Solid State Phenomena Vols. 205-206

Abstract: We report on a novel method of low pressure chemical vapor deposition of polycrystalline silicon layers used for external gettering in silicon substrate for semiconductor applications. The proposed method allowed us to produce layers of polycrystalline silicon with pre-determined residual stress. The method is based on the deposition of a multilayer system formed by two layers. The first layer is intentionally designed to have tensile stress while the second layer has compressive stress. Opposite sign of the residual stresses of the individual layers enables to pre-determine the residual stress of the gettering stack. We used scanning electron microscopy for structural characterization of the layers and intentional contamination for demonstration of the gettering properties. Residual stress of the layers was calculated from the wafer curvature.

Abstract: The influence of GBs contained in the channel of MOS-FETs - fabricated in thin SOI layers - is demonstrated. The drain current measured at room temperature increases about 50 times for nFETs and about 10 times for pFETs, respectively, as compared to reference devices. The observations might be interpreted as a strong increase of the mobility of charge carriers. Moreover, the observed stepwise changes of the drain current at 5 K may point to Coulomb blockades.

Abstract: Shallow dislocation-related electronic states near the bottom of the conduction band in n-type Si bonded sample have been investigated with deep-level transient spectroscopy (DLTS), isothermal transient spectroscopy (ITS) and energy-resolved DLTS. The effect of thermoemission (TE) enhancement in external electric field was found and the dependence of the TE activation energy reduction as a function of the filling grade was obtained for these states. A new model of dislocation-strain-related Poole-Frenkel effect that accounts for the own electric field of internal charge of dislocation line is suggested and compared with the experimental data.

Abstract: Structural defects induced by electron irradiation of n-Cz-Si wafers were identified. The influence of the annealing conditions in a chlorine-containing atmosphere on the structural and luminescent properties of the samples was examined and the optimal annealing conditions were found. Light-emitting diodes based on electron-irradiated and high-temperature-annealed wafers were fabricated by a vapour-phase epitaxy technique and their luminescence properties were studied. A high-intensity dislocation-related D1 line was observed at 1.6 μm in the room-temperature electroluminescence spectrum.

Abstract: Protons with energies of 1 MeV and 2.5 MeV were implanted into a p-doped silicon wafer and then the wafer was annealed at 350 °C for one hour. This resulted in two n-doped layers in the otherwise p-doped sample. The carrier concentration was measured using spreading resistance profiling while the positions of the four pn-junctions were measured using electron beam induced current measurements. The carrier concentration is not limited by the available hydrogen but by the concentration of suitable radiation induced defects.

Abstract: The free volume of the thermally stable vacancy center in n-FZ-Si:P has been probed by positrons. The defects were produced with 15 MeV protons, and then the irradiated material was subjected to the isochronal annealing. The positron lifetime has been determined over the temperature range ~ 30 K – 300 K; the samples-satellites have been characterized by Hall effect measurements. The microstructure of the center involves, at least, one atom of phosphorus and it manifests itself as a deep donor. The center is singly negatively charged and the cascade phonon-assisted trapping of positron proceeds over the length characteristic of the point defect, l₀ ~3.62 a. u. Obeying ~ T ^–3 law, the positron trapping cross section ranges 3∙10^–12 cm² (66K) to 2.5∙10^–14 cm² (266 K). The positron lifetimes ranging from ~240 ps to ~280 ps suggest that the atomic relaxation is directed inward towards the free volume of the deep donor involving, at least, two vacancies.

Abstract: The impact of Sn on the degradation of the nonequilibrium charge carriers lifetime (τ) and the change of free electrons concentration (n) in γ- or electron irradiated Czochralski (Cz) n-Si has been studied. It is shown that in some cases the low-resistivity Sn doped n-Si (n-Si:Sn) can be considered as a material with enhancement radiation tolerance. In this material the lifetime damage factor (k_τ) is in several times smaller compared to undoped n-Si while the conductivity damage factor (k_n) is close in both materials. We also compared the impact of Sn, Pb and Sn+Pb. It is found that the radiation damage of the carrier lifetime and the conductivity in n-Si:Pb is slightly smaller than in undoped n-Si and in n-Si:Sn+Pb is the same as in n-Si:Sn.

Abstract: It is well known that a smooth surface of Si wafers can be obtained by Si surface reconstruction via high-temperature annealing. However, there remains a possibility of smooth Si surfaces deteriorating by accidental oxidation (called reflow oxidation) during the unloading process, i.e., taking out Si wafers from a vertical furnace after high-temperature annealing. Therefore, we considered it important to investigate the atomic-scale effects of oxidation on surface steps and terraces on Si wafers during the unloading process. We examined the effect of unloading temperature on oxide formation on Si (100) and Si (110) surfaces. The change in surface roughness was also measured. Our results indicated a significant improvement in the root mean square values of the surface roughness of terraces on the reconstructed surface. Moreover, this improvement was dependent on the decrease in the oxidation layer thickness in the case of low-temperature unloading. Furthermore, for suppressing reflow oxidation, we replaced the injected Ar gas with H₂ in the cooling process during high-temperature Ar annealing and evaluated the thickness of the reflow oxidation layer and surface structure of Si (100) and Si (110). H₂ annealing during the cooling process resulted in the formation of H-terminated Si surfaces, and this formation effectively suppressed reflow oxidation. However, the H₂ atmosphere also caused etching of the reconstructed Si surfaces. Atomic force microscopy measurements revealed that in spite of the etching, Si (100) and Si (110) surface roughness drastically decreased because of subsequent roughness variation, regarded as being caused by oxidation. In the case of Si (110), characteristic line oxidation was effectively suppressed, resulting in a smooth terrace-and-step structure. In summary, the obtained results suggested that our method is effective for restraining the increase in atomic-scale surface roughness due to oxidation.

Abstract: The kinetics of hydrogen penetration through dislocation networks produced by silicon wafer bonding are investigated by means of reverse bias annealing (RBA) procedure. By using the combination of capacitance-voltage (CV) profiling of net-acceptor concentration and deep level transient spectroscopy (DLTS) the total concentration of H introduced in the samples during wet–chemical etching at room temperature was defined. Two processes with the different time constants τ₁ and τ₂ was found for the bonded sample. The slow process τ₁ with an activation energy of (1.25±0.13) eV was analogous to that in the reference sample and corresponded to the dissociation of boron-hydrogen pairs. The fast process τ₂ was found to exhibit a lower activation energy of (0.87±0.25) eV and was identified as the release of hydrogen bound at screw dislocations by their elastic strains.

Abstract: Effective reduction of front surface carrier recombination is essential for high efficiency silicon solar cells. Dielectric films are normally used to achieve such reduction. They provide a) an efficient passivation of surface recombination and b) an effective anti-reflection layer. The conditions that produce an effective anti-reflection coating are not necessarily the same for efficient passivation, hence both functions are difficult to achieve simultaneously and expensive processing steps are normally required. This can be overcome by enhancing the passivation properties of an anti-reflective film using the electric field effect. Here, we demonstrate that thermally grown silicon dioxide is an efficient passivation layer when chemically treated and electrically charged, and it is stable over a period of ten months. Double layers of SiO₂ and SiN also provided stable and efficient passivation for a period of a year when the sample is submitted to a post-charge anneal. Surface recombination velocity upper limits of 9 cm/s and 19 cm/s were inferred for single and double layers respectively on n-type, 5 Ωcm, Cz-Si.