Crystalline Tetrahedral Phases Al1-xBxPSi3 and Al1-xBxAsT3 (T = Si, Ge) Via Reactions of Al(BH4)3 and M(TH3)3 (M = P, As)
Authors: Sims Patrick , Aoki Toshihiro , Menendez Jose , Kouvetakis John 
Published: February 2020

Hybrid (III-V)-IV alloys with the general formula (III-V)x(IV)5-2x represent a new class of semiconductors that are designed to enhance the optoelectronic capabilities of current silicon and germanium-based technologies for applications in Si-based photovoltaics. Recent studies have produced mono-crystalline, phase pure alloys with compositions (AlP)xSi5-2x (x = 0-1.0) These materials are grown as thin layers directly on Si(001) substrates via reactions of molecular P(SiH3)3 and Al atomic beams using gas-source MBE techniques. The reactions generate AlPSi3 tetrahedral units that combine to form a diamond-like structure in which Al and P are imbedded as isolated bonded pairs within a host tetrahedral Si lattice. The presence of discrete Al-P dimers prevents phase segregation and AlP precipitation. The intact incorporation of the AlPSi3 building blocks fixes the material stoichiometry yielding bulk crystals with reproducible AlPSi3 compositions and Si rich analogues depending on the reaction temperature. However, difficulties associated with precise control of the Al flux in the MBE experiments can lead to minor variations of the atomic distribution at the nanoscale whose impact on material properties has yet to be determined. This issue can be potentially resolved by replacing the solid aluminum source by a chemically suitable molecular precursor that combines readily with P(SiH3)3 to produce the target materials using chemical vapor deposition (CVD). Development of CVD routes is highly desirable since they are more practical and cost effective than MBE for the large-scale, high-throughput fabrication of technologically relevant materials.
This study describes the use of Al(BH4)3, a carbon-free inorganic hydride of Al, as a molecular source to synthesize (AlP)xSi5-2x systems via reactions with P(SiH3)3. These reactions yield crystalline alloys with novel Al1-xBxPSi3(x = 0.04-0.06) compositions which are grown lattice-matched on Si(001) substrates. The new materials have been characterized for structure, composition, phase purity and optical response by spectroscopic ellipsometry, high-resolution XRD, XTEM, EELS and EDS, which indicate the formation of single-phase mono-crystalline layers with tetrahedral structures based on the AlPSi3 parent phase. Raman scattering of the Al1-xBxPSi3films supports the presence of substitutional B in place of Al and provides strong evidence that the boron is bonded to P in the form of isolated pairs, as expected on the basis of the AlPSi3 prototype. The substitution of small-size B atoms is facilitated by the stabilizing effect of the parent lattice, and it is highly desirable for promoting full lattice matching with Si as required for Si-based solar cell designs. The substitution of B also increases the bond-length disorder leading to a significantly enhanced absorption relative to Si and AlPSi3 at E < 3.3 eV which may be beneficial for PV applications. Analogous reactions of As(SiH3)3 with Al(BH4)3 produce Al1-xBxAsSi3 crystals in which the B incorporation is limited to doping concentrations at 1020 atoms/cm3. In both cases the classical Al(BH4)3 acts as an efficient delivery source of elemental Al to create crystalline group (III-V)-IV hybrid materials comprising light, earth-abundant elements with possible applications not only in the fields of Si-based photovoltaic technologies but also as light-element refractory solids.