Nitride-stressor and quantum-size engineering in Ge quantum-dot photoluminescence wavelength and exciton lifetime

Abstract

We report local nitride-stressor engineering in combination with quantum-size tunability in Ge quantum dots (QDs) for tailoring photoluminescence (PL) wavelength and exciton lifetime. Spherical-shaped Ge QDs with tunable diameters ranging from 25–90 nm embedded within layers of thermally-grown SiO2 and chemical-vapor-deposited Si3N4 were fabricated by thermal oxidation of lithographically-patterned poly-Si0.85Ge0.15 pillars on top of buffer layers of SiO2 and Si3N4, respectively, in a self-organization approach. The effects of local stressors and quantum confinement on the strain and optical properties of Ge QDs were systematically investigated using Raman and PL measurements. We observed that when Ge QD diameter gets smaller than 60 nm, quantum confinement sets in and has a predominant influence on PL wavelength and exciton lifetime. Our self-organized Ge QD/Si3N4 system provides a generic building block for the fabrication of active photonic devices on Si3N4 platform.

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“…Clear lattice fringes observed in high-resolution TEM micrographs and sharp diffraction spots observed in the selected area electron diffraction (SAED) patterns are testament to the good crystallinity of our Ge QDs [ 43 , 44 , 45 , 46 ]. Raman spectroscopy [ 46 , 47 ] and photoluminescence (PL) [ 47 , 48 ] measurements also confirm the high degree of crystallinity within the Ge QDs in terms of sharp Raman phonon lines and temperature-insensitive PL peaks, respectively.…”

“…Our systematic Raman measurements in combination with TEM/SAED examinations also reveal an important observation that the local environments of SiO 2 and Si 3 N 4 have a significant influence on the sign of the strain, tensile or compressive, which is imposed on the Ge QDs [ 46 , 47 ] That is, compressive and tensile strains can be generated in our Ge QDs depending on whether the Ge QD is embedded within Si 3 N 4 or SiO 2 layers. Measured Grüneisen parameters from temperature-dependent Raman frequencies suggest significant anharmonicity for small Ge QDs with possible distortions of the diamond cubic lattice, which have been confirmed by their lattice spacings through the transmission electron diffraction patterns.…”

“…Measured Grüneisen parameters from temperature-dependent Raman frequencies suggest significant anharmonicity for small Ge QDs with possible distortions of the diamond cubic lattice, which have been confirmed by their lattice spacings through the transmission electron diffraction patterns. We have also observed that quantum phonon confinement effect sets in when the Ge QD size is smaller than 40 nm [ 47 , 48 , 49 ]. Therefore, the valley degeneracy in our Ge QDs could be split by tailoring the local environmental materials of SiO 2 or Si 3 N 4 in combination with adjusting the QD sizes by design.…”

“…These locations are also where large geometric curvatures and higher film stress occur. The preferential formation of Ge QDs at the highly stressed ridge sidewall edges and their included-angle locations could also be due to the higher density of defects at these locations and the stress relief provided by the growing Ge QD [ 47 , 54 ]. The insertion of a S i3 N 4 overlayer with controllable thickness between the poly-SiGe spacer island and the c-Si ridge provides the tunability necessary for precise Ge QD location.…”

“…Our Ge QDs were created using the selective oxidation of poly-Si 1 − x Ge x lithographically-patterned structures with Si 3 N 4 in proximity. We have exploited the multi-dimensional parameter spaces of process conditions to grow Ge QDs with a high degree of controllability in the size, morphological shape, chemical purity, crystallinity, and spatial locations [ 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 ]. We have also proven the feasibility of paired DQDs embedded within SiO 2 /Si 3 N 4 matrices at each sidewall edge of lithographically-patterned Si ridges using spacer technology and thermal oxidation of poly-SiGe [ 37 , 38 ].…”