Photofield Emission Research Articles

Tunnel Currents in the Photo-Field Detectors and the Auger Transistor under Strong Electric Field

ABSTRACTThe photo-field emission properties of semiconductors at a very strong electric field together with tunnel electron emission in metal-insulator-semiconductor heterostructures with a tunneltransparent layer of an insulator was studied. It was found that a self-consistent quantum well near the surface of semiconductor emitter tips can change the spectral region of photosensitivity of the radiation semiconductor field emitter detectors, and leads to the significant increase in their photosensitivity [1]. Also the appearance of a self-consistent quantum well near the semiconductor surface is the key factor which allows to use the metal-insulator heterojunction in the development of an Auger transistor based on the Al-SiO2-n-Si structure - the fastest operation semiconductor bipolar transistor [2–4]. Conditions for appearance of a self-consistent quantum well under strong electric field in both the near-surface region of a vacuum semiconductor field-emitter and metal-insulator-semiconductor heterostructures (Auger transistor) were studied also.

Photofield emission spectroscopy and surface density of states

A novel procedure for the investigation of optical excitations on the metal surface by the photofield emission method is reviewed. A separation procedure of thermocurrent, accompanying the measured photocurrent, induced by the chopped light beam, is described. A differentiation of the true photocurrent curve allows one to obtain an energy distribution of photoelectrons. For the (0 0 0 1) face of Ti, it exhibits very small and densely spaced maxima, confirmed by theory as surface resonances. The dependence of the density on the thickness of the Ti crystal, grown by evaporation of Ti on the W field emitter surface, was verified experimentally. The possibility of observing surface resonances on a microcrystal of a homogeneous field emitter is discussed.

Photofield emission from a tipped silicon photocathode in IR with a new picosecond Nd–YAG laser at a frequency of 100 MHz

Tipped silicon photocathodes are tested with the IR wavelength (1.064 μm) of a new lock-mode Nd–YAG laser. The impulse width of 30 ps is repeated at a frequency of 100 MHz at a power of 122 W. A quantum yield of 2.5×10 −3 is obtained in a DC gun using the best continuous emission field of 20 MV/m for the photofield emission current. The ratio of the photofield emission current to the field emission current (dark current) is more than 10 4. A previous experiment with the second harmonic of a Nd–YAG at 10 Hz gave 700 pC per pulse of 35 ps width with a quantum yield of 10 −5 at the threshold emission field of 7.5 MV/m.

Surface density of states on the (0001) face of titanium

A titanium microcrystal has been grown by the evaporation of titanium onto a tungsten field emitter. The titanium surface was kept clean by maintaining a residual gas pressure of 10−9 Pa in the vacuum chamber. The surface density of states on the (0001) face of titanium was determined by the photofield emission method using photon energies hν=1.963, 2.409, 2.497, 2.541 and 2.602eV. The full-potential linearized augmented plane wave method was used to calculate the surface density of electron states of Ti(0001). It is concluded that the photocurrent is dominated by photofield emission from a surface state which lies just below the Fermi energy, and that fine structure in the photocurrent is due to modulation of the density of final states by a set of narrowly spaced surface resonances.

Optical transitions in the titanium 〈0 0 0 1〉 band structure

Optical transitions in the bulk band structure of the titanium in the 〈0001〉 direction have been observed using the photofield emission method. In the photocurrent–voltage characteristic the optical excitations appear as shoulders. From the voltages at which the shoulders occur, final and initial energies are estimated and compared with theory. Measurements were made for s-polarized light and therefore excitations in the bulk are predominant. The results obtained for photon energies hν=1.96, 2.409, 2.497, 2.541 and 2.602eV are compared with the calculated band structure. These experimental results show that the excitations occur only between two bands and the bands should be considerably shifted, and the minimum of the lower band should be about 0.5eV below that calculated. Additionally, excitations from the titanium (0001) face are observed. A single microcrystal of titanium was obtained by evaporation of titanium onto the tungsten tip emitter in ultra-high vacuum.

Photofield emission spectroscopy

A brief review of the investigations of optical excitations in the bulk and on the surfaces of metals by the photofield emission method is presented. The selected results show that the method is very useful for detailed penetration of band structure near the Fermi level. Examples for bulk band structures and surface density of states of tungsten, tantalum and titanium are presented and compared with theoretical results. The residual gas effect on surface states is described and discussed. Taking into consideration the thermocurrent produced by light radiation, a new and more detailed presentation of surface states observed for titanium (0001) face is shown as the density of states. The method of energy distribution of photofield emitted electrons is briefly mentioned. The inverse and two-photon photoemission methods are mentioned.

Theoretical approach of the photofield emission of a degenerated semiconductor. Case of tipped, weakly P-doped, silicon photocathodes

We study theoretically the field and photofield emission in a degenerated semiconductor: a tipped silicon is weakly doped with boron ( ϱ = 3000 Ω cm ) and the field near the tip is greater than 1 GV/m. Due to this important field, the conduction band bends down to the Fermi level near the surface. The degenerated electrons are confined in a well near the surface barrier. We develop a new theoretical approach using few discrete energy levels in the well. We calculate field emission current and compare with experimental results. The photoelectric transition probability is calculated for different values of the wavelength of the incident laser. A growth of the photofield current with the increase of the wavelength is shown theoretically, taking into account the optical properties of the semiconductor. Experimentally, we observe such a behaviour with continuous lasers [1–3]. With a pulsed picosecond laser (35 ps from a Nd-Yag) we observed with harmonic 4 (4.68 eV) a pure photoemission charge of 70 pC and a photofield charge with harmonic 2(2.34 eV) as high as 700 pC [4]. We hope to obtain more charges with harmonic 1 (1.17 eV). This original development should enable us to predict the behaviour of our tipped photocathodes in actual photoinjectors. An experiment with CANDELA is in progress.

Residual Gas Effect on Photofield Emission from Tungsten (111) Face

Using the photofield emission method, surface states and bulk excitations from clean and residual gas adsorbed (111) face of tungsten were measured. Residual gas shaded the surface states and diminished photoemission from the bulk excitations. Gas of the artificially increased background pressure 10 -8 Pa adsorbed during the measurement deforms the photocurrent—voltage characteristic; the optical excitations observed for clean surfaces are exhibited, diminished or lost. For the adsorbed (111) face the state related to the metal—gas surface has been observed. PACS numbers: 79.70.+q, 79.60.Dp, 73.20.Hb

Intensification of optical electric fields caused by the interaction with a metal tip in photofield emission and laser-assisted scanning tunneling microscopy

Numerical simulations of the interaction of optical fields with metal tips, such as in photofield emission and laser-assisted scanning tunneling microscopy, show that the electric field at the apex of the tip is much greater than that in the incident optical beam. Calculations were made using the method of moments with two models of the tip; a conical frustrum with hemispherical end caps, and a paraboloid. A large number of subvolumes (850–1000) was used to decrease the errors caused by finite matrix size with the method of moments, and it is shown that these errors have no significant effect on the results. The ratio of the optical electric field at the apex of the tip to that in the incident beam (field ratio) varies directly with the curvature at the apex, and is increased by raising the tip length and the optical wavelength. For the range of parameters used in the examples field ratios of 3–15 dB were found for incidence normal to the tip axis, and values as large as 32 dB were found for an angle of approximately 10° between the incident laser beam and the tip axis. Differences between the results for the two models and the dependence on the radius of curvature show that the field ratio must vary greatly from tip to tip.

Photofield emission from clean and titanium covered (111) plane of tungsten

Optical transitions are observed as connected with the occurrence of shoulders in the photofield emission characteristics. For the photon energy hυ = 1.96 eV, two optical transitions in the 〈111〉 direction of clean tungsten and three maxima of the surface density of states on the (111) plane are observed. One transition, in the bulk energy band, is connected with the Fermi level and the second one with a minimum in the empty band. They are in good agreement with the calculated electronic structure. Thin titanium films (0.5–1.5 monolayer) cause ‘smoothing up’ of the photofield emission characteristics and shoulders (due to optical transitions) are scarcely noticeable, if at all. Noticeable changes in the photofield characteristics are observed after annealing the emitter at a temperature of about 1600 K. At a temperature of ≈ 2100 K titanium is removed almost completely. Similar results have been obtained for the quantum energy hυ = 2.41 eV. Electron states connected with the titanium layer adsorbed on tungsten (111) face have not been observed.

Silicon photocathodes with array of tips in a photo-injector

Photo-injectors for linear accelerators and free electron laser need a high brightness photocathode with a dark current lower than 10 mA. Silicon with array of tips shows a photofield emission current with a continuous argon laser with a quantum yield of 1%. The ratio η between photofield and field emission (dark) current is 20 near the threshold of field emission. With a pulsed Nd-YAG laser ( λ = 1.02 μm pulses of 35 ps duration) we observe a pure photoemission charge (70 pC) with harmonic 4 (ℏ ω 4 = 4.68 eV) and a photofield charge above the threshold which is 700 pC with harmonic 2 (ℏ ω 2 = 2.34 eV). We calculate for the photo-injector CANDELA the dark current following the method of Loew and Wang, starting from the experimental Fowler-Nordheim curve and we find a dark current which is 1 μA at 3 GHz at E max = 30 MV/m.

Temperature Dependence of Photofield Emission from Tungsten ⟨001⟩ Direction

The initiation of the photofield emission method was aimed to investigate the band stucture that extends between the vacuum level and the Fermi level [1], which energy range is inaccessible for conventional photoemission. The method was employed to examine the position, in the energy scale, of singular band points such as the extrema lying above and below the Fermi level [2-4]. Recently, the position of the minimum in the energy band lying by approximately 0.2 eV above the Fermi level, which was theoretically predicted for the (001) crystallographic direction of tungsten [5], has been determined [3]. The aim of this brief paper is to demonstrate how temperature considerably affects the fllling up of the empty band minimum with electrons and how this is manifested in the photocurrent characteristic. Occasionally a rise of the Fermi level with increasing temperature was noted.

Electronic structure of silver overlayers on W(100).

The electronic structure of silver adsorbed on the (100) facet of a tungsten field emitter in the range of coverage from zero to three monolayers has been studied experimentally by means of field and photofield emission spectroscopy. An analysis of work function and total emission current data yields evidence that adsorbed silver reduces the surface density of states at the Fermi energy. It is found that adsorbed silver both suppresses the Swanson hump in the surface density of states of clean W(100) and also induces additional structures that depend largely on the number of complete overlayers. The experimental observations are interpreted by means of self-consistent fully relativistic supercell calculations of the layer densities of electronic states at the tungsten-vacuum and at the tungsten-silver-vacuum interface. The calculations predict silver-induced features in the surface density of states that are generally consistent with the experimental observations.

The dynamic, simultaneous imaging of both photon-and field-induced electron emission from “electron pin-holes” in planar vacuum-insulated HV electrodes

Details are given of a new photo-field electron emission microscopy technique that has been developed for the dynamic imaging of the prebreakdown electron emission processes that occurs on the surface of vacuum-insulated high-voltage electrodes. This new instrument can operate in: (a) a field-electron emission mode, (b) a photo-electron emission mode, and (c) the combined field- and photo-electron emission mode. Typical results obtained from the early application have confirmed the earlier finding that the prebreakdown electrons are emitted from isolated particulate surface structures on the surface of an electrode. These findings have also revealed that there are two types of associated emission process, i.e. a “surface” and an “edge” process. This successful early application has demonstrated the potential of this technique for the fundamental study of the physical origin of the prebreakdown electron emission from planar HV electrodes.

Photofield emission spectroscopy of the (111) and (100) tantalum band structure and surface density of states

Optical transitions near the Fermi level in the (111) and (100) directions of tantalum were observed using the photofield emission method. The final and initial energies of the photoelectrons have been estimated based on the quantum energy and lowering of the top of the surface potential barrier at which shoulders in currentvoltage characteristics appear. Electron energy values obtained for a few quantum energies are compared with the calculated band structure in which the total relativistic effect is taken into account. All excitations correspond to the band structure as direct transitions when the Fermi level indicated by theoretical calculation is shifted towards the high energy by about 0.4 eV. The experimental results confirm the existence of two bands close to each other near the Fermi level for the (111) direction. Besides, many peaks in the surface density of states lying below the Fermi level for the (111) and (100) planes of tantalum have been observed.

Photofield Emission Spectroscopy of Ba on W

Photofield Emission Spectroscopy of Ba on W

Aggregates of chemisorbed copper on the (110) and (100) surfaces of tungsten: electronic structure

In a certain range of temperature and coverage, copper atoms chemisorbed on low-index facets of a thermally annealed tungsten field emitter aggregate to form stable islands surrounded by regions of extremely low adsorbate density, yielding a spatially inhomogeneous overlayer. Spatially resolved field emission and photofield emission measurements of the surface density of electronic states have been carried out for copper aggregates on the (110) and (100) facets. Features detected in the surface density of states near the centre of an aggregate yield clues to its structure. On the (110) facet, the measurements reveal striking differences between the surface density of states near the edge of an aggregate and that near the centre. It is argued that the adsorbate configuration in the region between the aggregates might approximate single-atom adsorption.

Electronic structure of copper overlayers on the (100) and (111) surfaces of tungsten.

Field-emission and photofield-emission measurements of the surface density of electronic states have been carried out for copper chemisorbed on the (100) and (111) facets of a thermally annealed tungsten field emitter, in the range of temperature, and coverage where the adsorbed atoms form spatially homogeneous overlayers. Such overlayers are found to modify the intrinsic surface states of the substrate, and to introduce, on the (100) facet, additional features in the surface density of states. The observation of a final-state effect in surface photofield emission is reported, demonstrating that this technique can provide useful information on the surface density of states of an adsorbed overlayer in the energy range between the Fermi level and the vacuum level. The present work offers insight into earlier conflicting results concerning the effect of adsorbed copper on the intrinsic surface states of tungsten surfaces.

Photofield emission from the surface states of the tungsten (112) plane

Peaks of the surface density of states of the tungsten (112) plane, lying below the Fermi level, have been observed with photons of energy 1.96 eV. They supplement the results obtained earlier for other photon energies and indicate a complicated structure of the surface density of states. The results are in good agreement with photofield emission distributions obtained previously.

Image resonances at the Schottky barrier.

A density of barrier-image resonance states and the corresponding charge distributions are calculated---outside the metal---when an external negative electric field is applied to a free-electron-like tungsten surface. A possibility of detection of such resonances in the photofield emission experiment is discussed.