Czochralski Silicon Research Articles

Removing the effect of striations in n-type silicon solar cells

Several industrial n-type Czochralski silicon ingots were analysed on wafer and cell levels with ECN׳s bifacial n-type solar cell process. In some of the ingots, the solar cell performance in the very top drop of about 1% absolute with respect to cell from the middle part of the ingot. These cells show typical ring shaped pattern. After receiving a post-process anneal treatment at 200°C, the efficiency nearly completly recover. We demonstrated that the improvement is due to bulk lifetime enhancement. The recovery is stable in storage conditions, under illumination and high temperature treatments up to 600°C. The same effect cannot be reproduced in p-type Cz silicon solar cells with similar ring shaped patterns. This indicates that the defects responsible for lifetime and efficiency degradation in wafers affected by ring patterns differ in n-type and p-type.

Impact of Rapid Thermal Oxidation at Ultrahigh-Temperatures on Oxygen Precipitation Behavior in Czochralski-Silicon Crystals II

Oxygen precipitates (OPs) in a Czochralski silicon (Cz-Si) wafer can degrade the performance of semiconductor devices if they are in the active region of the Si wafer, i.e., the surface layer. On the other hand, they can improve device fabrication yield through their ability to enhance impurity gettering and through mechanical strengthening, both of which are important characteristics for semiconductor device fabrication. 1,2 Thus, the OPs must be controlled in an appropriate manner depending on the structure of the intended semiconductor devices and the production process employed. More precise and uniform control of OPs in Si wafers will be a significant factor for future advanced semiconductor devices. To understand this very important issue, we previously investigated the re-formation effect of OP nuclei using ultrahigh-temperature rapid thermal oxidation (RTO), at over 1300 ◦ C, and achieved wide and precise controllability of new OP nuclei. 3 This technique also demonstrated a remarkable ability to eliminate heterogeneity effects such as OP nuclei or related defects in the grown crystal. Ultrahigh-temperature annealing using rapid thermal processing (RTP) has significant advantages in terms of excellent temperature uniformity in the radial direction of the Si wafer as compared to the Cz-Si crystal growth process. As mentioned above, ultrahigh-temperature RTO showed a remarkable ability to control the effects of the OP nuclei. The behavior of OP nuclei in Cz-Si crystals is strongly related to point defects such as vacancies and Si interstitials. 4 Vacancies promote the formation of OP nuclei because these nuclei are formed as a complex between oxygen atoms and vacancies. It is generally known that annealing in an oxygen atmosphere is not a practical way to increase vacancy concentration because Si interstitials are dominant in the Si wafer due to the injection of Si interstitials from the oxidized surface. However, as we previously reported, 3 the behavior of OP nuclei can be controlled by changing the dominant point defects from Si interstitials to vacancies depending on the difference of each thermal equilibrium concentration at ultrahigh temperatures, even though the oxidation atmosphere is used for RTP. Furthermore, in the case of ultrahigh-temperature RTO, it is expected that Si interstitials also exist at high concentration due to the Si surface oxidation even though the dominant point defects are vacancies. Interstitial Si point defects are expected to annihilate vacancy-related defects such as void defects when enough interstitial Si atoms exist. 5 The void defects also influence the semiconductor device performance; therefore, in addition to OP control, annihilating void defects is a very important goal. It is well known that RTP in a nitrogen or argon atmosphere does not effectively annihilate void defects under the surface of the Si wafer. 6‐8 Ultrahigh-temperature RTO would gain a further advantage over RTP in a nitrogen or argon atmosphere if annihilation of void defects is confirmed. In this study, the annihilation behaviors of void defects in the case of the ultrahigh-temperature RTO were evaluated in detail.

Misfit strain of oxygen precipitates in Czochralski silicon studied with energy-dispersive X-ray diffraction

Annealed Czochralski Silicon wafers containing SiOx precipitates have been studied by high energy X-ray diffraction in a defocused Laue setup using a laboratory tungsten tube. The energy dispersive evaluation of the diffracted Bragg intensity of the 220 reflection within the framework of the statistical dynamical theory yields the static Debye-Waller factor E of the crystal, which gives access to the strain induced by the SiOx precipitates. The results are correlated with precipitate densities and sizes determined from transmission electron microscopy measurements of equivalent wafers. This allows for the determination of the constrained linear misfit ε between precipitate and crystal lattice. For samples with octahedral precipitates the values ranging from ε = 0.39 (+0.28/−0.12) to ε = 0.48 (+0.34/−0.16) indicate that self-interstitials emitted into the matrix during precipitate growth contribute to the lattice strain. In this case, the expected value calculated from literature values is ε = 0.26 ± 0.05. Further, the precise evaluation of Pendellösung oscillations in the diffracted Bragg intensity of as-grown wafers reveals a thermal Debye-Waller parameter for the 220 reflection B220(293 K) of 0.5582 ± 0.0039 Å2 for a structure factor based on spherically symmetric scattering contributions.

Enhanced internal gettering in n/n+ epitaxial silicon wafer: coaction of nitrogen impurity and vacancy on oxygen precipitation in substrate

The effects of the prior rapid thermal processing (RTP) at 1,250 °C in argon (Ar) and nitrogen (N2) atmosphere on oxygen precipitation (OP) and associated internal gettering (IG) in the heavily arsenic (As)-doped Czochralski (CZ) silicon substrates of n/n+ epitaxial silicon wafers have been comparatively investigated. It is found that OP in the heavily As-doped silicon subjected to the two-step anneal of 650 °C/16 h + 1,000 °C/16 h can be significantly enhanced by the prior RTP in either Ar or N2 atmosphere. By comparison, the prior RTP in N2 atmosphere exhibits much stronger enhancement effect on OP thus leading to a better IG capability of the epitaxial wafer. Secondary ion mass spectroscopy measurement reveals that nitrogen atoms of high concentration are injected into the heavily As-doped silicon substrate by the RTP at 1,250 °C in N2 atmosphere. It is believed that the vacancy- and nitrogen-assisted heterogeneous nucleation mechanisms are simultaneously operative for OP in the substrate subjected to the prior RTP in N2 atmosphere, thus leading to the more pronounced OP and therefore the better IG capability. It is expected that the present work offers a strategy feasible for improving the IG capability of n/n+ epitaxial silicon wafers.

Growth Dynamics of Single Void during Czochralski Silicon Crystal Growth Using Phase-Field Modeling

To investigate the growth dynamics of the single void during Czochralski silicon growth as well as capture the basic features of the diffusion-controlled dynamic mechanisms, a phase field method has been developed. The free energy of the system involving the chemical free energy and the gradient energy is presented. Numerical tests were performed to examine the capability of this model, and the results show that: the void grows due to the absorption of vacancies in the matrix, which essentially reduces the free energy of the system; with the growth of the void, there forms vacancy concentration gradient towards the void in the matrix; the increase of initial vacancy concentration contributes to a larger void size and growth rate.

Morphology of oxygen precipitates in silicon wafers pre-treated by rapid thermal annealing

The morphology of oxygen precipitates in Czochralski silicon wafers pre-treated by rapid thermal annealing (RTA) and subjected to a heat treatment in the temperature range between 800 °C and 1000 °C was investigated by scanning transmission electron microscopy. The samples were pre-treated by RTA in order to establish a defined supersaturation of vacancies. It was found that in such vacancy-rich samples subjected to an annealing at 800 °C three dimensional dendrites are formed. Until now, it was known that during annealing at 800 °C plate-like oxygen precipitates are formed.

Oxygen-Related Thermal Donor Formation in Dopant-Rich Compensated Czochralski Silicon

The thermal donor (TD) generation in dopant-rich compensated Czochralski silicon was studied by pulling an ingot from a feedstock containing large amounts of donors and acceptors. In a wafer located in the vicinity of the change of conductivity type, thermal donors were formed and the evolution of their concentration was similar to that in noncompensated lowly boron-doped silicon. Thus, simultaneous high densities of both boron and phosphorus do not have a significant impact on the TD formation. This brings the experimental evidence that for a given oxygen concentration and annealing temperature, the TD formation is controlled by the electron density.

Anisotropic study of thermal stresses induced by diameter fluctuation during Czochralski silicon single crystal growth

Instabilities in the Czochralski crystal growth process result in deviation from the desired crystal diameter i.e. uneven crystal surface. The excessive thermoelastic stress induced by diameter perturbation is studied by mean of numerical simulations. A set of 3D simulation has been performed for an axisymmetric crystal. The crystal anisotropy is taken into account. The influence of crystal diameter fluctuation on stress field inside the crystal has been studied. The resolved shear stresses have been calculated for 12 slip systems for three crystal orientations. Accumulated excess stress from its critical value is calculated. Simulation results suggest that crystal surface undulation affects both thermal field and stress distribution inside the crystal. The crystal region with high risk of dislocation generation is discussed for three crystal orientation. The stress level in [111] crystal orientation is found to be lower than other orientations.

The Application of CFZ-Si Monocrystal Material in Photovoltaic Field

As the main substrate materials for solar cell, Czochralskisilicon (CZ-Si) has more crystal defects, higher impurities content and so on, which limit the further improvement of conversion efficiency (h). Floating zone melting silicon (FZ-Si) has excellent performance but the feed rod cost is much higher, so it is hardly used for solar cell widely. To solve this problem, we developed CFZ silicon monocrystal (CFZ-Si). First the poly was made into poly rod withΦ110~Φ130mm by CZ process and then made into CFZ-Si by FZ method. During FZ process, the resistivity is controlled by gas doping and the process is adjustable. CFZ-Si combines advantages of CZ-Si with FZ-Si. It has less crystal defects, lower impurities content especially oxygen content (<0.2 ppm) and excellent consistency of axial resistivity. Meanwhile, poly rod used has lower price and easier fabrication, so CFZ-Si cost is much lower and the further cost reduction is easy. Therefore, CFZ-Si has significant comprehensive advantages and broader prospect in the future photovoltaic field.

Correlation between Copper Precipitation and Grown-In Oxygen Precipitates in 300~mm Czochralski Silicon Wafer

Correlation between Copper Precipitation and Grown-In Oxygen Precipitates in 300~mm Czochralski Silicon Wafer

Processing for Highly Emissive CZ-Silicon by Depositing Stressed Sol–Gel Films

Enhanced band-gap emission from Czochralski silicon substrates of up to ~100 times is reported. This was achieved by processing for a stressed interface resulting from baked and annealed silica films prepared by sol–gel processes. The active dopants include but are not limited to erbium and are prepared with tetraethylorthosilicate (TEOS) while forming the active precursors using oxide and nitrate forms of the rare earth. In addition, annealed films produce infrared emission in the 1.5-μm band from erbium ions in the film. Steady-state photoluminescence studies indicate that a strong correlation of the intensity of the emission at the band gap to the stress formed at the interface and is a direct function of the annealing temperature of the silica films, independent from the known erbium 4f emission bands.

Effect of dopant compensation on the temperature dependence of the transport properties in p-type monocrystalline silicon

In this paper, we investigate the temperature variations of the hole transport properties in initially uncompensated boron-doped Czochralski silicon progressively compensated through thermal donors activation. After each donor generation anneal, the boron and thermal donor concentrations in the samples are determined using (1) the change in carrier concentration at room temperature and (2) the analysis of the temperature variation of the carrier concentration in the range 77–350 K. By comparing both methods with theory, evidence is brought that down to 77 K the Hall factor is unaffected by compensation up to high compensation levels. This is of great interest for researchers working on new solar-grade materials since it nicely suggests that Hall factor models previously established for non-compensated silicon can be applied to compensated samples, for example, when extracting the individual dopant concentrations from the temperature variations of the hole concentration. At very high compensation levels, anomalous Hall data lead to erroneously low carrier mobility values. We showed that this artifact was due to the formation of a n-p-n transistor, arising from the preferential formation of thermal donors in the sample's subsurface. After rejecting these unphysical data from the analysis, we confirm that the hole mobility is greatly affected by compensation in the temperature range investigated. We eventually confront our experimental data to current mobility models and discuss the possible sources of discrepancy.

Impact of metal-organic vapor phase epitaxy environment on silicon bulk lifetime for III–V-on-Si multijunction solar cells

With the final goal of integrating III–V materials on silicon substrates for tandem solar cells, the influence of the metal-organic vapor phase epitaxy (MOVPE) environment on the minority carrier properties of silicon wafers has been evaluated. These properties will essentially determine the photovoltaic performance of the bottom cell in a III–V-on-Si tandem solar cell. A comparison of the base minority carrier lifetimes obtained for different thermal processes carried out in a MOVPE reactor on Czochralski silicon wafers has been carried out. An important degradation of minority carrier lifetime during the surface preparation (i.e. H2 anneal) has been observed. Three different mechanisms have been proposed for explaining this behavior: (1) the introduction of extrinsic impurities coming from the reactor; (2) the activation of intrinsic lifetime killing impurities coming from the wafer itself; and finally, (3) the formation of crystal defects, which eventually become recombination centers. The effect of the emitter formation by phosphorus diffusion has also been evaluated. In this sense, it has been reported that lifetime can be recovered during the emitter formation either by the effect of the P on extracting impurities, or by the role of the atomic hydrogen on passivating the defects.

Manipulation of K center charge states in silicon nitride films to achieve excellent surface passivation for silicon solar cells

High quality surface passivation (Seff < 5 cm/s) was achieved on polished float zone and textured p- and n-type solar grade Czochralski silicon substrates by externally injecting and storing positive or negative charges (>±8 × 1012 cm−2) into a dual layer stack of Plasma Enhanced Chemical Vapor Deposition (PECVD) Silicon Nitride (SiNx)/PECVD Silicon Oxide (SiO2) films using a corona charging tool. We demonstrate long term stability and uniform charge distribution in the SiNx film by manipulating the charge on K center defects while negating the requirement of a high temperature thermal oxide step.

Strip Detectors Processed on High-Resistivity 6-inch Diameter Magnetic Czochralski Silicon (MCz-Si) Substrates

Tracking detectors for future high-luminosity particle physics experiments have to be simultaneously radiation hard and cost efficient. This paper describes processing and characterization of p <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> /n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-</sup> /n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> (n-type silicon bulk) detectors made of high-resistivity Magnetic Czochralski silicon (MCz-Si) substrates with 6-inch wafer diameter. The processing was carried out on a line used for large-scale production of sensors using standard fabrication methods, such as implanting polysilicon resistors to bias individual sensor strips. Special care was taken to avoid the creation of Thermal Donors (TD) during processing. The sensors have a full depletion voltage of 120-150 V which are uniform over the investigated sensors. All of the leakage current densities were below 55 nA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 200 V bias voltage. A strip sensor with 768 channels was attached to readout electronics and tested in particle beam with a data acquisition (DAQ) similar to the system used by the CMS experiment at the CERN LHC. The test beam results show a signal-to-noise ratio greater than 40 for the test beam sensor. The results demonstrate that MCz-Si detectors can reliably be manufactured in the industrial scale semiconductor process.

Strategies to suppress A-center formation in silicon and germanium from a mass action analysis viewpoint

We investigate the impact of tin (Sn) doping on the formation and the thermal stability of the vacancy-oxygen (VO or A-center) in the electron-irradiated Czochralski silicon (Si) and its conversion to the VO2 defects. Previous experimental studies are consistent with the viewpoint that Sn (and other oversized isovalent atoms) doping suppresses the formation of the A-center. The results are discussed in view of recent density functional theory calculations, whereas we employ mass action analysis to calculate the impact of isovalent dopants on the suppression of the A-center. We propose point defect engineering strategies to suppress the concentration of the deleterious A-centers in Si and in related materials such as germanium.

Three-dimensional numerical simulation of flow, thermal and oxygen distributions for a Czochralski silicon growth with in a transverse magnetic field

A three-dimensional numerical simulation has been performed to understand the motion of the melt flow and thermal field during the Czochralski silicon single crystal growth process under the influence of a transverse magnetic field. With the application of a transverse magnetic field, the velocity, temperature and oxygen concentration fields in the melt become three-dimensional and asymmetric. Stronger downward flow motion is formed under the melt-crystal interface which can prevent the movement of oxygen impurities towards the melt-crystal interface. This may explain why the presence of a transverse magnetic field decreases the oxygen concentration level along the melt–crystal interface. The uniformity of the oxygen concentration at the melt–crystal interface is also improved when the magnetic field is applied. There is an increase in either the crystal diameter or the crucible rotation rate, and the oxygen concentration level becomes greater. The uniformity of oxygen concentration is better for higher crystal diameters.

Bifacial concentrator Ag‐free crystalline n‐type Si solar cell

AbstractWe report results obtained using an innovative approach for the fabrication of bifacial low‐concentrator thin Ag‐free n‐type Cz‐Si (Czochralski silicon) solar cells based on an indium tin oxide/(p+nn+)Cz‐Si/indium fluorine oxide structure. The (p+nn+)Cz‐Si structure was produced by boron and phosphorus diffusion from B‐ and P‐containing glasses deposited on the opposite sides of n‐type Cz‐Si wafers, followed by an etch‐back step. Transparent conducting oxide (TCO) films, acting as antireflection electrodes, were deposited by ultrasonic spray pyrolysis on both sides. A copper wire contact pattern was attached by low‐temperature (160°C) lamination simultaneously to the front and rear transparent conducting oxide layers as well as to the interconnecting ribbons located outside the structure. The shadowing from the contacts was ~4%. The resulting solar cells, 25 × 25 mm2 in dimensions, showed front/rear efficiencies of 17.6–17.9%/16.7–17.0%, respectively, at one to three suns (bifaciality of ~95%). Even at one‐sun front illumination and 20–50% one‐sun rear illumination, such a cell will generate energy approaching that produced by a monofacial solar cell of 21–26% efficiency. Copyright © 2014 John Wiley &amp; Sons, Ltd.

Effect of rapid thermal annealing on recombination centres in boron-doped Czochralski-grown silicon

Rapid thermal annealing in a belt furnace results in a dramatic change of the recombination properties of boron-doped Czochralski silicon: (1) the lifetime degraded by applying a prolonged illumination at room temperature was significantly improved, (2) after subsequent dark recovery, the lifetime has a remarkably high value, and (3) the permanent recovery, by annealing at 185 °C under illumination, is enormously accelerated, and the finally achieved stable lifetime acquires a record value of 1.5 ms, as compared to 110 μs after permanent recovery of not-annealed reference samples.

Boron deactivation in heavily boron-doped Czochralski silicon during rapid thermal anneal: Atomic level understanding

The changes in hole concentration of heavily boron (B)-doped Czochralski silicon subjected to high temperature rapid thermal anneal (RTA) and following conventional furnace anneal (CFA) have been investigated. It is found that decrease in hole concentration, namely, B deactivation, is observed starting from 1050 °C and increases with RTA temperature. The following CFA at 300–500 °C leads to further B deactivation, while that at 600–800 °C results in B reactivation. It is supposed that the interaction between B atoms and silicon interstitials (I) thus forming BI pairs leads to the B deactivation during the high temperature RTA, and, moreover, the formation of extended B2I complexes results in further B deactivation in the following CFA at 300–500 °C. On the contrary, the dissociation of BI pairs during the following CFA at 600–800 °C enables the B reactivation. Importantly, the first-principles calculation results can soundly account for the above-mentioned supposition.