Interfacial Cation-Defect Charge Dipoles in Stacked TiO₂/Al₂O₃ Gate Dielectrics

Area of Science:

• Materials Science
• Semiconductor Physics
• Surface Chemistry

Background:

• Atomic-layer deposition (ALD) enables precise control over thin film growth for advanced electronic devices.
• Titanium dioxide (TiO2) and aluminum oxide (Al2O3) are widely used high-k dielectrics in semiconductor technology.
• Interface properties significantly influence the performance of metal-oxide-semiconductor (MOS) devices.

Purpose of the Study:

• To investigate the impact of TiO2/Al2O3 dielectric stacks on interface charge dipole formation.
• To understand the relationship between interface dipoles, flat-band voltage shifts, and leakage current in germanium-based gate stacks.
• To explore the role of atomic intermixing in the observed electrical properties.

Main Methods:

• Fabrication of layered TiO2/Al2O3 dielectric stacks using atomic-layer deposition (ALD).
• Formation-gas annealing of the dielectric stacks on p-type germanium substrates.
• Electrical characterization, including flat-band voltage measurements and leakage current density analysis.
• Density functional theory (DFT) calculations to model interface dipole behavior and atomic intermixing.

Main Results:

• A significant interface charge dipole was observed in TiO2/Al2O3 stacks, causing a ~-0.2 V flat-band voltage shift.
• The interface dipole suppressed leakage current density for electron gate injection.
• Trilayer dielectric stacks eliminated these effects by canceling opposing dipoles.
• DFT calculations supported the presence of Al_Ti and Ti_Al point-defect dipoles due to intermixing.

Conclusions:

• The Al2O3 layer interposed between TiO2 and germanium induces a beneficial interface dipole.
• Atomic intermixing at the TiO2/Al2O3 interface is responsible for the observed dipoles and electrical properties.
• ALD and annealing conditions can be optimized to control interface dipole effects for improved gate stack performance.