Influence of Ge nanocrystals and radiation defects on C-V characteristics in Si-MOS structures

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 relatively 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.