Photoemission Current Research Articles

Absolute measurement of vacuum ultraviolet fluxes from inductively coupled plasmas via metal surface photoemission currents

Quantifying vacuum-ultraviolet (VUV) fluxes typically requires vacuum-compatible spectrometers and is often associated with significant cost and effort. A simple technique for the absolute measurement of local VUV fluxes from plasmas using the photoemission from a set of coated metal plates, is described. The radiant power from a 13.56 MHz hydrogen plasma operating at 40–87 mTorr and with an radio frequency (RF) input power from 100 to 120 W was investigated by irradiating a set of 2 cm diameter Au, Ag and Cu plates. The variation in photoemission currents was compared with the photoelectric yield curves to estimate the absolute flux incident on the surfaces in the 113–190 nm range. The measured fluxes were found to have an uncertainty of 5%–30% when compared with the VUV spectrometer measurements. The VUV output power was found to have a maximum at a pressure of 70–80 mTorr and to increase with RF power. In all cases, the VUV output power was measured to be approximately 12%–16% of the RF input power to the matching network, in good agreement with spectroscopy results.

Tuning quantum pathway interference in two-color laser photoemission using DC bias

Coherent control of quantum systems depends on the manipulation of quantum interference through external fields. In this work, we investigate the effects of DC bias field on coherent control of quantum pathways in two-color laser photoemission using exact analytical solutions of the one-dimensional time dependent Schrödinger equation. Increasing DC bias lowers and narrows the surface potential barrier, shifting the dominant emission to lower order multiphoton photoemission, photo-assisted tunneling and then direct tunneling. Those lower order photon absorption processes result in fewer possible pathways, and therefore modulation of photoemission current can be suppressed as DC field increases. It is shown that a maximum modulation depth of 99.4% can be achieved for a gold emitter at local DC bias F 0 = 0.5 V nm−1, fundamental (800 nm) laser field F 1 = 2.6 V nm−1 and second harmonic laser field F 2 = 0.25 V nm−1 . For a given set of input parameters, the total photoemission consists of different k-photon processes, each of which has their own different multiple possible pathways and interference effects. However, the quantum pathways and their interference for the dominant k-photon process and for the total photoemission probability show the same trends. This study demonstrates strong flexibility in tuning two-color lasers induced photoemission using a DC bias and provides insights into coherent control schemes of general quantum systems.

Estimating the optical depth of Saturn’s main rings using the Cassini Langmuir Probe

Abstract A Langmuir Probe (LP) measures currents from incident charged particles as a function of the applied bias voltage. While onboard a spacecraft the particles are either originated from the surrounding plasma, or emitted (e.g. through photoemission) from the spacecraft itself. The obtained current–voltage curve reflects the properties of the plasma in which the probe is immersed into, but also any photoemission due to illumination of the probe surface: As photoemission releases photoelectrons into space surrounding the probe, these can be recollected and measured as an additional plasma population. This complicates the estimation of the properties of the ambient plasma around the spacecraft. The photoemission current is sensitive to the extreme ultraviolet (UV) part of the spectrum, and it varies with the illumination from the Sun and the properties of the LP surface material, and any variation in the photoelectrons irradiance can be measured as a change in the current voltage curve. Cassini was eclipsed multiple times by Saturn and the main rings over its 14 yr mission. During each eclipse the LP recorded dramatic changes in the current–voltage curve, which were especially variable when Cassini was in shadow behind the main rings. We interpret these variations as the effect of spatial variations in the optical depth of the rings and hence use the observations to estimate the optical depth of Saturn’s main rings. Our estimates are comparable with UV optical depth measurements from Cassini’s remote sensing instruments.

Enhanced photoemission from surface modulated GaAs:Ge

AbstractThe present work reports the evolution and growth of GeGaAs(O) polytype nanoislands over GaAs p‐type substrate with photoemission application in mind. Several morphological transformations from NIs to simultaneously present nanopits/holes are observed as a function of annealing parameters that is, temperature (350‐800°C) and time (5‐90 minutes). Structural and elemental analyses are executed using atomic force microscopy, scanning electron microscopy and energy dispersive X‐ray spectroscopy. Photoemission current of the nanostructured surfaces, measured upon exposure from 265 nm light emitting diode, is found to depend on the nanoislands size, which in turn depends on the annealing parameters. A maximum photoelectric emission is obtained for structure annealed at 650°C for 60 minutes, upon which an increment of roughly two orders of magnitude is observed.

Role of photoelectric charge fluctuation in dust detachment from the lunar surface

Electrostatic processes are argued to be of fundamental importance in understanding the particle dynamics and complex dusty plasma environment over airless bodies—the Moon has been of particular interest. Based on the theory of electrostatic charge fluctuation corresponding to the photoemission current, the fundamental problem of dust detachment from the lunar surface is addressed. By applying the charge fluctuation at the microscopic scale, we have quantified the magnitude of fluctuating charge density over the sunlit lunar surface and illustrated that it could induce a sufficient electric field to overcome the dust–surface adhesive van der Waals bonding through the electrostatic Coulomb repulsion. The analysis takes into account the dynamic equations for the statistical variables, viz., the mean charge and the variance, corresponding to the charge distribution over the microscopic spots exposed to the solar radiation. The photoemission under the influence of extreme ultraviolet Lyman α radiation in the solar spectrum and subsequent collection of the emitted photoelectrons are accounted for as the dominant charging processes of the lunar surface. Based on analysis and calculations, the fluctuating charge is illustrated to be a significant function of the spot size, which may induce significantly high electric field fluctuations locally. As an illustrative example, it is shown that one square micrometer spot may acquire ∼15 electronic charges and might induce a local electric field equivalent to ∼10 kV/m, which can support the detachment of the submicrometer dust particles from the lunar surface.

Effects of post bonding annealing on GaAs//Si bonding interfaces and its application for sacrificial-layer-etching based multijunction solar cells

By using the sacrificial layer (SL) etching, GaAs substrates are separated from III–V epi substrate//Si substrate junctions that are made by surface activated bonding (SAB) technologies. The post-bonding low-temperature (300-∘C) annealing plays an essential role in achieving a promising (∼90%) bonding yield. The effects of the post-bonding annealing are investigated by hard X-ray photoemission spectroscopy and current–voltage measurements of GaAs//Si bonding interfaces. It is found that the concentration of oxygen atoms at interfaces is reduced and the resistance decreases to 1.6–2.1 mΩcm2 by the low-temperature annealing. Aluminum fluoride complexes are not observed by X-ray photoelectron spectroscopy on the exposed surfaces of separated GaAs substrates. The roughness average of the surfaces is ≈0.25–0.30 nm. The characteristics of double junction cells fabricated on the GaAs//Si junctions prepared by the SL etching are almost the same as those of cells fabricated by dissolving GaAs substrates after bonding. These results indicate that multijunction cells could be fabricated in a process sequence compatible with reuse of GaAs substrates by combining the SL etching and SAB.

Electrostatic charging of the sunlit hemisphere of the Moon under different plasma conditions

Accounting for the continuous interaction of the solar radiation, the development of electrostatic charge on the sunlit lunar regolith under extreme plasma conditions has been investigated—the ambient plasma around Moon corresponds to the plasma parameters associated with the solar wind, wake region, SEP events, and terrestrial magnetosphere. The photoemission of electrons from the lunar surface corresponding to the solar radiation spectrum, electron energy-dependent secondary electron emission and simultaneous collection of the ambient non-Maxwellian plasma electrons/ions have been considered the dominant charging mechanisms. The lunar surface potential has been derived using the dynamical balance of the photoemission and plasma accretion currents over its surface—the potential dependence of the charging currents has consistently been accounted for. In results, the lunar surface potential dependence on the plasma, surface, and radiation parameters have been parametrically derived. For the high temperature plasma composition, the sunlit locations over the lunar surface are predicted to acquire a negative potential. The outcome infers that depending on topography and location along with realistic plasma/surface parameters, the lunar surface may hold significant contrast in charging, and it may differ by the orders of magnitude. Such disparity in the surface charging may act as a source of the transport of charged dust and local atmospheric charge (ions/electrons) over the lunar regolith.

Extreme nonlinear strong-field photoemission from carbon nanotubes

Strong-field photoemission produces attosecond (10−18 s) electron pulses that are synchronized to the waveform of the incident light. This nonlinear photoemission lies at the heart of current attosecond technologies. Here we report a new nonlinear photoemission behaviour—the nonlinearity in strong-field regime sharply increases (approaching 40th power-law scaling), making use of sub-nanometric carbon nanotubes and 800 nm pulses. As a result, the carrier-envelope phase sensitive photoemission current shows a greatly improved modulation depth of up to 100% (with a total modulation current up to 2 nA). The calculations reveal that the behaviour is an interplay of valence band optical-field emission with charge interaction, and the nonlinear dynamics can be tunable by changing the bandgap of carbon nanotubes. The extreme nonlinear photoemission offers a new means of producing extreme temporal-spatial resolved electron pulses, and provides a new design philosophy for attosecond electronics and photonics.

Study on anomalous photoemission of LaB6 at high temperatures

Photoemission is used for a large variety of experimental techniques for study of material properties. Further applications photoemission finds for electron beam generation. Development of photocathodes with high quantum efficiency requires sufficient microscopic understanding of photoemission processes. The LaB6 is a well-known thermionic emitter, which can be used as a photocathode. Anomalous, thermally assisted increase of quantum efficiency was previously demonstrated for this material. The increase shows a bi-exponential rise with photon-energy dependent slopes. The photon-energy dependency indicates correlation of extracted current with electronic band structure. In this work, we have investigated possible mechanisms, which could be the reason for the anomalous dependence of photoemission current on the cathode temperature. In particular, the Fowler–DuBridge theory was modified to account for changes in the electron density of states. The results show negligible low influence of density of states structure. Finally, we make a suggestion regarding the modification of the electron escape probability, which can cause the increase in photoemission.

Relaxational kinetics of photoemission from Cs/GaAs and GaAs(Cs,O) surfaces at elevated temperatures

The relaxational kinetics of the photoemission from the Cs/GaAs and GaAs(Cs,O) surfaces at elevated temperatures is studied. It was found that heating to moderate temperatures of about 150°C leads to substantial changes in the amplitude and shape of the Cs coverage dependences of the photoemission current, along with the changes of its relaxational kinetics after the cesium deposition. After the oxygen exposure on the GaAs(Cs,O) surface, a photoemission relaxational increase is observed due to changes in the affinity. The temperature dependences of the amplitude and decay time of the relaxational kinetics on the Cs/GaAs and GaAs(Cs,O) surfaces were obtained.

Toward Plasmonic Tunnel Gaps for Nanoscale Photoemission Currents by On-Chip Laser Ablation.

We demonstrate that prestructured metal nanogaps can be shaped on-chip to below 10 nm by femtosecond laser ablation. We explore the plasmonic properties and the nonlinear photocurrent characteristics of the formed tunnel junctions. The photocurrent can be tuned from multiphoton absorption toward the laser-induced strong-field tunneling regime in the nanogaps. We demonstrate that a unipolar ballistic electron current is achieved by designing the plasmonic junctions to be asymmetric, which allows ultrafast electronics on the nanometer scale.

Thermally assisted photoemission effect on CeB6 and LaB6 for application as photocathodes

The thermally assisted photoemission (TAPE) effect was investigated for the hexaboride thermionic cathodes (${\mathrm{LaB}}_{6}$ and ${\mathrm{CeB}}_{6}$). It was found that the quantum efficiency of these cathodes can be increased by raising the cathode temperature along with the thermionic emission. In addition both materials can emit a measurable photoemission current by being irradiated with a laser having a photon energy below the work function at sufficiently high cathode temperatures. Our measurements indicate that this process is linear. These results show the significance of the TAPE effect. An additional effect of cathode heating is surface cleaning. This effect was detected as the hysteresis of the quantum efficiency which can be observed by heating up and cooling down the cathode. This effect slightly changes the quantum efficiency but not as much as the TAPE effect.

Two-color field enhancement at an STM junction for spatiotemporally resolved photoemission.

We report measurements and numerical simulations of ultrafast laser-excited carrier flow across a scanning tunneling microscope (STM) junction. The current from a nanoscopic tungsten tip across a ∼1 nm vacuum gap to a silver surface is driven by a two-color excitation scheme that uses an optical delay-modulation technique to extract the two-color signal from background contributions. The role of optical field enhancements in driving the current is investigated using density functional theory and full three-dimensional finite-difference time-domain computations. We find that simulated field-enhanced two-photon photoemission (2PPE) currents are in excellent agreement with the observed exponential decay of the two-color photoexcited current with increasing tip-surface separation, as well as its optical-delay dependence. The results suggest an approach to 2PPE with simultaneous subpicosecond temporal and nanometer spatial resolution.

New photocathode using ZnSe substrates with GaAs active layer

GaAs active layers were successfully fabricated on ZnSe substrates using a metalorganic vapor phase epitaxy system. As a photocathode, a GaAs active layer shows a high quantum efficiency (QE) of 9% at 532 nm laser light illumination, which is comparable to a QE of 11% from GaAs bulk. In addition, a photoemission current of 10 µA was obtained from this photocathode. One more important point is that this photocathode could realize back-side illumination of 532 nm laser light, and thus its widespread applications are expected in microscopy and accelerator fields.

Photoelectron detection from transient species in organic semiconducting thin films by dual laser pulse irradiation

An Nd3+:YAG pulsed laser was employed as a light source for two-photon photoemission from organic semiconducting thin films in low vacuum and air. Photoionization by the two-photon process was confirmed in both the environments by measuring photoemission current. By constructing a pump–probe system, photoemissions from transient species formed by the pump light irradiation were detected by probe light irradiation as a result of a linear increase in the photocurrent with the pump power via a one-photon process. Thus, we propose a novel method called two-photon photoelectron yield spectroscopy to determine the excited-state energy levels in ambient environments.

Single and double photoemission and generalizations

A unified diagrammatic treatment of single and double electron photoemission currents is presented. The irreducible lesser density-density response function is the starting point of these derivations. Diagrams for higher order processes in which several electrons are observed in coincidence can likewise be obtained. For physically relevant situations, in which the photoemission cross-section can be written as the Fermi Golden rule, the diagrams from the nonequilibrium Green's function approach can be put in direct correspondence with the diagrams of the scattering theory.

Kinetics of vertical transport and localization of electrons in strained semiconductor supperlattices

The kinetics of vertical electron transport in a semiconductor superlattice is considered taking into account partial localization of electrons. The time dependences of photoemission currents from samples based on a strained semiconductor superlattice calculated by numerically solving the kinetic equation are in good agreement with experimental data. Comparison of the theory with experiment makes it possible to determine the characteristic electron localization and thermoactivation times, the diffusion length, and losses of photoelectrons in the superlattice.

Probing hot-electron effects in wide area plasmonic surfaces using X-ray photoelectron spectroscopy

Plasmon enhanced hot carrier formation in metallic nanostructures increasingly attracts attention due to potential applications in photodetection, photocatalysis, and solar energy conversion. Here, hot-electron effects in nanoscale metal-insulator-metal (MIM) structures are investigated using a non-contact X-ray photoelectron spectroscopy based technique using continuous wave X-ray and laser excitations. The effects are observed through shifts of the binding energy of the top metal layer upon excitation with lasers of 445, 532, and 650 nm wavelength. The shifts are polarization dependent for plasmonic MIM grating structures fabricated by electron beam lithography. Wide area plasmonic MIM surfaces fabricated using a lithography free route by the dewetting of evaporated Ag on HfO2 exhibit polarization independent optical absorption and surface photovoltage. Using a simple model and making several assumptions about the magnitude of the photoemission current, the responsivity and external quantum efficiency of wide area plasmonic MIM surfaces are estimated as 500 nA/W and 11 × 10−6 for 445 nm illumination.

Nanoscale Photoelectron Mapping and Spectroscopy with an Atomic Force Microscope

The tip of an atomic force microscope is used as a local probe for photoelectrons excited by laser illumination. The tip-sample distance is precisely controlled by the van der Waals force and the pure photoemission current is measured without tunneling current contribution. The nanoscale photoelectron mapping with high current contrast is obtained on a cesium covered Au(111) surface. By sweeping the laser photon energy, the local photoelectron spectra are measured on Cs islands and terraces. The results reveal distinct electronic states and photoemission thresholds for different Cs coverage, providing the photoemission current contrast mechanism. The contrast in photoelectron mapping can be further tuned by the incident laser polarization exploiting the symmetry selection rules in the optical excitation.

Photoassisted electron emission from metal-oxide-semiconductor cathodes based on nanocrystalline silicon

This paper investigates the effect of optical pulses on the electron emission properties of metal-oxide-semiconductor (MOS) cathodes based on nanocrystalline silicon (nc-Si). The emission current is enhanced by about two orders of magnitude by the irradiation of 405 nm laser light. The increase of the emission current under irradiation was proportional to incident laser power. The differential quantum efficiency of the nc-Si based MOS diode itself was estimated to be 3 × 10−2. However, the value of the photoemission current was only 3 × 10−7 due to the short mean free path of hot electron for Pt used as the gate electrode. We obtained a pulsed electron beam from the cathode device by a pulsed laser. The result shows that MOS type cathodes have a suitable structure for optically generating a train of short electron bunches.