Synthesis and Structural and Optical Properties of Ga(As_1–xP_x)Ge₃ and (GaP)_yGe_5–2y Semiconductors Using Interface-Engineered Group IV Platforms

Abstract

Epitaxial synthesis of Ga(AsP)Ge alloys on Si(100) substrates is demonstrated using chemical vapor deposition reactions of [DGaN(CH)] with P(GeH) and As(GeH) precursors. These compounds are chosen to promote the formation of GaAsGe and GaPGe building blocks which interlink to produce the desired crystalline product. Ge-rich (GaP)Ge analogues have also been grown with tunable Ge contents up to 90% by reactions of P(GeH) with [DGaN(CH)] under similar deposition protocols. In both cases, the crystal growth utilized GeSi 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(AsP)Ge 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 GaAsGe, 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.