A. Laikhtman

Absolute quantum photoyield of diamond thin films: Dependence on surface preparation and stability under ambient conditions

Absolute quantum photoyield (QPY) measurements (140–210 nm) of chemical vapor deposited (CVD) diamond films are reported. The dependence of the QPY on hydrogenation by exposure to a hydrogen microwave (MW) plasma and oxidation by a mixture of acids or on exposure to air under ambient conditions have been studied. Films deposited by MWCVD display a higher QPY than those grown by hot filament (HF) CVD. The QPY values are found to depend on the state of the surface. Hydrogen-terminated films exhibit values above 12% at 140 nm, whereas even small amounts of oxygen strongly degrade the QPY. B-doping, at the level of 1500 ppm, has no apparent effect on the photoemission properties. Exposure of the hydrogenated films to ambient conditions results in oxygen adsorption, leading to degradation of the photoemission properties. Analysis of the data within the three-step model of photoemission clearly shows that the state of the surface is a dominant factor determining the QPY.

Sensitivity of near-edge x-ray absorption fine structure spectroscopy to ion beam damage in diamond films

In the present work, we study the sensitivity of the near-edge x-ray absorption fine structure (NEXAFS) spectroscopy to ion induced defects in polycrystalline diamond films. The ion bombardment of hydrogenated films is performed using 30 keV Xe+ ions at room temperature for doses ranging from 2×1013 ions/cm2, producing local point defects, to 2×1015 ions/cm2, which results in almost complete amorphization of the diamond surface. Auger electron spectroscopy measurements are not sensitive to the modifications induced by the lowest implantation dose. Whereas partial electron yield (PEY) NEXAFS measurements, applied in surface and bulk-sensitive modes, using 35, 15, and 8 eV secondary electrons, respectively, reveals the formation of a defective structure and gradual deterioration of diamond in the near-surface region. From PEY NEXAFS spectra measured using 15 eV secondary electrons, the position of C(1s) binding energy is measured. The x-ray photoelectron spectra of the samples were measured using an inciden...

Effect of moderate heating on the negative electron affinity and photoyield of air-exposed hydrogen-terminated chemical vapor deposited diamond

The effect of moderate heating (200–300 °C) in vacuum on the photoemission from air-exposed hydrogen-terminated chemical vapor deposited diamond films was studied in the photon spectral range of 140–210 nm (8.9–5.9 eV). A three- to fivefold enhancement was observed, stable in high vacuum and in some high purity gases, but unstable in air. The surfaces were also examined by x-ray induced photoelectron spectroscopy and ultraviolet induced photoelectron spectroscopy before and after the heating process and upon exposure to air and to oxygen. The results provide good evidence that the strong dipole originating from H2O molecules absorbed on the diamond surface is responsible for the observed effect. A simple model is presented for quantitative estimation of the effect.

Clarification of oxygen bonding on diamond surfaces by low energy electron stimulated desorption and high resolution electron energy loss spectroscopy

In this work we correlate between O− electron stimulated desorption (ESD) from hydrogenated and bare polycrystalline diamond films exposed to thermally activated oxygen for incident electron energies in the 4–23 eV range and vibrational electron energy losses. By utilizing the previously established mechanism of low energy ESD of O− from diamond surfaces and high resolution electron energy loss spectroscopy carried out at an incident energy below the threshold for ion desorption, the surface bonding configuration of oxygen is discussed. Such analysis reveals formation of C=O and C–O–C bonds on the bare diamond surface, as well as some adsorbed CO molecules. The bonding configuration of the hydrogenated surface before and after exposure to oxygen is found to be dependent on the hydrogenation method: ex situ microwave plasma versus in situ treatment using a hot tungsten filament for gas activation. Oxygen-related features on the ex situ hydrogenated surface are attributed to formation of C=O and C–O–H bonds...

Aspects of nucleation and growth of diamond films on ordered and disordered sp2 bonded carbon substrates

In the present work nucleation and growth of diamond by chemical vapor deposition (CVD) on highly oriented pyrolitic graphite (HOPG) and glassy carbon (GC) substrates have been investigated. These carbon substrates represent generic forms of well‐characterized ordered and disordered sp2 bonded carbon materials. The nature of the precursor to diamond CVD is assessed by studying nucleation and growth on substrates abraded with hard powders whose debris may act as initial growth centers, e.g., diamond and c‐BN, and hard powders onto which diamond CVD does not grow heteroepitaxially, e.g., alumina. Based on our experimental results it is concluded that the precursor to diamond nucleation may be debris left after the abrasion process and/or damage created preferentially on graphitic prism planes. A higher density of such damaged prism planes on GC than on HOPG resulted in a larger nucleation density on the former. Different morphologies of single particles deposited on HOPG and GC were found: well faceted on t...

NEXAFS spectroscopy of crystalline and ion beam irradiated diamond surfaces

Abstract In the present work we study the sensitivity of the near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to ion-induced defects in polycrystalline diamond films. The ion bombardment of hydrogenated films is performed using 30-keV Xe+ ions at room temperature for doses ranging from 2×1013 ions/cm2, producing local point defects, to 2×1015 ions/cm2 which results in almost complete amorphization of diamond. Partial electron yield (PEY) NEXAFS technique, applied in surface and bulk-sensitive modes, using 35, 15 and 8 eV secondary electrons, respectively, reveals the formation of a defective structure and gradual deterioration of diamond in the near-surface region. From PEY NEXAFS spectra measuring 15 eV secondary electrons the position of C(1s) binding energy is precisely measured. As determined from the NEXAFS spectra, C(1s) binding energy in the implanted samples has a positive shift of 0.6–1 eV, which is indicative of transformation of diamond to disordered carbon. The high sensitivity of NEXAFS spectroscopy to point defects induced by low-dose ion implantation is reflected by a sharp reduction in the intensity of the diamond core exciton peak and by the appearance of a new spectral feature in the region, below the C(1s)–π* resonance. Analysis of the NEXAFS spectra of ion implanted films is performed on the basis of the electronic band structure of diamond.

Decay of secondary electron emission and charging of hydrogenated and hydrogen-free diamond film surfaces induced by low energy electrons

In this work, the decay of secondary-electron emission (SEE) intensity and charging of hydrogenated and hydrogen-free diamond film surfaces subjected to incident electron irradiation at energies between 5 and 20 eV are investigated. Electron emission curves as a function of incident electron energy were measured. For the hydrogenated films, it was found that the SEE intensity decays in intensity under continuous electron irradiation, albeit maintains a nearly constant onset. The decay in time of the SEE intensity was measured for various incident electron energies. From these measurements, the SEE intensity decay rate from the hydrogenated diamond surface was calculated as a function of incident electron energy and found to display a broad peak at ∼9 eV. The decay of the SEE intensity is explained as due to electron trapping in the near-surface region of the hydrogenated diamond films resulting in the formation of a depletion layer and upward surface band bending while overall charge neutrality is maintai...

Interaction of thermally activated and molecular oxygen with hydrogenated polycrystalline diamond surfaces studied by synchrotron radiation techniques

Abstract In the present work we study the interactions between thermally activated and molecular (unactivated) oxygen gas with a hydrogen-terminated polycrystalline diamond film surface. As revealed by H + photodesorption measurements using excitation energies in carbon and oxygen K-edges, and X-ray photoelectron spectroscopy, molecular oxygen does not adsorb onto the hydrogenated diamond surface. However, thermally activated oxygen does adsorb onto it. A dominant reaction path leads to the formation of surface C–O–H bonds, although it is likely that some abstraction of chemisorbed hydrogen with formation of carbonyl bonds happens as well. The effect of oxygen on the electron emission properties of the films is studied by photon induced secondary electron emission (SEE) spectroscopy. The hydrogen-terminated, oxygen-free diamond surface exhibits negative electron affinity and high SEE intensity. Adsorption of thermally activated oxygen results in a slightly positive electron affinity (∼0.4 eV) and a decrease in the intensity of SEE, whereas electron emission properties of this sample following exposure to molecular oxygen seem to be unaffected.

Absolute quantum photoyield of ion damaged diamond surfaces

We report on the absolute quantum photoyield (QPY) measurements from defective diamond surfaces in the 140–200 nm spectral range. The effect of defects on the photoemission properties of polycrystalline diamond films is studied by intentionally introducing damage using room temperature 30 keV Xe+ ion bombardment at doses ranging from 2×1013 to 2×1015 ions/cm2. Ion bombardment results in a drastic degradation of the QPY, to less than 1% at 140 nm, even at the lowest implantation dose, compared to ∼11.5% measured for the unimplanted diamond film. The decrease in QPY is associated with a change of the electron affinity from negative to positive as determined by secondary electron emission measurements. Microwave hydrogen plasma treatment of the damaged diamond films results in complete regeneration of the photoemission properties for diamond films implanted to Xe+ doses up to 2×1014 ions/cm2; however, only partial recovery is obtained for films irradiated with higher ion dose.

Laser power effects on the Raman spectrum of isolated diamond chemical vapor deposition particles

In the present work the effect of incident laser power on the Raman spectra of diamond isolated particles and continuous films deposited on silicon and glassy carbon (GC) substrates by the chemical vapor deposition method is investigated. It is shown that the Raman line position measured for diamond particles shifts to lower wave numbers as a function of incident laser power. These shifts were most drastic for single particles deposited on GC that were examined using a Raman microprobe. In this case the diamond peak displayed a negative shift of ∼18 cm−1 when the laser power output was increased from 1 to 15 mW. The laser beam diameter was ∼2 μm and the diamond particle measured was 3–6 μm in diameter. Micro-Raman measurements of diamond particles deposited on a silicon substrate or continuous diamond films on GC display very small changes in the diamond Raman peak wavelength for the same laser power range. From our studies it is concluded that the negative shift of the Raman peak position is caused by la...