D.R. Chopra

Photoemission study of low pressure chemical vapor deposited and reactively sputtered titanium nitride in W/TiN/Si

Low‐pressure chemical vapor deposited (LPCVD) tungsten has been deposited onto LPCVD grown and reactively sputtered titanium nitride (TiN) films. The x‐ray photoelectron spectroscopy depth profiles and the chemical analysis suggest that the chemical interaction of TiN with W and with the Si/SiO2 substrates tends to be minimal.

Characterization of chemically vapor deposited tungsten on low‐pressure chemically deposited and reactively sputtered titanium nitride films

The growth of low‐pressure chemical vapor deposition (LPCVD) tungsten on LPCVD‐grown and reactively sputtered titanium nitride (TiN) films has been studied employing the x‐ray photoelectron spectroscopy depth profiling technique. Two types of gas ambients were used to grow tungsten (W) films at a deposition temperature of 400 °C. In one case, a very thin overlayer of W was formed on both types of TiN films by exposing the samples to WF6. The W thus deposited diffuses deep into the bulk of the reactively sputtered TiN film whereas it is confined to the W/TiN interface for the LPCVD‐grown TiN film. In the second case, both types of TiN films were exposed to the WF6, SiH4, and H2 ambient. No diffusion of W into any of the TiN films was observed.

Electronic structure of La—Ni intermetallics: a soft X-ray appearance-potential study

Abstract Soft X-ray appearance-potential spectra (SXAPS) of La—Ni intermetallic compounds are presented. SXAPS probes the binding energy (BE) of the core levels and provides information about the local density of conduction band states of the constituents as a function of alloy composition. Charge transfer from electropositive La to electronegative Ni is expected in La—Ni intermetallics. Ni metal has only 0.6 d vacancies and Ni spectra should disappear at high La concentrations as a result of charge transfer. Ni peak intensity decreases with increasing La concentration, however, in no case was a complete disappearance of the Ni spectra observed, which is interpreted as evidence for the d character in unoccupied states. The small BE shifts and narrowing of the SXAPS peaks in alloys indicate filling of the 3 d band of Ni due to hybridization of the wave functions centered on different atom sites instead of the charge transfer.

Comparison of light rare earths N4,5-level soft X-ray and auger electron appearance-potential spectra

Abstract Appearance-potential Spectroscopy (APS) probes the binding energy of core levels and local conduction band states of atoms in the surface region. Soft X-ray APS (SXAPS) and Auger electron APS (AEAPS), respectively, measure the differential X-ray fluorescence and secondary electron yields as a function of incident electron energy. We have obtained the N 4,5 -level SXAPS and AEAPS spectra of La, Ce, Pr, Nd and Sm metals. Both spectra exhibit multiplet structure below the expected 4 d excitation threshold and a broad, 10–20 eV wide peak above the threshold followed by small peaks of decreasing intensity. The data are used to gain an understanding of the decay mechanism following the excitation of the core levels in these spectroscopies. The strong similarity observed between the SXAPS and AEAPS indicates that the characteristic emission and not the bremsstrahlung dominates the spectral lineshape in APS.

A study of the electronic structure of GdMn2 by appearance potential spectroscopy (SXAPS)

Abstract The GdMn2 intermetallic has been studied using soft X-ray appearance potential spectroscopy (SXAPS) which exploits the abrupt changes in the total X-ray emission associated with the thresholds for the excitations of core levels of the atoms in the surface region of the materials. This technique is highly sensitive to the density of states at the Fermi level and is therefore particularly suitable for investigating the changes in the unoccupied density of states as a result of alloying rare earths with transition metals. The SXAPS spectra of the M4,5 levels of Gd and the L2,3 levels of Mn in the intermetallic are compared with the corresponding elemental spectra. These core levels exhibit a positive chemical shift. The experimentally observed core level shift is a sum of the shifts resulting from charge transfer and hybridization. The charge transfer shift is approximated by the Fermi energy shift. The Fermi energies in the pure elements and in the intermetallic have been estimated from the free electron theory model. Utilizing these Fermi level and core level shifts, the hybridization effects between 3d and 5d bands are obtained in this intermetallic. The 3d–5d hybridization plays a significant role in the magnetism observed in the intermetallic.

X-ray photoelectron spectroscopy characteristics of the W/TiN/Si and W/TiN/SiO2/Si structures

Abstract X-ray photoelectron spectroscopy (XPS) has been used to study the characteristics of titanium nitride (TiN) as a nucleation layer for the deposition of low pressure chemical vapor deposited tungsten (LPCVD-W) onto SiO 2 . The XPS depth profiles and the chemical analysis of the as-deposited W/TiN/Si (with sputtered TiN) and W/TiN/SiO 2 /Si (with LPCVD-TiN) structures suggest that the chemical interaction of TiN with W and with Si/SiO 2 substrates tends to be minimal.

X-ray photoelectron study of Al-Mn alloys

Abstract X-ray photoelectron Spectroscopy (XPS) has been employed to study the electronic structure of orthorhombic Al6Mn, Al-26 wt% Mn and Al-28 wt% Mn (icosahedral phase), and Al-35 wt% Mn (decagonal phase). From the plasmons associated with Al 2s peak the valence electron density at the Al site was calculated. An increase in the electron density was found in the quasicrystals as compared with the orthorhombic or pure Al. The total energy shift in the Al 2p and Mn 2p 3 2 peaks after relaxation energy subtraction showed the existence of sp-d hybridization in the Mn conduction band of the quasicrystals. We have also shown that the FWHM of the Al 2p peaks obtained by XPS serves as a measure of the interatomic distances and mass-density. The conclusions correlate very well with the reported experimental data and theoretical calculations.

Soft X-ray appearance potential spectroscopy study of Ni-Fe alloys

Abstract Soft X-ray appearance potential spectroscopy has been exploited to study the electronic structure of the NixFe100−x (x = 20, 33, 63, 74) alloy system. The binding energies of L3 levels of Ni and Fe exhibit negative and positive chemical shifts respectively with increasing Ni content. Also the full width at half maximum of the Fe L3-level peak increases systematically. The changes result from the charge transfer from Fe to Ni and are in agreement with the rigid-band model for the constituents of the alloys.

Study of the 4f-levels in rare earths by appearance potential spectroscopy

Abstract The yttrium M 4.5 -level Auger electron appearance potential spectrum (AEAPS) in the 150–190 eV energy range is presented. AEAPS is a surface sensitive technique that measures the differential secondary electron yield as a function of incident electron energy. The spectral features of AEAPS are determined by the width of the empty conduction band and the degree of localization of the conduction band wave functions. Yttrium (Y) is considered to be one of the rare earth metals with an absence of a 4 f state. In lanthanum (La), which has an identical valence electron configuration-to Y, the 4 f levels are localized. Comparison of the 3 d -level spectrum of Y with that of La shows that though the main peaks remain the same, additional fine structures appear in the spectrum of La. The existence of these structures is correlated with the presence of empty localized 4 f levels in La.

Appearance potential study of PrMn2 and SmMn2 intermetallics

Abstract Soft X-ray appearance potential spectroscopy (SXAPS) has been employed to study the changes in the electronic structure of PrMn2 and SmMn2 intermetallics as a result of aloying. In this technique the total X-ray emission associated with the thresholds for the excitations of core levels of the atoms in the surface region of the materials is measured. It sensitively provides information regarding the density of unoccupied states at the Fermi level. The SXAPS spectra of the M4,5 levels of Pr and Sm and the L2,3 levels of Mn in these intermetallics are compared with the corresponding elemental spectra. The observed shift in the core levels represents contribution from the shifts due to charge transfer and that due to the hybridization between the Mn 3d and the rare earth 5d bands. The charge transfer shifts have been estimated from the differences in the Fermi levels of the intermetallic and the element using the free electron model. These values have been utilized to obtain the hybridization shifts. The hybridization shifts show a decreasing trend in these intermetallics and is attributed to the decrease in the number of electrons in the 3d–5d hybridized bands. Our results correlating the Mn magnetic moment with the number of electrons are consistent with the reported experimental data. It is concluded that the 3d–5d hybridization plays a significant role in determining the magnetic properties of these intermetallics.