Fundamental experimental and theoretical aspects of high-order Ge-hydride chemistry for versatile low-temperature Ge-based materials fabrication

Abstract

We present an exploratory study to extend Ge-hydride chemistry and materials science beyond the classic GeH₄ case to higher order analogs. We emphasize the Ge₄H₁₀ and Ge₅H₁₂ compounds for possible applications as chemical sources for the next generation Ge-based semiconductor technologies. Quantum chemistry simulations, based on the hybrid B3LYP DFT functional, are used to predict fundamental structural, vibrational and thermochemical properties of Ge₅H₁₂ conformers and isomers for the first time. Structural results for seven distinct isomers including neo-Ge₅H₁₂, two branched conformers (i₁-Ge₅H₁₂ and i₂-Ge₅H₁₂) and four chained isomers derived from the linear chain n-Ge₅H₁₂ are presented, and reveal geometry and bonding motifs very similar to those found in tetragermane. This structural analogy is then used to interpret the vibrational spectrum of Ge₅H₁₂ in relation to Ge₄H₁₀ and identify key distinguishing features. Hindered-rotor corrected free-energies of the prototypical neo (tetrahedral) and n (linear) Ge₅H₁₂ isomers are predicted to be +3.4 kJ mol⁻¹ and +2.8 kJ mol⁻¹ above that of the branched ground state structure. This thermochemistry data is then combined with corresponding results for the Ge_nH_2n+2 (n = 1–4) compounds to study their sequential reactions via germylene insertion, which convert an n^th-order molecule to its (n + 1)^th relative, culminating in Ge₅H₁₂. The equilibrium for these reactions is then solved for a range of temperature and initial reactant concentrations and indicates that germane is the dominant species, followed by Ge₄H₁₀ and Ge₅H₁₂ in various ratios, depending on conditions. Our simulations may provide useful guidance for optimizing the production of Ge₃H₈, Ge₄H₁₀ and Ge₅H₁₂ as main products. The simulations are corroborated by initial experimental studies which also provide spectroscopic evidence for the formation of Ge₅H₁₂. The latter was subsequently used to undertake the first proof-of-principle depositions of high quality Ge structures on Si(100) indicating that this compound may represent both a viable and unique CVD precursor for ultra low-temperature and low-pressure fabrication.