On the Chemical Origin of the Gap Bowing in (GaAs)(1-x)Ge(2x) Alloys: A Combined DFT-QSGW Study

Materials Design and Optimization for Next-Generation Solar Cell and Light-Emitting Technologies

We review some of the most potent directions in the design of materials for next-generation solar cell and light-emitting technologies that go beyond traditional solid-state inorganic semiconductor-based devices, from both the experimental and computational standpoints. We focus on selected recent conceptual advances in tackling issues which are expected to significantly impact applied literature in the coming years. Specifically, we consider solution processability, design of dopant-free charge transport materials, two-dimensional conjugated polymeric semiconductors, and colloidal quantum dot assemblies in the fields of experimental synthesis, characterization, and device fabrication. Key modeling issues that we consider are calculations of optical properties and of effects of aggregation, including recent advances in methods beyond linear-response time-dependent density functional theory and recent insights into the effects of correlation when going beyond the single-particle ansatz as well as in the context of modeling of thermally activated fluorescence.

Synthesis and Structural and Optical Properties of Ga(As1-xPx)Ge3 and (GaP)yGe5-2y Semiconductors Using Interface-Engineered Group IV Platforms

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.