Solid State Phenomena Vols. 205-206

Abstract: The contributions of vacancies and self-interstitials to silicon (Si) self-diffusion are a matter of debate since many years. These native defects are involved in dopant diffusion and the formation of defect clusters and thus influence many processes that take place during Si single crystal growth and the fabrication of silicon based electronic devices. Considering their relevance it is remarkable that present data about the properties of native point defects in Si are still limited and controversy. This work reports recent results on the properties of native point defects in silicon deduced from self-diffusion experiments below 850°C. The temperature dependence of silicon self-diffusion is accurately described by contributions due to vacancies and self-interstitials assuming temperature dependent vacancy properties. The concept of vacancies whose thermodynamic properties change with temperature solves the inconsistency between self-and dopant diffusion in Si but further experiments are required to verify this concept and to prove its relevance for other material systems.

Abstract: Vacancies (and probably also self-interstitials) in silicon appear to exist in several forms (atomic configurations) some of them being fast diffusers and other slow diffusers. The data on enhanced self-diffusivity under proton irradiation, on vacancy and oxide precipitate profiles installed by Rapid Thermal Annealing, and on the self-diffusivity under equilibrium conditions suggest that there are at least two kinds of vacancy: 1) V_w - a fast-diffusing localized vacancy manifested in electron irradiated samples (Watkins vacancy), 2) V_s - a slow-diffusing extended vacancy manifested under hot proton irradiation. In RTA experiments, these two species behave as one equilibrated subsystem of a moderate effective diffusivity intermediate between those of V_w and V_s. There is also strong evidence in favor of a third kind of vacancy: V_f a fast extended species, which controls the grown-in voids in silicon crystals.

Abstract: For the development of the crystal pulling process for 450 mm-diameter defect-free Si crystals, the impact of thermal stress on intrinsic point defect behavior during crystal growth is studied using extensive density functional theory calculations. The impact of thermal stress on the so-called Voronkov criterion and on void formation is clarified and compared with published experimental results.

Abstract: In the framework of a unified approach the diffusion coefficient (the prefactor and activation barrier) of an interstitial oxygen O_i, the hydrogen molecule H₂, vacancy, oxygen dimer in silicon crystals and O_i in Si_1-xGe_x solid solutions, silicon nanotubes and nanowires has been calculated. For all the above cases, the calculated values of the diffusion coefficient are in good agreement with the experimental data. The calculated equilibrium structures, electrical activity, the vibrational spectrum, the mechanism of diffusion of oxygen dimer fully describe the experimental results. Our study has revealed that the diffusivity of impurities (defects) in alloys can decrease considerably and this variation results from the fact that the prefactor depends on the concentration of component elements of the alloy.

Abstract: The data obtained recently from combined deep-level-transient spectroscopy (DLTS), local vibrational mode (LVM) spectroscopy and ab-initio modeling studies on structure, electronic properties, local vibrational modes, reconfiguration and diffusion paths and barriers for trivacancy (V₃) and trivacancy-oxygen (V₃O) defects in silicon are summarized. New experimental results on the introduction rates of the divacancy (V₂) and trivacancy upon 4 MeV electron irradiation and on the transformation of V₃ from the fourfold coordinated configuration to the (110) planar one upon minority carrier injection are reported. Possible mechanisms of the transformation are considered and discussed.

Abstract: The inverted structure of the 1s ground state of lithium in silicon provides a unique opportunity to study inter-valley spin-orbit interactions of donor electrons. A study of lithium doped silicon enriched in the ²⁸Si isotope with a low oxygen content (N 210¹⁴ cm³) has demonstrated at low temperatures (T = 3.8 K) a family of electron spin resonance (ESR) spectra with anisotropic g factors associated with Li donor centres. The spectra were investigated without and with application of external stress to the sample and their g factors were found to be less then 2.000. The analysis of experimental data and numerical simulation of the spectra and their angular dependencies in the second order perturbation theory assuming the splitting of states due to internal strains in the crystal is larger than the Zeeman and spin-orbit splitting have shown that the spectrum, having g tensor components corresponding to the tetragonal symmetry, consists of two lines belonging to the triplet state T₂, and the other two lines in the spectrum have an angular dependence behaviour of the doublet states E. The ratio of the inter-valley spin-orbit coupling λ and λ' to the Δ parameters, characterizing the splitting of the states under internal strains have been defined. From the dependencies of the triplet ESR lines intensity on the compressive stress of the crystal along the [11 it was obtained the value of the internal strains, which allowed to determine the parameters of the spin-orbit coupling λ and λ'. Their values were found to be three orders of magnitude smaller than were obtained earlier for Li spectra with g > 2. Since experimentally observed Δ value was of the order of the Zeeman splitting parameter the spectra were analyzed using the full matrix of the spin Hamiltonian for the fivefold degenerate ground state. We found that angular dependencies of the spectra observed for the triplet and doublet states with g < 2 are well described by the solutions of the spin Hamiltonian with parameters λ and λ' obtained from our experimental data. At the same time, we are not able to find solutions that satisfy the data obtained for the spectra with g > 2.000 in previous studies.

Abstract: Vibrational spectroscopy has been used to investigate the properties of hydrogen in multicrystalline Si (mc-Si) and its interactions with carbon impurities that can be present with high concentration. The properties of point defects containing N and O have been investigated by way of their vibrational properties and their far-infrared electronic transitions.

Abstract: Unexpectedly large isotope effects have been reported for the vibrational lifetimes of the H-C stretch mode of the CH₂^* defect in Si and the asymmetric stretch of interstitial O in Si as well. First-principles theory can explain these effects. The results imply that defects trap phonons for lengths of time that depend on the defect and sometimes on its isotopic composition. Some consequences of phonon trapping are discussed.

Abstract: Deep level transient spectroscopy (DLTS) has been applied to study the formation of trivacancy-oxygen complexes in Si. Samples of p-type Czochralski grown (Cz) silicon, with a boron doping concentration of ~1x10¹⁵ cm^-3, have been irradiated at room temperature (RT) with 1.8 MeV protons to a dose of 5×10¹² cm^-2. Two new energy levels at E_v+0.24 eV and E_v+0.11 eV (E_v denotes the valence band edge) emerge when the divacancy (V₂) to divacancy-oxygen (V₂O) transition takes place during post-irradiation annealing. The concentration is ~30% relative to V₂ (or V₂O) and further, the two new levels exhibit an almost one-to-one correlation in strength. The present results strongly support that both levels are related to the same defect with a possible identification as single and double donor states of V₃O.

Abstract: The evolution of radiation-induced carbon-oxygen related defects with the fluence of MeV electrons and upon subsequent isochronal annealing (75-350 °C) in Si crystals with different carbon and oxygen content has been studied by means of Local Vibrational Mode (LVM) spectroscopy. In particular, the generation kinetics of the bands at 998 and 991 cm^-1 is considered and additional arguments supporting their previous assignment to the I₂C_iO_i and I₃C_iO_i complexes are found. An annealing behaviour of the LVMs related to the C4 (IC_iO_i) defect has been studied in the various Si crystals irradiated with different particles. In all the samples studied the bands at 940 and 1024 cm^-1 are found to disappear at about 200 °C while three new LVM bands, at 724 cm^-1 (O-related) and at 952 and 973 cm^-1 (both C-related) are emerging. Further increase in annealing temperature up to 250-275 °C results in a transformation of the latter bands into another set of LVM bands at 969 cm^-1 (O-related) and at 951 and 977 cm^-1 (both C-related). These bands disappear upon annealing in the temperature range 300-325 °C. The ratios of all the bands intensities as well as their transformation rates do not depend on the oxygen and carbon content in the Si samples nor on the kind of irradiation (2.5 and 10 MeV electrons, fast neutrons) and irradiation doses. These facts confirm our previous conclusion that all the above-mentioned LVMs arise from the C4 defect being in different configurations (IC_iO_i, IC_iO_i*, and IC_iO_i**).