Electrical properties of Ge nanocrystals grown in a SiO₂ matrix using MeV electron beam irradiation

The structural and electrical properties of Ge nanocrystals (Ge-NCs) grown in a SiO₂ matrix using high energy electron beam irradiation are investigated. The Ge-NCs in a SiO₂ were formed by irradiation of the electron beam on a tri-layers (SiO₂(20 nm)/Ge(5 nm)/SiO₂(10 nm)) grown on a Si(001) substrate and followed by annealing. For a sample exposed in the atmosphere by electron beam with energy of 1 MeV and a dose of ~5 × 10¹⁶ e/cm², and then annealed at 700 °C for 10 min, transmission electron microscopy (TEM) measurement revealed the formation of Ge-NCs with average size of ~20 nm in a SiO₂ matrix. For a metal-oxide-semiconductor (MOS) structure synthesized by Al deposition on the top of the sample, the capacitance-voltage (C-V) measurements showed the hysteresis loops with a flatband voltage shift (ΔV_FB) of ~5.0 V, which would originate mainly from Ge-NCs as the charge storage nodes. In contrast, for a sample annealed at above 900 °C for 10 min without electron beam irradiation, we could obtain a similar hysteresis loop of ~4.0 V. This indicates that the e-beam irradiation process can lead to temperature reduction of ~200 °C for Ge-NCs formation by annealing process. We propose that the temperature reduction would result from crystallity enhancement of Ge layers in the initial stage of Ge-NCs formation due to Ge atomic displacements induced by the high energy electron beam.