Advances in SiGeSn/Ge technology

We recently reported the CVD growth of binary Ge_1-ySn _y and ternary Ge_1-ySi_xSn_y alloys directly on Si wafers using SnD₄, Ge₂H₆ (di-germane), SiH₃GeH₃, and (GeH₃)SiH ₂ sources. Ge_1-ySn_y is an intriguing infrared material that undergoes an indirect-to-direct bandgap transition for y > 0.09. In addition, we have found that Ge_1-ySn_y layers have ideal properties as templates for the subsequent deposition of other semiconductors: (a) they are strain-relaxed and have low threading-defect densities (10⁵ cm ^-2) even for films thinner than 1 μm; (b) their low growth temperatures between 250°C and 350°C are compatible with selective growth, and the films possess the necessary thermal stability for conventional semiconductor processing (up to 750°C depending on composition); (c) they exhibit tunable lattice constants between 5.65 Å and at least 5.8 Å, matching InGaAs and related III-V systems; (d) their surfaces are extremely flat; (e) they grow selectively on Si and not on SiO ₂; and (f) the film surface can be prepared by simple chemical cleaning for subsequent ex-situ epitaxy. The incorporation of Sn lowers the absorption edges of Ge. Therefore, Ge_1-ySn_y is attractive for detector and photovoltaic applications that require band gaps lower than that of Ge. Spectroscopic ellipsometry and photoreflectance experiments show that the direct band gap is halved for as little as y = 0.15. Studies of a Ge_0.98Sn_0.02 sample yield an absorption coefficient of 3500 cm^-1 at 1675 nm (0.74 eV). Thus infrared detectors based on Ge_0.98Sn_0.02 could easily cover the U-(1625 nm-1675 nm), L-(1565 nm-1625 nm), and C-(1530 nm-1565 nm) telecomm bands. Photoluminescence studies show bandgap emission on thin GeSn layers sandwiched between higher bandgap SiGeSn barriers. We have made advances in p and n doping of GeSn and present results on electrical characterizations. GeSn also has application in band-to-band laser heterodiodes. The ternary system Ge_1-x-ySi _xSn_y grows on Ge_1-ySn_y-buffered Si. It represents the first practical group-IV ternary alloy, since C can only be incorporated in minute amounts to the Ge-Si network. The most significant feature of Ge_1-x-ySi_xSn_y is the possibility of independent adjustment of lattice constant and band gap. For the same value of the lattice constant one can obtain band gaps differing by more than 0.2 eV, even if the Sn-concentration is limited to the range y < 0.2. This property can be used to develop a variety of novel devices, from multicolor detectors to multiple junction photovoltaic cells. A linear interpolation of band gaps lattice constants between Si, Ge and α-Sn shows that it is possible to obtain SiGeSn with a band gap and a lattice constant larger than that of Ge. This feature can be exploited to make a tensile-strained Ge-on-SiGeSn telecomm detector with improved performance. A tensile-strain-induced direct gap of Ge should find applications in laser diodes and electroptical modulators.