Secondary Electron Emission Spectroscopy Research Articles

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

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.

Secondary electron emission characteristics of C(111) and the observation of double-peaked emission spectra

Secondary electron emission spectroscopy (SEES) is used to investigate the low-energy electron emission characteristics of the C(111) surface. A negative electron affinity (NEA) is observed at hydrogenated and cesiated C(111) surfaces, and very high secondary electron yields are measured from these surfaces. The emission from both surfaces is sharply peaked at low energy, although the cesiated surface produces greater energy spread than the hydrogenated surface. Yield measurements are uniform across the hydrogenated and cesiated surfaces, but energy distribution curves (EDCs) contain emission features that depend on the measurement position on the surface. Specifically, an intense secondary emission peak centered above Ec is observed in EDCs measured at all positions while a weaker peak lying completely below Ec appears only at specific regions of the surface. The intense peak is well understood and has been observed in EDCs taken from NEA surfaces of C(100) and chemical vapor deposited diamond. However, the weaker peak has not been observed in previous SEES studies of diamond. This peak corresponds to electron emission from surface or defect electronic states in the energy gap, and it is manifested in the EDCs only when χ is sufficiently lowered by the adsorption of H or Cs. Although the origin of the surface or defect states is not known, it appears to be associated with structural properties of the C(111) surface.

Secondary electron emission characteristics of single-crystal and polycrystalline diamond

Secondary electron emission spectroscopy (SEES) is used to examine the transport and emission of low-energy electrons in diamond. In particular, SEES measurements from single-crystal (100) and (111) diamond and polycrystalline chemical vapor deposited (CVD) diamond are compared in order to examine the effect of crystallographic orientation on the emission characteristics. Crystal orientation is found to influence the surface properties of the samples but not the low-energy transport properties. Specifically, very high yields are obtained from negative-electron-affinity (NEA) surfaces of all three samples, indicating that low-energy electrons are transported and emitted very efficiently regardless of crystal orientation. However, the energy distributions measured from adsorbate-covered C(111) surfaces are broader and shifted lower in energy than those measured from corresponding C(100) surfaces. In fact, the energy distributions measured from polycrystalline CVD diamond surfaces appear to be a superposition of the energy distributions measured from the (100) and (111) crystal faces. For all three samples, a broader, lower-energy distribution is measured from cesiated NEA surfaces than from hydrogenated NEA surfaces. This indicates that the electron emission process differs at the two types of surfaces. The emission characteristics observed for the different crystal orientations and adsorbate coverages can be understood by considering the role of surface structure in the emission process.

Cold emission characterization using secondary electron emission spectroscopy

Secondary electron emission spectroscopy is used to study the electron transport and emission properties of wide bandgap materials (e.g., diamond, BeO). The secondary electron distribution generated in diamond is found to be dominated by a high concentration of very low-energy electrons, and the emission characteristics of these low-energy electrons is used to deduce information about the cold emission properties of the material. In particular, analysis of the extremely high yields measured from diamond samples indicates that the low-energy electrons have long escape depths in the material. Furthermore, electron energy distribution measurements provide insight into the emission process at various surfaces. The electron transport and emission model inferred from the diamond measurements suggests that diamond and other wide bandgap materials may be promising cold emitter materials, in which case secondary electron emission spectroscopy can be a useful characterization tool.

Interpretation of low-energy feature in energy spectra measured from surfaces with low or negative electron affinity

The secondary electron distribution generated in wide bandgap material, such as diamond, is dominated by a high concentration of very low-energy electrons. Previous work has used the appearance of a low-energy feature in measured energy spectra to establish the presence of a negative electron affinity (NEA) at the emitting surface. In this letter, secondary electron emission spectroscopy is used to study the energy distribution of secondary electrons emitted from semiconductor surfaces having different electron affinities. We conclusively show that the emitted electron distribution depends strongly on the position of the vacuum level relative to the internal electron distribution. In fact, a low-energy peak is observed in energy spectra measured at GaN, diamond, and Si surfaces having a small but positive electron affinity, and the intensity and width of the peak increase steadily as the electron affinity decreases. Consequently, a low-energy structure in measured energy spectra is not sufficient evidence of a NEA.

Electronic properties of diamond for high-power device applications

The generation and transport of impact-ionized electrons in diamond are investigated using secondary electron emission spectroscopy. By studying secondary electron emission from diamond surfaces having a negative electron affinity (NEA), the total transmitted intensity and the full energy spectrum of the internal electrons are revealed in the measurements. Electron yield and energy distribution measurements from C(100) and CVD diamond samples are analyzed as a function of incident beam energy and information is obtained on both the internal gain and electron energy distribution following impact ionization as well as the effects of the transport process on the internal electron distribution. High internal gain and efficient electron transport are inferred from an analysis of the very high yields measured from both samples. In fact, by fitting calculated yield curves to the measured yield data, we deduce a lower limit on the electron escape depth of ∼5 μm and ∼1.3 μm in the C(100) and CVD diamond samples, respectively. Energy spectra measured from the diamond samples contain an intense, low-energy peak whose energy position and width are independent of incident beam energy. Based on an analysis of the energy distribution data, a model of the impact-ionization process in diamond is presented.

Electron transport and emission properties of diamond

The electron transport and emission properties of hydrogenated and cesiated single-crystal and chemical-vapor-deposited (CVD) diamond are investigated using secondary electron emission spectroscopy. The kinetic energy of the electrons and the height of the surface energy barrier are measured relative to the conduction band minimum, Ec, which is identified in the spectra. In spectra measured from hydrogenated and cesiated diamond surfaces, electron emission appears at energies E<Ec which gives direct evidence of a negative electron affinity. The strongest emission is observed from cesiated samples, which produced very high yields (δmax∼80–130 at Eb=2900 eV). The energy distributions from all three samples are sharply peaked at ∼0.50–0.65 eV above Ec and have a full width at half maximum ∼0.55–0.75 eV, except in the case of the cesiated CVD diamond samples. The energy distributions measured from cesiated CVD diamond are peaked at lower energy and are much broader due to lower emission-onset energies. An emission model, which invokes band bending near the surface, is deduced that accounts for the observed energy spectra from the samples in terms of the surface properties of the C(100) and CVD diamond and the internal electron energy distributions.

Magnetization of ultrathin Fe films deposited on Gd (0001)

The magnetization of ultrathin (1.5–6 ML) Fe films deposited on Gd (0001) was studied using spin polarized secondary electron emission spectroscopy. An anomalously high Curie temperature (>320 K) was observed for very low coverages of Fe (1.5 ML). The anomalously high Curie temperature results in part from the strong antiferromagnetic exchange interaction between the Fe and Gd at the interface. In addition, a spin reorientation transition of the Fe magnetization from in-plane to perpendicular was observed as the temperature is increased for Fe thickness between 1.5–4 ML. The spin reorientation is attributed to the competition between the perpendicular anisotropy of the Fe overlayer and the exchange interaction with the in-plane Gd bulk magnetization. Therefore the effects of varying the thickness of the Gd and hence the bulk Curie temperature on the spin reorientation transition can be studied.

Interaction of oxygen with a Cs-monolayer-covered Si(100) surface

Oxygen adsorption on a Cs-monolayer-covered Si(100) surface has been studied by ${\mathrm{Li}}^{\ensuremath{-}}$ ion spectroscopy and normally emitted secondary electron emission (SEE) spectroscopy. It is clearly shown that the oxygen lies above and below the Cs atoms, respectively, at low and high O exposures, disproving the dipole model for the work function change. The initial O adsorption induces a shoulder at the low-energy edge of the SEE spectra, indicating the existence of patches of a lower work function on the surface. The patches are explained as due to the Cs${\mathrm{O}}^{*}$ complexes formed by nonadiabatic chemisorption.

Spin-resolved electron spectroscopies of epitaxial magnetite (001) (abstract)

We will present the first spin-resolving electron spectroscopic studies of a magnetite (Fe3O4)(001) surface. Magnetite is a semimetal with a high density of states in the minority band, but a large band gap in the majority states at the Fermi energy. The polarization of the secondary emission cascade is measured using spin-resolved secondary electron emission spectroscopy (SRSEES), and reflects the semimetallic spin structure of Fe3O4. The polarization plateau of spin-resolved secondary emission (29.8%) matches the average 3D band polarization of stoichiometric Fe3O4 as determined from spin-resolved band structure calculations (34.2%). An enhancement of the polarization of the secondary electrons at lowest energies will also be discussed. Spin-resolved Auger emission spectroscopy (SRAES) of the Fe3O4 films have been measured and show correlation effects in the valence-valence Auger transitions. Suppressed intensity and polarization of M23M45M45 Auger emission relative to M1M45M45 Auger emission is observed, as well as strong resonant emission with shake-up. Conversely, no spin polarization is detected in the spin-resolved oxygen LMM Auger features, although oxygen Auger emission (in which we can distinguish between adsorbed and bonded oxygen) is used to verify surface cleanliness of the samples. The synthesis of Fe3O4 films grown on magnesium oxide (001) substrates using oxygen plasma-assisted molecular beam epitaxy will be discussed, as will thin-film characterization using SQUID magnetometry and x-ray and electron diffraction. A unique angle-, energy-, and spin-resolved electron spectrometer has been designed and built for the study of magnetic surfaces, and these studies represent its’ first use. That spectrometer is based on a tandem configuration of an energy-dispersive energy analyzer and Mott spin polarimeter.

Electronic dispersion relations of monolayer hexagonal boron nitride formed on the Ni(111) surface.

Angle-resolved ultraviolet-photoelectron spectroscopy and angle-resolved secondary-electron emission spectroscopy have been carried out for a film of single-crystalline hexagonal boron nitride (h-BN) formed on the Ni(111) surface to investigate both the valence- and conduction-band structures. The thickness of the film studied in this experiment was 1 ML. The observed electronic dispersion relations were compared with some theoretical ones reported for bulk h-BN. Among these theoretical calculations, the one by Catellani et al. [Phys. Rev. B 36, 6105 (1987)] is in the best agreement with the present results. We have discussed the strength of the interfacial bond and the influence of this bond upon the electronic states of the monolayer h-BN film on the basis of the observed band structures for the BN film and a film of monolayer graphite formed on Ni(111).

Influence Of Surface Terminating Species On Electron Emission From Diamond Surfaces

ABSTRACTChanges in electron affinity on the C(001) surface of type Ifb diamonds have been studied using a variety of surface analytical techniques, including ultraviolet photoemission spectroscopy, secondary electron emission spectroscopy and constant initial states photoemission. Following H-plasma exposure, an intense low-energy emission peak was observed with all spectroscopies. The emission intensity associated with the chemisorbed hydrogen was found to be a linear function of surface hydrogen coverage. The proposed mechanism for the hydrogen induced changes in electron affinity is the creation of a dipole on the surface by the addition of hydrogen which opposes the surface potential of the bare surface. A total change in electron affinity of 2.2 eV was measured upon hydrogen termination of the clean 2x1 surface. Constant initial states photoemission demonstrates that the intense low-energy electron emission observed arises from electrons emitted from bulk states at the conduction band edge. Oxygen, as an electronegative species, was found to have the opposite effect and the electron affinity was increased by ∼3.7 eV upon oxygen termination relative to the clean 2x I surface.

Ferromagnetism and growth of Ru monolayers on C(0001) substrates

The magnetic and growth properties of Ru monolayers on C(0001) are studied using spin-polarized secondary electron emission and Auger electron spectroscopy (AES). Using AES, we find that the initial growth of Ru on C(0001) occurs laterally until the first monolayer is completed. One monolayer-thin Ru film shows ferromagnetic order below a surface Curie temperature of approximately 250 K. The in-plane magnetization saturates in small applied fields of a few tenths of an Oe. This is the first observation of spontaneous, long-ranged, two-dimensional ferromagnetic order in an ultrathin film composed of a 4d transition metal.

Extended fine structure in the secondary electron emission spectra of graphite and glassy carbon

AbstractThe electron excited secondary electron emission (SEE) spectra of highly oriented pyrolytic graphite (HOPG) and glassy carbon (GC) were measured in the 0–150 eV electron kinetic energy range.For HOPG, the SEE spectrum was measured as a function of primary electron beam energy above and below the threshold for carbon core level 1s ionization. The SEE spectrum in the 0–70 eV range reflects high energy conduction band states, in agreement with band structure calculations and other spectroscopic techniques sensitive to the empty density of states. For electron kinetic energies above ∼ 70 eV the spectrum of HOPG displays a well defined structure which has not been studied previously. We found that the SEE fine structure in the low energy range up to ∼ 40 eV shows some dependence on primary electron energy, whereas for larger energies a much weaker dependence was found. It has been established that the SEE spectrum is strongly affected by low‐energy Ar ion irradiation of the HOPG surface. This result suggests that the SEE spectrum is very sensitive to crystal defects induced by the irradiation process. The lower‐energy range of the spectrum, up to ∼ 40 eV was found to be more sensitive to damage induced by the ion irradiation process than the higher range of the spectrum.For GC, the SEE spectrum in the 0–50 eV range is smeared out compared to that of HOPG. However, in the higher electron kinetic range some of the peaks measured for HOPG were also observed for GC. These results suggest that the lower‐energy range of the SEE spectrum is more sensitive to long‐range order, whereas the higher‐energy range of the spectrum reflects a shorter‐range order. It is suggested that SEE spectroscopy can be used as a very sensitive tool for the characterization of different carbon structures.

Fine structure in the secondary electron emission spectrum as a spectroscopic tool for carbon surface characterization

Electron-stimulated secondary electron emission (SEE) spectra of diamond type 2a, highly oriented pyrolytic graphite (HOPG), glassy carbon (GC) and diamond chemical vapour deposition (CVD) films were measured in the 0–75 eV electron kinetic energy range. It has been established that the SEE spectra are characteristic of the different carbon surfaces and very sensitive to crystal order. For crystalline carbon structures, HOPG and diamond type 2a the SEE spectra in the 0–75 eV range were found to be very different, reflecting high energy conduction band states in agreement with band structure calculations and other spectroscopic techniques sensitive to the electronic density of empty states. For GC the SEE spectrum in the 0–40 eV range is smeared compared with that measured for HOPG. However, in the higher electron kinetic energy range the SEE spectra of HOPG and GC are similar. The sensitivity of SEE relative to Auger electron spectroscopy in a case study of the near-surface region of diamond CVD films is discussed. It is concluded that SEE spectroscopy can be used as a very sensitive tool for the characterization of different carbon structures in the near-surface region.

Influence of surface atomic steps on in-plane magnetic anisotropy of ultrathin Fe films on W(001)

Previous magneto-optic Kerr effect studies of ultrathin epitaxial Fe films grown on stepped W(001) surfaces yielded evidence of in-plane uniaxial magnetic anisotropy with magnetization perpendicular to the steps. We report spin-polarized secondary electron emission spectroscopy studies of the same system that confirms this novel micromagnetic phenomena, and provides a more detailed characterization of the zero-field in-plane spin configuration as a function of initial applied field direction.

Electron energy loss and x-ray photoemission study of electron inelastic scattering in cadmium arachidate Langmuir Blodgett films

The inelastic scattering of electrons in Langmuir–Blodgett films of cadmium arachidate [(C19H39COO)2Cd] deposited on glass substrates has been studied by reflection electron energy loss spectroscopy (REELS) and by monitoring the loss features of the C 1s and O 1s core levels by x-ray photoemission spectroscopy (XPS). REELS studies with a primary beam of energy 95 eV revealed some vibrational modes of the cadmium arachidate molecules and features due to transition of valence electrons to the conduction band. Similar structures were also identified in the C 1s and O 1s loss spectra and interpreted as reflecting the density of states of the conduction band, in agreement with earlier findings using secondary electron emission spectroscopy. In contrast to the monolayer film, the 5 ML film showed reduced order as inferred from the lack of features in the loss spectra.

Auger electron, electron energy loss, secondary electron emission and secondary ion mass spectroscopic studies on the oxidation of hafnium at room temperature

Auger electron spectroscopy (AES), electron energy loss spectroscopy (EELS), secondary electron emission spectroscopy (SES) and secondary ion mass spectrometry (SIMS) have been used to investigate the oxidation of a hafniun surface at room temperature (298 K). The change in the electronic energies of the core and valence levels with the oxidation is discussed on the basis of the comparison between the observed spectra and the electronic structures calculated for a hafnium atom, ion, bulk metal and bulk oxide. The growth of oxide films to a thickness of ca. 1.5 nm is found from the decay in the intensity of Auger electrons emitted by the metal substrate and from the depth profile of the SIMS spectra.

Conduction band structure of VSe2 studied by inverse photoemission, secondary electron emission and total current spectroscopies

The authors have studied the unoccupied electronic states of VSe2 using three different techniques: inverse photoemission spectroscopy (IPES), secondary electron emission spectroscopy (SEES) and target current spectroscopy (TCS). The experimentally determined electron bands are in good agreement with band structure calculations, but on comparison the different techniques reveal significantly different aspects of the conduction band structure. The reasons for these differences are discussed.

Secondary electron emission spectroscopy and total electron yield measurements for the assessment of near-surface damage in diamond

In this work the sensitivity of secondary electron emission (SEE) spectroscopy in the 0–50 eV range and total electron yield (TEY) for characterising the near-surface region of diamond with varying levels of damage is examined. This is accomplished by SEE and TEY measurements of diamond surfaces subjected to ever increasing doses of 1 keV Ar ion irradiation. The irradiation gradually damages, amorphises and eventually graphitises the diamond surface thus providing a continuous spectrum of different forms of carbon ranging from crystalline sp 3 to amorphous sp 2 bonded carbon. A strong and abrupt charging effect is observed as a function of irradiation dose. This effect was correlated with changes in TEY and sample conductivity. It is found that TEY and SEE are very sensitive measures of the crystalline perfection of the near-surface region of diamond. It is suggested that SEE and TEY be used as sensitive tools for the characterisation of the near-surface region of diamond thin films.