FZ-Si Crystals Research Articles

Passivation of thermally-induced defects with hydrogen in float-zone silicon

In this study, passivation of thermally-activated recombination centers with hydrogen in n-type float zone (FZ) Si containing nitrogen has been investigated. Prior to hydrogenation samples were heated to 550 °C using rapid thermal annealing and conventional furnaces. A large decrease in minority carrier lifetime occurred upon the heat-treatments confirming previous reports. A sequence of electron traps created in this process have been detected in the deep level transient spectra and characterized. Significant changes in the spectra have occurred after treatments in remote hydrogen plasma and subsequent annealing of the hydrogenated samples in the temperature range 100 °C–400 °C. A total elimination of electrical activity of the thermally induced defects has been observed in the hydrogenated samples subjected to annealing in the temperature range 150 °C–300 °C. The results obtained suggest a simple way for an effective cure of the degraded FZ-Si-based solar cells. Possible defect reactions occurring in the FZ-Si crystals and the role of nitrogen and carbon upon the performed treatments are discussed.

Applicability of the three-dimensional Alexander-Haasen model for the analysis of dislocation distributions in single-crystal silicon

Applicability of the three-dimensional Alexander-Haasen (AH) model for the analysis of dislocation distributions in single-crystal silicon has been estimated. The numerical results obtained from the AH model agree well with the experimental data for both CZ-Si and FZ-Si crystals with the axis in the [001] direction but do not completely agree with the experimental data for the FZ-Si crystal with the axis in the [111] direction. The inapplicability of the AH model in a crystal with the axis in the [111] direction may arise from the neglect of dislocation propagation in this model, because the dislocation propagation in a crystal with the axis in the [111] direction is more active than that in a crystal with the axis in the [001] direction. Therefore, to increase the applicability of the AH model, it is necessary to include the effect of dislocation propagation.

Titanium in silicon: Lattice positions and electronic properties

Secondary ion mass spectroscopy (SIMS) and deep level transient spectroscopy measurements were carried out on Czochralski (Cz)- and float-zone-grown (FZ) Si crystals, which were implanted with Ti ions and annealed in the temperature range 600–900 °C. The electrical behavior of Ti atoms is found to be different in Cz- and FZ-Si annealed at 650 °C, although the Ti SIMS profiles are similar. It is argued that interstitial Ti atoms (Tii) in FZ-Si crystals interact with implantation-induced vacancies and take a substitutional position (Tis). No energy levels which can be assigned to Tis have been detected in this work or in previous experimental literature. However, previous calculations suggest that Tis is a deep acceptor in Si. We show from density functional calculations that by taking proper account of interactions within the d-shell of the Ti impurity the electronic structure of Tis has no levels in the band gap. The calculations show that Tii is more energetically favorable than Tis and that Tii binds more strongly to the silicon vacancy than interstitial oxygen does, explaining the observed differences between FZ- and Cz-irradiated materials.

The Way of Making Monoisotopic FZ-Si Crystals for the New kg Mass Unit Standard and for Basic Research

The naturally based redefinition of the kg unit being aimed by the Avogadro project ultimately requires a precise 1kg-sphere of mono isotopic silicon with practically perfect crystallinity. Silicon isotopes (28, 29, 30) can be effectively separated by the ultra-centrifugation technique. A large amount (ca.10kg) of gaseous 28SiF4 was extraordinarily highly enriched up to 99.996% using ca. 240 specially interconnected centrifuges over 6 months. Then, 28SiF4 was transformed to 28SiH4 and purified by cryo distillation. Polycrystalline 28Si was pyrolytically deposited to a cylindrical 6kg rod. After multiple FZ-purification, a dislocation-free 28Si FZ crystal of 4,5 kg mass was grown, just enough for two 1kg spheres and auxiliary samples. The enrichment was still 99,994%. Similarly, small (10-20g) 29Si and 30Si crystals were prepared for basic research needs. Si28 enables dramatic line narrowing improving deep and shallow defect spectroscopy. Under magnetic field, electronic and nuclear spins embedded in a zero-spin 28Si matrix could be individually determined.

Diffusion of implanted sodium in oxygen-containing silicon

The effect of oxygen on diffusion of sodium implanted into silicon is studied for the first time in the temperature range from 500 to 850°C. A high-resistivity p-Si (ρ > 1 kΩ cm) grown by the Czochralski method in a magnetic field (mCz) with the oxygen concentration ∼3 × 1017 cm−3 was used. For comparison, we used silicon grown by the crucibleless floating zone method (fz). Temperature dependences of the effective diffusion coefficient of sodium in the mCz-Si and fz-Si crystals were determined and written as DmCz[cm2/s] = 1.12exp(−1.64 eV/kT) cm2/s and Dfz[cm2/s] = 0.024exp(−1.29 eV/kT) cm2/s, respectively. It is assumed that larger values of diffusion parameters in oxygen-containing silicon are caused by formation of complex aggregates that contain sodium and oxygen atoms.

The Way of Making Monoisotopic FZ-Si Crystals for the New kg Mass Unit Standard and for Basic Research

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Influence of high pressure and temperature on defect structure of silicon crystals implanted with N or Si ions

(111) oriented Fz-Si crystals were implanted with N + or Si + at 150 keV energy to a dose of 2×10 16 cm −2. After implantation the samples were treated at a high temperature (up to 1400 K) under hydrostatic argon pressure (up to 1.1 GPa) for 1–5 h. Structural properties of Si:N and Si:Si before and after the treatment were investigated by high-resolution X-ray diffraction, synchrotron topography, transmission electron microscopy and photoluminescence methods. The structural properties of Si:N and Si:Si treated under hydrostatic pressure at 1270 K are different. A buried layer of uniform thickness with smooth interface was formed in Si:N under HP=1.1 GPa; the buried layer/Si interface becomes less smooth with rising treatment time. The shape of interference fringes and their changing distance at rocking curves are explained as an effect of nitrogen diffusion to the buried layer during the treatment. For the treated Si:Si samples, the observed strain changes are related to the out-diffusion of interstitial silicon atoms from the implanted layer to the sample surface and to creation of numerous extended defects near the implanted atoms range. Prolonged treatment (for 5 h) at 1.1 GPa and 1400 K results in creation of dislocations for Si:N while for Si:Si samples the structure of treated samples is more complex, involving also creation of amorphous-like surface film.

FZ-Si crystal growth and HREM study of new types of extended defects during in situ electron irradiation

High purity FZ-Si crystals with a very low secondary grown-in defects density (less than 10 2 cm −2) were grown under critical value V/ G ≈2×10 −5 cm 2/s K where V is the crystal growth rate in cm/s and G the axial temperature gradient in K/cm at the crystal-melt interface. Because of the coexistence self-interstitials and vacancies in this case, the structure of grown-in defects seems to be far from understood. To elucidate the point defect reaction at such a point we propose to use the data of in situ electron irradiation in a high resolution electron microscope (HREM) at room temperature which allow us to study a ‘frozen’ clustering of both types of point defects depending on the type of local point defect supersaturation in the thin FZ-Si samples covered with Si 3N 4 films. Here, we show that new types of extended defects are created upon an isolated and combined clustering of vacancies and self-interstitial atoms on {1 1 3} and {1 1 1} habit planes. The existence of an energy barrier against recombination of self-interstitials with the extended aggregates of vacancies results in the formation of unusual extended defects having a displacement vector close to zero. It is proposed that the combined clustering of point defects on {1 1 3} is able to provide the recombination of point defects at a higher temperature. However, the small energy of {1 1 1} defect and its highly ordered structure suggest the possibility of such a defect formation during vacancy-rich crystal growth.

Precision wavelength measurement of the 14.4 keV Mössbauer photon

The wavelength of the 14.4 keV Mössbauer photon has been determined by using undulator radiation and diffraction of FZ silicon crystals. For the wavelength determination a goniometer equipped with a laser rotary encoder and a sine-bar angle optical interferometer was developed; a temperature-monitoring system and X-ray optics related to the experiment were also developed. The mean wavelength was 0.08602557 nm with an uncertainty of 0.6 p.p.m., derived from 16 measurements of three pieces of FZ Si(840) crystals.

Study of the 1096.9 meV photoluminescent oxygen-related centre in neutron-irradiated CZ-Si: Formation and structure

The creation process and the structure of an oxygen-associated centre in Si are investigated by means of photoluminescence (PL). The centre is formed after annealing between 250°C; and 450°C in neutron-irradiated CZ-Si and is photoluminescent with a bound exciton emission at 1096.9 meV. We show that this PL line as well as the II PL line (1018.2 meV) are the major features of the PL spectra for samples annealed in this temperature range. The comparison of the PL spectra obtained in CZ-Si and FZ-Si crystals indicated that the 1096.9 meV PL line is formed at expenses of II, thus suggesting the associated centre is presumably created by complexing the trigonal II centre with one (or more) interstitial oxygen. Uniaxial stress measurements and the observed oxygen isotopic shift (0.037 meV) show that the PL line is associated with an oxygen centre of monoclinic I symmetry. The thermal activation energy of the binding exciton deduced from the intensity decay, ΔE = 5.1 meV, together with the stress behaviour suggest that the centre probably corresponds to an iso-electronic centre.

Optical Absorption Study of the Interaction between Group II Acceptors and Hydrogen in Si

The interaction between hydrogen (H) and Zn was studied from the measurements of optical absorption spectra of H and Zn-doped Si crystals. FZ Si crystals were doped with Zn by annealing the crystals in Zn vapor at 1200 °C followed by quenching. After Zn-doping, the specimens were doped with H and/or deuterium (D) by annealing them in a H and/or D ambient followed by quenching. Optical absorption spectra of the above specimens were measured at about 6 K with an FT-IR spectrometer. Hydrogen atoms are known to passivate Zn acceptors partially since the optical absorption intensity associated with electronic transitions becomes much weaker after hydrogenation. Instead, other absorption peaks appear at around 2130 cm−1. From the comparison of separations of those peaks, they are identified to be due to electronic transitions associated with Zn–H acceptors. Optical absorption peaks associated with Zn–D acceptors have higher energies by about 12.1 cm−1 than those of Zn–H acceptors.

Observation of vacancy clustering in FZ-Si crystals during in situ electron irradiation in a high voltage electron microscope

Vacancy accumulation in pure FZ-Si crystals covered with thin Si 3N 4 films leads upon irradiation with a high intensity electron beam in a high voltage electron microscope (HVEM) at temperatures between 20 and 250°C, to the suppression of the formation of 113 -defects of interstitial type in the central part of the irradiated area. Very small secondary clusters of interstitial type with a density of about 10 11 cm −2 appear, however, in this area after prolonged irradiation. Small stacking fault tetrahedra and Frank loops of vacancy type connected with interstitial clusters were analysed on an atomic scale in the thinnest part of the irradiated area in a 400 keV high resolution electron microscope. Vacancy clusters might act as nucleartion centres for the observed secondary defect formation.

Identification of vacancy clusters in FZ-Si crystals

D defects in FZ-Si crystals are identified by XAFS method, in conjunction with the Cu decoration technique. The atomic structure of the Cu precipitates on the D defects is analyzed and their nature is referred. The Cu precipitates on the D defects are Cu clusters. This result suggests that Cu precipitation takes place at vacancy-rich sites and the D defects are vacancy-clusters. Mittels XAFS-Methode kombiniert mit der Cu-Dekorationstechnik werden D-Defekte in FZ-Si-Kristallen identifiziert. Die atomare Struktur der Cu-Prazipitate auf den D-Defekten wird analysiert und ihre Ursache diskutiert. Die Cu-Prazipitate auf den D-Defekten sind Cu-Cluster. Diese Ergebnisse zeigen, das Cu-Prazipitation an leerstellenreichen Platzen stattfindet und die D-Defekte Leerstellencluster sind.

Hydrogen Diffusion and Complex Formation in Silicon

AbstractThe kinetics of hydrogen diffusion and complex formation in crystalline silicon are investigated using a model previously developed to explain the influence of dopant type and concentration on observed deuterium profiles in silicon. The predictions of the model have been shown to be in close agreement with recent SIMS profiles of deuterated FZ-Si crystals, with the in-diffusion process dominated by trapping at impurity sites and by the formation of immobile hydrogen molecules.Previous studies have treated the surface concentration of hydrogen and the capture radii of hydrogen at complexes as free kinetic parameters. We present an analytic relationship between the diffusion coefficient D of neutral hydrogen and both the hydrogen surface concentration and the capture radius for molecule formation. The consequences of fixing D at the known high-temperature value for the diffusion coefficient in the model are determined. The existence of this analytic relation reduces the number of free parameters in the kinetic model and leads to an improved understanding of hydrogen reactions in silicon.

Preservation of Point Defect Distribution in FZ Silicon

Evidences for the preservation of point defect distribution in FZ Si crystals have been observed by two experimental methods. One is divided selective oxidation and the other is divided backside selective oxidation with Si3N4 as the oxidation resistant films. In both experiments, the selective oxidation is divided into several oxidation steps, and the effects of oxidation division on the range of oxidation effects on impurity diffusion is investigated. The results show that, both laterally and vertically from the oxidizing Si–SiO2 interface, the effect of oxidation on impurity diffusion is determined only by total oxidation time. This suggests that point defect distribution is preserved after heat treatment.

The Range of Diffusion Enhancement of B and P in Si during Thermal Oxidation

The lateral range of oxidation enhanced diffusion (OED) of B and P in Si is investigated using selective oxidation at 1100°C with directly formed Si3N4 films as oxidation resistant masks and FZ Si crystals as substrates. It is found that this OED range of B agrees well with that of P and that the range increases with oxidation time. The value of the range is found to be much larger than previously. The results can be explained using a model in which the range is determined by interstitial diffusion.