Edward H. Poindexter

Characterization of Si/SiO2 interface defects by electron spin resonance

Abstract The defect structure of the Si/SiO 2 interface is increasingly important as the size of metal-oxide-semiconductor (MOS) integrated circuits shrinks into the submicron regime. Extensive electrical studies of the interface over the past two decades, interpreted via indirect physico-chemical theory, have yielded useful, but empirical, interface models. Electron spin resonance (ESR) has now given a spectroscopic identification of the trivalent silicon or P b center (·Si≡Si 3 ) at the interface. This center is perhaps the most important characteristic defect at the interface. Its specific detection and identification allow a better diagnosis of interface and oxide electrical properties. This paper reviews the present status and historical development of ESR application to the Si/SiO 2 system, and includes a background of relevant ESR research on other materials systems. A very brief overview of popular electrical characterization methods is included, and also a short review of the basic principles of ESR spectroscopy. The detection and identification of the critical trivalent silicon defect (·Si≡Si 3 ) on oxidized Si wafers (111, 110, 100 orientations) is presented in detail. The correlation of this center with interface traps is shown over a variety of device-pertinent thermal processes. The nearly 1:1 quantitative relation between ·Si≡Si 3 and interface trap concentration is emphasized. The response of ·Si≡Si 3 to light and electric field is explored in order to define its physical and electrical nature. These results, in comparison with similar defects in bulk Si and SiO 2 , are interpreted to yield a tentative working model of the ·Si≡Si 3 interface defect. It is thus shown to be a plausible source for the majority of interface bandgap traps. A variety of other pertinent ESR centers in oxidized Si, including radiation-induced defects, is surveyed briefly. A few oft-expected centers, such as the silica E′ center, are not found in significant concentration.

Paramagnetic defects in silicon/silicon dioxide systems

Abstract Paramagnetic defect centers in Si/SiO2 systems have been observed by direct ESR, optically-induced ESR, and NMR relaxation of liquids at the outer oxide surface. In general, all the defects reported elsewhere were confirmed, but with some significant discrepancies in character. The PB center was observable even at room temperature. The PC center was found to exist much deeper in the silicon than previously determined, and it is tentatively identified to be neutral iron. Surface liquid relaxation is very strong on oxidized crushed silicon, is not dependent on liquid composition, and suggests a strong wide-line spin center in the outer oxide surface. The optically activated spin center created by HF/HNO3 etches was found not to involve H2O or OH functionalities, and appears to be a nitrogenous radical. The optical defect center lies within the silicon, and its presence warrants caution in use of HNO3-based etches in wafer processing. Oxides prepared at elevated pressures show fewer PA and PC defects than those produced by conventional processing, which indicates potential merit in pressure oxidation methods.

Dynamic Polarization of Fluorine Nuclei in Solutions of Selected Free Radicals

Dynamic polarization of fluorine nuclei has been examined with 14 free radicals dissolved in hexafluorobenzene (C6F6) and 1,1,1‐trifluoro‐2,2,2‐trichloroethane (CF3CCl3). The radicals may be broadly grouped by their tendency to produce either positive or negative fluorine NMR enhancement. A large positive enhancement indicates that Fermi‐contact coupling between the nucleus and the radical electron is the dominant nuclear relaxation mechanism; a large negative enhancement indicates that dipolar coupling is dominant. The correlation times for dipolar and contact relaxation components are comparable, and approximately the values expected from molecular diffusion. The intermolecular contact coupling may be transmitted by exchange polarization of bonding electrons on the fluorocarbon, or by charge transfer between radical and fluorocarbon, or by a combination of both mechanisms. Steric shielding appears to weaken the contact interaction in certain cases; but it is not the sole physical influence governing the...

Dynamic Nuclear Polarization by Nitroxide, Perhalocarbon, Semiquinone, and Verdazyl Radicals

Dynamic nuclear polarization has been examined in liquid solutions of 16 different radicals. Hexafluorobenzene was used to gauge the intermolecular hyperfine interaction. The scalar coupling reveals transient complexation between radical and receptor and various bond‐forming tendencies. Perhalocarbon radicals show very weak scalar coupling with fluorine nuclei. Verdazyls show strong scalar coupling, with negligible effects from substituents on the outer phenyl rings. Phenoxyl and pyrylyl radicals show a wide range of coupling which is only partly explained by planar complexation. Semiquinones show increasing scalar coupling with increasing chlorination. DNP with aliphatic nitroxide radicals is dominated by the NO group, but aromatic nitroxides show a strong ring substituent effect, exceeding intramolecular hyperfine variation in ESR. It is suggested that DNP can provide unique signatures of substituents on phenyl rings.

Dynamic Nuclear Polarization: Collision Mechanics in Fluorocarbon Solutions

Dynamic polarization measurements at three widely different magnetic field strengths are presented for an array of chemically different free radicals and fluorocarbons. Observed fluorine NMR signal enhancements ranged from −u2009225 to +u2009430. The scalar hyperfine correlation time, which is shown to be the major factor governing the enhancement, is systematically longer for systems showing higher positive enhancement at low field. A generalized pulse‐transform collision model is applicable to all cases, and it is found that the slope of the scalar spectral cutoff becomes progressively less steep as low‐field scalar coupling increases. In extreme cases, the spectrum may be interpreted as a superposition of at least two component spectra, each corresponding to a pulse from a stereospecific collision attitude. The observed behavior is consistent with molecular orbital calculations. The range of computed hyperfine coupling energies is also in good qualitative agreement with observed differences between fluorocarbo...

Overhauser Effect in Asphalt Solutions

We have discovered that solutions of asphalt in a suitable solvent will show an Overhauser effect. The effect is qualitatively similar to that in crude oil. It arises from a dipolar interaction between the unpaired spins in the asphaltic free radicals, and the protons of the solvent molecules. The contribution to proton relaxation from the asphalt is a linear function of asphalt concentration. The function relating proton signal enhancement to electron saturating rf field strength is found to vary with asphalt concentration. A plot of enhancement versus electron‐stimulating frequency at constant H0 yields a curve similar to the EPR absorption line. The large size of the asphalt radicals does not appear to have any direct significance with respect to the Overhauser interaction.

Dynamic polarization in fluorocarbons: Effect of scalar correlation time

Multi-field nuclear-electron double resonance experiments with octafluoronaphthalene (OFN) solutions containing organic free radicals have led to the conclusion that independent fluorine scalar and dipolar correlation times are necessary to characterize these systems. Scalar correlation times were longer than dipolar times for systems with strong scalar coupling and shorter than dipolar times for systems with weak scalar coupling. This trend may be associated with the availability of the odd electron at the radical edge in conjunction with the ability of particular radical-solvent pairs to form transient complexes. Further, it signifies that a large part of the variation in F signal enhancement arises from changes in scalar correlation time. Largest observed F enhancements were + 390 for OFN with 2,3,5,6-tetrachlorosemiquinone in acetone at 1 gauss and + 435 for OFN with 2,4,6-triphenyl pyrylyl in acetone at 74 gauss.

Relaxation of fluorine nuclei by collisions with free radicals

Intermolecular nuclear relaxation during liquid‐state collisions has been examined for 16 combinations of four free radicals and four fluorocarbons. The spectra of electron‐induced scalar relaxation transitions are resolved into high‐ and low‐frequency components. All samples are found to have a spectral component with a short correlation time of around 1×10−11 sec, corresponding to fast diffusion‐controlled collisions. Those samples which show stronger scalar hyperfine coupling show a second component with time up to 44×10−11 sec, corresponding to sluggish stereospecific collisions or transient complexation. Strong scalar coupling is found to occur with a proportionally strong dipolar coupling, which is augmented well beyond that expected from feasible molecular approach distances. Monopoly of spin‐rich sites by the fluorocarbon might account for part of the dipolar augmentation but does not satisfactorily account for the entire range of observations. An induced anisotropic hyperfine coupling, after the ...

Dynamic Polarization in Asphaltene Solutions

Dynamic polarization experiments in solutions of asphaltene in xylene have been performed at both low and high magnetic fields. The interpretation of the experiments leads to the conclusion that the EPR line of the asphaltene is inhomogeneously broadened. The characteristics of the spin packet are deduced. In particular, the spin‐packet width is found to be constant for many different experimental conditions. Specific information is obtained on the residence time of a xylene molecule on the asphaltene particle.

Dynamic Polarization of 31P Nuclei: Some Evidence for Stereospecific Hyperfine Interactions in Liquids

Dynamic nuclear polarization parameters were compared for all combinations of three free radicals and eight representative phosphorus compounds. The chemical nature of the phosphorus compound is shown to be much more significant than that of the radical in determining the degree of scalar coupling. This contrasts sharply with fluorine, where the radical was equally important, and reflects the more complex geometry and bonding associated with phosphorus compounds. The degree of scalar coupling can be correlated with stereospecific interactions between phosphorus substituents and sites of high spin density on the radical, suggesting the formation of virtual transient bonds.