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1.
We have measured the surface photovoltage (SPV) of intrinsic (i.e., undoped) and phosphorus-doped amorphous Si : H between ?168 and 25°C in the spectral range from 0.5 to 2.5 eV. The a-Si : H was grown in a silane glow discharge. Vibrating Kelvin probe techniques were used for the SPV measurements; Auger spectroscopy was used for monitoring surface cleanliness and chemistry. At all temperatures and for both materials, (1) the SPV was invariably negative, (2) there was no correlation between the spectral, thermal and response-time properties of the SPV and the bulk photoconductivity, and (3) surface treatments such as sputtering and oxygen physisorption strongly affected the SPV but not the photoconductivity. These facts indicated that the SPV was due to the emptying of surface-states via surface transitions, and corresponded to the flattening of bands which, when unilluminated, were bent upwards. Intrinsic material showed a maximum SPV of about 0.2 V. The SPV was characterized at ?168°C by strong electronic isolation between surface-states and valence band (i.e., once light was removed, there was no surface-state refilling or decay of the SPV), slow rise times (~min), saturation at photon fluxes of about 1011/cm2 · s, and a SPV spectral threshold occurring at 0.7 eV. At 25°C, all SPV responses were much faster (<0.5 s) and the optical threshold was 0.9 eV. The thermal activation energies associated with the SPV were 0.11 eV for surface-state emptying and 0.22 eV for surface-state refilling. For P-doped material the maximum SPV at ?168°C was 0.3 V and its properties indicated less electronic isolation between surface-states and valence band. There was no SPV at room temperature. Our results are discussed in terms of an energy level scheme which contains a distribution of filled surface states isolated from both conduction and valence bands. The surface-state density is estimated to be about (1?2) × 1011/ cm2, a relatively low value which is consistent with the observed lack of Fermi level pinning. In both materials there is a very fast component of the SPV which suggests the presence of additional surface states below the valence band edge. 相似文献
2.
The electronic surface states of cleaved and annealed Ge(111) surfaces have been investigated by photoemission yield spectroscopy and contact potential measurements on a set of differently doped samples. On the 2 × 1 cleaved surface, a surface state band centered about 0.7 eV below the valence band maximum is found. The variations of the work function with the doping level show that an empty surface state band exists above the Fermi level. After annealing at temperatures of the order of 350°C, this surface exhibits a 2 × 8 superstructure. A new surface state band is then found closer to the valence band maximum. This variation of the surface state distribution is correlated to a change in the surface potential. The variation of the electronic characteristics upon oxygen adsorption are also reported and an evaluation of the sticking coefficient is made for both structures. 相似文献
3.
The spectral dependence of surface photovoltage and surface photoconductance both under continuous illumination as well as LEED I/V spectra were studied with cleaved Si(111)-2 × 1 surfaces at 130 K. Between 0.23 and 0.5 eV a doubly peaked absorption band was found with opposite sign compared to the SPV and SPC signals at higher photon energies. This band is due to electronic transitions from occupied to empty dangling-bond states located at the raised and the lowered rows of atoms in the 2 × 1 reconstruction, respectively. This absorption shows a pronounced dependence on the polarization of the incident light which correlates with the spatial symmetry of the dangling-bond states. Anneals at up to 500 K remove the low-energy absorption peak and equalize the 2 × 1 reconstruction: The homogeneous Si(111)-2 × 1 structure exhibits a buckling of 0.3 Å and a dangling-bond absorption with a threshold at 0.42 eV and a maximum at 0.47 eV. An anneal at 750 K, forming the 7 × 7 structure, destroys the peak of opposite sign in SPV and SPC and only leaves a broad tail with a threshold of 0.32 eV. 相似文献
4.
Electronic excitations on Cu(001) and Cu(001)c(2 × 2)-Cl have been investigated by angle-resolved electron energy loss spectroscopy at an angular resolution of Δθ = ±1° and an energy resolution of ΔE = 60 meV. Primary energies in the 50–100 eV range were chosen and the specular reflection was studied for angles 35° ? θ ? 71° with respect to the surface normal. The results are summarized as follows: The specular Cu(001) spectra are compared to optical data and good agreement is found for the energetic position of direct transitions. The electronic losses observed for the ordered overlayer system may be interpreted by one-electron excitations from occupied surface bands (known from angle-resolved photoemission results) into an empty band with a minimum energy at 0.4 eV above the Fermi level. 相似文献
5.
P. Jiricek M. Cukr I. Bartos M. Adell T. Strasser W. Schattke 《Czechoslovak Journal of Physics》2006,56(1):21-26
The electron structure of GaAs(100)-c(4 × 4) has been studied by means of angular-resolved photoelectron spectroscopy for photon energies (20–40) eV. The sample
was prepared by molecular beam epitaxy in-situ at the BL41 beamline of the MAX I storage ring of the Max-lab in Lund.
Photon energy variation helped in separating dispersing bulk features from nondispersing surface features in the energy distribution
curves recorded at normal emission. Two sets of peaks were related to bulk transitions from the two topmost E(k
⊥) branches of the valence band of GaAs and one more set came from the surface state in the center of the 2D Brillouin zone.
Good agreement was found between experimental bulk dispersion branches and theoretical calculations based on realistic final
state dispersion. The surface state peak, hardly visible at 20 and 22 eV photon excitations, gets clearly enhanced at higher
excitation energies.
In contrast to earlier measurements of this kind, two major differences have been found: (i) clearly developed surface state
peak just below the top of the v alence band, (ii) absence of a large peak in the electron energy distribution at around −6.5eV
below the valence band top.
Presented at the X-th Symposium on Suface Physics, Prague, Czech Republic, July 11–15, 2005. 相似文献
6.
We report photoemission results from which we directly determined the density of states g(E) in the gap of a-Si:H between the top of the valence band Ev and the Fermi level. At 0.4 eV above Ev, g(E) was found to be ≈1×1020 cm-3 eV-1 in the undoped film; P-doping increased g(E) in this region whereas annealing reduced it. The photoconductivity-derived optical absorption spectrum matched the shape of the photoemission spectrum, and thus supports the explanation that the photoconductivity shoulder at photon energies in the region of 1.3 eV is due to transitions from localized states above the valence band to the conduction band. 相似文献
7.
8.
The electron energy loss spectra (EELS) of a pure metallic lanthanum surface and variations in these spectra at the initial stages of surface oxidation were studied. The measurements were performed at primary-electron beam energies E p from 200 to 1000 eV. A very pronounced peak at a loss energy of about 7.5 eV arises due to transitions from the La4d electronic states of the valence band into the empty La4f electronic states of the conduction band at 5.0–5.5 eV above the Fermi level. Marked changes are observed in the EELS during the oxidation of lanthanum: the peak at an energy of 7.5 eV disappears, and the peak at 13.5 eV corresponding to bulk collective energy loss in lanthanum oxide becomes more pronounced. The results obtained are discussed in terms of the electronic structure of lanthanum and lanthanum oxide. 相似文献
9.
External differential reflection measurements were carried out on clean Si(100) and (110) surfaces in the photon energy range of 1.0 to 3.0 eV at 300 and 80 K. The results for Si(100) at 300 K showed two peaks in the joint density of states curve, which sharpened at 80 K. One peak at 3.0 ± 0.2 eV can be attributed to optical transitions from a filled surface states band near the top of the valence band to empty bulk conduction band levels. The other peak at 1.60 ± 0.05 eV may be attributed to transitions to an empty surface states band in the energy gap. This result favours the asymmetric dimer model for the Si(100) surface. For the (110) surface at 300 K only one peak was found at 3.0 ± 0.2 eV. At 80 K the peak height diminished by a factor of two. Oxygen adsorption in the submonolayer region on the clean Si(100) surface appeared to proceed in a similar way as on the Si(111) 7 × 7 surface. For the Si(110) surface the kinetics of the adsorption process at 80 K deviated clearly. The binding state of oxygen on this surface at 80 K appeared to be different from that on the same surface at 300 K. 相似文献
10.
High-resolution X-Ray Photoemission Spectroscopy (XPS) and Bremsstrahlung Isochromat Spectroscopy (BIS) measurements are reported for UO2. Clear evidence is found for the localization of the 5f electrons. The Coulomb correlation energy and the p-d gap are determined to be 4.6 ± 0.8 and 5.0 ± 0.4 eV, respectively. A satellite observed 7 eV below the Uranium core levels is assigned to a final state excitation from the valence p band to the empty 5f3 state. 相似文献
11.
R. Ludeke 《Solid State Communications》1977,24(10):725-728
We have observed a unique, pressure-dependent adsorption isotherm of oxygen on the ZnSe (100) surface, which consists of an unmeasurable uptake followed by an irreversible, step-like uptake for pressures exceeding a critical value of ~ 0.08 torr at room temperature. A partial depletion of Se accompanies this adsorption process. For incomplete oxidation, oxygen induced (2 × 1) and (3 × 1) surface reconstructions may be generated, the first such structures to be observed for semiconductors. The electron-energy-loss spectra for these surfaces and for the clean ZnSe (100)c(2 × 2) surface are presented. The clean surface exhibits a dangling-bond-derived empty surface state ~ 1 eV above the conductor band edge, and filled surface states near 3.2, 6.5, and 15 eV below the valence band edge. 相似文献
12.
Photo-absorption spectra of formaldehyde (HCHO) is recorded in the range of 6–11.5 eV at various pressures (<0.001–2 mbar) at an average resolution of 1.2 Å using Photophysics beam line at the 450 MeV Indus-1 synchrotron radiation facilities at RRCAT Indore, India. The spectrum is found to consist exclusively of n → Rydberg transitions converging to the ground state of HCHO+. The highest identified Rydberg states, observed up to the first ionization limit of HCHO, correspond to 7s, 11p, 9d, and 12f orbitals. Analyzed electronic spectrum along with the intensities and quantum defects are presented. To interpret the observed weak valence transitions instead of strong valence transitions, a theoretical study of Rydberg and valence electronic states of HCHO is performed in the framework of single configuration interaction (CIS) and time-dependent density functional theory (TDDFT) using different basis sets. Electronic transition energies of high-lying singlet and triplet valence states as calculated using TDDFT (B3LYP) level of theory are found to give fairly-good agreement with the experimental data. 相似文献
13.
14.
We performed measurements of the optical reflectivity in the energy range 0.007–30 eV
on the clathrate-VIII type compound α-Eu8Ga16- xGe30 x in order to investigate its electronic band
structure. The very low charge carrier concentration as well as ferromagnetic ordering
of the divalent Eu ions below 10.5 K characterize the spectra at photon energies below
≃0.4 eV in accordance with the results of band structure calculations. Disorder
induced bound states have been identified to affect the optical conductivity at energies
between 10 and 100 meV. 相似文献
15.
The band structure, the density of states, the partial electron densities, and optical functions (such as permittivity, refraction index, reflection and absorption coefficients) of sodium nitrite, nitrate, carbonate, chlorate, sulfite, perchlorate, and sulfate are calculated in a local approximation of the density-functional theory using the Troullier-Martins pseudopotentials in the basis of numerical pseudoatomic orbitals. The nature of the upper valence bands and the lower empty bands is established. It is shown that the specific features of the optical functions at energies of up to 8 eV and at E> 8 eV are due to the excitation of electrons into a localized anionic conduction band and into the bands of anion-cation states, respectively. The results are compared to experimental photoelectron spectra and reflection and absorption spectra. 相似文献
16.
Photoemission from evaporated films of LiF were measured at photon energies of 10-27 eV. The photoelectron spectra exhibit features that can be identified as density-of-states structures in the valence and conduction bands of LiF. Regions of high density of states can be seen at ca. 3.3 and 7.8 eV above the vacuum level. The valence-band spectrum shows a doublet structure similar to the calculated density of states for the F?2p band of LiF. The base width of this structure is found to be 4.6 ± 0.3 eV. The photoelectron spectra for photon energies > 15 eV indicate that the highest occupied states of the F?2p band are located at 11.8 ± 0.3 eV below the vacuum level. The photoelectron spectra in the exciton region, however, show photo-emission from higher occupied states. 相似文献
17.
S. Nannarone P. Chiaradia F. Ciccacci R. Memeo P. Sassaroli S. Selci G. Chiarotti 《Solid State Communications》1980,33(6):593-595
Surface states in Si(111)2×1 and Ge(111)2×1 are detected by the method of the change of external reflectivity, both at energies below and above the gap. Optical transitions at 2.6 eV in Si and at 1.8 eV and 3.1 eV in Ge, as well as the already known transitions below the gap are observed. 相似文献
18.
Walsh A Da Silva JL Wei SH Körber C Klein A Piper LF DeMasi A Smith KE Panaccione G Torelli P Payne DJ Bourlange A Egdell RG 《Physical review letters》2008,100(16):167402
Bulk and surface sensitive x-ray spectroscopic techniques are applied in tandem to show that the valence band edge for In2O3 is found significantly closer to the bottom of the conduction band than expected on the basis of the widely quoted bulk band gap of 3.75 eV. First-principles theory shows that the upper valence bands of In2O3 exhibit a small dispersion and the conduction band minimum is positioned at Gamma. However, direct optical transitions give a minimal dipole intensity until 0.8 eV below the valence band maximum. The results set an upper limit on the fundamental band gap of 2.9 eV. 相似文献
19.
G. Hollinger F.J. Himpsel B. Reihl P. Pertosa J.P. Doumerc 《Solid State Communications》1982,44(8):1221-1225
W4f, Na2p photoelectron spectra and valence band spectra are reported for a series of cubic metallic NaxWO3 (0.4 < x < 0.85) bronzes in the 20–130 eV photon energy range. From a comparison of experimental and theoretical conduction band densities of states it is found that in the measured composition range the trends of the rigid band model behavior are respected. 相似文献
20.
Synchrotron radiation ultraviolet photoemission experiments at photon energies of 150 and 49 eV were performed on an epitaxial layer of (1 1 1) In2O3 with good crystallinity as established by a standard scanning probe and diffraction methods. Valence band (VB) and band gap photoemission spectra were monitored under separate oxygen, water and carbon monoxide exposures (100 L) at different activation temperatures within the range utilized for chemiresistive gas sensors (160-450 °C). Large changes in photoemission response within the whole VB were observed for all gases. Regular shifts of the valence band edge relative to the Fermi energy were found under gas exposures on two kinds of surface (partially reduced or partially oxidized), and are interpreted as changes of surface potential. Treatments in oxygen resulted in upward band bending (∼0.5 eV at T = 320 °C). Regardless of activation temperature, treatments in water resulted in downward band bending, but with small changes (<0.1 eV). Reduction properties of carbon monoxide were observed only at high temperatures of T ? 370 °C. At temperatures of 160 and 250 °C unusual “oxidizing” behavior of CO was observed with upward band bending of ∼0.7 eV (160 °C). Oxidizing and reducing effects of the gas interactions with the (1 1 1) In2O3 surface in all cases were accompanied by a corresponding behavior, i.e., a decrease or increase in photoemission response from so-called defect states in the band gap near the top of the valence band. The increases of photoemission within a band gap with maxima at binding energies (BE) of 0.4 (O2-induced peak) and 1.0 eV (CO-induced peak) were, respectively, found for interactions with O2 and CO for low temperatures (T = 160 and 250 °C). These responses were ascribed to acceptor-like electronic levels of O2 and CO chemisorption states, respectively. A definite split of the top VB peak (BE ∼ 4.0 eV) was found under CO dosing at 160 °C. Established knowledge of the CO interaction with the (1 1 1) In2O3 surface explains earlier revealed acceptor-like behavior of In2O3 film conductivity during CO detection at operational temperatures lower than 250 °C through the formation of acceptor-like electronic levels of adsorbed CO molecules. 相似文献