Density-functional theory molecular dynamics simulations of a-HfO2/Ge(100)(2 × 1) and a-ZrO2/Ge(100)(2 × 1) interface...

Area of Science:

• Materials Science
• Computational Physics
• Semiconductor Device Physics

Background:

• Germanium (Ge) is a promising material for advanced transistors due to its high carrier mobility.
• High-k dielectrics are essential for scaling down gate oxides in Ge-based metal-oxide-semiconductor field-effect transistors (MOSFETs).
• The interface properties between high-k dielectrics and Ge significantly impact device performance, particularly concerning interfacial states.

Purpose of the Study:

• To investigate the structural and electronic properties of amorphous hafnium oxide (a-HfO2) and zirconium oxide (a-ZrO2) interfaces with Ge(001).
• To evaluate the role of a germanium oxide (GeOx) interface interlayer on the interface properties.
• To determine the conditions for forming electronically passive interfaces for Ge-based devices.

Main Methods:

• Utilized density-functional theory (DFT) molecular dynamics (MD) simulations.
• Employed a hybrid classical-DFT MD 'melt-and-quench' approach to generate realistic oxide/Ge stacks.
• Simulated annealing at 700 K, followed by cooling and relaxation, to form realistic interfaces.

Main Results:

• Stacks with a GeOx interlayer exhibited band-edge states, reducing band gaps by 0%-30%, primarily due to under-coordinated Ge atoms.
• These band-edge states resulted from deformation, intermixing, and bond-breaking within or near the GeOx layer.
• All investigated stacks were free of midgap states; however, the presence of GeOx introduced undesirable electronic states.

Conclusions:

• Electronically passive interfaces can be formed either directly between high-k dielectrics and Ge or with a GeO2 monolayer.
• Careful processing is required to avoid creating or to properly passivate under-coordinated Ge atoms and distorted Ge bonds.
• The study confirms that while GeOx interlayers are common, forming an oxide/Ge interface without GeOx is feasible and potentially beneficial.