Synthesis and properties of monocrystalline Al(As _1-xP _x)Si ₃ alloys on Si(100)

Semiconductors with the general formula Al(As _1-xP _x)Si ₃ have been theoretically studied and synthesized. These new materials are grown on Si(100) at 490-500 °C as single-phase epitaxial layers with monocrystalline diamond-like structures using reactions of Al atomic beams with As(SiH ₃) ₃ and P(SiH ₃) ₃ molecular sources. An intriguing outcome of the reaction behavior is that there appears to be no preference in the interaction between Al and the As(SiH ₃) ₃ and P(SiH ₃) ₃ coreactants, leading to the organized assembly of the corresponding Al-As-Si ₃ and Al-P-Si ₃ building blocks into a tetrahedral lattice in which the P/As atoms are arranged in a common third nearest neighbor sublattice in a manner that precludes the formation of energetically unfavorable Al-Al bonds. The translation of these molecular building blocks into the crystalline structures is elucidated using quantum chemistry. A general crystallographic description for the primitive cell is introduced and used to carry out first principle calculations of the structural, thermodynamic and electronic properties of AlP _1-xAs _xSi ₃. The results indicate that these materials are indirect gap semiconductors with indirect gaps similar to that of Si but with smaller direct gaps, which should increase their absorption coefficients. Raman spectroscopy provides a qualitative confirmation of these predictions as well as important clues on the orientational order of the tetrahedral building blocks that make up these novel materials. In the context of device design, strain analysis of Al(As _1-xP _x)Si ₃ indicates that the films are tetragonally compressed due to an inherent 0.8% (or less) lattice mismatch with the Si substrate. At the highest mismatch of 1.6%, strain relaxation is observed for thicknesses exceeding 40 nm, producing prototype layers with bulk-like behavior. Collectively, the results demonstrate that it may be feasible to design and prepare a host of similar systems in this general class of semiconductors with potential optoelectronic applications, including photovoltaics.