Impurity Gettering Research Articles

Impurity Gettering by Boron‐ and Phosphorus‐Doped Polysilicon Passivating Contacts for High‐Efficiency Multicrystalline Silicon Solar Cells

Highly doped polysilicon (poly‐Si) on ultra‐thin oxide layers are highlighted as they allow both carrier collection efficiency with a low contact resistivity and an excellent surface passivation. Their integration at the rear surface of a high‐quality single‐crystalline silicon solar cell allows to achieve a record conversion efficiency of 25.7% for a double‐side contacted device. However, so far, only a very few studies investigate the interactions between poly‐Si passivating contacts and low‐quality cheaper silicon wafers. Thus, this study focuses on the external gettering response of both boron (B) and phosphorus (P) in situ doped poly‐Si passivating contacts on high‐performance multicrystalline silicon. Wafers are extracted from five ingot heights and experience P‐ and B‐doped poly‐Si passivating contact fabrication processes. Subsequently, the bulk carrier lifetime and interstitial iron (Fei) concentration are characterized and compared with conventional POCl3 and BCl3 thermal diffusion steps, and as‐cut references. The P‐doped poly‐Si contact fabrication process results in gettering more than 99% of the Fei, which leads to an increase in the bulk carrier lifetime. Interestingly, the B‐doped poly‐Si contact also develops a substantial external gettering action, and allows removing 96% of the Fei from the bulk.

The getters in silicon

The processes of gettering of fast-diffusing metal impurities and structure defects in silicon, mainly used in the production of integrated circuits, power high-voltage devices, nuclear-doped silicon, are considered. The getters based on structural defects and gas-phase getters based on chlorine-containing compounds are analyzed. It is noted that for the formation of getters on the basis of structural defects, it is necessary to create internal sources for generation of dislocations and formation of precipitate — dislocation clusters. It is shown that dislocations are generated in the mouths of microfractures, which then form a sedentary dislocation grid on the non-working side of the plates. In the second case, defects are created in the area of the plate adjacent to the active layer of the electronic component. The process of creating an internal getter is based on the decomposition of a supersaturated solid oxygen solution in silicon, due to which a complex defect medium consisting of various precipitate-dislocation clusters is formed in the crystal. The packing defect as oxide precipitate with a cloud of Frank’s loops is formed. Two variants of creating an internal getter are considered — first is associated with the distillation of an oxygen impurity from the near-surface region of the plate, the second is associated with a fine adjustment of the distribution of vacancies along the plate thickness. The analysis of the influence of the getter as the defect structure reducing the magnitude of mechanical stress of the beginning of the generation of dislocations, which ultimately can determine the mechanical strength of the silicon wafer.This paper also considers the mechanism of gas-phase medium impurities and defects gettering with the addition of chlorine-containing compounds. It is shown that at elevated temperatures, due to the interaction of silicon atoms with chlorine in the near-surface region of the plate, it is possible to create vacancies that penetrate the sample volume with some probability. As a result, the case DСv > 0, DCi £ 0 is realized, that leads to a change in the composition of microdefects and their density. The examples of practical application of heat treatment in chlorine-containing atmosphere silicon wafer during application of the oxide film, in the case of the target the need for dissolution of the microdefects and of the withdrawal of fast diffusing impurities from the crystal volume, and to prevent the formation of generation-recombination centers in the manufacturing process of devices and in a nuclear doping silicon.

Physicochemical fundamentals of phase formation in silicon layers implanted with oxygen and carbon

The thermodynamic and kinetic regularities of processes occurring during heat treatment in silicon layers implanted with oxygen and carbon ions have been considered. We have analyzed the regularities of silicon deformation, impurity distribution and defect formation after different annealing modes. Diffusion smearing of implanted impurities in these layers has not been observed during carbon and oxygen implantation. As-annealed carbon does not occupy sites of the silicon lattice, in contrast to the implantation behavior of other impurities, e.g. boron and phosphorus. Phase formation regularities in implanted layers during subsequent heat treatment have been analyzed. Changes in the free energy of the system during heterogeneous and homogeneous precipitate nucleation have been compared. Sequential implantation with carbon and oxygen ions has been found to initiate diffusion flows of carbon and oxygen toward the center of the ion doped layer (the uphill diffusion phenomenon). The possibility of uphill diffusion has been analyzed from the standpoints of the Onsager theory. We show that the contribution of the chemical interaction between oxygen and carbon is far greater than the entropy contribution to the diffusion flux. We have demonstrated the high efficiency of ion doping with oxygen and carbon for gettering of uncontrolled impurities from active regions of silicon structures. The efficiency of this gettering process has been assessed for epitaxial structures in which layers had been grown on silicon wafers implanted with these impurities. Uphill diffusion in the layers after double doping with carbon and oxygen has led to the formation of more defects which may provide for efficient gettering. We have found the optimal oxygen and carbon implantation dose ratio for maximal gettering efficiency.

Getters in silicon

Gettering of rapidly diffusing metallic impurities and structural defects in silicon which is the main material for IC fabrication, high-power high-voltage devices and neutron doped silicon has been studied. Structural defect based getters and gas phase getters based on chlorine containing compounds have been analyzed. Formation of structural defect based getters requires producing intrinsic sources of dislocation generation and precipitate/dislocation agglomerate formation. We show that dislocations are generated at microcrack mouths and form a low-mobility dislocation network at inactive wafer sides. In the latter case the defects are generated in the wafer region adjacent to the active layer of the electronic component. The generation of intrinsic getters is based on the decomposition of the supersaturated oxygen solid solution in silicon which favors the formation of a complex defect system in silicon that consists of various precipitate/dislocation agglomerates. Stacking faults also form, i.e., oxide precipitates with Frank’s dislocation loop clouds. Two intrinsic getter formation methods have been considered: one is related to oxygen impurity drain from the wafer surface region and the other implies accurate control of vacancy distribution over wafer thickness. We have analyzed the effect of getters as defect structures on the reduction of the mechanical stress required for dislocation generation onset which may eventually determine the mechanical strength of silicon wafers. The mechanism of impurity and defect gettering by gas phase medium with chlorine-containing compound additions has been considered. We show that silicon atom interaction with chlorine in the surface wafer region at high temperatures may cause the formation of vacancies which may penetrate to the specimen bulk with some probability. This leads to the case ∆Сv > 0 and ∆Ci ≤ 0, which changes the composition and density of the microdefects. Examples have been given for practical use of heat treatment of silicon wafers in a chlorine-containing atmosphere during oxide film application with the aim to dissolve microdefects, drain rapidly diffusing impurities from crystal bulk and prevent the formation of generation/recombination centers during device fabrication and silicon neutron doping.

Innovative technology for the production of high-purity sand silica by thermal treatment and acid leaching process

The production of high purity silica out of natural sand plays a critical role as a starting material in the industry of glass and high grade silicon. In this paper, we expose a novel processing method for the purification of sand silica. This method is a subsequent combination of thermal treatment and acid leaching. Firstly, samples of Tunisian natural sand were submitted to a rapid thermal annealing in an infra-red furnace under O2 atmosphere at fixed temperature and time 1000 °C and 1 h, respectively. Subsequently, the samples were soaked in an aqueous acid solution composed hydrofluoric acid and a hydrochloric acid concentration. This last acid leaching step was intended to remove the gettered impurities during the annealing step. Various characterization techniques such as optical absorption, energy dispersive X-ray analysis, electronic paramagnetic resonance analysis and atomic absorption spectroscopy were used aiming to evaluate the effectiveness of this impurity gettering technology. The results show substantial reduction in all metallic impurities in silica after three successive purification cycles improving the purity from 99.7 to 99.9%.

Gettering Effects of Silicon Nitride Films From Various Plasma-Enhanced Chemical Vapor Deposition Conditions

This paper investigates and compares the impurity gettering effects of silicon nitride (SiN x ) films that are synthesized by plasma-enhanced chemical vapor deposition (PECVD) under various conditions. Both industrial- and laboratory-scale PECVD systems are employed to deposit SiN x films with a wide range of properties (with refractive indices from 1.93 to 2.45 at 632 nm), which covers the entire range of SiN x used for silicon solar cells. The gettering effects are quantified by monitoring the reduction kinetics of the interstitial iron concentration in the silicon wafer bulk as iron becomes gettered to the surface SiN x layers during cumulative annealing at 400 °C. The results show that the very different SiN x films generate similar gettering kinetics, indicating that the impurity gettering effect is likely present in most PECVD SiN x films for silicon solar cells. The gettering kinetics and the SiN x film properties of refractive index, Si–N, Si–H, N–H bond densities, and H content, are found to have no clear correlations.

Gettering Efficacy of APCVD-Based Process Steps for Low-Cost PERT-Type Multicrystalline Silicon Solar Cells

Gettering of impurities plays a crucial role in production processes of multicrystalline silicon (mc-Si) solar cells. In industry this is commonly done via POCl3 diffusion gettering during emitter or back surface field formation. We report about the gettering efficacy of an alternative approach using doped glasses deposited prior to diffusion via atmospheric pressure chemical vapor deposition (APCVD). Not only effective high-temperature diffusion gettering is shown but also low-temperature internal gettering, which takes place during deposition. Both mechanisms were found to reduce interstitial iron concentrations considerably leading to significantly enhanced minority charge carrier lifetimes. Despite structure and composition of APCVD, phosphorus silicate glasses (PSG) may differ considerably from POCl3 PSG, its overall gettering efficacy is found to be comparable or even superior. Resulting uniform sheet resistances and doping profiles are suitable for a cost-effective industrial APCVD-based codiffusion passivated emitter, rear totally diffused solar cell production process, whose applicability is demonstrated on 156 × 156 cm2 p-type mc-Si.

(Invited) Proximity Gettering Design of Hydrocarbon Molecular Ion Implanted Silicon Wafers Using Direct Bonding Technique for Advanced CMOS Image Sensors: A Review

We developed high gettering capability silicon wafers for advanced CMOS image sensors using hydrocarbon molecular ion implantation and surface activated direct wafer bonding (SAB). We found that this novel wafer has three unique characteristics for the improvement of CMOS image sensor device performance. The first is metallic impurity gettering capability in the hydrocarbon ion implantation projection range during CMOS device fabrication. The second is the oxygen out-diffusion barrier effect; this wafer can control out-diffusion to the device active region from the CZ grown silicon substrate during CMOS device heat treatment. The third is the hydrogen passivation effect; hydrogen passivates to the Si/SiO2 gate oxide interface state defects which out-diffuse to the device active region from the hydrocarbon ion implantation projection range during the CMOS device fabrication. Moreover, we demonstrated that this novel wafer can improve the pn-junction leakage current under the actual device fabrication.

The Effect of High Temperature Annealing on the Photoluminescence of ZnMgO Alloys

The effect of thermal annealing on the luminescence of wurtzite ZnMgO alloys in the films and ceramics produced by solid state reaction route is investigated. Formation of hexagonal ZnMgO phase is proved by X‐ray diffraction, micro‐Raman, photoluminescence (PL), and PL excitation methods. Mg starts incorporating into ZnO lattice at 700 °C, and its content increases with the increase of the annealing temperature up to 1000 °C. Nevertheless, in the PL spectra of the films annealed in air at 1000 °C, excitonic and defect‐related emissions from residual ZnO phase dominate, while those from the ZnMgO alloy are weak. The annealing in Zn vapor results in even weaker ZnMgO PL. In ceramics sintered in the air at 1000 °C, the PL from wurtzite ZnMgO phase is found to be strongly supressed on the surface and bright in the depth of a pellet. The gettering of Na, K, and Ca residual impurities at the surface of the samples under annealing is proved by Secondary ion mass spectrometry. It is proposed that degradation of ZnMgO PL in the near surface region is caused mainly by intense thermal decomposition of wurtzite ZnMgO phase accompanied by generation of non‐radiative defects.

Low dislocation density MBE process for CdTe-on-GaSb as an alternative substrate for HgCdTe growth

This work demonstrates a low dislocation density molecular beam epitaxial process (average etch pit density ∼1.4 × 105 cm−2) for the growth of CdTe buffer layers on GaSb (211)B alternative substrates for subsequent growth of HgCdTe infrared materials. This dislocation density is much lower than that for CdTe layers grown on other alternative substrates (mid-106 to low-107 cm−2 range for Si, Ge and GaAs), is well below the critical level required for fabricating high performance long-wave infrared HgCdTe detectors (5 × 105 cm−2), and is close to that achieved on lattice-matched CdZnTe substrates (mid-104 to low-105 cm−2 range). The low dislocation density is achieved by inserting a ZnTe/CdTe-based transitional buffer layer between the GaSb substrate and the CdTe buffer layer. The main purpose of this transitional buffer layer is to better accommodate the 6.1% lattice mismatch between the GaSb substrate and the CdTe epitaxial layer, which is evidenced by X-ray diffraction reciprocal space mapping. Additional benefits of this transitional buffer layer include possible blocking/filtering of misfit dislocation propagation, as well as gettering of defects and impurities. More importantly, an even lower dislocation density can be expected by increasing the thickness of the CdTe epitaxial layer and implementing a thermal annealing cycle for more efficient gettering. The results of this study indicate the great potential of GaSb as an alternative substrate for growing next generation HgCdTe infrared materials to meet the focal plane array requirements of higher device yield, lower cost and larger array format size.

Direct Observation of the Impurity Gettering Layers in Polysilicon-Based Passivating Contacts for Silicon Solar Cells

The formation of certain types of doped polysilicon passivating contacts for silicon solar cells is recently reported to generate very strong impurity gettering effects, revealing an important additional benefit of this passivating contact structure. This work investigates the underlying gettering mechanisms by directly monitoring the impurity redistribution during the contact formation and subsequent processes, via a combination of secondary ion mass spectrometry (SIMS), transmission electron microscopy (TEM), and minority carrier lifetime techniques. Microscopic features of the phosphorus and boron diffusion-doped polysilicon passivating contacts are also presented. Iron is used as a marker impurity in silicon to enable direct quantification of its concentration change in the bulk of the silicon wafers and in the surface layers that compose the contact structure. The results conclusively show that, for phosphorus-doped polysilicon passivating contacts, impurities are relocated from the silicon wafer bul...

Impurity Gettering by Atomic‐Layer‐Deposited Aluminium Oxide Films on Silicon at Contact Firing Temperatures (Phys. Status Solidi RRL 3/2018)

Impurity Gettering by Atomic‐Layer‐Deposited Aluminium Oxide Films on Silicon at Contact Firing Temperatures (Phys. Status Solidi RRL 3/2018)

Black silicon significantly enhances phosphorus diffusion gettering

Black silicon (b-Si) is currently being adopted by several fields of technology, and its potential has already been demonstrated in various applications. We show here that the increased surface area of b-Si, which has generally been considered as a drawback e.g. in applications that require efficient surface passivation, can be used as an advantage: it enhances gettering of deleterious metal impurities. We demonstrate experimentally that interstitial iron concentration in intentionally contaminated silicon wafers reduces from 1.7 × 1013 cm−3 to less than 1010 cm−3 via b-Si gettering coupled with phosphorus diffusion from a POCl3 source. Simultaneously, the minority carrier lifetime increases from less than 2 μs of a contaminated wafer to more than 1.5 ms. A series of different low temperature anneals suggests segregation into the phosphorus-doped layer to be the main gettering mechanism, a notion which paves the way of adopting these results into predictive process simulators. This conclusion is supported by simulations which show that the b-Si needles are entirely heavily-doped with phosphorus after a typical POCl3 diffusion process, promoting iron segregation. Potential benefits of enhanced gettering by b-Si include the possibility to use lower quality silicon in high-efficiency photovoltaic devices.

Impurity Gettering by Atomic‐Layer‐Deposited Aluminium Oxide Films on Silicon at Contact Firing Temperatures

Aluminium oxide (Al2O3) thin films deposited on silicon surfaces, synthesised by plasma‐assisted atomic layer deposition, are recently reported to possess impurity gettering effects for the silicon wafer bulk during annealing at 425 °C, a typical temperature used for activating the surface passivation quality of the Al2O3 films. This paper investigates the gettering effects of Al2O3 films at higher temperatures of 700–900 °C, which are commonly used for contact firing in silicon solar cell fabrication. Iron is used as a marker impurity in silicon to study the gettering effectiveness. Results show that Al2O3 films also generate strong impurity gettering effects at 700–900 °C, through a segregation gettering mechanism. The as‐deposited Al2O3 films are found to be more effective at gettering than the 425 °C‐activated Al2O3 films, demonstrating gettering processes that are largely limited by the impurity diffusivity in silicon. For both as‐deposited and activated Al2O3 films, gettering during high temperature annealing occurs by impurity accumulation at the Al2O3/Si interfaces, similar to the gettering action at 425 °C. However, some iron is found to redistribute into the bulk of the Al2O3 films after long annealing at a high temperature.

Effective impurity gettering by phosphorus- and boron-diffused polysilicon passivating contacts for silicon solar cells

This paper presents direct experimental evidence for the strong impurity gettering effects associated with the formation of both phosphorus and boron doped polysilicon/oxide passivating contacts for silicon solar cells, doped via thermal diffusion from POCl3 or BBr3 sources. Ion-implanted iron is used as a marker to quantify the gettering effectiveness via carrier lifetime measurements. The process conditions for fabricating optimum polysilicon passivating contacts are found to remove more than 99.9% of the iron from the silicon wafer bulk. The gettering effects of POCl3 and BBr3 diffused polysilicon/oxide contacts mainly arise from the dopant diffusions, as opposed to gettering by structural defects in the polysilicon films. The thin oxide interlayer hinders the gettering effectiveness at low diffusion temperatures, although its blocking effect becomes small at the moderate temperatures used to fabricate optimum polysilicon contacts. The gettering effectiveness increases with increasing diffusion temperature. The gettering of iron from the silicon wafer bulk to the surface layers is found to have a negligible impact on their ability to suppress recombination at the interface with the silicon wafer. Therefore, the formation of polysilicon/oxide passivating contacts, via thermal diffusion from POCl3 and BBr3 sources, not only achieves high quality surface and contact passivation but also has the net additional benefit of achieving very effective gettering of unwanted impurities in the silicon wafer bulk.

Ambient-temperature diffusion and gettering of Pt atoms in GaN with surface defect region under 60Co gamma or MeV electron irradiation

Abstract Generally, the diffusion and gettering of impurities in GaN needs high temperature. Calculated with the ambient-temperature extrapolation value of the high temperature diffusivity of Pt atoms in GaN reported in literature, the time required for Pt atoms diffusing 1 nm in GaN at ambient temperature is about 19 years. Therefore, the ambient-temperature diffusion and gettering of Pt atoms in GaN can hardly be observed. In this work, the ambient-temperature diffusion and gettering of Pt atoms in GaN is reported for the first time. It is demonstrated by use of secondary ion mass spectroscopy that in the condition of introducing a defect region on the GaN film surface by plasma, and subsequently, irradiated by 60Co gamma-ray or 3 MeV electrons, the ambient-temperature diffusion and gettering of Pt atoms in GaN can be detected. It is more obvious with larger irradiation dose and higher plasma power. With a similar surface defect region, the ambient-temperature diffusion and gettering of Pt atoms in GaN stimulated by 3 MeV electron irradiation is more marked than that stimulated by gamma irradiation. The physical mechanism of ambient-temperature diffusion and gettering of Pt atoms in a GaN film with a surface defect region stimulated by gamma or MeV electron irradiation is discussed.

Impurity phases and their removal in Si purification with Al–Si alloy using transition metals as additives

In order to discuss the effects of transition metals such as Zr, Hf, and Ti on the removal of impurities (B, P, Fe, Al, Ca, Ti, V, Cu, Ni, Mn, Zr, and Hf) in the purification of metallurgical-grade Si (MG-Si) with Al–Si alloy, the impurity phases formed after solidification were investigated by electron probe microanalysis (EPMA) and their removal by acid leaching was performed. EPMA investigation showed that β-Al5FeSi was the main impurity phase aside from Al in the sample without additives. However, after adding Zr, Hf, or Ti, the ZrSi2, HfSi2, or TiSi2compound dissolved in small amounts of Al was the most observed phase besides β-Al5FeSi and Al. Impurities such as V, Ni, and Mn were more likely to coexist with β-Al5FeSi when Hf or Zr was employed as the impurity getter. However, V preferentially coexisted with TiSi2 when Ti was added. The leaching results showed that the addition of Zr, Hf, or Ti significantly enhanced the extraction of B and slightly enhanced that of other impurities. The added Zr, Hf, and Ti were eliminated efficiently with the other impurities, reaching extraction ratios of 99.998%, 99.9994%, and 99.997%, respectively.

Effect of Ti addition to Cu-Si alloy on the boron distribution in various phases

In this study, titanium was introduced to a Cu-Si alloy as an impurity getter to redistribute boron. A small amount of Ti was melted with Cu-Si alloy, followed by cooling of the alloy to precipitate purified Si crystals which were later separated through multi-step leaching. The effects of Ti content on the distribution of various elements in different phases and microstructure of the solidified alloy were investigated. It was found that Ti in Cu-Si alloy facilitates boron removal by several mechanisms including formation of TiB2 and decreasing the segregation coefficient of B between Si and Cu-Si. Addition of Ti to a Cu-50 wt%Si alloy, compared to silicon metal, showed a significant increase in B removal. An increase in Ti in the range of 1–5% increased B removal, reaching ∼85% at 5% Ti addition. Ti in excess of that required to form TiB2 reacts with Si to generate TiSi2, which collects impurities such as P and Al.

Saw Damage Gettering for industrially relevant mc‐Si feedstock

The silicon photovoltaic industry is currently shifting towards lightly doped emitters. These have electrical properties that benefit solar cells, compared to the traditional heavily doped emitters. This move brings new challenges, as gettering efficiencies of impurities are lowered as the doping reduces. This is particularly problematic in multicrystalline silicon (mc‐Si) since cell performance is typically boosted by the effective gettering of such impurities. In prior work, we proposed the novel gettering technique, saw damage gettering (SDG), which improved effective carrier lifetime of standard performance mc‐Si red zone material. In this work, we expand the study of SDG to various types of industrially relevant mc‐Si: upgraded metallurgical grade (UMG), high performance bottom red zone (HPRZ), and diamond sawn high performance (DHP). The optimal condition for SDG is found to be an annealing temperature of 850 °C. With this condition it was demonstrated that the effective carrier lifetime can be increased in all silicon types upon SDG. The largest increase was observed for HPRZ material by a factor of 10, and the largest final effective lifetime post SDG was that of UMG, with τeff = 61.3 µs. SDG is a potentially viable gettering method to work in conjunction with lightly doped emitters in removing the impurities of mc‐silicon feedstock and thus, improving the efficiency of the cells made therefrom.

Siにおける不純物のゲッタリング : III. プロセス誘起不純物偏析

Siにおける不純物のゲッタリング : III. プロセス誘起不純物偏析