Atomic Step-Terrace Ordering Enables Unprecedentedly Low Pop-in Stress Scatter in GaN (0001)

Area of Science:

• Materials Science
• Nanotechnology
• Solid Mechanics

Background:

• Atomic-level surface features significantly influence crystal plasticity at the nanoscale.
• Controlling surface atomic arrangements is challenging, especially for difficult-to-process materials like Gallium Nitride (GaN).
• Previous nanoindentation studies were limited by surface preparation techniques and reproducibility.

Purpose of the Study:

• To create ideal step-terrace Gallium Nitride (GaN) surfaces approaching atomic flatness.
• To investigate the impact of controlled atomic-scale surface topography on nanoindentation-induced plasticity.
• To establish new standards for evaluating incipient plasticity in crystals.

Main Methods:

• Catalyst-referred etching (CARE) was employed to achieve monoatomic step-terrace GaN surfaces.
• Nanoindentation was performed on surfaces with varying degrees of atomic ordering.
• Equivalent-radius contact analysis was used to decouple topographic effects from plasticity.

Main Results:

• CARE-treated surfaces exhibited pop-ins at ideal strength (16.15 GPa) with minimal stress scatter (2.3%).
• As-received and mechanically buffed surfaces showed reduced reproducibility and heterogeneous dislocation nucleation.
• Atomic step-terrace structure was identified as a key factor, not just surface roughness, for plasticity initiation.

Conclusions:

• Controlled atomic-scale surface topography is essential for reliable nanoindentation studies of crystal plasticity.
• CARE provides a method to prepare surfaces for fundamental studies of material behavior at the nanoscale.
• This work redefines nanoindentation evaluation standards towards atomic structure.