Characterization of Ge1-x-ySixSny Ternary Alloys – Comparison of UHV-CVD and Gas Source MBE Growth
Authors: Claflin Bruce , Kiefer Arnold , Beeler Richard , Fang Z.-Q. , Grzybowski Gordon 
Published: February 2020

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 Ge1-x-ySixSny 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 Ge1-x-ySixSny 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 Ge1-x-ySixSny 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 Ge1-x-ySixSny 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 Ge1-x-ySixSny 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 Ge1-x-ySixSny 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 Ge1-x-ySixSny 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 Ea=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.

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