Solid State Phenomena Vols. 131-133

Abstract: Density functional theory (DFT) with local density approximation has been used to calculate the formation energy (EF) of the neutral vacancy in germanium single crystal. It was shown that careful checking of convergence with respect to the number of k-points is necessary when calculating the formation energy of the intrinsic point defects in Ge. The formation energy of the single neutral vacancy was estimated at 2.35 eV which is in excellent agreement with published experimental data.

Abstract: Using first principles molecular dynamics and Nudged Elastic Band calculations, we have investigated the effect of irradiation on cubic silicon carbide at the atomic scale, and in particular the formation of Frenkel pairs, and the crystal recovery after thermal treatment. Threshold displacement energies have been determined for C and Si sublattice, and the stability and structure of the formed Frenkel pairs are described. The activation energies for annealing these defects have then been computed and compared with experiments.

Abstract: The properties of point defects introduced by low temperature electron irradiation of germanium are investigated by first-principles modeling. Close Frenkel pairs, including the metastable fourfold coordinated defect, are modelled and their stability is discussed. It is found that damage evolution upon annealing below room temperature can be consistently explained with the formation of correlated interstitial-vacancy pairs if the charge-dependent properties of the vacancy and self-interstitial are taken into account. We propose that Frenkel pairs can trap up to two electrons and are responsible for conductivity loss in n-type Ge at low temperatures.

Abstract: Recently, the interaction of copper with dislocations in p-type Si/SiGe/Si structures has been investigated experimentally and a new dislocation related DLTS-level at Ev +0.32 eV was detected after intentional contamination with copper. To determine the origin of this newly detected level, in this work we present first density functional calculations of substitutional copper at 90◦ and 30◦ partial dislocations in silicon. Defect–dislocation binding energies are determined and electrical gap levels are calculated and compared with the experimental data. As a result, the observed level at Ev + 0.32 eV is tentatively assigned to the single acceptor level of substitutional copper at the dislocation.

Abstract: In this paper we investigate the formation of interstitial nitrogen trimers N3 which have been suggested as a fast-diffusing species in silicon recently. Out-diffusion profiles of nitro- gen show the involvement of at least two independent nitrogen related defects in the diffusion process depending on the nitrogen concentration at different depths of the sample. When the nitrogen concentration is small it is proposed that nitrogen trimers are formed in a two step process. We present the structural properties of such a defect using density functional theory and examine the energetics of the two proposed reactions leading to the formation of N3.

Abstract: A theoretical modeling of the diffusion of self-interstitials in silicon and germanium crystals both at normal and high hydrostatic pressure has been carried out using molecular mechanics, semiempirical (PM3, PM5) and ab-initio (SIESTA) methods. According to the simulation for the Si and Ge neutral interstitials (I0) both in silicon and germanium crystals more stable configuration is <110> split interstitial. T is the stable configuration for the double positive interstitial I++, but the interstitial is displaced from the high-symmetry site. Stability of <110> splitinterstitial is not changed under hydrostatic pressure. The activation barriers for the diffusion of interstitials were determined and equal to ΔEa(Si)(<110> -> T1)=0.69 eV; ΔEa (Ge)(<110> -> T1)=1.1 eV. For mixed interstitials the calculated activation barriers equal Si Emix = 1.06 eV, Ge Emix = 0.86 eV. Hydrostatic pressure decreases the activation barriers ΔEa(Si), ΔEa (Ge).

Abstract: We have developed a diffusion and activation model for implanted arsenic in silicon. The model includes the dynamic formation of arsenic-vacancy complexes (As4V) as well as the precipitation of a SiAs phase. The latter is mandatory to correctly describe concentrations above solid solubility while the former are needed to describe the reduced electrical activity as well as the generation of self-interstitials during deactivation. In addition, the activation state after solid-phase epitaxy and the segregation at the interface to SiO2 are taken into account. After implementation using the Alagator language in the latest version of the Sentaurus Process Simulator of Synopsys, the parameters of the model were optimized using reported series of diffusion coefficients for temperatures between 700 °C and 1200 °C, and using several SIMS profiles covering annealing processes from spike to very long times with temperatures between 700 °C and 1050 °C and a wide distribution of implantation energies and doses. The model was validated using data from flash-assisted RTP and spike annealing of ultra-low energy arsenic implants.

Abstract: The features of microdefect formation during dislocation-free Si single crystals are considered in connection with the specific thermal CZ growing conditions. For this purpose the thermal crystal growth histories are calculated by means of a global thermal mathematical model and then on their basis the intrinsic point defect recombination and microdefect formation are modeled numerically. Difficulty of such integrated approach is explained by of the complicated and conjugated thermal modeling and a presence of various temperature zones in growing single crystal, answering to various defect formation mechanisms.

Abstract: The coherent agglomeration of interstitial oxygen into single-plane and double-plane plates can explain the two peaks in the M-shaped nucleation curves in Czochralski silicon. The density of nucleation sites for the double-plane plates corresponds to the VO2 concentration. Ab initio calculations have shown that the agglomeration of oxygen atoms in single-plane and doubleplane plates is energetically favorable. These plates are under compressive strain. VO2 agglomeration plays only a minor role for modeling the M-shaped nucleation curves because of prior homogenization treatments. It is of much higher impact if as-grown wafers are subjected to nucleation anneals because of the higher vacancy concentration which was frozen in during crystal cooling. This results in higher nucleation rates at higher temperatures. Because the oxygen diffusivity below 700 °C is important for the nucleation rate and many controversial results about the diffusivity in this temperature range were published, we have analyzed the data from literature. We have demonstrated that the effective diffusivity of oxygen at temperatures below 700 °C which corresponds to the quasi equilibrium dimer concentration is very similar to the extrapolation from oxygen diffusivity at high temperature. The high effective diffusivities from out-diffusion and precipitation experiments, and the somewhat lower effective diffusivities from dislocation locking experiments are the result of an ongoing formation of fast diffusing dimers because the equilibrium is disturbed as the result of the strongly increasing difference in the diffusion length between interstitial oxygen and the fast diffusing dimer with decreasing temperature.