Float Zone Silicon Research Articles

Effect of Crystal Rotation on Melt Convection and Resistivity Distribution in 200 mm Floating Zone Silicon Single Crystal Growth.

Floating silicon is particularly suitable for the production of power devices and detectors due to its high purity and high resistivity. However, when the crystal diameter increases to 200 mm, the inhomogeneous distribution of dopants in the radial direction of the crystal becomes an important factor affecting the quality of the crystal. In this paper, the melt flow and crystal interface dopant distribution of 200 mm floating zone silicon under different crystal rotation modes using 2D axisymmetric models was calculated. In the simulation, the crystal rotates unidirectionally clockwise or alternates between clockwise and counterclockwise rotations and is compared with the corresponding experimental results. The results of alternate rotation show a significant improvement in the distribution of resistivity, which is explained from the perspective of melt flow, providing ideas for the industrial production of high-quality FZ silicon crystals.

A scalable method for fabricating monolithic perovskite/silicon tandem solar cells based on low-cost industrial silicon bottom cells

Tandem solar cells composed of perovskite and silicon (PVSK/Si TSCs) exhibit significant potential for improving the power conversion efficiency (PCE) and reducing the levelized cost of electricity. Currently, most tandem cells demonstrating high efficiencies utilize costly float zone (FZ) silicon wafers, which pose limitations for large-scale production due to their high expense. This study presents a scalable approach for manufacturing perovskite/silicon tandem solar cells using industrial silicon bottom cells. We employ a double-layer intrinsic amorphous silicon passivation layer to enhance the carrier lifetime of the bottom silicon cell. Additionally, we introduce a novel indium oxide doped with transition metals (IMO) transparent electrode to enhance near-infrared (NIR) light absorption. To achieve silicon bottom cells with a polished front surface, we utilize a cost-effective and time-saving saw damage etching process. The perovskite absorber is then deposited on the polished surface using slot-die coating. Furthermore, we coat the perovskite absorber with a mixed solution of FAI and mF-PEAI via slot-die coating to eliminate excess PbI2 on the surface and passivate surface defects. Through the integration of top and bottom sub-cells, we obtained a PCE exceeding 24.22 % from a 5 cm × 5 cm TSCs device (with an aperture area of 3.8 cm × 3.8 cm) and a PCE of 28.68 % from a 2.5 cm × 2.5 cm TSCs device (with an aperture area of 1 cm × 1 cm).

Numerical modelling of melting front structures during floating zone silicon crystal growth

We propose a mathematical model of the thin molten silicon film to describe the quasi-steady shape of the inhomogeneous structures on the open melting front of polycrystalline feed rod during the floating zone process for single crystal silicon growth. As a result, precise shapes of the phase boundaries in a three-phase environment on a local scale were determined with the use of moving meshes. The presence of the local structures influences the heat transfer on the open melting front. Consequently, the global phase boundary model can be adjusted, resulting in improved agreement with the experimentally measured interface shape.

Enhancing dielectric-silicon interfaces through surface electric fields during firing

Minimising charge losses at silicon interfaces is a major development area for highly efficient solar cells. Here we report on the interface improvements achieved by establishing a surface electric field during low-temperature firing of dielectric thin films. By inducing a corona electric field on the surface of a thin film stack, we observe significant modifications to the silicon-dielectric interface upon annealing, which correlate with the characteristics of interface defects. The passivation properties of the interfaces strongly depend on the polarity and strength of the electric field during firing, as well as the dielectric materials in the layer stack. We show that the surface electric fields not only influence surface carrier population but also affect the resulting chemical interface properties post-annealing. It is postulated that hydrogen migration plays a role in these observed effects. Leveraging the corona-induced electric field enables fine-tuning of both the chemical and field-effect passivation in thin film surface dielectrics, resulting in recombination current densities as low as 2.8 fA cm−2 in research-grade float zone silicon, and 14 fA cm−2 in industrial-grade textured silicon. The simplicity and versatility of the thin film electric polarisation enable a new strategy for controlling and exploiting the chemical enhancement of interfaces in solar cell devices, from current TOPCon and PERC devices to future multijunction silicon-based cells.

Multi-label oxide classification in float-zone silicon crystal growth using transfer learning and asymmetric loss

AbstractFloat-Zone (FZ) crystal growth process allows for producing higher purity silicon crystal with much lower concentrations of impurities, in particular low oxygen content. Nevertheless, the FZ process occasionally faces the problem of small contamination from oxidation. This can come in the form of a thin oxide layer that may form on un-melted polysilicon surface. The appearance of the oxide layer indicates degraded machine performance and the need for machine maintenance. Therefore, oxide investigation is important for improving both the FZ process and FZ machines, and the first step is oxide recognition. In this study, we characterized oxide into mainly three varieties, according to their surface texture characteristics, which are: (i) spot (ii) shadow and (iii) ghost curtain. We leveraged FZ images captured from the vision system integrated on the FZ machine to establish an oxide dataset. Targeted for data imbalance problem in our dataset, a method based on transfer learning and asymmetric loss for multi-label oxide classification is presented in this work. The results showed that the pre-trained model and the asymmetric loss used for training outperformed the baseline models and improved the classification performance. Furthermore, this study deeply investigated the effectiveness of the components of asymmetric loss. Finally, Gradient-weighted Class Activation Mapping (Grad-CAM) was employed to explain decision process of the models in order to adopt them in the industry.

Investigation of high resistivity [formula omitted]-type FZ silicon diodes after 60Co [formula omitted]-irradiation

In this work, the effects of 60Co γ-ray irradiation on high resistivity p-type diodes have been investigated. The diodes were exposed to dose values of 0.1, 0.2, 1, and 2MGy. Both macroscopic (I–V, C–V) and microscopic investigations, by means of Thermally Stimulated Current (TSC) and Deep Level Transient Spectroscopy (DLTS) techniques, were conducted to characterize the radiation-induced changes. The investigated diodes were manufactured on high resistivity p-type Float Zone (FZ) silicon and were further classified into two types based on the isolation technique between the pad and guard ring: p-stop and p-spray. After irradiation, the macroscopic results of current–voltage and capacitance–voltage measurements were obtained and compared with existing literature data. Additionally, the microscopic measurements focused on the development of the concentration of different radiation-induced defects, including the Boron interstitial-Oxygen interstitial (BiOi) complex, the Carbon interstitial-Oxygen interstitial (CiOi) defect, the H40K, and the so-called IP∗.To investigate the thermal stability of induced defects in the bulk, isochronal annealing studies were performed in the temperature range of 100°C to 300°C. These annealing processes were carried out on diodes irradiated with doses of 1 and 2MGy. Furthermore, in order to investigate the unexpected results observed in the C–V measurements after irradiation with high dose values, the surface conductance between the pad and guard ring was measured as a function of both dose and annealing temperature.

Activation of Al2O3 surface passivation of silicon: Separating bulk and surface effects

Understanding surface passivation arising from aluminium oxide (Al2O3) films is of significant relevance for silicon-based solar cells and devices that require negligible surface recombination. This study aims to understand the competing bulk and surface lifetime effects which occur during the activation of atomic layer deposited Al2O3. We demonstrate that maximum passivation is achieved on n- and p-type silicon with activation at ∼ 450 °C, irrespective of annealing ambient. Upon stripping the Al2O3 films and re-passivating the surface using a superacid-based technique, we find the bulk lifetime of float-zone and Czochralski silicon wafers degrade at annealing temperatures > 450 °C. By accounting for this bulk lifetime degradation, we demonstrate that the chemical passivation component associated with Al2O3 remains stable at activation temperatures of 450─500 °C, achieving an SRV of < 1 cm/s on n- and p-type silicon. In conjunction with the thermal stability, we show that films in the range of 3–30 nm maintain an SRV of < 1 cm/s when annealed at 450 °C. From atomic-level energy dispersive X-ray analysis, we demonstrate that, post deposition, the interface has a structure of Si/SiO2/Al2O3. After activation at > 300 °C, the interface becomes Si/SixAlyO2/Al2O3 due to diffusion of aluminium into the thin silicon oxide layer.

The NIST silicon lattice comparator upgrade.

The NIST silicon lattice comparator has been in service, in various forms, since the 1970s. It is capable of measuring the difference in lattice spacing between specimens of high-quality float-zone silicon to Δd/d ≈ 6 × 10-9. It has recently undergone a thorough update of its control systems and mechanics. These upgrades result in the ability to collect data with improved stability, less settling time of the instrument, and less operator intervention.

Terahertz integration platforms using substrateless all-silicon microstructures

The absence of a suitable standard device platform for terahertz waves is currently a major roadblock that is inhibiting the widespread adoption and exploitation of terahertz technology. As a consequence, terahertz-range devices and systems are generally an ad hoc combination of several different heterogeneous technologies and fields of study, which serves perfectly well for a once-off experimental demonstration or proof-of-concept, but is not readily adapted to real-world use case scenarios. In contrast, establishing a common platform would allow us to consolidate our design efforts, define a well-defined scope of specialization for “terahertz engineering,” and to finally move beyond the disconnected efforts that have characterized the past decades. This tutorial will present arguments that nominate substrateless all-silicon microstructures as the most promising candidate due to the low loss of high-resistivity float-zone intrinsic silicon, the compactness of high-contrast dielectric waveguides, the designability of lattice structures, such as effective medium and photonic crystal, physical rigidity, ease and low cost of manufacture using deep-reactive ion etching, and the versatility of the many diverse functional devices and systems that may be integrated. We will present an overview of the historical development of the various constituents of this technology, compare and contrast different approaches in detail, and briefly describe relevant aspects of electromagnetic theory, which we hope will be of assistance.

Silicon lenses with HDPE anti-reflection coatings for low THz frequency range

Presented in this paper have been the design, fabrication, and testing of the high resistance floating-zone silicon (HRFZ-Si) optics with the anti-reflection (AR) high-density polyethylene (HDPE) coatings, for the low part of terahertz (THz) frequency range (ν ≈ 0.14 THz), by using the precision press molding. Experimental results of the transmission of the wafers and lenses with double-sided anti-reflection HDPE coatings for radiation frequency 0.14 THz showed an increase in the transmittance T values up to ≈1.45 times as compared to T magnitudes in wafers and lenses without HDPE coatings. The capability to use terahertz lenses with HDPE interference films and the technology of press molding, as a cheaper alternative to the horn antenna applications for the terahertz range of 0.14 THz was shown. With advancements in THz imaging and 6G communication technologies, further implementation of these Si optical elements is possible.

The Impact of Different Hydrogen Configurations on Light- and Elevated-Temperature- Induced Degradation

In this article, the impact of different hydrogen configurations and their evolution on the extent and kinetics of light- and elevated-temperature-induced degradation (LeTID) is investigated in float-zone silicon via charge carrier lifetime measurements, low-temperature Fourier-transform infrared spectroscopy, and four-point-probe resistance measurements. Degradation conditions were light soaking at 77 °C and 1 sun-equivalent illumination intensity and dark anneal at 175 °C. The initial configuration of hydrogen is manipulated by varying the wafer thickness, the cooling ramp of the fast-firing process, and the dopant type (B- or P-doped). We find lower hydrogen concentrations in thinner samples and samples with a slower cooling ramp. This suggests that hydrogen diffuses out of the sample during the cool-down, which strongly affects the final concentration of hydrogen molecules H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , and to a smaller degree the concentration of boron-hydrogen (BH) pairs. A regeneration of potential LeTID defects and a presumed LeTID degradation during dark annealing is found in n-type Si. In p-type Si, the LeTID extent was found to scale with H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , suggesting a direct link between both. The temporal evolution of BH pairs, LeTID degradation/regeneration, and surface degradation depends on wafer thickness and the cooling ramp of the fast-firing process. Based upon these findings, we formulate a theory of the hydrogen-related mechanism behind LeTID: Hydrogen originating from H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> moves between different temporary traps. First, hydrogen binds to LeTID precursors and acceptor atoms in the silicon bulk, later moving toward the surface. This leads first to the LeTID degradation and regeneration and then to the degradation of surface passivation.

Precision mapping of a silicon test mass birefringence

Excellent mechanical and thermal properties of silicon make it a promising material for the test masses in future gravitational wave detectors. However, the birefringence of silicon test masses, due to impurity and residual stress during crystal growth or external stress, can reduce the interference contrast in an interferometer. Using the polarization–modulation approach and a scanning system, we mapped the birefringence of a float zone silicon test mass in the ⟨100⟩ crystal orientation to assess the suitability of such material for future gravitational wave detectors. Apart from the stress-induced birefringence at the supporting area due to the weight of the test mass, the high resolution birefringence map of the silicon test mass revealed a high birefringence feature in the test mass. At the central 40 mm area, birefringence is in the range of mid 10−9 to low 10−8, which satisfy the requirement for future gravitational wave detectors.

Formation of light-emitting defects in silicon by swift heavy ion irradiation and subsequent annealing

High-resistivity floating-zone silicon (FZ-Si) and n-type (4.5 Ohm·cm) Czochralski silicon (CZ-Si) wafers were irradiated at room temperature with 167 MeV Xe ions to fluences ranging from 5·1010 to 5·1011 cm−2. Temperature dependent photoluminescence (PL) measurements were performed to characterize the as-implanted and annealed silicon wafers. It was found that irradiation with swift heavy ions (SHI) leads to the formation of light-emitting defects in both types of silicon wafers. Low-temperature annealing (400 °C) leads to a significant increase of the photoluminescence intensity. In addition, the photoluminescence decreases for increasing measurement temperatures and eventually quenching is observed for temperatures larger than 80 K. The type of silicon wafer (FZ or CZ) does not significantly influence the defect-related light-emitting properties. This implies that low fluence SHI irradiation is a promising method for creation of light-emitting defects in silicon.

Numerical Simulation of Species Segregation and 2D Distribution in the Floating Zone Silicon Crystals

The distribution of dopants and impurities in silicon grown with the floating zone method determines the electrical resistivity and other important properties of the crystals. A crucial process that defines the transport of these species is the segregation at the crystallization interface. To investigate the influence of the melt flow on the effective segregation coefficient as well as on the global species transport and the resulting distribution in the grown crystal, we developed a new coupled numerical model. Our simulation results include the shape of phase boundaries, melt flow velocity and temperature, species distribution in the melt and, finally, the radial and axial distributions in the grown crystal. We concluded that the effective segregation coefficient is not constant during the growth process but rather increases for larger melt diameters due to less intensive melt mixing.

Electronic Properties of Light- and Elevated Temperature-Induced Degradation in Float-Zone Silicon

Light- and elevated temperature-induced degradation (LeTID) causes long-term instabilities, especially in passivated emitter and rear cells, leading to severe performance loss of commercial modules. Despite the hundreds of LeTID reports available in the literature, there is little consensus regarding the underlying defects and defect formation mechanism responsible for this degradation and its exact electronic properties. Recently, it has been shown that a form of carrier-induced degradation similar to that of LeTID is also observed in some high-purity silicon crystals grown by the float-zone method. In this work, using deep level transient spectroscopy and lifetime spectroscopy, we study the role of nitrogen on the degradation. Intentional contaminated samples with different and known level of nitrogen have been specially grown and characterized for this study. We detect the appearance of a set of majority carrier traps in the degraded state of the samples with activation energies 0.1 (H85), 0.43 (H270A), 0.39 (H270B), and 0.46 eV (H200) with respect to the valence band, from which H270A appears to correlate with the degradation. The results show that the extent of degradation in the nitrogen-rich samples is at least double that of the nitrogen-lean samples.

Quasi-monocrystalline silicon for low-noise end mirrors in cryogenic gravitational-wave detectors

Mirrors made of silicon have been proposed for use in future cryogenic gravitational-wave detectors, which will be significantly more sensitive than current room-temperature detectors. These mirrors are planned to have diameters of $\ensuremath{\approx}50$ cm and a mass of $\ensuremath{\approx}200$ kg. While single-crystalline float-zone silicon meets the requirements of low optical absorption and low mechanical loss, the production of this type of material is restricted to sizes much smaller than required. Here we present studies of silicon produced by directional solidification. This material can be grown as quasi-monocrystalline ingots in sizes larger than currently required. We present measurements of a low room-temperature and cryogenic mechanical loss comparable with float-zone silicon. While the optical absorption of our test sample is significantly higher than required, the low mechanical loss motivates research into further absorption reduction in the future. While it is unclear if material pure enough for the transmissive detector input mirrors can be achieved, an absorption level suitable for the highly reflective coated end mirrors seems realistic. Together with the potential to produce samples much larger than $\ensuremath{\approx}50$ cm, this material may be of great benefit for realizing silicon-based gravitational-wave detectors.

Quantitative analysis of boron–hydrogen pair dynamics by infrared absorption measurements at room temperature

The ability of hydrogen quantification in crystalline silicon in concentrations as low as 1014cm−3 becomes fairly important in regard to hydrogen-related degradation phenomena in silicon devices generally and solar cells particularly. The method presented here allows for direct boron–hydrogen pair quantification and, therefore, allows inference on total hydrogen content. Hydrogen-rich amorphous silicon nitride was deposited on stripes of boron-doped float-zone silicon (1 Ωcm), which were exposed to a rapid high temperature step to introduce relatively high amounts of hydrogen into the wafer. Infrared absorption spectra, which have been corrected for multiple reflection and free-carrier absorption, show absorption related to the boron–hydrogen stretching mode at ν~=1868cm−1 with varying strengths during formation and subsequent dissociation of boron–hydrogen pairs triggered by annealing in the dark at 220°C. Since the measurements were performed at room temperature, this method allows investigations with little effort and standard laboratory equipment. Furthermore, the change in free-carrier absorption (described by Drude’s theory) is used to derive the change in hole concentration concurring with the formation and dissociation of boron–hydrogen pairs. The latter is found to fairly match not only the changing strength in absorption of the stretching mode, but also the change in hole concentration obtained by highly sensitive resistivity measurements. The comparison of stretching mode absorption strength and change in resistivity allows for a calibration of specific absorption, yielding a calibration factor ABH. This calibration was performed with the absorption α [ABHα=(4.2±0.3)×1015cm−1] as well as with the quotient of absorption and wavenumber α/ν~ [ABHα/ν~=(7.8±0.6)×1018cm−2].

On the nature of thermally activated defects in n-type FZ silicon grown in nitrogen atmosphere

n-type float-zone silicon grown in a nitrogen atmosphere contains defects which are activated by temperatures between 450 and 700 °C. We use deep level transient spectroscopy (DLTS) to study the nature of these defects and the impact of the nitrogen content and the polysilicon feed stock type. We find four dominant DLTS peaks with activation energies of Ena = 0.16 eV (E1), Ena = 0.21 eV (E2), Ena = 0.34 eV (E4), and Ena = 0.64 eV (E6). We tentatively assign the two DLTS peaks E1 and E2 to single acceptor and single donor levels of the same defect, a complex of nitrogen with an impurity. Furthermore, we tentatively assign the two DLTS peaks labeled E4 and E6 to two levels of the off-center substitutional nitrogen. Based on the apparent electron capture cross sections and an analysis of the electric field effect on the emission rates, we propose them to be double and single acceptor levels, respectively. Due to its position at midgap and the competing electron and hole emission, the apparent concentration of E6 is reduced to one fifth of the total defect concentration. Correcting for these processes, we find the activation energies for electron and hole emission to be En = 0.50 eV and Ep = 0.68 eV.

Selection of the silicon sensor thickness for the Phase-2 upgrade of the CMS Outer Tracker

During the operation of the CMS experiment at the High-Luminosity LHC the silicon sensors of the Phase-2 Outer Tracker will be exposed to radiation levels that could potentially deteriorate their performance. Previous studies had determined that planar float zone silicon with n-doped strips on a p-doped substrate was preferred over p-doped strips on an n-doped substrate. The last step in evaluating the optimal design for the mass production of about 200 m2 of silicon sensors was to compare sensors of baseline thickness (about 300 μm) to thinned sensors (about 240 μm), which promised several benefits at high radiation levels because of the higher electric fields at the same bias voltage. This study provides a direct comparison of these two thicknesses in terms of sensor characteristics as well as charge collection and hit efficiency for fluences up to 1.5 × 1015 neq/cm2. The measurement results demonstrate that sensors with about 300 μm thickness will ensure excellent tracking performance even at the highest considered fluence levels expected for the Phase-2 Outer Tracker.

All-silicon, low-cross-talk terahertz waveguide crossing based on effective medium.

All-silicon effective-medium-clad waveguides are a promising candidate for an integrated terahertz platform with high efficiency and broad bandwidth. Waveguide crossings are essential circuit components, allowing for wave routing over shorter paths to increase circuit density. However, the simple intersection of two orthogonal effective-medium-clad waveguides results in terahertz wave scattering, leading to relatively high cross talk. In this work, a low-loss, 40% fractional bandwidth crossing utilizing Maxwell-Garnet effective-medium theory and wavefront planarization techniques is proposed. This monolithic structure is fabricated on a single high-resistivity float-zone silicon wafer using a deep reactive ion etching process with a modest 4.4 mm diameter (4.03λ0) structure footprint. Experimentally verified results show low insertion loss, less than 1 dB, and average cross talk level of -39dB for both E11x and E11y operating modes, over 220-330 GHz with a 40% fractional bandwidth. This waveguide crossing can be foreseen as a useful routing component for terahertz all-silicon integrated circuits. The proposed techniques are applicable to other dielectric waveguide platforms at infrared and optical frequencies.