Germanium nanocrystal non-volatile memory devices: fabrication, charge storage mechanism and characterization

The widespread proliferation and increasing use of portable electronic devices and wearables, and the recent developments in artificial intelligence and internet-of-things, have fuelled the need for high-density and low-voltage non-volatile memory devices. Nanocrystal memory, an emergent non-volatile memory (NVM) device that makes use of the Coulomb blockade effect, can potentially result in the scaling of the tunnel dielectric layer to a very small thickness. Since the nanocrystals are electrically isolated, potential charge leakage paths via localized defects in the thin tunnel dielectric can be substantially reduced, unlike that in a continuous polysilicon floating gate structure. The equivalent oxide thickness of the tunnel dielectric layer can be further reduced by using high dielectric constant materials to replace silicon dioxide, thus giving rise to faster program/erase during device operation and better charge retention performance. In this review on germanium (Ge) nanocrystal NVM devices, a brief historical perspective of semiconductor NVM devices will first be presented. Fabrication techniques for synthesizing Ge nanocrystals and those for Ge nanocrystal capacitor and transistor devices in a tri-layer insulator gate stack structure will then be presented. Investigations into the charge storage mechanism and electrical performance of Ge nanocrystal memory devices will be discussed. The application of a scanning probe microscopy-based nano-characterization method, that of scanning capacitance spectroscopy/microscopy, to analyze carrier charging in Ge nanodots and the passivation of hole and electron traps after forming gas annealing will be highlighted. This has led to a better understanding of the charge storage mechanism in the Ge nanocrystals. The use of high dielectric constant materials in the tri-layer gate structure to minimize Ge penetration into the substrate during the high temperature annealing synthesis step will also be presented in this article. Traps/defects within the Ge nanocrystals play an important role in the charge storage and retention mechanism. This article will also show how the trap energy level could be modulated using high dielectric constant materials in the tunnel dielectric and cap oxide layers for improved device performance.