SiC formation for a solar cell passivation layer using an RF magnetron co-sputtering system
© Joung et al; licensee Springer. 2012
Received: 19 September 2011
Accepted: 5 January 2012
Published: 5 January 2012
In this paper, we describe a method of amorphous silicon carbide film formation for a solar cell passivation layer. The film was deposited on p-type silicon (100) and glass substrates by an RF magnetron co-sputtering system using a Si target and a C target at a room-temperature condition. Several different SiC [Si1-xCx] film compositions were achieved by controlling the Si target power with a fixed C target power at 150 W. Then, structural, optical, and electrical properties of the Si1-xCx films were studied. The structural properties were investigated by transmission electron microscopy and secondary ion mass spectrometry. The optical properties were achieved by UV-visible spectroscopy and ellipsometry. The performance of Si1-xCx passivation was explored by carrier lifetime measurement.
Semiconductor technology or microelectronics including solar cells has been adopted to form micro- or nano-sized state-of-the-art structures which can reduce system size, improve its performance, achieve lower system cost, and so on [1–3]. Generally, a semiconductor structure or system is manufactured by a combination of additive (film deposition) and subtractive (etching) processes. These days, many research have tried to utilize the microelectronic technology (especially in the deposition of thin film layers) to get more efficient and cost-effective solar cells [4, 5].
Amorphous silicon-based thin film layers (SiO2, SiN, a-SiC:H, and so on) for antireflection coatings, diffusion barriers, passivation layers, and silicon bulk materials have been broadly researched in the solar cell industry. Among the film layers, SiO2 and SiN passivation layers have highly attracted to fabricate high-efficiency silicon solar cells. However, they have negative demerits such as the need for a high-temperature process, difficulty in the photolithography process, and poor thermal stability [6–8]. Hydrogenated amorphous silicon carbide [a-SiC:H] has been studied for solar cell passivation layers due to its wide bandgap, excellent coefficient of thermal expansion that matches with silicon wafers, relatively good thermal and mechanical stabilities, superior cost-of-ownership compared to other materials, and so on. The formation or deposition of the a-SiC:H film has mainly been done by plasma-enhanced chemical vapor deposition [9, 10]. However, the thermal stability of the hydrogen-containing film is degraded during a post-high-temperature firing process. To avoid the hydrogen molecule's void generation, the authors have proposed an a-SiC deposition method by radio frequency [RF] sputtering which was performed by a single silicon-carbide composite target in an argon environment . In this paper, we introduce a deposition method of amorphous silicon carbide [a-Si1-xCx] which was done by RF magnetron co-sputtering. The method can utilize multiple targets (in this paper, Si and C) simultaneously in order to deposit complex compositional coatings. The compositional ratio can be controlled by the differences in sputtering yield, the relative ability of the materials to stick to the substrate, the deposition temperature, and the relevant percent of sputtered elements to reach the substrate without being scattered from the plasma. In this paper, we investigate several properties of the compositional films by controlling the Si target's RF power. Film thickness was measured using a field-emission scanning electron microscope [FE-SEM]. Reflective index of the film was obtained using an ellipsometer, and carrier lifetime of the film on the doped p-type silicon wafer was obtained by a silicon wafer lifetime tester.
Deposition conditions of the a-Si1-xCx passivation layer
Glass and Si substrates
C target, 150 W
Si target, 100; 150; 175; and 200 W
1, 700 rph
4-inch Si and C
Ar, 40 sccm
The deposited a-Si1-xCx passivation layer's thickness and crystal structure were measured using an FE-SEM (S-4800, Hitachi, Tokyo, Japan) and a transmission electron microscope [TEM] (JEM-2100F, JEOL, Seoul, South Korea), respectively. Optical properties of the transmittance and bandgap were measured by UV-visible spectroscopy (S-3100, Scinco, Seoul, South Korea), the refractive indexes were obtained using an ellipsometer (M2000D, Woollam, Uiwang-si, South Korea), and the electrical performance of the a-Si1-xCx passivation layer was analyzed by carrier lifetime measurement (WCT-120, Sinton Consulting Inc., Boulder, CO, USA).
Results and discussion
We demonstrate a formation method of an a-Si1-xCx passivation layer for Si solar cells. The method was performed by RF magnetron co-sputtering with a Si target and a C target. The a-Si1-xCx passivation layer was deposited on p-type silicon (100) and glass substrates with the reaction of argon (Ar) gas. In this work, we have checked the effect of the Si-to-C ratio on the a-Si1-xCx passivation layers with variation on the RF powers of the Si target (100, 150, 175, and 200 W) and with fixation on the RF power of the C target (150 W). However, the thicknesses of all a-Si1-xCx passivation layers were kept constant at 100 nm. The SIMS profile showed that the film composition is changed with a variation on the RF power of the Si target. The refractive index analysis indicated that a Si-rich thin film has higher refractive index. We could obtain the highest refractive index of 3.7 with a 200-W RF power of the Si target. The optical bandgap analysis showed that the bandgap of the Tauc plot was observed to decrease gradually from 1.4 to 0.9 eV. The carrier lifetime analysis showed that the carrier lifetime of the a-Si1-xCx passivation layer was observed to decrease gradually from 8.9 to 6.0 μs as the carbon ratio in the film is decreased by changing the Si target power. Therefore, we could conclude that the RF magnetron co-sputtering method for SiC can deposit a thin film passivation layer for solar cells with various compositional ratios of Si and C.
This work was supported by the New IT Project for global competitiveness strengthening of the advanced mobile devices and equipments of the Chungcheong Leading Industry Office of the Korean Ministry of Knowledge Economy.
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