Josef Kuběna

X-ray diffuse scattering from defects in nitrogen-doped Czochralski grown silicon wafers

Nitrogen doping during crystal growth is used to create nitrogen–vacancy(–oxygen) complexes. These complexes enhance the nucleation of silicon oxide precipitates. The precipitates and other volume defects in silicon wafers serve as gettering centres for metal impurities during the device processing. We have studied nitrogen-doped silicon wafers (001) from the origin, middle and end of the ingot, annealed at low (750°C) and high (1050°C) temperatures, using triple-axis high-resolution x-ray diffraction. The reciprocal space intensity distributions from clusters, stacking faults and dislocation loops were modelled using the Krivoglaz theory and a continuum model of the defect deformation field. Good agreement of the theory with the experimental data was achieved for the model of dislocation loops. The symmetry of measured reciprocal space maps determines the type of dislocation loops and from extracted linear scans in the 111 direction we can obtain the radius and concentration of the loops. These parameters were combined with the results from selective etching and infrared absorption spectroscopy. Concentration of interstitial oxygen, shape, stoichiometry and volume fraction of precipitates were obtained from absorption spectra taken at room and liquid nitrogen temperature.

Stacked Josephson Junction Based on Nb/Si Superlattice

We report on the technology and basic electrical properties of ten-fold stacked Nb/Si/Nb Josephson junction (JJ). The problem of making a large number of stacked JJs with identical properties was solved by metallic superlattice preparation technology. The uniformity of Si barriers thickness was examined by low angle X-ray diffraction and cross-sectional transmission electron microscopy. To prevent pinholes in the Si barriers the Nb/Si superlattice was sputtered at the regime at which smoothing of interfacial roughness occurs. The stacked junction exhibit both ac and dc Josephson effects. In the Ic(B) diffraction pattern there is an extra periodicity of about 2–3 G in addition to a larger period of about 21 G.

The diffuse X-ray scattering in real periodical superlattices

Abstract The X-ray diffraction in real periodical superlattices has been described using a four-dimensional formalism on the basis of the Takagi diffraction theory and an optical coherence approach. This method enabled us to study the diffraction properties of real superlattices containing rough interfaces and other structural defects (precipitates etc.). The interfaces have been described by a statistical model, which takes into account lateral correlations at a rough interface. The theoretical conclusions have been verified by X-ray diffraction measurements on a GaAs AlAs superlattice using a double crystal and a triple crystal diffractometers. From the reflection curve of the 0-th diffraction satellite and from the integrated reflectivities of non-zero satellites the averaged parameters of the lattice have been determined. The statistical parameters of the interfaces were estimated from the shapes of non-zero diffraction peaks.

Analysis of vacancy and interstitial nucleation kinetics in Si wafers during rapid thermal annealing.

The successful application of the rapid thermal annealing (RTA) process for creation of a magic denude zone in individual Czochralski silicon wafers is based on vacancy controlled oxygen precipitation. The kinetics of the vacancy and self-interstitial processes in Si wafers are studied in this paper. Detailed insight into nucleation processes, out-diffusion and vacancy-interstitial recombination during the RTA leads to a new model of interaction between vacancies and oxygen. The calculation of the distribution function of these defects follows from modified Becker-Döring equations transformed for vacancies and interstitials and extended by diffusion and recombination terms. The new model, which includes the vacancy influence on oxygen nucleation and which follows from this theoretical analysis, corresponds very well to the experimental properties of formation of bulk micro-defects during RTA processes.

Development of oxide precipitates in silicon: calculation of the distribution function of the classical theory of nucleation by a nodal-points approximation

The classical theory of nucleation in solids is mathematically expressed by a system of differential equations for temporal development of cluster distribution (sizes and their concentration). Cluster sizes reach hundreds of nanometers during long annealing times, requiring us to deal with up to 107–108 differential equations. The full numerical simulation grows linearly with the number of equations, making the numerical solution extremely time-consuming. In this paper we develop a nodal-points approximation method with a logarithmic efficiency, which allows us to calculate the cluster distribution very quickly. The method is based on modified Becker–Doring equations solved precisely only within a given set of nodal points and approximated in between them. Availability of the method is shown by monitoring the kinetics of oxygen precipitation in Czochralski silicon for the case of a three-stage annealing for 8 h at 600 °C+4 h at 800 °C+8 h at 1000 °C, where the number of monomers in the clusters reaches more than 2 × 107. Examples are discussed, mainly about the development of a concentration gap and concentration wavelet of the cluster distribution and about interstitial oxygen concentration.

Spectroscopic ellipsometry of strained Si1-x Gex layers

Abstract Spectroscopic ellipsometry was used for the determination of optical parameters and thicknesses of Si-capped Si1−x Gex layers with x = 0.10, 0.16 and 0.20, grown by ultrahigh vacuum chemical vapour deposition. The parameters of the E1 critical were determined together with the thicknesses of the surface natural oxide, the Si cap and the Si1−x Gex layers. The results were correlated with cross-section transmission electron microscopy, X-ray diffraction and reflectance measurements and compared with available data from literature.

Vacancies and self-interstitials dynamics in silicon wafers

Successful theoretical models of vacancies, self-interstitials and oxygen dynamics during an ingot growth lead us to apply these models results also to silicon wafers. The modern silicon technology uses successfully the RTA (rapid thermal annealing) in order to form MDZ (magic denude zone) in individual CZ Si wafers. The effect of RTA is based on the utilization of vacancies for control of oxygen precipitation. The question about the theory of kinetics of vacancies and interstitials, which describes its behavior within RTA, is still opened up to now. This work deals mainly with the nucleation of vacancies during RTA concerning various cooling rates and initial states.

Thermal Stability of SIS Tunnel Junction with Silicon Barrier

The thermal treatment of Nb/Si/Nb Josephson junction with high resistive amorphous silicon barrier (2nm thick) involves the interdiffusion at Nb/Si interfaces even at 250°C. In order to study these tiny interdiffusion process a superlattice [Nb(6nm)/Si(2nm)]*10 (prepared at the same technological conditions as the Josephson junction) was used for low angle x-ray spectroscopy. The interdiffusion yielding a sublayer of superconducting Nb−Si amorphous mixture at the interfaces was detected. This represents in the Josephson junction the decrease of Si-barrier thickness and consequently the increase of critical current in measured I–V characteristic.

X-ray diffraction on Fibonacci superlattices

The exact diffraction curve of the Fibonacci superlattice is calculated using the semi-kinematical approximation of dynamical X-ray diffraction. The properties of the discrete Fourier transform of quasiperiodically arranged layers are employed to derive explicit approximate formulae for the diffracted intensity and the angular positions of peaks. The exact and approximate curves are compared by a numerical simulation and a good agreement is found. The measurement of the diffraction curve was performed on the generalized Fibonacci superlattice built by stacked Fibonacci generations. This superlattice belongs to the same class of local isomorphism as the Fibonacci superlattice if both are infinitely thick. The explicit approximate formulae enabled the fitting of the structural parameters of the superlattice even in the low-resolution experimental set-up when the fitting of the whole measured diffraction curve was not possible.