Crystalline Tetrahedral Phases Al1-xBxPSi3 and Al1-xBxAsT3 (T = Si, Ge) Via Reactions of Al(BH4)3 and M(TH3)3 (M = P, As)
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-xBxPSi3 films 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.