Nanoscale assembly of silicon-like [Al(As1_-xN^x)] ^ySi5_-2y alloys: Fundamental theoretical and experimental studies of structural and optical properties

Ab initio theoretical simulations of Al(As1_-xN ^x)Si³ alloys, a new class of optoelectronic materials, confirm that these compounds are likely to be disordered via a mechanism that preserves the integrity of the constituent III-V-Si³ tetrahedra but randomizes their orientation in the average diamond lattice of the compound. This type of disorder is consistent with experimental structural data and with the proposed growth mechanism for such alloys, according to which "III:V(SiH³)³" intermediate complexes are formed in the gas phase from reactions between group-III atomic beams and V(SiH ³)³ molecules, delivering the entire III-V-Si³ tetrahedra to the growing film. Experimental optical studies of these Al(As1_-xN^x)Si³ alloys as well as more general [Al(As1_-xN^x)]^ySi5_-2y compounds grown on Si substrates were carried out using spectroscopic ellipsometry. The resulting dielectric functions are found to be similar to broadened versions of their counterparts in pure Si. This broadening may have important practical applications, particularly in photovoltaics, because it dramatically enhances the optical absorption of Si in the visible range of the electromagnetic spectrum. A critical point analysis reveals the existence of direct optical transitions at energies as low as 2.5 eV, well below the lowest direct absorption edge of Si at 3.3 eV. Such transitions are predicted theoretically for perfectly ordered III-V-Si³ compounds, and the experimental results suggest that they are robust against tetrahedra orientational disorder.