Synthesis and Structural and Optical Properties of Ga(As_1-xP_x)Ge₃ and (GaP)_yGe_5-2y Semiconductors Using...

Epitaxial synthesis of Ga(As_1-xP_x)Ge₃ alloys on Si(100) substrates is demonstrated using chemical vapor deposition reactions of [D₂GaN(CH₃)₂]₂ with P(GeH₃)₃ and As(GeH₃)₃ precursors. These compounds are chosen to promote the formation of GaAsGe₃ and GaPGe₃ building blocks which interlink to produce the desired crystalline product. Ge-rich (GaP)_yGe_5-2y analogues have also been grown with tunable Ge contents up to 90% by reactions of P(GeH₃)₃ with [D₂GaN(CH₃)₂]₂ under similar deposition protocols. In both cases, the crystal growth utilized Ge_1-xSi_x buffer layers whose lattice constants were specifically tuned as a function of composition to allow perfect lattice matching with the target epilayers. This approach yielded single-phase materials with excellent crystallinity devoid of mismatch-induced dislocations. The lattice parameters of Ga(As_1-xP_x)Ge₃ interpolated among the Ge, GaAs, and GaP end members, corroborating the Rutherford backscattering measurements of the P/As ratio. A small deviation from the Vegard's law that depends on the As/P ratio was observed and corroborated by ab initio calculations. Raman scattering shows evidence for the existence of Ga-As and Ga-P bonds in the Ge matrix. The As-rich samples exhibited photoluminescence with wavelengths similar to those observed for pure GaAsGe₃, indicating that the emission profile does not change in any measurable manner by replacing As by P over a broad range up to x = 0.2. Furthermore, the photoluminescence (PL) data suggested a large negative bowing of the band gap as expected on account of a strong valence band localization on the As atoms. Spectroscopic ellipsometry measurements of the dielectric function revealed a distinct direct gap transition that closely matches the PL emission energy. These measurements also showed that the absorption coefficients can be systematically tuned as a function of composition, indicating possible applications of the new materials in optoelectronics, including photovoltaics.