Dhruva Kulkarni

First multicharged ion irradiation results from the CUEBIT facility at Clemson University

A new electron beam ion trap (EBIT) based ion source and beamline were recently commissioned at Clemson University to produce decelerated beams of multi- to highly-charged ions for surface and materials physics research. This user facility is the first installation of a DREEBIT-designed superconducting trap and ion source (EBIS-SC) in the U.S. and includes custom-designed target preparation and irradiation setups. An overview of the source, beamline, and other facilities as well as results from first measurements on irradiated targets are discussed here. Results include extracted charge state distributions and first data on a series of irradiated metal-oxide-semiconductor (MOS) device targets. For the MOS devices, we show that voltage-dependent capacitance can serve as a record of the electronic component of ion stopping power for an irradiated, encapsulated oxide target.

Encapsulating Ion-Solid Interactions in Metal-Oxide-Semiconductor (MOS) Devices

We report on a measurement of low energy ion irradiation effects on as-grown films of SiO2 on a Si substrate. Beams of normally incident Na+ ions with kinetic energies of 2 keV to 5 keV were focused onto ~ 1900 Å SiO2 films. Aluminum top metal contacts were subsequently deposited onto these targets such that irradiated regions and unexposed (pristine) regions of the target could be compared using capacitance-voltage (C-V) measurements of individual metal-oxide-semiconductor (MOS) devices. The C-V data reveal an energy-dependent shift in the flatband voltage ( VFB) that can be returned to its near-pristine value by a low temperature anneal. An increase in the density of interface states ( Dit) inferred from the C-V curves is found to have a superlinear dependence on the incident kinetic energy. These data are consistent with previously observed UV radiation effects on MOS oxides, where transferred energy leads to electron-hole pair production and the diffusion and trapping of holes throughout the oxide. Our measured trapped hole densities are compared with calculated densities, which are based on the incident ion dose and the predicted ion implantation range, to arrive at a fractional yield for hole survival and measurement within an encapsulated MOS device.

Effects of slow highly charged ion irradiation on metal oxide semiconductor capacitors

Measurements were performed to characterize and better understand the effects of slow highly charged ion (HCI) irradiation, a relatively unexplored form of radiation, on metal oxide semiconductor (MOS) devices. Si samples with 50 nm SiO2 layers were irradiated with ion beams of ArQ+ (Q = 4, 8, and 11) at normal incidence. The effects of the irradiation were encapsulated with an array of Al contacts forming the MOS structure. High frequency capacitance–voltage (CV) measurements reveal that the HCI irradiation results in stretchout and shifting of the CV curve. These changes in the CV curve are attributed to dangling Si bond defects at the Si/SiO2 interface and trapped positive charge in the oxide, respectively. Charge state dependencies have been observed for these effects with the CV curve stretchout having a dependence of Q∼1.7 and the CV curve shifting with a dependence of Q∼1.8. These dependencies are similar to the results of previous studies focused on the Q-dependence of the stopping power of HCIs.

Probing kinetically excited hot electrons using Schottky diodes

Hot electron generation was measured under the impact of energetic Ar and Rb ions on Ag thin film Schottky diodes. The energy- and angular-dependence of the current measured at the backside of the device due to ion bombardment at the frontside is reported. A sharp upturn in the energy dependent yield is consistent with a kinetic emission model for electronic excitations utilizing the device Schottky barrier as determined from current–voltage characteristics. Backside currents measured for ion incident angles of ±30° are strongly peaked about 0° (normal incidence) and resemble results seen in other contexts, e.g., ballistic electron emission microscopy. Accounting for the increased transport distance for excited charges at non-normal incidence, the angular results are consistent with the accepted mean free path for electrons in Ag films.