In-situ fabrication and characterization of ordered Ge QDs in Si₃N₄ matrix without barrier layers by rf-magnetron sputtering

In this work, in-situ fabrication and characterization of ordered arrays of germanium (Ge) quantum dots (QDs) embedded in silicon nitride matrix were studied. A single layer of Ge and Si₃N₄ composite was deposited by rf-magnetron sputtering with substrate heating. Transmission electron microscopy (TEM), X-ray diffraction (XRD) and Raman spectroscopy were used to investigate structural properties of the Ge QDs. TEM images showed that uniformly sized Ge QDs were formed in diagonal QD arrays grown from the substrate. The average size estimated from XRD was in agreement with the QD size observed in TEM images. Chemical composition of the films was analyzed by X-ray photoelectron spectroscopy (XPS), confirming the Ge⁰ state of the Ge–Ge bond in the QDs. Phonon confinement in the QDs was demonstrated by phonon peak broadening in Raman spectra. The confinement and stress effect resulted in phonon peak shifting to 297 cm⁻¹. With decreasing QD size, E₁ and E₂ transitions estimated from spectroscopic ellipsometry were blue-shifted, indicating the quantum confinement effect. This closely spaced Ge QD structure would be potentially advantageous for third generation photovoltaic applications as this structure could improve electrical conductivity while confining the QD growth.