Molecular approaches to p- and n-nanoscale doping of Ge_1-ySn_y semiconductors: Structural, electrical and transport properties

We report the development of practical doping protocols via designer molecular sources to create n- and p-type doped Ge_1-ySn_y layers grown directly upon Si(1 0 0). These materials will have applications in the fabrication of advanced PIN devices that are intended to extend the infrared optical response beyond that of Ge by utilizing the Sn composition as an additional design parameter. Highly controlled and efficient n-doping of single-layer structures is achieved using custom built P(GeH₃)₃ and As(GeH₃)₃, precursors containing preformed Ge-As and Ge-P near-tetrahedral bonding arrangements compatible with the structure of the host Ge-Sn lattice. Facile substitution and complete activation of the P and As atoms at levels ∼10¹⁷-10¹⁹ cm^-3 is obtained via in situ depositions at low temperatures (350 °C). Acceptor doping is readily achieved using conventional diborane yielding carrier concentrations between 10¹⁷-10¹⁹ cm^-3 under similar growth conditions. Full activation of the as-grown dopant concentrations is demonstrated by combined SIMS and Hall experiments, and corroborated using a contactless spectroscopic ellipsometry approach. RTA processing of the samples leads to a significant increase in carrier mobility comparable to that of bulk Ge containing similar doping levels. The alloy scattering contribution appears to be negligible for electron carrier concentrations beyond 10¹⁹ cm^-3 in n-type samples and hole concentrations beyond 10¹⁸ cm^-3 in p-type samples. A comparative study using the classical lower-order hydrides PH₃ and AsH₃ produced n-doped films with carrier densities (up to 9 × 10¹⁹ cm^-3) similar to those afforded by P(GeH₃)₃ and As(GeH₃)₃. However, early results indicate that the simpler PH₃ and AsH₃ sources yield materials with inferior morphology and microstructure. Calculations of surface energetics using bond enthalpies suggest that the latter massive compounds bind to the surface via strong Ge-Ge bonds and likely act as "retardants" that moderate surface diffusion of the reactions species, thereby promoting layer-by-layer growth leading to thick, atomically smooth films, particularly in the case of P(GeH₃)₃. Furthermore, the intact incorporation of the Ge₃P and Ge₃As molecular cores results in highly uniform compositional, strain and doping profiles at the atomic level.