V. I. Orlov

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.

Influence of the Dislocation Travel Distance on the DLTS Spectra of Dislocations in Cz-Si

We investigated the development of dislocation-related DLTS spectra in n-CZ-Si crystals with small (about 7.104 cm-2) number of long individual dislocations depending on the distance L that dislocations traveled during deformation at 600oC and on the velocity of dislocations. We found that a typical dislocation-related DLTS signal appeared only when dislocations traveled a significant distance that is more than 150-200μm, and it depended strongly on dislocation velocity. The results were interpreted on the assumption that the DLTS signal corresponds to some core defects and atomic impurities accumulated on the dislocations during their slow motion. At high concentration of deep level defects on dislocations a strange “negative DLTS” signal was observed. This can be explained by electron tunneling between deep defects along dislocations.

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.

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.

A Comparison of EBIC, LBIC and XBIC Methods as Tools for Multicrystalline Si Characterization

A comparative study of multicrystalline Si based solar cells and plastically deformed single crystalline Si by the EBIC, LBIC and XBIC methods as well as a computer simulation were carried out. The XBIC measurements were realized on a laboratory X-ray source. Simulations of LBIC and XBIC contrast values for grain boundaries, dislocations and spherical precipitates were carried out for different diffusion length and beam diameter values. It is shown by a computer simulation that the LBIC and XBIC contrast of two-dimensional defects in the crystals with a large enough diffusion length can be a few times higher than that in the EBIC mode, i.e. these methods in recent multicrystalline Si structures allow to reveal grain boundaries with the lower recombination strength. The contrast of dislocations perpendicular to the surface can be comparable in all three methods. The XBIC and LBIC contrast of precipitates usually is essentially smaller than that in the EBIC mode and could approach it in the structures with the small diffusion length only. Experimental data confirming the results of simulations are presented.

Determination of the nonequilibrium concentration of vacancies in silicon crystals by measuring the concentration of nickel atoms at lattice sites

A method is proposed for determining the nonequilibrium concentration of vacancies and vacancy complexes in silicon crystals by measuring the concentration of electrically active nickel atoms at the sites of the silicon lattice, [Nis], after the diffusion of nickel at temperatures from 550 to 650°C. It is shown experimentally that, after the diffusion of nickel from the surface into silicon samples with different initial nonequilibrium concentrations of vacancy complexes, [V]init, formed during crystal growth, the concentration [Nis] in the bulk of a sample to a good degree of accuracy corresponds to the vacancy concentration [V]init determined by a standard method based on the analysis of the concentration profiles of [Aus] after the diffusion of gold from the surface. This method for determining the concentration of vacancies is much simpler than the standard method and allows one to use lower temperatures and a lower thermal budget.

Silicon Layers Grown on Siliconized Carbon Net: Producing and Properties

The paper describes the elaboration of a method for producing composite Si/SiC wafers and investigation of their properties. The known two-shaping elements (TSE) method was used to produce the material. Pilot tests show that this composite material can be used for production of solar cells. The structure of silicon grains is elongated relative to the growth direction, the dislocation density in grains is of about (5÷8) ×104 cm-2, the average lifetime of minority carriers is 4÷6 µs.

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.

Residual Stress Distribution and Silicon Phase Transformation Induced by Rockwell Indentation at Different Temperatures, Studied by Means of Micro-Raman Spectroscopy

Rockwell microindentations obtained with a load of 1.5 N were made on (110) Cz-silicon substrates at temperatures of 70 and 600°C. The residual stress fiel ds and the silicon polymorphs induced by loading and unloading were studied by micro-Raman spe ctro copy. Mapping of the indented zones showed that the residual stress varies along the imprint radius. The indentations made at different temperatures show different str ss distributions. At 600°C the region of tensile stress is broad and the entire magnitude of t h residual stress variation is small. High compressive stress was measured at the imprint center. The stress becomes tensile in the pileup region and alters to compressive again away from the i ndentation. At 70°C the region with tensile stress is completely missing and there i s no residual stress outside the imprint. The size of the dislocation rosette around the imprint was correlated to the size of the stressed region outside the indentation. Polymorphs of silicon (Si-III, Si-XII, Si-IV), as well as amorphous material were observed within the indenter contact area. The variety of crystal phases s hows a tendency to diminish with increasing temperature. Scanning Electron Microscopy was applied to investigate the impri nt morphology as a function of the indentation temperature. Introduction The stress distribution in large diameter silicon wafers induce d by their weight during the thermal treatment has been the subject of some recent studies [1,2]. However, there is a lack of studies regarding the processes occurring in microscopic scale at th area of contact with the supporting pins. The indentation of silicon and its related microstructural changes have been a focus of research over the last decade [3-9]. Imprints in silicon prepar ed t room temperature have been studied extensively by a variety of techniques [3-5, 10]. Silicon has been indented by Solid State Phenomena Online: 2003-09-30 ISSN: 1662-9779, Vols. 95-96, pp 513-518 doi:10.4028/www.scientific.net/SSP.95-96.513