We suggest that the difference noted between Fig. and Fig. can be discussed in terms of precursor diffusion. As the precursors are introduced into the reactor, their partial pressures instantly increase. Along the rising edge of the precursor partial pressure, the precursor molecules diffuse towards the bottom of the trench structure, with a driving force to even out local differences in partial pressures. From Graham's law of diffusion, the diffusion velocities of two gases in thermal equilibrium are depending on their molecular masses. Lighter molecules diffuse faster than heavier molecules. The small difference between the molecular mass of TMA (72 amu) and Kr (84 amu) should have a very small effect on the diffusion of TMA. But the large difference in mass between NH3 (17 amu) and Kr should, according to Graham's law of diffusion, cause a significant difference in their diffusion down the trench. This would explain the reduction in GPC at the trench opening as the nitridation of the TMA monolayer is reduced by the higher diffusion velocity of NH3, decreasing the partial pressure of NH3 at the trench opening. On the other hand, towards the bottom, the partial pressure of NH₃ is maintained, or possibly increased, maintaining the GPC. An alternative explanation is that as inert Kr atom collides with the surface as it diffuses down the trench, the heavy Kr atom can act to enhance the surface desorption of adsorbed precursors. Especially in the case of lesser reactive NH3, which takes more time to saturate the surface reaction. This would lead to a decreased GPC and the desorbed precursors would be available for film deposition when they reach a new surface site. It could possibly be argued that this enhanced surface desorption by the Kr addition would render a lower GPC at the top of the trench while maintaining a saturated GPC (0.8 Å) closer to the bottom surfaces. We propose that the explanation for the observed results is likely a combination of, at least, these two factors and that further studies and modeling are needed to fully understand the results. The lower GPC at the opening of the trench is most likely not explained by a surface inhibiting effect from the Kr, as the Kr atoms are expected to be inert.