Size-tunable strain engineering in Ge nanocrystals embedded within SiO₂ and Si₃N₄

We report a unique ability to control the sign and size of the stress within Ge nanocrystals or nanodots fabricated using a complementary metal-oxide-semiconductor-compatible process within SiO₂ and Si₃N₄ layers. Very large (as much as 4.5%), size-dependent compressive and tensile strains can be generated depending on whether the dot is embedded within either a Si₃N₄ or a SiO₂ layer. Raman measurements reveal significant anharmonicity for smaller Ge dots and possible distortions of the diamond cubic lattice as evidenced by the measured Grünesien parameters and confirmed by their transmission electron diffraction patterns. Two completely different mechanisms are proposed to explain the formation of the tensile and compressive strain states, respectively.