Structure and electronic properties of epitaxial fluorite-type ${\text{IrSi}}_2$ on Si(001)

An epitaxially stabilized Ir-silicide phase was grown in ultrathin two-phase films on Si(001). Using transmission electron microscopy it was found to have the fluorite structure. Due to the misfit between this epitaxially stabilized phase and the silicon substrate, elastic and plastic strain relaxation can be observed. Optoelectronic measurements of transmission, resistivity, and Schottky barrier height show a transition from infrared absorbing to infrared transparent films depending on thickness and reaction temperature. First-principles calculations confirm the experimental data on the structure and electronic properties of fluorite-type Ir disilicide.

Figure 1

Plan view in [001] zone axis of a ${\text{Ir}}_x{\text{Si}}_y{\text{-IrSi}}_2$ film grown from 2.0 nm deposited Ir at $700°\text{C}$.

Figure 2

Experimental diffraction pattern in [001] zone axis of the fluorite-type ${\text{IrSi}}_2$ with additional spots from silicon substrate.

Figure 3

Kinematical simulation of a diffraction pattern of fluorite-type ${\text{IrSi}}_2$ in [001] zone axis. Diffraction vectors are denoted as $g_1=\left[\overline{1}00\right]$ and $g_2=\left[010\right]$.

Figure 4

Experimental diffraction pattern of the fluorite-type ${\text{IrSi}}_2$ in [100] zone axis from a sample prepared in cross section. The diffraction pattern shows additional spots from the silicon substrate. The inset shows diffraction spots from the film only. Both patterns correspond in arrangement and intensity distribution.

Figure 5

Diffraction pattern in [110] zone axis of the fluorite-type ${\text{IrSi}}_2$ with additional spots from the silicon substrate.

Figure 6

Kinematical simulation of diffraction pattern in [110] zone axis. Diffraction vectors are denoted as $g_1=\left[1\overline{1}0\right]$ and $g_2=\left[001\right]$.

Figure 7

Cross-section micrograph of ${\text{IrSi}}_2$ taken in [100] zone axis. The magnified view inset shows structure in accordance with simulations (see text for details), and also the perfect epitaxial relationship between film and Si substrate.

Figure 8

(Color online) (a) Fluorite structure of ${\text{IrSi}}_2$. Ir atoms form the face-centered cell, Si atoms occupy the eight tetrahedral interstices. (b) Projection of the structure in [100] zone axis. (c) Calculated high-resolution contrast in [100] zone axis for a lattice constant of 0.565 nm. The contrast varies with thickness and defocus. Correspondence between lattice and image can be seen at particular thickness and defocus values, e.g., at 2.83 and 3.39 nm thickness.

Figure 9

HAADF micrograph of ${\text{IrSi}}_2$ in [100] zone axis.

Figure 10

HAADF micrograph of ${\text{IrSi}}_2$ taken in [110] zone axis.

Figure 11

Elastic strain in ${\text{IrSi}}_2$ on Si(001). Dark field STEM image of ${\text{IrSi}}_2$. Insets show Fourier transforms of film and substrate from which the strain was determined.

Figure 12

Cross-section micrograph of surface undulations of the film grown from 2.0 nm Ir deposit at $700°\text{C}$ (${\text{IrSi}}_2$ taken in [100] zone axis).

Figure 13

Periodic faults in ${\text{IrSi}}_2$ in [110] zone axis. (a) Cross-section micrograph showing faults generated by adjacent $A$- and $B$-type cubic ${\text{IrSi}}_2$, which appear periodically in the film. (b) The respective experimental diffraction pattern of the silicide film showing the extra spots typical for $A$- and $B$-type in front of each other in the electron beam (Ref. 24).

Figure 14

Optical absorption of Ir silicide/Si(001) at $\lambda =4\mu \text{m}$ vs the deposited Ir thickness for illumination from silicon and silicide side. The gray bar marks the critical thickness to which the high absorption is limited. The lines should guide the eye.

Figure 15

Resistivity vs temperature (K). The phases a-IrSi and c-IrSi grow at $500°\text{C}$ annealing temperature dependent on the thickness of the initially deposited iridium layer. In thicker films c-IrSi grows. The phases ${\text{Ir}}_x{\text{Si}}_y$ and ${\text{IrSi}}_2$ grow at $700°\text{C}$ annealing temperature.

Figure 16

Schottky barrier ${\Phi }_B$ vs annealing temperature determined from photoemission and $I\text{−}V$ curves. The thickness of the initially deposited iridium layer is $d_{\text{Ir}}=0.5\text{nm}$.

Figure 17

Illustration of the “dense” zincblende-type structure. As in Fig. 8 the origin is chosen so that the Ir atoms are located at the corners. In contrast to the rocksalt structure half of the interstitial sites are occupied with Si atoms.

Figure 18

Total energy per primitive cell as a function of lattice constant for ${\text{IrSi}}_2$ in the fluorite structure and a “dense” zincblende structure. The total energy of the fluorite structure is lower than that of the dense zincblende structure.

Figure 19

Electronic density of states (DOS) versus energy for ${\text{IrSi}}_2$ in the fluorite structure. The Fermi energy $E_F$ is taken at the zero of energy.