Raman spectroscopy of germanium nanoparticles in amorphous silicon oxide films

Silicon and germanium nanocrystals display unusual and fascinating properties such as visible photoluminescence. These properties have led to a tremendous amount of research in nanostructures as they create new possibilities for applications in optoelectronics and microelectronics. One of these potential applications is the use of nanocrystals as storage elements within the gate oxide of memory devices. One of the major challenges for the generation of such devices is the fine-tuning of the nanocrystals in terms of size and position. Usually transmission electron microscopy is employed for obtaining this information. We have explored Raman spectroscopy as an alternative non-destructive and less time consuming tool for the characterisation of germanium nanocrystals.
The germanium nanocrystals were produced by ion implantation into a 500 nm thick silica layer followed by thermal annealing at various temperatures and for different durations. Raman spectroscopy was performed at room temperature using excitation wavelengths ranging from 468 nm to 530 nm. The Raman spectra were obtained in the 001(110,110)001 backscattering configuration with respect to the silicon substrate. This orientation of the silicon substrate is crucial since the second order Raman peak of silicon at about 300 cm-1 is supressed and does not mask the Raman peak arising from the germanium nanoparticles. Samples annealed for one hour at 700°C to 800°C show a broad band centred at 280 to 300 cm-1 similar to amorphous germanium, whereas samples annealed at higher temperatures always exhibited sharp well defined peaks which indicate crystalline material. TEM measurements confirmed the presence of Ge nanocrystals. The Raman peak position was found to depend on the annealing time. Samples annealed at 950°C for 15 min exhibited a peak at 298 cm-1 whereas samples annealed for 1 hour displayed a peak at 303.5 cm-1. The Raman peak position of a Ge single crystal was measured at 300 cm-1; therefore the peaks are shifted. They were also found to be asymmetrically broadened in comparison to bulk germanium. A negative shift and broadening of the Raman peak is characteristic of a phonon confinement effect and tensile stress while a positive shift indicates the presence of compressive stress. Accordingly we have analysed the spectra in terms of both, stress effects and phonon confinement. Our model uses an improved description of the phonon dispersion and produces excellent results for silicon nanocrystals. The mean cluster size was measured by TEM. Ge nanocrystals grown for 15 min at 950°C are about 6nm in diameter and experience tensile stress of 200 MPa. Nanocrystals grown for 60 min have a mean diameter of 14 nm and are under compressive stress of about 800 MPa.
Finally Raman interferometry experiments are planned to measure the spatial organization of a nanocrystal plane within an ultrathin oxide layer. This technique has already been used as a powerful method to probe local order (disorder) in quantum wells and dots.