Float Zone Silicon Research Articles

Platinum in-diffusion controlled by radiation defects for advanced lifetime control in high power silicon devices

The low-temperature (∼700 °C) in-diffusion of platinum (Pt) into the n-type float zone silicon guided and enhanced by radiation damage produced by implantation of helium ions was used to shape the profile of ideal recombination centers—platinum substitutionals (Pt s). Implantation of helium ions with energies up to 11 MeV introducing different profiles of radiation defects were applied for this purpose. Both the platinum silicide (PtSi) and implanted Pt layers were compared as the sources of Pt diffusion. The full-depth distribution of in-diffused Pt was studied by monitoring the acceptor level of Pt s −/0 ( E C − E T = 0.23 eV) by the current transient spectroscopy. Results show that the helium implantation significantly enhances platinum diffusion and allows its control up to the depths of hundreds of micrometers. The resulting Pt s distribution can be controlled by the profile of radiation damage produced by helium ions while the amount of in-diffused Pt s is set by the dose of platinum implantation. Application of the method using both the implanted and PtSi sources is demonstrated on optimization of turn-off properties of high power PiN diodes.

Effect of emitter deposition temperature on surface passivation in hot-wire chemical vapor deposited silicon heterojunction solar cells

Low substrate temperature (< 150 °C) during initiation of amorphous silicon emitter deposition by hot-wire chemical vapor deposition is found to be crucial for reaching high open-circuit voltage ( V oc) in an amorphous/crystalline silicon (a-Si/c-Si) heterojunction solar cell. Low-temperature results in immediate a-Si deposition and a smooth interface to the c-Si substrate. The smooth heterojunction leads to effective passivation of the c-Si surface by the a-Si intrinsic layer through a much-reduced interface recombination velocity, and V oc is consistently above 620 mV. We obtain a V oc above 640 mV and a fill factor of 80% on Al-backed p-type Czochralski wafers with emitters deposited at temperatures below 135 °C. Energy conversion efficiencies of 14.8% and 15.7% are obtained on a polished p-type Czochralski silicon wafer and a polished p-type float-zone silicon wafer, respectively.

Investigation of type inversion of n-bulk in 10 MeV proton-irradiated FZ silicon detectors using a scanning electron microscope

Based on the results of capacitance–voltage measurements and transient current technique, it was earlier deduced that the n-type bulk of float zone silicon radiation detectors changes type in heavy irradiation. This paper describes the results of measuring the voltages and electric fields with a scanning electron microscope using the voltage–contrast effect, inside radiation detectors that were irradiated with 10 MeV protons with several fluences. The results confirm the earlier observations and give more accuracy to the electric field measurements.

Characterization of standard and oxygenated float zone Si diodes under radiotherapy beams

The dosimetric response of silicon diodes made from high resistivity float zone (FZ) and diffusion oxygenated FZ (DOFZ) silicon has been studied with a 60Co clinical radiotherapy gamma source. To investigate the changes in sensitivity with the accumulated dose, the diodes have been exposed to doses of 137Cs gamma-rays up to 6 kGy. As expected, Si diodes showed a degradation in the signal response with the accumulated dose due to the formation of radiation-induced defects acting as lifetime killers. Nonetheless, the DOFZ Si diode appeared to be moderately radiation harder than the standard FZ sample, with a decay of sensitivity less pronounced.

High energy proton damage effects in thin high resistivity FZ silicon detectors

The proposed luminosity upgrade of the Large Hadron Collider (S-LHC) will demand the inner most layers of the vertex detector to sustain fluences of about 10 16 charged hadrons/cm 2. Due to the high multiplicity of tracks, the required spatial resolution and the extremely harsh radiation field thin silicon detector assemblies are proposed as a possible solution for this challenge. The radiation tolerance of 50 μm thin high resistivity float zone (FZ) devices has been studied for 24 GeV/ c protons in the fluence range between 4×10 13 cm −2 and 8.6×10 15 cm −2. For the manufacturing of such thin devices, a technology based on direct wafer bonding and deep anisotropic etching was used. Annealing measurements at 80 °C have shown that the introduction rate g C in the stable damage component is about the same as observed for oxygen enriched FZ detectors and that the fluence dependence of the reverse annealing amplitude N Y exhibits a saturating function. It is also demonstrated that the annealing behavior of the reverse current related damage parameter α is independent on the fluence and the silicon material parameters. Charge collection efficiency (CCE) measurements were performed using 5.8 MeV α-particles. After fully annealing for about 31 days at 80 °C CCE was determined by extrapolation to be 66% at 10 16 p/cm 2.

Elimination and formation of electrically active defects in hydrogenated silicon particle detectors irradiated with electrons

The influence of preliminary treatment in hydrogen plasma on elimination of radiation defects and formation of thermal donors has been studied in detector structures made of standard float zone silicon. The detectors were irradiated with 3.5 MeV electrons and annealed at temperatures of 50–350 °C. It has been found that preliminary hydrogenation at 300 °C leads to disappearance of divacancies and vacancy–oxygen complexes at lower annealing temperatures. The annealing of hydrogenated and irradiated crystals is accompanied by hydrogen redistribution and formation of hydrogen-related donors.

New Topographic Method of Detecting Microdefects Using Weak-Beam Topography with White X-Rays

We describe a new topographic method of detecting microdefects in nearly perfect crystals. By this method, faint kinematical images of microdefects are observed with minimized dynamical background intensity using the interference effects of X-rays in a sample crystal. We demonstrate the capability of the method by observing “A-swirl” defects in floating-zone (FZ) silicon. The dynamical background intensity is markedly reduced, and weak kinematical images could be observed.

Characterization of magnetic Czochralski silicon radiation detectors

Silicon wafers grown by the Magnetic Czochralski (MCZ) method have been processed in form of pad diodes at Instituto de Microelectrònica de Barcelona (IMB-CNM) facilities. The n-type MCZ wafers were manufactured by Okmetic OYJ and they have a nominal resistivity of 1 kΩ cm. Concentrations of oxygen and carbon in MCZ wafers were measured by Secondary Ion Mass Spectroscopy (SIMS) technique to be compared to standard and oxygenated float zone (FZ) silicon material. The diodes were characterized by leakage current and capacitance measurements. The average full depletion voltage, V FD, of the pad detectors is 290±10 V, while the leakage current density at V FD+20 V is 7±3 nA/cm 2. Minority carrier lifetime of MCZ, standard FZ and oxygenated FZ substrates was measured by using a quasi steady-state photoconductance technique. The uniformity of the MCZ wafers was studied before the fabrication by mapping the resistivity of the wafer and after the fabrication process by measuring the full depletion voltage and leakage current as a function of diode position on the wafer. It was found that the fabrication process did not change the resistivity of the material although no donor-killing heat treatment was performed on the wafers.

Punch through float-zone silicon phototransistors with high linearity and sensitivity

In this paper, we propose, analyze and demonstrate a high-purity float-zone (FZ) silicon phototransistor operating at the punch through state with high linearity and sensitivity. Those phototransistors were fabricated on high-purity FZ silicon substrates; the dependence of the sensitivity on incident optical power and bias voltages has been investigated to light with a wavelength of 0.83 μm from a laser diode. In accordance with theoretical prediction, it shows that the linearity reaches the highest when the base of the phototransistor is completely depleted (punch through), and the fitting goodness of output R 2 is 0.9954 over a 40 dB range from 0.15 to 1500 nW. The sensitivity of the phototransistor can be as large as 40–70 A/W at the punch through state, which is equivalent to an internal current conversion gain of about 75–130. The stability of 1% in the sensitivity for the punch through phototransistor with an internal current conversion gain of 130 can be obtained if the bias voltage and operating temperature can be stable to about 2.5% (1 in 40 V) and the temperature to ±2 °C.

Oxygen and Nitrogen Transport in Silicon Investigated by Dislocation Locking Experiments

The behavior of oxygen and nitrogen impurities in silicon has been investigated using a novel dislocation locking technique. The locking effect of oxygen in Czochralski silicon (CZ-Si) was investigated in the temperature range and was found to display five well-defined regimes as a function of annealing time. Results indicate that enhanced transport of oxygen to dislocations takes place for temperatures below . Numerical simulations of the enhanced oxygen transport indicate that the effective diffusivity becomes dependent on oxygen concentration with an activation energy of approximately . The same technique has been used to investigate nitrogen transport in nitrogen-doped float-zone silicon in the temperature range and shows nitrogen to have a comparable locking effect to oxygen in CZ-Si, despite being present in a concentration that is 2 orders of magnitude lower. The stress required to unlock dislocations at which have previously been immobilized by nitrogen during an annealing step, initially increases approximately linearly with the duration of the anneal before saturating to a steady-state value of approximately for all anneal temperatures investigated. An expression for the transport of nitrogen to the dislocations was deduced, which has an activation energy of .

Early stages of iron precipitation in silicon

The early stages of iron precipitation in p-type float-zone silicon have been studied by means of Deep Level Transient Spectroscopy, DLTS, for iron concentrations in the range of 1014 – 3 × 1015 cm3. A DLTS line showing the signatures of extended localized states is observed. The emission characteristics of the associated defect is very similar to that of the interstitial iron donor. It is concluded that large clusters consisting of interstitial iron atoms give rise to the observed DLTS line. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Positron annihilation studies of detector grade n-type silicon irradiated with 140 MeV oxygen (O 6+) ions

Detector grade n-type float-zone silicon irradiated with 140 MeV O 6+ ions to a dose of 5×10 15 particles/cm 2 was studied by positron annihilation spectroscopy. Vacancy clusters containing five vacancies were detected in the irradiated sample. Isochronal annealing studies of the irradiated sample reveal the dissociation of five-vacancy clusters and formation of vacancy agglomerates V 3, V 6 and vacancy–oxygen complexes in the temperature range 250–500 °C. The vacancy–oxygen complexes dissociate at 550 °C and released vacancies agglomerate around 600 °C to form voids comprising 10 vacancies. In the temperature range 600–700 °C, the voids dissociate and around 750 °C advanced vacancy–oxygen complexes V m O n ( n> m) form.

Pressure stimulated creation of oxygen-related defects in oxygen-implanted and neutron-irradiated silicon

The present work reports some experimental results based on capacitance measurements on float-zone silicon (Fz-Si) and Czochralski-grown silicon (Cz-Si) 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 acceptors and donors in silicon. 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 720 K. The most striking result of the study is finding that high-pressure thermal anneals result in extra donor formation even for low oxygen concentration. The effects mentioned above lead to changes in the type of conductivity (p-n junction formation) depending on oxygen content in the material, hydrostatic pressure and an extent of damage caused by the irradiation.

Radiation-hard semiconductor detectors for SuperLHC

An option of increasing the luminosity of the Large Hadron Collider (LHC) at CERN to 10 35 cm −2 s −1 has been envisaged to extend the physics reach of the machine. An efficient tracking down to a few centimetres from the interaction point will be required to exploit the physics potential of the upgraded LHC. As a consequence, the semiconductor detectors close to the interaction region will receive severe doses of fast hadron irradiation and the inner tracker detectors will need to survive fast hadron fluences of up to above 10 16 cm −2. The CERN-RD50 project “Development of Radiation Hard Semiconductor Devices for Very High Luminosity Colliders” has been established in 2002 to explore detector materials and technologies that will allow to operate devices up to, or beyond, this limit. The strategies followed by RD50 to enhance the radiation tolerance include the development of new or defect engineered detector materials (SiC, GaN, Czochralski and epitaxial silicon, oxygen enriched Float Zone silicon), the improvement of present detector designs and the understanding of the microscopic defects causing the degradation of the irradiated detectors. The latest advancements within the RD50 collaboration on radiation hard semiconductor detectors will be reviewed and discussed in this work.

Particle detectors made of high-resistivity Czochralski silicon

We have processed pin-diodes and strip detectors on n- and p-type high-resistivity silicon wafers grown by magnetic Czochralski method. The Czochralski silicon (Cz-Si) wafers manufactured by Okmetic Oyj have nominal resistivity of 900 Ω cm and 1.9 kΩ cm for n- and p-type, respectively. The oxygen concentration in these substrates is slightly less than typically in wafers used for integrated circuit fabrication. This is optimal for semiconductor fabrication as well as for radiation hardness. The radiation hardness of devices has been investigated with several irradiation campaigns including low- and high-energy protons, neutrons, γ-rays, lithium ions and electrons. Cz-Si was found to be more radiation hard than standard Float Zone silicon (Fz-Si) or oxygenated Fz-Si. When irradiated with protons, the full depletion voltage in Cz-Si has not exceeded its initial value of 300 V even after the fluence of 5×10 14 cm −2 1-MeV eq. n cm −2 that equals more than 30 years operation of strip detectors in LHC experiments.

Radiation enhanced diffusion of implanted platinum in silicon guided by helium co-implantation for arbitrary control of platinum profile

The in-diffusion of platinum into a low-doped n-type float zone silicon guided and enhanced by radiation damage produced by co-implantation of helium ions was investigated. The implantation of 1MeV platinum ions at different doses ranging from 5×1011 to 5×1012cm−2 was used to produce a finite source for platinum diffusion. Single and multiple energy implantation of helium ions with energies 7, 9, 11 and 13MeV introducing different profiles of radiation defects were applied to enhance and shape the diffusion of platinum atoms performed by 20min annealing at 725°C in vacuum. The distribution of in-diffused platinum was studied by monitoring the acceptor level of substitutional platinum Pts-/0 (EC−ET=0.23eV) by deep level transient spectroscopy. Results show that the helium co-implantation significantly enhances platinum diffusion and allows its control up to the depths of hundreds of micrometers. The resulting Pts distribution is given by the profile of radiation damage produced by helium ions while the amount of in-diffused Pts can be controlled by the dose of platinum implantation. It is also shown that an extra annealing at 685°C performed prior to helium implantation substantially increases the amount of in-diffused platinum.

High-Performance Amorphous Silicon Emitter for Crystalline Silicon Solar Cells

AbstractThin hydrogenated amorphous silicon (a-Si:H) layers deposited by hot-wire chemical vapor deposition (HWCVD) are studied for use as the emitter in silicon heterojunction (SHJ) solar cells on p-type crystalline silicon wafers. Low interface recombination velocity and high open-circuit voltage are achieved by a low substrate temperature (&lt;150°C) intrinsic a-Si:H deposition which ensures immediate amorphous silicon deposition. This is followed by deposition of n-type a-Si:H at a higher temperature (&gt;200°C) which improves dopant activation and other properties. A prolonged atomic H pretreatment to clean the c-Si surface is actually detrimental because it creates additional defects in the c-Si lattice. However, a brief H pretreatment is beneficial and may render the intrinsic interlayer unnecessary. The n-type a-Si:H thickness must be limited to ~5 nm to minimize current loss, because the phosphorous doped a-Si:H layer has significant absorption in the usable solar spectrum. Using the optimized a-Si:H emitter, we obtain efficiency of nearly 17% on planar float-zone (FZ) silicon and 15% on planar Czochralski (CZ) silicon substrates with aluminum back-surface-field (Al-BSF) and contacts.

High efficiency screen-printed EFG Si solar cells through rapid thermal processing-induced bulk lifetime enhancement

This paper shows that one second (1 s) firing of Si solar cells with screen-printed Al on the back and SiN x anti-reflection coating on the front can produce a high quality Al-doped back-surface-field (Al-BSF) and significantly enhance SiN x -induced defect hydrogenation in the bulk Si. Open-circuit voltage, internal quantum efficiency measurements, and cross-sectional scanning electron microscopy pictures on float-zone silicon cells revealed that 1 s firing in rapid thermal processing at 750°C produces just as good a BSF as 60s firing, indicating that the quality of Al-BSF region is not a strong function of RTP firing time at 750°C. Analysis of edge-defined film-fed grown (EFG) Si cells showed that short-term firing is much more effective in improving the hydrogen passivation of bulk defects in EFG Si. Average minority-carrier lifetime in EFG wafers improved from ∼3 to ∼33 ps by 60s firing but reached as high as 95μs with 1 s firing, resulting in 15.6% efficient screen-printed cells on EFG Si.

Radiation damage effects on CMS sensors quality assurance and irradiation tests

The large hadron collider (LHC) at the Centre Europeenne pour la Recherche Nucleaire (CERN), Geneva, Switzerland, is a proton-proton collider with a luminosity of 10/sup 34//cm/sup 2/s and will be working for ten years (starting in 2007). Compact muon solenoid (CMS) will be one of the four general-purpose detectors. The CMS tracker consists of ten barrel layers, plus 2 /spl times/ 9 end cap discs, which amounts to a total of 24 328 silicon sensors with a total area of 206 m/sup 2/ silicon, covering a pseudorapidity of |/spl eta/|/spl les/2.5. For the sensors close to the beam pipe, fluences of 1.6/spl middot/10/sup 14/n/sub 1 MeV//cm/sup 2/ are expected over the ten-year lifetime. To guarantee the functionality of the single-side silicon sensors during the runtime of the LHC, quality assurance was developed. In the two Irradiation Qualification Centers (IQCs) in Karlsruhe, Germany, and Louvain-la-Neuve, Belgium, a fraction of 1% of the sensors are electrically qualified. In Karlsruhe, the sensors are irradiated with 26-MeV protons and in Louvain-la-Neuve with neutrons at an average energy of 20 MeV. For reasons of flux uncertainties in the CMS tracker, sensors are irradiated with a fluence 50% higher than predicted. For standard float zone silicon, large material dependencies have been observed under irradiation resulting in changes to various electrical parameters. To guarantee the functionality of the sensors in the tracker, it is very important to know these parameters. These electrical parameters have been determined before and after irradiation and are discussed in the following sections.

High-energy electron irradiation of different silicon materials

The effects of 900 MeV electron irradiation on different types of silicon substrates (standard and oxygenated float-zone, Czochralski, and epitaxial silicon) have been experimentally investigated. Irradiations up to a fluence of 2.1/spl times/10/sup 15/ e/cm/sup 2/ have been performed with the electron beam of the LINAC injector at the synchrotron light facility Elettra in Trieste (Italy). Irradiated devices have been electrically characterized by reverse I-V and C-V measurements. Substrate type inversion has been observed for standard and oxygenated float-zone but not for Czochralski and epitaxial devices. The effects of isothermal annealing cycles at 80/spl deg/C have also been studied, and the hardness factor of 900 MeV electrons, with respect to 1 MeV neutrons, has been experimentally estimated from the measurement of the reverse leakage current after annealing.