Negative Electron Affinity Surface Research Articles

Development of angle-resolved spectroscopy system of electrons emitted from a surface with negative electron affinity state.

We developed an angle-resolved photoemission spectroscopy system for the analysis of conduction-band electrons. By forming a negative electron affinity surface on a semiconductor surface, electrons in conduction bands are emitted into a vacuum and measured by using an analyzer. This method enables us to determine the energy and momentum of the conduction electrons. Furthermore, it can be used to determine unoccupied conduction band structures. The main challenges of this method are that the energies of the emitted electrons are extremely low and the trajectories of the electrons change due to various influences. We overcame these problems by placing the shielding mesh close to the sample and parallel to the sample surface. The entire chambers, including the shielding mesh, were grounded, and a negative bias voltage was applied only to the sample. This configuration realizes the acceleration of electrons while preserving the momentum component parallel to the sample surface. Another problem is the establishment of a method for converting a detected angle into the corresponding wavevector. We focused on the emission angle of electrons emitted from a sample and their minimum energy and then established an analytical method for converting detected angles into corresponding wavevectors on the basis of the minimum energy.

Rb adsorbate-induced negative electron affinity on quartz

SynopsisWe have investigated Rb adsorbates on the SiO2 (0001) surface. Using Rydberg atom electromagnetically induced transparency, we investigate the electric fields resulting from Rb adsorbed on the quartz surface, and measure the activation energy of the Rb adsorbates. We show that the adsorbed Rb induces a negative electron affinity (NEA) on the quartz surface. The NEA surface allows low energy electrons to bind to the surface and cancel the electric field from the Rb adsorbates. Our results are important for integrating Rydberg atoms into hybrid quantum systems and the fundamental study of atom-surface interactions, as well as applications for electrons bound to a surface.

NEA surface activation of GaAs photocathode with different gases

The GaAs photo-cathode is a unique device which is able to generate highly polarized electron beam up to 90% and an extremely small emittance beam. The photo-electron emission is possible with IR (Infrared Red) and visible light and the quantum efficiency can be more than 20%. These unique features depend on the negative electron affinity (NEA) surface made by adsorption of Cs and O2/NF3, but this surface is easily damaged by residual gas adsorption, ion back-bombardment, etc. The exact structure of the NEA surface is not known, even there are several hypotheses. In this study, we performed the NEA activation on a cleaned GaAs surface with CO2, CO, N2, and O2 gases and compared the results to improve our understanding on the NEA surface. We found that the NEA activation with CO2 is similar to that with O2, but the maximum QE value is significantly lower than that with O2. We also found that N2 and CO gases did not contribute to NEA activation at all. By analyzing CO2 activation, we found that atomic oxygen activates the NEA surface and CO degrades the NEA surface simultaneously. We found that the NEA activation ability of atomic oxygen is almost a half of that of O2 molecule.

SiV center photoluminescence induced by C=O termination in nanocrystalline diamond and graphite loops hybridized films

We performed a series of thermal oxidation at different temperatures on nanocrystalline diamond (NCD) films to construct various surface termination states of NCD grains and investigated their effects on silicon-vacancy (SiV) photoluminescence (PL) at 738 nm. Experiments and first principles calculations show that the negative electron affinity surface induced by C-H bond termination quenches the SiV PL, while the positive electron affinity surface originating from C=O bond termination removes this quenching. Moreover, oxidation at 600 °C results in the transition from amorphous carbon to graphite loops with an interlayer space of 0.4 nm, so that NCD and graphite loops' hybridized structure is formed. This allows oxygen atoms to contact with inside NCD grains to form more C=O bonds on the surface, producing much larger positive electron affinity in the surface. It traps the excited state electrons, lets them scatter back to the ground state, and emits SiV PL. These results reveal that C=O bonds play a crucial role in SiV PL of NCD grains and well explain the experimentally observed quenching effect. A novel way by changing the surface termination states is proposed to control the PL of NCD grains with SiV centers for potential quantum information processing and biological sensing.

Observation of relaxation time of surface charge limit for InGaN photocathodes with negative electron affinity

A thin p-type InGaN with a negative electron affinity (NEA) surface was used to measure the relaxation time of a surface charge limit (SCL) by irradiating rectangular laser beam pulses at changing time interval. The p-type InGaN film was grown by metal organic vapor phase epitaxy and the NEA activation was performed after the sample was heat cleaned. 13 nC per pulse with 10 ms width was obtained from the InGaN photocathode. The current decreased exponentially from the beginning of the pulse. The initial current value after the laser irradiation decreased with the time interval. As a result, the SCL relaxation time was estimated through the InGaN photocathode measurements at 100 ms.

Electric Field Cancellation on Quartz by Rb Adsorbate-Induced Negative Electron Affinity.

We investigate the (0001) surface of single crystal quartz with a submonolayer of Rb adsorbates. Using Rydberg atom electromagnetically induced transparency, we investigate the electric fields resulting from Rb adsorbed on the quartz surface, and measure the activation energy of the Rb adsorbates. We show that the adsorbed Rb induces negative electron affinity (NEA) on the quartz surface. The NEA surface allows low energy electrons to bind to the surface and cancel the electric field from the Rb adsorbates. Our results will be important for integrating Rydberg atoms into hybrid quantum systems, as fundamental probes of atom-surface interactions, and for studies of 2D electron gases bound to surfaces.

Electron emission from conduction band of heavily phosphorus doped diamond negative electron affinity surface

Hydrogen (H)-terminated surfaces of diamond have attracted significant attention due to their negative electron affinity (NEA), suggesting high-efficiency electron emitters. Combined with n-type doping technique using phosphorus (P) as donors, the unique NEA surface makes diamond a promising candidate for vacuum cold-cathode applications. However, high-electric fields are needed for the electron emission from the n-type doped diamond with NEA.Here we have clarified the electron emission mechanism of field emission from P-doped diamond having NEA utilizing combined ultraviolet photoelectron spectroscopy/field emission spectroscopy (UPS/FES). An UP spectrum has confirmed the NEA of H-terminated (1 1 1) surface of P-doped diamond. Despite the NEA, electron emission occurs only when electric field at the surface exceeds 4.2 × 106 V cm−1. Further analysis by UPS/FES has revealed that the emitted energy level is shifted, indicating that the electron emission mechanism of n-type diamond having NEA surface does not follow a standard field emission theory, but is dominated by potential barrier formed within the diamond due to upward band bending. The reduction of internal barrier is the key to achieve high-efficiency electron emitters using P-doped diamond with NEA, of which application ranges from high-resolution electron spectroscopy to novel vacuum nanoelectronics devices.

Electron affinity of cubic boron nitride terminated with vanadium oxide

A thermally stable negative electron affinity (NEA) for a cubic boron nitride (c-BN) surface with vanadium-oxide-termination is achieved, and its electronic structure was analyzed with in-situ photoelectron spectroscopy. The c-BN films were prepared by electron cyclotron resonance plasma-enhanced chemical vapor deposition employing BF3 and N2 as precursors. Vanadium layers of ∼0.1 and 0.5 nm thickness were deposited on the c-BN surface in an electron beam deposition system. Oxidation of the metal layer was achieved by an oxygen plasma treatment. After 650 °C thermal annealing, the vanadium oxide on the c-BN surface was determined to be VO2, and the surfaces were found to be thermally stable, exhibiting an NEA. In comparison, the oxygen-terminated c-BN surface, where B2O3 was detected, showed a positive electron affinity of ∼1.2 eV. The B2O3 evidently acts as a negatively charged layer introducing a surface dipole directed into the c-BN. Through the interaction of VO2 with the B2O3 layer, a B-O-V layer structure would contribute a dipole between the O and V layers with the positive side facing vacuum. The lower enthalpy of formation for B2O3 is favorable for the formation of the B-O-V layer structure, which provides a thermally stable surface dipole and an NEA surface.

Analysis of electron emission from GaAs(Cs,O) by low energy electron microscopy

Low-energy electron microscopy was carried out to study the electron emission process from a GaAs photocathode with a negative electron affinity (NEA) surface. The relationship between emission current and electron affinity was investigated in detail to obtain information regarding the electron tunneling in the vacuum barrier and the electron distribution in the interior of GaAs, especially with respect to photoelectron capture in the band bending region. A comparison of the calculated quantized sub-band energies in the band bending region confirmed that the majority of photoelectrons fell within sub-bands, from where a large portion of the photoelectrons escape into the vacuum.

Nitrogen termination of single crystal (100) diamond surface by radio frequency N2 plasma process: An in-situ x-ray photoemission spectroscopy and secondary electron emission studies

In this letter, we report the electronic and chemical properties of nitrogen terminated (N-terminated) single crystal (100) diamond surface, which is a promising candidate for shallow NV− centers. N-termination is realized by an indirect RF nitrogen plasma process without inducing a large density of surface defects. Thermal stability and electronic property of N-terminated diamond surface are systematically investigated under well-controlled conditions by in-situ x-ray photoelectron spectroscopy and secondary electron emission. An increase in the low energy cut-off of the secondary electron energy distribution curve (EDC), with respect to a bare diamond surface, indicates a positive electron affinity of the N-terminated diamond. Exposure to atomic hydrogen results in reorganization of N-terminated diamond to H-terminated diamond, which exhibited a negative electron affinity surface. The change in intensity and spectral features of the secondary electron EDC of the N-terminated diamond is discussed.

Low energy electron microscopy and Auger electron spectroscopy studies of Cs-O activation layer on p-type GaAs photocathode

Work function, photoemission yield, and Auger electron spectra were measured on (001) p-type GaAs during negative electron affinity (NEA) surface preparation, surface degradation, and heating processes. The emission current sensitively depends on work function change and its dependence allows us to determine that the shape of the vacuum barrier was close to double triangular. Regarding the NEA surface degradation during photoemission, we discuss the importance of residual gas components the oxygen and hydrogen. We also found that gentle annealing (≤100 °C) of aged photocathodes results in a lower work function and may offer a patch to reverse the performance degradation.

Photoemission lifetime of a negative electron affinity gallium nitride photocathode

A photocathode electron source using p-type GaN semiconductor with a negative electron affinity (NEA) surface has been studied for its ability to maintain an extended NEA state. The key technology of NEA photocathodes is the formation of electric dipoles by cesium and gallium atoms on the surface, which makes it possible for photoexcited electrons in the conduction band minimum to escape into the vacuum. This means that in order to keep the electron energy spread as small as possible, the excitation photon energy should be tuned to the band gap energy. However, the NEA surface is damaged by the adsorption of residual gas and the back-bombardment of ionized residual gas by photoelectrons. The p-type GaN semiconductor was measured time evolution in quantum yield during NEA surface activation, and a lifetime of quantum yield of excitation energy corresponding to the band gap energy in comparison to the p-type GaAs as the conventional NEA photocathode. In NEA surface activation process, the quantum yield of the GaN was more than 3 orders of magnitude higher than that of the GaAs by only cesium deposition. The exposure amount of cesium in the NEA surface activation of the GaAs was 1.5 times as that of the GaN, even though the quantum yield of the GaAs was the same value as the GaN. Lifetime of NEA-photocathodes using the GaN was 21 times longer than that using the GaAs. The decrease of quantum yield of the GaAs was well correlated in the form of the exponential decrease function with a decrease time of 4.4 h, while the decrease of quantum yield of the GaN was well correlated in the form of the exponential decrease function with two decrease times of 47 and 174 h.

Direct observation of phonon emission from hot electrons: spectral features in diamond secondary electron emission

In this work we use high-resolution synchrotron-based photoelectron spectroscopy to investigate the low kinetic energy electron emission from two negative electron affinity surfaces of diamond, namely hydrogenated and lithiated diamond. For hydrogen-terminated diamond electron emission below the conduction band minimum (CBM) is clearly observed as a result of phonon emission subsequent to carrier thermalization at the CBM. In the case of lithiated diamond, we find the normal conduction band minimum emission peak is asymmetrically broadened to higher kinetic energies and argue the broadening is a result of ballistic emission from carriers thermalized to the CBM in the bulk well before the onset of band-bending. In both cases the spectra display intensity modulations that are the signature of optical phonon emission as the main mechanism for carrier relaxation. To our knowledge, these measurements represent the first direct observation of hot carrier energy loss via photoemission.

Use of energy-filtered photoelectron emission microscopy and Kelvin probe force microscopy to visualise work function changes on diamond thin films terminated with oxygen and lithium mono-layers for thermionic energy conversion

Kelvin probe force microscopy (KPFM) and energy-filtered photoelectron emission microscopy (EF-PEEM) with vacuum UV (VUV) excitation have been used to study the work function of p -type diamond films treated to exhibit a negative electron affinity (NEA) surface. NEA was generated by a lithium-oxygen monolayer termination. This monolayer was achieved in two different ways: thermally evaporated films 50 nm thick were either treated by in situ vacuum annealing or by a subsequent water wash. The work function values obtained from these samples by EF-PEEM were compared with KPFM measurements to establish which of the two fabrication techniques was most effective in activating a NEA surface. The washing method was shown to be more effective and the work function values obtained by the two techniques were comparable, as they showed the same work function peaks at 4.54 eV in their respective histograms. It was found that neighbouring polycrystalline facets could show a large variation in work function of up to 400 meV.

Negative electron affinity of diamond and its application to high voltage vacuum power switches

AbstractThis paper reviews the electron emission properties of hydrogen terminated diamond surfaces with a negative electron affinity (NEA), and presents the recent development of 10 kV vacuum switches based on diamond PIN junction electron emitters that utilize such NEA surfaces. The background to this topic, including the electron emission mechanism through NEA, the electronic states of diamond with free excitons, and free carrier injection from a heavily doped layer with hopping conduction into a high purity intrinsic layer with band conduction, is also discussed. Our 10 kV vacuum power switch featuring an NEA diamond PIN junction electron emitter exhibited a breakthrough power transmission efficiency of 73% at 9.8 kV. This result validates the principle of such vacuum power switches, which have the potential to operate at 100 kV with an efficiency beyond 99.9%.

In situ Observation of Formation Process of Negative Electron Affinity Surface of GaAs by Surface Photo-Absorption

We have used surface photo-absorption (SPA) to investigate the formation of negative electron affinity (NEA) surfaces on p-GaAs during the Yo-Yo method, under an alternating supply of Cs and O2. The SPA spectra showed that the surface during the first Cs step was different from those in the following Cs and O2 steps. This suggests that the surface structure did not change after the initial surface was formed, indicating that there could be two Cs adsorption sites on the GaAs surface, which is different from previously proposed models.

Analysis of Cs/GaAs NEA surface by XAFS

By exposing a GaAs surface to Cs and O2, a surface which is called negative electron affinity (NEA) surface can be obtained where the vacuum level shifts lower than the bottom of GaAs conduction band. The Cs/GaAs NEA surface has been attracting great attention in recent years, because of its availability for a highly functional electron source which can be controlled by excitation light. In this work, XAFS and X-ray fluorescence measurements were conducted for Cs/GaAs NEA surface prepared on GaAs (001) and (110) substrates. As the result, As and O were considered as near neighbor atoms of center Cs for both of the samples. Most of Cs formed some kind of Cs-oxide and the Cs atoms at interface between Cs-oxide and GaAs were observed as forming Cs-As bond. Taking account of the ratios of coordination numbers and the fluorescence X-ray yield, the Cs-oxide on GaAs (001) surface forms island like structure in which the body/interface ratio is large, and the Cs-oxide on GaAs (110) surface forms layer like structure in which the body/interface ratio is small.

Doping-induced spectral shifts in two-dimensional metal oxides

Doping of strongly layered ionic oxides is an established paradigm for creating novel electronic behavior. This is nowhere more apparent than in superconductivity, where doping gives rise to high-temperature superconductivity in cuprates (hole doped) and to surprisingly high Tc in HfNCl (Tc = 25.5 K, electron doped). First-principles calculations of hole doping of the layered delafossite CuAlO2 reveal unexpectedly large doping-induced shifts in spectral density, strongly in opposition to the rigid-band picture that is widely used as an accepted guideline. These spectral shifts, of similar origin as the charge transfer used to produce negative electron affinity surfaces and adjust Schottky barrier heights, drastically alter the character of the Fermi level carriers, leading in this material to an O-Cu-O molecule-based carrier (or polaron, at low doping) rather than a nearly pure-Cu hole as in a rigid-band picture. First-principles linear response electron-phonon coupling (EPC) calculations reveal, as a consequence, net weak EPC and no superconductivity rather than the high Tc obtained previously using rigid-band expectations. These specifically two-dimensional dipole-layer–driven spectral shifts provide new insights into materials design in layered materials for functionalities besides superconductivity.

30-kV spin-polarized transmission electron microscope with GaAs–GaAsP strained superlattice photocathode

A spin-polarized electron beam has been used as the probe beam in a transmission electron microscope by using a photocathode electron gun with a photocathode made of a GaAs–GaAsP strained superlattice semiconductor with a negative electron affinity (NEA) surface. This system had a spatial resolution of the order of 1 nm for at 30 keV and it can generate an electron beam with an energy width of 0.24 eV without employing monochromators. This narrow width suggests that a NEA photocathode can realize a high energy resolution in electron energy-loss spectroscopy and a longitudinal coherence of 3 × 10−7 m.

Development of Spin-Polarized Pulsed TEM

Recent nanointegration of magnetic storage and spintronic devices is expected to lead to the development of systems that can analyze magnetic and spin states with a nanometer-order spatial resolution. Development of a polarized transmission electron microscope (SPTEM) that employs a spin-polarized electron source (PES) has commenced at Nagoya University. A spin-polarized electron beam is extracted from a photocathode that has a negative electron affinity surface. The temporal profile of the electron beam can be modulated by varying the temporal profile of the driving laser. The SPTEM can obtain temporal and spatial information about spin. The design and construction of the PES were optimized for the SPTEM. A beam energy of less than 40 keV (a low energy for a TEM) was used because the spin interaction with condensed matter is very small compared with the Coulomb interaction. A polarized electron gun was realized in an extreme high vacuum by applying a high field gradient of 4 MV/m to the photocathode surface. A 40-keV polarized electron beam with sub-millisecond pulses was generated using a backside-excited photocathode. The illumination system of the TEM was designed with the aim of producing a spin-polarized electron beam. The system contains two components that rotate spin: one performs azimuthal rotation and the other performs in-plane rotation in the condenser magnet system. The SPTEM has the potential to dynamically generate magnetic-field images with high contrast and high temporal resolution.