P. G. Coleman

Leakage current and charge trapping behavior in TiO2/SiO2 high-kappa gate dielectric stack on 4H-SIC substrate

The TiO2∕SiO2 gate dielectric stack on 4H‐SiC substrate has been studied as a high-κ gate dielectric for metal-oxide semiconductor devices. X-ray photoelectron spectroscopy confirmed the formation of stoichiometric TiO2 films. The leakage current through the stack layer was investigated and it has been shown to be a double conduction mechanism. At low fields, the current is governed by properties of the interfacial layer with a hopping like conduction mechanism, while at relatively high electric field, carriers are modulated by a trap assisted tunneling mechanism through traps located below the conduction band of TiO2. The current-voltage characteristics, time evolution of charge transport, and capacitance-voltage behaviors under constant voltage stressing suggest the composite effect of electron trapping and positive charge generation in the dielectric stack layer.

Microvoids in chalcogenide glasses studied by positron annihilation

Abstract Microvoids have been detected in glassy As2Se3 by positron lifetime spectroscopy. The positron lifetime in perfect and defected crystalline As2Se3 was calculated and the results correlated with the size of the (open volume) defects. This correlation suggests that the observed mean lifetime τ = 345(2) ps for the glassy material corresponds to microvoids of on average about three missing atoms (i.e., 2.5–3 A radius). The lower limit of the microvoid concentration per atom is estimated to lie between 0.1 and 100 ppm (corresponding to a volume fraction 10−5−10−2%), well below the detection threshold of small-angle X-ray scattering. The microvoids are stable on heating to 260°C, i.e., after annealing at temperatures 80°C above the glass transition temperature. The present results are in conflict with earlier suggestions that positrons are trapped only in the vicinity of point-like negatively charged defects in amorphous As2Se3. Frequent observation of long positron lifetimes leads to the conclusion that microvoids are a feature common to a wide range of chalcogenide glasses. The consequence of these results for the recent model of Elliott, in which the first sharp diffraction peak in the total structure factor of covalently bonded glasses is attributed to the chemical ordering of microscopic voids around cation-centred units, is discussed.

Optical attenuation in defect-engineered silicon rib waveguides

The excess optical attenuation at wavelengths around 1550nm induced by subamorphous dose ion implantation of silicon-on-insulator rib waveguides has been quantified. Optical attenuation is related to the introduction of lattice defects such as the silicon divacancy. After 2.8MeV Si+ implantation at a dose of 2.5×1014cm−2, the attenuation is greater than 1000dBcm−1. Using positron annihilation spectroscopy to determine the vacancy-type defect concentration, it is demonstrated that the absorption component of the excess attenuation can be predicted using a simple analytical expression. Additional losses are suggested to result from a defect induced change in the real part of the refractive index of the silicon waveguide. A processing strategy for ensuring that the absorption component dominates the excess attenuation is described, and it is shown that selective implantation of a relatively low dose of inert ions is an efficient method for the reduction of optical cross talk in silicon photonic circuits.

The equivalence of vacancy-type damage in ion-implanted Si seen by positron annihilation spectroscopy

The response of the Doppler-broadened annihilation linewidth parameter S to ion dose has been measured using a controllable-energy positron beam for n-type Fz–Si(100) implanted with 120 keV Ge+, 400 keV O+, 200 keV He+, and 450 keV H+ ions. For all ions S increases with dose, indicating the presence of vacancy-type defects. It is found that the maximum S values for the Ge+, O+, and H+ implants can be normalized to one well-defined distribution using values of predicted vacancy concentration from the Monte Carlo code TRIM. This result implies that vacancy-type defects from these three implantations are similar in both structure and interstitial recombination rate. The He+ data do not lie perfectly on the universal distribution; it is suggested that this is a result of defect passivation by the implanted He.

Growth temperature dependence for the formation of vacancy clusters in Si/Si0.64Ge0.36/Si structures

The incorporation of vacancy clusters and vacancy point defects during the growth of Si/Si0.64Ge0.36/Si structures has been observed for growth temperatures between 250 °C and 550 °C using positron annihilation spectroscopy. A strong correlation between the electrical characteristics of the structures and the size and concentration of the clusters is observed. For the onset of two-dimensional hole gas behavior, a defect concentration less than 5×1016 cm−3 is required. A further reduction in concentration below 1×1016 cm−3 results in optimum electrical performance. The depth at which defects are observed increases with decreasing growth temperature indicating defect mobility during growth or subsequent annealing.

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.

Post-implantation annealing of SiC studied by slow-positron spectroscopies

The effects of post-implantation annealing of damage in 6H-SiC caused by ion implantation at two different fluences have been studied by monoenergetic positron Doppler broadening and lifetime techniques. The measurements are supported by new calculations of positron lifetimes in vacancy clusters in SiC. At both fluences two defected layers are identified and characterized by depth and defect type as a function of annealing temperature. The results indicate that it is impossible to remove the radiation damage by annealing at temperatures up to .

Fluorine-vacancy complexes in ultrashallow B-implanted Si

Shallow fluorine-vacancy (FV) complexes in Si have been directly observed using variable-energy positron annihilation spectroscopy and secondary ion mass spectrometry. The FV complexes, introduced to combat the deactivation and transient-enhanced diffusion of ultrashallow boron, were observed in preamorphized Si wafers implanted with 0.5keV B and 10keV F ions at a dose of 1015cm−2, and then annealed isothermally at 800°C for times ranging from 1to2700s. The results are in agreement with a model which predicts that the complexes are of the form F3nVn, with n most probably being 1 and/or 2.

Silicon carbide: a new positron moderator

Observation of copious positron re-emission from crystalline 6H-SiC, with no pre-treatment and without the need for ultra-high-vacuum conditions, suggests that this material may form the basis of an important new moderator for the production of monoenergetic positrons. Its positron work function is measured to be eV. Its electrical characteristics point to SiC as a prime candidate for development as a field-assisted positron moderator, producing moderately intense slow-positron beams in laboratory-based systems and enabling a new generation of positron experimentation.