Mechanisms of Gas Phase Reactions in Germanium-Tin Chemical Vapor Deposition Processes Determined By in Situ Mass Spectrometric Investigation
Authors: Yang Enbo , Grant Joshua M , Golden Alexander , Margetis Joe , Du Wei , Yu Shui-Qing 
Published: December 2024

Keywords: GeSn, Mass spectrometry, Residual gas analyzer (RGA), Chemical vapor deposition (CVD), gas phase reaction

GeSn, an alloy in the group IV category, has garnered significant research interest. Its appeal lies in its compatibility with Si photonics platforms and its engineerable bandgap covering the mid-infrared (MIR) wavelengths[1]. These properties make GeSn a promising candidate for various applications, including high-speed optical communications, bio-medical sensing, and long-range light detection and ranging (LiDAR) systems. Chemical vapor deposition (CVD) was one of the main methods of synthesizing device-grade GeSn materials. However, the fundamental understanding of the growth mechanism is lacking. Our group has recently demonstrated the mass spectrometric method as an effective method for investigating chemical species generated by chemical reactions during CVD, where room temperature reactions between SnCl4 and GeH4 in the gas phase have been confirmed[2]. As extended research, this paper systematically studied the chemical reaction mechanisms for the GeSn CVD processes, emphasizing the gas phase reaction regime by sampling the reactant gases of SnCl4 and GeH4 using a residual gas analyzer (RGA). Reactions at room temperature, 100°C, 150°C, and 200°C with various gas partial pressure ratios (pSnCl
4 /pGeH
4 ) of 0.002, 0.005, 0.01 and 0.042 have been investigated. Figure 1 shows a set of representative mass spectra of the mixture gases of SnCl4 and GeH4 with different gas ratios at 100°C, where typical species of HxSnCl4-x and HxGeCl4-x are observed, indicating the reactions have taken place in the experimental conditions indicated in the figure.
The resulting original mass spectra and derived fragmentation pattern data for the species generated from those reactions have been obtained compared to the National Institute of Standards and Technology (NIST) data of SnCl4 [3] and GeCl4 [4], respectively. Based on the analysis of the data, it is determined that gas phase reactions occurred in all scenarios investigated.
Moreover, the reactions include both direct interactions between SnCl4 and GeH4, as well as the decomposition of GeH4 itself. The main products of the first type of reactions are the Tin hybrid of Sn, H3SnCl/SnCl, and the Germanium hybrid of H3GeCl/GeCl and H2GeCl2/GeCl2, respectively. The second type of reaction generates GeHx and hydrogens (H, H2, and H3
+), mainly occurring at the chamber surfaces. These products are the crucial intermediate species produced in the GeSn CVD process. Overall, the reactions exhibited a more pronounced response with increasing temperature. This agrees, at least qualitatively, with previous[2,5] data and the latest thermochemistry calculations using the Schrodinger software package, which employs the Jaguar program for molecular-orbital calculations [6]. It is based on the density functional theory (DFT) method, where the proposed chemical reactions were either exothermic or minimally endothermic reactions according to the calculations of ΔG(T)=ΔH-TΔS.
The experimental and theoretical findings of this study offer valuable insights into the growth mechanism involved in GeSn CVD processes. Subsequent investigations will involve the incorporation of an RGA system into the CVD chamber during growth processes. This will facilitate a more comprehensive exploration of the chemical reaction mechanisms occurring throughout the entire process, encompassing both gas-phase and surface reactions.

Acknowledgments

The work is supported by the Air Force Office of Scientific Research (AFOSR) (Grant No.: FA9550-19-1-0341, FA9550-20-1-0168).

References:

[1] Shui-Qing Yu et al., SiGeSn Technology for All-Group-IV Photonics, 2022 Device Research Conference (DRC).
[2] J. M. Grant, E. Yang, N. Masoumi, A. Golden, J. Margetis, A. Chizmeshya, W. Du and S.-Q. Yu, "In-situ Mass Spectrometric Investigation to Probe GeSn Growth Dynamics and Mechanisms in the Chemical Vapor Deposition Processes," submitted to the Journal of Vacuum Science and Technology B, under review.
[3] https://webbook.nist.gov/cgi/cbook.cgi?ID=C7646788&Mask=200#Mass-Spec, retrieved April.1, 2024.
[4] https://webbook.nist.gov/cgi/cbook.cgi?ID=C10038989&Mask=200#Mass-Spec, retrieved April.1, 2024.
[5] J. Margetis, S.-Q. Yu, B. Li and J. Tolle, "Chemistry and kinetics governing hydride/chloride chemical vapor deposition of epitaxial Ge1- xSnx," Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, vol. 37, p. 021508, 2019.
[6] Jaguar, version 11.0, Schrodinger, Inc., New York, NY, 2020. A. D. Bochevarov, E. Harder, T. F. Hughes, J. R. Greenwood, D. A. Braden, D. M. Philipp, D. Rinaldo, M. D. Halls, J. Zhang, R. A. Friesner, "Jaguar: A High-Performance Quantum Chemistry Software Program with Strengths in Life and Materials Sciences", Int. J. Quantum Chem., 2013, 113(18), 2110-2142.

Figure 1