CVD Growth of GeSnSiC Alloys Using Disilane, Digermane, Tin Tetrachloride and Methylsilane

Ge-Sn-Si-C alloys offer a large range of strain engineering which gives the possibility to tailor the material properties for advanced photonic, electronic, photovoltaic and thermoelectric applications. The benefits of Ge-Sn-Si-C alloys are possibilities to obtain a direct-to-indirect transition (for Sn-based alloys) [1], high carrier mobility [2] and low cost process [3].

  In this study, Ge_1-x-y-zSn_xSi_yC_z layers (0.01≤x≤ 0.24, 0≤y≤0.12 and 0≤z≤0.01) have successfully grown on Ge and Si by using RPCVD technique. It was demonstrated that the quality of epitaxial layers is dependent on the growth parameters, layer thickness and the quality of Ge virtual layer. It was found that a proper strain balance in the matrix during the epitaxy where the Si and Si-C is adjusted carefully compared to Sn flux improves the incorporation of Sn in Ge matrix. This is explained by the compensation of tensile strain induced by Si or Si-C with the compressive strain caused by Sn in order to obtain the minimum energy in Ge matrix. Fig. 1a and 1b show RBS spectra of two GeSnSi samples when the first one has a constant Si flux whereas the sample in Fig. 1b has a graded Si flux within GeSnSi layer. The Sn incorporation improved from 3% to 24% when Si flux was graded.

Incorporation of P and B in GeSnSiC matrix has been studied and the effect of dopant concentrations on Sn content has been investigated. A low boron concentration in range of 10¹⁷ cm^-3 could be incorporated in the epi-layers whereas P-doping in range of 10¹⁹ cm^-3could be achieved.    

   All Ge_1-x-ySn_xSi_yC_z layers were grown at 280-330 °C at 20 torr in a RPCVD reactor. The gas precursors were Si₃H₈, Ge₂H₆, 1% B₂H₆ in H₂, 5% PH₃ in H₂, and SnCl₄as Si, Ge, B, P and Sn sources, respectively. The strain in the epi-layers was measured by high-resolution x-ray reciprocal lattice maps (HRRLMs). Transmission electron microscopy (TEM) was applied to detect the defects in the epi-layers and to observe the interfacial quality. The Sn, Si and C content in the samples was measured by Rutherford backscattering spectrometry (RBS) and HRRLMs. The atomic dopant concentration in the epi-layers was measured by secondary ion mass spectrometry (SIMS). Four-probe measurements were done to determine the resistivity of the doped layers.

The quality of carrier and reactant gases in terms of oxygen and water vapor was investigated by residual gas analyzer (RGA). The RGA instrument was connected to the exhaust gas output and was situated in 20 mm distance from the reactant gas chamber. A RGA spectrum reveals any absorption or desorption of molecules with accuracy of ppt range. The results showed that the level of oxygen during epitaxy was as low as 10 ppb and the contamination concetration was found as low as 10¹⁷ cm^-3. This low oxygen level during epitxy is due to the high purity of trisilane, digermane and tetrachloride precursors as well as the hydrogen carrier gas.

 Acknowledgment

The Swedish Research Council (VR) is greatly acknowledged for supporting this work.

References

[1] P. Moontragoon, Z. Ikonic and P.Harrison, J. Semicond. Sci. Technol. 22, p.742, 2007.

[2] S. Gupta, R. Chen, B. Magyari-Kope, H. Lin, B. Yang, A. Nainani, Y. Nishi, J. S. Harris, and K. C. Saraswat, in Proc. IEDM, 11, p. 398, 2011.

[3] A. Jamshidi, M. Noroozi, M. Moeen, A. Hallen, B. Hamawandi, J. Lu,  L. Hultman, M. Ostling, H. H. Radamson, Surface & Coatings Technology, v 230, p.106, 2013.

Fig. 1 RBS spectra for GeSnSi layers grown with a) constant Si and Sn flux b) a grading Si flux but constant Sn flux during epitaxy