Datasets:

pdf2dataset

/

88a0bec02ea556058c2c01085abb7ae2

like 0

2202469 (1 of 7) © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH Gmb H www. advmatinterfaces. de Improved Silicon Surface Passivation by ALD Al 2O3/Si O 2 Multilayers with In-Situ Plasma Treatments Armin Richter,* Hemangi Patel, Christian Reichel, Jan Benick, and Stefan W. Glunz DOI: 10. 1002/admi. 202202469total fixed negative charge density Q tot of ≈1 × 1013 cm-2 in combination with a low interface defect density D it of ≈1 × 1011 e V-1 cm-2. [4-9] While the low D it rep-resents a rather good chemical surface passivation, the high negative Qtot causes a reduction of the electron density at the surface, which results in an important field effect contribution to the c-Si sur-face passivation. Thus, this high negative Q tot induces an inversion layer on n-type Si surfaces, while an accumulation layer is formed on p-type surfaces. The inver-sion layer at n-type Si surfaces makes its application prone to parasitic shunting effects at n-type metal contacts. [10] There-fore, Al 2O3 is predominantly applied to p-type c-Si surfaces, such as the rear surface of passivated emitter and rear cell (PERC) passivated emitter and rear cell solar cells-the current mainstream cell design in high-volume production [11,12]-or the front-side boron-doped p+ emitter of n-type c-Si tunneling oxide passivating contact (TOPCon) solar cells, which are becoming currently increasingly attractive due to their higher efficiency potential. [11,13-15] Al2O3 is also very inter-esting for advanced cell designs with efficiencies in the range of 26%, such as the rear emitter (TOPCon) cell[16] or the poly-crystalline Si on oxide-interdigitated back contact (POLO-IBC) cell,[17] where a highly effective but transparent passivation of the bare (undiffused) p-type c-Si front surface is required. Recently, a direct comparison of different surface passivation schemes indicated that there is still room for improvement for Al 2O3[3] which becomes increasingly important as device per-formance improves. In contrast to single layers, multilayers with thicknesses of only a few nanometers of the individual layers open the opportunity to modify material properties on a nanometer scale. One interesting example are the so-called interface dipole layers, which are currently intensively inves-tigated especially for the application in metal-oxide-semicon-ductor field-effect transistor (MOSFETs) to adjust the desired flat-band voltage. [18-20] They are multilayers consisting of two or three different dielectric layers and can provide the possibility of increasing the flat-band voltage simply by varying the number of the bi-or trilayers. The origin of this flat-band voltage shift are dipoles, which are formed only at specific interfaces of this multilayer with only one polarity. For instance, Si O 2/Al 2O3 stacks have been reported, where dipoles are formed only at the Si O 2/Al 2O3 interfaces with one polarity but not at the Al 2O3/ Si O 2 interfaces with the opposite polarity. [19]Al2O3 is one of the most effective dielectric surface passivation layers for silicon solar cells, but recent studies indicate that there is still room for improvement. Instead of a single layer, multilayers of only a few nanometers thickness offer the possibility to tailor material properties on a nanometer scale. In this study, the effect of various plasma treatments performed at different stages during the ALD deposition of Al 2O3/Si O 2 multilayers on the silicon surface passivation quality is evaluated. Significant improvements in surface passivation quality for some plasma treatments are observed, particularly for single Al 2O3/Si O 2 bilayers treated with a H 2 plasma after Si O 2 deposition. This treatment resulted in a surface recombination parameter J 0 as low as 0. 35 f A cm-2 on (100) surfaces of 10 Ω cm n-type silicon, more than a factor of 5 lower than that of Al 2O3 single layers without plasma treatment. Capacitance-voltage measurements indicate that the improved surface pas-sivation of the plasma-treated samples results from an enhanced chemical interface passivation rather than an improved field effect. In addition, a superior temperature stability of the surface passivation quality is found for various plasma-treated multilayers. A. Richter, H. Patel, C. Reichel, J. Benick, S. W. Glunz Fraunhofer Institute for Solar Energy Systems (ISE)Heidenhofstrasse 2, 79110 Freiburg, Germany E-mail: armin. richter@ise. fraunhofer. de H. Patel now with Department of Aerogels and Aerogel Composites German Aerospace Center (DLR)Institute for Materials Research Linder Höhe 51147 Cologne, Germany S. W. Glunz Department of Sustainable Systems Engineering (INATECH)University of Freiburg Emmy-Noether-Str. 2, 79110 Freiburg, Germany © 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH Gmb H. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited. Resea Rch a R ticle 1. Introduction The electronic passivation of crystalline silicon (c-Si) surfaces with dielectric layers is highly important for various semicon-ductor devices, in particular for c-Si solar cells. One of the most prominent and effective dielectric passivation layers is Al 2O3, which has been intensively investigated and highly opti-mized. [1-3] The effectiveness of Al 2O3 is attributed to its high Adv. Mater. Interfaces 2023, 10, 2202469

10.1002admi.202202469.pdf

www. advancedsciencenews. com© 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH Gmb H 2202469 (2 of 7) www. advmatinterfaces. de In a previous study, we investigated the c-Si surface passi-vation quality of such Al 2O3/Si O 2 multilayers synthesized via atomic layer deposition (ALD)[21]-a deposition technique with a high relevance for high-volume production. [22] In that study, we did not find enhanced surface passivation quality of these layers when compared to Al 2O3 single layers, nor evidence of dipole effects in these layers, which would enhance the field-effect pas-sivation. Instead, we found that the negative Q tot of the ALD Al2O3/Si O 2 multilayers could be quite substantially increased upon voltage stress. However, the increased negative Q tot did not result in an improved surface passivation quality because the chemical passivation degraded upon voltage stress. [21] In this work, we study systematically the effect of plasma treatments during the ALD multilayer deposition on the sur-face passivation quality of these Al 2O3/Si O 2 multilayers. In general, the effect of plasma treatments on the surface pas-sivation quality of dielectric layers has hardly been studied so far, in particular for layers prepared by ALD. Therefore, we studied the effect of different plasma treatments (H 2, N 2 O2, N2/H2, or Ar), with specific focus on whether they can induce a modification of the Al 2O3/Si O 2 or Si O 2/Al 2O3 interfaces so that interface dipoles are formed analogously to Ref.,[19] and which might potentially enhance the field-effect passivation. Instead of starting these multilayers with Si O 2 at the c-Si interface, as usually done for MOSFET applications,[18-20] we studied in this work multilayers starting with Al 2O3, which we found to be beneficial for c-Si surface passivation with our ALD layers. [23] 2. Results and Discussion In the first experiment, we studied the influence of plasma treatments on the surface passivation quality for single and triple Al 2O3/Si O 2 multilayers, i. e., c-Si/(Al 2O3/Si O 2)n with n  = 1 and n = 3, which were deposited with plasma-assisted ALD (PA-ALD). For the activation mainly of the chemical interface passi-vation, a post-deposition forming gas anneal was performed at temperatures between 350 and 550 °C. During this activation, the c-Si/Al 2O3 interface is hydrogenated with hydrogen mainly released from the Al 2O3. [1,25] Our approach to provide similar hydrogen sources for all different multilayers was to deposit a 10  nm thick Al 2O3 layer on top of the multilayer stacks. This 10 nm thick Al 2O3 layer provides a quite good passivation and therefore also served as a reference passivation scheme in this study. The plasma treatments consisting either of pure H 2, N 2 O2, or Ar or a mixture of N 2/H2 were conducted either i) after every Al 2O3 deposition, or ii) after every Si O 2 deposition, or iii) after both, as schematically illustrated in Figure 1 exemplarily for the single Al 2O3/Si O 2 multilayers. The resulting effective minority charge carrier lifetime τ eff as a measure for the surface passivation quality is shown in Figure 2 as a function of the post-deposition annealing temperature T an; on the left for the single and on the right for the triple Al 2O3/Si O 2 multilayers, where the top row shows the results for plasma treatments after every Al 2O3 deposition, the middle row for plasma treatments after every Si O 2 deposition and the bottom row for plasma treatments after both, Al 2O3 and Si O 2 depositions. In addi-tion, the results for multilayers and Al 2O3 single layers without plasma treatment are shown as a reference (gray data points). The τ eff data is evaluated at an excess charge carrier density of Δn = 1 × 1015 cm-3, which represents charge carrier injection conditions close to the maximum power point condition of high-efficiency silicon solar cells, [13], i. e., this τ eff assesses the surface passivation quality close to operation conditions of real devices. At annealing temperatures below 450-500 °C, τeff of all multilayers is strongly increasing with increasing annealing temperatures, indicating a strongly improving surface passiva-tion. This trend is consistent with thermal activation results of PA-ALD Al 2O3 single layers[29] and can be mainly attributed to the thermal activation of the chemical surface passivation, i. e., the reduction of interface defect states. [25,30] At higher annealing temperatures, τeff of most multilayers starts to saturate and the difference in passivation quality of the various plasma-treated samples becomes visible. It can be clearly observed that there are plasma treatments which result in a significantly higher τ eff compared to the reference multilayers without plasma treat-ment, indicating improved surface passivation and thus, a ben-eficial effect of the plasma treatments. These effects are most pronounced for the single Al 2O3/Si O 2 multilayers, while sig-nificantly weaker effects are obtained for the triple Al 2O3/Si O 2 multilayers. Almost all of these plasma-treated triple Al 2O3/Si O 2 multilayers show a maximum τ eff level, which is not significantly higher than that of the reference Al 2O3 single layer without plasma treatment. There is one clear exception, which is the triple Al 2O3/Si O 2 multilayer treated with H 2 plasma after both the Al2O3 and Si O 2 depositions (Figure  2f), which exceeds τ eff of the Al 2O3 single layer reference by about 40%. In contrast, there are various plasma-treated single Al 2O3/Si O 2 multilayers which show an even higher τ eff, which is up to 65% higher than that of the Al 2O3 single layer reference, as observed, for instance, for the O2 plasma treatment after Si O 2 deposition (Figure 2c). The lower performance of the plasma-treated triple Al 2O3/Si O 2 multilayers might to some extent be attributed to their overall lower sur-face passivation quality even without plasma treatments, which is almost a factor of 2 lower than that of the single Al 2O3/Si O 2 multilayers without plasma treatments. Figure 1. Schematic cross-section of the processed samples. The yellow arc indicates the interfaces at which plasma treatments were performed during depositions. Adv. Mater. Interfaces 2023, 10, 2202469 21967350, 2023, 16, Downloaded from https://onlinelibrary. wiley. com/doi/10. 1002/admi. 202202469 by Cochrane China, Wiley Online Library on [08/09/2024]. See the Terms and Conditions (https://onlinelibrary. wiley. com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License

10.1002admi.202202469.pdf

www. advancedsciencenews. com© 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH Gmb H 2202469 (3 of 7) www. advmatinterfaces. de It can be also observed from Figure 2 that most plasma-treated samples show superior high-temperature stability. While τeff of the reference multilayers and the single Al 2O3 layer without plasma treatments saturates at T an = 450-500 ° C and even tends to degrade slightly above 500 ° C, τeff of various plasma-treated multilayers are at 550 ° C not even in the saturation regime. With respect to the different plasma treatments being studied, it is found that almost every plasma can result in a beneficial effect. It appears, however, to be important at which interface the respective plasma treatment is performed. For instance, a N 2/H2 plasma is most beneficial when applied after the Al 2O3 deposition (Figure  2a,b), while a H 2 plasma is very beneficial when applied after both, the Al 2O3 and the Si O 2 dep-osition (Figure 2e,f). For plasma treatments after Si O 2 deposi-tion, the O 2 plasma, the H 2 plasma, and even the N 2 plasma show quite substantial positive effects in the case of single Al 2O3/Si O 2 multilayers (Figure  2c). Interestingly, the smallest positive or even detrimental effects are in almost any case observed for Ar plasma treatments. This indicates that the ben-eficial effects of the other plasma treatments might be related to chemical modifications taking place at the plasma-treated inter-faces, rather than ion bombardment or UV irradiation-induced effects. In our ALD reactor with its remote plasma source, the kinetic energy of the ions was found to be low enough to pre-vent substantial film damage. [31-33] In the case of the H 2 con-taining plasma treatments even an incorporation of hydrogen into the layer might take place. Interestingly, we found that the surface passivation quality of 15 nm thick PA-ALD Al 2O3 single layers is not significantly modified when exposed to a post-dep-osition plasma treatment (see Figure S5, Supporting Informa-tion), which indicates that the beneficial effects observed here are a special property of these PA-ALD Al 2O3/Si O 2 multilayers. In order to study the underlying mechanism, we measured in a second experiment the interface properties using the capac-itance-voltage (CV) technique. In this experiment, we studied the same variations as in the previous experiment, except that the multilayers were activated with a single annealing process at 450 °C for 25 min (instead of the 10 min used in the previous experiment), which is more compatible with our TOPCon solar cell processing. [14,16] Figure 3 displays the total fixed charge Figure 2. Effective lifetime τ eff as a function of the annealing temperature Tan for single (left) and triple (right) Al 2O3/Si O 2 multilayers, where the different plasma treatments were either performed after Al 2O3 deposition a,b), after Si O 2 deposition c,d) or after Al 2O3 and Si O 2 deposition e,f). τeff for Al 2O3 single layer and stack reference samples without plasma treat-ment are shown as well (gray data points). All samples were made of planar 10 Ω cm n-type FZ Si wafers and were sequentially annealed at the dif-ferent temperatures for 10 min in forming gas. Figure 3. Effective lifetime τ eff and total fixed charge density Q tot for single Al2O3/Si O 2 multilayers treated with different plasmas either after Al 2O3 deposition, after Si O 2 deposition or after Al 2O3 and Si O 2 deposition. All samples were made of planar 10 Ω cm n-type FZ Si wafers. Note that the samples were annealed at 450 ° C for 25 min in forming gas after deposi-tion. Note further that Q tot results are not available for all samples. Results for reference samples without plasma treatment are shown as well. Adv. Mater. Interfaces 2023, 10, 2202469 21967350, 2023, 16, Downloaded from https://onlinelibrary. wiley. com/doi/10. 1002/admi. 202202469 by Cochrane China, Wiley Online Library on [08/09/2024]. See the Terms and Conditions (https://onlinelibrary. wiley. com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License

10.1002admi.202202469.pdf

www. advancedsciencenews. com© 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH Gmb H 2202469 (4 of 7) www. advmatinterfaces. dedensity Qtot of this experiment together with τ eff for single (left) and triple (right) Al 2O3/Si O 2 multilayers treated with the dif-ferent plasmas. Results of the Al 2O3 and Al 2O3/Si O 2 multilayer reference samples without plasma treatments are also shown on the very left of each graph. The measured τ eff clearly confirms the beneficial effect of various plasma treatments when compared to the reference multilayers without plasma treatment (black data points in Figure 3a,b), in particular for the single Al 2O3/Si O 2 multilayers. With respect to the Al 2O3 single layer reference (gray data points), these beneficial effects are, however, less pronounced than in the previous experiment. However, in the previous experiment, the beneficial effect was most pronounced at the highest annealing temperatures, while they were also less pro-nounced at the lower annealing temperature of 450 °C used in this experiment. A detailed comparison of the surface passiva-tion qualities observed in both experiments indicate for some variations slightly different trends. The fact that the trends are reasonably reproduced in the next experiment (see the com-parison in Figure S1, Supporting Information) indicates that the annealing conditions in particular the annealing duration is causing these differences. With respect to the fixed charge density, the single Al 2O3/ Si O 2 multilayers tend to have a higher negative Q tot than the triple Al 2O3/Si O 2 multilayers, which contribute to their overall higher surface passivation quality as observed from their higher τeff. Interestingly, the observed Q tot of all plasma-treated multi-layers is significantly lower than that of the Al 2O3 and Al 2O3/ Si O 2 multilayer reference samples without plasma treatment, in particular with respect to the Al 2O3 single layer, which reveals that they have a lower field-effect component contributing to the surface passivation. The fact that various plasma-treated multilayers display at the same time a superior surface passiva-tion, indicates that they have a chemical passivation component which is significantly improved. However, exact D it values could not be extracted from our CV data due to the low D it level in combination with a limited measurement accuracy. Thus, this finding supports the hypothesis that, especially in the case of the H 2 containing plasma treatments, hydrogen is incorpo-rated into the layers and/or at the c-Si/Al 2O3 interface, which then contributes to the chemical surface passivation during the thermal activation anneal. [34] Such H 2 plasma-induced sil-icon interface hydrogenation effects have also been reported, for instance, for amorphous silicon [35] or Si O 2[36,37] layers. The mechanism behind the beneficial O 2 or N 2 plasma treatments is yet to be determined and requires further analysis. One hypothesis might be that the plasma treatments result in more mobile hydrogen during the post-deposition thermal annealing, where the interface hydrogenation takes place. [25,30] The fact that the plasma-treated multilayers show a lower Qtot means also that the plasma treatments did not result in a formation of interface dipoles which enhance the field-effect passivation. Instead, this might indicate that the plasma treat-ment induces interface dipoles of the opposite polarity reducing thereby the apparent Q tot, as speculated by Irikawa et  al. for their Qtot reduction observed for c-Si/Al 2O3 interfaces after H 2 plasma exposure. [38] Alternative explanations for the Q tot reduc-tion might be that the reduction of interface defects inherently results also in a reduction of trapped charges at the interface, or that oxide charging effects take place upon UV irradiation during plasma treatments, [39] although for UV radiation of the c-Si/Al 2O3 interface an increase of Q tot is expected. [40,41] So far, the plasma exposure time was constant at 10 s. In a third experiment, we studied the influence of the plasma exposure time for H 2, O 2, and N 2 plasma treatments. Figure 4 displays the result of this experiment where the 10 s exposure is compared to a 60 s exposure for single Al 2O3/Si O 2 multi-layers annealed at either at 450 or 500 °C. The experiment confirms again that a H 2 plasma treatment shows the most significant improvement, in particular when applied after the Si O 2 deposition. It can also be clearly observed that there are plasma treatments for which the longer plasma exposure time results in increased τ eff, i. e., further improved surface passiva-tion. This is particularly the case for the H 2 plasma after Si O 2 deposition, where the 60 s plasma results in ≈40% increased τ eff. With respect to the annealing temperature, it is interesting that for 10 s plasma treatments annealing at 500 °C results in superior passivation quality for all variations consistently with the results of the first experiment, while for the 60 s plasma treatments annealing at 500 °C is only beneficial if the plasma treatment is performed after Si O 2 deposition. Thus, the highest τ eff of 23. 8 ms is observed for 60 s H 2 plasma treat-ment after Si O 2 deposition annealed at 500 °C. This translates to a surface recombination parameter J 0 of 0. 35 ± 0. 34 f A/cm2 for this sample while the Al 2O3 reference with a max. τ eff of 13. 2 ms has a corresponding J 0 of 2. 09 ± 0. 90 f A/cm2 (see also Figure S2, Supporting Information and Table S1), which high-lights the quite substantially improved surface passivation. However, for other plasma treatments such as the O 2 or the N 2 plasma after Al 2O3 deposition, τ eff of the extended Figure 4. τeff for single Al 2O3/Si O 2 multilayers treated either with an O 2, H2 or N 2 plasma for 10 s or 60 s. The plasma treatment was either applied after Al 2O3 deposition, after Si O 2 deposition or after Al 2O3 and Si O 2 depo-sition. τeff for Al 2O3 single layer and multilayer reference samples without plasma treatment are shown as well. All samples were made of planar 10 Ω cm n-type FZ Si wafers and were annealed either at 450 ° C or 500 ° C, both for 25 min in forming gas. Adv. Mater. Interfaces 2023, 10, 2202469 21967350, 2023, 16, Downloaded from https://onlinelibrary. wiley. com/doi/10. 1002/admi. 202202469 by Cochrane China, Wiley Online Library on [08/09/2024]. See the Terms and Conditions (https://onlinelibrary. wiley. com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License

10.1002admi.202202469.pdf

www. advancedsciencenews. com© 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH Gmb H 2202469 (5 of 7) www. advmatinterfaces. de60 s exposure is actually lower than after 10 s exposure. In the case of the 60 s N 2 plasma treatment, τ eff is even significantly lower than that of the Al 2O3/Si O 2 reference multilayer without plasma treatment, which indicates that this long plasma treat-ment can also degrade the surface passivation quality. Such plasma-induced degradation effects can be caused by the high energy of the vacuum UV photons, as reported, for instance, for O 2 plasma treatments. [4,32] The fact that these detrimental effects are most pronounced when the plasma treatment was performed after Al 2O3 deposition (see, e. g., Figure  4: 60 s N 2 plasma), i. e., when it took place very close to the c-Si interface (see Figure 1), might, however, also indicate that some kind of plasma-induced damage occurs even if a remote plasma source with low ion energies is used. Since the H 2 plasma treatment showed the strongest surface passivation improvement after extended plasma exposure, we studied the effect of plasma duration for the H 2 plasma treat-ment in more detail. Figure 5 shows the resulting τeff and Qtot of a systematic plasma duration variation between 10 s and 120 s again for single Al 2O3/Si O 2 multilayers annealed at 450 °C. It can be clearly observed that for the plasma treatment after Si O 2 deposition, as well as after Al 2O3 and Si O 2 deposi-tion, there is a pronounced maximum τ eff for a plasma exposure time ≈60 s to 80 s. The positive effect of plasma treatments after Al2O3 deposition seems to saturate after 60 s at a slightly lower τeff. The Q tot determined for these samples is not significantly affected by the plasma exposure time, indicating again that the improved surface passivation being observed by the maximum τ eff is caused by an improved chemical passivation, i. e., interface hydrogenation, rather than by improved field-effect passivation. So far, the surface passivation was studied on planar sur-faces. However, the surfaces of c-Si solar cells are often textured with random pyramids in particular on the front surface for improved light absorption. Therefore, we studied in a further experiment the effect of H 2 plasma treatments for single Al 2O3/ Si O 2 multilayers on textured surfaces. Figure 6 summarizes the resulting τ eff of this experiment, where in addition to the plasma duration (10 s/60 s), the ALD deposition temperature (180 °C/250 °C) was also varied. It can be observed that for the multilayers deposited at 250 °C, the 60 s plasma treatment after Si O 2 resulted again in the most significantly improved surface passivation, as indicated by a τ eff which is almost a factor of 4 higher than that of the Al 2O3/Si O 2 multilayer reference without plasma treatment, and almost a factor of 2 higher than that of the Al 2O3 reference. The samples deposited at 180 °C are much less affected by the plasma treatments. Interestingly, the 10 s plasma treatments did not show any effect on the surface passivation of these textured samples, neither after Si O 2 nor after Al 2O3 deposition, indicating that on textured surfaces with larger surface area longer plasma exposure times are required. The same variations have also been studied on p-type FZ Si wafers (1 Ω cm, 250  µm thick) with textured surfaces, where very similar trends have been obtained (see Supplementary Figure S4). This shows that the enhanced surface passivation is not only obtained for n-type Si surfaces but also for p-type Si surfaces. 3. Conclusion We have studied the effect of interface plasma treatments during the ALD deposition of Al 2O3/Si O 2 multilayers on their Figure 5. τeff and Q tot for single Al 2O3/Si O 2 multilayers as a function of the H 2 plasma exposure time either after Al 2O3 deposition, after Si O 2 deposition or after Al 2O3 and Si O 2 deposition. All samples were made of planar 10 Ω cm n-type FZ Si wafers and were annealed at 450 °C for 25 min in forming gas after deposition. Figure 6. Effective lifetime τ eff for single Al 2O3/Si O 2 multilayers as a func-tion of the deposition temperature T dep exposed to H 2 plasma of either 10 s or 60 s duration after Al 2O3 or Si O 2 deposition. All samples were made of textured 1 Ω cm n-type FZ Si wafers and were annealed at 425 °C for 25 min in forming gas after deposition. The results for reference multilayer and Al 2O3 single layer without plasma treatment are also shown. Adv. Mater. Interfaces 2023, 10, 2202469 21967350, 2023, 16, Downloaded from https://onlinelibrary. wiley. com/doi/10. 1002/admi. 202202469 by Cochrane China, Wiley Online Library on [08/09/2024]. See the Terms and Conditions (https://onlinelibrary. wiley. com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License

10.1002admi.202202469.pdf

www. advancedsciencenews. com© 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH Gmb H 2202469 (6 of 7) www. advmatinterfaces. dec-Si surface passivation properties. By varying, for instance, the plasma gas (H 2, O2, N2, or Ar), the interfaces where the plasma treatment took place, or the number of Al 2O3/Si O 2 bilayers, we identified plasma treatments resulting in a significant improvement in the surface passivation quality in different independent experiments. These positive effects were found to be most pronounced for a single Al 2O3/Si O 2 bilayer, where a 60 s H 2 plasma treatment was applied after Si O 2 deposition. This treatment resulted in J 0 values as low as 0. 35 f A cm-2 which is more than a factor of 5 lower than that of the Al 2O3 single layer reference without plasma treatment. This is among the lowest J 0 values reported in the literature. [1] Interestingly, such improved surface passivation upon plasma treatments was not observed for the Al 2O3 references, indicating that the positive effect strongly depends on the type of layer system and the interface, at which the treatment is applied during film deposition. CV measurements indicate that the positive effect of the plasma-treated multilayers is not caused by an increased field-effect passivation. In fact, their fixed charge density was found to be significantly lower than that of the Al 2O3 reference without plasma treatment, indicating an even lower field-effect contribution. Instead, the CV results indicate an improved chemical surface passivation for multilayers with plasma treat-ments, which supports the hypothesis that in the case of the H 2 plasma-treated samples hydrogen is incorporated into the film and/or at the c-Si/Al 2O3 interface, which can then con-tribute to the interface hydrogenation during the post-deposi-tion thermal anneal. In addition, we found indications that the plasma-treated multilayers show a superior high-temperature stability. Although the multilayers were deposited in a lab-type PA-ALD reactor, they are of high relevance for mass production as large-batch PA-ALD reactors are available. [22] As such, these plasma-treated dielectric multilayers with superior c-Si surface passivation properties identified in this study might pave the path to new surface passivation schemes for high-efficiency c-Si solar cells with a high transparency as usually required for the front side. 4. Experimental Section The surface passivation of Al 2O3 single layers and Al 2O3/Si O 2 multilayer stacks was mainly studied on shiny-etched (100)-oriented n-type FZ Si wafers with a resistivity of 10 Ω cm and a thickness of 200  µm. Some of the wafers were textured with random pyramids in an alkaline solution to study the surface passivation quality on surfaces similar to those usually utilized at the front surface of silicon solar cells. These textured samples were made of (100)-oriented n-type FZ Si wafers with a resistivity of 1 Ω cm and a thickness of 200  µm. First, the wafers were subjected to a standard RCA cleaning procedure, followed by a thermal oxidation at 1050 °C. This high-temperature process was performed to remove the recombination-activity of the common FZ defects. [24] Prior to the multilayer deposition, the thermally-grown Si O 2 was etched in buffered oxide etching solution. The Al 2O3/Si O 2 bilayers were then deposited with plasma-assisted ALD (PA-ALD) using trimethylaluminum (TMAl) and bis-diethyl aminosilane (BDEASi) as aluminum and silicon precursors, respectively. The deposition was performed in a single wafer reactor (Flex AL, Oxford Instruments) with a remotely placed inductively coupled plasma source operated at a frequency of 13. 56  MHz. Two different multilayer variations were prepared: single and triple sequences of Al 2O3/Si O 2, i. e., c-Si/(Al 2O3/Si O 2)n with n  = 1 and n  = 3. If not stated otherwise, the samples were deposited at 250 °C and with fixed Al 2O3 and Si O 2 layer thicknesses of 3 and 2 nm respectively, which were found to result in a promising surface passivation. [21] Some of the samples were treated with a plasma during the deposition, either i) after every Al 2O3 deposition, or ii) after every Si O 2 deposition, or iii) after both, as schematically illustrated in Figure 1 exemplarily for the single Al 2O3/Si O 2 multilayers. The plasma consists either of pure H 2, N 2 O2, or Ar or a mixture of N 2/H2 and was performed at 150  W for 10 s, if not stated otherwise. The gas flow rates during plasma treatments were either 30 sccm H 2, 50 sccm N 2, 60 sccm O 2, 60 sccm Ar or 15 sccm/15 sccm N2/H2. The pressure during these treatments was around 50 m Torr. The deposition of the complete multilayer stack, including the plasma treatments, took place without breaking the vacuum. After deposition, the samples were annealed in forming gas (N 2/H2 mixture) in a tube furnace (from ASM). Photoconductance decay (PCD) measurements were performed with the lifetime tester WCT-120 (Sinton Instruments) to determine the effective minority charge carrier lifetime (τ eff), using either the transient mode or the quasi-steady-state mode (with generalized analysis). [26,27] For PCD measurements, symmetrically coated (passivated) samples were prepared. All τ eff values were evaluated at an excess charge carrier density of Δ n = 1 × 1015 cm-3. The surface recombination parameter J0 was determined via modeling the full injection-dependent effective lifetime data τ eff as described in Ref.,[42] considering also the intrinsic bulk recombination model from Niewelt et al..[28] This J 0 fitting was performed in a Δ n range from 6 × 1014 cm-3 to 8 × 1015 cm-3. The J 0 values were evaluated for a temperature of 25 ° C taking the effective intrinsic charge carrier concentration n i,eff as a function of Δ n according to Altermatt et al. [44] (with the equilibrium intrinsic charge carrier concentration n i,0 = 8. 29 × 109 cm-3 at 25 ° C[45] and band gap narrowing according to Schenk[46]) into account. In addition to the PCD measurements, we also performed τ eff calibrated photo-luminescence imaging (PLI) using the Fraunhofer ISE modulum setup. [43] Reasonable homogeneity was observed in the PLI data, thus a dominating random effect on the observed trends in surface passivation quality due to inhomogeneity issues can be excluded. Exemplary PLI data is shown in Figures S6 and S7, Supporting Information. Metal-insulator-semiconductor (MIS) structures were prepared for capacitance-voltage (CV) measurements by thermally evaporating Al metal dots (≈ 1 mm diameter) on the multilayer surface and full area Al contact (≈300 nm thick) on the rear side. The CV measurements were performed with the semiconductor analyzer B1500A (Agilent Technologies) to determine Q tot, which was evaluated from the flat band voltage and the maximum capacitance of the Al 2O3 layer or Al 2O3/Si O 2 multilayer stacks was attained by the low-frequency curve of the samples. The low-frequency curve was measured at 1 k Hz and the high-frequency curve at 1 MHz. Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements The authors would like to thank A. Leimenstoll, R. Neubauer, F. Schätzle, and K. Zimmermann for support during the preparation of the samples and P. Redmond for proofreading. This work was funded by the German Ministry of Economic Affairs and Climate Action under Grant No. 03EE1031A “Pa Sodoble”. Open access funding enabled and organized by Projekt DEAL. Conflict of Interest The authors declare no conflict of interest. Adv. Mater. Interfaces 2023, 10, 2202469 21967350, 2023, 16, Downloaded from https://onlinelibrary. wiley. com/doi/10. 1002/admi. 202202469 by Cochrane China, Wiley Online Library on [08/09/2024]. See the Terms and Conditions (https://onlinelibrary. wiley. com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License

10.1002admi.202202469.pdf

www. advancedsciencenews. com© 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH Gmb H 2202469 (7 of 7) www. advmatinterfaces. de Data Availability Statement The data that support the findings of this study are available from the corresponding author upon reasonable request. Keywords aluminum oxide, atomic layer deposition, plasma treatment, silicon oxide, silicon surface passivation Received: December 14, 2022 Revised: March 22, 2023 Published online: April 18, 2023 [1] R. S.  Bonilla, B.  Hoex, P.  Hamer, P. R.  Wilshaw, Phys. Status Solidi A 2017, 214, 1700293. [2] G.  Dingemans, W. M. M.  Kessels, J. Vac. Sci. Technol., A 2012, 30, 40802. [3] T.   Niewelt, A.   Richter, T. C.   Kho, N. E.   Grant, R. S.   Bonilla, B.  Steinhauser, J.-I.  Polzin, F.  Feldmann, M.  Hermle, J. D.  Murphy, S. P.  Phang, W.  Kwapil, M. C.  Schubert, Sol. Energy Mater. Sol. Cells 2018, 185, 252. [4] G.   Dingemans, N. M.   Terlinden, D.   Pierreux, H. B.   Profijt, M. C. M.  van de Sanden, W. M. M.  Kessels, Electrochem. Solid-State Lett. 2011, 14, H1. [5] B.  Hoex, J. J. H.  Gielis, M. C. M.  van de Sanden, W. M. M.  Kessels, J. Appl. Phys. 2008, 104, 113703. [6] P.  Saint-Cast, D.  Kania, M.  Hofmann, J.  Benick, J.  Rentsch, R.  Preu, Appl. Phys. Lett. 2009, 95, 151502. [7] D.   Schuldis, A.   Richter, J.   Benick, P.   Saint-Cast, M.   Hermle, S. W.  Glunz, Appl. Phys. Lett. 2014, 105, 231601. [8] F.   Werner, B.   Veith, D.   Zielke, L.   Kühnemund, C.   Tegenkamp, M.  Seibt, R.  Brendel, J.  Schmidt, J. Appl. Phys. 2011, 109, 113701. [9] These interface parameters vary to some extend with the different deposition techniques. [10] S.  Dauwe, L.  Mittelstädt, A.  Metz, R.  Hezel, Prog Photovolt 2002, 10, 271. [11] C.  Ballif, F.-J.  Haug, M.  Boccard, P. J.  Verlinden, G.  Hahn, Nat. Rev. Mater. 2022, 7, 597. [12] R.   Preu, E.   Lohmüller, S.   Lohmüller, P.   Saint-Cast, J. M.   Greulich, Appl. Phys. Rev. 2020, 7, 041315. [13] F.   Feldmann, M.   Bivour, C.   Reichel, M.   Hermle, S. W.   Glunz, Sol. Energy Mater. Sol. Cells 2014, 120, 270. [14] A.  Richter, J.  Benick, F.  Feldmann, A.  Fell, M.  Hermle, S. W.  Glunz, Sol. Energy Mater. Sol. Cells 2017, 173, 96. [15] P.  Zheng, J.  Yang, Z.  Wang, L.  Wu, H.  Sun, S.  Chen, Y.  Guo, H.  Xia, S. P.   Phang, E.-C.   Wang, J.   Stuckelberger, H. C.   Sio, X.   Zhang, D.  Macdonald, H.  Jin, Cell Rep. Phys. Sci. 2021, 2, 100603. [16] A.   Richter, R.   Müller, J.   Benick, F.   Feldmann, B.   Steinhauser, C.  Reichel, A.  Fell, M.  Bivour, M.  Hermle, S. W.  Glunz, Nat. Energy 2021, 6, 429. [17] F.   Haase, C.   Hollemann, S.   Schäfer, A.   Merkle, M.   Rienäcker, J.  Krügener, R.  Brendel, R.  Peibst, Sol. Energy Mater. Sol. Cells 2018, 186, 184. [18] Y.   Hotta, I.   Kawayama, S.   Miyake, I.   Saiki, S.   Nishi, K.   Yamahara, K.  Arafune, H.  Yoshida, S.-I.  Satoh, N.  Sawamoto, A.  Ogura, A.  Ito, H.   Nakanishi, M.   Tonouchi, H.   Tabata, Appl. Phys. Lett. 2018, 113, 012103. [19] H.  Kamata, K.  Kita, Appl. Phys. Lett. 2017, 110, 102106. [20] K.  Kita, A.  Toriumi, Appl. Phys. Lett. 2009, 94, 132902. [21] H.  Patel, C.  Reichel, A.  Richter, P.  Masuch, J.  Benick, S. W.  Glunz, Appl. Surf. Sci. 2022, 579, 152175. [22] B.  Liao, et al., Prog Photovolt 2022, 3607, https://doi. org/10. 1002/pip. [23] A.   Ek, C.   Reichel, A.   Richter, J.   Benick, J. Appl. Phys. 2020, 127, 235303. [24] N. E.  Grant, V. P.  Markevich, J.  Mullins, A. R.  Peaker, F.  Rougieux, D.  Macdonald, J. D.  Murphy, Phys. Status Solidi A 2016, 213, 2844. [25] G.   Dingemans, F.   Einsele, W.   Beyer, M. C. M.   van de  Sanden, W. M. M.  Kessels, J. Appl. Phys. 2012, 111, 093713. [26] H.  Nagel, C.  Berge, A. G.  Aberle, J. Appl. Phys. 1999, 86, 6218. [27] R. A.  Sinton, A.  Cuevas, Appl. Phys. Lett. 1996, 69, 2510. [28] T.   Niewelt, B.   Steinhauser, A.   Richter, B.   Veith-Wolf, A.   Fell, B.  Hammann, N. E.  Grant, L.  Black, J.  Tan, A.  Youssef, J. D.  Murphy, J.  Schmidt, M. C.  Schubert, S. W.  Glunz, Sol. Energy Mater. Sol. Cells 2022, 235, 111467. [29] A.  Richter, J.  Benick, M.  Hermle, S. W.  Glunz, Appl. Phys. Lett. 2014, 104, 061606. [30] J.   Benick, et  al., in Photovoltaic Specialists Conference (PVSC), 2010 35th IEEE, xx, Honolulu, HI, USA 2010, pp. 891-896. [31] T.   Faraz, K.   Arts, S.   Karwal, H. C. M.   Knoops, W. M. M.   Kessels, Plasma Sources Sci. Technol. 2019, 28, 024002. [32] H. B.  Profijt, P.  Kudlacek, M. C. M.  van de Sanden, W. M. M.  Kessels, J. Electrochem. Soc. 2011, 158, G88. [33] H. B.  Profijt, W.  Kessels, ECS Trans. 2013, 50, 23. [34] G.  Dingemans, W.  Beyer, M. C. M.  van de Sanden, W. M. M.  Kessels, Appl. Phys. Lett. 2010, 97, 152106. [35] A.   Descoeudres, L.   Barraud, S.   De Wolf, B.   Strahm, D.   Lachenal, C.   Guérin, Z. C.   Holman, F.   Zicarelli, B.   Demaurex, J.   Seif, J.  Holovsky, C.  Ballif, Appl. Phys. Lett. 2011, 99, 123506. [36] J. S.   Herman, F. L.   Terry, IEEE Electron Device Lett. 1991, 12, 236. [37] T.   Seo, H.   Park, G.   Jeon, J.   Yun, S.   Park, S.   Seong, Y.   Chung, ACS Appl. Electron. Mater. 2020, 2, 3320. [38] J.  Irikawa, S.  Miyajima, S.  Kida, T.  Watahiki, M.  Konagai, Jpn. J. Appl. Phys. 2011, 50, 012301. [39] C.  Cismaru, J. L.  Shohet, J. L.  Lauer, R. W.  Hansen, S.  Ostapenko, Appl. Phys. Lett. 2000, 77, 3914. [40] R.  Hezel, K.  Jaeger, J. Electrochem. Soc. 1989, 136, 518. [41] B.  Liao, R.  Stangl, T.  Mueller, F.  Lin, C. S.  Bhatia, B.  Hoex, J. Appl. Phys. 2013, 113, 024509. [42] B.  Hammann, B.  Steinhauser, A.  Fell, R.  Post, T.  Niewelt, W.  Kwapil, A.  Wolf, A.  Richter, H.  Höffler, M. C.  Schubert, Quantifying Surface Recombination-Improvements in Determination and Simulation of the Surface Recombination Parameter J0s, submitted for publica-tion accepted for publication in IEEE Journal of Photovoltaics. [43] J. A.   Giesecke, M. C.   Schubert, B.   Michl, F.   Schindler, W.   Warta, Sol. Energy Mater. Sol. Cells 2011, 95, 1011. [44] P. P.  Altermatt, A.  Schenk, F.  Geelhaar, G.  Heiser, J. Appl. Phys. 2003, 93, 1598. [45] K.  Misiakos, D.  Tsamakis, J. Appl. Phys. 1993, 74, 3293. [46] A.  Schenk, J. Appl. Phys. 1998, 84, 3684. Adv. Mater. Interfaces 2023, 10, 2202469 21967350, 2023, 16, Downloaded from https://onlinelibrary. wiley. com/doi/10. 1002/admi. 202202469 by Cochrane China, Wiley Online Library on [08/09/2024]. See the Terms and Conditions (https://onlinelibrary. wiley. com/terms-and-conditions) on Wiley Online Library for rules of use; OA articles are governed by the applicable Creative Commons License

10.1002admi.202202469.pdf