Advances in SiGeSn technology
Authors: Soref Richard , Kouvetakis John , Tolle John , Menendez Jose , D’Costa Vijay 
Published: November 2007

We recently reported the chemical vapor deposition growth of binary Ge1–ySny and ternary Ge1–ySixSny alloys directly on Si wafers using SnD4, Ge2H6 (di-germane), SiH3GeH3, and (GeH3)2SiH2 sources. Ge1–ySny 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 Ge1–ySny 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, Ge1–ySny 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 Ge0.98Sn0.02 sample yield an absorption coefficient of 3500 cm−1 at 1675 nm (0.74 eV). Thus, IR detectors based on Ge0.98Sn0.02 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 Ge0.98Sn0.02 and ∼500 nm, respectively. GeSn also has application in band-to-band laser heterodiodes. The ternary system Ge1–x–ySixSny grows on Ge1–ySny-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 Ge1–x–ySixSny 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.