Purushottam Chakraborty

XPS and SIMS analysis of gold silicide grown on a bromine passivated Si(111) substrate

Abstract When a thin film of Au (∼100 nm) deposited under high vacuum conditions on a chemically prepared Br-passivated Si(111) substrate was annealed around 363°C, epitaxial layer-plus-island mode growth of gold silicide was observed along with some unreacted gold in stringy patterns. This unreacted gold was removed by etching the sample in aqua regia. X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) measurements were carried out on these samples. SIMS results reveal that the height of the islands is about 1.2 μm and the silicide/Si interface is abrupt. XPS measurements were made after sputtering the sample surface at constant intervals of time. Si 2 p , Au 4 f , C 1 s and O 1 s photoelectrons were detected. XPS spectra of Si 2 p are resolved into three peaks corresponding to bulk Si, Si in silicide and Si in oxide. The Au 4 f 7/2 peak in the silicide is shifted by 1–1.2 eV towards higher binding energy compared to metallic Au. The shift of Si 2 p towards the higher binding energy in the silicide is understood from the higher electronegativity of Au, while the shift of Au 4 f 7/2 peak towards higher binding energy is known to be due to d-electron depletion to form an sd hybrid. The XPS peak intensity profile with sputtering time indicates that the thin uniform layer (∼5.5 nm) of gold silicide is sandwiched between a thin (∼2.8 nm) SiO 2 layer and the Si(111) substrate.

Narrowing of band gap and effective charge carrier separation in oxygen deficient TiO2 nanotubes with improved visible light photocatalytic activity.

Oxygen vacancies are introduced into hydrothermally processed TiO2 nanotube by vacuum calcination. Formation of oxygen vacancies modifies the local coordination in TiO2 as evident from Raman spectroscopy and secondary ion mass spectrometry (SIMS) results. The surface area is increased from 172.5m(2)/g in pure to 405.1m(2)/g in defective TiO2 nanotube. The mid-band gap electronic states created by oxygen vacancies are mostly responsible for the effective narrowing of band gap. Charge carrier separation is sufficiently prolonged as the charged oxygen defect states inhibit facile carrier recombination. With high surface area, narrowed band gap and separated charge carriers defective TiO2 nanotube is a suitable candidate in the photodegradation of methylene blue (MB) and phenol under visible light illumination. Photosensitized electron transfer from MB to the conduction band of TiO2 and the photodegradation of MB is facilitated in presence of high density of oxygen vacancies. Unlike MB, phenol absorbs in the UV region and does not easily excited under visible light. Phenol shows activity under visible light by forming charge transfer complex with TiO2. Defect trapped carriers become available at the phenol-TiO2 interface and finally interact with phenol molecule and degrade it.

Interfacial profile of a Bragg Mirror

Abstract X-ray reflectivity and secondary ion mass spectrometry studies of a Bragg Mirror are presented. We find that the AlAs-on-AlGaAs interfaces are diffused due to formation of continuously varying composition of AlxGa1−xAs at the interfaces. On the other hand, the AlGaAs-on-AlAs interfaces are found to be sharp.

Towards an understanding of MCsn+ formation mechanism in SIMS

Abstract The present work is aimed towards understanding the formation mechanisms of MCs + and MCs 2 + molecular ions in the SIMS process. Energy distribution studies of MCs + under varying Cs surface concentrations have been made for Au, Cu and Ni. From the peak shifts of the energy distribution curves it is evident that the surface binding of MCs + is stronger at higher Cs surface concentration for elements having greater atomic polarisability. As far as the MCs 2 + formation mechanisms are concerned, we feel that the most probable formation probability could be due to the association of a sputtered M − ion with two self-sputtered Cs + ions. Energy distribution study for MCs + , MCs 2 + and Cs + ions seems to be an effective diagnostic for the plausible explanation of the above mechanism.

Structural and morphological characterization of molecular beam epitaxy grown Si=Ge multilayer using x-ray scattering techniques

Si/Ge multilayers are of great technological importance as is evident from the research studies of the past two decades. Here, we have presented a method for the morphological and structural characterization of such MBE grown epitaxial Si/Ge superlattice structures using simultaneous analysis of x-ray reflectivity and x-ray diffraction data, respectively. The consistent analysis of the data collected in the Indian Beamline at Photon Factory Synchrotron have allowed for the determination of electron density and strain profile as a function of depth.

On the intensity of secondary Cu+, Cu+2, and Mo+ ions as a function of bombarding ion energy

The effect of Zn+ and Cd+ primary ions with energies between 3 and 10 keV on the emission of secondary Cu+, Cu+2, and Mo+ ions from polycrystalline copper and molybdenum target surfaces has been investigated. It was found that secondary ion intensity increases almost linearly with bombarding ion energy, attains a broad maximum, and then decreases steadily. It has also been observed that the location of the maximum depends strongly on the ion‐target combination. The positions of maxima were found to be around 7.5, 6.5, and 5.0 keV for Zn+‐Cu, Cd+‐Cu, and Cd+‐Mo combinations, respectively. Our finding could be experimental evidence of the fact that the nontextured surfaces may become textured under ion bombardment.

Studies of bonding and valence states in YBa2Cu3O7-x through secondary-ion-mass spectrometry

The superconducting compound YBa2Cu3O7−x (YBCO) and oxides of copper (CuO and mixture of CuO and Cu2O) have been examined by secondary‐ion‐mass spectrometry (SIMS). The results support the fact that two kinds of copper (from the consideration of bonding and valence state) exist in YBCO—one in the basal planes of charge reservoir layers and the other in the conduction layers. The bonding of Cu‐O in the CuO2 conduction layer unit is similar to that in CuO. No Cu+++ secondary ions could be detected in the SIMS spectrum, in agreement with electron spectroscopy for chemical analysis, extended x‐ray‐absorption fine structure, and electron probe microanalysis measurements reported earlier. A plausible explanation has been given for this.

On the mechanism of small Sin+andAln+ (n = 1–3) cluster emission from 3–10 keV Ar+ bombarded Al and Si surfaces at increased target current densities

Abstract Secondary emission of the ion types M n q + ( M = Si or Al ; q = 1, 2; n = 1–3) from 3–10 keV Ar + bombarded polycrystalline Si and Al samples at target current densities ( J p ) in the range of 60–800 μ A cm 2 has been studied in detail. For each projectile, the yield ratio of the cluster ions with respect to the singly-charged monomers ( I n + I 1 + ) is observed to be steady only for primary energies ( E ) > 9 keV. Below this energy, the ratio rapidly increases in more or less an exponential manner with I p , the effect being most prominent at the lowest E value. This increase in yield ratio with target current density appears to be rather inconsistent with the recombination model, but can be, to some extent, explained within the framework of the direct emission model. Briefly speaking, our results, at least at high J p and low E values are in apparent contradiction to the standing notion that emission of small clusters is best understood within the framework of the recombination model proposed by Konnen.

Evidence of primary atom accumulation on Si under 3–10 keV O+2 bombardment

Abstract Positive secondary ion signals of the type Si n O + m ( n = 1–3, m = 1, 2) from a polycrystalline Si target under O + 2 bombardment in high vacuum at primary energies in the range 3–10 keV have been monitored as a function of target current density J p ( I p / A , where I p is the target current and A the spot area). Most of the secondary ion clusters (but not the monomers) show pronounced chemical enhancement effects with an increase in I p . Moreover, the intensity ratios of the higher oxygen content clusters, e.g. SiO + /Si + or SiO + /Si 2 O + , also showed a marked increase under the same conditions. Our results suggest that reactive primary atoms are gradually accumulated at the target surface under energetic ion bombardment at increased target current densities. The phenomenon stands rather in contradiction to the theoretical concepts where the surface stoichiometry is not expected to change with target current density once steady state is attained. Our observation also gives an insight into the cluster emission process at increased target current densities. It appears that our current ideas regarding the emission of small clusters need to be modified, at least in the high target current density regime.

Diffusion mechanism and photoluminescence of erbium in GaN

Abstract Erbium has been diffused into GaN for the first time. A weak spontaneous emission is observed in the photoluminescence spectra after the diffusion process during 168 h at 800 °C under N2 atmosphere. The diffusion coefficient of erbium in GaN is obtained in Arrenhius expression to be D=1.8±1.3×10 −12 exp (−1±0.4 eV /kT) cm2/s. The result shows that the Er diffusion mechanism might be an interstitial-assisted process. The luminescence characteristics of the Er-diffused GaN is compared with the Er-implanted GaN. The methods to enhance the emission intensity of the Er-diffused GaN are discussed.