(Invited) Solid Phase Epitaxy of GeSn Alloys on Silicon and Integration in MOSFET Devices

Single crystalline Ge compounds (SiGe, GeSn) have interesting electrical and optical properties [1], rendering them well-suited to a wide variety of applications. GeSn has been predicted to exhibit carrier mobilities exceeding that of Ge [2], which makes this material suitable as a channel material for high-speed complementary metal-oxide semiconductor (CMOS) technology [3]. In addition to increased carrier mobility, the GeSn material system is interesting for its optical properties. GeSn exhibits a direct band gap between 0.61 and 0.35 eV for Sn concentrations between 6 and 15 % [4]. This makes it much better fit for optoelectronic applications than Si and Ge, especially at long wavelengths.

Epitaxial growth of GeSn on Si substrates poses several challenges including a limited solubility of Sn in Ge (0.5%), compositional fluctuations, Sn segregation and a large lattice mismatch (>4%). It is critical to suppress these effects while simultaneously optimizing the GeSn layer characteristics for the basis of high performance devices.

Recently, we demonstrated the growth of single crystalline GeSn layers with excellent structural quality grown on Si(111) substrates, by solid phase epitaxy (SPE) of amorphous GeSn alloys[5]. This technique has an advantage in the realization of thin GeSn layers directly on Si. We do not use surfactants, a capping layer, nor a buffer layer. Amorphous GeSn layers are obtained by the co-evaporation of Ge and Sn atoms, with beam equivalent pressures of  1.33 x 10^-7 Torr for Ge and 9 x 10^-9 Torr for Sn, while exposing the substrate to a N₂ flux with a beam equivalent pressure of about 1 x 10^-5 Torr, at a deposition temperature around RT. Subsequent annealing transforms the amorphous GeSn into a single crystalline layer via solid phase epitaxy. Excellent structural quality is demonstrated for layers with up to 6.1 % of Sn. The GeSn layers show tensile strain (up to +0.34%), which lowers the difference between the direct and indirect band transition and makes this method promising for obtaining direct band gap group IV layers. GeSn with 4.5% Sn shows increased optical absorption compared to Ge and an optical band gap of 0.52 eV. Furthermore, the surface and interface are smooth (RMS roughness < 1 nm), despite the relaxation of the layers.

Additional structural investigation by XRD and TEM showed the presence of twin defects in the GeSn layers after crystallization. These defects can have a significant impact on the carrier mobility and it is therefore important to suppress the formation of twin defects during crystallization. The formation of twin defects is likely related to the presence of contaminants, in particular oxygen, at the interface between the crystalline substrate and the amorphous layer leading to the roughening of the amorphous-crystalline interface during SPE growth. We have developed a 2-step deposition process which successfully suppresses the formation of such twin defects.

Additionally, we demonstrate excellent electrical properties of the GeSn layers by the fabrication of depletion-mode GeSn pMOSFETs on Si(111) using SPE of amorphous GeSn layers, TaN/Al₂O₃ metal-gate/high-k gate stacks, and Ni-based metal S/D contacts. The GeSn MOSFET devices show +100% improvement in hole mobility with respect to bulk Si and good transfer characteristics with On/Off ratio of ~100 for ultrathin (<10 nm) GeSn layers on Si [6].

Finally, we also demonstrate successful crystallization of amorphous SiGe layers, deposited by plasma enhanced chemical vapor deposition (PECVD) on Si substrates and annealing in N₂ atmosphere. The SiGe layers show excellent structural quality for compositions ranging from Ge- to Si-rich.

[1] J. Kouvetakis et al., Ann. Rev. Mater. Res. 36, 497 (2006)

[2] J. D. Sau and M. L. Cohen, Phys. Rev. B 75, 045208 (2007)

[3] C. Merckling et al., Appl. Phys. Lett. 11, 192110 (2011)

[4] G. He and H. A. Atwater, Phys. Rev. Lett. 79, 1937 (1997)

[5] R. R. Lieten et al., Appl. Phys. Lett. 102, 052106 (2013)

[6] R. R. Lieten et al., Appl. Phys. Express 6, 101301 (2013)