Characterization of Ge1-x-ySixSny Ternary Alloys – Comparison of UHV-CVD and Gas Source MBE Growth

Group-IV semiconductor alloys are attractive materials for advanced optoelectronic device applications such as integrated Si photonics, multi-junction solar cells, telecom, and infrared sensing.  Over the past decade, ternary alloys of Ge_1-x-ySi_xSn_y have been developed with attractive materials properties for these applications.  These alloys show great promise for strain engineering or as compliant buffer layers since their band gap energy and lattice parameter can be tuned independently [1].  In addition, for sufficient levels of Sn incorporation, the fundamental optical band gap undergoes a crossover from indirect to direct transition [2-4].

The first demonstrated synthesis of Ge_1-x-ySi_xSn_y was achieved via a UHV-CVD process using deuterated stanane (SnD₄) and digermane (Ge₂H₆) precursors [5].  Low growth temperatures (T < 350 ^◦C) are required to stabilize the substitutional Sn in the SiGe lattice because of the large disparity in atomic size.  These deposition processes have subsequently been improved and extended to include the use of higher order group-IV hydride precursors such as Ge₃H₈, Si₃H₈, Ge₄H₁₀, and Si₄H₁₀ [6-9].  Alternative growth processes have also been developed to synthesize Ge_1-x-ySi_xSn_y alloys including gas source molecular beam epitaxy (GS-MBE) [10] and ion implantation of Sn into a SiGe matrix [11].

In this work, we report the electrical, optical, structural, and material characteristics of Ge_1-x-ySi_xSn_y films over a range of alloy compositions.  We compare the properties of films deposited by UHV-CVD with those grown by GS-MBE.  The elemental compositions of these Ge_1-x-ySi_xSn_y alloys, determined by X-ray photoelectron spectroscopy (XPS), are in good agreement with values measured using Rutherford backscattering (RBS).  The XPS measurements also indicate that these alloys are susceptible to surface oxidation when exposed to laboratory ambient.  Crystalline structural characteristics were measured with X-ray diffraction, two-dimensional reciprocal space mapping and X-ray reflectivity.  Thin Ge_1-x-ySi_xSn_y films (~ 300 nm) grown on Ge substrates showed coherent strain, while thicker films (~ 800 nm) showed partial strain relaxation.  The thickest films (~ 1000 nm) were completely relaxed.  No thickness fringes were observed in X-ray reflectivity due to negligible difference in electron density between the film and substrate.

Temperature-dependent Hall-effect measurements show highly conductive layers although care must be exercised to avoid parallel conduction through the substrate for samples grown on Si or Ge wafers.  We also report the first measurement of electronic traps in Ge_1-x-ySi_xSn_y via deep level transient spectroscopy (DLTS) on p-n junction devices. Samples grown by UHV-CVD show two electron traps at low temperature while samples grown by GS-MBE show only one which is more than 10x weaker.  Both the UHV-CVD and GS-MBE samples exhibit an activation energy of E_a=0.40 eV for the dark current for 200 K < T < 350 K.  In addition, the leakage current under reverse bias conditions increases with increasing Sn content in the films.

[1] V. R. D’Costa, Y.-Y. Fang, J. Tolle, J. Kouvetakis, and J. Menendez, Phys. Rev. Lett. 102, 107403 (2009).

[2] D. W. Jenkins and J. D. Dow, Phys. Rev. B 36, 7994 (1987).

[3] R. A. Soref and C. H. Perry, J. Appl. Phys. 69, 539 (1991)

[4] J. D. Gallagher, C. Xu, L. Jiang, J. Kouvetakis, and J. Menendez, Appl. Phys. Lett. 103, 202104 (2013)

[5] M. Bauer, J. Taraci, J. Tolle, A. V. G. Chizmeshya, S.Zollner, D. J. Smith, J. Menendez, C. Hu and J. Kouvetakis, Appl. Phys. Lett., 81, 2992 (2002).

[6] J. Tolle, A. V. G. Chizmeshya, Y. Y. Fang, J. Kouvetakis, V. R. D’Costa, C. W. Hu, J. Men_endez, and I. S. T. Tsong, Appl. Phys. Lett. 89, 231924 (2006).

[7] Y.-Y. Fang, J. Xie, J. Tolle, R. Roucka, V. R. D’Costa, A. V. G. Chizmeshya, J. Menendez, and J. Kouvetakis, J. Am. Chem. Soc. 130, 16095 (2008).

[8] J. Xie, J. Tolle, V. R. D’Costa, A. V. G. Chizmeshya, J. Men_endez, and J. Kouvetakis, Appl. Phys. Lett. 95, 181909 (2009).

[9] C. Xu, L. Jiang, J. Kouvetakis, and J. Men_endez, Appl. Phys. Lett. 103, 072111 (2013).

[10] H. Lin, R. Chen, W. Lu, Y. Huo, T. I. Kamins, and J. S. Harris, Appl. Phys. Lett. 100, 141908 (2012).

[11] G. H. Wang, E.-H. Toh, X. Wang, S. Tripathy, T. Osipowicz, T. K. Chan, K.-M. Hoe, S. Balakumar, G.-Q. Lo, G. Samudra, and Y.-C. Yeo, Appl. Phys. Lett. 91, 202105 (2007).