The influence of fast neutron irradiation on the structure and spatial distribution of Ge nanocrystals (NC) embedded in an amorphous SiO2 matrix has been studied. The investigation was conducted by means of laser Raman Scattering (RS), High Resolutio
n Transmission Electron Microscopy (HR-TEM) and X-ray photoelectron spectroscopy (XPS). The irradiation of NC-Ge samples by a high dose of fast neutrons lead to a partial destruction of the nanocrystals. Full reconstruction of crystallinity was achieved after annealing the radiation damage at 800 deg. C, which resulted in full restoration of the RS spectrum. HR-TEM images show, however, that the spatial distributions of NC-Ge changed as a result of irradiation and annealing. A sharp decrease in NC distribution towards the SiO2 surface has been observed. This was accompanied by XPS detection of Ge oxides and elemental Ge within both the surface and subsurface region.
Metal-Oxide-Semiconductor (MOS) structures containing 74Ge nanocrystals (NC-Ge) imbedded inside the SiO_2 layer were studied for their capacitance characterization. Ge atoms were introduced by implantation of 74Ge+ ions with energy of 150 keV into re
latively thick (~640nm) amorphous SiO_2 films. The experimental characterization included room temperature measurements of capacitance-voltage (C-V) dependences at high frequencies (100 kHz and 1 MHz). Four groups of MOS structures have been studied: The 1st - initial samples, without Ge atoms (before ion implantation). The 2nd - implanted samples, after Ge+ ion implantation but before annealing, with randomly distributed Ge atoms within the struggle layer. The 3rd - samples after formation of Ge nanocrystals by means of annealing at 800 degree C (NC-Ge samples), and the 4th - final samples: NC-Ge samples that were subjected by an intensive neutron irradiation in a research nuclear reactor with the integral dose up to 10^20 neutrons/cm^2 followed by the annealing of radiation damage. It is shown that in initial samples, the C-V characteristics have a step-like form of S-shape, which is typical for MOS structures in the case of high frequency. However, in implanted and NC-Ge samples, C-V characteristics have U-shape despite the high frequency operation, In addition, NC-Ge samples exhibit a large hysteresis which may indicate charge trapping at the NC-Ge. Combination of the U-shape and hysteresis characteristics allows us to suggest a novel 4-digits memory retention unit. Final samples indicate destruction of the observed peculiarities of C-V characteristics and recurrence to the C-V curve of initial samples.
