Thien-Truong-Doped-Poly-SiSiOx-Passivating-Contacts-Hydrogenation-and-Its-Mechanisms

See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/351132476 Doped Poly-Si/SiOx Passivating Contacts: Hydrogenation and Its Mechanisms Conference Paper · December 2020 CITATIONS0 READS88 5 authors, including: Some of the authors of this publication are also working on these related projects: High DC Voltage From AC in Voltage Multiplier Circuit View project Oxygen in Silicon View project Di Yan Australian National University 86 PUBLICATIONS 1,863 CITATIONS SEE PROFILE Daniel Harold Macdonald Australian National University 347 PUBLICATIONS 7,278 CITATIONS SEE PROFILE Hieu T. Nguyen Australian National University 90 PUBLICATIONS 1,265 CITATIONS SEE PROFILE All content following this page was uploaded by Thien Ngoc Truong on 28 April 2021. The user has requested enhancement of the downloaded file. Doped Poly-Si/SiOx Passivating Contacts: Hydrogenation and Its Mechanisms Thien N. Truong1, Di Yan1, Andres Cuevas1, Daniel Macdonald1 and Hieu T. Nguyen1 1The Australian national University, Canberra, ACT 2601 Australia Abstract: Polycrystalline silicon (poly-Si) passivating-contact solar cells are predicted to replace the everyday solar technology in the market in the next few years, with some pilot lines in production now. However, the poly-Si films possess two limitations: (i) They absorb strongly light in the ultra-violet to blue region, resulting in relatively large parasitic loss when employed on the front side of a solar cell, and (ii) They often contain a high density of defects which can potentially affect the quality of poly- Si/SiOx passivating contacts and limit the conversion efficiency of solar cells. Therefore, reducing defect densities inside the poly-Si films and improving their optoelectronic performances are critical to the commercialization of this technology. Here, we examine different hydrogen treatments to reduce the defect concentration inside the doped poly-Si films and to improve the passivation quality of the poly- Si/SiOx stacks. We demonstrate significant improvements in the passivation quality (presented by an increase of the implied open circuit voltage) of both boron and phosphorus-doped poly-Si/SiOx stacks formed on both n and p-type silicon substrates, with either planar or textured surfaces. Furthermore, combining various characterization techniques, we unlock key optical and electrical properties of the passivating-contact structures, providing critical insights about the mechanisms of the hydrogenation processes. Introduction: Beyond the Passivated Emitter and Rear Cell (PERC) crystalline silicon (c-Si) technology with potential commercial cell conversion efficiencies in the range of 21 - 24% [1], c-Si solar cells incorporating poly-Si/SiOx passivating contacts have attracted significant interest as one of the most promising pathways to overcome the efficiency limits of the PERC technology [2]–[5]. In general, poly- Si films often contain a high density of defects which may affect the passivating-contact performance and thus the conversion efficiency of solar cells. This stems from the fact that poly-Si films, formed by recrystallizing amorphous Si (a-Si) films via a high-temperature processing step, often contains both amorphous and crystalline phases. In the a-Si phase, each Si atom is surrounded by four other Si atoms whose bonds are stretched, twisted, or broken (known as dangling bonds). These bonds then create energy levels inside the material bandgap and are responsible for Shockley-Read-Hall (SRH) carrier recombination. One way to overcome this SRH recombination is to neutralize each defective bond inside the films with the addition of a hydrogen atom, that is, by performing a hydrogenation step after the formation of the poly-Si film. On the other hand, the benefits of hydrogenation processes are well-known for c-Si wafers and solar cells. Therefore, in principle, hydrogenation techniques could also be used to passivate defects within the doped poly-Si films themselves, in a similar manner to the c-Si substrates, hence improving the overall performance of doped poly-Si/SiOx passivating contacts. This work investigates effective methods to reduce the defect concentration inside the doped poly-Si films and to improve the passivation quality of the doped poly-Si/SiOx stacks. We first study the structural and optoelectronic properties of the doped poly-Si/SiOx passivating contacts on different substrates. We then demonstrate that atomic hydrogen, supplied by a SiNx:H or AlOx:H capping layer, can be effectively driven in to passivate defects inside various types of poly-Si films, thus improving the quality of the passivating-contact structures. Finally, we elucidate the underlying mechanisms of various post- treatments of doped poly-Si/SiOx, including AlOx:H and/or SiNx:H films followed by FGA, and FGA alone, by tracking the evolution of their optoelectronic properties after each process. Results and Analysis A. Structural and optoelectronic properties Figure 1A shows a high-resolution transmission electron microscopy (TEM) image of the interface. The interfacial oxide film can be observed as a thin stripe of amorphous structure between the poly-Si and c-Si, as noted in the figure. The doped poly-Si layer shows both crystalline and amorphous phases, whereas as expected, the c-Si substrate shows a very uniform crystalline structure. Figure 1B shows the evolution of the PL spectra from the phosphorus-doped poly-Si samples before and after hydrogenation. There is a clear PL peak associated with the a-Si:H content after hydrogenation, indicating that the atomic hydrogen in the SiNx:H films has been driven into the doped poly-Si films [6]. Figure 1C shows a high-angle annular dark-field (HAADF) image of a vertical cross-section of one pyramid at the surface. The interfacial silicon oxide layer can be observed as a thin amorphous SiOx stripe in the TEM image (Figure 1C-1). However, there is a clear disruption of the oxide interface, and the thickness of the poly-Si film is ~40 nm. On the other hand, Figure 1C-2 shows a TEM image at the pyramid’s peak area (area 2). The poly-Si film is thinner (~20 nm) and there are is no clear boundary between the poly-Si film and c-Si substrate. There is no feature that represents the amorphous phase of the SiOx interfacial layer either. Surprisingly, the luminescence properties of the stacks are remarkably different. There is no clear a-Si:H PL peak emitted from the phosphorus-doped poly-Si film after the hydrogen treatment on n-textured substrates (Figure 1D). Also, the phosphorus-doped poly-Si PL peak intensity is minimal, although it slightly increases after the treatment. We hypothesize that the disappearance of the a-Si:H PL peak and the minimal intensity of the poly-Si PL peak, after the hydrogenation treatment, on the textured substrate are due to the absence of the ultrathin oxide interface around the pyramid peaks and the oxide disruption on the pyramid sides. The oxide absence and disruption cause a lack of photoinduced carrier build-up inside the doped-poly film [7]. Note that, even with the oxide absence and disruption, the implied open circuit voltage (iVoc) values are still 700 mV before and after hydrogenation. This could be due to the very small area of the disrupted and absent oxide regions compared to the normal oxide region. Moreover, Figure 1E shows FTIR results from the hydrogenated phosphorus-doped poly-Si sample at different stages: the poly-Si film is still present, only the film is removed by TMAH etching, and both the film and oxide layer are removed. The captured spectrum presents a clear stretching mode of Si− H bonds (1900~2200 cm−1) in the sample with the hydrogenated poly-Si layer. When the layer is removed, the peak also disappears regardless of the presence of the oxide interface. This demonstrates that the amount of hydrogen in the poly-Si layer is still relatively high whereas that in the oxide interface is little or below the detection limit of our FTIR tool. Figure 1. (A) High-resolution TEM image of an 830 °C phosphorus-doped poly-Si/SiOx/c-Si structure on a planar substrate. (B) PL spectra captured from the sample before and after hydrogenation by SiNx:H + FGA. (C) HAADF image of a vertical cross-section of a pyramid at the sample surface: (1) TEM image of the selected area at the edge and (2) at the peak of the pyramid. (D) PL spectra captured from the textured 830 °C phosphorus-diffused poly-Si/SiOx/c-Si sample before and after hydrogenation. (E) FTIR absorbance spectra captured from a hydrogenated 830 °C phosphorus-doped poly-Si/SiOx/c-Si sample with the poly-Si film present, and with the poly-Si film and/or SiOx layer removed. All PL measurements conducted using an excitation laser of 405 nm at 80 K. B. Effects of hydrogen treatments on the performance of passivating contacts Figure 2A and 2B show a significant improvement in iVoc at 1-sun intensity from various planar phosphorus-doped poly-Si samples after a SiNx:H assisted hydrogen treatment. All samples, initially with either low or high passivation qualities (some with 700 mV for p-type 100 Ω.cm c-Si substrates), have iVoc boosted. The improvement can also be observed for all investigated planar boron-doped poly-Si samples, as shown in Figure 2C and 2D. Figure 2E shows iVoc of phosphorus doped samples hydrogenated by thermal assisted atomic layer deposition (ALD) AlOx:H + FGA, SiNx:H + FGA, and FGA only. All samples after the phosphorus diffusion Poly - Si SiO x interface c - Si A 2 m m A 20 nm 1 B 2 0.5 m m C 2 1 10 nm poly - Si c - Si c -Si poly -Si Pt Disr upted SiO x 10 nm 800 1000 1200 1400 1600 0 10000 20000 30000 40000 50000 PL I nt ens it y (a. u. ) W av elength (nm ) H y d ro g e n a t e d As di f f u se d c-Si p h o sp h o ru s do p e d p o l y -Si a -Si : H 900 1050 0 750 1500 1400 1600 0 750 1500 800 1000 1200 1400 1600 0 10000 20000 30000 40000 50000 a-Si: H phos phrus doped poly -Si c -Si PL I nt ens it y (a. u. ) W av elength (nm ) H y drogenated As dif f us ed 1000 2000 3000 4000 0. 00 0. 05 0. 10 0. 15 S i-Si S i-H Abs orbanc e (a. u. ) W av enum ber (c m -1 ) Po ly - S i ( r e mo v e d ) /SiO x /c - S i Po ly - S i ( r e mo v e d ) /SiO x ( r e mo v e d ) /c - S i Po ly - S i/ S iO x /c - S i h y d r o g e n a ted S i-H 1500 2000 2500 0.000 0.002 E D B show good initial passivation qualities (iVoc ~ 685–700 mV). Some of the phosphorus-doped poly-Si/SiOx samples were then annealed in forming gas at 400 oC for 30 min without any capping layer. The samples with a high resistivity (100 Ω.cm) showed a dramatic increment in iVoc values after the annealing (ΔiVoc ~ 28 mV), whereas on low-resistivity samples (2 Ω.cm), the change was minimal. The other samples, annealed in forming gas in the presence of an AlOx:H capping layer, showed a significant increment in iVoc values (iVoc 725 mV) regardless of the initial conditions. To compare the AlOx:H + FGA and SiNx:H + FGA hydrogen treatments, we continued capping the already hydrogenated samples (via FGA alone and AlOx:H + FGA) with SiNx:H films and subsequently annealed them in forming gas (SiNx:H + FGA), as shown in Figure 2E. There was a slight reduction in the passivation quality of the samples initially hydrogenated by AlOx:H + FGA (of both low- and high- resistivity substrates) but a significant improvement of samples initially hydrogenated by FGA only (especially low-resistivity substrates). The surface passivation reduction of the samples with pre-AlOx:H + FGA after the second hydrogenation step by SiNx:H + FGA (after having removed the AlOx:H layer) could be due to two possible reasons. First, the hydrogenated poly-Si films may have been damaged by the plasma from the PECVD SiNx:H deposition. Second, the samples may have been injected with too much hydrogen, which could cause a hydrogen-induced degradation. We can conclude that the former is the main reason for the decreasing passivation quality, as we subsequently injected even more hydrogen by a third hydrogenation step (SiNx:H + FGA 2) but did not observe any passivation change. Despite that, iVoc values were all very high (715–727 mV) after the second hydrogenation step. Figure 2. Implied open circuit voltages iVoc of various poly-Si/SiOx/c-Si samples before and after different hydrogenation steps. The poly-Si films are doped with (A, B, E) phosphorus and (C, D) boron. The c-Si substrates are planar. (A) 1 Ω.cm n-type substrate, 250 oC and 300 oC deposition temperatures of a-Si:H films, 830 oC diffusion temperature to form poly-Si films. (B) 100 Ω.cm p-type substrate, 300 oC deposition temperatures of a-Si:H films, 830 oC diffusion temperature to form poly-Si films. (C) 1 Ω.cm n-type and (D) 100 Ω.cm p-type substrates, 300 oC deposition temperatures of a-Si:H films, 830 oC and 860 oC diffusion temperature to form poly-Si films. (E) Samples before and after various hydrogenation methods: FGA alone and AlOx:H or SiNx:H followed by FGA. The iVoc are measured at a 1-sun equivalent intensity. C. Mechanisms of different hydrogen post-treatments Figure 3A presents the PL spectra captured from the poly-Si/SiOx/c-Si sample at 80 K using the 405 nm excitation laser after various processing steps: as diffused and subsequently annealed in forming gas with and without the AlOx:H capping layer. In all cases, the spectra are nearly identical. In contrast, it is known that the poly-Si films contain both amorphous and crystalline phases, and the hydrogenated a- Si (a-Si:H) phase will emit a distinct peak located at ~950 nm, as shown in Figure 1B [8]. Surprisingly, there is no PL peak from the a-Si:H phase after the AlOx:H deposition and subsequent FGA (Figure 3A), albeit there being a big improvement in the passivation quality (Figure 2E). These results demonstrate that the 40 nm AlOx:H film had injected some amount of atomic hydrogen into the sample, which sufficiently passivated defects at the SiOx/c-Si or poly-Si/SiOx interfaces. However, the hydrogen had not neutralized the non-radiative defects inside the poly-Si films or the dangling bonds associated with the a-Si phase. Figure 3B shows the PL spectra captured from the samples after the second hydrogenation step (SiNx:H + FGA) at 80 K. Compared to the as-diffused sample, the poly-Si PL intensity (FGA + (SiNx:H + FGA)) also increases indicating that some non-radiative defects inside the poly-Si layer have been passivated. Interestingly, the PL spectrum from the sample with pre-AlOx:H + FGA even shows a much more significant increment in the PL intensities of both a-Si:H and poly-Si (AlOx:H + FGA + (SiNx:H + FGA)). These results suggest that as more hydrogen is injected, more non-radiative defects inside the poly-Si films and their a-Si phase are passivated. Phos p h or u s d op e d Bor on d op e d 650 675 700 725 iV oc ( m V ) n, 50 0 o C a- S i :H( 1) n, 50 0 o C a- S i :H( 2) n, 40 0 o C a- S i :H( 1) n, 40 0 o C a- S i :H( 2) 30