Development of light emitting group IV ternary alloys on Si platforms for long wavelength optoelectronic applications

This paper describes preparation of a new class of Ge _1-x-ySi_xSn_y direct-gap semiconductors grown on Ge-buffered Si substrates via depositions of trigermane (Ge₃H ₈), tetragermane (Ge₄H₀), tetrasilane (Si ₄H₁₀), and stannane (SnD₄) hydride precursors. These react at ultralow temperatures 320-290 °C to produce thick (∼500 nm) monocrystalline films with concentrations closely reflecting the gas phase molar ratio of the coreactants. A series of Ge-rich samples with a fixed 3-4% Si content and progressively increasing Sn content in the 4-10% range are grown to explore the possibility of obtaining direct gap materials for the first time in this semiconductor system. The resultant films also exhibit residual compressive strains that are largely relaxed via rapid thermal annealing between 550 and 700°C depending on composition. These temperatures are 50-75°C above the thermal decomposition threshold of Ge_1-ySn_y/Ge materials with same Sn content, indicating that the ternaries are significantly more robust than the binary analogues. The compositional dependence of the strain relaxations is similar to that found for Ge_1-xSi_x alloys grown directly on Si. Cross hatch patterns are formed on the surface of these materials as a consequence of interface misfit dislocations generated due to the mismatch with the underlying Ge buffer. The annealed films exhibit low defect densities and atomic scale chemical uniformity as indicated by STEM and element-selective EELS mapping, allowing a meaningful study of the optical properties as a function of Sn concentration. The results show that the separation of the direct and indirect edges can be made smaller than in Ge even for the non-negligible 3-4% Si content, confirming that with a suitable choice of Sn compositions the ternary Ge_1-x-ySi_xSn_y reproduces all features of the electronic structure of the binary Ge _1-ySn_y, including the sought after indirect-to-direct gap cross over. The alloys produced in this study represent an attractive alternative to Ge_1-ySn_y for applications in IR optoelectronic technologies requiring high thermal budget processing and harsh operating conditions due to their enhanced thermal stability conferred by the incorporation of Si in the diamond-like structure.