Solid State Phenomena Vols. 131-133

Abstract: The influence of Cu contamination on radiation-induced defect reactions in n-type Czochralski-grown silicon (Cz-Si) crystals has been studied by means of the Hall effect technique, deep level transient spectroscopy (DLTS) and high-resolution Laplace DLTS with supporting theoretical modeling of defects. It is found that the contamination of Cz-Si samples with Cu does not influence significantly the energy spectrum and introduction rates of the principal electrically active defects induced by electron irradiation. The vacancy-oxygen (VO) centre, divacancy (V2) and a complex consisting of a silicon self-interstitial with the oxygen dimer (IO2) are found to be the dominant radiation-induced defects in Cu-contaminated samples as well as in uncontaminated ones. An isochronal annealing study has shown that the presence of Cu affects the annealing behaviour of the vacancy-related defects. In Cu-doped samples the VO centre disappears upon annealing at significantly lower temperatures (175-250°C) compared to those of the VO disappearance in the uncontaminated samples (300-375°C). The disappearance of the VO centres in the Cu-doped samples occurs simultaneously with an anti-correlated introduction of a defect with an energy level at about Ec- 0.60 eV. It is suggested that this defect is formed by the interaction of a mobile Cu atom with the VO complex. According to results of quantum-chemical modelling, in the most stable configuration of the Cu-VO defect a Cu atom occupies a tetrahedral interstitial position nearest to the elongated Si-Si bond of the VO centre. The presence of the Cu atom is found to result in the further elongation of the Si-Si bond and a shift of the VO acceptor level to the middle of the gap. The annealing behaviour of V2 has also been found to be different in the irradiated Cu-doped samples compared to that in the uncontaminated ones. The most probable reason for this difference is an interaction of mobile Cu atoms with di-vacancies. An energy level at about Ec-0.17 eV has been tentatively assigned to a complex consisting of a Cu atom and a di-vacancy.

Abstract: In this work we discuss an original analysis about a method to reduce the dislocation density in the devices that use the Shallow Trench Isolation (STI). It is well known that a high mechanical stress in silicon combined with an amorphizing implantation damage can generate many dislocations. So we propose to release the mechanical stress in silicon before implanting. A high temperature treatment indeed can trigger the viscous behaviour of the filling oxide inducing the relaxation of the stress field in silicon. For the first time a systematic study of the effect of different furnace and RTP annealings in the stress relaxation was done by Raman measurements. Different temperatures (from 3000C to 11000C) and different durations (from few seconds to one hour) were explored and the experimental results were compared with the numerical simulation with a good agreement. Finally we study the effect of the most promising annealings selected by Raman in a complete process flow.

Abstract: The effect of hydrostatic argon pressure equal to 105 Pa and 1.1 GPa applied to processing at up to 1270 K (HT) of Si:Cr samples prepared by Cr+ implantation (dose 1x1015 cm-2, 200 keV) into (001) oriented Czochralski silicon, has been investigated by Secondary Ion Mass Spectrometry, photoluminescence, X-ray and SQUID methods. Cr+ implantation at this energy and dosage produces amorphous silicon (a-Si) near the implanted ions range. Solid phase epitaxial re-growth (SPER) of a-Si takes place at HT. The Cr profile does not depend markedly on HP applied during processing at 723 K. Si:Cr processed at up to 723 K indicates magnetic ordering. Annealing under 105 Pa at 873 K, 1070 K and 1270 K results in a marked diffusion of Cr toward the sample surface. In the case of processing under 1.1 GPa this diffusion is less pronounced, SPER of a-Si is retarded and the a-Si/Si interface becomes enriched with Cr. The Cr concentration in Si:Cr sample processed at 1270 K under 1.1 GPa forms two distinct maxima, the deeper one at 0.35 μm depth.

Abstract: Low-temperature diffusion of palladium (450–700 oC) from an implanted layer (9.5 MeV, 1⋅1013cm-2) deep into the volume of a high-power P-i-N diode (2.5kV, 150A) is compared with that of a sputtered palladium layer (50nm thick), both under the enhancement by the radiation defects from He2+ implantation (10 MeV, 1⋅1012cm-2). Annealing after the palladium implantation (800oC, 60min.) prior to the He2+ implantation is shown to increase the concentration of palladium related deep levels in the damaged region with a subsequent improvement of diode dynamic parameters. The concentration of palladium in-diffusing from the implanted layer with the dose of 1⋅1013cm-2 does not sufficiently reduce excess carrier lifetime close to the anode junction as is the case of the devices with sputtered palladium that give better dynamic parameters like lower maximal reverse recovery current and recovery losses for the same magnitude of leakage current.

Abstract: The time dependence of thermal donor (TD) concentration, N(t), during annealing at 450oC was measured in samples cut from a single slab of silicon containing bands of grown-in microdefects of different types. An enormous impact of the microdefect type on the kinetic curve was observed. Samples from the interstitial region showed simple linear rise in N(t). The samples from an inner part of the vacancy region showed a complicated oscillating variation with an abrupt disappearance of the TDs at some moment followed by an immediate restoration of a linear rise. In samples from the marginal H-band of the vacancy region, an initial anneal does not produce TDs. However if this anneal was followed by a quench, subsequent anneals produce a linear rise in N(t). On the other hand, if the sample was slowly cooled, the subsequent production of TDs remained almost negligible. These observed peculiarities are accounted for by enhanced TD growth in the presence of self-interstitials (I) - due to IO species serving as vehicles for oxygen transport.

Abstract: The thermal donor formation at 425oC - 450oC in Ge doped Czochralski (GCZ) silicon having about 1016 cm-3 Ge content pretreated by rapid thermal annealing (RTA) and conventional furnace annealing (CFA) has been investigated using low-temperature infrared spectroscopy (LT-IR). The measurements prove that lightly Ge doping can enhance the formation of thermal double donors in the initial stage of the low temperature annealing after RTA process. Ge induced additional grown-in oxygen precipitates during silicon ingot growth and the abundant self-interstitials during RTA may be the reason for the enhancement. However, after extending the annealing time at the low temperatures, the thermal donor concentration in the GCZ silicon is lower than that in the conventional CZ silicon. In final, the mechanism is also discussed.

Abstract: The effectiveness of phosphorus diffusion gettering (PDG) and related segregation coefficients for different metal impurities were measured applying thermal treatments in the temperature range 800-950 °C for different times. We used multi-crystalline and mono-crystalline CZ p-type wafers with different boron concentrations and different levels of dislocations and bulk micro-defects (BMD). In all sample types, for Cu and Ni we found complete gettering in the temperature range investigated. In the case of Fe, the segregation coefficient increases with both increase in temperature and extension of time. The increase is qualitatively changing when going above 900 °C. At 950 °C the segregation coefficient increases faster at shorter diffusion time but at extended diffusion time it increases slower as compared to diffusion at 900 °C. At the same temperature and time of phosphorus diffusion the segregation coefficient is found to be independent of the metal impurity concentration in the range of 1012-1015 cm-3 investigated. We have shown that the presence of BMD and dislocations in bulk silicon does not impede the ability of PDG to completely remove Fe, Ni and Cu metal impurities from the bulk. Further analysis suggests that the PDG has the same gettering efficiency for mono-crystalline silicon and multi-crystalline silicon. We conclude that if any bulk precipitation of Fe, Ni and Cu impurities is present in multi-crystalline silicon it cannot seriously compete with PDG. However we found that increasing the boron concentration in the samples reduces the segregation coefficient of Fe, and this reduction is more severe at lower temperatures. Finally, by applying a post anneal ramp down from 900 °C to 700 °C after phosphorus diffusion, we found that the Fe segregation coefficient increases by a factor of 36 for lightly B doped samples, from 53 to 1919, leading to a significant reduction of Fe in the bulk after 2 hours ramp down anneal.

Abstract: IR-spectroscopy with computer analysis of the shape of the Si-O absorption band, electron microscopy, X-rays diffraction and measurements of unsteady photoconductivity timedecay under band-to band excitation were used to investigate the influence of defects in different diameter (40 – 300 mm) Si ingots on the oxygen precipitation due to two-stage annealing (750 оС + 1050 оС). It is shown that large size Cz-Si ingots have a relatively low concentration of electrically active micro-defects, containing small (0.06 – 0.1 μm) dislocation loops. During thermal treatments this leads to the formation of a low stressed oxide phase (SiO2) with an enhanced thermo-stability. The precipitates in small size ingots, however, contain distorted 4-fold rings of SiO4 tetrahedra.

Abstract: The gravitational induced shear stresses in 200 mm silicon wafers supported in verticaltype or horizontal-type furnace were calculated using 3D-FEM analysis of the displacement vector assuming linear elastic behavior of the anisotropic material. For comparison of the two complex loading cases and for relating the effect of gravitational constraints to the mechanical strength of the wafers, the invariant von Mises shear stress τM was chosen. The computed maximum values of τM demonstrate that the gravitational induced stress for vertical processing is approximately one order of magnitude less than the gravitational induced stress for horizontal processing. The experimental results obtained from processing of 200mm wafers with different oxygen concentration in horizontal and vertical boats at 1200°C are in an excellent agreement with the theoretical simulations.

Abstract: Aim of this work is to study the electrical properties and the minority charge carrier recombination behaviour of extended defects in multicrystalline silicon (mc-Si) ingots grown from solar grade silicon (SoG-Si) feedstock. The pure metallurgical SoG-Si feedstock has been produced directly by carbothermic reduction of very pure quartz and carbon without subsequent purification processes.This mc SoG-Si is studied by temperature-dependent Electron Beam Induced Current measurements and PhotoLuminescence spectroscopy and the potentiality of the combination of these two techniques in the identification of the defects which limit the quality of the base material is shown. The EBIC mapping technique shows the presence of electrically active grain boundaries at room temperature while dislocations result inactive. Dislocations become active only at temperatures lower than 250K, indicating a moderate level of metal decoration. The most detrimental defects in this material seem to be the grain boundaries and impurities dissolved in the matrix. Furthermore, the PL spectra reveal the presence of oxygen and carbon related complexes. In this work we show that the knowledge about the defect related recombination processes acquired by a combined application of EBIC measurements and PL-spectroscopy is of particular importance to tune the proper solar cell process step to be applied on such material.