Czochralski-grown Silicon Wafers Research Articles

Passivation of Ring Defects in Czochralski-Grown Silicon Using Magnesium Fluoride Films

This work investigates the potential application of magnesium fluoride (MgFx) films and compares its effectiveness to conventional silicon nitride (SiNx:H) for passivating ring defects, which sometimes occur in n-type Czochralski-grown silicon wafers subjected to high-temperature processes. We find that the passivation of ring defects in MgFx coated samples is fully activated only at 600 °C, whereas passivation in SiNx:H coated samples can be activated by 5 min anneals at 500 °C. Consistent with the passivation activation temperature, energy-dispersive X-ray shows that MgFx films significantly lose fluorine at temperature ≥ 600 °C. Further, subsequent annealing of the previously passivated samples reveals that the ring defects reappear and become recombination-active again. However, this depassivation of the ring defects occurs significantly faster in SiNx:H passivated samples than in MgFx passivated samples at temperatures ≤ 500 °C, whereas the depassivation happens at a rate similar to that at temperatures ≥ 600 °C. Furthermore, in situ monitoring of the change in band–band photoluminescence intensity during annealing also confirms that the passivation and depassivation of ring defects is faster in SiNx:H passivated samples. Based on these observations, we suggest the possibility that the dominant passivating agent in the MgFx and SiNx:H films is fluorine and hydrogen, respectively. Finally, minority carrier lifetime measurements indicate that both the passivation methods improve the bulk lifetime significantly.

Optical-electrical characteristics of Al/Gd2O3/(CZ-pSi)/Al diodes under gamma ray irradiation

This article aimed to report the results on the properties of Gd2O3/(CZ-pSi) diodes under various gamma ray irradiation dose rates (3, 6, 12, 18, 24, 30, 36, 42, 50 Gy). n-type Gd2O3 films were deposited on a p-type Czochralski-grown monocrystalline silicon (CZ-pSi) wafers by pulsed laser deposition (PLD) technique with varying building laser energies (500, 600, 700, 800, 900 mJ) at room temperature and pressure (3.3 × 10−2 Pa). Amorphous structures were perceived for all films by XRD. Band gaps of the films were modified by controlling the amount of Gd2O3 on CZ-pSi wafers by laser power during deposition and determined to be between 5.30 and 5.75 eV. The Gd2O3/(CZ-pSi) films were irradiated by a Co-60 gamma-ray source system. The current and voltage (I-V) measurements, before and after irradiation (0–50 Gy), indicated that the current decreases toward zero with increasing doses. Capacitance and voltage (C-V) measurements at frequencies of 10–1000 kHz before and after irradiation showed a shift towards the right side of the pre-irradiation curves for each film. The shift was observed in both I-V and C-V curves with increasing radiation doses at 100 kHz as the average of the frequencies used. Contrary to the decrease in current curves, there was an upward increase in capacitance curves of all films with increasing radiation dose. The best build-in potentials of 0.1 V (800 mJ), 1.2 V (500 mJ) and 1.4 V (600 mJ) were obtained for the laser deposition powers of the Gd2O3/(CZ-pSi) diodes. These results are promising for diodes to be used in the radiation detection applications such as in neutron and alpha particles detectors.

Photoluminescence Spectroscopy of Thermal Donors and Oxygen Precipitates Formed in Czochralski Silicon at 450 °C

Photoluminescence spectra have been measured at 80 K in Czochralski-grown silicon wafers annealed at 450 °C, causing thermal donor and oxygen-precipitate generation. Six subbandgap luminescence peaks were identified after the annealing. One peak, centered at 1206 nm, was found to increase with a similar time constant to the thermal donor concentration measured by changes in the resistivity. We propose that the other five peaks are related to the formation of oxygen precipitates (OPs), or their nuclei. These peaks remain after a thermal donor annihilation (TDA) step, while the 1206 nm peak disappeared. This is consistent with our previous observations that the growth of OPs occurs concurrently with the growth of thermal donors at low temperatures. The annihilation of the thermal donors leads to an incomplete recovery of the interstitial oxygen concentration, consistent with the formation of precipitates. Recombination-active ring-defects formed during the annealing also remain after TDA.

Impact of hydrogen on the boron-oxygen-related lifetime degradation and regeneration kinetics in crystalline silicon

We examine the impact of hydrogen on the boron-oxygen-related lifetime degradation and regeneration kinetics in boron-doped p-type Czochralski-grown silicon wafers. We introduce the hydrogen into the silicon bulk by rapid thermal annealing. The hydrogen source are hydrogen-rich silicon nitride (SiNx:H) layers. Aluminum oxide (Al2O3) layers of varying thickness are placed in-between the silicon wafer surfaces and the SiNx:H layers. By varying the Al2O3 thickness, which acts as an effective hydrogen diffusion barrier, the hydrogen bulk content is varied over more than one order of magnitude. The hydrogen content is determined from measured wafer resistivity changes. In order to examine the impact of hydrogen on the degradation kinetics, all samples are illuminated at a light intensity of 0.1 suns near room temperature. We observe no impact of the in-diffused hydrogen content on the degradation rate constant, confirming that hydrogen is not involved in the boron-oxygen degradation mechanism. The regeneration experiments at 160°C and 1 sun, however, show a clear dependence on the hydrogen content with a linear increase of the regeneration rate constant with increasing bulk hydrogen concentration. However, extrapolation of our measurements toward a zero in-diffused hydrogen content shows that the regeneration is still working even without any in-diffused hydrogen. Hence, our measurements demonstrate that there are two distinct regeneration processes taking place. This is in good agreement with a recently proposed defect reaction model and is also in agreement with the finding that the permanent boron-oxygen deactivation also works on non-fired solar cells, though at a lower rate.

On the Correlation between Light-Induced Degradation and Minority Carrier Traps in Boron-Doped Czochralski Silicon.

Boron-doped Czochralski-grown silicon wafers dominate the photovoltaic market. Light-induced degradation of these wafers is one of the most significant roadblocks for high-efficiency solar cells. Despite a very large number of publications on this topic, only a few studies have directly investigated the precursor of the defect responsible for this degradation. In this study, using the photoconductance decay measurement method, we identify the precursor of the defect responsible for light-induced degradation. By comparing the photoconductance decay of samples in the different states, we observe the presence of a minority carrier trap in the annealed state, which is not present after degradation. Trap annihilation shows a clear anticorrelation with the generation of the recombination-active boron-oxygen defect, as determined from minority carrier lifetime measurements. Furthermore, it is concluded that a model based on a single-level trap cannot explain the doping-dependent measurements, meaning that the detected trap has two or more energy levels.

Photoconductance Determination of Carrier Capture Cross Sections of Slow Traps in Silicon Through Variable Pulse Filling

The capture cross sections are a key metric to characterize carrier traps in semiconductors. In this article, photoconductance measurements are used to estimate the capture cross sections of traps that are induced by thermal donors in Czochralski-grown silicon wafers. Here, we use a pulse-filling technique with light-emitting diode excitation to measure the capture rate. The measured capture rates are shown to be similar to the values that were measured previously by deep-level transient spectroscopy. Our method allows the contactless measurements of the capture cross sections. The minority carrier cross sections for the two distinct trap states that are studied here are of the order of 10−18 cm2 and the majority carrier cross sections are orders of magnitude smaller. Therefore, it is concluded that the slow traps that are originating from the thermal donors are dominated by the minority carrier trapping.

Pathways for efficiency improvements of industrial PERC silicon solar cells

The global manufacturing capacity of Passivated Emitter and Rear Cell (PERC) devices on p-type Czochralski-grown silicon (Cz-Si) wafers is increasing rapidly. This paper analyses various industrial process improvements carried out in our lab to improve the efficiency of large-area Cz-Si PERC solar cells from 20.7% to 21.9%. The key improvements presented in this paper include the transition from line to dot-shaped laser openings in the rear dielectric stack, a dual printing approach for the front fingers and implementation of a laser-doped selective-emitter structure. A nanosecond (ns) pulsed laser source with 532 nm wavelength was used for both the rear dielectric ablation and the front laser doping. By adopting a systematic power loss analysis approach and targeting the largest power loss mechanisms at various stages of development, the best efficiency of the solar cells in this experimental study was improved from 20.7% to 21.9%. Pathways for further improvements in efficiency are analysed, including the application of passivated-contact structures in a PERC-like cell.

Impact of germanium doping on the mechanical strength of low oxygen concentration Czochralski silicon wafers

ABSTRACT The lack of interstitial oxygen for locking and pinning of dislocations has made the ultra-high resistivity silicon wafers for radio frequency applications prone to slip and warpage during electronic device manufacturing. In this work, we investigate the role of germanium doping on the dislocation mobility in very low oxygen concentration, Czochralski-grown silicon wafers. Mechanical bending tests are used to study the average dislocation velocity and unlocking stress for germanium-doped, nitrogen-doped, and undoped silicon wafers with similar oxygen levels. In contrast to the previously reported effect of germanium on retarding dislocation motion in high oxygen concentration silicon wafers, we find that germanium by itself at a high concentration (7–9 × 1019 atoms/cm3) does not lock dislocations by solid solution strengthening, nor does it appreciably change the oxygen dislocation locking behaviour in silicon. The lack of dislocation pinning and locking effects by germanium in silicon is explained from the small lattice mismatch (4%) and low chemical interactions of substitutional Ge atoms with silicon comparing to interstitial impurities, such as oxygen and nitrogen.

A 22.3% Efficient p‐Type Back Junction Solar Cell with an Al‐Printed Front‐Side Grid and a Passivating n+‐Type Polysilicon on Oxide Contact at the Rear Side

The fabrication of a silicon solar cell on 6 in. pseudo‐square p‐type Czochralski grown silicon wafers featuring poly‐Si‐based passivating contacts for electrons at the cell rear side and screen‐printed aluminum fingers at the front side is demonstrated. The undiffused front surface is passivated with an Al2O3/SiNx stack, and the rear surface is covered with a thin oxide/n+‐poly‐Si/Al2O3/SiNx layer system, contacted by screen‐printed silver fingers. A loss analysis shows that the recombination losses at the metal contacts on both cell sides dominate the total energy losses. A voltage of 700 mV as the highest open‐circuit voltage from a batch of seven cells is achieved, and the best cell efficiency is 22.3%, independently confirmed.

Onset of ring defects in n-type Czochralski-grown silicon wafers

This paper presents experimental studies on the formation of ring defects during high-temperature annealing in both electronic-grade and upgraded metallurgical-grade (UMG) Czochralski-grown silicon wafers. Generally, a faster onset of ring defects, or shorter incubation time, was observed in the UMG samples ([Oi] = 6.3 × 1017 cm−3) in comparison to the electronic-grade samples ([Oi] = 3.9 × 1017 cm−3) used in this work. By applying a tabula rasa (TR) treatment prior to annealing, the incubation time can be increased for both types of wafers. We show that TR temperatures above 1000 °C are necessary to effectively dissolve grown-in oxygen precipitate nuclei and limit the subsequent formation of ring defects. A 30 min TR treatment at 1000 °C resulted in the longest incubation time for both types of samples used in this work, as it achieved the best balance between precipitate nuclei dissolution and precipitate re-growth/ripening. Finally, a nitrogen ambient TR step showed a short incubation time for the formation of ring defects in comparison to an oxygen ambient TR step.

The effects of pre-annealing on oxygen precipitation in silicon P/P-epitaxial wafers

Abstract Herein, the effect of pre-annealing on oxygen precipitation in a silicon P/P-epitaxial wafer was investigated using an experimental approach. Czochralski-grown silicon wafers were annealed at 800–900 °C to enhance their bulk micro defects (BMDs), before the epitaxial growth process. The effect of the pre-annealing temperature on oxygen precipitation was greater than that of the pre-annealing time, and the effect of pre-annealing temperature varied between polished and epitaxial wafers. Regarding pre-annealed epitaxial wafers that were annealed at low temperatures, the pre-annealing time also had as large an effect on BMD formation as the pre-annealing temperature. These experimental results were used to determine the optimal pre-annealing temperature for enhancing oxygen precipitation in epitaxial wafers. The experimental results regarding the optimal conditions were consistent with prior hypotheses. Crystal originated particles (COPs) and COP-free crystals exhibited their highest BMD density values at similar pre-annealing temperatures.

Formulation of mobile dislocation density in oxygen-precipitated silicon by crystal plasticity model

The relationship between mobile dislocation density and oxygen precipitation characteristics in Czochralski-grown silicon wafers has been investigated using the crystal plasticity model based on an extended Haasen–Alexander–Sumino model in combination with the experimental method. A formula of initial mobile dislocation density for oxygen-precipitated wafers has been deduced from the relationship obtained through the correlation of three-point bending test results and the simulation of a finite element model using the plasticity model. The formula obtained in this work indicates that the size of oxide precipitates has a dominant effect on the strength of precipitated wafers. It is also found from the formulation that the size of oxide precipitates responsible for dislocation slip is larger than about 200 nm, which is consistent with other studies. Analysis of the results also suggests a relationship between upper yield stress and the initial dislocation density which can be used as a tool to predict the yield stress of oxygen-precipitated silicon wafers.

Ring defects in n-type Czochralski-grown silicon: A high spatial resolution study using Fourier-transform infrared spectroscopy, micro-photoluminescence, and micro-Raman

We investigated ring defects induced by a two-step anneal in n-type Czochralski-grown silicon wafers using a combination of high spatial resolution Fourier Transform Infrared Spectroscopy (FTIR), micro-photoluminescence (PL) mapping, and micro-Raman mapping. Through FTIR measurements, we show the inhomogeneous loss in interstitial oxygen with a positive correlation with the inverse lifetime. Using high-resolution micro-PL mapping, we are able to distinguish individual recombination-active oxygen precipitates within the rings with a decreasing density from the center to the edge of the sample. The radial inhomogeneity of the oxygen precipitates is likely to be related to variations in the distribution of grown-in defects. We also demonstrate that micro-Raman mapping reveals the oxygen precipitates without the smearing effects of carrier diffusion that are present in micro-PL mapping.

Reassessment of intrinsic lifetime limit in n-type crystalline silicon and implication on maximum solar cell efficiency

Unusually high carrier lifetimes are measured by photoconductance decay on n-type Czochralski-grown silicon wafers of different doping concentrations, passivated using plasma-assisted atomic-layer-deposited aluminum oxide (Al2O3) on both wafer surfaces. The measured effective lifetimes significantly exceed the intrinsic lifetime limit previously reported in the literature. Several prerequisites have to be fulfilled to allow the measurement of such high lifetimes on Al2O3-passivated n-type silicon wafers: (i) large-area wafers are required to minimize the impact of edge recombination via the Al2O3-charge-induced inversion layer, (ii) an exceptionally homogeneous Al2O3 surface passivation is required, and (iii) very thick silicon wafers are needed. Based on our lifetime measurements on n-type silicon wafers of different doping concentrations, we introduce a new parameterization of the intrinsic lifetime for n-type crystalline silicon. This new parameterization has implications concerning the maximum reachable efficiency of n-type silicon solar cells, which is larger than assumed before.

In-situ characterization of electron-assisted regeneration of Cz-Si solar cells

We examine the regeneration kinetics of passivated emitter and rear solar cells (PERCs) fabricated on boron-doped p-type Czochralski-grown silicon wafers in darkness by electron injection via application of a forward bias voltage at elevated temperature (140 °C) in order to discriminate between electronic and photonic effects. Based on these dark regeneration experiments, we address the existing inconsistency regarding the measured linear dependence of the regeneration rate constant on the excess carrier density. Using the method of dark regeneration by current injection into the solar cell, we are able to measure the total recombination current of the solar cell at the actual regeneration temperature under applied voltage, i.e., at the physically relevant regeneration conditions. The direct comparison of the regeneration rate constant as a function of electronically injected carrier concentration in the dark and the regeneration rate constant during illumination clearly shows that the regeneration is a purely electronically stimulated effect and that photons are not directly involved.

Gettering Sinks for Metallic Impurities Formed by Carbon-Cluster Ion Implantation in Epitaxial Silicon Wafers for CMOS Image Sensor

Gettering sinks for metallic impurities formed by carbon-cluster ion implantation in epitaxial silicon wafers have been investigated using technology computer-aided design and atom probe tomography (APT). We found that the defects formed by carbon-cluster ion implantation consist of carbon and interstitial silicon clusters (carbon-interstitial clusters). Vacancy-type clusters are not dominant gettering sinks for metallic impurities in the carbon-cluster ion implanted region. APT data indicated that the distribution of oxygen atoms in the defects differs between Czochralski-grown silicon and epitaxial silicon wafers. The high gettering efficiency observed in carbon-cluster ion implanted epitaxial silicon wafers in comparison with Czochralski-grown silicon wafers is due to the distribution of oxygen atoms in the defects. Defects not containing O atoms provide strong gettering sinks for metallic impurities. These defects are formed by only carbon-interstitial clusters. Oxygen atoms inside the defects modify the amount of carbon-interstitial cluster formation on the defects. It is suggested that the gettering efficiency for metallic impurities in carbon-cluster ion implanted epitaxial silicon wafer is determined by the amount of carbon-interstitial clusters.

Proximity gettering technology for advanced CMOS image sensors using carbon cluster ion‐implantation technique: A review

A new technique is described for manufacturing advanced silicon wafers with the highest capability yet reported for gettering transition metallic, oxygen, and hydrogen impurities in CMOS image sensor fabrication processes. Carbon and hydrogen elements are localized in the projection range of the silicon wafer by implantation of ion clusters from a hydrocarbon molecular gas source. Furthermore, these wafers can getter oxygen impurities out‐diffused to device active regions from a Czochralski grown silicon wafer substrate to the carbon cluster ion projection range during heat treatment. Therefore, they can reduce the formation of transition metals and oxygen‐related defects in the device active regions and improve electrical performance characteristics, such as the dark current, white spot defects, pn‐junction leakage current, and image lag characteristics. The new technique enables the formation of high‐gettering‐capability sinks for transition metals, oxygen, and hydrogen impurities under device active regions of CMOS image sensors. The wafers formed by this technique have the potential to significantly improve electrical devices performance characteristics in advanced CMOS image sensors.

Lifetime improvement in silicon wafers using weak magnetic fields

We improve the lifetime of n-type Czochralski-grown silicon wafers using weak magnetic fields. This processing is found to increase carrier lifetimes by up to a factor of 2, from about 3μs to 7μs in our samples. Employing atomic and magnetic force microscopy, surface photovoltage transients, and X-ray photoelectron spectroscopy technique, we show that the effect can be explained by the magnetic field stimulated impurity diffusion from a bulk into the crystal surface, which forms impurity nanoclusters on the surface that can serve as centers of absorption of chemical elements from the environment. This, in turn, increases the oxide film thickness. We furthermore assume that the growth of SiO2 leads to negatively charged oxygen species in the vicinity of the Si/SiO2 interface. The existence of a local electric field generated by the charged areas can thus cause surface gettering by the positively charged metal ions, such as K+, Na+, Ca+, Al+, moved from the wafer bulk. Exposure to weak magnetic fields is therefore assumed to be important for the cost effective overall gettering efficiency during processing of silicon wafers for solar cell production.

Kinetics of the permanent deactivation of the boron-oxygen complex in crystalline silicon as a function of illumination intensity

Based on contactless carrier lifetime measurements performed on p-type boron-doped Czochralski-grown silicon (Cz-Si) wafers, we examine the rate constant Rde of the permanent deactivation process of the boron-oxygen-related defect center as a function of the illumination intensity I at 170°C. While at low illumination intensities, a linear increase of Rde on I is measured, at high illumination intensities, Rde seems to saturate. We are able to explain the saturation by assuming that Rde increases proportionally with the excess carrier concentration Δn and take the fact into account that at sufficiently high illumination intensities, the carrier lifetime decreases with increasing Δn and hence the slope of Δn(I) decreases, leading to an apparent saturation. Importantly, on low-lifetime Cz-Si samples no saturation of the deactivation rate constant is observed for the same illumination intensities, proving that the deactivation is stimulated by the presence of excess electrons and not directly by the photons.

An Improved Methodology for Extracting the Interface Defect Density of Passivated Silicon Solar Cells

The interface properties of the dielectric layer passivated silicon wafers can be characterized by capacitance–voltage, surface photovoltage, or contactless corona–voltage measurements. Conventionally, U-shaped interface defect density distributions ${D_{{\rm{it}}}}{\rm{(}}E{\rm{)}}$ are reported. However, in this study, the reported interface defect density toward the silicon conduction or valence band edges is proven to be an artefact, or at least strongly overestimated. This stems from the fact that the formula used for ${D_{{\rm{it}}}}$ extraction is valid only at very low temperatures, whereas measurements are typically performed at ∼300 K. We propose an improved methodology for ${D_{{\rm{it}}}}{\rm{(}}E{\rm{)}}$ extraction, which can self-consistently reproduce the raw data of contactless corona–voltage measurements. Applying this advanced methodology, the interfaces of several dielectric layers passivated n-type Czochralski-grown silicon wafers are investigated. In addition, the extracted ${D_{{\rm{it}}}}{\rm{(}}E{\rm{) }}$ distributions of these samples are then used to analyze their effective carrier lifetime performance.