GaAs Photocathode Research Articles

Charge lifetime measurements at high average current using aK2CsSbphotocathode inside a dc high voltage photogun

Two ${\mathrm{K}}_{2}\mathrm{CsSb}$ photocathodes were manufactured at Brookhaven National Lab and delivered to Jefferson Lab within a compact vacuum apparatus at pressure $\ensuremath{\sim}{10}^{\ensuremath{-}11}\text{ }\text{ }\mathrm{Torr}$. These photocathodes were evaluated using a dc high voltage photogun biased at voltages up to 200 kV, and illuminated with laser light at wavelengths 440 or 532 nm, to generate dc electron beams at currents up to 20 mA. Some conditions produced exceptionally large photocathode charge lifetimes, without measurable quantum efficiency decay, even from the center of the photocathode where operation using GaAs photocathodes is precluded due to ion bombardment. Under other conditions the charge lifetime was poor due to extensive ion bombardment under severely degraded vacuum conditions, and as a result of localized heating via the laser beam. Following beam delivery, the photocathodes were evaluated using a scanning electron microscope to determine surface morphology.

Monte Carlo charge transport and photoemission from negative electron affinity GaAs photocathodes

High quantum yield, low transverse energy spread, and prompt response time make GaAs activated to negative electron affinity an ideal candidate for a photocathode in high brightness photoinjectors. Even after decades of investigation, the exact mechanism of electron emission from GaAs is not well understood. Here, photoemission from such photocathodes is modeled using detailed Monte Carlo electron transport simulations. Simulations show a quantitative agreement with the experimental results for quantum efficiency, energy distributions of emitted electrons, and response time without the assumption of any ad hoc parameters. This agreement between simulation and experiment sheds light on the mechanism of electron emission and provides an opportunity to design novel semiconductor photocathodes with optimized performance.

Quantum efficiency conversion from the reflection-mode GaAs photocathode to the transmission-mode one

In the article, the transmission-mode negative electron affinity GaAs photocathode is compared with the reflection-mode one in structure, working principle and the quantum efficiency equation. The key point to establish relation between the two modes is that the active-layer is in full accord and the biggest difference is that the incident direction of photons is inverse. Based on the relation and difference above, we obtain the electron escape probability P and the spectrum absorption coefficient α of the reflection-mode GaAs photocathode by multilayer film matrix theory. The quantum efficiency of the transmission-mode GaAs photocathode can be estimated through the coefficients P and α of the reflection-mode one. Two groups of samples are analyzed, the one group contains uniform doping reflection-mode and transmission-mode GaAs photocathodes and the other contains exponential doping ones. Through the analysis and curve fitting, we find that the conversion difference of the uniform-doping is about 11.22% while that of the exponential-doping is about 5.46%. Another four groups of samples with exponential-doping active layer are experimentalized and the differences of them are all less than 6%. The results prove the suggested method is feasible and effective and it is more suitable for the conversion of GaAs photocathode with exponential-doping active layer.

Numerical calculation method of modulation transfer function for preproximity focusing electron-optical system

In order to explore the relation between the modulation transfer function (MTF) and the halo effect for low-light-level (LLL) image intensifiers, the MTF for the preproximity focusing electron-optical system is calculated according to the electron distribution on the microchannel plate input face. During the calculation, the halo effect from secondary scattered electrons is not treated. By tracing the trajectory of photoelectrons emitted from one point on a GaAs photocathode into the preproximity focusing electron-optical system, the electron distribution is calculated, namely the point spread function. The MTF for the preproximity focusing electron-optical system is numerically calculated according to the electron distribution, which is fitted. The results show that the fitting curve of the MTF is in agreement with the analytic expressions. When the spatial frequency is less than 50 lp/mm, the relative error is below 5%. This research provides theoretical support for further development of LLL night-vision technology.

Relationship between X-Ray Relative Diffraction Intensity and Integral Sensitivity of GaAlAs/GaAs Photocathode

To establish a methode for predicting the integral sensitivity of transmission-mode GaAs photocathodes, the relationship between X-ray relative diffraction intensity and integral sensitivity of GaAlAs/GaAs photocathode material is researched. After thermocompression bonding Si3N4/GaAlAs/GaAs/GaAlAs/GaAs epitaxial material to glass window in the vacuum condition, and chemically etching the GaAlAs buffer-layer and GaAs substrate, the glass/Si3N4/GaAlAs/GaAs photocathode module is formed. The X-ray relative diffraction intensity of the photocathode module is tested and calculated respectively, then the photocathode surface was activated in the ultrahigh vacuum chamber using the Cs-O activation technique. Following that, the integral sensitivity of the transmission-mode GaAs photocathode is measured by the spectral response measurement instrument in situ. It is found that the GaAlAs/GaAs photocathode material and photocathode module have similar X-ray relative diffraction shapes. The higher the similar degree of X-ray relative diffraction shape is, the bigger the X-ray relative diffraction intensity of photocathode module is, which results in the better photoemission capability and higher photocathode integral sensitivity. This method can be used as an evaluation criterion for the quality of transmission-mode GaAs photocathode module material.

Search for spin-polarized photoemission from GaAs using light with orbital angular momentum

Laser light with photon energy near the band gap of GaAs and in Laguerre-Gaussian modes with different amounts of orbital angular momentum was used to produce photoemission from unstrained GaAs. The degree of electron spin polarization was measured using a micro-Mott polarimeter and found to be consistent with zero with an upper limit of \ensuremath{\sim}3$%$ for light with up to $\ifmmode\pm\else\textpm\fi{}5\ensuremath{\hbar}$ of orbital angular momentum. In contrast, the degree of spin polarization of 32.3 \ifmmode\pm\else\textpm\fi{} 1.4$%$ using circularly polarized laser light at the as the same wavelength, which is typical for bulk GaAs photocathodes.

Study on X-Ray Integral Diffraction Intensity of GaAs Photocathode

In order to precisely predict the sensitivity of Ф18 mm transmission-mode GaAs photocathode, a concept of integral diffraction intensity is proposed based on X-ray diffraction principle after analyzing the predecessors′ limitations of testing the micro-area of such photocathode and GaAs photocathode of image intensifier tube is plane electron source in this paper. The integral diffraction intensity on the entire photocathode surface was obtained by multi-points detection in the effective area of the photocathode with integral method. The crystal quality of entire photocathode surface will be taken with the integral diffraction intensity. According to the principle, X-ray diffraction testing for 4 samples of GaAs photocathode modules was executed with high-resolution four-wafer X-ray Diffractometer whose test spot size is 4 mm×5 mm. The diffraction curves were obtained and the integral diffraction intensity was calculated. Subsequently the 4 photocathode modules was activation processed with Cs-O in ultra-high vacuum system simultaneity the photocurrent of photocathode modules was measured. Comparing the variation of diffraction curve with integral diffraction vs photocathode photocurrent curve, they show that the greater the integral diffraction intensity of is, the more photocurrent is in the photocathode module. The variation relation curve between X-ray integral diffraction intensity and photocurrent in the photocathode was fitted with least square method. The curve, which accords with logarithm curve and whose fitting degree is 0.878, was achieved. Since photocathode sensitivity is direct proportion to photocathode photocurrent. The above results prove that A Practicality Φ18mm photocathode of image intensifier tube is plane electron source, GaAs photocathode sensitivity and other photoelectric performance lies on entire photocathode surface crystal quality, the photocathode module integrality reflected by the integral diffraction intensity plays crucial role of GaAs photocathode sensitivity. So integral sensitivity of Ф18 mm transmission-mode GaAs photocathode can be precisely predicted with X-ray integral diffraction intensity, some feasible ideas for further research of GaAs photocathode was obtained in this paper.

Comparison between exponential-doping reflection-mode GaAlAs and GaAs photocathodes

A reflection-mode GaAlAs photocathode and a reflection-mode GaAs photocathode using exponential-doping technique are prepared by metal organic chemical vapor deposition, and the Al content of GaAlAs emission layer is 0.63. The two photocathodes are activated in an ultra-high vacuum system, and the spectral response curves are measured after activation. The quantum efficiency formula for exponential-doping reflection-mode photocathode is used to fit the experimental curves of the two photocathodes respectively, and the effects of some performance parameters on photoemission are analyzed, such as electron diffusion and drift length, back-interface recombination velocity, surface electron escape probability, etc. The results show that the Al content of the GaAlAs photocathode plays a bad role in the photoemission compared with that the GaAs photocathode, but it solves the problem that the GaAs photocathode cannot be well used in the area of detecting the narrow wavelength light due to the broad spectral response. The reflection-mode GaAlAs photocathode prepared is responsive to the blue and green light.

Comparative research on indium seal process for transmission-mode GaAs photocathodes

For night vision devices and other applications, the transmission-mode photocathodes must be sealed to tube by indium seal process (ISP) in practical application. But in early research, the integral sensitivity has large drop to about 30% amplitude after ISP process than after activation process. In order to well study the influence of ISP on surface barriers of activated GaAs photocathodes, we used the comparative research method by surface photovoltage spectroscopy (SPS) and spectral response current (SRC). Through fitting calculation, we can find that the impurity gas sources by micro channel plate and fluorescent screen have deep influence on the amplitude and width of surface barriers which lead to the large drop on SRC curves before and after ISP process. This on-line comparative research method can help to optimize ISP technique and vacuum degree optimization for transmission-mode GaAs photocathodes in the future.

Theoretical Modification on Optic Properties for Transmission-Mode GaAs Photocathode Module

To explore the optic properties of transmission-mode GaAs photocathode module, experimental and theoretical values of reflectance and transmittance of photocathode module has been compared. it showns that experemental curves cannot tally with theoretical curves completely. The variation range of initial values of thickness is firstly setted. Modifing transmittance formula by a fitting coefficient A, optical properties is fitted using the method of error control. R-T combined error reduced from 15.2% to 4.9% using R-T combined error control scheme. Optimal fitting values of thickness of photocathode module are obtained, which are d1= 110 nm, d2= 1019 nm, d3= 1491 nm. And the error of total thickness is 2.9%.

GaAs光阴极高温热清洗

GaAs光阴极高温热清洗

Compact noninvasive electron bunch-length monitor

A compact rf cavity was constructed that simultaneously resonates at many harmonic modes when excited by a repetitive bunched electron beam passing through its bore. The excitation of these modes provides a Fourier description of the temporal characteristics of the bunch train. The cavity was used to noninvasively characterize electron bunches produced from thin and thick GaAs photocathodes inside a DC high voltage photogun illuminated with 37 ps (full width half maximum, FWHM) laser pulses at repetition rates near 1500 MHz, at average beam current from 5 to $500\text{ }\ensuremath{\mu}\mathrm{A}$, and at beam energy from 75 to 195 keV. The cavity bunch-length monitor could detect electron bunches as short as 57 ps (FWHM) when connected directly to a sampling oscilloscope, and could clearly distinguish bunches with varying degrees of space-charge induced growth and with different tail signatures. Efforts are under way to detect shorter bunches by designing cavities with increased bandwidth. This demonstration lends credibility to the idea that these cavities could also be used for other applications, including bunching and shaping, when driven with external rf.

Comparative research on Cs/O activation source ratio for GaAs photocathodes

Through the comparative research on cathode material by surface photovoltage spectroscopy (SPS) before activation and spectral response current (SRC) after activation, the value of surface escape probability (SEP) can be well measured. By the experiments and calculations for the designed cathode material, the influence of Cs/O activation source ratio on SRC and SEP can be well fitted. The lower and excessive source ratio can not get well SEP value and only the appropriate source ratio can get the better SEP value. Through the analysis of surface barrier layer model, the influence of Cs/O source ratio on the formation of surface barrier layer were well discussed. The appropriate source ratio can help to form the best double dipole arrange structure and better depth of Cs–O layer. This comparative research method can help to well study and optimize the Cs/O activation technique and varied doping structure design for GaAs photocathodes in the future.

Energy distributions of electrons emitted from reflection-mode Cs-covered GaAs photocathodes

By calculating the energy distributions of electrons reaching the photocathode surface and solving the Schrödinger equation for an electron tunneling through the surface potential barrier, we have obtained an equation to calculate the energy distributions of electrons emitted from reflection-mode Cs-covered GaAs photocathodes based on a two-minima diffusion model. According to the equation, we studied the effects of incident photon energies, diffusion lengths, and surface potential barrier on the electron energy distributions. The equation was also used to fit the measured electron energy distribution curves and the cathode performance parameters were obtained from the fitting. The Γ and L peaks in the theoretical curves are in agreement with the peaks in the experimental curves. The fitted barrier thickness 1.7 Å exactly reflects the GaAs-Cs dipole layer thickness.

Research on quantum efficiency of reflection-mode GaAs photocathode with thin emission layer

The conventional quantum efficiency formula for the reflection-mode GaAs photocathode is not applicable to the photocathode with a thin emission layer. The revised quantum efficiency formula has been solved from one-dimension continuity equations, in which the electrons generated in the GaAlAs buffer layer are considered. According to the revised formula, we analyze the impact of some relational performance parameters on the quantum efficiency, such as the thickness of emission layer, the interface recombination velocity etc. Besides, we use the revised and conventional formula to fit the experimental curves of two photocathodes which have the different-thickness emission layers respectively. The results show that compared with the conventional quantum efficiency formula, the revised formula is more suitable to the reflection-mode GaAs photcathode with a thin emission layer, and the GaAlAs buffer layer takes part in the photoemission.

Study on the influence of recombination velocity to quantum efficiency of reflection-mode GaAs photocathode

The experiment result of American ITT company shows that the quick recombination velocity on the front surface of GaAs photocathode affects the quantum efficiency seriously. In order to explain this phenomenon theoretically, the quantum efficiency formula which include front surface recombination velocity is firstly deduced for reflection-mode GaAs photocathode by using integral deduction method, realize the evaluation of the photocathode parameters which cannot be detected directly, especially the quantum efficiency, provide a guidance to further study GaAs photocathode.

Quantum efficiency dependence on built-in electric fields in exponential-doped and graded-doped gallium arsenide photocathodes

We calculated the intensities and widths of the built-in electric fields of exponential-doped and graded-doped gallium arsenide (GaAs) photocathode for the first time. We analyzed the quantum efficiency (QE)for both samples with these two factors, along with the absorption coefficient and found out that although the exponential-doped sample was more complicated in structure than the graded-doped sample, the QE was not enhanced in the full experimental waveband as we expected, especially for those electrons with lower energy. It shows that the escape probability and diffusion length mainly depend on the intensity of the electric fields, which is believed the most decisive factor for the QE enhancement.

Surface science analysis of GaAs photocathodes following sustained electron beam delivery

Degradation of the photocathode materials employed in photoinjectors represents a challenge for sustained operation of nuclear physics accelerators and high power free electron lasers (FEL). Photocathode quantum efficiency degradation is due to residual gases in the electron source vacuum system being ionized and accelerated back to the photocathode. These investigations are a first attempt to characterize the nature of the photocathode degradation, and employ multiple surface and bulk analysis techniques to investigate damage mechanisms including sputtering of the Cs-oxidant surface monolayer, other surface chemistry effects, and ion implantation. Surface and bulk analysis studies were conducted on two GaAs photocathodes, which were removed from the JLab FEL DC photoemission gun after delivering electron beam, and two control samples. The analysis techniques include helium ion microscopy, Rutherford backscattering spectrometry (RBS), atomic force microscopy, and secondary ion mass spectrometry (SIMS). In addition, two high-polarization strained superlattice GaAs photocathode samples, one removed from the continuous electron beam accelerator facility (CEBAF) photoinjector and one unused, were also analyzed using transmission electron microscopy (TEM) and SIMS. It was found that heat cleaning the FEL GaAs wafer introduces surface roughness, which seems to be reduced by prolonged use. The bulk GaAs samples retained a fairly well organized crystalline structure after delivering beam but show evidence of Cs depletion on the surface. Within the precision of the SIMS and RBS measurements, the data showed no indication of hydrogen implantation or lattice damage from ion back bombardment in the bulk GaAs wafers. In contrast, SIMS and TEM measurements of the strained superlattice photocathode show clear crystal damage in the wafer from ion back bombardment.

Mean Transverse Energy Measurement of Negative Electron Affinity GaAs-Based Photocathode

A negative electron affinity GaAs photocathode electron source is characterized by high brightness, high quantum efficiency, and a moderate temporal response. The initial emittance depends on the mean transverse energy (MTE) of the electrons on the cathode surface. We evaluated the MTE based on emittance measurements obtained using the waist scan method with three types of cathodes: bulk GaAs, thickness-controlled samples with active-layer thicknesses of 100 and 1000 nm, and a GaAs/GaAsP superlattice sample. The dependence of the cathode quantum efficiency, the laser wavelength, and the thickness of the GaAs cathode active layer on the MTE are described. In the case of the bulk GaAs and the thickness-controlled samples, it was determined that the thickness and cathode quantum efficiency do not affect the MTE within the measurement error. The laser wavelength, on the other hand, affects the MTE of all cathodes.

Mean Transverse Energy Measurement of Negative Electron Affinity GaAs-Based Photocathode

A negative electron affinity GaAs photocathode electron source is characterized by high brightness, high quantum efficiency, and a moderate temporal response. The initial emittance depends on the mean transverse energy (MTE) of the electrons on the cathode surface. We evaluated the MTE based on emittance measurements obtained using the waist scan method with three types of cathodes: bulk GaAs, thickness-controlled samples with active-layer thicknesses of 100 and 1000 nm, and a GaAs/GaAsP superlattice sample. The dependence of the cathode quantum efficiency, the laser wavelength, and the thickness of the GaAs cathode active layer on the MTE are described. In the case of the bulk GaAs and the thickness-controlled samples, it was determined that the thickness and cathode quantum efficiency do not affect the MTE within the measurement error. The laser wavelength, on the other hand, affects the MTE of all cathodes.