Negative Electron Affinity Photocathodes Research Articles

Cs/O co-adsorption on C-doped GaAs surface: From first-principles simulation to experiment

C-doped GaAs is considered a potential material for negative electron affinity photocathodes, where the p-type doped property is beneficial to photoemission. To clarify the stability and efficiency during Cs/O activation, the gradient concentration of Cs adsorption and Cs/O co-adsorption models of C-doped GaAs are established. The work function, adsorption energy, and surface dipole moment are intensified by first principles calculation based on density functional theory. Experimental results demonstrate that Cs/O activation effectively enhances the performance of C-doped GaAs photocathodes, resulting in high levels of quantum efficiency. Therefore, we conclude that C-doped GaAs photocathodes have the potential to significantly improve the photoelectric emission performance and stability of GaAs photocathodes, making them a viable candidate for future applications.

Adsorption structure deteriorating negative electron affinity under the H2O environment

Photocathodes with negative electron affinity (NEA) characteristics have various advantages, such as small energy spread, high spin polarization, and ultrashort pulsing. Nitride semiconductors, such as GaN and InGaN, are promising materials for NEA photocathodes because their lifetimes are longer than those of other materials. In order to further prolong the lifetime, it is important to better understand the deterioration of NEA characteristics. The adsorption of residual gases and back-bombardment by ionized residual gases shorten the lifetime. Among the adsorbed residual gases, H2O has a significant influence. However, the adsorption structures produced by the reaction with H2O are not comprehensively studied so far. In this study, we investigated adsorption structures that deteriorated the NEA characteristics by exposing InGaN and GaAs to an H2O environment and discussed the differences in their lifetimes. By comparing the temperature-programmed desorption curves with and without H2O exposure, the generation of CsOH was confirmed. The desorption of CsOH demonstrated different photoemission behaviors between InGaN and GaAs results. InGaN recovered its NEA characteristics, whereas GaAs did not. Considering the Cs desorption spectra, it is difficult for an NEA surface on InGaN to change chemically, whereas that for GaAs changes easily. The chemical reactivity of the NEA surface is different for InGaN and GaAs, which contributes to the duration of photoemission. We have attempted to prolong the lifetime of InGaN by recovering its NEA characteristics. We found that InGaN with NEA characteristics can be reused easily without thermal treatment at high temperatures.

Electronic Behavior of GaN Nanowire Photocathode Surface Coated with n‐Type GaN Capping Layer via First Principles

Conventional negative electron affinity photocathodes require alternate activation of Cs/O, and the instability of Cs makes the cathode surface easily inactive. A Cs‐free GaN photocathode structure is reported, in which band engineering of the photocathode surface induced by Si doping eliminates the need to use Cs for photocathode activation. This heterojunction structure can obtain a low‐barrier surface without activation. GaN nanowires with n‐type capping layers are established using first principles, and their stability, work function, and optoelectronic structures are analyzed. Si atoms replace Ga atoms more easily, thereby forming an n‐type capping layer. The formation energy of the n‐type capping layer of the nanowires surface is lower than that of the surface capping layer of the thin film. The addition of the n‐type capping layer reduces the work function of the GaN surface, which will help to enhance photoemission capability. The results clearly illustrate the positive effect of the n‐type capping layer on the electronic properties of the device, so this Cs‐free activation structure can be further considered.

The photoemission characteristics of a NEA InGaN photocathode by simultaneously supplying Cs and O2

A high quantum efficiency (QE) can be obtained on negative electron affinity (NEA) surfaces. It is well-known that NEA surfaces can be formed on semiconductor materials such as GaAs by the alternating supply of cesium (Cs) and oxygen (O2), which is called the yo-yo method. While GaN and related compounds such as InGaN are expected to realize an NEA photocathode with a long lifetime, the surface reactions between GaAs and nitride semiconductors are completely different with respect to the increasing rate of QE induced by the supply of O2. In addition, the surface processes of photoemission from NEA nitride semiconductors have not yet been elucidated. In the present study, a higher QE was achieved in InGaN by simultaneously supplying Cs and O2 instead of using the conventional yo-yo method. The possible Cs adsorption states in relation to the photoemission are also discussed based on the QE tendencies and the temperature programmed desorption (TPD) spectra of the NEA surfaces formed under elevated temperature conditions. This study suggests that the Cs oxide species, which is one of the key compounds for imparting the NEA nature, decomposes at approximately 350 °C and that the InGaN-Cs2O2 structure is a possible candidate for NEA photocathodes.

Brightness evaluation of pulsed electron gun using negative electron affinity photocathode developed for time-resolved measurement using scanning electron microscope

Temporal changes in carrier relaxations, magnetic switching, and biological structures are known to be in the order of ns. These phenomena can be typically measured by means of an optical-pump & electron-probe method using an electron microscope combined with a pulsed electron source. A photoemission-type pulsed electron gun makes it possible to obtain a short-pulsed electron beam required for high temporal resolution. On the other hand, spatial resolution is restricted by the brightness of the pulsed electron gun used in electron microscopes when a low brightness electron source is used and an irradiation current larger than a certain value is required. Thus, we constructed a prototype pulsed electron gun using a negative electron affinity (NEA) photocathode for time-resolved measurement using a scanning electron microscope (SEM) with high spatiotemporal resolution. In this study, a high-speed detector containing an avalanche photodiode (APD) was used to directly measure waveforms of the pulsed electron beam excited by a rectangular-shape pulsed light with a variable pulse duration in the range of several ns to several μs. The measured waveforms were the same rectangular shape as incident pulsed excitation light. The maximum peak brightness of the pulsed electron beam was 4.2×107 A/m2/sr/V with a pulse duration of 3 ns. This value was larger than that of the continuous electron beam (1.6 × 107 A/m2/sr/V). Furthermore, an SEM image with image sharpness of 6.2 nm was obtained using an SEM equipped with a prototype pulsed electron gun at an acceleration voltage of 3 kV.

A new imaging concept in spin polarimetry based on the spin-filter effect.

The concept of an imaging-type 3D spin detector, based on the combination of spin-exchange interactions in the ferromagnetic (FM) film and spin selectivity of the electron-photon conversion effect in a semiconductor heterostructure, is proposed and demonstrated on a model system. This novel multichannel concept is based on the idea of direct transfer of a 2D spin-polarized electron distribution to image cathodoluminescence (CL). The detector is a hybrid structure consisting of a thin magnetic layer deposited on a semiconductor structure allowing measurement of the spatial and polarization-dependent CL intensity from injected spin-polarized free electrons. The idea is to use spin-dependent electron transmission through in-plane magnetized FM film for in-plane spin detection by measuring the CL intensity from recombined electrons transmitted in the semiconductor. For the incoming electrons with out-of-plane spin polarization, the intensity of circularly polarized CL light can be detected from recombined polarized electrons with holes in the semiconductor. In order to demonstrate the ability of the solid-state spin detector in the image-type mode operation, a spin detector prototype was developed, which consists of a compact proximity focused vacuum tube with a spin-polarized electron source [p-GaAs(Cs,O)], a negative electron affinity (NEA) photocathode and the target [semiconductor heterostructure with quantum wells also with NEA]. The injection of polarized low-energy electrons into the target by varying the kinetic energy in the range 0.5-3.0 eV and up to 1.3 keV was studied in image-type mode. The figure of merit as a function of electron kinetic energy and the target temperature is determined. The spin asymmetry of the CL intensity in a ferromagnetic/semiconductor (FM-SC) junction provides a compact optical method for measuring spin polarization of free-electron beams in image-type mode. The FM-SC detector has the potential for realizing multichannel 3D vectorial reconstruction of spin polarization in momentum microscope and angle-resolved photoelectron spectroscopy systems.

Structural and electronic properties of Cs‐adsorbed GaN monolayer and bilayer based on first principles

A negative electron affinity photocathode can be obtained by adsorbing Cs on the emission layer surface of photocathode, which greatly improves the number of electrons escaped and the quantum efficiency. Based on first principles, this paper studies the structural and electronic properties of Cs-adsorbed GaN monolayer and bilayer. The results reveal that the Cs-adsorbed GaN monolayer is the most stable when Cs atom is adsorbed at TGa site. There is a significant charge transfer between Cs atom and GaN monolayer, and the difference in electronegativity makes electrons of Cs atom transfer to GaN monolayer system. Cs adsorption can reduce band gap, work function, and electron affinity, which is conducive to the escape of electrons and improvement of quantum efficiency. Cs adsorption is helpful for tuning optical properties of GaN monolayer. For the GaN bilayer, greater stability and lower work function make the Cs adsorbed above the top layer of GaN bilayer be more beneficial to the improvement of photoelectric performance. Novelty Statement Based on first principles, this paper studies the structure and electric properties of Cs-adsorbed GaN monolayer and bilayer. Cs adsorption can reduce band gap, work function, and electron affinity, which is conducive to the escape of electrons and improvement of quantum efficiency. And for the GaN bilayer, greater stability and lower work function make the Cs adsorbed above the top layer of GaN bilayer be more beneficial to the improvement of photoelectric performance.

Spectral detection of spin-polarized ultra low-energy electrons in semiconductor heterostructures

The circularly polarized cathodoluminescence (CL) technique has been used to study the free spin-polarized electron injection in semiconductor heterostructures with quantum wells (QWs). A polarized electron beam was created by the emission of optically oriented electrons from the p-GaAs(Cs,O) negative electron affinity (NEA) photocathode. The prepared beam was injected in a semiconductor QW target, which was activated by cesium and oxygen to reduce the work function. To study the spin-dependent injection, we developed a spin-detector prototype, which consists of a compact proximity focused vacuum tube with the source and target placed parallel to each other on the opposite ends of the vacuum tube (photodiode). The injection of polarized low-energy electrons into the target by varying the kinetic energy in the range of 0.5-5.0 eV and temperature in the range of 90-300 K was studied. The CL was polarized to 2 % by the injection of 20 % spin-polarized electron beam with the energy of 0.5 eV at room temperature. The asymmetry (Sherman function) of spin detection was estimated. It was shown that the dependence of the CL polarization degree on the injected electron energy is satisfactory described by the model that considers the electron spin relaxation in the heterostructure matrix and QWs. The results demonstrate that semiconductor detectors are promising for the spin-polarimetry applications based on the optical detection of free-electron spin polarization.

Resolution improvement of low-voltage scanning electron microscope by bright and monochromatic electron gun using negative electron affinity photocathode

The scanning electron microscope (SEM) is a powerful tool for analyzing the surface of various specimens. SEM observation at low voltage, which is typically lower than the 1 kV range, is very effective for obtaining surface information of specimens. However, at the low voltage range, resolution is restricted by the brightness and energy spread of electron sources. Therefore, to improve the resolution, a bright and monochromatic electron source is indispensable. In this work, we focused on an electron gun with a negative electron affinity photocathode using p-type GaAs and Cs–O adsorbates. The resolution of a prototype SEM was improved by approximately 16% compared with a SEM using a conventional Schottky emission (SE)-type emitter at the acceleration voltage of 1 kV. In addition, results showed that the measured brightness was as high as an SE-type emitter (∼107 A/m2/sr/V) and the estimated energy spread was less than 0.2 eV, which was narrower than that of a cold field emission-type emitter.

New electron gun improves imaging resolution of low-voltage scanning electron microscopy

A negative electron affinity photocathode used to develop a prototype.

Mie-type GaAs nanopillar array resonators for negative electron affinity photocathodes.

This paper presents modeling results of Mie-type GaAs nanopillar array resonant structures and the design of negative electron affinity photocathodes based on Spicer's three-step model. For direct-bandgap GaAs with high intrinsic absorption coefficient in the 500 ∼ 850 nm spectral range, photoelectrons were found to be highly localized inside the nanopillars near the top and side surfaces where electrons can be efficiently transported and emitted into vacuum, and the light reflectance can be reduced to ∼1% level at resonance wavelengths. Predictions of spectrally resolved photoemission indicate that these nanophotonics resonators, when properly optimized, can increase the photo-electron emission quantum efficiency at resonance wavelengths to levels limited only by the surface-electron escape probability, significantly outperforming traditional flat wafer photocathodes. Ultrafast photoelectric response is also expected from these nanostructured photocathodes due to the much shorter photoelectron transport distance in nanopillars compared to flat wafers. Given these unique optoelectronic properties, GaAs nanophotonic resonance structured photocathodes represent a very promising alternative to photocathodes with flat surfaces that are widely used in many applications today.

Analysis of negative electron affinity InGaN photocathode by temperature-programed desorption method

A III–V semiconductor with a few monolayers of alkali metals (e.g., Cs) forms a negative electron affinity (NEA) surface, for which the vacuum level lies below the conduction band minimum of the base semiconductor. The photocathodes that form an NEA surface (NEA photocathodes) have various advantages, such as low emittance, a large current, high spin polarization, and ultrashort pulsed operation. The NEA-InGaN photocathode, which is sensitive to blue light, has been studied as a material for the next-generation robust photocathode. However, the proper conditions for forming NEA surfaces remain unknown. The authors consider whether the suitable process for NEA surfaces can be understood by investigating the relationship between the electron emission and the adsorption state of alkali metals. In this study, the relationship between the electron emission and the adsorption state of Cs on the p-type InGaN (0001) was analyzed by the temperature-programed desorption (TPD) method using a quadrupole mass spectrometer. From the results of the TPD measurements, it was shown that there were several adsorption states of Cs on InGaN. The quantum efficiency (QE), which indicates the ratio of emitted electrons to incident photons, increased while Cs desorption occurred. The authors divided the formation process of an NEA surface into several sections to investigate the adsorption states of Cs related to the electron emission and to discuss the reasons why the QE increased despite the desorbed Cs. From the results of the NEA activation in each section, it was shown that there were sections where the QE increased by reacting with O2 after Cs supply stopped. There is a possibility that several layers reacting with O2 and those not reacting with O2 are formed by performing NEA activation until the QE saturates. From the results of the TPD measurements in each section, it was suggested that there was a Cs peak at above 700 °C when the TPD method was carried out immediately after confirming the electron emission. Therefore, the adsorption state of Cs that formed a peak at above 700 °C had a close relation to the electron emission. It is considered that the increase of the QE in the TPD was affected by adsorbed Cs compounds that reacted with O2. Although the mechanism is not understood, it is known that the QE was increased by the reaction of Cs adsorbed compounds and O2 in previous studies. It was suspected that layers that reacted with O2 appeared from TPD and then the QE increased by reacting with O2.

Surface sensitization mechanism on negative electron affinity p-GaN nanowires

The surface sensitization is the key to prepare negative electron affinity photocathode. The thesis emphasizes on the study of surface sensitization mechanism of p-type doping GaN nanowires utilizing first principles based on density function theory. The adsorption energy, work function, dipole moment, geometry structure, electronic structure and optical properties of Mg-doped GaN nanowires surfaces with various coverages of Cs atoms are investigated. The GaN nanowire with Mg doped in core position is taken as the sensitization base. At the initial stage of sensitization, the best adsorption site for Cs atom on GaN nanowire surface is BN, the bridge site of two adjacent N atoms. Surface sensitization generates a p-type internal surface with an n-type surface state, introducing a band bending region which can help reduce surface barrier and work function. With increasing Cs coverage, work functions decrease monotonously and the “Cs-kill” phenomenon disappears. For Cs coverage of 0.75 ML and 1 ML, the corresponding sensitization systems reach negative electron affinity state. Through surface sensitization, the absorption curves are red shifted and the absorption coefficient is cut down. All theoretical calculations can guide the design of negative electron affinity Mg doped GaN nanowires photocathode.

Study on the electronic and optical properties of zinc interstitial doped In0.53Ga0.47As (100) β2 (2 × 4) photocathode

Zn-doped In0.53Ga0.47As (100) β2 (2 × 4) photocathode surface is an important process for forming the performance of the negative electron affinity (NEA) photocathode. In this paper, the first-principles calculation is used based on the density functional theory (DFT) within scope of the generalized gradient approximation (GGA). The electronic and optical properties of zinc interstitial doped on five sites of In0.53Ga0.47As (100) β2 (2 × 4) photocathode surface are chosen and investigated. The band structure, density of state, dielectric function and absorption coefficient of zinc interstitial doping on In0.53Ga0.47As (100) β2 (2 × 4) photocathode surface are calculated. Results show that the zinc interstitial doping on In0.53Ga0.47As (100) β2 (2 × 4) photocathode surface makes energy band bend and some new energy bands are generated. The band gaps of zinc interstitial doping on In0.53Ga0.47As (100) β2 (2 × 4) surface are all becoming narrow. And it is beneficial for the formation of the NEA photocathode when zinc atom is interstitial doping at 1 position of In0.53Ga0.47As (100) β2 (2 × 4) photocathode surface, which researches dielectric constant and absorption coefficient. The theoretical calculations and analysis provide a guide for designing the near-infrared material.

Reducing the contribution of the photoemission process to the unwanted beam in photoelectron sources at accelerators

Negative electron affinity (NEA) GaAs photocathodes show different pulse responses depending on the wavelength of photoexcitation. The pulse response at 800 nm shows a long and relatively intense tail, whereas at 400 nm, a tail of similar shape but with an intensity lower by around two orders of magnitude is observed. We explain this behavior with the specific properties of NEA photocathodes and compare it with the response of a positive electron affinity photocathode.

Electronic and optical properties of Zn-doped InGaAs emission layer with vacancy defects: A DFT study

In0.53Ga0.47As is an important material for the shortwave near-infrared negative electron affinity photocathodes. Zn atoms were doped in the intrinsic InGaAs to form p-type In0.53Ga0.44 Zn0.03As emission layer. Models of Zn-doped In0.53Ga0.44 Zn0.03As bulk with In, Ga or As vacancy defect were built in the article. Emphasis was put on how the defects influence the microstructure and the optical properties of the Zn-doped InGaAs emission layer. Combined effect of both vacancy defects and doped Zn atoms for the photoelectron emission performance was further studied. It is indicated that the In or Ga vacancy defect is easier to be generated than the As from the analysis of formation energy. However, As vacancy defect is more benefit for photoelectrons emission considering the material polarity because As vacancy defect interacting with doped Zn atoms is a positive center and generates acceptor level to narrow the energy gap. At the same time Zn ion will be easier to accept electrons when the As vacancy defect appears. The absorption coefficient of the InGaAs emission layer with As vacancy defect is also bigger than that with In or Ga defect. In conclusion, As vacancy defect is benefit and will promote the photoemission of InGaAs photocathode while the In or Ga vacancy defect should be avoided when the emission layer is grown.

Atomic structure and optoelectronic properties of In0.53Ga0.47As (100) reconstruction surfaces: A DFT study

There are kinds of reconstructions on InGaAs(100) surface, such as As-rich β2(2×4), In-rich α(2×4), and Ga-rich α(2×4) reconstructions. In this article, the three kinds of In0.53Ga0.47As(100) reconstruction surfaces are studied based on first-principles calculations, and atomic structures, formation energies, band structures, electron distributions, and absorption coefficients were obtained. Atomic structures of the surfaces were calculated and results show that the relaxation of As-rich β2(2×4) surface is the smallest. The As-rich β2(2×4) surface also has the smallest formation energy, which means the As-rich β2(2×4) surface is the steadiest among the three surfaces. Furthermore, the energy band structures and the work function are analyzed. The less the work function is, the easier the photoelectrons escape from the surfaces. Therefore, it is the easiest for photoelectrons to escape from As-rich β2(2×4) surface due to its lowest work function. The same conclusion is also confirmed based on the calculations of the electronic structure. The negative electron affinity photocathode is mainly sensitive to the infrared region where the absorption coefficient of the As-rich β2(2×4) surface is bigger than those of the In-rich α(2×4) and Ga-rich α(2×4) surfaces. The As-rich β2(2×4) surface is benefit for the formation of the negative electron affinity In0.53Ga0.47As photocathode.

Theoretical study on the electron structure and optical properties of Cs adsorption on In0.53Ga0.47As (100) β2 (2 × 4) reconstruction surface

Cs activation on In0.53Ga0.47As (100) β2 (2×4) reconstruction surface is an important process for improving the performance of the negative electron affinity (NEA) photocathode. Using the first-principles method based on the density functional theory (DFT), within scope of the generalized gradient approximation (GGA), the electronic structure and optical properties of Cs adsorption on In0.53Ga0.47As (100) β2 (2×4) reconstruction surface models are calculated and analyzed in this article. The results show that adsorption energy is negative suggesting that the structure of Cs adsorption on In0.53Ga0.47As (100) β2 (2×4) surface is stable. The Cs adsorption on In0.53Ga0.47As (100) β2 (2×4) surface makes work function lower and energy band bend, which is benefit for electrons escape to the surface. At the same time, new energy bands are generated. Combined with study of the Al0.5Ga0.5As (100) β2 (2×4) reconstructed surface, two different elements of Al and In are compared under the Cs atomic adsorption. The work function and band gap of Cs atom adsorption on In atom are all smaller than adsorption on Al atom. And the absorption coefficient, dielectric function and energy loss function of Cs adsorption on In0.53Ga0.47As (100) β2 (2×4) surface and clean In0.53Ga0.47As (100) β2 (2×4) surface are researched and compared. Cs adsorption on surface makes the peaks of absorption coefficient, dielectric function and energy loss function of surface higher than that of clean In0.53Ga0.47As (100) β2 (2×4) surface. The analysis is helpful to further strengthen the study for near-infrared material photoemission.

GaN(Mg)-Cs]:[O-Cs] model for the Negative Electron Affinity GaN (0001) surface

Because of the limitations of the existing Negative Electron Affinity (NEA) photocathode surface emission models, [GaN(Mg)-Cs]:[O-Cs] photoemission model based on dual-dipole model is established, which can explain the photoemission mechanism of the NEA GaN photocathode well. Cs, O adsorption process on the GaN (0001) surface during the activation is discussed, and the GaN(Mg)-Cs dipole layer is found having a unified direction which is conducive for photoelectron escaping, so photocurrent has a significant increase when Cs is introduced. There is not a unified direction for the O-Cs dipole layer. Because of the surface defects, part of the O-Cs dipoles have the direction which is conducive for photoelectron escaping, and the photocurrent has a modest growth after introducing O. Finally, the performance of GaN and GaAs photocathode are compared based on the photoemission model, and the reason of better stability of NEA GaN photocathode is interpreted.

Study of Negative Electron Affinity Property for Varied Doping GaN Photocathode

Negative electron affinity (NEA) property means that the vacuum level is lower than the conduction band minimum in the bulk. For GaN photocathode, the effective electron affinity of the material is smaller than zero. According to Spicer “3-step model”, the photoemission mechanism for varied GaN photocathode was studied. For the varied doping GaN photocathode, the photoelectrons can be transported to the surface by diffusing and drifting in the directional inside electric field, and the higher photoemission performance can be obtained. By covering with low escape power materials such as Cs or O on an atomically clean surface, Negative electron affinity property can be achieved for the varied doping GaN photocathode. Using the activation and evaluation system for NEA photocathode, the varied doping GaN photocathode was activated with Cs and O, and the photocurrent curve for varied doping GaN photocathode was gotten.