Gettering Effect Research Articles

Impurity Gettering in (112)B HgCdTe/CdTe/Alternate Substrates

The crystalline structure and impurity profiles of HgCdTe/CdTe/alternate substrate (AS; Si and GaAs are possibilities) and CdTe/AS were analyzed by secondary-ion mass spectrometry, atomic force microscopy, etch pit density analysis, and scanning transmission electron microscopy. Impurities (Li, Na, and K) were shown to getter in as-grown CdTe/Si epilayers at in situ Te-stabilized thermal anneal (~500°C) interfaces. In HgCdTe/CdTe/Si epilayers, indium accumulation was observed at Te-stabilized thermal anneal interfaces. Impurity accumulation was measured at HgCdTe/CdTe and CdTe/ZnTe interfaces. Processing anneals were found to nearly eliminate the gettering effect at the in situ Te-stabilized thermal anneal interfaces. Impurities were found to redistribute to the front HgCdTe/CdTe/Si surface and p–n junction interfaces during annealing steps. We also investigated altering the in situ Te-stabilized thermal anneal process to enhance the gettering effect.

Enhanced boron gettering effect of n-type solar grade Si wafers by in situ oxidation

To investigate the gettering effect of n-type (phosphorous-doped) crystalline Czochralski-silicon (Cz-Si) wafers, we made boron emitters by diffusing boron into them and, subsequently, oxidized the boron-diffused n-type crystalline silicon wafers by in situ wet and dry oxidation. After oxidation, we measured the minority carrier lifetime by using microwave photoconductivity decay (μ-PCD), open-circuit voltage by quasi-steady-static photoconductance and the sheet resistance by a 4 point probe, respectively. We suggested the boron diffusion and in situ oxidation conditions to achieve longer lifetimes. We can obtain the equivalent lifetime to conventional oxidation through the quartz tube designed for boron doping and in-situ oxidation. The uniformity of sheet resistance under 3% was achieved at relatively low temperature and the lifetime of 21.6 μs was also obtained by boron gettering effect and passivation of oxide layer.

Improvements to the solar cell efficiency and production yields of low-lifetime wafers with effective phosphorus gettering

This research focuses on the improvement of solar cell efficiencies in low-lifetime wafers by implementing an appropriate gettering method of the diffusion process. The study also considers a reduction in the value of the reverse current at −12 V, an important electrical parameter related to the hot-spot heating of solar cells and modules, to improve the product's quality during commercial mass production. A practical solar cell production case study is examined to illustrate the use of the proposed method. The results of this case study indicate that variable-temperature gettering significantly improves solar cell efficiencies by 0.14% compared to constant-temperature methods when the wafer quality is poor. Moreover, this study finds that variable-temperature gettering raises production yields of low quality wafers by more than 30% by restraining the measurement value of the reverse current at −12 V during solar cell manufacturing.

Enhanced performance in the deteriorated area of multicrystalline silicon wafers by internal gettering

The deteriorated area of the multicrystalline silicon (mc-Si) ingots grown by directional solidification, commonly known as the Red Zone, is usually removed before wafering. This area, characterized by poor minority carrier lifetime, is located on the sides, at the top, and the bottom of the mc-Si ingots. In this study, the effect of internal gettering by oxygen precipitates and structural defects has been investigated on the bottom zone of a mc-Si ingot. Nucleation and growth of oxygen precipitates as well as low temperature annealing were studied. Photoluminescence imaging, lifetime mapping, and interstitial iron measurements performed by μ-PCD reveal a considerable reduction of the bottom Red Zone. An improvement of lifetime from below 1 µs to about 20 µs and a reduction of interstitial iron concentration from 1.32 × 1013 at/cm3 to 8.4 × 1010 at/cm3 are demonstrated in this paper. Copyright © 2013 John Wiley & Sons, Ltd.

Novel and efficient texturing approach for large-scale industrial production line of large-area monocrystalline silicon solar cell

Texturing monocrystalline silicon (c-Si) for solar cell fabrication is still a key issue due to consumption of a large amount of costly isopropyl alcohol (IPA) in conventional NaOH/KOH solution. Controlling IPA amount in NaOH/KOH-texturing bath for longer life of the solution in industrial batch production is a difficult task. This article reports the use of a chemical bath with a composition of NaOH–NaOCl (sodium hydroxide–sodium hypochlorite) for texturing monocrystalline silicon surface for solar cell fabrication. We report such a novel concept of texturing of the c-Si surface without using IPA and discarding the saw damage removal step. The optimized etching condition with NaOH–NaOCl-containing solution does not have any negative effect on monocrystalline silicon solar cell efficiency. Easy control of the process due to the self-controlled reaction is one of the major advantages and there is no need of sophisticated temperature-controlling system during texturing in the industrial batch process. Moreover, FTIR analysis shows the formation of Si–Cl bond during texturing, which improves the quality of diffused junction due to the gettering effect of Cl. Solar cell of conversion efficiency 14.5–15.8 % on CZ–Si substrate of area 125 mm × 125 mm was successfully fabricated using a novel texturing approach in industrial production line with yield >85 %.

Phosphorus diffusion gettering of substitutional metal contaminants at a small-angle grain boundary in n-type silicon: The case of gold

In this paper, we have studied the effect of phosphorus diffusion gettering on the Au impurities at a model small-angle grain boundary (GB) in n-type silicon by combining spreading resistive profiling and current–voltage characteristic techniques. It is found that at a low level contamination, a single-step phosphorus gettering can be effective for most of Au impurities at the GB by extending the diffusion time. However, at a high level contamination, a two-step phosphorus gettering should be utilized to getter out the gold impurities at the GB. After the effective phosphorus gettering for Au impurities, the GB electrical characteristics can be recovered to the original level.

Effects of getters on hermetically sealed micromachined cesium–neon cells for atomic clocks

The wafer-level integration technique of PageWafer® (SAES Getters’ solution for getter film integration into wafer to wafer bonded devices) has been tested in hermetically sealed miniature glass-Si-glass cells filled with Cs and Ne, e.g. for microelectromechanical systems (MEMS) atomic clock applications. Getter effects on the cell atmosphere are analyzed by quadruple mass spectroscopy and coherent population trapping (CPT) spectroscopy. The quadruple mass spectroscopy revealed that the residual gases (H2, O2, N2 and CO2) that are attributed to anodic bonding process are drastically reduced by the getter films while desirable gases such as Ne seem to remain unaffected. The impurity pressure in the getter-integrated cells was measured to be less than 4 × 10−2 mbar, i.e. pressure 50 times lower than the one measured in the cells without getter (2 mbar). Consequently, the atmosphere of the getter-integrated cells is much more pure than that of the getter-free cells. CPT signals obtained from the getter-integrated cells are stable and are, in addition, similar to each other within a cell batch, suggesting the strong potential of applications of this getter film and especially for its wafer-level integration to MEMS atomic clocks and magnetometers.

Effects of crystal defects and their interactions with impurities on electrical properties of multicrystalline Si

We investigated the effects of different crystal defects and their interactions with impurities on the electrical properties of multicrystalline Si (mc-Si) using samples with unique defect patterns and impurities. By using the floating cast method, a single grain boundary (GB), identified as a Σ27 boundary, was first formed with a high density of impurities from atmosphere, leading to an inefficient external gettering of impurities during phosphorus (P) diffusion. During crystal growth, the Σ27 GB splits into the Σ3 and Σ9 GBs with accompanying generation of dislocations and reduction in the density of impurities. The external gettering of impurities became efficient for removing impurities as evidenced by an increase in average minority carrier lifetime. At the final stage of crystal growth, the decrease in minority carrier lifetime was significant, which could not be improved by phosphorus diffusion because of the high densities of segregated impurities and crystal defects originating from the strong contact with the crucible. The increase in Σ number was found to result in more enhanced precipitation of impurities, which led to the poor gettering effect of P diffusion. These results further confirmed the importance of the reduction in the densities of impurities and dislocations for the quality and yield improvement of mc-Si ingots for solar cells.

Evaluation of Titanium Direct Bonding Mechanism

Direct metal bonding represents an advanced joining technology that allows vertical stacking with electrical conduction and even heat dissipation. For most metals used as bonding layers, direct bonding when operating at ambient air involves metal oxides. The bonding interface saddles with a trapped oxide layer that might affect electrical conduction and even complete sealing of bonding interface. Titanium especially, because of its high affinity with oxygen, makes oxide free direct bonding very difficult. In the mean time, the remarkable getter effect of Ti matrix allows the dissolution of oxygen during post bonding annealing. In this paper, the bonding limits with regards to the titanium thickness have been investigated. The key role of layer roughness on the bonding quality and energy has been pointed out. A titanium thickness below 10nm appears as a limit for an oxide free bonding.

Evaluation of the influence of an embedded porous silicon layer on the bulk lifetime of epitaxial layers and the interface recombination at the epitaxial layer/porous silicon interface

ABSTRACTPorous silicon plays an important role in the concept of wafer‐equivalent epitaxial thin‐film solar cells. Although porous silicon is beneficial in terms of long‐wavelength optical confinement and gettering of metals, it could adversely affect the quality of the epitaxial silicon layer grown on top of it by introducing additional crystal defects such as stacking faults and dislocations. Furthermore, the epitaxial layer/porous silicon interface is highly recombinative because it has a large internal surface area that is not accessible for passivation. In this work, photoluminescence is used to extract the bulk lifetime of boron‐doped (1016/cm3) epitaxial layers grown on reorganised porous silicon as well as on pristine mono‐crystalline, Czochralski, p+ silicon. Surprisingly, the bulk lifetime of epitaxial layers on top of reorganised porous silicon is found to be higher (~100–115 µs) than that of layers on top of bare p+ substrate (32–50 µs). It is believed that proper surface closure prior to epitaxial growth and metal gettering effects of porous silicon play a role in ensuring a higher lifetime. Furthermore, the epitaxial layer/porous silicon interface was found to be ~250 times more recombinative than an epitaxial layer/p+ substrate interface (S ≅ 103 cm/s). However, the inclusion of an epitaxially grown back surface field on top of the porous silicon effectively shields minority carriers from this highly recombinative interface. Copyright © 2013 John Wiley & Sons, Ltd.

Precipitated iron: A limit on gettering efficacy in multicrystalline silicon

A phosphorus diffusion gettering model is used to examine the efficacy of a standard gettering process on interstitial and precipitated iron in multicrystalline silicon. The model predicts a large concentration of precipitated iron remaining after standard gettering for most as-grown iron distributions. Although changes in the precipitated iron distribution are predicted to be small, the simulated post-processing interstitial iron concentration is predicted to depend strongly on the as-grown distribution of precipitates, indicating that precipitates must be considered as internal sources of contamination during processing. To inform and validate the model, the iron distributions before and after a standard phosphorus diffusion step are studied in samples from the bottom, middle, and top of an intentionally Fe-contaminated laboratory ingot. A census of iron-silicide precipitates taken by synchrotron-based X-ray fluorescence microscopy confirms the presence of a high density of iron-silicide precipitates both before and after phosphorus diffusion. A comparable precipitated iron distribution was measured in a sister wafer after hydrogenation during a firing step. The similar distributions of precipitated iron seen after each step in the solar cell process confirm that the effect of standard gettering on precipitated iron is strongly limited as predicted by simulation. Good agreement between the experimental and simulated data supports the hypothesis that gettering kinetics is governed by not only the total iron concentration but also by the distribution of precipitated iron. Finally, future directions based on the modeling are suggested for the improvement of effective minority carrier lifetime in multicrystalline silicon solar cells.

Tradeoffs Between Impurity Gettering, Bulk Degradation, and Surface Passivation of Boron-Rich Layers on Silicon Solar Cells

The suitability of using a boron-rich layer (BRL) formed during boron diffusion as a gettering layer for n-type silicon solar cells is investigated. We have studied the gettering effectiveness, generation of dislocations and associated bulk lifetime degradation, and the impact of the BRL on the saturation current density, for different thickness of BRL and postoxidation conditions. Our results show that a BRL deposited using BBr3-based furnaces is very effective at gettering interstitial Fe, removing more than 99.9% of Fe, but that the gettered Fe is released back into the wafer when the BRL is oxidized thermally. While we have detected no significant bulk degradation due to dislocations for the diffusion conditions used, there remains a tradeoff between the gettering effect and the recombination in the boron-doped region. Although the BRL can be oxidized chemically at low temperature using boiling nitric acid without losing the gettering effect, the lowest saturation current density is obtained by means of thermal oxidation, thanks partly to a lower boron surface concentration in thermally oxidized samples.

Evaluation of Titanium Direct Bonding Mechanism

Direct metal bonding represents an advanced joining technology that allows vertical stacking with electrical conduction and even heat dissipation. For most metals used as bonding layers, direct bonding when operating under ambient conditions involves metal oxides. The bonding interface saddles with a trapped oxide layer that might affect electrical conduction and even complete sealing of bonding interface. Titanium especially because of its high affinity with oxygen makes oxide free direct bonding very difficult. In the mean time, the remarkable getter effect of Ti matrix allows the dissolution of oxygen during post bonding annealing. In this paper, the bonding limits with regards to the titanium thickness have been investigated. The key role of layer roughness on the bonding quality and energy has been pointed out. A titanium thickness below 10 nm appears as a limit for an oxide free bonding in our conditions.

Aluminum-silicon Interdiffusion in Screen Printed Metal Contacts for Silicon based Solar Cells Applications

In this work we propose a detailed investigation of the Al – Si interdiffusion that occurs during the firing process of the Al-Si back contact of silicon based solar cells. The investigation is based on high resolution scanning electron microscopy (SEM) and compositional microanalysis with energy dispersive X-Ray microanalysis (EDX). We have found a dependence of Si precipitation in the Al matrix depending on the microstructure of the Al screen printable paste. We suggest a gettering effect promoted by the larger Al particles lying within the Al paste being able to affect the Al paste resistivity, the Al distribution within the BSF region of the solar cell, thus affecting the solar cell performances and finally the Al paste thermal expansion coefficient. Finally we demonstrate that the presence of the glass frit reduces the surface tension and, homogenizes the diffusion process. Reduction of surface tension decreases the internal pressure and increases the Si interdiffusion in Al.

The Study of Different Structuring Techniques for Creation of Non-Evaporable Getters

The results of observation of different structuring techniques of thin metal layers applied in micro system technologies are presented. The Ti V getter films formed by magnetron sputtering have been explored using scanning electron and atomic-force microscopy, Brunauer-Emmett-Teller method, thermogravimetric analysis and fractal geometry. The film sorption capacity for hydrogen given by thermogravimetry was of 7.7 m3·Pa·g-1. To estimate the effective surface area, the fractal geometry tools were used and the calculated value of the specific surface area was about 155 m2/m3. The second object under investigation was a structure composed of micro- and mesoporous silicon and copper layer deposited electrochemically on the pore walls. Porous silicon when coupled with a reactive metal or alloy is expected to be an effective getter for micro system techniques. The use of porous silicon and specific conditions of depositions allows to form the structure of complex fractal type with a specific surface area of 167 m2/cm3.

Research on Effects of Annealing and Al Gettering on Electrical Properties of Upgraded Metallurgical Grade Multicrystalline Silicon

Annealing and Al gettering were performed on upgraded metallurgical grade multicrystalline silicon (UMG multi-Si) wafers with a purity of 99.999%. The dislocation and grain boundaries of samples were characterized by optical microscopy and electron back scattering diffraction (EBSD), respectively. The minority carrier lifetime and resistivity of the Si wafers were measured using microwave photoconductance decay and four-point probe techniques, respectively. The results show that the number of dislocations in Si wafers reduced obviously after annealing and Al gettering for 2 hours at 600～1100°C. The proportion of Σ3 grain boundary increases. But the minority carrier lifetime and resistivity of the Si wafers after annealing decreases. However, the minority carrier lifetime and resistivity of the Si wafers after Al gettering increases firstly and then decreases with increasing of the annealing temperature. It is considered that the metal impurities determine electrical properties of UMG multi-Si wafers rather than dislocations and grain boundary. However, Al gettering can enhances the properties of Si wafers effectively and the optimal effect of Al gettering has been achieved at 800°C.

Investigating Internal Gettering of Iron at Grain Boundaries in Multicrystalline Silicon via Photoluminescence Imaging

In this paper, we present measurements and modeling of the reduction in dissolved iron Fe; concentrations near grain boundaries in multicrystalline silicon (mc-Si) wafers. The measurements of the interstitial Fe concentrations are obtained via photoluminescence images taken before and after iron-boron pair dissociation. A simple diffusion-capture model was developed to characterize the removal of interstitial Fe by the gettering sites. The model is based on a numerical solution to the 1-D diffusion equation with two fitting parameters: the diffusion length of dissolved Fe atoms and the effective gettering velocity at the gettering site. By comparing the simulation with a controlled phosphorous gettering process, the model is shown to give good estimation of the diffusion length of Fe atoms. For as-cut multicrystalline silicon wafers from different parts of the ingot, that is, wafers with different average dissolved Fe concentrations [Fei], the diffusion lengths of Fe atoms are found to decrease with decreasing average [Fei] This suggests the presence of relaxation precipitation during the internal gettering of dissolved Fe by the grain boundaries in mc-Si during ingot cooling.

Gettering effect in grain boundaries of multi‐crystalline silicon

AbstractIn this work, we analyze the effect of three gettering procedures on the variation of the grain boundaries (GBs) defect density in multicrystalline silicon (mc‐Si). The effective defect density (NB) was calculated using a theoretical model where we consider the potential barrier induced by the GB as being due to structural defects and impurities. Results are compared to those obtained from C‐V measurements. The potential barrier was evaluated from the dark current‐voltage (I‐V) characteristic performed across the GB. In addition to the Rapid Thermal Annealing (RTA), we use aluminum (Al) in the first gettering procedure, in the second we use porous silicon (PS), whereas in the third one, we realize a chemical damage (grooving). Mc‐Si wafers were annealed in an infrared furnace in the same conditions, at temperatures ranging from 600 °C to 1000 °C (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Defect engineering during the contact co‐firing step in an industrial belt furnace

AbstractThis work implements an optimization of the phosphorus gettering effect during the contact co‐firing step by means of both simulations and experiments in an industrial belt furnace. An optimized temperature profile, named ‘extended co‐firing step’, is presented. Simulations show that the effect of the short annealing on the final interstitial iron concentration depends strongly on the initial contamination level of the material and that the ‘extended co‐firing’ temperature profile can enhance the gettering effect within a small additional time. Experimental results using sister wafers from the same multicrystalline silicon ingot confirm these trends and show the potential of this new defect engineering tool to improve the solar cell efficiency (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influences of Phosphorous Gettering on Minority Carrier Lifetime Distribution of Polycrystalline Silicon Wafer

The influences of the phosphorus gettering in PN junction formation on the distribution of minority carrier lifetime of polycrystalline silicon wafer in solar cell production process were studied. The experimental results shows that the distribution of internal defects and impurities in polycrystalline silicon wafer significantly impact on the effectiveness of the phosphorus gettering during preparation of PN junction through the phosphorus diffusion technology. Especially when the impurities were transition metal elements, it caused a large difference of the phosphorus gettering effectiveness in PN junction preparation, which was due to the presence and interaction of defects and impurities on polycrystalline silicon wafer, such as grain boundaries, dislocations, oxygen and carbon, and other metal elements. In addition, the uneven distribution of impurities and defects of also influenced the phosphorus gettering effectiveness.