Diamond Photocathodes Research Articles

Investigation of phosphorus-doped nanocrystalline diamond films for photocathode application

This work is devoted to an experimental study of electron photoemission from phosphorus-doped nanocrystalline diamond (NCD) films under the effect of laser radiation with a wavelength of 266 nm and a pulse duration of 15 ns. Homogeneous NCD films with a thickness of 50–1200 nm were grown on conductive silicon substrates using the CVD method. The phosphorus content in the films was changed by varying the phosphine content in the hydrogen–methane gas mixture and the substrate temperature. A relation has been established between the growth conditions, the thickness of NCD films, and the measured quantum efficiency of diamond photocathodes. It was shown that 50 nm thick heavily doped NCD films with H-terminated surface demonstrated the maximum quantum efficiency of 3⋅10−5.

Spectral emission properties of a nitrogen-doped diamond (001) photocathode: Hot electron transport and transverse momentum filtering

The electron emission properties of a single-crystal nitrogen-doped diamond(001) photocathode inserted in a 10kV DC photoelectron gun are determined using a tunable (235-410nm) ultraviolet laser radiation source for photoemission from both the back nitrogen-doped substrate face and the front homo-epitaxially grown and undoped diamond crystal face. The measured spectral trends of the mean transverse energy (MTE) and quantum efficiency (QE) of the emitted electrons are both anomalous and non-monotonic, but are shown to be consistent with (i) the known physics of electron photoexcitation from the nitrogen substitution states into the conduction bands of diamond, (ii) the energy position and dispersion characteristics of the conduction bands of diamond in the (001) emission direction, (iii) the effective electron affinity of the crystal faces, (iv) the strong electron-(optical)phonon coupling in diamond, and (v) the associated hot electron transport dynamics under energy equipartition with the optical phonons. Notably, the observed hot electron emission is shown to be restricted parallel to the photocathode surface by the low transverse effective masses of the emitting band states - a transverse momentum filtering effect.

Quantum efficiency, photoemission energy spectra, and mean transverse energy of ultrananocrystalline diamond photocathode

The quantum efficiency and mean transverse energy of electrons emitted from a cathode determine the quality of beams generated from photoinjectors. The nitrogen-incorporated ultrananocrystalline diamond, (N)UNCD, is a new class of robust semiconductor photocathodes, which has been considered in photoinjectors for high peak current extraction. In this work, we measure the spectral response in quantum efficiency, photoemission energy spectra, and mean transverse energy of the (N)UNCD photocathode using a photoemission electron microscope. The observed quantum efficiency was comparable to that of copper photocathodes. Photoemission spectra showed the evidence of scattering of electrons before emission. This relaxation of electrons due to scattering is also observed in the spectral response of the mean transverse energy. The mean transverse energy is limited to ∼70 meV at the threshold. We attribute this to the physical and chemical roughness of the (N)UNCD photocathode and, hence, smoother films will be required to further reduce the mean transverse energy obtained from the (N)UNCD photocathode.

Calculation and optimization of the limiting characteristics of a single-channel dual-spectrum image receiver of objects emitting in the ultraviolet range

A single-channel, two-spectral image receiver of objects emitting in UV radiation, made in the image intensifier tube architecture, was proposed and investigated. With the help of the COMSOL Multiphysics software package, search optimal measurements of the potential on the elements of the image receiver (silicon membrane, germanium and diamond photocathode, MCP input and output sensors) were implemented, which provides the possibility of registering and presence of UV objects in relation to the terrain. Keywords: image intensifier tube, diamond photocathode, germanium photocathode, ultraviolet radiation, object imager, photoelectron emission, secondary electron emission.

Image converter tubes with diamond photocathodes and electron flow multipliers

The results of investigations of solar-blind image converter tubes (ICTs), sensitive in the vacuum-ultraviolet (VUV) spectral range are presented. Sensitive-conversion layers of photocathodes based on boron-doped polycrystalline diamond films were grown up on sapphire substrates for the first time. Electron flow multipliers (EFMs) were fabricated in the form of diamond grid. Solar-blind VUV ICTs without the EFM are characterized by spectral sensitivity range of 180…250 nm, estimate of the threshold sensitivity value ~10-9 W/Hz0.5 and current sensitivity ~12 - 15 mA/W. Solar-blind VUV ICTs comprising the electron flow multipliers are characterized by extended spectral sensitivity range of 180…270 nm, improved estimate of the threshold sensitivity value 10-11… 5 × 10-12 W/Hz 0.5 and current sensitivity 50 mA/W.

Diamond Photocathodes As Field-Emission Electrodes for Vacuum Microelectronics

Diamond Photocathodes As Field-Emission Electrodes for Vacuum Microelectronics

Алмазные фотокатоды как полевые катоды для вакуумной микроэлектроники

The use of diamond photocathodes and electron flow amplifiers in high-frequency vacuum micro and nanoelectronics is analyzed. The main devices are a vacuum microtriode and an electron gun for an integral traveling wave tube.

Demonstration of nitrogen-incorporated ultrananocrystalline diamond photocathodes in a RF gun environment

Quantum efficiency (QE), intrinsic emittance, and robustness are the three most important figures of merit for photocathodes, the first two determine the ultimate achievable brightness of an electron beam, and the third one directly correlates with the complications of a beamline design. Nitrogen-incorporated ultrananocrystalline diamond [(N)UNCD] materials are promising candidates for photocathode applications due to their remarkable electron emission performance as well as the moderate vacuum requirement. Two (N)UNCD photocathodes have been characterized in a realistic RF gun environment with the nicely balanced performance of all three figures of merit. The QE of the first (N)UNCD cathode (stored in air for two years before the test) was found to be 3.8 × 10−4 using a 262 nm UV laser and a cathode surface field of 30 MV/m. It was found that the QE of the second (N)UNCD sample (grown days before the test) was nearly the same and, therefore, demonstrates the exceptional environmental tolerance of the material. The intrinsic emittance of (N)UNCD was measured to be 1.00 μm/mm.

Analysis and experimental research on graphene's electron transparency and its application for the development of micro- and nanoelectronic devices

Theoretical and experimental data of the electron beam passage through graphene taking into account reflection of electrons are obtained. Theoretical estimates were made by considering the motion of an electron in a central force field which was modeled as a positively-charged carbon nucleus screened by the Thomas-Fermi potential. Along with the theoretical estimates, the results of experimental studies, obtained on the basis of a specially developed stand in which the electron beam was generated by irradiation of a diamond photocathode by vacuum ultraviolet, are presented. A comparison of the theoretical and experimental data showed that the experimental estimate of the passage of an electron through graphene is in good agreement with the calculated data when the first reflection is taken into account. The greatest discrepancies were observed for small values of the electron energy. The obtained results show that at the electron energies of >35 eV the electron almost freely penetrates through the graphene membrane and it corresponds to the results published earlier, and at energies < 20 eV the data differ strongly. This fact shows that in this energy range it is necessary to continue studying the graphene's transparency.Major part of the work is devoted to the use of graphene membrane's electron transparency for developing a number of unique devices and systems. These are vacuum ultraviolet radiation matrix detectors that are not sensitive to solar radiation, flat terminals with the properties of cathode ray tubes, image converter based on a vacuum emission triode, and self-excited oscillation memory cells for archival memory. It is noted that these devices and systems can be developed on the basis of existing technologies for the graphene films production.

Studying the Transparency of Graphene for Low-Energy Electrons

The transparency of graphene membranes for electrons with energies in the range from 5 to 50 eV has been studied with a view to using graphene as an electrode stimulating field-induced emission in microand nanoelectronic devices. The behavior of electrons reflected from a membrane was analyzed with allowance for their return under the action of a retarding electric field. Low-energy electrons were represented by photoelectrons emitted from a diamond photocathode under the action of vacuum ultraviolet radiation.

Исследование электронной прозрачности графена для малых энергий электрона

AbstractThe transparency of graphene membranes for electrons with energies in the range from 5 to 50 eV has been studied with a view to using graphene as an electrode stimulating field-induced emission in microand nanoelectronic devices. The behavior of electrons reflected from a membrane was analyzed with allowance for their return under the action of a retarding electric field. Low-energy electrons were represented by photoelectrons emitted from a diamond photocathode under the action of vacuum ultraviolet radiation.

High quantum efficiency ultrananocrystalline diamond photocathode for photoinjector applications

We report results of quantum efficiency (QE) measurements carried out on a 150 nm thick nitrogen-incorporated ultrananocrystalline diamond terminated with hydrogen; abbreviated as (N)UNCD:H. (N)UNCD:H demonstrated a remarkable QE of ∼10−3 (∼0.1%) at 254 nm. Moreover, (N)UNCD:H was sensitive in visible light with a QE of ∼5 × 10−8 at 405 nm and ∼5 × 10−9 at 436 nm. Importantly, after growth and prior to QE measurements, samples were exposed to air for about 2 h for transfer and loading. Such design takes advantage of a key combination: (1) H-termination proven to induce negative electron affinity on the (N)UNCD and to stabilize its surface against air exposure; and (2) N-incorporation inducing n-type conductivity in intrinsically insulating UNCD.

Transmissive diamond photocathodes

The first studies are reported on the characteristics of ultra-violet transmissive mode, CVD diamond photocathodes, in which photoelectrons are emitted from a diamond membrane via the opposite interface from that of the incident radiation. For membranes in the thickness range 5–40 μm, the transmissive photoyield at threshold wavelengths is shown to be less but comparable to that measured for reflective, thick film diamond photocathodes. The characteristics are found to be insensitive to the phase purity of differing diamond membranes used, but are very dependent on whether the growth or substrate interfaces of the diamond membrane face the incident UV radiation. The factors influencing the characteristics observed are discussed.

UV photoemission efficiency of polycrystalline CVD diamond films

The absolute quantum efficiency of polycrystalline diamond films grown on silicon substrates by chemical vapor deposition (CVD) is reported in the range of 25–200 nm. The efficiency of boron-doped and hydrogen-activated by microwave plasma reflective photocathodes peaked at 37% at 40 nm with the sensitivity cutoff observed at ∼190 nm. We confirmed that hydrogen activation is relatively stable in air: the efficiency of the photocathode degraded by less than 15% after an 18-h air exposure. It was found that diamond photocathodes can be reactivated multiple times to the same high QE values even after extended ultrasonic cleaning in both water and alcohol. The sensitivity of a diamond photocathode coated with a high dipole moment 4-nm-thick CsI layer was found to be slightly better than that of hydrogenated films, especially at longer wavelengths, while the sensitivity cutoff shifted to ∼200 nm. The observed high efficiency of thin CVD diamond films and relatively high stability of surface activation makes them a very attractive alternative to many existing UV photocathodes, especially taking into account other useful features of diamond (mechanical and chemical stability and radiation hardness).

Diamond photocathodes in gaseous environments

The performance of UV diamond photocathodes in gaseous environments has been explored with regard to the potential applications, which exist for such devices. Stable performance is exhibited even at gas pressures approximately 0.5 bar for hydrogenated diamond, in gaseous environments such as He, H2, O2, CH4 and CF4. However, for chlorocarbon gases, UV stimulated photodissociation of the ambient gas causes the adsorption of chlorine on the diamond surface, and rapidly degrades device performance. Thermal dissociation of ambient gases brings about a similar effect. Thus, although diamond photocathodes do have potential use in gaseous environments, it is clear that care is required to ensure that the hydrogenated surface layer is not destroyed when in use.

Diamond photocathodes in gaseous environments

The performance of UV diamond photocathodes in gaseous environments has been explored with regard to the potential applications, which exist for such devices. Stable performance is exhibited even at gas pressures approximately 0.5 bar for hydrogenated diamond, in gaseous environments such as He, H 2, O 2, CH 4 and CF 4. However, for chlorocarbon gases, UV stimulated photodissociation of the ambient gas causes the adsorption of chlorine on the diamond surface, and rapidly degrades device performance. Thermal dissociation of ambient gases brings about a similar effect. Thus, although diamond photocathodes do have potential use in gaseous environments, it is clear that care is required to ensure that the hydrogenated surface layer is not destroyed when in use.

Picosecond laser-induced ultrashort electron emission from carbon-based photocathodes

Photoelectric properties of carbon-based thin films are investigated by UV pulsed laser-induced photoelectric charge measurements. Photocathodes are thin films deposited on different substrates by vacuum deposition methods. Laser irradiation consists of fifth harmonic (λ=213 nm) picosecond pulses of a Nd-YAG laser. Samples are irradiated at low laser intensities (approx. a few MW/cm2) in order to avoid pronounced structural transformations at their surface. Quantum yield (QY) values are determined at 213 nm; UV laser-induced surface cleaning process and air contamination effects on electron emission are studied. The highest QY value is obtained for the hydrogenated diamond photocathode according to its low surface potential barrier (NEA emitter). This QY value is decreasing after air exposure of the post-irradiated sample; on the other hand, electron emission from nanostructured fullerene films can be enhanced by a UV surface cleaning process.

Influence of material properties on the performance of diamond photocathodes

Variations in photoyield at 186 and 252 nm have been measured from CVD diamond photocathode materials grown on silicon substrates, as the growth conditions and surface termination were changed. The photoyield increased rapidly for short growth times when the surface comprised a mixture of diamond and non-diamond phases in intimate contact, but this increase occurred at both wavelengths, so poor spectral selectivity resulted. Further growth caused the photoyield to decay as the diamond content of the film improved, before rising again at longer growth times, whence improved wavelength discrimination was also observed. Caesiation enhanced the photoyield observed compared to hydrogenated interfaces.

Influence of Surface Properties on the Quantum Photoyield of Diamond Photocathodes

The quantum efficiency and chemical stability of CVD diamond photocathodes has been examined. As-grown or microwave plasma hydrogenated boron-doped diamond films display a quantum photoyield of approximately 0.05% at 190 nm, which degrades gradually as the material is left in ambient atmosphere, due to slow oxidation. Rapid degradation in performance occurs when exposed to atomic or electronically excited oxygen. X-ray photoelectron spectroscopy shows that the yield drops close to zero at around monolayer oxygen coverage, and that the main oxygen species on the surface is hydroxyl or isolated ether linkages.

An investigation of the surface reactivity of diamond photocathodes with molecular and atomic oxygen species

The surface reactivity of CVD diamond films, which function as UV photocathodes, with thermal and electronically excited O 2, as well as atomic oxygen, has been explored. The CVD films show a low, but measurable reactivity with O 2, which increases markedly with electronic excitation, to form dilute oxygen adlayers which are stable thermally to 800 K. In contrast, reaction with atomic O produces significantly higher surface concentrations of oxygen, although these adlayers decompose thermally below 500 K, evolving CO 2. Greater surface reaction probabilities are observed if the diamond surface is pre-hydrogenated before interaction with oxygen. The consequences of these observations for the stable operation of H-terminated diamond photocathodes are discussed.