Molecular strategies for configurational sulfur doping of group IV semiconductors grown on Si(100) using S(MH₃)₂ (M = Si,Ge) delivery sources: An experimental and theoretical inquiry

We report a new doping protocol of pure Ge films grown on Si and related Si/Sn materials based on S delivered from high reactivity hydride molecules S(MH₃)₂ (M = Si,Ge). The new doping strategy targets next generation semiconductor applications requiring enhanced IR optical performance as well as high-mobility field effect transistors fully integrated with silicon. To explore this paradigm, we first developed a practical and straightforward synthesis approach, which avoids the use of toxic starting materials and yields viable quantities of the title compounds. These were then used to carry out proof-of-concept low-temperature depositions of Ge/Si(100) and GeSn/Si(100) films, doped with "double donor" S atoms for the first time. These systems are characterized using standard materials science techniques via RBS, XTEM, SIMS, and XRD for structure, composition, and crystallinity, and their electrical properties are measured by the Hall method. Thermally robust dopant levels <10¹⁸ are systematically obtained using a range of process protocols, which can be further optimized for practical applications. Complementary first-principles density functional theory simulations were then used to study the stability and strain associated with the incorporation of S within the parent lattice as molecular fragments such as Ge-S-Ge, Ge-S, Si-S-Si, and Si-S derived from the S(SiH₃)₂ and S(GeH ₃)₂ molecular sources. Completely incorporated Ge-S-Ge or Ge-S units in Ge in which S resides in either substitutional (tetrahedral) or near-substitutional (3-coordinate) sites are predicted to be strongly bound (-2.36 eV and -1.49 eV, respectively) relative to interstitial S and isolated Ge vacancies, while the corresponding binding in Si-S-Si and Si-S analogs is slightly enhanced. By considering the induced lattice strain, binding energy, and the structural accommodation of molecular core bonds, plausible defect-clusters are identified and tentatively used to correlate sulfur concentrations and carrier concentrations trends. In the case of the S(GeH ₃)₂ grown films, a mixture of deep and shallow donor centers must be invoked to account for the observed carrier concentration enhancement.