Plasmonic properties of nonstoichiometric zirconium nitride, oxynitride thin films, and their bilayer structures

Nonstoichiometric ${\mathrm{ZrN}}_x$ (ZrN) thin films, ${\mathrm{ZrO}}_x{\mathrm{N}}_y$ (ZrON) thin films, and bilayer ZrN/ZrON structures were prepared, and the effects of stoichiometry and interface on their plasmonic properties were investigated. We find that the samples are all B1 structured with similar lattice constants. Higher nitrogen and oxygen content can reduce the screened plasma frequency ${\omega }_c$. Interestingly, the bilayer ZrN/ZrON structures with some ZrON thickness are more metallic than ZrN films, which should be reasonable since the mutual diffusion through the ZrN/ZrON interface may cause a ZrON buffer zone with substitute oxygen atoms generating more free carriers. The postulation is further confirmed by the calculations about the band structure, which indicates that substitute oxygen atoms can depress the interband transition level and cause more carriers in conduction band. This work implies that oxygen substitution is an effective method to enhance the performances of nitride-based plasmonic materials and devices.

Figure 1

XRD $\theta \text{−}2\theta$ patterns of the ZrN films (a), ZrON films (b), and bilayer ZrN/ZrON structures (c). The ZrN films were prepared with different flow rate ratio of nitrogen gas $R_{\text{N}}$; the ZrON films were prepared with different oxygen flow rate $F_{\text{O}}$ ($R_{\text{N}}=4\%$); the thickness of ZrON layer $d_o$ of the bilayer ZrN/ZrON structures is different, but their total thickness is the same (150 nm). In the preparation of ZrN/ZrON structures, $R_{\text{N}}=4\%$ and $F_{\text{O}}=1.5$ sccm.

Figure 2

Cross-sectional SEM image of the bilayer ZrN/ZrON structure with $d_o=100\mathrm{nm}$. The dotted line indicates the estimated interface.

Figure 3

N $1s$ (a), O $1s$ (b), and Zr $3d$ (c) XPS spectra of ZrON films prepared with different oxygen flow rate $F_{\text{O}}$ ($R_{\text{N}}=4\%$).

Figure 4

N $1s$ XPS spectra of the bilayer ZrN/ZrON structures with different ZrON layer thickness $d_o$.

Figure 5

Real part ${\varepsilon }_1$ (a) and imaginary part ${\varepsilon }_2$ (b) of the permittivities of the ZrN films deposited with different nitrogen flow rate ratio $R_{\mathrm{N}}$.

Figure 6

Real part ${\varepsilon }_1$ (a) and imaginary part ${\varepsilon }_2$ (b) of the permittivities of the ZrON films deposited with different oxygen flow rate $F_{\text{O}}$.

Figure 7

Real part ${\varepsilon }_1$ (a) and imaginary part ${\varepsilon }_2$ (b) of the permittivities of the bilayer Zr/ZrON structures with different ZrON layer thickness $d_o$. The inset shows the $\hslash {\omega }_c$ as a function of $d_o$. The thicknesses of 0 and 150 nm correspond to monolayer ZrN and ZrON films, respectively.

Figure 8

Crystal cells of stoichiometric ZrN (a) and ${\mathrm{ZrO}}_{0.5}{\mathrm{N}}_{0.5}$ (b).

Figure 9

Electronic band structures and the density of states of ZrN (a) and ${\mathrm{ZrO}}_{0.5}{\mathrm{N}}_{0.5}$ (b).

Figure 10

The density of states of ZrN and ${\mathrm{ZrO}}_{0.5}{\mathrm{N}}_{0.5}$.