Improving Passivation Properties using a Nano-crystalline Silicon Oxide Layer for High-Efficiency TOPCon Cells

See discussions, stats, and author profiles for this publication at https//www.researchgate.net/publication/350369573 Improving Passivation Properties using a Nano-crystalline Silicon Oxide Layer for High-Efficiency TOPCon Cells Article in Infrared Physics Accepted 19 March 2021 Infrared Physics and Technology 115 2021 103723 2 absorption losses, rear-side films must be highly transparent, while providing excellent transport and superior passivation for the selected charge carrier. To minimize absorption loss, we report a novel rear side- area passivating electron contact that improves the transparency by replacing a part of the doped poly-Si layer. The novel contact structure includes a thin thermally grown SiO 2 layer on the c-Si wafer surface capped by a phosphorus-doped nanocrystalline silicon oxide nc-SiO x . The plasma-enhanced chemical vapor deposition PECVD system of SNTEK company with a frequency of 13.56 MHz, was used for the growth of the nc-SiO x layer. The PECVD process has the advantage that it allows single-sided deposition instead of the wrap-around deposition observed during the low-pressure chemical vapor deposition process [22]. Moreover, the temperature stability of passivation is improved owing to the presence of a thermally grown SiO 2 layer, resulting in a broad thermal processing window. A wide bandgap material, nc-SiO x , was introduced to reduce optical absorption. The optical and electrical characteristics could be varied by changing the oxygen content. The conductive properties of the n-doped nc-SiO x were found at low oxygen concentrations with phosphorus doping. The annealing process is an important process for activating the dopant and developing films required for emitter contact. During the annealing process, the surface morphology varied throughout. The Si precipitates into small grains of nano-Si, while the O 2 acts as insulation and forms SiO 2 on the grain boundaries [23]. The consequences of O 2 assimilation are the result of a reduction in grain growth. For higher O 2 concentrations, Carrier transport is resulted by tunneling via thin SiO x shells for higher O 2 concentrations [24]. The junction properties were analyzed by varying the thickness and annealing time of the deposited layer. Hydrogen passivation in forming gas annealing FGA was investigated after the influence of each parameter. Additionally, our research focused on the experimental de- signs to explain the physical mechanisms accountable for melioration and optimization of the passivation characteristics of symmetric struc- tures using a Quasi-Steady-State Photoconductance QSSPC measure- ment system. 2. Experimental Czochralski-grown phosphorus-doped n-type wafers 200 m thick with 3–5 Ω -cm, were used in this study. The first step was the removal of surface defects by employing an etching solution comprising sodium hydroxide NaOH at 70–80 C for approximately 10 min. Further, samples were consecutively cleaned by employing the standard RCA-1 and RCA-2 cleaning processes to remove organic and metallic impu- rities [25]. Afterward, the samples were dipped in HCl solution and then in HF solution for 2 min each. Finally, the deionized water was used for the cleaning of samples. The cleaned samples were immediately placed in a thermal oxide furnace. The thermally growth of a SiO 2 tunnel layer with a thickness of approximately 1.5 nm on the front and back sides of the cleaned Si wafer was conducted in a furnace. The SiO 2 layer was grown by thermal oxidation of the cleaned silicon wafer at a tempera- ture of 600 C with an O 2 flow rate of 10 L/min for 30 min. Finally, the samples were detached from the thermal furnace for the deposition of nc-SiO x . The deposition was performed using PECVD in the presence of SiH 4 , H 2 , CO 2, and PH 3 gases at a temperature of approximately 110 C, and thicknesses of 30, 40, and 50 nm were obtained. For deposition of nc-SiO x , we used 1.5 T pressure and 35 mW/cm 2 power, respectively. Optimized PECVD deposition conditions of phosphorus-doped nc-SiO x material were reported in our previous paper [26]. Electrical and optical properties of n-type nc-SiO x were optimized by introducing SiH 4 , H 2 , CO 2 , and PH 3 at ratios of 4, 500, 2, and 0.08, respectively [26]. All samples synthesized in this study were exposed to post-deposition annealing PDA at 850, 900, 950, or 1000 C for more crystallization. Finally, FGA was performed for 2 h at 400 C. The parameters used to obtain passivated nc-SiO x contacts are listed in Table 1. The procedure is described in detail below. The symmetric structures synthesized through these two approaches are shown in Fig. 1. The ultrathin SiO 2 tunnel oxide layer and nc-SiO x layer with various thicknesses were confirmed through high-resolution transmission electron microscopy HRTEM and spectroscopic ellips- ometry VASE, JA Woollam. The passivation characteristics were evaluated using a QSSPC measurement system WCT Sintron, 120. The transfer length method TLM was used to obtain the contact resistivity ρ c . We used the TLM method by etching the polysilicon to measure the contact resistivity. To investigate the current pathway, the region be- tween the TLM pads was repetitively etched in SF 6 /O 2 /Ar plasma using the aluminum pads as an etching mask, and the samples were measured after each etch process by TLM. The plasma-enhanced chemical vapor deposition PECVD system of SNTEK company with a frequency of 13.56 MHz and thermal furnace of EM TECH company are used during the experiment. Structural characteristics were evaluated by Raman spectroscopy Ramboss 500i, Dongwoo Optron Co., Ltd.. X-ray photo- electron spectroscopy XPS experiments were carried out on a Physical Electronics PHI5000 Versa Probe II photoelectron spectrometer with a mono-chromated Al-Ka source operated in a high-power mode at 100 W. To increase the probing depth, the takeoff angle was set to 85. The Si2p peak was measured with a pass energy of 5.85 eV and 25 meV/step. 3. Results and discussion 3.1. Observation of structure characteristic and passivation characteristics at various thicknesses of nc-SiO x on different annealing temperature The HRTEM image of the ultrathin SiO 2 layer capped with the nc- SiO x layer observed at a scale of 1 nm is shown in Fig. 2. The thickness of this layer was recorded as 1.5 nm. The SiO 2 layer with a thickness greater than 1.6 nm has previously been reported to exhibit poor passivation properties due to the reduced degree of dopant diffusion from nc-SiO x to the c-Si wafer owing to the thicker SiO 2 layer [27]. The structural analyses of such nc-SiO x layers using Raman spectroscopy are shown in Fig. 3a and b. Crystalline volume factor X c as shown in Fig. 3c was derived from the data using the relation X c I 518,519 / I518,519 I 480 , in which I 518, 519 and I 480 are the integrated areas extracted from Gaussian fitting at 518, 519 and 480 cm 1 , respectively. We can see in Fig. 3a that the nc-SiO x layer structure before annealing appears to present a mix phase of crystalline and amorphous, which is character- ized by a Raman peak at 480 cm 1 and 518 cm 1 , mainly visible at thicknesses above the value of 50 nm thick. On the contrary, for thick- nesses in the range of 30–50 nm, the structure can be regarded essen- tially as an amorphous phase, as defined by a broad spectral range centered around 480 cm 1 . It has been known that the growth of crys- talline phase in silicon films during plasma deposition is generally initiated through a first incubation amorphous phase followed by Table 1 Parameters and results of symmetric structure for SiO x /nc-SiO x passivated contacts after FGA. Sample Annealing Temp. C FGA C Thickness nm Lifetime s J O fA/ cm 2 i-V oc mV 1 1000 400 30 1466 2.92 717 2 1000 40 2035 1.51 730 3 1000 50 2397 1.35 736 4 950 30 1919 1.86 726 5 950 40 2530 1.1 731 6 950 50 2918 1.1 739 7 900 30 1013 3.97 709 8 900 40 1547 1.93 729 9 900 50 1877 1.6 733 10 850 30 1096 5.15 705 11 850 40 1118 3.85 711 12 850 50 611 8.17 698 M.Q. Khokhar et al. Infrared Physics and Technology 115 2021 103723 3 nucleation and, finally, crystalline phase formation [27–29]. In this process, nucleation-growth aggregation plays an important role to prompt subsequent crystalline particles. As shown in Fig. 3b, raman spectra showing a clear tendency towards lower amorphous fraction with increasing temperature. As shown in Fig. 3b, the crystalline phase of n-nc-SiO x layer can be initiated and more prominent at all thickness of 30, 40, and 50 nm after annealing at 950 C in thermal furnace. We done Raman spectroscopy only at 950 C because we got ameliorated passivation properties at 950 C. Already in the as-deposited state, the Raman spectrum is dominated by the transverse optical TO c-Si phonon with a peak around 518 cm 1 . However, the signal still shows a pronounced shoulder at wavenumbers between 400 cm 1 and 510 cm 1 , which can be attributed to amorphous Si. After 1-hour higher annealing temperature at 950 C, the amorphous fraction is decreasing further and lateral inhomogeneities vanish not shown here. The amorphous contribution almost disappears for a dwell temperature of 950 C, meaning that the layer is almost fully crystalline. The annealing temperature plays significant role in prompting the rapid growth of crystalline phase in the nc-SiO x layer even with a low or higher thick- ness. Based on the nucleation kinetics, the crystalline phase may grow rapidly and increase in thickness. This is observable in Fig. 3c where the value of X c increases with thickness of nc-SiO x layer as well as increases with annealing temperature. Examination of the chemical state of the silicon by XPS depicted an oxidation induced splitting of the Si2p peak as described in Fig. 4. Investigation of the oxidation states was executed by deconvolution of the silicon 2p peak. It is known as a rule of thumb that the peak shifts 1 eV per oxidation state. The exact parameters were taken from Ref. 37 Si 1 0.9 eV, Si 2 1.7 eV, Si 3 2.7 eV, Si 4 3.7 eV. The deconvolution of the peak revealed the presence of silicon suboxides in the material. The observed evolution of the Si n distribution with 50 nm thick layer of nc-SiO x , despite the fixed oxygen content in the present study, indicates a phase separation. Deviation in the passivation char- acteristics of samples deposited by nc-SiO x were analyzed. The passiv- ation properties at thicknesses of 30, 40, and 50 nm were equated. Then, the nc-SiO x deposited samples were annealed at various temperatures in the range of 850–1000 C. The results of the PDA clarified the breaking of the silicon-hydrogen bond as well as accompanying hydrogen diffu- sion within the c-Si substrate and phosphorous diffusion via the tunnel oxide layer within the n-type substrate. As mentioned earlier, the passivation quality was enhanced by the insertion of the tunnel oxide layer. In this study, passivation of the tunnel oxide junction structure was observed to be excellent for the sample subjected to 950 C thermal annealing. However, the passivation quality decreased with heat treat- ment above 950 C, which means at 1000 C. This is reportedly attrib- uted to the local disruption of the tunnel oxide layer. This disruption is not easy to observe through TEM and XPS measurements due to the fine area analysis on the atomic scale. A bifacial structure such as used for the QSSPC measurement was employed. Substantial improvements in passivation characteristics were observed after PDA when the nc-SiO x layer thickness was approximately 50 nm at 950 C. As the nc-SiO x layer thickness approached 50 nm, considerable improvements were detected in both lifetime τ eff and i-V OC . Fig. 5a depicts the τ eff and i-V OC of the deposited samples from the evaluated thicknesses as a function of the post-deposition annealing temperature T PDA . Both τ eff and i-V OC of the deposited annealed samples were found to be more prominent as compared to the as-deposited sample. The results of τ eff and i-V OC improved at all examined thicknesses as the annealing temperature increased to 950 C, after which they diminished for higher annealing temperature around 1000 C. Fig. 5b demonstrates the J o of the three different nc-SiO x thickness samples in response to the T PDA . J o dimin- ished with an increase in annealing temperature up to 950 C and then J o starts to increase as the annealing temperature increased further up to 1000 C. Particularly high increases in τ eff and i-V OC as well as re- ductions in J o compared to the as-deposited state of the nc-SiO x layer, at the annealing temperature of approximately 950 C, were ascribed to the ameliorated chemical passivation of ultrathin SiO 2 due to the decreased field-effect passivation owing to carrier selectivity [28]. The best passivation characteristics were observed for the deposited sample with the 50 nm thick nc-SiO x layer after the PDA at 950 C, with the highest lifetime and i-V OC of 2446 μs and 728 mV, respectively, and lowest J o of 1.33 fA/cm 2 . Fig. 1. Schematic diagrams of nc-SiO x passivation a wafer characteristics, b passivation characteristics of the SiO x tunnel layer τ eff , iV oc , J o , and thickness, c passivation properties τ eff , iV oc , J o , and thickness and recombination current density J o of SiO x /nc-SiO x passivated contact, and d effective contact resistivity ρ of SiO x /nc-SiO x passivated contacts. Fig. 2. HRTEM images of ultrathin 1.5 nm SiO 2 layer. M.Q. Khokhar et al. Infrared Physics and Technology 115 2021 103723 4 3.2. Improvement in passivation after FGA treatment The passivation characteristics of symmetric structure enhanced further when FGA was performed at 400 C for 2 h via rapid thermal processing on three different thicknesses of 30, 40, and 50 nm nc-SiO x layer which were already exposed to post-deposition annealing PDA at 850, 900, 950 C or 1000 C. The highest lifetime and the i-V OC were achieved up to 2.918 ms and of 739 mV on 50 nm thick nc-SiO x layer Fig. 3. Raman spectra of n-nc-SiO x layers a Before and b after annealing at 950 C; and c the crystalline volume factors X c of n-nc-SiO x layers before and after annealing at 950 C as a function of thickness. Fig. 4. XPS spectra of Si 2p peak photo-electron spectrum of a nc-SiO x layer deposited. Fig. 5. a. Minority carrier lifetime and implied open-circuit voltage as func- tions of T PDA for three nc-SiO x layer thicknesses at injection level 1 10 15 . b. Recombination current density as a function of T PDA depending on nc-SiO x layer thickness. M.Q. Khokhar et al.