C. Carbone

Synchrotron radiation studies of the effect of thermal treatment on the Si(111)-Yb interfaces

Synchrotron radiation photoemission results (80–200 eV) from Si(111)-Yb interfaces in the coverage range 0.2–2 monolayers are presented for interfaces (prepared at room temperature) before and after heating at 300°C and 450°C. The composition of the interface reaction products is calibrated against photoemission from bulk silicides photoemission. At θ ≳ 1 the interfaces are extremely sensitive to thermal treatment with the formation of Si-rich reaction products while at submonolayer coverages they are basically not sensitive to the same thermal treatment. The results are discussed in connection with interface growth problems.

Synchrotron radiation study of the photoionization cross sections for the whole valence band of 2H-MoS2

Abstract The first complete analysis of photoionization cross section in the whole valence band of 2H-MoS2 is reported. The measurements, taken with synchrotron radiation in the 65–190 eV energy range, allow the identification of the Mo 4d contribution to the electron states in the different regions of the valence density of states. In particular it is found that the deepest valence peak contains a considerable 4d contribution.

Photoemission studies of mixed valence in Yb3Si3, YbSi and Yb5Si3: Equivalent versus inequivalent Yb sites

Abstract We present angle integrated 4ƒ photoemission from Yb3Si5, YbSi and Yb5Si3 taken with synchrotron radiation (hv = 80 eV, hv = 450 eV, and hv at the 4d-4ƒ threshold). The Yb valence is determined and Si2p shifts are also given. Yb3Si5 and YbSi are homogeneous mixed valent in the bulk; in Yb5Si3, which has inequivalent Yb sites, one Yb specie is divalent and the other is mixed valent. This shows the connection between mixed valence and local steric effects due to crystal structure. In all compounds the Yb is divalent in the surface region.

Photoemission studies of Si(1 1 1)-Yb interfaces: What can be learned from Yb mixed valence

Abstract The 4 f photoemission spectra taken with syncrotron radiation from Si(1 1 1)-Yb interface are given vs metal coverage and annealing temperature. It is shown that the Yb atoms which are fully coordinated by Si go into a well defined mixed valent state ( v = 2.37 ± 0.02) which is always the same independently of the coverage and of the annealing temperature. This shows that the growth is chemically driven by the existence of a provileged interface site. The mixed valence of this site is definitely lower than that of stoichiometric bulk silicides.

Surface and bulk valence in Yb3Si5

We present the first photoemission study of mixed valence Yb3Si5. A monophasic, polycrystalline sample has been studied with synchrotron radiation in the range 65–450 eV and with MgKα radiation. The wide hv range allows the surface sensitivity to be changed strongly due to the energy dependence of escape depth. The ratio of the intensities from Yb2+ and Yb3+ is higher when the surface sensitivity increases; this trend can be fitted accurately with a simple model which gives valence ≈ 2.6 at room temperature in the bulk and at least one surface layer with divalent Yb. The fitting gives also the energy dependent escape depth. We give also the shifts of the Yb bulk and surface f states upon Yb3Si5 formation which explain the different valence in the bulk and at the surface. Si2p photoemission shows the effect of inequivalent Si sites in Yb3Si5.

X-ray absorption spectroscopy of platinum silicides: The L2,3 and M2,3 edges of platinum

Abstract The following platinum X-ray absorption edges were measured in platinum silicides: Pt2Si L2,3, PtSi L2,3 and PtSi M2,3. The results are compared with the edges measured in platinum metal. As well as the known peak at the edges of index 3, a new strong peak absent in the metal appears at the edges of index 2 of the silicides. This effect is very strong in the M2 edge in spite of the linewidth which is greater than that in L2. The results are interpreted in terms of transitions to empty d states typical of the hybridization (Si p)-(Pt d) which gives rise to states with projections onto j= 3 2 and j= 5 2 so that the peak due to p-d transitions is present in the edges of index 2 and of index 3. The different localization involved in M and L transitions is also discussed.

Resonant photoemission at the 5p threshold in La, Pr, Sm and Tb

Abstract The 5 p -valence band interaction in the rare earth metals La, Sm, Pr and Tb is studied with resonant ultraviolet photoelectron Spectroscopy in the photon energy range hv = 10–40 eV. For all four metals the valence band (5 d 6 s ) photoionization cross-section has a minimum at the 5 p 3 2 threshold, with a strong delayed, 10 eV wide, maximum above threshold. An O 3 VV Auger channel opens at the 5 p 3 2 threshold.

Photoelectron Spectroscopy of the Si/Eu Interface Using Synchrotron Radiation

Rare earth metal/semiconductor interfaces have attracted recent interest for properties that are distinct from those observed in other, more extensively studied, metal/semiconductor systems. Rare earth silicide/silicon junctions have the lowest measured Schottky barrier [1] of any metal silicide contact to n-type Si. The thin film reaction kinetics are often controlled by nucleation phenomena [2] instead of atomic diffusion as in many d metals [3]. The contrast in these properties, and others such as the thermodynamics of metal/Si compound formation [4], makes the study of these interfaces important to the general understanding of the formation of Schottky barriers in microscopic terms.