G. V. Ravi Prasad

Ionoluminescence decay measured with single ions

A new ion beam analysis-based, single ion technique called the time to first photon has been developed to measure the decay of the luminescence signal of phosphors. Such measurements are currently needed to study luminescence decay mechanisms following high-density excitations and to identify strongly luminescent phosphor coatings with short lifetimes for ion photon emission microscopy (IPEM). The samples for this technique consist of thin phosphor layers placed or coated on the surface of PIN diodes. Single ions from an accelerator strike this sample and simultaneously create ion beam induced luminescence (IBIL) from the phosphor that is measured by a single-photon-detector, and an ion beam induced charge collection (IBICC) signal in the PIN diode. In this case, the IBICC signal provides the start pulse and the IBIL signal the stop pulse to a time to amplitude converter. It is straightforward to show that this approach also measures a signal proportional to activity versus time with an accuracy of 5% as long as the number of detected photons per ion is less than 0.1, which usually requires the use of absorbers for the IBIL detector or electronic discrimination for the IBIL signals. Details of the new analysis are given together with examples of luminescence decay measurements of several ceramic phosphors being considered to coat IPEM samples. IPEM is currently being developed at Sandia National Laboratory (SNL), the University of North Texas in Denton, and the Universities and INFN of Padova and Torino.

Status of the AMS project at IOP Bhubaneswar

Abstract The 3 MV tandem Pelletron accelerator facility at the Ion Beam Laboratory of the Institute of Physics (IOP), Bhubaneswar is being augmented by the installation of a fast bouncer system for routine 14 C measurements. The old injector involving a single cathode SNICS source and a 45° bending magnet has been replaced by a new system consisting of a 40 cathode MC-SNICS source and a 90° analyzing bending magnet. The system is being tested for transmission. The status of the facility with test results is presented.

Decadal Changes of Radiocarbon in the Surface Bay of Bengal: Three Decades after GEOSECS and One Decade after WOCE

Radiocarbon was measured in the surface seawater dissolved inorganic carbon (DIC) of the Bay of Bengal during November 2006. A meridional transect of the ∆14C in DIC was obtained from measurements in closely spaced samples collected roughly along 88°E. The ∆14C of these samples ranged from 44‰ to 57.7‰ (mean 51.8 ± 1.1‰, n = 12), and 38‰ at one station in the northern Bay of Bengal. The overall pattern of 14C distribution in DIC of surface Bay of Bengal during 2006 was roughly similar to that during the WOCE expedition of 1995. These results indicate a ∆14C decline rate of ~4‰ per decade since WOCE in the surface Bay of Bengal, which is much smaller compared to a decline rate of ~25‰ per decade observed in the 2 decades between the GEOSECS and WOCE expeditions, due to the smaller atmosphere-ocean ∆14C gradient.

Impurity measurements in semiconductor materials using trace element accelerator mass spectrometry

Abstract Accelerator mass spectrometry (AMS) is commonly used to determine the abundance ratios of long-lived isotopes such as 10 B, 14 C, 36 Cl, 129 I, etc. to their stable counterparts at levels as low as 10−16. Secondary ion mass spectrometry (SIMS) is routinely used to determine impurity levels in materials by depth profiling techniques. Trace-element accelerator mass spectrometry (TEAMS) is a combination of AMS and SIMS, presently being used at the University of North Texas, for high-sensitivity (ppb) impurity analyses of stable isotopes in semiconductor materials. The molecular break-up characteristics of AMS are used with TEAMS to remove the molecular interferences present in SIMS. Measurements made with different substrate/impurity combinations demonstrate that TEAMS has higher sensitivity for many elements than other techniques such as SIMS and can assist with materials characterization issues. For example, measurements of implanted As in the presence of Ge in GexSi1−x/Si is difficult with SIMS because of molecular interferences from 74 GeH, 29 Si30Si16O, etc. With TEAMS, the molecular interferences are removed and higher sensitivities are obtained. Measured substrates include Si, SiGe, CoSi2, GaAs and GaN. Measured impurities include B, N, F, Mg, P, Cl, Cr, Fe, Ni, Co, Cu, Zn, Ge, As, Se, Mo, Sn and Sb. A number of measurements will be presented to illustrate the range and power of TEAMS.

The loss of boron in ultra-shallow boron implanted Si under heavy ion irradiation

Heavy ion impact has been known to cause a loss of light elements from the near-surface region of the irradiated sample. One of the possible approaches to a better understanding of the processes responsible for the release of specific elements is to irradiate shallow-implanted samples, which exhibit a well-known depth distribution of the implanted species. In this work, the samples studied were produced by implantation of Si<1 0 0>wafers with 11B at implantation energies of 250 and 500 eV and fluence of 1.0×1015 atoms/cm 2. Elastic Recoil Detection Analysis was applied to monitor the remnant boron fluence in the sample. Irradiation of the samples by a 14.2 MeV 19F 4+ beam resulted in a slow decrease of boron remnant fluence with initial loss rates of the order of 0.05 B atom per impact ion. Under irradiation with 12 MeV 32S 3+ ions, the remnant boron fluence in Si decreased exponentially with a much faster loss rate of boron and became constant after a certain heavy ion irradiation dose. A simple model, which assumes a finite desorption range and corresponding depletion of the near-surface region, was used to describe the observations. The depletion depths under the given irradiation conditions were calculated from the measured data.

A novel study of charged carbon clusters using a tandem Van de Graaff accelerator

A conventional tandem Van de Graaff accelerator is used to produce charged carbon cluster beams. The unique capability of the method for studying highly charged clusters unaccessible to other methods of producing cluster beams is demonstrated.

Depth Profiles of Mg, Si, and Zn Implants in GaN by Trace Element Accelerator Mass Spectrometry

GaN is one of the most promising electronic materials for applications requiring high-power, high frequencies, or high-temperatures as well as opto-electronics in the blue to ultraviolet spectral region. We have recently measured depth profiles of Mg, Si, and Zn implants in GaN substrates by the TEAMS particle counting method for both matrix and trace elements, using a gas ionization chamber. Trace Element Accelerator Mass Spectrometry (TEAMS) is a combination of Secondary Ion Mass Spectrometry (SIMS) and Accelerator Mass Spectrometry (AMS) to measure trace elements at ppb levels. Negative ions from a SIMS like source are injected into a tandem accelerator. Molecular interferences inherent with the SIMS method are eliminated in the TEAMS method. Negative ion currents are extremely low with GaN as neither gallium nor nitrogen readily forms negative ions making the depth profile measurements more difficult. The energies of the measured ions are in the range of 4-8 MeV. A careful selection of mass/charge ratios of the detected ions combined with energy-loss behavior of the ions in the ionization chamber eliminated molecular interferences.

Dose Measurements of Ultra‐Shallow Implanted As and B in Si by RBS and ERD

Continuous miniaturization of integrated circuits requires narrower dopant profile depth in the Si channel and consequently the use of ultra‐shallow implants in the manufacturing process. Secondary Ion Mass Spectroscopy (SIMS) is routinely used to measure the boron depth concentration profiles. However, due to the altered nature of the near‐surface sputtering process inherent to SIMS, it underestimates the B implanted doses for implantation energies below 2 keV. Alternate ion beam methods for absolute dose measurements of ultra‐shallow implanted As and B in Si are presented in this study. The dopant implant energies ranged from 250 eV, to 5 keV for boron and from 500 eV to 5 keV for arsenic. Implanted doses for both B and As varied from 2 × 1013 to 1 × 1015 atoms/cm2. The arsenic implants were studied with Rutherford Backscattering Spectrometry (RBS) using 2 MeV carbon ions. The absolute arsenic implanted doses were measured to an accuracy of better than 5%. The 1 keV arsenic implants were extensively stu...