A photocathode epitaxial structure. The photocathode epitaxial structure includes an improved substrate stack. The improved substrate stack includes a GaAs substrate and one or more additional layers formed on the GaAs substrate. The one or more additional layers are configured to provide an improved substrate stack surface with predetermined characteristics for forming a semiconductor device on the improved substrate stack surface. The photocathode epitaxial structure further includes an InGaAs p-type photocathode formed on the improved substrate stack surface. The InGaAs p-type photocathode has a predetermined percentage of In.

A photocathode structure, which can include an alkali halide, has a protective film on an exterior surface of the photocathode structure. The protective film includes ruthenium. This protective film can be, for example, ruthenium or an alloy of ruthenium and platinum. The protective film can have a thickness from 1 nm to 20 nm. The photocathode structure can be used in an electron beam tool like a scanning electron microscope.

A tunable photocathode for use in vacuum electronic devices includes a nanostructured photoemission layer including quantum confined nanostructures, such as quantum dots. The quantum confined nanostructures can be tuned (e.g., prepared to have various characteristics or parameters) in order to independently optimize various characteristics of the electron beam emitted by the photocathode. For example, by changing the material composition, size and geometry of the quantum confined nanostructures, the energy levels of the quantum confined nanostructures in the photoemission layer can be tuned to provide a photocathode having a high quantum efficiency, low emittance, fast response time to incident light pulses, long operational lifetime, and increased environmental stability compared with conventional photocathodes and cathodes in vacuum electronic devices.

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.

A photocathode. The photocathode includes an absorber. The absorber a p-type bulk active layer and a plurality of nanostructures formed on the p-type bulk active layer. The Photocathode further includes the plurality of nanostructures, such that the plurality of nanostructures are formed at a band bending region between the bulk active layer and the vacuum.

A photocathode epitaxial structure. The photocathode epitaxial structure includes a binary compound substrate material. The photocathode epitaxial structure further includes an active device absorber layer forming a portion of a p-type device photocathode formed on the binary compound substrate material. The active device absorber layer comprising at least a quaternary or greater material structure configured to be lattice matched with the substrate material to reduce strain to allow charge carriers to go further in the active device absorber layer implemented in the photocathode of a nightvision system.

A photocathode for use in vacuum electronic devices has a bandgap gradient across the thickness (or depth) of the photocathode between the emitting surface and the opposing surface. This bandgap gradient compensates for depth-dependent variations in transport energetics. When the bandgap energy E.sub.BG(z) is increased for electrons with shorter path lengths to the emitting surface and decreased for electrons with longer path lengths to the emitting surface, such that the sum of E.sub.BG(z) and the scattering energy is substantially constant or similar for electrons photoexcited at all locations within the photocathode, the energies of the emitted electrons may be more similar (have less variability), and the emittance of the electron beam may be desirably decreased. The photocathode may be formed of a III-V semiconductor such as InGaN or an oxide semiconductor such as GaInO.

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.

Provided are an electron gun that can extend the lifetime of a photocathode, an electron beam applicator on which the electron gun is mounted, and an irradiation position moving method. This object can be achieved by an electron gun including: a light source; a photocathode that emits an electron beam in response to receiving light from the light source; an anode; a motion device that moves excitation light irradiating the photocathode; and a control unit, the control unit controls the motion device to move an irradiation position of the excitation light from a position R.sub.n (n is a natural number) on the photocathode to a position R.sub.n+1 outside an excitation light irradiation-caused deteriorated range associated with the position R.sub.n, the excitation light irradiation-caused deteriorated range is a range where the photocathode is deteriorated due to irradiation with the excitation light, and the distance between the center of a spot of the excitation light at the position R.sub.n and the center of a spot of the excitation light at the position R.sub.n+1 is at least three or more times a spot diameter of the excitation light on the photocathode.

A photocathode structure, which can include an alkali halide, has a protective film on an exterior surface of the photocathode structure. The protective film includes ruthenium. This protective film can be, for example, ruthenium or an alloy of ruthenium and platinum. The protective film can have a thickness from 1 nm to 20 nm. The photocathode structure can be used in an electron beam tool like a scanning electron microscope.