Deposition of cobalt atoms onto ${\mathrm{Alq}}_3$ films: A molecular dynamics study

The charge and spin injections into semiconductors are determined by the quality of the electrode/semiconductor interface. The latter is defined by the atomic penetration depth when growing metal electrodes on organic semiconductor thin films by the physical deposition method. Although the interfaces between top electrodes and a typical organic material, tris(8-hydroxyquinoline) aluminum $\left({\mathrm{Alq}}_3\right)$ thin films, have been intensively investigated by several groups, their results on the atomic penetration depth into ${\mathrm{Alq}}_3$ film diverged from each other. In this paper we study the deposition of cobalt atoms onto ${\mathrm{Alq}}_3$ thin films using the molecular dynamics method. The intermixing between Co and ${\mathrm{Alq}}_3$ is calculated to be limited to the first ${\mathrm{Alq}}_3$ layer for low Co initial kinetic energies, but spread to a few layers for high initial energies. The results of our calculation indicate that a proper deposition method with low injection energy helps to reduce the amount of intermixing at interfaces and improve the magnetoresistance of an organic-based magnetic junction.

Figure 1

Optimized Lennard-Jones potential (solid lines) and first-principles binding energies (circles) along two lines perpendicular to and through the center of an aromatic ring (left) and a N(O) triangle (right).

Figure 2

A $10\times 10{\mathrm{nm}}^2$ AFM-like image of the surface of the ${\mathrm{Alq}}_3$ thin film, with a root-mean-square roughness of $0.3\mathrm{nm}$ and a peak-to-peak roughness of $2.0\mathrm{nm}$. The white dashed rhombus shows the unit cell of our model system.

Figure 3

(a) Frequency distribution of the $z$ component of the final position and (b) penetration depth of cobalt atoms for different initial energies. The dotted line in (a) shows the average height of the ${\mathrm{Alq}}_3$ thin film.

Figure 4

Snapshots at (a) $1.6\mathrm{ps}$, (b) $1.7\mathrm{ps}$, and (c) $3.7\mathrm{ps}$ of one trajectory with an initial energy of $10\mathrm{eV}$ and penetration depth of $1.5\mathrm{nm}$. The largest sphere in each snapshot represents the cobalt atom. Three ${\mathrm{Alq}}_3$ molecules adjacent to the trajectory of the cobalt atom are shown in each snapshot. Atoms of the ${\mathrm{Alq}}_3$ molecule, which interacts directly with the cobalt atom, are in different colors according to their local temperatures. The other two ${\mathrm{Alq}}_3$ molecules are in light gray in order to make the figures clear. The atoms in ${\mathrm{Alq}}_3$ molecules are illustrated by spheres with radii Co $>$ Al $>$ O $>$ N $>$ C $>$ H. The local temperatures of atoms are defined by Eq. (1). The coordinate system orientations for (a)–(c) are the same.

Figure 5

Two different final positions of cobalt with respect to the ${\mathrm{Alq}}_3$ molecule: (a1) and (b1) spin density ${\rho }_{\uparrow }\left(\vec{r}\right)-{\rho }_{\downarrow }\left(\vec{r}\right)$ contour plots, positive for blue (heavy gray) and negative for yellow (light gray), and (a2) and (b2) projected density of states for two ${\mathrm{Alq}}_3$-Co clusters: the dashed line is for carbon, the dotted line is for cobalt, and the solid line is for oxygen. In (a1) and (b1), oxygen atoms are labeled.

Figure 6

Density profiles of carbon (dashed lines) and cobalt (shadow) elements in arbitrary units for different cobalt initial energies. The corresponding experimental deposition methods are shown.