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

摘要

Metal-Oxide-Semiconductor (MOS) structures containing 74Ge nanocrystals(NC-Ge) imbedded inside the SiO_2 layer were studied for their capacitancecharacterization. Ge atoms were introduced by implantation of 74Ge+ ions withenergy of 150 keV into relatively thick (~640nm) amorphous SiO_2 films. Theexperimental characterization included room temperature measurements ofcapacitance-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 Geatoms within the struggle layer. The 3rd - samples after formation of Genanocrystals by means of annealing at 800 degree C ("NC-Ge" samples), and the4th - "final" samples: NC-Ge samples that were subjected by an intensiveneutron irradiation in a research nuclear reactor with the integral dose up to10^20 neutrons/cm^2 followed by the annealing of radiation damage. It is shown that in "initial" samples, the C-V characteristics have astep-like form of "S-shape", which is typical for MOS structures in the case ofhigh frequency. However, in "implanted" and "NC-Ge" samples, C-Vcharacteristics have "U-shape" despite the high frequency operation, Inaddition, "NC-Ge" samples exhibit a large hysteresis which may indicate chargetrapping at the NC-Ge. Combination of the "U-shape" and hysteresischaracteristics allows us to suggest a novel 4-digits memory retention unit."Final" samples indicate destruction of the observed peculiarities of C-Vcharacteristics and recurrence to the C-V curve of "initial" samples.