined shapes, so that the blanks form the bimetal layers one of which is not in contact with the outer surface of the input window 6 in the finished photocathode assembly. The manufactured element 8 for coupling the input window 6 with the housing of the vacuum photoelectronic device is vacuum-tightly attached to the outer surface of the input window 6 at its periphery, for example by thermo-compression bonding using an intermediate layer of aluminum. The thus formed photocathode assembly of the vacuum photoelectronic device with the semi-transparent photocathode is subjected to vacuum heating up to a temperature of 600-620 C. and, thus, the surface of the layers 7 of the heteroepitaxial structure of gallium nitride compounds is cleaned. The cleaned surface of the heteroepitaxial structure of gallium nitride compounds is activated with cesium and oxygen by known methods, thereby ensuring a high level of quantum yield of the semi-transparent photocathode of the photocathode assembly of the vacuum photoelectronic device.

(5) The thus manufactured photocathode assembly of the vacuum photoelectronic device is characterized, in contrast to the technical solution of the closest prior art, by a wider application area, by a higher level of the quantum yield of the semi-transparent photocathode, and by the ability to meet the requirement for uniform resolving power over the operational field of the screen of the vacuum photoelectronic device in the case of using the claimed photocathode assembly within a proximity-focused direct view electron-optical converter, which is evidenced by the results of tests of photocathode assembly samples. Thus, the results of the tests performed show that the photocathode assembly samples of the vacuum photoelectronic device embodying the technical solution of the closest prior art and comprising the semi-transparent photocathode with a standard diameter of 18 mm lose their vacuum tightness in three percent of the tests, and those with a standard diameter of 25 mm in one hundred percent of the tests and, moreover, this happens after a single-time heating to temperatures of 600-620 C. In this case, the out-of-flatness of the sapphire disk of the input window in the photocathode assembly samples of the closest prior art is 50 m. In contrast to this, the photocathode assembly samples of the vacuum photoelectronic device which have been manufactured in accordance to the claimed technical solution and which comprise the semi-transparent photocathode with a standard diameter of 25 mm retain the vacuum tightness in one hundred percent of the tests even when heated to the temperatures of 600-620 C. up to ten times. These test results confirm the wider application area of the claimed technical solution of the photocathode assembly of the vacuum photoelectronic device with the semi-transparent photocathode, in contrast to the technical solution of the closest prior art. At the same time, these test results confirm the feasibility of temperature conditions of heating-up the heteroepitaxial structure prior to its activation which are necessary for causing a high level of quantum yield of the semi-transparent photocathode, while maintaining the vacuum tightness at these temperature conditions and hence the suitability of the photocathode assembly for use thereof within the vacuum photoelectronic device. Moreover, in all the cases of testing the claimed photocathode assembly samples by heating to the temperatures of 600-620 C., the out-of-flatness of the sapphire disk of the input window thereof does not exceed 10 m. Such a small degree of the out-of-flatness of the sapphire disk of the input window and, accordingly, of the surface of the semi-transparent photocathode of the claimed photocathode assembly of the vacuum photoelectronic device ensures a sufficient degree of uniformity of the resolving power distribution over the operational field of the screen of the proximity-focused direct view electron-optical converter, in the case the photocathode assembly according to the claimed technical solution is used therein. Thus, the test results show a better technical and operational performance of the claimed technical solution of the photocathode assembly of the vacuum photoelectronic device with the semi-transparent photocathode as compared to the technical solution of the closest prior art.