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 SiO₂ layer were studied for their capacitance characterization. Ge atoms were introduced by implantation of ⁷⁴Ge⁺ ions with energy of 150 keV into relatively thick (∼640 nm) amorphous SiO₂ 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 °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 10²⁰ neutrons/cm² followed by the annealing of radiation damage. It is shown that in "initial" samples, the C-V characteristics have a step-like form or "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.