Yu. L. Iunin

Influence of Nitrogen on Dislocation Mobility in Czochralski Silicon

Additions of oxygen, boron, and phosphorus are well known to affect appreciabl y the dislocation dynamics in silicon single crystals. However, there are few reports on the effect of nitrogen on the mechanical strength, despite it’s using as dopant w hen growing large diameter silicon crystals CZ-Si. We have studied the effect of nitrogen (N ) doping on the plastic properties and mobility individual dislocations in 300 mm CZ-silicon. Three point loadings were used for dislocation mobility and starting stresses measurements in the t emperature range of 500 – 700 oC. It is shown that N-doping causes the decrease of dislocation mobility and increase starting stress. Possible mechanisms of hardening of silicon single crystals are discussed due to interaction of dislocations with impurities. Introduction The interaction of dislocations with intrinsic and impurity point defec ts in silicon single crystals is still being an object of an intensive study [1-3]. One of the reasons why researches in this domain are urgent is the trend toward increasing the diameter of silic on single crystals being grown [4, 5]. This contributes appreciably to the probability of formation and motion of dislocations in wafers in the process of high temperature technologic operations owing both to an increase of ther mal str sses and weight gain wafer themselves. Therefore, the problem of mecha nical strength of silicon wafers of larger diameter is of prime consideration. Currently nitrogen is of considerable use to strengthen silicon single crystals of larger diameter. It is well known that doping with nitrogen of silicon single cry stals significantly increases the upper and lower yield points. A number of works have been concerned with this f act [6, 7]. However, the effect of nitrogen doping on the mobility of individual dislocations in si licon is less studied. So, the authors of [7] showed that in a float-zone silicon single crystal ni rogen doping does not affect the velocity of individual dislocations but it leads to immobilization of dis locations while the crystal is kept under a low or zero applied stress at elevated temperatures. T he authors of [8] studied the effect of nitrogen doping on the process of formation and motion of dis locations from inner and surface (Knoop indentations) dislocation sources in CZ-Si. It was shown in a umber of works [9, 10] that nitrogen doping of CZ-Si leads to a diminution of the dislocat ion rosette around the indentation. Thus, today there are no data (or we are unaware of them) on the e ffect of nitrogen on the mobility and starting stress of individual dislocations in CZ-Si thoug , as is known, precisely the properties of individual dislocations make it possible to gain informati on on micromechanisms of the dopant-dislocation core interplay [11]. In this paper we have studie d the mobility of individual dislocations in nitrogen doped CZ-Si. This work is a follow-up to [6] where the macroplastic properties of the same silicon specimens were examined. Solid State Phenomena Online: 2003-09-30 ISSN: 1662-9779, Vols. 95-96, pp 465-472 doi:10.4028/www.scientific.net/SSP.95-96.465

Mechanical properties and deformation of fullerites

Mechanical tests of C60/C70 single crystals allow one to assign them to the category of soft and fragile materials. Their mechanical properties are comparable with those of graphite but, in contrast, they are isotropic. The dependence of C60/C70 microhardness on the temperature in the range 77–570 K and on the influence of solvent residues has been examined. Preliminary data on microhardness of pure C60 and C70 have been obtained.

Strain-rate sensitivity of the hardness of crystalline materials under dynamic nanoindentation

The coefficients of the strain-rate sensitivity of plastic characteristics (including microhardness) carry useful information concerning the nature of elementary carriers of plastic deformation and their mobility in a solid. In macrotests of various ductile materials (see, e.g., [1]), a wide range of strain rates (between 10–8 and 106 s–1) was investigated. However, fracture of many brittle materials (in particular, single crystals with covalent bonds, ceramics, glasses, etc.) begins before noticeable plastic strain. The plastic properties of such materials are usually studied by the methods of local deformation or microindentation. In recent years, the method of nanoindentation has also been extensively used in this field [2–4]. Famous firms (MTS, Micromaterials, CSEM, Hysitron, etc.) produce commercial nanotesters only for small values (10–3–10–1 s–1). At the same time, very high rates of local deformation in submicron areas (@10–1 s–1) are characteristic for many processes, including dry friction between rough surfaces, abrasive and erosive wear, atomic-force microscopy, nanolithography by the methods of imprinting and scribing, and fine grinding [5]. Thus, the area of the mechanical properties of materials that is characterized by both short loading time intervals and small deformation zones is little studied. Under these conditions, the ordinary mechanisms (in particular, dislocation mechanisms) of plastic flow can be strongly impeded or suppressed.

Effect of a Magnetic Field on the Starting Stress and Mobility of Individual Dislocations in Silicon

The strong effect of a magnetic field on the starting stress and mobility of individual dislocations is discovered in silicon grown by the Czochralski method with a high concentration of dissolved oxygen. It is shown that exposure of dislocations preliminarily introduced into the sample to a magnetic field considerably reduces the starting stresses for the motion of these dislocations. The effect is not observed in samples with a low oxygen concentration. It is assumed that the magnetic field induces singlet-triplet transitions in thermally excited states of silicon-oxygen complexes in the dislocation core, thus stimulating a change in the state (atomic configuration) of oxygen already located at dislocations. As a result, the mean binding energy of oxygen with a dislocation decreases.

Modes of kink motion on dislocations in semiconductors

Abstract Analysis is given of the changes of dislocation motion modes with stress and temperature variation. Different regimes of dislocation kink pair formation and spreading (motion in the random potential, in the field of random forces, the quasi-localization) are considered. Discrepancies are discussed between the theory and experimental data on dislocation velocities.

Influence of Magnetic Field on Critical Stress and Mobility of Dislocations in Silicon

We have found the effect of strong influence of magnetic field tre atm nt on the starting stresses of individual dislocations in Czochralski-gr own silicon (CZ-Si). It is shown that the exposure of CZ-Si samples with dislocations at room temperatur to magnetic field reduces essentially the starting stresses for dislocation motion. T he effect is absent in FZ-Si samples. We suppose that magnetic field causes the singlet-triplet transiti o in thermally exited states of oxygen complexes in a dislocation core that changes the state of oxy gen already situated on dislocations in such a way that the mean binding energy of oxygen w ith a dislocation is diminished.

Possible Polymerisation at Dislocations in C60 Crystals

It is known that under hydrostatic pressures of about 1 GPa the polymerisation of molecules in C 60 crystals occurs in a temperature range of about 400-600 K. The same strain of a lattice can be achieved in the vicinity of a dislocation core. Therefore, one can suppose that the polymerisation of C 60 molecules can be possible in the vicinity of dislocations in C 60 crystals at elevated temperature. Here we report the results of stress-strain measurements made on C 60 single crystals and the photoluminescence (PL) spectra of plastically deformed samples. A hardening of samples with increase of the deformation temperature from 20 to 300 °C was observed together with a strong enhancement of some defect-related lines in PL spectra. We suppose that the appearance of covalently bonded C 60 molecules at dislocations is a possible reason for that.

Dislocation and kink motion study in the bulk SiGe alloy single crystals

The individual dislocation mobility in bulk single SiGe crystals has been studied both with conventional and intermittent loading (IL) techniques. A model is used connecting the experimental data on dislocation paths with values of kink displacements under IL. The experimental data are compared with two models describing the interaction of a dislocation with point defects. It is shown that with small Ge concentration Cottrell atmosphere determines the dynamical drag of the dislocation giving rise to threshold immobilisation. With higher Ge content the specific mode of kink drift along the dislocation line (the motion in the field of random forces) takes place.

Dynamics of Kinks on Dislocations in SiGe Single Crystals

The method of intermittent pulse loading is used for obtaining the dependences of the mean free path of individual dislocations in SiGe single crystals with various concentration of Ge (0–5.5 at. %) on the duration of loading pulses and time intervals between them. It is found that these dependences change qualitatively upon an increase in the Ge concentration. It is shown that the motion of dislocations in SiGe crystals under small shear stresses is characterized by a nonlinear drift of kinks and the formation of superkinks. A theory of the motion of dislocations under the action of intermittent pulse loading under the conditions of heterogeneous kink dynamics is developed. Extended quasi-one-dimensional defects repeating the shape of a part of a segment of a moving dislocation are discovered in SiGe crystals containing 0.96 at. % Ge. The mechanism of formation of such defects as the result of the shedding of a part of the impurity atmosphere by a dislocation segment during overcoming of a local obstacle is proposed.

Dislocation-point defects interaction in semiconductors and kink mobility

The results are presented of an experimental study of the mobility of individual dislocations in silicon single crystals under a periodic two-level loading. The loadings have been employed to study a mean distance covered by dislocations as a function of load pulse duration and intervals between them at different shear stress levels. A description of the experimental data obtained for the kink migration in the field of random force has been examined. The qualitative agreement of the experimental data with calculations is shown.