Urgent attention has been devoted to the establishment of observation techniques for inhomogeneous distribution of dopants at an atomic scale, because nano-scale devices have been realized with the development of advanced fabrication technology. Recent development of scanning tunneling microscopy (STM) has capacitated to visualize not only atomic structures but also dopant species near the surface. Nishizawa et al. have observed individual B or P dopant atom in STM images for the H-terminated Si(111) surfaces [1]. Our aims are to simulate theoretically the STM images and to reveal the observation condition for n-type and p-type substitutional dopants near the surface. In particular, we discuss the visible depth of the substitutional dopant.
In order to extract the knowledge of dopant species located at the subsurface region from the STM image, we performed calculations for the B- and P-doped Si(111) surfaces terminated with H atoms. We executed total energy calculations based on the first-principles pseudo-potential method within the density functional theory (DFT). STM images were calculated from isosurfaces of the local density of state (LDOS) with the Tersoff-Hamann approximation [2].
For the B-doped surface, protrusions and depressions appear around the dopants in the calculated STM images at the negative and positive sample biases, respectively. On the other hand, the P-doped surface shows the opposite features to those for the B-doped surface. Such features on the STM images are explained by the change of the local electrostatic potential in the vicinity of the screened negatively-charged B ion or the screened positively-charged P ion. Further, we present details of the depth dependence of dopants and effects of the dangling bond at the surface on STM images. We discuss, especially, the ability of the dangling bond as a surface charge indicator, considering the surface charge dependence of the STM image.
[1] M. Nishizawa, L. Bolotov, T. Toda, and T. Kanayama, J. Vac. Sci. Technol. B 24, 365 (2006)
[2] J. Tersoff and D. R. Hamann, Phys. Rev. B 31, 805 (1985) |