Monocrystalline Al(As1–xNx)Si3 and Al(P1–xNx)ySi5–2y Alloys with Diamond-like Structures: New Chemical Approaches to Semiconductors Lattice Matched to Si

Abstract

The reaction of M(SiH3)3 (M = P, As) with Al atoms from a Knudsen cell has recently led to an entirely new approach to the synthesis of (III–V)–(IV) alloys on Si substrates using gas-source molecular beam epitaxy. The main feature of these new materials is the presence of isolated “donor–acceptor” pairs within fully interlinked III–V–(IV)3 “building blocks”, leading to highly stable and crystalline structures with average diamond-like symmetry. Examples include stoichiometric compounds Al(P–As)Si3 as well as silicon-rich alloys (Si)5–2y(AlP)y. While these materials can be grown on Si with small mismatch strains, the complete elimination of the lattice mismatch may be important for applications such as photovoltaics which require thick films. Accordingly, in this paper we expand the above compositional space and explore the optical tuning potential in this new class of materials by introducing nitrogen onto the group V sublattice. We demonstrate that the addition of N(SiH3)3 into the reaction mixture under appropriate conditions readily yields novel Al(As1–xNx)Si3 and Al(P1–xNx)ySi5–2y alloys which exhibit enhanced optical absorption with respect to silicon as evidenced by spectroscopic measurements indicating potential applications in Si-based photovoltaics. These materials can be grown as monocrystalline layers completely lattice matched on Si via substitution of As and P by the markedly smaller N atoms. First-principles studies of the reactivity trends are consistent with the finding that a large excess of the less reactive N(SiH3)3 is needed to incorporate even minor amounts of the N atoms in the alloy products, up to ∼11% in the case of AlAs1–xNxSi3 but only ∼3% in the case of AlP1–xNxSi3. Molecular calculations are then combined with solid-state simulations to elucidate the path from “molecule to solid” by quantifying the structural changes associated with the incorporation of the Al–P–Si3, Al–As–Si3, and Al–N–Si3 tetrahedral cores into the solid. The main conclusions of this study are that severe departures from tetrahedral symmetry in the Al–N–Si3 building block thwart the assembly of a regular diamond lattice. This is in contrast to the AlAsSi3 counterpart, which is found to exhibit near perfect tetrahedral symmetry in the crystalline state. All compounds are predicted to be thermodynamically stable relative to their elemental reference states.