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

As deformation is increasingly probed at nanometer scales, atomic-level surface features become decisive in triggering crystal plasticity. Direct experimental elucidation of such effects has long been hindered by limited control over surface atomic arrangements. This challenge is particularly pronounced for GaN, a promising wide-bandgap semiconductor that is hard to process, making ideal planarization nontrivial. Here, step-terrace GaN surfaces with monoatomic topography approaching the theoretical limit were realized using catalyst-referred etching (CARE). On these surfaces, nanoindentation pop-ins occurred in all 100 indents at the ideal strength (16.15 GPa) with a record-low stress scatter of only 2.3%. As-received surfaces, conventionally regarded as flat but lacking step-terrace ordering, exhibited pop-ins at similar stress levels yet larger scatter, indicating that even atomic-scale surface irregularities can perturb plasticity initiation. Mechanically buffed surfaces produced only 28/100 pop-ins, evidencing a marked loss of reproducibility. Pop-ins occurred at much lower stresses and with smaller bursts, consistent with heterogeneous dislocation nucleation and reduced elastic energy release. Equivalent-radius contact analysis elucidated how local curvature affects stress estimation, decoupling topographic effects from incipient plasticity. These results shift nanoindentation evaluation standards from roughness metrics to atomic step-terrace structure and provide a reliable framework for analyzing incipient plasticity in crystals.