A. P. Pathak

Synthesis, characterization and radiation damage studies of high-k dielectric (HfO2) films for MOS device applications

The current trend in miniaturization of metal oxide semiconductor devices needs high-k dielectric materials as gate dielectrics. Among all the high-k dielectric materials, HfO2 enticed the most attention, and it has already been introduced as a new gate dielectric by the semiconductor industry. High dielectric constant (HfO2) films (10 nm) were deposited on Si substrates using the e-beam evaporation technique. These samples were characterized by various structural and electrical characterization techniques. Rutherford backscattering spectrometry, X-ray reflectivity, and energy-dispersive X-ray analysis measurements were performed to determine the thickness and stoichiometry of these films. The results obtained from various measurements are found to be consistent with each other. These samples were further characterized by I–V (leakage current) and C–V measurements after depositing suitable metal contacts. A significant decrease in the leakage current and the corresponding increase in device capacitance are observed when these samples were annealed in oxygen atmosphere. Furthermore, we have studied the influence of gamma irradiation on the electrical properties of these films as a function of the irradiation dose. The observed increase in the leakage current accompanied by changes in various other parameters, such as accumulation capacitance, inversion capacitance, flat band voltage, mid-gap voltage, etc., indicates the presence of various types of defects in irradiated samples.

An experimental study of stopping power for MeV heavy ions

A 60 MeV Ag primary ion beam was used to generate secondary recoil ion beams of Cl, K, Ca, Sc, Ti, V, Mn and Cu in the energy range 0.1-0.6 MeV . The stopping power in the carbon absorber was measured and compared with Lindhard, Scharff and Schiott (LSS) theory, the Northcliffe and Schilling model and semiempirical estimates of Ziegler et al (1985). Data for heavier ions of Ag, I and Au from other sources was also considered in order to extend the scope of such a comparison. LSS theory was found to satisfy the data within a narrow mid-velocity range (-, where is Bohr velocity). Estimates made by Ziegler et al were seen to generally agree to within a few per cent except for energies below 0.2 MeV , where deviations as high as 25% were seen. Expected oscillations could not be discerned down to the lowest ion velocities so far attempted.

Stopping power of carbon for Si, Fe, Ni and Cu ions using the ERDA technique

Abstract We have been carrying out systematic random stopping power experiments in the low velocity region ( ν 0 to 5 ν 0 , where ν 0 is the Bohr velocity) using heavy ions ( Z 1 = 8 to 29). The objective is to supplement in a significant way the currently available scanty data in this region. In order to optimise the data collection for a variety of heavy ions, we have used our recently developed technique of adopting a twin detector in conjunction with Elastic Recoil Detection Analysis (ERDA) technique. In continuation, we now report energy losses of Si, Fe, Ni and Cu ions in carbon foil in the energy range 0.1-1.1 MeV/u. Here we are reporting experimental stopping power data for Ni ions in the velocity region of 0.1-0.6 MeV/u. Most of the experimental data have an error of around 6%. Comparison of the now available data have been made with existing theoretical and semiempirical models.

Stopping power of carbon for heavy ions upto copper

Abstract Stopping powers for ions of O, Cl, Sc, Ti, Cr, Mn, Fe and Cu in the energy range 0.2–1.0 MeV/n have been measured in carbon foils using the elastic recoil ion technique and a twin detector system. Very heavy ions of Gold and Iodine of energy around 100 MeV have been employed to generate the various recoil ions using pure thin (≈ 1000A) elemental/compound targets. The data have an overall average accuracy of 6%. The stopping power for Sc, Mn and Fe ions in the energy region 0.2–1.0 MeV/n as well as those for Ti and Cu ions in the energy region below 0.45 MeV/n are perhaps the first such measurements. The limitations of LSS theory even within the range of its applicability has been demonstrated. Varelas and Biersack estimates agree well in the region of their applicability lying above the ion velocity limit set for LSS theory. Northcliffe and Schilling predictions as well as the TRIM estimates provide the best fit with data. However, one does notice variations upto 25% for some ions at lower energi...

Studies on linear, nonlinear optical and excited state dynamics of silicon nanoparticles prepared by picosecond laser ablation

We report results from our studies on the fabrication and characterization of silicon (Si) nanoparticles (NPs) and nanostructures (NSs) achieved through the ablation of Si target in four different liquids using ∼2 picosecond (ps) pulses. The consequence of using different liquid media on the ablation of Si target was investigated by studying the surface morphology along with material composition of Si based NPs. The recorded mean sizes of these NPs were ∼9.5 nm, ∼37 nm, ∼45 nm and ∼42 nm obtained in acetone, water, dichloromethane (DCM) and chloroform, respectively. The generated NPs were characterized by selected area electron diffraction(SAED), high resolution transmission microscopy(HRTEM), Raman spectroscopic techniques and Photoluminescence(PL) studies. SAED,HRTEM and Raman spectroscopy data confirmed that the material composition was Si NPs in acetone, Si/SiO2 NPs in water, Si-C NPs in DCM and Si-C NPs in chloroform and all of them were confirmed to be polycrystalline in nature. Surface morphological information of the fabricated Si substrates was obtained using the field emission scanning electron microscopic (FESEM) technique. FESEM data revealed the formation of laser induced periodic surface structures (LIPSS) for the case of ablation in acetone and water while random NSs were observed for the case of ablation in DCM and chloroform. Femtosecond (fs) nonlinear optical properties and excited state dynamics of these colloidal Si NPs were investigated using the Z-scan and pump-probe techniques with ∼150 fs (100 MHz) and ∼70 fs (1 kHz) laser pulses, respectively. The fs pump-probe data obtained at 600 nm consisted of single and double exponential decays which were tentatively assigned to electron-electron collisional relaxation ( 1 ps). Large third order optical nonlinearities (∼10−14 e.s.u.) for these colloids have been estimated from Z-scan data at an excitation wavelength of 680 nm suggesting that the colloidal Si NPs find potential applications in photonic devices.

MeV heavy ion stopping power measurements using NSC Pelletron

Abstract Surface modification and characterization are being undertaken increasingly using the MeV heavy ion beams. Interpretation of such data requires reliable and precise values of stopping powers involved. Sufficient data exists above 2.5 MeV/n but less information is available at lower energies. The present series of experiments report the results of stopping power measurements for various ion species between Z = 8 and 29, covering the energy range ∼ 0.2–1.0 MeV/n in carbon. The elastic recoil ion detection system provided a unique method to generate variety of variable energy ion species utilising a fixed energy heavy (Z = 47, 53, 79) projectile beam from the NSC Delhi 15 MV Pelletron facility. The data has been compared with available theoretical and semiempirical fitting treatments. Relative merits and demerits of these formulations have been highlighted. Future projections to our line of investigation have been indicated in the light of some recent innovative theoretical predictions.

Three-dimensional hybrid silicon nanostructures for surface enhanced Raman spectroscopy based molecular detection

Three-dimensional silver nanoparticles decorated vertically aligned Si nanowires (Si NWs) are effective surface-enhanced Raman spectroscopy (SERS) substrates for molecular detection at low concentration levels. The length of Si NWs prepared by silver assisted electroless etching is increased with an increase in etching time, which resulted in the reduced optical reflection in the visible region. These substrates were tested and optimized by measuring the Raman spectrum of standard dye Rhodamine 6G (R6G) of 10 nM concentration. Further, effective SERS enhancements of ∼105 and ∼104 were observed for the cytosine protein (concentration of 50 μM) and ammonium perchlorate (oxidizer used in explosives composition with a concentration of 10 μM), respectively. It is established that these three-dimensional SERS substrates yielded considerably higher enhancement factors for the detection of R6G when compared to previous reports. The sensitivity can further be increased and optimized since the Raman enhancement was...

Clinical profile and short-term outcome of hypoxic ischemic encephalopathy among birth asphyxiated babies in Katihar medical college hospital

Objectives: The aim was to determine the selected maternal and fetal risk factor for adverse short-term outcomes in babies with birth asphyxia. Materials and Methods: This was a prospective observational study, including all birth asphyxiated babies born between January 1, 2013 and December 1, 2013 who fulfill the inclusion criteria. Detailed maternal information and progress of babies were noted regularly until the time of death or discharge. Cord blood was collected after delivery for measurement of nucleated red blood cell/100 white blood cells (NRBC/100 WBC) for Katihar Medical College born babies only. Babies were categorized into different stages of hypoxic ischemic encephalopathy according to Sarnat and Sarnat staging. On the basis of outcome, they were divided into two groups, group one who survived and was discharged with stable sign and group two who died. Effects of risk factors on both groups were compared and result were expressed as (P < 0.05 was taken significant), using the Chi-square test. Results : During the study period, out of 5481 neonatal admissions 600 babies (10.94%) were asphyxiated. Of the total asphyxiated babies 60 babies (10%) expired. Maternal factors significantly associated with mortality included antenatal check-up, premature rupture of membranes, meconium stained liquor, parity and place of delivery. Birth injury, hypotension, hypothermia, hypoglycemia, hypoxemia, cord accidents and increased NRBC/100 WBC count were the baby factors. Conclusion : Early identification of high risk mothers and timely referral to tertiary care center can reduce the mortality. Furthermore, there is need to carefully evaluate and monitor the babies with low APGAR scores immediately after birth.

Influence of activation of Si29+ ion-implantation in GaAs on ohmic contact resistance and electrical performances of MESFETs

The active layers of Metal Semiconductor Field Effect Transistors (MESFETs) are obtained by Si29+ ion implantation in GaAs. Implantation was done at 35 keV with a higher dose near the wafer surface for facilitating easier formation of ohmic contacts, and at 180 keV with a lower dose for obtaining the device channel. Post-implantation annealing was carried out in a rapid thermal processor for activating the implants. Very high activation levels of about 60% for the n+ GaAs layer, and 85% for the n-GaAs channel layer were achieved by annealing at 955 °C for 25 s. Activation was characterized using C–V profiling, secondary ion mass spectrometry and by electrical device data of fabricated MESFETs. We attempt an experimental correlation between the ohmic contact resistance (R c) and activation of both the n+ and the channel layer. It was found that very high and simultaneous activation of the n+ and channel layers results in very low contact resistances. The conduction of source-drain current into the channel is easily facilitated due to reduction in the resistance of the transition region at the interface of n+-contact and n-channel layers.

z1 oscillations in the stopping powers of silicon and tungsten for low-velocity channelled heavy ions

A model of stopping for low-velocity heavy ions moving through an electron gas incorporating a shell model charge density has been used to calculate the stopping powers of silicon and tungsten for ions channelled along silicon (110) and tungsten (100) single crystals. The energy loss of the heavy ions is attributed to the scattering of target electrons in the potential field of a moving projectile. So the stopping cross section is proportional to the momentum transfer (or transport) cross section. A comparison of the results with experimental data and earlier theoretical calculation shows good agreement. The present model is most suited to projectile velocities greater than the Fermi velocity of target electrons.