Negative Electron Affinity Condition Research Articles

Photoemission characterization of N-polar III-nitride photocathodes as candidate bright electron beam sources for accelerator applications

We report on the growth and characterization of a new class of photocathode structures for use as electron sources to produce high brightness electron beams for accelerator applications. The sources are realized using III-nitride materials and are designed to leverage the strong polarization field, which is characteristic of this class of materials when grown in their wurtzite crystal structure, to produce a negative electron affinity condition without the use of Cs, possibly allowing these materials to be operated in radio frequency guns. A Quantum Efficiency (QE) of about 1×10−3 and an emitted electrons’ Mean Transverse Energy (MTE) of about 100 meV are measured at a wavelength of 265 nm. In a vacuum level of 3×10−10 Torr, the QE does not decrease after more than 24 h of continuous operation. The lowest MTE of about 50 meV is measured at 300 nm along with a QE of 1.5×10−5. Surface characterizations reveal a possible contribution to the MTE from surface morphology, calling for more detailed studies.

Method for monitoring GaAs photocathode heat cleaning temperature

Before a GaAs photocathode can be activated to achieve a negative electron affinity condition, the GaAs crystal must be cleaned. This is most commonly done by ohmic, radiative, or electron bombardment heating. We report a new technique to monitor the temperature of heated GaAs photocathodes by observation with a camera. The method is robust and yields the same temperatures for different GaAs samples heated using different methods in different mounting configurations.

Fluorescent Nanodiamonds: Effect of Surface Termination

AbstractIt has been reported that physico-chemical properties of diamond surfaces are closely related to the surface chemisorbed species on the surface. Hydrogen chemisorption on a chemical vapor deposition grown diamond surface is well-known to be important for stabilizing diamond surface structures with sp3 hybridization. It has been suggested that an H-chemisorbed structure is necessary to provide a negative electron affinity condition on the diamond surfaces. Negative electron affinity condition could change to a positive electron affinity by oxidation of the H-chemisorbed diamond surfaces. Oxidized diamond surfaces usually show characteristics completely different from those of the H-chemisorbed diamond surfaces. The unique electron affinity condition, or the surface potential, is strongly related to the chemisorbed species on diamond surfaces. The relationship between the surface chemisorption structure and the surface electrical properties, such as the surface potential of the diamond, has been modelled using DFT based calculations.

Water-Induced Negative Electron Affinity on Diamond (100)

The interaction of water vapor on clean diamond (100) has been studied using ultraviolet photoemission spectroscopy (UPS), X-ray photoemission spectroscopy (XPS), X-ray-absorption near-edge structure (XANES) spectroscopy, and high-resolution electron energy loss spectroscopy (HREELS). It is shown that water dissociates at room temperature on clean diamond forming C−H and C−OH bonds, resulting in a surface dipole layer which produces the condition of negative electron affinity (NEA). The strong polarization dependence of the O K XANES could be associated with the out-of-plane orientation of the OH bonds. Density functional theory (DFT) calculations confirm the existence of NEA on this surface with a mixture of hydrogen and hydroxyl (OH) terminations.

A spectroscopic study of the negative electron affinity of cesium oxide-coated diamond (111) and theoretical calculation of the surface density-of-states on oxygenated diamond (111)

The modification of the electron affinity of clean and oxygenated C(111) with cesium has been studied using ultra-violet photoelectron spectroscopy. Oxygenated C(111) shows strong valence emission features at 4.2 eV attributable to CO bonding orbital. Adsorption of cesium on the oxygenated diamond results in the formation of cesium oxide features at 4.2 eV and 8 eV and the condition of negative electron affinity (NEA). The cesium oxide adlayer is thermally stable to 500 °C and the NEA condition is not removed even at saturated O exposures, suggesting that cesium oxide-modified C(111) may act as a good photo-cathode. In order to evaluate the effect of oxygen atom adsorption on the surface states of C(111) 2×1 surface, the density of states of the O:C(111) surface adopting two different oxygen binding configurations have been calculated using the periodic density functional theory. The surface gap states on clean C(111) 2×1 surface are passivated by the adsorption of 1/2 monolayer oxygen in a bridging ‘epoxy’ fashion (C–O–C) across the carbon in the Pandey chain. At 1 monolayer oxygen coverage, the 2×1 reconstruction is lifted and an ‘on-top’ carbonyl (CO) binding mode results. Layered-resolved partial DOS calculation reveals that the surface band gap is closed with the adoption of the CO bonding configuration due to the introduction of quasi-continuous surface band states.

Negative electron affinity of cubic boron nitride

Abstract We have verified that the condition of negative electron affinity (NEA) exists on both single crystal cubic boron nitride (001)-(1×1) and polycrystalline cubic boron nitride (c-BN). The NEA condition has been linked to the presence of chemisorbed hydrogen on the surface. Annealing the c-BN to temperatures in excess of 1200 °C converts the surface to a positive electron affinity condition, which is believed to be due to the desorption of hydrogen from the surface. Subsequent exposure of the annealed surface to atomic hydrogen or cesium metal can regenerate the NEA condition.

Surface conditioning of chemical vapor deposited hexagonal boron nitride film for negative electron affinity

The surface conditions favoring a negative electron affinity (NEA) on hexagonal boron nitride (h-BN) grown by radio-frequency plasma-assisted chemical vapor deposition (CVD) have been investigated by ultraviolet photoelectron spectroscopy. The NEA condition on the h-BN film appears to be resistant to oxygen-plasma or in-vacuo atomic oxygen treatment. It is not certain whether the segregation of bulk hydrogen onto the surface helps to promote the NEA; the depth profile of the deposited film reveals about 0.01%–0.1% atomic concentration of hydrogen. High temperature annealing at 1100 °C results in a positive electron affinity surface (PEA). Reexposure of PEA surface to atomic hydrogen at room temperature regenerates the NEA condition. This is evident of the role of superficial hydrogen in promoting NEA on the h-BN film.

Thermal stability of the negative electron affinity condition on cubic boron nitride

We have verified that the condition of negative electron affinity (NEA) exists on hydrogen-terminated polycrystalline cubic boron nitride (c-BN) grown by the high-pressure high-temperature method using ultraviolet photoelectron spectroscopy. The NEA condition is thermally stable to 950 °C. At higher temperatures, the surface reverts to a positive electron affinity condition due to the desorption of surface bound hydrogen. Repeated annealing at high temperatures results in the degradation of the surface crystallinity, which manifests in the growth of a π→π* feature attributable to sp2-type bonds. Complete regeneration of initial valence band features and NEA conditions along with the suppression of the π→π* features can be achieved by subjecting the surface to atomic hydrogen etching. It is discovered that the He (II)-excited valence band spectra of c-BN display significant differences between the hydrogen-terminated and hydrogen-free surface.

Tuning the electron affinity of CVD diamond with adsorbed caesium and oxygen layers

Abstract The adsorption of Cs on CVD diamond is observed to produce the condition of negative electron affinity (NEA). The intensity of the NEA photoelectron peak together with the thermal and chemical stability of the NEA condition are measured and are found to be sensitive to the chemical species terminating the diamond surface prior to caesiation. Diamond surfaces that have been etched with acid and annealed to 1400 K, or simply oxidised at 300 K before caesiation display strong NEA characteristics that are chemically stable in air and thermally stable to 650 K. Diamond surfaces that have been hydrogenated prior to caesiation show a comparatively weaker NEA condition that is thermally and chemically unstable. The results are rationalised on the basis of band bending and reactive interface formation.

Experimental study of the response time of GaAs as a photoemitter.

An experimental investigation was carried out to measure the response time of GaAs in negative electron affinity conditions as a photoemitter. During the experiment, the photocathode was excited by a short-pulse (38 ps rms) frequency-doubled Nd:YLF laser. Results show that the rms response time of GaAs is shorter than 40 ps.

Surface analysis of a GaAs electron source using Rutherford backscattering spectroscopy

We experimented with Rutherford backscattering spectroscopy to measure the amount of cesium on the surface of a GaAs photocathode activated in negative electron affinity (NEA) conditions. The surface analysis was performed both immediately after activation of the source in NEA condition and also some time after its total spoiling. Results showed that cesium did not desorb from the GaAs surface. With any further cesium feeding to reactivate the surface, the cesium accumulated on the surface until a saturation level was reached.

Integration of microstructures onto negative electron affinity cathodes: Fabrication and operation of an addressable negative electron affinity cathode

A new cathode technology which integrates isolated multilayer structures onto negative electron affinity (NEA) cathodes has been developed. The process is designed to have the emission areas protected throughout the processing steps. With this process, a three-metal layer structure has been integrated onto a GaAs chip to fabricate an addressable-array NEA cathode. The metal layers are sputter-deposited TiW, the topmost layer is SiN. The addressable-array NEA cathodes were tested in an ultrahigh vacuum chamber, where they were coated with cesium and oxygen to achieve the NEA condition on the lithographically selected areas. Electron emission from the addressable cathodes has been projected onto a phosphorous screen. The photocurrent was modulated by applying a voltage to each control electrode. Pictures demonstrating the addressing capability are presented. The addressable-array NEA cathode technology offers the possibility of new projection electron-beam architectures for high throughput electron-beam lithography.

Optical properties of GaAsP NEA cold cathodes

Photovoltage and photoemission experiments have shown that the GaAsP cold cathode negative electron affinity condition was inhibited by a surface defect state. Operationally, the NEA condition was further restricted by cesium coating of nonsemiconductor surfaces resulting in a reduction of band bending at the p surface, unless a negative potential of 50-70 V was applied to the cathode. >

Comparative study of the preparation of negative electron affinity GaAs photocathodes with O2 and with NF3

We discuss the possibility of replacing oxygen with nitrogen trifluoride in the preparation of stable GaAs negative electron affinity (NEA) photocathodes. The different role of O2 versus NF3 exposure during the activation is examined by means of quantum yield and photocurrent lifetime measurements. Quantum yield results clearly demonstrate that NEA conditions can be attained also with NF3. Measurements performed on the same samples and in the same experimental conditions, show a remarkably different time evolution of the activation process in the two cases. However, as a result of a huge number of activations, no evidence of significantly larger lifetimes for NF3 prepared photocathodes can be deduced.

Electron emission properties of laser pulsed GaAs negative electron affinity photocathodes

The emitted electron energy distribution of a laser pulsed GaAs negative electron affinity (NEA) photocathode was measured with a retarding grid analyzer. The total electron energy distribution was found to depend strongly on laser intensity and cathode thickness. A model based on the separation of the excess photogenerated carriers within the cathode structure has been developed and tested to explain this behavior. Pulsed electron emission patterns were obtained by directing the emitted electrons to a phosphorous screen. The patterns indicate that uniform emission occurs and that the emission develops uniformly as the cathode is activated to a NEA condition. An extrapolated plot of the angular intensity FWHM versus V1/2B indicates that the pulsed cathode possesses a natural semicone angle of approximately 10°. The angular distribution is narrow because of the electron refraction which occurs upon emission due to differences in the effective mass and vacuum rest mass of the electrons.

Electron optical characteristics of negative electron affinity cathodes

Stable continuous electron emission has been obtained from GaAs electron cathodes which have been activated to a negative electron affinity condition. Negative electron affinity (NEA) photocathodes offer one means of producing very short electron pulses with a high peak brightness necessary for time-resolved metrology. Despite concerns regarding the stability of such cathodes, we have demonstrated a high degree of short-term stability through the ability to take scanning electron microscope photographs and Auger spectra using a NEA cathode. We have also demonstrated an effective brightness measured at the sample of 1×105 A/cm2 sr at 3 kV using our prototype electron gun. Room-temperature operation and laser excitation of this cathode make it particularly well-suited for low-voltage time-resolved electron probe and electron spectroscopy applications.

Photoelectron spectroscopic determination of the structure of (Cs,O) activated GaAs (110) surfaces

p-GaAs (110) surfaces activated to negative electron affinity (NEA) have been examined with photoelectron spectroscopy. A typical activated GaAs surface is found to consist of both a layer of oxygen bonded to GaAs and a (Cs+,O−2) layer. The GaAs-O layer was not anticipated prior to this work. A GaAs-O-Cs dipole plus the polarization of a Cs+-O−2-Cs+ sandwich layer is proposed to explain the NEA condition based on the structure of activated surfaces found in this work. The identification of the O-GaAs bonding layer explains the better yield achieved by the two-step activation process compared to that achieved by a single-step process. Possible optimization of the activation process by forming the O-GaAs layer and the (Cs+,O−2) layer is also discussed. Adsorption of OH from the residual gas in an ultrahigh vacuum chamber is identified as one degradation mechanism of the GaAs photocathodes.

Photoemission and electron backscattering from monocrystalline boron

Auger effect, electron backscattering and external photoemission were studied at room temperature on clean, cesiated and cesium oxide-covered monocrystalline boron samples in ultra-high vacuum. The diffusion recombination length of thermalized photoelectrons and their surface escape probabilities under negative electron affinity conditions were determined. Photoelectron energy distribution curves show that boron is an indirect bandgap semiconductor.