GaAs Photocathode Research Articles

Study on the time-resolved characteristics of the transmission-mode GaAs photocathode

Study on the time-resolved characteristics of the transmission-mode GaAs photocathode

On the energy resolution of a GaAs-based electron source for spin-resolved inverse photoemission

The spin resolution in inverse photoemission spectroscopy is achieved by injecting spin-polarized electrons, usually produced by GaAs-based cold cathodes that replace hot-filament electron guns of spin-integrated setups. The overall energy resolution of the system can be enhanced by adjusting either the optical bandpass of the photon detector or the energy distribution of the electron beam. Here we discuss the influence of the photocurrent and the photocathode temperature on the energy broadening of the electron beam through the inverse photoemission spectra of the spin-splitted Shockley surface state of Au(111). First, we find that cooling down the GaAs photocathode to 77 K increases the band gap and reduces the number of allowed vertical transitions, monochromatizing the electron beam with an enhancement of about 30 meV for the energy resolution. Second, we observe a correlation between the generated photocurrent at the electron source, and the space-charge effects at the sample as a reduction of lifetime and spin asymmetry of a polarized bulk state. These observations allow defining a threshold of current density for the optimum acquisition in the measurements of spin-resolved inverse photoemission in Au.

Effect of Improved 'yo-yo' Cs/O Alternate Activation Method on Stability of GaAs Photocathode

Effect of Improved 'yo-yo' Cs/O Alternate Activation Method on Stability of GaAs Photocathode

Photoelectron transportation dynamics in GaAs photocathodes

We report here a general theory describing photoelectron transportation dynamics in GaAs semiconductor photocathodes. Gradient doping is incorporated into the model through the inclusion of directional carrier drift. The time-evolution of electron concentration in the active layer upon the injection of an excitation pulse is solved both numerically and analytically. The predictions of the model are compared with experiments via a carrier-induced transient reflectivity change, which is measured for gradient-doped and uniform-doped photocathodes using femtosecond pump–probe reflectometry. An excellent agreement is found between the experiments and the theory, leading to the characterization of key device parameters, such as diffusion constant and electron decay rates. Comparisons are also made between uniform doping and gradient doping for their characteristics in photoelectron transportation. Doping gradient is found to be able to accelerate electron accumulation on the device surface. These results offer new insights into the dynamics of III–V photocathodes and potentially open a new avenue toward experimental characterization of device parameters.

Creating gallium arsenide photocathodes with high quantum efficiencies

Method for activating GaAs photocathodes using an initial surface heat-cleaning step.

High quantum efficiency GaAs photocathodes activated with Cs, O2, and Te

GaAs photocathodes are the primary choice for generating spin-polarized electron beam with high brightness, high polarization, and fast polarization reversal. However, it suffers from short lifetime due to the highly reactive nature of the emission surface, resulting in substantial operational difficulties. Activating GaAs with a more robust material, such as Cs2Te, shows comparable polarization to that of Cs–O activation and increases the lifetime due to the robustness of the Cs2Te layer. However, previously reported photocathodes based on Cs–Te activation on GaAs suffer from 10× lower quantum efficiency (QE) compared to that activated with conventional Cs–O activation. Herein, we report activation recipes for GaAs photocathodes using Cs, O2, and Te. For Cs–Te activation, the QE was 6.6% at 532 nm. For Cs–O–Te activation, the QE was 8.8% at 532 nm and 4.5% at 780 nm. The negative electron affinity of the activated GaAs was directly measured and confirmed by low energy electron microscopy. We also report the activation layer chemical states and stoichiometry using in situ micro-spot synchrotron radiation x-ray photoelectron spectroscopy.

Theoretical study on the optoelectronic properties of GaAs nanostructures with Al component gradient change

In order to improve the photoemission characteristics of the GaAs photocathode, GaAs nanostructures (nanoholes and nanowires) with three sublayers and Al component of gradient distribution are designed in this article. Finite element method was used to study the effects of Al component gradient interval, component range, height, diameter, and other parameters on light absorption of nanostructures. And the quantum efficiency of these nanostructures was calculated. The results show that the wide band absorption can be achieved with small gradient interval and component variation range, and the nanostructure can enhance the light capture ability with certain angle inclination. In the gradient component nanostructures, the thickness of the top most layer has a great influence on the quantum efficiency. Highlights The nanohole and nanowire structures with three sublayers and Al component gradient distribution were designed. The effects of gradient interval, range of Al components, and geometric parameters on the optical properties of nanostructures are discussed.

Effect of excessive Cs and O on activation of GaAs(100) surface: From experiment to theory

The surface Cs–O activation process directly determines quantum efficiency and stability of negative-electron-affinity photocathodes. To investigate the effects of excessive Cs and O supply on activation and to explore a more effective Cs–O activation recipe, Cs–O activation experiments of GaAs(100) photocathodes are carried out based on the current-driven solid Cs and O dispensers. By a comparison of differences in activation photocurrent, quantum efficiency, and photocurrent decay, it is found that the recipe of excessive O and non-excessive Cs is not suitable for activating GaAs photocathodes, while the recipe of continuous and completely excessive Cs along with intermittent and non-excessive O can achieve the most excellent photoemission performance, including the highest quantum efficiency in the long-wave threshold region and best stability under intense light irradiation after activation. Furthermore, this improved activation recipe with the least Cs–O alternating cycles is easier to operate. Combined with density functional calculations and dipole layer model, it is found that the activation recipe of completely excessive Cs and non-excessive O can form effective dipoles to the greatest extent, and avoid the direct interaction between As atoms and O atoms to form As–O–Ga oxides on the GaAs(100) reconstructed surface.

Improved preparation techniques for preparing high-performance GaAs photocathodes.

Efficiency and lifetime are always problems raised with photocathodes during operation. With the purpose of obtaining high-performance GaAs photocathodes with high sensitivity and long operational lifetime, it is necessary to investigate the preparation techniques during both the cleaning and the activation procedure. By comparison with the classical preparation techniques, the improved preparation techniques with an optimized chemical etching method and activation procedure are proposed. The experimental results show that the optimized chemical etching solution is more effective in removing oxide and carbon contamination, which can help photocathodes obtain higher sensitivity. On this basis, better long wavelength response and longer operational lifetime can be obtained with the help of the more competitive activation procedure. The proposed preparation techniques will be useful for applications as a source of spin-polarized electrons.

First-principles investigation of Cs-NF3 co-adsorption on GaAs(100)-β2(2 × 4) surface

The mechanism of Cs-NF3 adsorption on the GaAs(100)-β2(2 × 4) reconstructed surface is investigated by using the first-principles method based on density functional theory. Cs-NF3 adsorption cases at ten different sites named as D, D′, T2, T2′, T3, T3′, T4, T4′, H and H′ with two opposite vertical directions are simulated. The results show that the vertical layered direction can remarkably affect surface properties. Compared with the NF3-up models, the Cs-up models can possess lower adsorption energy, smaller work function and stronger dipole moment. Meanwhile, it is found that in the Cs-up models, the trench site T4 is the energy-preferred position for its lowest adsorption energy while the topmost As-dimer bridge site D is the most unstable for adsorption for its highest adsorption energy. Besides, the hollow site H′ can achieve the maximum drop of work function. Furthermore, Mulliken charge distribution indicates that the NF3 molecule gets negatively charged in Cs-up models while that is neutrally charged in NF3-up models. Combined with the change of atomic geometric structure, it is inferred that the [Cs+-NF3-] dipole is formed in Cs-up models while only the [Cs+-As-] dipole is formed in NF3-up models. These results can help to understand Cs-NF3 co-activation behavior for GaAs photocathode.

Revisiting heat treatment and surface activation of GaAs photocathodes: In situ studies using scanning tunneling microscopy and photoelectron spectroscopy

The lifetime of GaAs photocathodes in polarized electron guns is limited due to the delicate activation layer. An atomically clean and smooth GaAs surface is needed to deposit a robust activation layer, such as Cs2Te, with longer lifetime compared to traditional (Cs,O) activation. A previous experiment with Cs2Te activation on GaAs used heat cleaning temperatures around 400°C to avoid an increase in surface roughness [Bae et al., Appl. Phys. Lett. 112, 154101 (2018)]. High-temperature heat cleaning around 580°C, which results in a relatively contamination-free surface, could be one possible way to improve quantum efficiency. However, one should be cautious about surface roughness degradation during high-temperature heat cleaning. In this paper, we report results of surface roughness measurements on native, heat cleaned, and (Cs,O) activated GaAs photocathodes under vacuum. The results, measured by ultrahigh vacuum scanning tunneling microscopy, show that the surface roughness improves as the heat cleaning temperature is increased, by at least a factor of three for 580°C heat cleaning, compared to the native sample. Activation with (Cs,O) is shown to increase surface roughness by a factor of four compared to a 580°C heat cleaned sample. This confirms that high-temperature heat cleaning can be useful for depositing good quality robust activation layers on GaAs. We also report chemical analysis for each step of preparation for p-doped GaAs photocathodes using X-ray photoelectron spectroscopy (XPS), angle-resolved XPS, and ultraviolet photoelectron spectroscopy. Our results indicate that the (Cs,O) activation layer forms a sandwich structure consists of Cs and oxygen. We found no formation of any specific compound such as Cs2O or Cs11O3.

Enhancement of Photoemission on p-Type GaAs Using Surface Acoustic Waves.

We demonstrate that photoemission properties of p-type GaAs can be altered by surface acoustic waves (SAWs) generated on the GaAs surface due to dynamical piezoelectric fields of SAWs. Multiphysics simulations indicate that charge-carrier recombination is greatly reduced, and electron effective lifetime in p-doped GaAs may increase by a factor of 10× to 20×. It implies a significant increase, by a factor of 2× to 3×, of quantum efficiency (QE) for GaAs photoemission applications, like GaAs photocathodes. Conditions of different SAW wavelengths, swept SAW intensities, and varied incident photon energies were investigated. Essential steps in SAW device fabrication on a GaAs substrate are demonstrated, including deposition of an additional layer of ZnO for piezoelectric effect enhancement, measurements of current–voltage (I–V) characteristics of the SAW device, and ability to survive high-temperature annealing. Results obtained and reported in this study provide the potential and basis for future studies on building SAW-enhanced photocathodes, as well as other GaAs photoelectric applications.

Recovery of quantum efficiency on Cs/O-activated GaN and GaAs photocathodes by thermal annealing in vacuum

In this paper, the authors describe the effectiveness of thermal annealing in vacuum for quantum efficiency (QE) recovery from Cs/O-activated GaN and GaAs photocathodes. The QE of Cs/O-activated GaN photocathodes at 3.4 eV dropped from 1.0% to <0.001% upon exposure to nitrogen and then increased to 0.6% upon annealing. On the other hand, the QE of Cs/O-activated GaAs at 1.42 eV did not increase after annealing. In addition, after Cs/O activation, the sample was exposed to normal laboratory air and installed in an X-ray photoemission spectroscopy system. Upon annealing at 330 °C, three key results were confirmed as follows: (1) the work function decreased by 0.32 eV, (2) the chemical states of Cs 4d and Ga 3d were unchanged, and (3) the intensities of O 1s and C 1s on the high-binding-energy side decreased. In conclusion, the experimental results indicate that the annealing recovers the QE of Cs/O-activated GaN photocathode.

The electron emission characteristics of GaAs photocathode with vacuum-channel structure

The photocathode was an ideal electronic source with fast response, centralized distribution of electronic capabilities, and many other advantages. But for its low emission current density and short lifetime at ion bombardment environment, it will not be used in power devices and large scientific devices. Although the emission current of the photocathode can be increased with the aid of electric filed, it was not reach the milliamperelevel. In the field emission cathode, the strong electric field can effectively reduce the surface barrier of the emitter, so that the cathode produces electron emission. Based on the photoelectric conversion mechanism of photocathode and the electron emission theory of field emission cathode, a vacuum channel structure photocathode module is designed. The cathode modules with GaAs substrate were fabricated by the peritoneal and etching process. The parameters of vacuum channel were 3 μm × 700 μm, and its array structure was 1 × 100. The deep of the vacuum channels were 400 nm. It can be seen from the test result, the emission current of the photocathode was 26.12 mA with the laser beam power 5 W, and its emission current density was 5.33 A/cm<sup>2</sup>. During cathode operation, the laser beam and emission will cause the temperature of the photocathode material to rise, and this could icreasing the conductivity of the GaAs, the efficiency of electron supplement from negative electrode to electron emission area of the GaAs will also be improving. Its emission current could reach 89.69 mA at 400 ℃. Because there is no active atom on the photocathode surface, its lifetime was longer than the traditional photocathode that astivated by Cs/O. The life test of the photocathode modules was carried out, its emission current was 4.5 mA ± 0.3 mA within the 144 h, and its performance was not attenuate significantly. The vacuum channel was the main area of the photocatode electron emission, and the shape of electron beam can be easily obtained by adjusting the structure of the vacuum channel parameters. This advantage can improve the applicability of high current density photocathode in vacuum electronic devices and equipment.

Pre-Emission Study of Photoelectron Dynamics in a GaAs/AlGaAs Photocathode

Understanding the carrier dynamics in III-V semiconductor photocathodes is of great importance to the development of many novel optoelectronic systems such as low-light imagers and coherent x-ray sources. Past experimental effort to study ultrafast response of GaAs photocathodes has relied exclusively on post-photoemission measurement, i.e. via characterization of the temporal properties of emitted electron bunches. Such an indirect approach is not only difficult to achieve experimentally but also liable to sample surface conditions when assessing key carrier dynamics such as photoelectron diffusion and decay. In this report, we present a pre-photoemission investigation of photoelectron transport and population decay in a GaAs/AlGaAs photocathode based on pump-probe transient reflectometry. By measuring the ultrafast behaviors of reflectivity after the excitation of a femtosecond optical pulse, we are able to evaluate the transient evolution of photoelectron population near the device surface, which provides a direct picture of electron diffusion and decay. A carrier diffusion model shows excellent agreement with experiment. Experiment-theory comparisons also suggests a diffusion coefficient much greater than some of the post-photoemission predictions. In addition, the free-electron population decay near the surface is found to be of bi-exponential nature. Possible physical mechanisms underlying these decay lifetimes are discussed.

Comparison of photoemission performance of a GaAs photocathode between white light and monochromatic light illumination during activation.

To obtain higher quantum efficiency and longer operational lifetime of negative-electron-affinity GaAs-based photocathodes, illumination conditions using different light sources during the activation process are explored. GaAs photocathodes were activated under white light and 633 nm monochromatic light with different intensities, and experimental quantum efficiencies and photocurrent degradations were compared after activation and recaesiation. The results show that GaAs photocathode can obtain higher quantum efficiency and longer lifetime by using illumination of 633 nm monochromatic light, and this advantage can hardly be achieved by adjusting the intensity of white light. This work verifies an improved solution for preparing GaAs-based photocathodes with satisfactory capability.

Functionalized GaAs Photocathodes for Spin-Dependent Charge Transfer Studies

Chiral-Induced Spin Selectivity (CISS) is a well-documented phenomenon occurring at electrochemical interfaces, where chiral functional groups can hinder the transfer of electrons with a specific spin state, while allowing the electrons with an opposite spin state to be transferred to the electroactive species in the solution.1,2 Spin dependent electron transfer has also been observed at p-GaAs/electrolyte interface without chiral functionalities, relying on the spin selectivity between polarised electrons in the conduction band (CB) of the p-GaAs and spin-oriented radical species in the solution.3,4 Here we report spin-dependent charge transfer in electrochemistry, where the spin selectivity is achieved by manipulating the electron spins of a radical redox mediator in the sample solution using microwaves (MW) under resonance conditions. This work has required us to characterise the photocathodic behaviour of p-GaAs in contact with methyl viologen radical cation (MV+•) in organic solvents under very low light intensities, a domain not traditionally covered in semiconductor electrochemistry. We have also contrasted the behaviour of the bare p-GaAs to biphenyl- and peptide-functionalised surfaces, showing that the parasitic valence band (VB) and surface state (SS) mediated processes can be completely avoided. In the future, we hope that these functionalised surfaces allow us to increase the spin-dependent signal intensity, as the probability of electron’s spin state relaxing due to parasitic processes can be minimised. Figure 1

Monte Carlo modeling of thin GaAs photocathodes

A Monte Carlo model was developed to simulate electron transport and emission from thin GaAs photocathodes with different active layer thicknesses and dopant concentrations. The simulation accurately predicts expected behavior, namely, quantum efficiency (QE) is enhanced for thicker GaAs photocathodes and for higher dopant concentrations. More significantly, the simulation predicts that electrons excited to the conduction band of the GaAs can be reflected by the band bending regions of the AlGaAs barrier layer, which contributes to enhance QE. The simulation also predicts that electrons in the conduction band suffer more scattering for thicker GaAs photocathodes and for higher dopant concentration, leading to longer emission response time. This Monte Carlo model will improve our understanding and predicting of the performance of more complicated GaAs-based heterojunction structures composed of multiple thin layers.

Increasing charge lifetime in dc polarized electron guns by offsetting the anode

Charge lifetime of strained superlattice GaAs photocathodes in DC guns is limited by ion back bombardment. It needs to be improved at least an order of magnitude to meet the requirements for future colliders such as Electron-Ion Collider (EIC). In this work, we propose and present simulation results for an offset anode scheme to increase charge lifetime in DC guns. This scheme eliminates the bombardment of high energy ions on the cathode and enables maximum usage of the available cathode area. Depending on the size of the available cathode area, this method can increase the charge lifetime by an order of magnitude compared to the current best alternative method. An anode assembly capable of in-vacuum movement is required for this method, which has been designed and fabricated at Brookhaven National Laboratory.

A test system for optimizing quantum efficiency and dark lifetime of GaAs photocathodes

Gallium-arsenide-based photocathodes are used for a variety of applications, including electron-beam production for particle accelerators. An ultra-high vacuum teststand has been developed for cleaning and preparation of photocathodes. First studies using the apparatus were carried out investigating quantum efficiency and vacuum (dark) lifetimes of GaAs photcathodes that were prepared using different procedures involving Cs, Li, and O2. While the two-stage activation procedure with Cs and O2 yielded the highest quantum efficiency of up to (25.9± 1.3) % at wavelength 405 nm, the vacuum lifetime of cathodes activated with Cs, Li, and O2 using a two-stage process exceeded those obtained in single-stage process by more than a factor of three.