An assessment of germane and tin tetrachloride for GeSn epitaxy

Abstract

We have screened at 325 °C chamber pressures, precursor flows and H₂ carrier flows to overcome some of the limitations encountered when growing GeSn layers with a GeH₄ + SnCl₄ + H₂ chemistry (e.g. a reduced Sn content compared to Ge₂H₆, a definite lack of uniformity at 400 Torr and so on). Layers grown at 300 Torr and 400 Torr were smooth and of high crystalline quality provided that GeH₄ and SnCl₄ flows were high and low enough, respectively. Meanwhile, layers grown at 100 Torr and 200 Torr were milky and of lesser quality. The best tradeoff in terms of GeSn growth rates, Sn contents and layer uniformity, which was vastly improved, was reached at 300 Torr. A halving of the H₂ carrier flow yielded even higher growth rates and Sn concentrations. Over the 301 °C–349 °C range, we succeeded in depositing, at 300 Torr, layers with growth rates that were similar to that with our reference Ge₂H₆ + SnCl₄ process (at 100 Torr). Activation energies associated to the growth rate increase with the temperature were slightly higher with GeH₄ than with Ge₂H₆ (12–13 kcal. mol.⁻¹ versus 10 kcal. mol.⁻¹). Meanwhile, Sn contents did not drop as fast, as the temperature increased, with GeH₄ than with Ge₂H₆ (−1.1 %/10 °C versus −1.6 %/10 °C slopes). The highest Sn content achievable with our curent GeH₄ process conditions was otherwise not as high than with Ge₂H₆, however (10.3 % versus 14.2 %). Switching to temperatures lower than 301 °C to reach even higher Sn contents was unfortunately not an option with our current GeH₄ + SnCl₄ + H₂ process, as layers were islanded, then.