Growth and Characterization of Epitaxial Ge1-XSnx Alloys and Heterostructures Using a Commercial CVD System

Group IV alloys incorporating Sn have several possible applications ranging from optoelectronics to stress engineering in FinFET based CMOS designs.  From an optoelectronics perspective the benefits of incorporating Sn into Ge is realized in the reduction of the difference between the direct and indirect band gap transitions resulting in more efficient luminescence/ absorption.  Furthermore, alloys of Ge_1-xSn_x are theoretically expected to become a direct band gap material for Sn contents of approximately 10 at. % [1].  With regards to logic applications of Sn containing alloys the possibilities include the passive function as a common strain relieved buffer (SRB) for both NMOS and PMOS applications as well as PMOS S/D stressor for an active GeSn channel FinFET devices [2].  Given these potential application one of the key issues lies in the lack of viable growth process on a commercial deposition system.

The Ge_1-xSn_x alloys and Ge buffer layers presented here were grown in an ASM Epsilon^® 2000 Plus chemical vapor deposition (CVD) system This system is ideally suited for the low deposition temperatures required to form these metastable alloys. A specialized growth approach has been adopted to grow the Ge_1-xSn_x layers at temperatures of less than 450°C.

Material and optical characterization of these alloys indicate that the materials are of high crystal quality.  Figure 1 shows a series symmetrical (004) 2θ-ω XRD scans for Ge_1-xSn_x alloys with Sn contents ranging from 0.5 to 10 at. % in which the perpendicular lattice constant increases with Sn content. Rutherford Backscattering Spectroscopy (RBS) in combination with ion channeling was used to verify the thickness and composition of the alloys as well as to determine substitutionality of the Sn within the Ge lattice (Fig.1).  The thickness and composition of these alloys was in close agreement with those obtained from SIMS.  Further structural characterization of the Ge_1-xSn_x alloys was obtained by cross-sectional Transmission Electron Microscopy (XTEM) analysis.  Figure 3 shows a bright field XTEM image of a Ge_0.93Sn_0.07/Ge/Si heterostructure in which no threading defects are observed propagating through the GeSn epilayer.  Room temperature photoluminescence studies of the as-deposited alloys show a strong well-defined peak which shifts to lower energy with increasing Sn contents.    

Growth of heterostructures with different alloy compositions have also been demonstrated in which abrupt compositional transitions are observed.   Optical devices based on these alloys have been fabricated.

References

[1] Y. Chibane et al., J. Appl. Phys. 107, 053512 (2010)

[2] S. Gupta et al., IEDM proceedings (2014)

Fig. 1. ω-2q XRD (004) scans for a series of Ge_1-xSn_x alloys with x = 0.005 to 0.1 grown on Ge buffered Si. 

Fig. 2. RBS random and aligned spectra of Ge_0.93Sn_0.07 epilayer grown on Ge buffered Si. The low yield observed for the aligned spectra indicates a high degree of crystalline perfection in which the Sn occupies a substitutional lattice site.

Fig. 3. Diffraction contrast image of a Ge_0.93Sn_0.07 epilayer grown on a Ge buffered Si in which no threading defects are observed penetrating through the GeSn epilayer.