C. P. Burrows

Simple expression for vacancy concentrations at half ion range following MeV ion implantation of silicon

Mean concentrations CD of aggregated vacancy-type point-defect structures in float-zone Si implanted with H+, B+, Si+, O+, and Ge2+ ions at energies between 0.45 and 4.0 MeV have been measured as a function of ion dose φ at depths ∼RP/2 (half projected ion range) by beam-based positron spectroscopy. By adjusting φ to φA using factors given by the code TRIM, one arrives at the universal expression CD=(2.79×1010) φA0.63; CD (cm−3) can be estimated to ±50% for MeV ions implanted for φA from 109×1013 cm−2, which corresponds to an upper limit dose approaching 1014 cm−2 for 2 MeV Si+ implantation.

Characterization of thermally oxidized Ti/SiO2 gate dielectric stacks on 4H-SiC substrate

The structural and electrical characteristics of thermally oxidized Ti∕SiO2 gate dielectric stacks on 4H–SiC substrates have been investigated. X-ray photoelectron spectroscopy shows a good stoichiometry of TiO2 films formed by thermal oxidation of evaporated Ti. No evidence of the formation of titanium silicide at the surface as well as in the interfacial layer was observed. Electrical measurements show, in particular, no signature of an increase in interface state density towards the conduction band edge of 4H–SiC. The improved leakage current with higher breakdown field of 11MV∕cm makes TiO2∕SiO2 stacks a potential gate insulator for high-power SiC devices.

Photoluminescence response of ion-implanted silicon

This work was supported by EPSRC Grant No. GR/ R10820/01 and by the EU Co-ordination Action programme CADRES. One of the authors J.W.-L. acknowledges the support of the Australian Research Council.

Identification by photoluminescence and positron annihilation of vacancy and interstitial intrinsic defects in ion-implanted silicon

This work was supported by EPSRC Grant No. GR/R 10820/01 and by the EU Coordination Action program CADRES. One of the authors J.W.L. thanks the Australian Research Council for a fellowship.

Vacancy-type defects in TiO2/SiO2/SiC dielectric stacks

Open-volume (vacancy-type) point defects have been observed in ∼80‐nm-thick titanium dioxide films grown on silicon dioxide∕4H silicon carbide substrates as stacks with high dielectric constant for power device applications, using variable-energy positron annihilation spectroscopy. The concentration of vacancies decreases as the titanium dioxide growth temperature is increased in the range from 700to1000°C, whereas grain boundaries form in the polycrystalline material at the highest growth temperatures. It is proposed that the optimal electrical performance for films grown at 800°C reflects a balance between decreasing vacancy concentration and increasing grain boundary formation. The concentration of vacancies at the silicon dioxide∕silicon carbide interface appears to saturate after 2.5h oxidation at 1150°C. A supplementary result suggests that the quality of the 10‐μm-thick deposited silicon carbide epilayer is compromised at depths of about 2μm and beyond, possibly by the migration of impurities and∕o...

Enhanced nitrogen diffusion in 4H-SiC

Experimental evidence is given for boron (B) enhanced diffusion of nitrogen (N) in ion-implanted 4H silicon carbide (4H-SiC), when a nitrogen implant is co-doped within an existing boron p-type well. The co-implanted nitrogen is shown to diffuse continuously with time when samples are annealed at 1600 °C—with little movement of the boron p-well implant profile. An effective nitrogen in boron diffusivity at 1600 °C is determined to be at least 60 times larger than that of a mono-doped nitrogen implant.

Trap assisted conduction in high K dielectric capacitors on 4H-SiC

The characteristic of trap assisted conduction and interface states for a Pd/TiO2/SiO2/SiC structure has been investigated at temperatures up to 500 °C. Thermally oxidized Ti/SiO2 gate capacitors fabricated by dry oxidation in O2 were studied. The electrical measurements show the current conduction through this capacitor structure is controlled by a trap assisted conduction mechanism at low bias and the barrier height (φA) between the metal and the TiO2 was extracted. The current density in the dielectric stacks is also shown to be strongly temperature dependent. The results demonstrate that the formation of a charge dipole under the Pd contact is responsible for barrier height and not any changes in the behaviour of the TiO2 film itself, such as a change in concentration of trapping centres. The reported results indicate electron trapping property across the SiO2 layer is consistent with fitting experimental results to the trap assisted conduction model.

Optimization of measurement parameters in Doppler broadening spectroscopy

Abstract The choice of regions of interest (ROIs) in the annihilation gamma line spectrum, which define the S and W parameters commonly used in the study of vacancy-type defects in Si, has been optimized by simulations using theoretical Doppler-broadened spectra. The figure of merit used is the difference between the S ( W ) parameter and its value in bulk Si expressed as multiple of the standard deviation in S ( W ). The optimum ROIs depend on the crystalline orientation of the Si and the energy resolution of the gamma ray detector used. Experimental data on B + -implanted Si were found to be consistent with the simulation results, which can therefore be used reliably to guide further experimental work.

Room-temperature evolution of vacancy-type damage created by 2 keV B+ implantation of Si

Beam-based positron annihilation spectroscopy has been applied to the study of near-surface vacancies created by 2 keV B+ ions implanted into Cz Si. The use of a controllable-energy positron beam means that the probe can be tuned to maximize the response to the subsurface damage. Time-dependent changes have been observed in the near-surface vacancy concentration profile. For example, after one week at room temperature, exposure of an implanted sample to white light for 1 h resulted in the migration of ∼95% of the measurable damage to sinks—primarily, it is assumed, to the surface. The relative importance of temperature, air, and light has been investigated.

A new tool for nondestructive monitoring of ion implantation

We describe recent research designed to assess the potential of charged particle beams as an implant-monitoring tool. The technique currently under development, based on positron annihilation spectroscopy, is totally nondestructive, sensitive to implant doses as low as 10/sup 9/ cm/sup -2/, and most importantly, can be tuned to obtain depth-resolved information. A collaborative project between the Surrey Ion Beam Centre and researchers at the University of Bath is underway, aimed at developing a tool suitable for use in the semiconductor industry. A prototype instrument is described, together with preliminary measurements using a laboratory based instrument. Applications to other vacancy-mediated processes, and the value of the new instrument in a research and development environment is discussed.