Fundamental Studies of P(GeH₃)₃, As(GeH ₃)₃, and Sb(GeH₃)₃: Practical n-Dopants for New Group IV Semiconductors

We introduce a new chemical approach to the incorporation of high concentrations of active n-dopant atoms into group-IV semiconductors via low temperature, low cost, high efficiency routes involving carbon-free single-source inorganic hydrides. Controlled substitution of As into Ge-based semiconductors is made possible by the use of As(GeH₃)₃, which furnishes structurally and chemically compatible AsGe₃ molecular cores. As(GeH₃)₃ is synthesized in high purity yields (∼75%) via a new single-step method based on reactions of GeH ₃Br and As[Si(CH₃)₃]₃, circumventing the need for toxic and unstable starting materials as used in earlier approaches. We demonstrate the development of a viable route to the entire family of compounds M(GeH₃)₃ {M = P. As, Sb}, suitable for doping or superdoping applications of a wide range of functional materials. The structural, vibrational, and thermochemical properties of M(GeH ₃)₃ are simulated for the first time via density functional theory calculations using both all-electron and effective core potentials. The vibrational calculations are in excellent agreement with the observed infrared spectra and the thermochemical stability is predicted to decrease with increasing molecular mass, in accord with experimental observations. The simulated structures show that the Ge-M-Ge angles decrease with increasing M size and further resolve inconsistencies with earlier gas electron diffraction measurements of P(GeH₃)₃. Bulk supercell calculations are then used to study the formation free energy of P, As, and Sb incorporation in bulk Ge, as well as the bond and lattice strains induced by the dopant atoms in the host diamond-structure lattice. As a first example of the usability of the M(GeH₃)₃ family, we demonstrate the successful doping of metastable Ge_1-ySn_y alloys. This represents a crucial step toward the goal of developing photonic devices, such as photodetectors and photovoltaic cells, based on Ge _1-ySn_y. Infrared ellipsometry experiments demonstrate high carrier concentrations and excellent resistivities in As(GeH₃) ₃-doped Ge_1-ySn_y. The latter are only moderately higher than those measured in pure Ge for the same dopant levels.