Plasma nitridation of Ge(100) surface studied by scanning tunneling microscopy

The geometric and electronic structures of the surface species on Ge(100) after plasma nitridation were investigated in this study. An electron cyclotron resonance (ECR) plasma source was used to directly nitride Ge(100), and scanning tunneling microscopy and spectroscopy (STM/STS) were employed to study the structures of the nitrided surface. Nitridation at room temperature generated a large diversity of adsorbate sites on the surface containing N, O, and displaced Ge atoms, differentiated by annealing between 200 °C and 450 °C. Conversely, nitridation at 500 °C produced Ge-N adsorbate sites which formed ordered and disordered structures on the surface free from oxygen. Density functional theory (DFT) simulations were performed focusing on the ordered nitride structure, and the simulated surface structure showed a good correspondence with the STM data. DFT calculations also found an increase of density of states near the Fermi level on the ordered nitride structure, which is consistent with the Fermi level pinning observed in the STS results. The DFT results predict H-passivation can unpin the Fermi level of the nitrided surface by reducing the dangling bonds and the bond strain, but the residual plasma damage and the low nitridation rate in UHV are challenges to obtain complementary experimental results.