Float Zone Silicon Research Articles

Out- and in-diffusion of oxygen 16O in silicon

Oxygen was in-diffused into the float zone silicon either during oxidation or upon annealing in argon of previously oxidized Si wafers. The resultant in-depth profiles of the main isotope 16O were measured by secondary ion mass spectrometry (SIMS) with detection limit below 1016 cm−3. The oxygen solubility Cs, as determined from the 16O surface concentration, was found to be insensitive to the annealing ambient. This observation seems to contradict the effect of enhanced solubility under oxidizing conditions, reported for the in-diffused 18O isotope. Over a temperature range of 900–1250 °C, the solubility of oxygen can be expressed by a relationship Cs = 9.1 × 1022 cm−3 exp(−1.57 eV/kT). The surface concentration of oxygen out-diffused from supersaturated CZ Si appears to be significantly higher than that of in-diffused 16O. This is because the out-diffusion process is accompanied by precipitation of oxygen atoms into immobile SiO2 particles. As a consequence, SIMS measurements of the out-diffusion profile cannot provide reliable information on the solid solubility of oxygen.

Effect of Combined Oxygenation and Gettering on Minority Carrier Lifetime in High-Resistivity FZ Silicon

High oxygen concentration in silicon increases the resistance of radiation detectors to high radiation doses. Unfortunately, high-resistivity float zone (FZ) silicon, needed for radiation detectors, has too low an oxygen content. The solution to this is silicon oxygenation. There are different ways of incorporating oxygen into silicon. The most accepted one is by high-temperature diffusion from a thick layer. In this paper, we investigate the impact of this silicon oxygenation technique as well as the application of different gettering techniques to improve the minority carrier lifetime of high-resistivity FZ silicon substrates for radiation detectors. The minority carrier lifetimes before and after surface etching have been measured on samples subjected to different oxygenation and gettering treatments by using a quasi steady-state photoconductance technique. A lifetime improvement efficiency factor is defined for each treatment process. The lifetime efficiency factors behave independently, so that the lifetime efficiency factors associated with different sequential combinations of treatments can be estimated by a multiplicative combination. Different gettering techniques that improve or degrade the minority carrier lifetime are analyzed, and the best options for silicon radiation detector fabrication are determined. Oxygenated silicon with a minority carrier lifetime close to 1 ms can be obtained. © 2004 The Electrochemical Society. All rights reserved.

Observation of fluorine-vacancy complexes in silicon

We show direct evidence, obtained by positron annihilation spectroscopy, for the complexing of fluorine with vacancies in silicon. Both float zone and Czochralski silicon wafers were implanted with 30keV fluorine ions to a fluence of 2×1014ions∕cm2, and studied in the as-implanted condition, and after annealing to 650°C for 10 and for 30min. The “2-detector” background reduction technique for positron annihilation was applied. The spectra reveal a significant concentration of fluorine-vacancy complexes after annealing, for both Czochralski and float zone material, supporting the results of computer simulations of the implantation and annealing process.

Radiation hardness of high resistivity magnetic Czochralski silicon detectors after gamma, neutron, and proton radiations

High resistivity magnetic Czochralski Si detectors were irradiated with /sup 60/Co gamma rays, neutrons, and protons to various doses/fluences, along with control float zone Si detectors. 1) It has been found that for gamma radiation, magnetic Czochralski Si detectors behave similarly to the high-temperature, long-time (HTLT) oxygenated float zone Si detectors. There is no space charge sign inversion and there is a buildup of positive space charges. The rate for this buildup is much higher than that for the oxygenated Si detectors and is proportional to the oxygen concentration. 2) For neutron radiation, there is little difference between magnetic Czochralski and control float zone silicon detectors. Space charge sign inversion is observed for both materials. The introduction rate of deep acceptors (beta) for magnetic Czochralski Si detectors is slightly less than that for control float zone Si detectors, and 3) for proton radiation (10 and 20 MeV), although the space charge sign inversion is also observed for magnetic Czochralski Si detectors, the 1-MeV neutron-equivalent space charge sign inversion fluence is about three times higher than that of magnetic Czochralski Si detectors irradiated with neutrons. Also, the acceptor introduction rate beta is about half of that for oxygenated Si detectors. Thus, high resistivity magnetic Czochralski Si behaves in a similar manner to the HTLT oxygenated float zone Si detectors and is even more radiation resistant to damage caused by charged particles.

Defects and diffusion in high purity silicon for detector applications

In this contribution we review some recent results on defects occurring after irradiation and thermal treatment of silicon detectors for ionizing radiation. In particular, the annealing of the prominent divacancy centre and the concurrent growth of a new double acceptor centre, assigned to the divacancy–oxygen pair, is treated in detail. The detectors were fabricated using oxygenated high purity float zone (FZ) silicon wafers of n-type in order to improve the radiation hardness. The results obtained have important implication on our current understanding of the degradation of silicon detectors at high radiation fluences and suggest that a new concept for optimised impurity/defect engineering needs to be developed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Terahertz time-domain spectroscopy characterization of the far-infrared absorption and index of refraction of high-resistivity, float-zone silicon

The far-infrared absorption and index of refraction of high-resistivity, float-zone, crystalline silicon has been measured by terahertz time-domain spectroscopy. The measured new upper limit for the absorption of this most transparent dielectric material in the far infrared shows unprecedented transparency over the range from 0.5 to 2.5 THz and a well-resolved absorption feature at 3.6 THz. The index of refraction shows remarkably little dispersion, changing by only 0.0001 over the range from 0.5 to 4.5 THz.

Study of the defects in oxygen implanted silicon subjected to neutron irradiation and high pressure annealing

This paper reports on capacitance measurements on Czochralski-grown and float-zone silicon subjected to oxygen implantation, subsequent neutron irradiation and finally high pressure thermal anneals. The purpose of this work was the study of the effect of irradiation on the formation of thermal donors in silicon. The observed changes in the C-V characteristic curves and profiles are discussed. We found that oxygen-ion implantation followed by neutron irradiation results in shallow and deep level acceptor-like defects formation. Prolonged heat treatment leads to thermal donor generation as usual in Cz-Si annealed at 450 °C. The most striking result of the study is finding that high pressure thermal anneals result in extra donor formation. The effects mentioned above may lead to changes in the type of conductivity depending on oxygen content in the material, hydrostatic pressure and an extent of damage caused by the irradiation.

Influence of deep levels on space charge density at different temperatures in γ-irradiated silicon

In this work, it is shown that the analysis of thermally stimulated currents (TSC) and current transients (CT) at constant temperature can be a suitable tool to study the influence of deep levels on space charge density N( T) in irradiated silicon diodes . In particular, the occurrence of space charge sign inversion (SCSI) can be related to signal discontinuities in TSC and CT measurements. This approach has been adopted in this work to study devices made of standard Float Zone (FZ) and Diffusion Oxygenated Float Zone silicon, irradiated by γ-rays up to a dose of 300 Mrad. Our study shows that all the samples are inverted at 50 K after a low-temperature excitation. Several space charge sign inversions, from positive to negative and vice versa, have been observed between cryogenic and room temperature, and have been related to carriers emission from dominating deep traps. Only standard FZ silicon remains inverted at room temperature after a dose of 300 Mrad.

Radiation hardness of different silicon materials after high-energy electron irradiation

The radiation hardness of diodes fabricated on standard and diffusion-oxygenated float-zone, Czochralski and epitaxial silicon substrates has been compared after irradiation with 900 MeV electrons up to a fluence of 2.1×10 15 e/ cm 2 . The variation of the effective dopant concentration, the current related damage constant α and their annealing behavior, as well as the charge collection efficiency of the irradiated devices have been investigated.

Diffuse x-ray scattering and transmission electron microscopy study of defects in antimony-implanted silicon

Ion implantation followed by laser annealing has been used to create supersaturated and electrically active concentrations of antimony in silicon. Upon subsequent thermal annealing, however, these metastable dopants deactivate towards the equilibrium solubility limit. In this work, the formation of inactive antimony structures has been studied with grazing incidence diffuse x-ray scattering, and transmission electron microscopy, and the results are correlated to previous high-resolution x-ray diffraction data. We find that at a concentration of 6.0×1020 cm−3, small, incoherent clusters of radius 3–4 Å form during annealing at 900 °C. At a higher concentration of 2.2×1021 cm−3, deactivation at 600 °C occurs through the formation of small, antimony aggregates and antimony precipitates. The size of these precipitates from diffuse x-ray scattering is roughly 15 Å in radius for anneal times from 15 to 180 seconds. This value is consistent with the features observed in high-resolution and mass contrast transmission electron microscopy images. The coherent nature of the aggregates and precipitates causes the expansion of the surrounding silicon matrix as the deactivation progresses. In addition, the sensitivity of the diffuse x-ray scattering technique has allowed us to detect the presence of small clusters of radius ∼2 Å in unprocessed Czochralski silicon wafers. These defects are not observed in floating zone silicon wafers, and are tentatively attributed to thermal donors.

Radiation hardness of Czochralski silicon, Float Zone silicon and oxygenated Float Zone silicon studied by low energy protons

We processed pin-diodes on Czochralski silicon (Cz-Si), standard Float Zone silicon (Fz-Si) and oxygenated Fz-Si. The diodes were irradiated with 10, 20, and 30 MeV protons. Depletion voltages and leakage currents were measured as a function of the irradiation dose. Additionally, the samples were characterized by TCT and DLTS methods. The high-resistivity Cz-Si was found to be more radiation hard than the other studied materials.

Effect of Oxygen and Nitrogen Doping on Mechanical Properties of Silicon Using Nanoindentation

AbstractThe effects of oxygen and nitrogen on the mechanical properties of Czochralski (CZ) and float zone silicon have been studied using nano-indentation. Nitrogen free FZ Si exhibited low hardness of 6.49 GPa and elastic modulus of 104 GPa. When doped with 2×1015cm−3 nitrogen, FZ Si hardness and elastic modulus increased to 8.2 and 182 GPa, respectively. In the near-surface denuded zone of N-doped CZ Si (N-CZ) the hardness correlates well with the O and N profiles. Distinct high hardness points, found in the O- and N- rich subsurface region, were attributed to precipitates. Nano-scratch tests of N-CZ Si confirmed the existence of hard phases, mostly small precipitates, whose density, estimated to be 2×1013cm−3, is in the range of previously suggested nuclei density in as-grown N-CZ silicon.

Effect of Oxygen on the Diffusion of Nitrogen Implanted in Silicon

Nitrogen implants were performed in Si wafers of variable purity, i.e., epitaxial layers, float zone, and Czochralski silicon (CZ-Si). The diffusion behavior of nitrogen reported by Hockett in Appl. Phys. Lett., 54, 1793 (1989), where nitrogen diffuses for several micrometers at temperatures as low as 750°C and the profiles assume a double-peak structure, is peculiar of CZ-Si. In contrast, in pure epitaxial-Si, nitrogen is immobile at low temperature but, at a higher temperature (850°C), the broadening of the nitrogen profile never assumes the shape observed in CZ-Si. Our results suggest that oxygen determines the shape of the nitrogen diffusion profiles.

Particle Detectors made of High Resistivity Czochralski Grown Silicon

We describe the fabrication process of fullsize silicon microstrip detectors processed on silicon wafers grown by magnetic Czochralski method. Defect analysis by DLTS spectroscopy as well as minority carrier lifetime measurements by µPCD method are presented. The electrical and detection properties of the Czochralski silicon detectors are comparable to those of leading commercial detector manufacturers. The radiation hardness of the Czochralski silicon detectors was proved to be superior to the devices made of traditional Float Zone silicon material.

Three-dimensional simulation of heat flow, segregation, and zone shape in floating-zone silicon growth under axial and transversal magnetic fields

Three-dimensional simulation is carried out to illustrate the effects of magnetic fields on the heat flow, dopant segregation, and the zone shape in a small-scale floating-zone silicon growth. With an axial magnetic field, a steady growth can be found. However, with the increasing magnetic field strength, there is a steady solution with a four-fold symmetry before reaching an axisymmetric state. With a transversal field, although the molten zone is asymmetric, the minimum field strength required for a steady growth is found much lower than the axial one. The calculated results show a simple two-fold symmetry with respect to the magnetic direction. Although the convection on the plane parallel to the transversal field can be significantly suppressed, the flow on the perpendicular plane is not weakened at all due to the buildup of electric potential. This leads to a non-uniform zone and an elliptic crystal shape. Besides, due to the significant dopant mixing on the perpendicular plane, for the same field strength, the radial segregation in the transversal field is found smaller than that in the axial field.

Radiation hardness of Czochralski silicon studied by 10-MeV and 20-MeV protons

We have processed pin-diodes on Czochralski silicon (Cz-Si), standard float zone silicon (Fz-Si), and diffusion oxygenated float zone silicon (DOF) and irradiated them with 10- and 20-MeV protons. Evolutions of depletion voltage and leakage current as a function of irradiation dose were measured. Space charge sign inversion (SCSI) was investigated by an annealing study and verified by transient current technique (TCT). Czochralski silicon was found to be significantly more radiation hard than the other materials.

Second-order generation of point defects in highly irradiated float zone silicon—annealing studies

Isothermal and isochronal annealing experiments have shown that the I defect (the main cause for the changes in silicon diodes characteristics after high levels of gamma-irradiation) is stable up to 325–350°C. Possible reactions, which can explain the behavior of different defects formed during the annealing experiments, are discussed. All the experimental results concerning the I center (second-order irradiation induced defect, formed most likely via VO center, suppressed in oxygen rich material, high thermal stability) make it the favorite for being associated with the V2O2 complex.

Radiation-induced junction formation behavior of boron-doped Czochralski and float zone silicon crystals under 3 MeV proton irradiation

A comparative study was performed on the junction formation behavior of boron-doped p-type Czochralski (Cz) and float zone (Fz) Si wafers, which differed mainly in interstitial oxygen concentration, upon 3 MeV proton irradiation with fluences of up to 2×1015 cm−2. The region around the projected range in both the Cz and Fz Si wafers converted its conduction type to n type at fluences between 1×1013 and 3×1013 cm−2, which is most probably due to the formation of hydrogen-related donors. The main difference between the Cz and Fz Si wafers was in the susceptibility of the proton track region to type conversion. The proton track region of the Cz Si wafer converted to n type at fluence between 1×1013 and 3×1013 cm−2, whereas that of the Fz Si wafer showed only an increase in resistivity without any type conversion as the fluence increased up to 2×1015 cm−2, which was attributed to oxygen-related donor formation in the case of the Cz Si wafer. The present results are discussed with respect to the radiation-induced failure mechanisms of n+/p/p+-structured Si space solar cells based on boron-doped Cz and Fz Si.

Erratum: “Second-order generation of point defects in gamma-irradiated float-zone silicon, an explanation for ‘type inversion’ ” [Appl. Phys. Lett. 82, 2169 (2003)

First Page

The Effect of Oxygen Partial Pressure on Marangoni-Flow-Induced Dopant Striations in Floating-Zone Silicon Crystals

A silicon crystal was grown after the floating-zone (FZ) method under oxygen partial pressures (Po2 in) of 2.0×10-8 MPa and 5.8×10-6 MPa, which were measured at the entrance of a furnace. It was found that dopant striations showed a single-peak spectrum at 0.19 Hz in this crystal, and a crystal grown at Po2 in of 2.0×10-8 MPa showed multiple frequencies. This result is explained by a decrease in the Marangoni number with increasing oxygen partial pressure. It has thus been concluded that Marangoni flow in FZ silicon crystal growth can be controlled by introducing oxygen gas, independently of the temperature field control in the vicinity of the crystal/melt interface.