Revealing and reducing growth-induced interfacial disorder in preferentially aligned nitrogen-vacancy centers in diamond

Nitrogen-vacancy (NV) centers in diamond are powerful tools for quantum sensing, but their performance is often limited by decoherence caused by lattice defects and strain. These defects frequently arise at the interface between the buffer layer and the NV-doped layer during chemical vapor deposition (CVD) growth, particularly when nitrogen gas is introduced. This study investigates how different nitrogen-injection methods influence the formation of these interfacial defects and the resulting spin-coherence properties of the NV ensembles.

The researchers grew two sets of 12C-enriched, preferentially aligned NV layers on (111) diamond substrates using two distinct nitrogen-doping procedures. The first sample (S1) utilized mass flow controllers (MFCs) for smooth, gradual nitrogen injection, while the second sample (S2) used a traditional pulsed injection method. The team employed nanometer-scale selective plasma etching to create a series of layers at varying distances from the interface. They then combined optical emission spectroscopy (OES), time-of-flight secondary-ion mass spectrometry (ToF-SIMS), and advanced spin-coherence measurements—including Ramsey, double-quantum Ramsey (DQR), and double electron-electron resonance (DEER)—to map the concentration of defects and their impact on NV spin decoherence.

The pulsed injection method (S2) caused a significant, non-equilibrium nitrogen overshoot within a 60–80 nm interfacial region, leading to excessive lattice strain and spin noise that persisted even after 250 nm of overgrowth. In contrast, the smooth injection method (S1) minimized this overshoot, resulting in a homogeneous spin environment throughout the NV layer. The researchers found that in regions of high nitrogen concentration, not all nitrogen atoms are converted into P1 centers (substitutional nitrogen). Instead, the excess nitrogen induces other spin-noise-producing defects. By optimizing the growth conditions, the team achieved coherence properties in S1 that approached the theoretical limit imposed by spin-bath noise, which they successfully demonstrated by performing proton nuclear magnetic resonance (NMR) detection.