Float Zone Silicon Research Articles

Magnetic Czochralski silicon strip detectors for Super-LHC experiments

High resistivity and high oxygen concentration of silicon wafers can be beneficial for the radiation hardness of silicon detectors. Wafers of Magnetic Czochralski silicon (MCz-Si) can be grown with a resistivity of a few k Ω cm and with well-controlled, high oxygen concentration. According to the beam test results presented in this paper, n-type MCz-Si bulk, p-strip readout detectors with can be operated with acceptable signal-to-noise ratio up to the irradiation fluence of 1×10 15 cm −2 1-MeV neutron equivalent. The improved radiation hardness compared to that of traditional p-in-n Float Zone silicon (p-in-n FZ-Si) detectors can be explained by better electric field distribution inside MCz-Si detectors. The difference between the distributions is clearly shown by Transient Current Technique (TCT) measurements, presented in this paper. Thus, strip detectors made on n-type MCz-Si are a feasible option for the outer tracker layers of the potential upgrade of the Large Hadron Collider (LHC), the Super-LHC. This corresponds approximately 95% of the total area of silicon detectors in the Super-LHC.

Dependence of silicon PIN diode electrical characteristics on proton fluence

A comparative analysis of capacitance–voltage (C–V ) and current–voltage (I–V ) characteristics dependent on proton irradiation fluence in the range from 7·1012 to 7·1014 p/cm2 has been performed in float zone (FZ) silicon PIN diodes. A δ-shape and triangle profiles of radiation induced defects were formed by irradiation with protons having energy in the range from 2.0 to 2.7 MeV. Temperature-dependent variations of current (I–T) at fixed reverse voltage VR enable one to extract the thermal activation parameters. These values are found to be in a quantitative agreement with trap activation parameters measured by DLTS technique. The role of point and extended defects in correlated modifications of C–V and I–V characteristics of the irradiated diodes is discussed. Keywords: radiation defects, Si PIN diodes, capacitance–voltage, current–voltage characteristics

Breakdown of silicon particle detectors under proton irradiation

Silicon particle detectors made on Czochralski and float zone silicon materials were irradiated with 7 and 9 MeV protons at a temperature of 220 K. During the irradiations, the detectors were biased up to their operating voltage. Specific values for the fluence and flux of the irradiation were found to cause a sudden breakdown in the detectors. We studied the limits of the fluence and the flux in the breakdown as well as the behavior of the detector response function under high flux irradiations. The breakdown was shown to be an edge effect. Additionally, the buildup of an oxide charge is suggested to lead to an increased localized electric field, which in turn triggers a charge carrier multiplication. Furthermore, we studied the influences of the type of silicon material and the configuration of the detector guard rings.

Electrical properties of nitrogen-doped Float-Zoned silicon annealed in a range of 200 to 900 °C

Nitrogen in Float-Zoned silicon is electrically inactive in as-grown state, but upon annealing, numerous deep centres are generated. In the past, only selected annealing temperatures were tried. In the present study a wide temperature range was inspected. The effect of anneal was sensitive to the cooling rate. With slow cooling following anneals, the room-temperature hole concentration (in boron-doped samples with an initial resistivity of 1200 Ohm cm) was almost unchanged. The temperature dependence of the hole concentration (monitored by resistivity and Hall effect) showed however that various deep donors were generated. Depending on the anneal (and on the cooling rate), levels at 0.36, 0.3, 0.2 or 0.1 eV (above the top of the valence band) were found. These levels are assigned to the vacancy-nitrogen species VN 5, VN 7, VN 9 and VN 11, respectively.

Design and simulation of blue/violet sensitive photodetectors in silicon-on-insulator

According to Lambert's law, a novel structure of photodetectors, namely photodetectors in silicon-on-insulator, is proposed. By choosing a certain thickness value for the SOI layer, the photodetector can absorb blue/violet light effectively and affect the responsivity of the long wavelength in the visible and near-infrared region, making a blue/violet filter unnecessary. The material of the SOI layer is high-resistivity floating-zone silicon which can cause the neutral N type SOI layer to become fully depleted after doping with a P type impurity. This can improve the collection efficiency of short-wavelength photogenerated carriers. The device structure was optimized through numerical simulation, and the results show that the photodiode is a kind of high performance photodetector in the blue/violet region.

Electrical Characterization of Deep-Lying Donor Layers Created by Proton Implantation and Subsequent Annealing in N-Type Float Zone and Czochralski Silicon

The deep-lying donor layers created by proton implantation and subsequent isochronal annealing were investigated in silicon substrates with oxygen concentration changing from 2x10^16 cm-3 to 1.4x10^18 cm-3. Implantation was performed with 700 keV and 1.8 MeV protons to fluence ranging from 1x10^10 to 1x10^15 cm-2. Results of C-V measurement showed that proton implantation introduces a Gaussian like distribution of shallow hydrogen donors that corresponds to the profile of implanted hydrogen obtained from the secondary ion mass spectroscopy measurement. Subsequent isochronal annealing leads to annealing out of hydrogen donors (~250{degree sign}C) and formation of a series of hydrogen thermal donors (250-350{degree sign}C). In substrates with enhanced concentration of oxygen, the radiation enhanced thermal donors and ordinary thermal donors (>400{degree sign}C) were registered. It was shown that the introduction rate of hydrogen and thermal hydrogen donor layers is enhanced in materials with higher oxygen concentration. However, low oxygen concentration is preferred when better control over spatial distribution of the excess donor profiles is required.

Electron paramagnetic resonance spectroscopy of lithium donors in monoisotopic silicon

Abstract Electron paramagnetic resonance (X-band EPR) spectra are reported for lithium-related donors in monoisotopic silicon. High resolution EPR spectra of lithium donor centers in monoisotopic silicon, enriched by 28 Si isotope (99.99%) with very narrow individual lines are observed. In monoisotopic silicon sample ( 28 Si enriched floating zone silicon with low concentration of lithium ~10 16 cm −3 ), the trigonal EPR spectrum, with well resolved 7 Li hyperfine structure is recorded in the temperature range 3.5–20 K. This spectrum was attributed to LiO complex. At high concentration of lithium (about 10 18 cm −3 ) in monoisotopic silicon two types of spectra are observed. The trigonal one has the same feature as for low concentration of lithium with g -values: g ∥ =1.9974 and g ⊥ =1.9989. Another spectrum consists of two lines and has tetragonal symmetry with g ∥ =1.9992 and g ⊥ =1.9983. This spectrum is more intensive than the trigonal one and has no resolved hyperfine structure probably due to time averaging of the hyperfine interaction caused by hopping motion of electrons.

Effects of activation by proton irradiation on silicon particle detector electric characteristics

After irradiation with 7 and 9 MeV protons, activation-induced effects were encountered in measurements of current-voltage (IV) and capacitance-voltage (CV) characteristics for Czochralski and float-zone grown silicon particle detectors prepared on printed circuit boards with copper electrodes. With the present detector construction, the S30i(p,n)P30 and C63u(p,n)Z63n reactions induce dominant interference in such measurements. The daughter nuclides are positron emitters with half-lives of 2.5 and 38.5 min, respectively, and the slowing down of the emitted positrons generates a significantly large concentration of electron-hole pairs in the detector volume increasing the leakage current level and decreasing the breakdown voltage. The observed time-dependent characteristics were verified by modeling the activation of the detector structure and the resulting leakage current. As a result, the electrical measurements cannot be performed immediately after irradiation due to silicon activation, and, generally, materials becoming easily activated should be avoided in the detector concept.

Electrical Characterization of Deep-Lying Donor Layers Created by Proton Implantation and Subsequent Annealing in n-Type Float Zone and Czochralski Silicon

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Measurements of Dislocation Locking by Near-Surface Ion-Implanted Nitrogen in Czochralski Silicon

A modified dislocation unlocking technique is used to measure dislocation locking due to nitrogen implanted into Czochralski silicon. The results show that near-surface dislocations can be locked by implanted nitrogen. The magnitude of the locking measured suggests that nitrogen transport proceeds by a dissociative mechanism, where transport occurs by the splitting of immobile dimers into fast monomers, rather than movement of nitrogen dimers. In other experiments, nitrogen-doped float-zone silicon is investigated using the standard dislocation unlocking technique. The results give an activation energy for effective nitrogen diffusion in silicon of at . Using the assumption that the dislocation locking strength per nitrogen atom is the same as that of oxygen, a value of can be inferred for the effective diffusivity prefactor. If analyzed using the dissociative model, an activation energy of 1.1–1.4 eV is found for nitrogen monomer diffusion, with a diffusivity prefactor of .

The geometrical dependence of radiation hardness in planar and 3D silicon detectors

The radiation hardness of planar and 3D silicon detectors fabricated on Float-Zone and epitaxial silicon substrates is compared after exposure to neutron equivalent fluences greater than 10 15 cm −2. Following irradiation, the Signal Efficiency (SE), expressed as the ratio of the maximum signal after irradiation divided to the maximum signal before irradiation, is shown to depend only on the geometrical distance, L, between the p + and n + electrodes. The Signal Efficiency is independent of the silicon substrate used for the various detectors. A formalism describing the dependence of Signal Efficiency on L is derived and used to fit the data. The Signal Efficiency dependence on inter-electrode distance L and the fluence φ follow the same inverse proportionality law.

Characterization of Radiation Hard Silicon Materials

Segmented silicon detectors are widely used in modern high-energy physics (HEP) experiments due to their excellent spatial resolution and well-established manufacturing technology. However, in such experiments the detectors are exposed to high fluences of particle radiation, which causes irreversible crystallographic defects in the silicon material. Since 1990’s, considerable amount of research has gone into improving the radiation hardness of silicon detectors. One very promising approach is to use magnetic Czochralski silicon (MCz-Si) that has been found to be more radiation hard against charged hadrons than traditional Float Zone silicon material (Fz-Si) used in the current HEP applications. Other approaches include operating the devices at cryogenic temperatures and designing special detector structures such as p-type detectors or semi-3D detectors. In order to demonstrate that the developed technologies are suitable for the HEP experiments, it is necessary to extensively characterize the potentially radiation hard detectors. We have an excellent instrument for this, the Cryogenic Transient Current Technique (C-TCT) measurement setup, which is an effective research tool for studying heavily irradiated silicon detectors. With the C-TCT setup it is possible to extract the full depletion voltage, effective trapping time, electric field distribution and the sign of the space charge in the silicon bulk in the temperature range of 45-300 K. This article

Broadband antireflective structures applied to high resistive float zone silicon in the THz spectral range

The optimal structural parameters for an antireflective structure in high resistive float zone silicon are deduced for a rectangular and a hexagonal structure. For this the dependence of the effective index from the filling factor was calculated for both grating types. The structures were manufactured by the Bosch-process. The required structural parameters for a continuous profile require an adaption of the fabrication process. Challenges are the depth and the slight positive slope of the structures. Starting point for the realization of the antireflective structures was the manufacturing of deep binary gratings. A rectangular structure and a hexagonal structure with period 50 mum and depth 500 mum were realized. Measurements with a THz time domain spectroscopy setup show an increase of the electric field amplitude of 15.2% for the rectangular grating and 21.76% for the hexagonal grating. The spectral analysis shows that the bandwidth of the hexagonal grating reaches from 0.1 to 2 THz.

Degradation of high-resistivity float zone and magnetic Czochralski n-type silicon detectors subjected to 2-MeV electron irradiation

Particle-tracking detectors made on high-resistivity (HR) float zone (FZ) silicon are widely used in high-energy physics experiments. It is known that the incorporation of oxygen in the FZ Si can lead to some improvement in the radiation hardness of the material. In this contribution we investigate the effects of 2 MeV electron irradiation, up to a fluence of 5×10 16 e/cm 2, on the electrical and carrier lifetime properties of p-on-n silicon diodes fabricated on different substrate materials, including HR standard and oxygenated FZ, as well as HR magnetic Czochralski silicon, with a higher intrinsic oxygen contents. A progressive degradation of the characteristics is observed for all devices, pointing to a generation of bulk damage. Interestingly, a significant increase of the effective carrier concentration is observed after the highest fluences for all materials. Under the limited experimental conditions studied, no significant changes are observed for diode characteristics subjected to a thermal annealing treatment at 80 °C. This degradation in the electrical properties should be taken into account for the use of such HR Si materials under high-energy electron environments.

Distribution of Hydrogen- and Vacancy-Related Donor and Acceptor States in Helium-Implanted and Plasma-Hydrogenated Float-Zone Silicon

AbstractThe formation and evolution of hydrogen- and vacancy-related donor and acceptor states were studied in helium-implanted and subsequently hydrogen plasma-treated n-type Float-Zone (FZ) silicon wafers by means of two-point-probe Spreading Resistance (SR) measurements. He+-implantation was executed at 3.75 MeV and 11 MeV at fluences of 1×1014 cm−2. Post-implantation 13.56-MHz RF-plasma hydrogenations were carried out at 150 W either for 15 min or 1 hour, applying substrate temperatures between 350 °C and 500 °C. Enhanced donor concentrations as well as acceptor-like states were observed in the subsurface layers of the treated FZ Si samples after 15-min post-implantation H-plasma exposures. Under appropriate process conditions, the latter ones compensated for the n-type doping, so that even buried p-type layers were created. The experimental results indicated that oxygen played a central role in the formation of the acceptor-like states.

Unsteady 3D and analytical analysis of segregation process in floating zone silicon single crystal growth

Unsteady 3D and analytical analysis of segregation process in floating zone silicon single crystal growth

Nitrogen diffusion and interaction with dislocations in single-crystal silicon

The results of dislocation unlocking experiments are reported. The stress required to unpin a dislocation from nitrogen impurities in nitrogen-doped float-zone silicon (NFZ-Si) and from oxygen impurities in Czochralski silicon (Cz-Si) is measured, as a function of the unlocking duration. It is found that unlocking stress drops with increasing unlocking time in all materials tested. Analysis of these results indicates that dislocation locking by nitrogen in NFZ-Si is by an atomic species, with a similar locking strength per atom to that previously deduced for oxygen atoms in Cz-Si. Other experiments measure dislocation unlocking stress at 550 °C in NFZ-Si annealed at 500–1050 °C. The results allow an effective diffusivity of nitrogen in silicon at 500–750 °C to be inferred, with an activation energy of 3.24 eV and a diffusivity prefactor of approximately 200 000 cm2 s−1. This effective diffusivity is consistent with previous measurements made at higher temperatures using secondary ion mass spectrometry. When the results are analyzed in terms of a monomer-dimer dissociative mechanism, a nitrogen monomer diffusivity with an activation energy in the range of 1.1–1.4 eV is inferred. The data also show that the saturation dislocation unlocking stress measured at 550 °C in NFZ-Si is dependent on the anneal temperature, peaking at 600–700 °C and falling toward zero at 1000 °C.

Study of the response to minimum ionising particles of microstrip detectors made with float zone and magnetic Czochralski silicon after neutron irradiation

The use of various silicon substrates to improve the radiation hardness of tracker detectors for high energy physics experiments has been investigated for several years. Single crystal silicon grown with different methods has different concentrations of impurities (mainly O and C). It has been found that relatively high concentrations of particular impurities in the silicon crystal can affect the degradation of some of the detector electrical properties as a function of the radiation dose. When high resistivity Czochralski (Cz) silicon became available after refinement of the Cz method for growing single crystal silicon ingots, the new material (magnetic Cz, MCz) was tested for radiation hardness with promising results obtained with simple pad diodes. A crucial measurement for evaluating the radiation tolerance of the detectors is though the response to minimum ionising particles (MIPs) of finely segmented devices. To this purpose, miniature microstrip sensors made on n and p-type MCz substrates have been produced and compared to identical ones made on n and p-type standard floating zone (FZ) detector grade silicon. The study has been performed evaluating the signal induced by beta electrons from a 90Sr source and measured with an LHC speed (40 MHz clock) analogue electronics readout. The comparison of MCz and FZ n and p-type detectors has been carried out after several doses of neutron irradiation (up to 1×10 15 n cm −2).

Formation and Annihilation of Hydrogen Related Donor States in Proton Implanted and Subsequently Plasma Hydrogenated N-Type Float Zone Silicon

The formation of hydrogen-related donor states was studied in proton-implanted and hydrogen plasma-treated Float-Zone silicon. H-implantations were executed at energies of 1 or 3 MeV, and fluences of 1x10E14 1/cm2. 13.56-MHz RF-plasma hydrogenations were done at 150 W for 15 min or 1 hour at substrate temperatures between 350 C and 500 C. Analysis was done by two-point-probe spreading-resistance measurements. H-plasma exposure for 15 min caused an enhanced donor concentration close to the projected ion range Rp. The donor states can be related to hydrogenated vacancy defects. Near the wafer surface n-type doping is also enhanced. Applying higher substrate temperatures (> 400 C) during plasma exposure, acceptor-like states overcompensate for the enhanced n-type doping near the surface. After 1-hour H-plasma exposure excessive donor states in the wafers subsurface region are completely annealed. At the surface n-type doping is still overcompensated for by acceptor-like defects attributed to hydrogenated vacancy defect clusters.

Measurements of Dislocation Locking by Near-Surface Ion-Implanted Nitrogen in Czochralski Silicon

A modified dislocation unlocking technique is used to measure dislocation locking due to nitrogen implanted into Czochralski silicon. The results show that near-surface dislocations can be locked by implanted nitrogen. The magnitude of the locking measured suggests that nitrogen transport proceeds by a dissociative mechanism, where transport occurs by the splitting of immobile dimers into fast monomers, rather than movement of nitrogen dimers. In other experiments, nitrogen-doped float-zone silicon is investigated using the standard dislocation unlocking technique. The results give an activation energy for effective nitrogen diffusion in silicon of 3.24{plus minus}0.25eV at 500 to 750{degree sign}C. Using the assumption that the dislocation locking strength per nitrogen atom is the same as that of oxygen, a value of 200,000cm2s-1 can be inferred for the effective diffusivity pre-factor. If analysed using the dissociative model, an activation energy of 1.1 to 1.4eV is found for nitrogen monomer diffusion, with a diffusivity pre-factor of 30cm2s-1.