Negative Electron Affinity State Research Articles

Scanning electron microscope imaging by selective e-beaming using photoelectron beams from semiconductor photocathodes

Pulsed electron beams from a photocathode using an InGaN semiconductor have brought selectively scanning technology to scanning electron microscopes, where the electron beam irradiation intensity and area can be arbitrarily selected within the field of view in SEM images. The p-type InGaN semiconductor crystals grown in the metalorganic chemical vapor deposition equipment were used as the photocathode material for the electron beam source after the surface was activated to a negative electron affinity state in the electron gun under ultrahigh vacuum. The InGaN semiconductor photocathode produced a pulsed electron beam with a rise and fall time of 3 ns, consistent with the time structure of the irradiated pulsed laser used for the optical excitation of electrons. The InGaN photocathode-based electron gun achieved a total beam operation time of 1300 h at 15 μA beam current with a downtime rate of 4% and a current stability of 0.033% after 23 cycles of surface activation and continuous beam operation. The InGaN photocathode-based electron gun has been installed in the conventional scanning electron microscope by replacing the original field emission gun. SEM imaging was performed by selective electron beaming, in which the scanning signal of the SEM system was synchronized with the laser for photocathode excitation to irradiate arbitrary regions in the SEM image at arbitrary intensity. The accuracy of the selection of regions in the SEM image by the selective electron beam was pixel by pixel at the TV scan speed (80 ns/pix, 25 frame/s) of the SEM.

Multiple electron beam generation from InGaN photocathode

In this study, we generated 25 multielectron beam (MEB) using an InGaN photocathode with a negative electron affinity state irradiating with 25 multilaser beam. The uniformity of the MEB and the total electron beam current were evaluated. A laser beam was split into 25 laser beams using a spatial light modulator. The coefficient of variation (CV) of laser power was 20%. The CV of quantum efficiency was 1.1%. The CV of electron beam current was 12%, and the total current was about 1.2 μA. These results will enhance the development of the MEB-defect inspection using the InGaN photocathode.

Photoemission and photon-enhanced thermionic emission: Effect of jump in electron mass

The effect of jump in electron mass on the electron transfer through interfaces with positive and negative potential steps is considered in relation with practically important photoemission and photon-enhanced thermionic emission from semiconductors with positive and negative effective electron affinities. For a zero potential step, a direct similarity is established between the expressions for electron reflection and transmission and Fresnel equations for p-polarized light, with the ratio of masses replacing the ratio of dielectric constants. For non-zero potential steps of both signs, due to the jump in mass, there are critical electron energies that separate the energy regions with qualitatively different angular dependences of the transmission coefficient. The influence of the jump in mass on the spectra of photoemission quantum yield, which are measured experimentally under transition from negative to positive electron affinity at the p-GaAs(Cs,O)-vacuum interface, is considered. It is shown that, for a decisive proof of the jump-in-mass contribution to the electron transfer through semiconductor-vacuum interface, measuring the energy and angular distributions at the state of positive electron affinity is more advantageous, as compared to the state of negative electron affinity.

Photoemission from p-GaAs(Cs,O) under transition from negative to positive electron affinity

The evolution of the electron escape probability from p-GaAs(Cs, O) to vacuum was measured under the transition from the state of negative to the state of positive electron affinity. The transition was induced by the deposition of excess cesium or oxygen on the p-GaAs(Cs, O) surface activated to the state of negative electron affinity. Under deposition of excess cesium, the escape probability was significantly lower as compared to excess oxygen, presumably, due to reflection and scattering of electrons on two-dimensional cesium metal clusters. Under Cs-induced transition, at positive values of affinity, a new peak was observed below the band gap in the photoemission quantum yield spectra. The peak stems, presumably, from: (i) the effective capture of sub-bandgap light in the structure, which consists of the heavily doped p +-GaAs epilayer grown on the semi-insulating GaAs substrate, due to diffuse backside reflection from the rough back surface of the substrate; (ii) absorption of the captured light near the emitting p +-GaAs surface due to the Franz-Keldysh effect in the strong surface electric field; (iii) emission of the electrons generated near the surface.

Study on work function and corresponding electron emission during NEA activation of GaAs surfaces

We have investigated the relation between the work function and corresponding quantum efficiency during the negative electron affinity (NEA) activation processes of GaAs surfaces qualitatively and determined the regions of positive electron affinity (PEA) surface and NEA surface. We found that the two surface states of NEA and PEA were switched during the alternating supply of Cs and O2 in the NEA activation sequence. After the supply of O2 following Cs deposition onto the clean GaAs, the surface still showed the PEA state for a few minutes and changed to the NEA state with increase of electron emissions. It should be noted that once the NEA surface was formed, there were two regions where the quantum efficiency continued increasing, both with and without changing the shape of spectrum. As one possibility, we believe that the increase in quantum efficiency is related to the number of electron emission sites and that there are various electron emission structures.

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.

Early stage of Cs activation mechanism for In0.53Ga0.47As (0 0 1) β2 (2 × 4) surfaces: Insights from first-principles calculations

The negative electron affinity (NEA) InxGa1−xAs photocathodes are of particular importance as a fundamental building block for the near infrared sensitivity image intensifier. However, a comprehensive understanding is still lacking of the activation mechanism in order to form NEA state. Here we perform a detailed theoretical research on the early stage of Cs adsorption on the As-rich β2(2 × 4) reconstruction of In0.53Ga0.47As (0 0 1) surface by using first-principles calculations within the framework of density functional theory. Adsorption energies, work functions, dipole moments, band structures and density of states of In0.53Ga0.47As (0 0 1) β2 (2 × 4) surface corresponding to different Cs coverages are investigated. Calculation results indicate that Cs adsorption on In0.53Ga0.47As (0 0 1) β2(2 × 4) surface becomes more and more difficult as Cs coverage increases. Cs-induced [Csn+-In0.53 Ga0.47Asn-] dipoles are perpendicular to the substrate, which will decrease the work function and make the electrons easier to escape to the vacuum. However, excessive Cs adatoms will lead to the dipoles repelling each other, and some electrons return back to the substrate due to the electrostatic depolarization, resulting in the appearance of “Cs-kill” phenomenon and the work function rises again. Consequently, the optimal Cs coverage corresponding to the peak value of the InGaAs photocurrent should be considered in order to prevent the absence of the “Cs-kill” phenomenon. Meanwhile, there emerge new valence bands induced by Cs adsorption ranging from −25 eV to −22 eV and from −13 eV to 9 eV. Due to the combined effect of Cs 5s and 5p state electrons, the Fermi level moves into the conduction band, leading to the adsorption models showing more metallic properties.

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.

Temperature dependence of carrier relaxation time in gallium phosphide evaluated by photoemission measurements

For understanding of carrier behavior in semiconductors, it is important to measure the carrier relaxation time. In the present study, the relaxation times of inter-valley transition from the Γ valley to the X valley in GaP were evaluated by near-band-gap photoemission spectroscopy of electrons emitted from a surface with a negative electron affinity state. In the energy distribution curves, two peaks, which originate from the electron population accumulated in the Γ valley and the X valley, were observed. From the temperature dependence of the energy of these two peaks, we could successfully evaluate the temperature dependence of the energies of the Γ valley and the X valley. Furthermore, the relaxation times of the inter-valley transition from the Γ valley to the X valley were estimated from the ratio of the electron concentration of the Γ valley and the X valley. The values of the relaxation time are good agreement with the previous studies. These results indicate that the near-band-gap photoemission spectroscopy can directly investigate conduction electrons and also evaluate the carrier dynamics in semiconductor.

Evolution of the transverse and longitudinal energy distributions of electrons emitted from a GaAsP photocathode as a function of its degradation state

We present measurements of the transverse and longitudinal energy spread of photoelectrons emitted from a GaAsP photocathode as a function of its degradation state. The cathode was initially activated to a state of negative electron affinity in our photocathode preparation facility, achieving a quantum efficiency of 3% at a wavelength of 532 nm. It was then transferred under XHV conditions to our transverse energy spread spectrometer, where energy spread measurements were made while the photocathode was progressively degraded through a controlled exposure to oxygen. Data have been collected under photocathode illumination at 532 nm, and the changing photoelectron energy distribution associated with the changes in the level of electron affinity due to quantum efficiency degradation through an exposure to 0.25 L of oxygen has been demonstrated. Our experiments have shown that GaAsP boasts a significantly higher resilience to degradation under exposure to oxygen than a GaAs photocathode, though it does exhibit a higher level of mean transverse energy. Coupled with the favourable published data on GaAsP photoemission response times, we conclude that GaAsP is a viable candidate material as a particle accelerator electron source.

Extremely high negative electron affinity of diamond via magnesium adsorption

We report large negative electron affinity (NEA) on diamond (100) using magnesium adsorption on a previously oxygen-terminated surface. The measured NEA is up to $(\ensuremath{-}2.01\ifmmode\pm\else\textpm\fi{}0.05)$ eV, the largest reported negative electron affinity to date. Despite the expected close relationship between the surface chemistry of Mg and Li species on oxygen-terminated diamond, we observe differences in the adsorption properties between the two. Most importantly, a high-temperature annealing step is not required to activate the Mg-adsorbed surface to a state of negative electron affinity. Diamond surfaces prepared by this procedure continue to possess negative electron affinity after exposure to high temperatures, air, and even immersion in water.

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.

Comparison of the photoemission behaviour between negative electron affinity GaAs and GaN photocathodes

In view of the important application of GaAs and GaN photocathodes in electron sources, differences in photoemission behaviour, namely the activation process and quantum yield decay, between the two typical types of III—V compound photocathodes have been investigated using a multi-information measurement system. The activation experiment shows that a surface negative electron affinity state for the GaAs photocathode can be achieved by the necessary Cs—O two-step activation and by Cs activation alone for the GaN photocathode. In addition, a quantum yield decay experiment shows that the GaN photocathode exhibits better stability and a longer lifetime in a demountable vacuum system than the GaAs photocathode. The results mean that GaN photocathodes are more promising candidates for electron source emitter use in comparison with GaAs photocathodes.

Semiconductor surfaces with negative electron affinity

The review of the studies of GaAs and GaN surfaces performed by the authors at the Institute of Semiconductor Physics (Novosibirsk) are presented. The results of these studies are used for further elucidation of the physics of photoemission from the surfaces with negative electron affinity (NEA) and improving the performance of NEA-photocathodes. In particular, the requirements to III-V semiconductor surfaces, which are necessary for the preparation of NEA photocathodes with ultimate parameters, are analyzed. To meet these requirements, the experimental procedures aimed at preparing atomically clean and flat surfaces with certain reconstructions, which are subsequently activated to the state of NEA in ultra-high vacuum by coadsorption of cesium and oxygen, are developed. The surface morphological, structural, and electronic phenomena occurring on various stages of these preparation procedures are studied, and the relation of these phenomena to the physical limits of NEA-photocathode performance are discussed. [DOI: 10.1380/ejssnt.2007.80]

Epitaxial growth, electronic properties, and photocathode applications of strained pseudomorphic InGaAsP/GaAs layers

The results of experimental and theoretical investigations directed toward the development of highly efficient sources of spin-polarized electrons are reported. The sources are based on heteroepitaxial elastically strained films of the quaternary InGaAsP solid solution grown by liquid-phase epitaxy on GaAs substrates. The InGaAsP films synthesized were 0.1–0.2 µm thick with the band gap being within the range of 1.4–1.9 eV and having elastic strains as high as 1%. This provided splitting of the valence band top by 40–60 meV and a degree of the spin polarization P of the electrons photoemitted as high as 80%. The films have a high quantum yield of photoemission Y upon activating to the negative electron affinity state due to the adsorption of Cs and O. Record values for the effective figure of merit P2Y are achieved.

STM, STS, and local work function study of Cs/p-GaAs(1 1 0)

Various surface configurations of Cs/p-GaAs(1 1 0) have been studied by scanning tunneling microscopy and scanning tunneling spectroscopy with an interest in relation to the photoelectron emission. The local work function is found to be 4.8, 3.3, and 3.3 eV from clean p-GaAs(1 1 0), Cs one-dimensional (1D) lines (Cs coverage of 0.23 ML), and partially-ordered Cs polygons (Cs coverage of 0.5 ML), respectively. It is understood that only a coherently c(4×4)-ordered Cs-polygon surface (Cs coverage of 0.6 and 0.7 ML) can emit photoelectrons due to a sufficient reduction of the local work function down to 1.3 eV to get the negative electron affinity state. The local work function image shows that the boundary of Cs atoms has a lower local work function than at the top of Cs atoms.

Resonance enhancement of the photoemission from semiconductors with negative electron affinity

The variation of photoemission from negative electron affinity semiconductor surface with the position of the surface Fermi level is theoretically studied. Resonance maxima both in photoyield and surface recombination rate (when the depth and the width of the quantum well in the band bending region (BBR) are close to the appearance of new quantum level) are predicted as a result of quantum character of the electron motion in the BBR. The resonance BBR transmission is expected to manifest itself in the temperature, doping, and thickness of the activation layer photoyield dependence. Experimental studies of the dependencies should help to monitor the evolution of the BBR during surface activation to the negative electron affinity state.

Evolution of electronic properties at the p-GaAs(Cs,O) surface during negative electron affinity state formation

The evolution of surface band bending and surface photovoltage was monitored in situ by photoreflectance spectroscopy during activation of the surface of epitaxial GaAs to the state of negative electron affinity by successive deposition of cesium and oxygen in a standard ‘‘yo-yo’’ technique. Considerable variations of the band bending (by approximately 0.3 eV) and surface photovoltage (by three orders of magnitude) were observed. It was found that the maximum of photoemission quantum yield corresponded to unexpectedly small value of the band bending φs=0.3 eV, as compared to widely accepted value of approximately 0.5 eV.

Liquid phase epitaxial growth of elastically strained InGaAsP layers for spin-polarized electron sources

InGaAsPGaAs(111)B pseudomorphic layers have been grown with elastic strains up to 1% and bandgaps ranging from 1.4 to 1.8 eV by liquid phase epitaxy (LPE) for the first time. Splitting of the valence band up to 50 meV was observed in these films. Photoemitters with the negative electron affinity state have been prepared by CsO activation of the strained InGaAsP films in ultrahigh vacuum (UHV).

New material for photoemission electron source: semiconductor alloy InGaAsP grown on GaAs substrate

Wide-bandgap epitaxial In x Ga 1− x As y P 1− y layers grown on GaAs substrates were investigated as a material for photoemission electron sources for the first time. By variation of x and y, both thick (of about one μm) lattice-matched unstrained layers and thin (of about 0.2 μm) lattice-mismatched strained layers with direct energy gap from 1.78 to 1.92 eV were grown. The layers were chemically treated in a glove-box, loaded into UHV via a loading chamber without exposure to air, heat cleaned, and activated to the state of negative electron affinity (NEA). For InGaAsP it was possible to reach the NEA-state by an activation with cesium only, without oxygen. The estimated e-folding lifetime of InGaAsP photocathodes operating at DC current of 400 μA was above one thousand hours in the regime with automatical additional cesiations and above one hundred hours without additional cesiations. Lifetime limitation factors are discussed. Strain-induced splitting of the valence band up to 32 meV was observed in the lattice-mismatched films; this observation gives prospects for an increase of spin polarization beyond the 50% limit.