Structural and Optical Behavior of Germanium Quantum Dots

Controlled growth, synthesis, and characterization of a high density and large-scale Ge nanostructure by an easy fabrication method are key issues for optoelectronic devices. Ge quantum dots (QDs) having a density of ～10¹¹ cm^?2 and a size as small as ～8 nm are grown by radio frequency magnetron sputtering on Si (100) substrates under different heat treatments. The annealing temperature dependent structural and optical properties are measured using AFM, XRD, FESEM, EDX, photoluminescence (PL) and Raman spectroscopy. The effect of annealing is found to coarsen the Ge QDs from pyramidal to dome-shaped structures as they grow larger and transform the nanoislands into relatively stable and steady state configurations. Consequently, the annealing allows the intermixing of Si into the Ge QDs and thereby reduces the strain energy that enhances the formation of larger nanoislands. The room temperature PL spectra exhibits two strong peaks at ～2.87 eV and ～3.21 eV attributed to the interaction between Ge, GeO_x and the possibility of the presence of QDs core-shell structure. No reports so far exist on the red shift ～0.05 eV of the strongest PL peak that results from the effect of quantum confinement. Furthermore, the Raman spectra for the pre-annealed QDs that consist of three peaks at around ～305.25 cm^?1, 409.19 cm^?1 and 515.25 cm^?1 are attributed to Ge-Ge, Ge-Si, and Si-Si vibration modes, respectively. The Ge-Ge optical phonon frequency shift (～3.27 cm^?1) associated with the annealed samples is assigned to the variation of shape, size distribution, and Ge composition in different QDs. The variation in the annealing dependent surface roughness and the number density is found to be in the range of ～0.83 to ～2.24 nm and ～4.41 to ～2.14 × 10¹¹ cm^?2, respectively.