Advances in SiGeSn technology

We recently reported the chemical vapor deposition growth of binary Ge_1-y Sn_y and ternary Ge_1-y Si_ySn_y alloys directly on Si wafers using SnD₄, Ge₂H₆ (di-germane), SiH₃GeH₃, and (GeH₃)₂SiH₂ sources. Ge_1-y Sn_y is an intriguing infrared (IR) material that undergoes an indirect-to-direct band-gap transition for y < 0.1. In addition, we have found that Ge_1-y Sn_y layers have ideal properties as templates for the subsequent deposition of other semiconductors: (i) they are strain-relaxed and have low threading-defect densities (10⁵ cm^-2 ) even for films thinner than 1 μm; (ii) their low growth temperatures between 250 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); (iii) they exhibit tunable lattice constants between 5.65 Å and at least 5.8 Å, matching InGaAs and related III-V systems; (iv) their surfaces are extremely flat; (v) they grow selectively on Si and not on SiO₂; and (vi) 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-y Sn_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_0.98 Sn_0.02 sample yield an absorption coefficient of 3500 cm^-1 at 1675 nm (0.74 eV). Thus, IR detectors based on Ge_0.98 Sn_0.02 could easily cover the L-(1565-1625 nm) and C-(1530-1565 nm) telecomm bands. Photoluminescence studies show band-gap emission on thin GeSn layers sandwiched between higher band-gap SiGeSn barriers. We have made advances in p- and n-doping of GeSn and present results on electrical characterizations. Hall measurements reveal mobilities as high as of 600 cm²/V-s and background p-dopant concentrations in the 10¹⁶ cm^-3 range for samples with nominal composition and thickness of Ge_0.98 Sn_0.02 and -500 nm, respectively. GeSn also has application in band-to-band laser heterodiodes. The ternary system Ge_1-x-y Si_xSn_y grows on Ge_1-y Sn_y-buffered Si. It represents the first practical group IV ternary alloy, because C can only be incorporated in minute amounts to the Ge-Si network. The most significant feature of Ge_1-x-y Si_xSn_y is the possibility of independent adjustment of the lattice constant and band gap. For the same value of the lattice constant, one can obtain band gaps differing by >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-gap 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. We shall use this feature to make a tensile-strained Ge-on-SiGeSn telecomm detector with improved performance. To date, record high tensile strain (0.40%) has been achieved in Ge layers grown on GeSn-buffered Si where the strain is systematically tuned by adjusting the lattice constant in the buffer. A tensile-strain-induced direct gap of Ge can be used also for laser diodes and electroptical modulators.