Phosphorous Diffusion Gettering Research Articles

Improvement of charge carrier lifetime in heat exchange method multicrystalline silicon wafers by extended phosphorous gettering process

External phosphorous diffusion gettering were applied using homogenous and extended schemes on multicrystalline silicon (Mc-Si) wafers obtained from solar grade (SOG) silicon feedstock by the heat exchange method (HEM) growth technique. X-ray fluorescence (XRF) characterization shows the presence of transition metals like Chromium (Cr), manganese (Mn) and iron (Fe) in the analyzed as-cut Mc-Si substrates with a predominance of Mn and Fe elements. Before and after gettering process, we have observed that the Cr atoms concentration drop from 4.1015 at.cm-3 in the as-cut Mc-Si samples to 2.5 1015 at.cm-3 with homogeneous gettering and to 3.1014 at.cm-3 in the wafer undergoes extended gettering. The quasi-steady-state photo conductance (QSSPC) technique is used to characterize the minority charge carrier’s lifetime tn before and after gettering process. It was found that the response of Mc-Si wafers is better (form point of view of lifetime improvement) to the extended process but in some regions it is less effective and the homogenous process give the best amelioration due probably to the spatial distribution of crystalline defects and metallic precipitates densities. With tn initial values before gettering in the range of 2.5-8 usec we have measured lifetime values in the range of 15 to 37 usec with extended process and from 10 to 30 µsec with the homogenous one which ensure cell efficiencies in the range of 15 to 16%.

Boosting Charge Carrier Mobilities in Upgraded Metallurgical Grade Silicon by Phosphorous Diffusion Gettering

Herein, it is demonstrated how the carrier mobility and carrier lifetime of upgraded‐metallurgical grade silicon (UMG‐Si), a feedstock alternative to electronic‐grade, high‐purity polysilicon dominating photovoltaic technology, can be largely improved upon the gettering action of industrially compatible phosphorous diffusion gettering (PDG) process at the wafer level. The results, based on ultrafast THz spectroscopy and inductively coupled photoconductive decay measurements, show outstanding increments in the carrier lifetime of PDG‐treated multi‐crystalline UMG‐Si wafers from 50 ps to 25 μs and a boost in intra‐grain charge carrier mobility from 283 ± 37 up to 726 ± 23 cm2 Vs−1. Most remarkably, the latter figure parallels the carrier mobility observed in monocrystalline wafers manufactured from polysilicon, thereby demonstrating the effectiveness of a simple pre‐conditioning step in upgrading the electronic properties of UMG‐Si up to the device level.

Reduction of trapping and recombination in upgraded metallurgical grade silicon: Impact of phosphorous diffusion gettering

Upgraded metallurgical grade (UMG) silicon (Si) has raised interest as an alternative material for solar cells due to its low cost, low environmental impact and low CAPEX. Maximum cell efficiencies at the level of those obtained from high purity poly-Si have been reported. However, a higher defect density and the compensated doping character result in UMG-based cell efficiencies varying over wider ranges in frequency distribution charts. In this report we characterize mc-Si UMG samples with different defect densities, comparing them with mono-Si UMG and commercial high-performance mc-Si samples, analysing the impact of carrier trapping by means of photoconductance (PC) decay measurements, and its evolution after applying a phosphorous diffusion gettering (PDG) process. When analyzing the decay time constant of the PC measurements, slow (66.8+-14.3 ms) and fast (16.1+-3.5 ms) traps are found in mc-Si samples, while no evidence of trapping is found in mono-UMG samples. Slow traps are effectively removed after the PDG process, while fast traps do remain. The influence of dislocations clusters and the possible role of oxygen, as revealed by Fourier-transform infrared spectroscopy (FTIR) is discussed. Finally, the improvement in minority carrier lifetime due to the PDG treatment is reported for each sample type, reaching values up to 140 us in mc-Si samples with no slow traps neither interstitial oxygen FTIR-peaks

Phosphorous Diffusion Gettering of Trapping Centers in Upgraded Metallurgical‐Grade Solar Silicon

Experimental evidence indicating the beneficial impact of a phosphorous diffusion gettering (PDG) in the reduction of trapping centers is shown, as observed by means of inductively coupled photoconductance (PC) decay and lifetime measurements carried out on upgraded metallurgical‐grade silicon (UMG‐Si) wafers. The presence of trapping species dominating the long time range of the PC decay of UMG material (slow traps), which is effectively removed after a PDG conducted at 780 °C, is detected. Notwithstanding, a second trapping mechanism, characterized by a shorter time constant, still governs the response at very low injection levels after the gettering. Furthermore, the beneficial effect of the PDG is studied as a function of processing time, showing minority carrier bulk lifetime improvements up to 18‐fold, up to the range of 70 μs. Thereby, the way for developing gettering strategies capable of successfully removing trap centers and improving the bulk lifetime of unconventional Si material is paved.

Silicon Nitride Barrier Layers Mitigate Minority-Carrier Lifetime Degradation in Silicon Wafers During Simulated MBE Growth of III–V Layers

We observe a degradation of the minority-carrier lifetime in silicon substrates after a temperature cycle in a molecular beam epitaxy (MBE) chamber that is representative of the growth of III–V materials. This decrease in the lifetime is from milliseconds to microseconds, and in some cases into the sub-microsecond range. The degradation appears to be caused by thermally activated diffusion of metals from the back side of the substrate, occurring at temperatures above 500 °C. This impacts the ability to achieve high-performance monolithic III–V/Si multi-junction solar cells, since the epitaxial growth usually requires high-temperature steps (over 700 °C) for surface de-oxidation or surface reconstruction and temperatures of 400–600 °C during the epitaxial growth. We show that, through phosphorous diffusion gettering, the lifetimes of degraded wafers can be recovered to the millisecond range. Further, we demonstrate that a silicon nitride coating functions both as a diffusion barrier and as an interfacial gettering or hydrogenation agent, enabling high minority-carrier lifetimes directly out of the MBE chamber. This approach allows for the silicon minority-carrier lifetime to be maintained in the millisecond range without the need for post-growth recovery, providing a path to achieve high-efficiency III–V/Si solar cells.

A new single-component low-cost emitter etch-back process for silicon wafer solar cells

A way of achieving lightly doped emitter is a combination of a heavy emitter diffusion and emitter etch back, which has an added advantage of phosphorous diffusion gettering. However, this chemical emitter etch-back process must fulfil some critical requirements, e.g. cost-effectiveness, near-conformal Si etching even after deep emitter etch back, controlled Si etch rate, post-etch clean Si surface and lowest safety issues in chemical handling and drainage. In this work, we report a new low-cost (less than 1 US Cents/wafer), single-chemical, non-acidic, high-throughput emitter etch-back process for tube-diffused emitters for crystalline Si wafers. This process uses only sodium hypochlorite solution at 80 °C as the Si etchant. This process is versatile with its applications on phosphorous and boron tube-diffused monocrystalline Si and phosphorous tube-diffused multicrystalline Si wafers. The preparation, usage and drainage of this highly diluted solution are easy and safe. The Si etching process leads to excellent spatial uniformity over large-area Si wafers (243 cm2). With deep etch back resulting in a change of sheet resistance by ~60 Ω/sq, the standard deviation value changes by only 2.7%. High surface conformity in the etch-back surface is evident from reflectance studies. Quasi-steady-state photoconductance and photoluminescence imaging are used to demonstrate improved electrical parameters of the etch-back wafers.

Solar Cell Efficiency and High Temperature Processing of n-type Silicon Grown by the Noncontact Crucible Method

The capital expense (capex) of conventional crystal growth methods is a barrier to sustainable growth of the photovoltaic industry. It is challenging for innovative techniques to displace conventional growth methods due the low dislocation density and high lifetime required for high efficiency devices. One promising innovation in crystal growth is the noncontact crucible method (NOC-Si), which combines aspects of Czochralski (Cz) and conventional casting. This material has the potential to satisfy the dual requirements, with capex likely between that of Cz (high capex) and multicrystalline silicon (mc-Si, low capex). In this contribution, we observe a strong dependence of solar cell efficiency on ingot height, correlated with the evolution of swirl-like defects, for single crystalline n-type silicon grown by the NOC-Si method. We posit that these defects are similar to those observed in Cz, and we explore the response of NOC-Si to high temperature treatments including phosphorous diffusion gettering (PDG) and Tabula Rasa (TR). The highest lifetimes (2033μs for the top of the ingot and 342μs for the bottom of the ingot) are achieved for TR followed by a PDG process comprising a standard plateau and a low temperature anneal. Further improvements can be gained by tailoring the time-temperature profiles of each process. Lifetime analysis after the PDG process indicates the presence of a getterable impurity in the as-grown material, while analysis after TR points to the presence of oxide precipitates especially at the bottom of the ingot. Uniform lifetime degradation is observed after TR which we assign to a presently unknown defect. Future work includes additional TR processing to uncover the nature of this defect, microstructural characterization of suspected oxide precipitates, and optimization of the TR process to achieve the dual goals of high lifetime and spatial homogenization.

Phosphorous diffusion gettering of n-type CZ silicon wafers for improving the performances of silicon heterojunction solar cells

Phosphorous diffusion gettering (PDG) for different kinds of commercial n-type CZ silicon wafers was investigated for the application in silicon heterojunction (SHJ) solar cells. It was found that surplus phosphorus diffusion was effective in improving the effective minority carrier lifetime (τeff). Highest τeff was obtained at the optimal diffusion temperature of 840°C, which was almost independent on the diffusion duration at a wide region from 5 to 60min, corresponding to a sheet resistance region from 85 to 21Ω/sq. The segregation coefficient and gettering sites were considered responsible for the gettering mechanism. When the optimized PDG process was applied to SHJ solar cells with three different groups of commercial Si wafers, the average efficiencies were improved from 21.2% to 22.4%, 21.5% to 22.4%, and 22.1% to 22.5%, respectively, where the efficiency gains were mainly contributed by the improvements of open-circuit voltage and fill factor.

Optimized Temperature in Phosphorous Diffusion Gettering Setup of Chromium Transition Metal in Solar Grade Multicrystalline p-Type Silicon Wafer

We have investigated in this work the effect of the temperature profile during homogeneous phosphorous diffusion gettering (PDG) on multicrystalline (mc-Si) silicon p-type wafers destined for photovoltaic solar cells. Temperatures were varied from 800 ◦C to 950 ◦C with time cycle of 90 minutes. Phosphorous profile of np junction was measured by secondary ion mass spectroscopy (SIMS) from 0.45 μm to 2.4 μm. Chromium concentration profile measured on the same samples by SIMS shows a high accumulated concentration of Cr atoms in the gettering layer at 900 ◦C and 950 ◦C, compared to samples obtained at 800 ◦C and 850 ◦C. The effective lifetime (τeff) of minority charge carriers characterized by quasi-steady state photoconductance (QSSPC) is in correlation with these results. From the QSSPC measurements we have observed an amelioration of τeff from 7 μs before PDG to 26 μs in the samples after PDG, processed at 900 ◦C. This indicates the extraction of a non-negligible concentration (5× 10 cm−3 to 5× 10 cm−3) of Cr from the bulk to the surface gettering layer, as observed in the chromium SIMS profiles. A light degradation of τeff (18 μs) is observed in the samples treated at 950 ◦C due probably to a partial dissolution of the metallic precipitates, especially at the grain boundaries and in the dislocations vicinity. The related τCr-Impurity lifetime value of about 8.5 μs is extracted, which is the result of interstitial Cri or CriBs pairs, proving their strongest recombination activity in silicon.

Impact of structural defect density on gettering of transition metal impurities during phosphorus emitter diffusion in multi-crystalline silicon solar cell processing

The impact of structural defect density on gettering of transition metal impurities during phosphorous emitter diffusion has been investigated using a pair of multi-crystalline silicon (mc-Si) wafers. Chromium (Cr) impurities incorporated during growth were identified by deep level transient spectroscopy (DLTS) and used to evaluate the gettering efficiency. The Cr impurity concentration in the low defect density region of mc-Si wafers was reduced from ~3.5 × 1013 cm−3 to ~1.7 × 1012 cm−3 after phosphorous diffusion gettering (PDG), while for the high defect density region, there is no appreciable variation in the Cr concentration which only changed from ~3.0 to ~2.2 × 1012 cm−3 following PDG. It was concluded that the gettering process is not effective for highly defective regions of mc-Si wafers due to the ineffective impurity release from structural defects during the PDG process. Open image in new window

Synchrotron-based analysis of chromium distributions in multicrystalline silicon for solar cells

Chromium (Cr) can degrade silicon wafer-based solar cell efficiencies at concentrations as low as 1010 cm−3. In this contribution, we employ synchrotron-based X-ray fluorescence microscopy to study chromium distributions in multicrystalline silicon in as-grown material and after phosphorous diffusion. We complement quantified precipitate size and spatial distribution with interstitial Cr concentration and minority carrier lifetime measurements to provide insight into chromium gettering kinetics and offer suggestions for minimizing the device impacts of chromium. We observe that Cr-rich precipitates in as-grown material are generally smaller than iron-rich precipitates and that Cri point defects account for only one-half of the total Cr in the as-grown material. This observation is consistent with previous hypotheses that Cr transport and CrSi2 growth are more strongly diffusion-limited during ingot cooling. We apply two phosphorous diffusion gettering profiles that both increase minority carrier lifetime by two orders of magnitude and reduce [Cri] by three orders of magnitude to ≈1010 cm−3. Some Cr-rich precipitates persist after both processes, and locally high [Cri] after the high-temperature process indicates that further optimization of the chromium gettering profile is possible.

Lifetime improvement after phosphorous diffusion gettering on upgraded metallurgical grade silicon

Wide experimental evidence of the phosphorus diffusion gettering beneficial effect on solar grade silicon is found by measuring electron effective lifetime and interstitial iron concentration in as-grown and post-processed samples from two ingots of upgraded metallurgical grade silicon produced by Ferrosolar. Results after two different P-diffusion processes are compared: P emitter diffusion at 850°C followed by fast cool-down (called “standard process”) or followed by slow cool-down (called “extended process”). It is shown that final lifetimes of this low cost material are in the range of those obtained with conventional material. The extended process can be beneficial for wafers with specific initial distribution and concentration of iron, e.g. materials with high concentration of big Fe precipitates, while for other cases the standard process is enough efficient. An analysis based on the comparison of measured lifetime and dissolved iron concentration with theoretical calculations helps to infer the initial iron distribution and concentration, and according to that, choose the more effective type of gettering.

A novel method to enhance the gettering efficiency in p-type Czochralski silicon by a sacrificial porous silicon layer

A new two-step phosphorous diffusion gettering (TSPDG) process using a sacrificial porous silicon layer (PSL) is proposed. Due to a decrease in high temperature time, the TSPDG (PSL) process weakens the deterioration in performances of PSL, and increases the capability of impurity clusters to dissolve and diffuse to the gettering regions. By means of the TSPDG (PSL) process under conditions of 900°C/60 min + 700°C/30 min, the effective lifetime of minority carriers in solar-grade (SOG) Si is increased to 14.3 times its original value, and the short-circuit current density of solar cells is improved from 23.5 o 28.7 mA/cm2, and the open-circuit voltage from 0.534 to 0.596 V along with the transform efficiency from 8.1% to 11.8%, which are much superior to the results achieved by the PDG (PSL) process at 900°C for 90 min.

Effect of Oxygen in Low Temperature Boron and Phosphorus Diffusion Gettering of Iron in Czochralski-Grown Silicon

Low temperature boron and phosphorous diffusion gettering (BDG and PDG) of iron in Czochralski-grown silicon were experimentally studied. Differences and similarities between the gettering techniques were clarified by using intentionally iron contaminated wafers emphasizing especially the effect of oxygen. Experiments showed that the surprisingly high gettering effects of BDG could be explained by B-Si precipitates. Oxygen precipitation was seen to decrease minority carrier diffusion length after long gettering at low temperatures in both BDG and PDG. In the case of BDG oxygen precipitation affected more as a higher thermal budget was needed to obtain similar sheet resistance to that of PDG. According to experiments the efficiency of BDG can not be concluded from the sheet resistance, whereas the efficiency of PDG can. This has practical influences in a process control environment.

Investigation of iron impurity gettering at dislocations in a SiGe/Si heterostructure

This article examined the electrical activities of dislocations in a SiGe/Si heterostructure by deep level transient spectroscopy (DLTS) after iron contamination and phosphorous diffusion gettering. DLTS of iron contaminated samples revealed a peak at 210 K, which was assigned to individual iron atoms or very small (<2 nm) precipitates decorated along dislocations, considering that the iron contamination annealing was terminated by quenching and transmission electron microscopy did not reveal any precipitate at dislocations. Arrhenius plot of the 210 K peak yielded a hole capture cross section of 2.4×10−14 cm2 and an energy level of 0.42 eV above the valance band. The relatively large hole capture cross section indicates strong interactions between iron-related deep levels and the dislocation shallow bands. DLTS of the iron contaminated sample revealed that 6×1014 cm−3 of boron can more effectively trap interstitial iron at room temperatures than the strain field/defect sites at 107–108 cm−2 dislocations. Phosphorous diffusion experiments revealed that the gettering efficiency of iron impurities depends on the dislocation density. For regions of high dislocation density, phosphorous diffusion cannot remove all iron impurities decorated at dislocations, suggesting a strong binding of iron impurities at dislocation core defects.

Photoluminescence study on defects in multicrystalline silicon

We report on spatially resolved luminescence measurements on ribbon-grown silicon samples. It is found that the band-edge luminescence shows anomalous temperature behavior, namely an increase in the radiation intensity with temperature. Phosphorous diffusion gettering is found to enhance this effect. The anomalous temperature behavior is attributed to nonradiative recombination governed by shallow traps. A shift in the phonon replica of the band edge luminescence peak has been observed and associated with tensile stress.

Ge composition dependence of the minority carrier lifetime in monocrystalline alloys of Si 1−xGe x

In this work we have investigated the influence of the Ge composition on the minority carrier lifetime in Czochralski (Cz) grown monocrystalline Si 1− x Ge x alloys in the composition range of 0< x<0.07. The lifetime measurements were performed by both microwave photo conductance decay ( μ-PCD) measurements and the quasi-steady-state photoconductance (QssPC) technique. Low-temperature (450 °C) anneals as well as phosphorous diffusion gettering were applied in order to investigate the possibility of improving the minority carrier lifetime in this material. A surface passivation method based on deposition of a double layer stack consisting of a hydrogenated amorphous silicon thin film capped by a silicon nitride anti-reflection coating was used. It is established that the minority carrier lifetime in the investigated alloys decreases dramatically with increasing Ge concentration. It is further shown that phosphorous gettering is the most effective method for the enhancement of bulk lifetime values in the Si 1− x Ge x alloys studied.

Phosphorous Gettering on Spherical Si Solar Cells Fabricated by Dropping Method

Si spheres for spherical Si solar cells are produced by a dropping method. In the current state, the Si spheres are generally multicrystalline. In the dropping method, some impurities, mainly iron, which act as minority carrier recombination centers can be introduced in the Si spheres. To remove the impurities, phosphorous diffusion gettering (P-gettering) has been performed on spherical Si solar cells at 925 °C for 40 min. The effect of P-gettering was evaluated by solar cell performance and external quantum efficiency (EQE). The increase in the EQE of a long wavelength region was confirmed, which indicates that the deleterious impurities in a bulk region (not only the surface region) would be effectively reduced. Also, a minority carrier diffusion length estimated by the surface photovoltage method increased after P-gettering. Consequently, the efficiency of the spherical Si solar cell was improved. These results confirmed that P-gettering is effective to improve the solar cell performance of spherical Si solar cells.

Efficiency-limiting defects in silicon solar cell material

The precipitation rate of intentionally introduced iron during low-temperature heating is studied among a variety of single-crystal and polycrystalline silicon solar cell materials. A correlation exists between the iron precipitation rate and the carrier recombination rate in dislocation-free as-grown material, suggesting that diffusion-length-limiting defects in as-grown material are structural defects which accelerate iron precipitation. Phosphorous diffusion gettering was found to be particularly ineffective at improving diffusion length after intentional iron contamination in materials with high iron precipitation rates. We propose that intragranular structural defects in solar cell silicon greatly enhance transition metal precipitation during cooling from the melt and become highly recombination-active when decorated with these impurities. The defects then greatly impair diffusion length improvement during phosphorus gettering and limit carrier lifetimes in as-grown material.