Photoemission Quantum Yield Research Articles

Пленение излучения и субзонный пик в спектрах квантового выхода фотоэмиссии из p-GaAs(Cs,O)

In the spectra of the photoemission quantum yield of p-GaAs(Cs,O) measured in the reflection geometry, a peak was found at a photon energy lower than the band gap of GaAs. It is shown that the appearance of the peak is due to the trapping of weakly absorbed radiation, which scatters diffusely on the rough back face of the epitaxial structure. Possible microscopic mechanisms of the appearance of the peak are discussed: the Franz-Keldysh effect in the surface electric field and adsorption-modified optical transitions in cesium adatoms.

Light trapping and subbandgap maximum in photoemission quantum yield spectra of p-GaAs(Cs,O)

In the spectra of the photoemission quantum yield of p-GaAs(Cs,O) measured in the reflection geometry, a peak was found at a photon energy lower than the band gap of GaAs. It is shown that the appearance of the peak is due to the trapping of weakly absorbed radiation, which scatters diffusely on the rough back face of the epitaxial structure. Possible microscopic mechanisms of the appearance of the peak are discussed: the Franz-Keldysh effect in the surface electric field and adsorption-modified optical transitions in cesium adatoms. Keywords: photoemission, GaAs, negative electron affinity, quantum yield spectra.

Localization Effect in Photoelectron Transport Induced by Alloy Disorder in Nitride Semiconductor Compounds.

Near-band-gap photoemission spectroscopy experiments were performed on p-GaN and p-InGaN/GaN photocathodes activated to negative electron affinity. The photoemission quantum yield of the InGaN samples with more than 5% of indium drops by more than 1 order of magnitude when the temperature is decreased while it remains constant for lower indium content. This drop is attributed to a freezing of photoelectron transport in p-InGaN due to electron localization in the fluctuating potential induced by the alloy disorder. This interpretation is supported by the disappearance at low temperature of the peak in the photoemission spectrum that corresponds to the contribution of the photoelectrons relaxed at the bottom of the InGaN conduction band.

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.

Strategies to increase the quantum yield: Luminescent methoxylated imidazo[1,5-a]pyridines

A series of methoxylated imidazo[1,5-a]pyridines is presented and their optical and electrochemical properties investigated and interpreted on the basis of density functional theory calculations. The photophysical properties are discussed in relation to the chemical structure. The key role of the 1,3 substitutions on the imidazo[1,5-a]pyridine nucleus on rotational barriers and on frontier molecular orbitals is discussed in relation to the experimental hyperchromic effect, photoemission quantum yield and electrochemical properties. Depending on the position of the introduced methoxy substituents on the imidazo[1,5-a]pyridine nucleus, we are able to tune the Stokes shift and to increase the emission quantum yield from 22% to 50%.

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.

Evolution of photoemission properties on Cs/GaAs and GaAs(Cs,O) surfaces under thermal cycling

The photoemission properties on Cs/GaAs and GaAs(Cs,O) surfaces prepared at room temperature were studied under thermal cycling. The evolution of electron affinity and escape probability to vacuum was measured using photoemission quantum yield spectroscopy for the surfaces with various Cs-O overlayer compositions. It was found that an increase in the oxygen exposure led to the improvement in the thermal stability of electron affinity.

Electron emission from GaAs(Cs,O): Transition from negative to positive effective affinity

The evolution of the escape probability of hot and thermalized photoelectrons to vacuum from GaAs(001) with adsorbed layers of cesium and oxygen under the transition from the negative to positive effective electron affinity is studied by photoemission quantum yield spectroscopy. A minimum in the dependence of the escape probability of thermalized electrons at zero affinity is observed. The minimum is caused by a photoelectron capture into the two-dimensional subband states in the near-surface band bending region. Lower escape probability values for the cesium deposition, as compared to the oxygen deposition on the GaAs(Cs,O) surface with negative electron affinity, are explained by the photoelectron reflection or scattering at two-dimensional “metallic” Cs clusters.

Relaxational kinetics of photoemission and photon-enhanced thermionic emission from p-GaAs surface with nonequilibrium Cs overlayers

Photoreflectance and photoemission quantum yield spectroscopies are used for the experimental study of direct photoemission, photon-enhanced thermionic emission (PETE), surface band bending and photovoltage, effective electron affinity, and probabilities of electron escape into a vacuum under cesium deposition on the Ga-rich p-GaAs(0 0 1) surface and the subsequent structural relaxation in the absorbed overlayer. The relaxational decrease of the direct photoemission and PETE at small Cs coverages is caused by the band bending decrease, while the photocurrent relaxational increase at large coverages is due to electron affinity relaxation.

Electron emission from Cs/GaAs and GaAs(Cs, О) with positive and negative electron affinity

The evolution of probabilities of escape of hot and thermalized electrons from GaAs(001) with adsorbed cesium and oxygen layers to vacuum at the transition from positive to negative effective electron affinity is studied by the photoemission quantum yield spectroscopy. A minimum of the probability of escape of thermalized electrons near zero electron affinity is revealed and explained.

Photon-enhanced thermionic emission from p-GaAs with nonequilibrium Cs overlayers

Photon-enhanced thermionic emission (PETE), which is promising for increasing the efficiency of solar energy conversion, is studied during cesium deposition on the As- and Ga-rich p-GaAs(001) surfaces and subsequent relaxation in the nonequilibrium Cs overlayer by means of photoemission quantum yield spectroscopy adapted for systems with time-variable parameters. Along with direct photoemission of “hot” electrons excited by light above the vacuum level, the spectra contain PETE contribution of “thermalized” electrons, which are excited below the vacuum level and emit in vacuum due to thermalization up in energy by phonon absorption. Comparing the measured and calculated spectra, the effective electron affinity and escape probabilities of hot and thermalized electrons are obtained as functions of submonolayer Cs coverage. The minima in the affinity and pronounced peaks in the escape probabilities are observed for Cs deposition on both the As- and Ga-rich surfaces. Possible reasons for the low mean values of the electron escape probabilities and for the observed enhancement of the probabilities at certain Cs coverages are discussed, along with the implications for the PETE device realization.

Influence of temperature and photon energy on quantum yield of photoemission from real iron surfaces

The influence of temperature and incident photon energy on the photoemission quantum yield, Ys, of real iron surfaces has been investigated by thermally assisted photoemission (TAPE). Measurements were carried out using a Geiger counter under a gaseous atmosphere of He containing 1 % isobutane vapor at normal atmospheric pressure in the temperature range 25–353 °C under UV irradiation with wavelengths of 200, 210, 220, and 230 nm. The Ys obtained under irradiation at a given wavelength was found to increase with temperature, particularly more rapidly with wavelengths of greater photon energy. From the Arrhenius plots, the Ys values were found to have activation energies of 0.112–0.040 eV, depending on the photon energy. The chemical composition of the surfaces after TAPE measurements at different temperatures was examined by X-ray photoelectron spectroscopy (XPS). The intensity of species thermally removed from the surface was also measured by temperature desorption spectroscopy (TDS). XPS and TDS results showed the removal of the surface water and weakly bound carbon material from the surface with temperature. It was concluded that the removal of these species with increasing temperature enabled the incident light to penetrate through the surface into the metal, causing the increase in the Ys. The dependence of the activation energies on the photon energy was explained by the change of UV light absorption spectra of the surface water, and the enhancement of the Ys with temperature was also attributed to the influence of iron cations (Fe3+) corresponding to positive holes produced in the surface oxide layer by UV light.

Photoemission from InAs/GaAs quantum dots decorated with cesium adatoms

An array of non-overgrown InAs/GaAs quantum dots has been decorated with adsorbed metal atoms in situ in ultrahigh vacuum. Their electron and photoemission properties have been studied. The radical modification of the spectra of the threshold emission from the quantum dots with increasing cesium coating has been found. Two photoemission channels have been established; they are characterized by considerably different intensities, spectral locations, and widths of the selective bands. It has been shown that the decoration of the quantum dots makes it possible to control the electronic structure and quantum yield of photoemission, the nature of which is related to the excitation of the electronic states of the GaAs substrate and InAs/GaAs quantum dots.

High energy photocathodes for laser fusion diagnostics

Laser fusion experiments at the National Ignition Facility require time-resolved x-ray images of the ignition target self-emission. The photon energies are expected to be greater than 10 keV. Photoemission quantum yield measurement data and photoelectron energy spectrum data are presently unavailable in this photon energy range, but are essential in the design of x-ray imaging diagnostics. We developed an apparatus to measure the quantum efficiency of primary and secondary photoelectron emission and to estimate the energy spectrum of the secondary photoelectrons. The apparatus has been tested using photon energies less than 10 keV to allow comparisons with prior work. A method for preparing photocathodes with geometrically enhanced photoefficiency has been developed.

Self-organization of nanostructures on the n-GaN(0001) surface in the Cs and Ba adsorption

A regular self-organized 2D nanostructure of a new type of nanocombs has been created in situ in an ultrahigh vacuum on the n-GaN(0001) surface. The nanostructure is formed as a result of multilayer adsorptions of Cs and Ba. The structure is highly regular in the microrange, arranged in the form of combs 60–70 nm in diameter with a wall height of about 7 nm. It has been revealed that the nanostructure has a quasi-metallic conduction, a low work function of about 1.4 eV, and a high quantum yield of photoemission under light excitation in the GaN transparency region. A self-organization model is proposed, which implies the formation of a surface 2D longperiod incommensurate phase interacting with the superstructure of the Cs+ and Ba2+ ion clusters with allowance for the polaron compensation on the GaN surface.

Влияние электронной структуры нанокластеров на квантовый выход фотоэмиссии магниевых сплавов

Влияние электронной структуры нанокластеров на квантовый выход фотоэмиссии магниевых сплавов

Photoelectron emission caused by surface plasmons in silver nanoparticles

A new intense band of photoelectron emission peaked at a wavelength of 490 nm has been observed in granular silver films with a nanoparticle size of 10–50 nm. Coinciding with an optical reflection band, the new photoelectron emission peak is related to the excitation of surface plasmons in silver nanoparticles. The quantum yield of photoemission (0.25%) for the new band is approximately equal to that for a longwave band of the classical silver-oxygen-cesium photocathode. It is demonstrated that the photoelectron emission from both photoemitters has a common nature and a time constant on the order of several femtoseconds can be obtained.

Low current mode of negative coronas: mechanism of electron emission

Photo-induced non-self-sustained discharges are studied in point–plane gaps in air. Point electrodes made of different materials and with different non-metallic coatings (including TiO2 and HfO2) are tested with varying intensity of radiation and voltage. It is found that the quantum yield of photoemission (electrons per photon) changes with time and depends on the radiation intensity and applied voltage. It is assumed that the tunnelling of electrons through the barrier at the metal–dielectric interface and the finite capacitance of the electron reservoir at the dielectric–gas interface limit the current. The results of the model fit with the experimental findings.

Quantum yield of photoemission of Ag–BaO thin films for detecting ultrashort infrared laser pulses

The Ag–BaO thin films are a new kind of photoemissive thin films, which have transient optical and ultrafast photoelectronic response. The properties of the Ag–BaO thin films establish the application foundation of detecting ultrashort infrared laser pulses. The measurement of the quantum yield of photoemission for ultrashort laser pulses is different from that for normal visible light. The experimental results present that the quantum yield of Ag–BaO thin films is of the order of magnitude of 10 −6 under the action of a single laser pulse when the incident infrared laser wavelength is 1.06 μm. It is two orders of magnitude higher than that of pure metals at the same conditions. The quantum yield under the action of a single-chain laser pulse, which comprises 11 single pulses of duration 40 ps separated by 10 ns, increases rapidly following increasing incident energy of the laser. In this case, the Ag–BaO thin films present multiphoton emission of 2-photon process and higher quantum yield of 10 −5.

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.