Developed is the granulated, Au film-based, semitransparent photocathode consisting of spherical Au nanoparticles.
The granulated Au films are activated by a thin layer of cesium and oxygen of about two monolayer thicknesses to
decrease the work function down to about 1 eV and gain the photoemission effect in the visible spectrum range. The
sensitivity maximum equal to about one mA/W is located in the green spectrum range. The nanoparticles formation and
photocathode surface structure are studied with the use of the X-ray photoelectron spectroscopy technique. Those studies
have shown that the photoemission effect in the wavelength range &lgr;> 450 nm is conditioned by excitation of the surface
plasmons in quasi-spherical Au nanoparticles. This has allowed manufacturing of a streak tube with the introducible, Au
nanoparticles-based photocathode, stability of which has been remaining invariant.
Spectral dependences of photoemission (PE), absorption and reflection from Ag and Au granular films are studied experimentally together with their structure and physical properties using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPES). It is found that a new intensive PE band in the visible spectral range (l = 500 ÷ 600 nm) appears when such films are activated with Cs and O and this PE band coincides with the absorption and reflection bands. Theoretical calculations of PE spectra based on absorption spectrum of metallic oblate spheroidal nanoparticles are also carried out. Such calculations indicate that the appearance of this PE band can be explained by excitation of the surface plasmons in spheroidal nanoparticles with the major axes approximately equal to 50 nm and minor axes approximately equal to 5 nm. Similar calculations carried out for an S-1 photocathode indicate that the shape and the position of the measured long wavelength PE band with the peak maximum at λ ≈ 800 nm can also be explained by excitation of the surface plasmons in Ag spheroidal nanoparticles with the axes equal to 25 and 0.9 nm correspondingly. Degradation with time of PE from Ag and Au granular films is also studied and it is shown that while Ag nanoparticles degrade due to desorption of Cs, Au nanoparticles degrade due to its adsorption.
Photoelectron emission in the studied metallic nanostructures can be explained by the surface photoeffect caused by excitation of the surface plasmons in nanoparticles. Therefore, photocathodes with subfemtosecond-range temporal resolution and quantum yield equal to several percent in the visible wavelength range can be fabricated from such nanostructures.
Comprehensive studies upon the structure and physical properties of Ag-O-Cs photocathode with the AES and XPES techniques are presented. The photoelectron emission spectrum has been measured in situ on each stage of the photocathode preparation. Similar investigations have been carried out as to nanoparticles of Ag with size several tens of nanometers. Being activated with Cs and O, those particles are located on the thin sapphire film surface. It may be concluded that the photoemission from the Ag-O-Cs photocathodes has the same nature as the photoemission from the nanoparticles of Ag on sapphire, namely it is conditioned by the excitation of surface plasmons with concurrent emission of electrons. It also means that the nanoparticles of Ag activated with Cs and O and investigated in this paper may be conceived as a 2-dimensional photocathode. The external photoemission from Ag-O-Cs photocathodes is of strongly pronouned surface nature, with no process of transportation of electrons towards the surface. In this case the characteristic time during which a light wave is able of exciting plasmons at the nanoparticles of Ag is less than 1 fs, which makes the Ag-O-Cs photocathode most perspective from the viewpoint of getting limiting subfemtosecond-range time resolution.
Some technique is presented for IR photocathodes manufacturing on the basis of In0.53Ga0.47As/InP heterostructures with Shottky barrier. Discussed is a method for ultra-high vacuum transfer of the photocathodes into vacuum devices. It is shown that the sensitivity of photocathodes in a sealed out device at (lambda) equals 1.55 micrometers is two orders of magnitude higher comparing to the sensitivity of traditional Ag-Cs-O photocathodes. The developed photocathodes may be used in time analyzing image tubes covering the spectral range from 0.9 to 1.7 micrometers .
A possibility of temporal analysis of picosecond light pulses in the IR region with the help of photocathodes based on semiconductor superlattices (SL) of type I (InP/InGaAs) with Schottky barrier is discussed. A new principle of avalanche photoelectron emission from such an SL at interband absorption of light is suggested. The principle is based on the electrons free length path increasing in a SL with narrow quantum wells under high electric field applied to the SL. The idea makes it possible to develop a new device - avalanche photocathode with internal amplification for the IR region of 0.9-2 micrometers and temporal resolution better than 30 ps. It is proposed to use doped as well as undoped SL as basis for photocathodes sensitive to the IR radiation in the range of up to 10 micrometers . The photoemission from such structures is caused by the intersubband absorption of light in quantum wells. The use of undoped SL greatly reduced the thermoemission current of the photocathode but requires additional excitation of the SL by light pulses with energy approximately corresponding to the band gap of the narrow band gap material of the SL. The temporal resolution of such photocathodes is supposed to be less than 30 ps. The conditions for the avalanche photoelectron emission obtaining are determined, and the SL parameters which meet the requirement of maximum quantum efficiency of the photocathode are calculated.
The properties of IR photocathodes intended for streak tubes as well as InGaAs/InP heterostructures were investigated. It has been shown that optical transmittance, photoluminicence, photovoltaic and C-V measurements made possible to control the heterostructure composition with the precision better than 1% and to determine its type of conductivity and carriers concentration. Since these techniques are nondestructive ones and they do not require any electric contacts deposited onto the structure they could be used for controlling of the grown heterostructures. The properties of InGaAs/InP heterostructures with Schottky barriers were studied by scanning electron microscope (SEM) working in electron beam induced current (EBIC) mode. It has been shown that for the investigated photocathodes the photoemission current value strongly depends on the applied anode voltage, and therefore at high electric field 104 V/cm the photoemission quantum efficiency is substantially increased. Preliminary study of the photoemission current stability were carried out for a sealed volume serving as a model of a streak tube.
Photocathodes based on In0.53Ga0.47As/InP heterostructures (HS-photocathodes) with Schottky barriers for a spectral range of 0.9 - 1.6 micrometers were investigated. The maximum external quantum yield was 0.5% at (lambda) equals 1.5 micrometers and dark current was Ied equals 3*10-8 A/cm2. It has been shown that in such photoemitters under reverse bias of about U equals 30 V, the electric field completely penetrates into the working layer of the photocathode. Since the dark current does not depend on the value of the reverse bias, HS-photocathodes may be used for time analyzing tubes to record picosecond pulses with milliwatt peak intensity. To increase the signal/noise ratio we suggest using InP/In0.53Ga0.47As superlattice (SL) for designing a SL-photocathode with internal amplification.
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