Reactive Force-Field Molecular Dynamics Study for Analysis of Silicon Nanocrystal Formation Process in Silicon Oxide Film
Authors: Tamura Genta , Uene Naoya , Gotoh Kazuhiro , Usami Noritaka , Tokumasu Takashi 
Published: December 2024

Recently, Tunnel Oxide Passivated Contact (TOPCon) solar cells using ultra-thin silicon oxide (SiO
x
) films as passivation films have attracted attention due to the high conversion efficiency of 25.7% [1]. Generally, thick SiO
x

films exhibit excellent passivation performance and lead to long-term reliability of the solar cells[2]. However, in TOPCon solar cells, the thickness of the passivation film is limited to 1~2 nm due to the high electrical resistance of the SiO
x
film. Therefore, a new SiO
x
film (named NATURE contact) with improved conductivity through silicon nanocrystals (Si NCs) have been developed by Tsubata et al. [3]. The left figure shows the structure of NATURE contact. It consists of three SiO
x
layers with different oxygen concentrations. The Si NCs formed in the thin Si-rich layer in the center functions as a carrier transport pathway. The passivation film with the NATURE contact has shown a good passivation performance and conductivity even for relatively thick SiO
x
films. Since the effects of multidimensional process parameters during NATURE contact formation have been neither elucidated nor optimized, there is significant room for performance improvement in NATURE contact. Therefore, it is of crucial importance to understand the atomic level growth process of Si NCs during formation of NATURE contact and its impact of the oxygen concentration and temperature.

We employed Reactive force-field molecular dynamics (ReaxFF MD) method, which reproduces chemical reactions using a bond order-dependent concept. The method has been used in studies such as Si deposition simulations [4]. The periodic boundary conditions were applied to the simulation box for all directions in the size of 65 Å × 40 Å × 45 Å. Crystalline silicon was placed throughout the simulation box and the atoms of the simulated Si NC section were fixed in the center of the box. Crystals other than the fixed portion are then made amorphous using the Melt-Quench method. Finally, the fixation was released to allow the entire box to react. Crystallinity was evaluated by the average radius calculated from the number of atoms constituting the crystal. It is noted that the average radius is evaluated up to 25 Å due to the size of the simulation box. Annealing was performed in the following three processes: pre-moistening, amorphization, and crystal growth. We varied the temperature of crystal growth at 1500 - 2500 K and the oxygen concentration at 0 - 5 %. We investigated the effects of temperature and oxygen concentration on the growth of Si NCs in a-SiO
x
.

The right figure shows the transition of the mean radius of the Si NCs at different temperatures. As a result, when the temperature was varied, a peak in the size of crystal growth was observed at 2250 K, and the crystals disappeared at 2500 K. The trend of maximum crystal growth at a certain temperature and the qualitative agreement were experimentally confirmed [5]. This result suggests that the change in the size of Si NCs occurred because the kinetic energy of Si atoms is larger than the energy barrier required from a-Si to c-Si. The initially placed Si NC of 269 atoms was appropriately shaped or grown. This is consistent with the nucleation theory[6]. When the oxygen concentration was increased, no crystal growth was observed. Experiments have shown that increasing oxygen concentration tends to suppress crystallization [3]. This result suggests that the change in the size of Si NCs was caused by the interdiffusion of silicon and oxygen.

References

[1] A. Richter et al., Elsevier Oceanogr. Ser. 2017

[2]D. L. Young et al., Energy Procedia, vol. 55, pp. 733–740, Jan. 2014.

[3] R. Tsubata et al., ACS Appl. Nano Mater., vol. 5, no. 2, pp. 1820–1827, Feb. 2022.

[4] N. Uene, et al., Comput. Mater. Sci., vol. 204, p. 111193, Mar. 2022.

[5] A. T. Voutsas, et al., J. Appl. Phys., vol. 78, pp. 6999–7006, Dec. 1995.

[6] S. Izumi, et al., J. Cryst. Growth, vol. 274, no. 1, pp. 47–54, Jan. 2005.

Figure 1