Far infrared spectroscopy of ${\text{Pb}}_{1-x}{\text{Eu}}_x\text{Te}$ epitaxial layers

Transmission and reflection coefficients of epitaxial layers of solid solutions ${\text{Pb}}_{1-x}{\text{Eu}}_x\text{Te}$ $\left(0\le x\le 0.37\right)$ on ${\text{BaF}}_2$ and Si substrates are measured in a wide frequency range from 7 to $4000{\text{cm}}^{-1}$ at temperatures between 5 and 300 K. Although several phonon and impurity absorption lines of the substrate and buffer layers dominate the absorption spectra, a local ${\text{Pb}}_{1-x}{\text{Eu}}_x\text{Te}$ mode is observed around 110 to $114{\text{cm}}^{-1}$. In addition, a soft transverse-optical phonon mode of ${\text{Pb}}_{1-x}{\text{Eu}}_x\text{Te}$ is detected with the frequency decreasing from 32 to $18{\text{cm}}^{-1}$ upon cooling from room temperature to 5 K.

Figure 1

(Color online) Transmission coefficient spectra of an epitaxial layer ${\text{Pb}}_{1-x}{\text{Eu}}_x\text{Te}$ ($x=0.076$, thickness $0.67\mu \text{m}$) grown on ${\text{BaF}}_2$ substrate (thickness 1.05 mm), measured at 300 and 10 K. Arrows show absorption lines in ${\text{BaF}}_2$ and PbEuTe. Stars indicate maxima due to interference of radiation in the layer.

Figure 2

(Color online) Transmission coefficient spectra of epitaxial layers ${\text{Pb}}_{1-x}{\text{Eu}}_x\text{Te}$ with $x=0.06$ (thickness $0.6\mu \text{m}$) and $x=0.09$ (thickness $6\mu \text{m}$), grown on Si substrates (thickness 0.36 mm and 0.54 mm, respectively), measured at 300 K. Arrows show absorption lines in PbEuTe and in ${\text{BaF}}_2$ buffer layer.

Figure 3

(Color online) Transmission coefficient spectra of an epitaxial layer ${\text{Pb}}_{1-x}{\text{Eu}}_x\text{Te}$ ($x=0.076$, thickness $0.67\mu \text{m}$) grown on ${\text{BaF}}_2$ substrate (thickness 1.05 mm) at different temperatures. Lines show least-square fits (see text). Temperature-dependent minimum corresponds to absorption due to a soft mode.

Figure 4

(Color online) Temperature dependence of parameters of the soft mode observed in epitxial layer ${\text{Pb}}_{1-x}{\text{Eu}}_x\text{Te}$ $\left(x=0.076\right)$: (a) frequency position, (b) damping, and (c) dielectric contribution. Closed circles correspond to a layer on ${\text{BaF}}_2$ substrate, open circles—on Si substrate. Triangles in panel (a) represent the data from Ref. 19 corresponding to the soft-mode position in PbTe. Lines are guide to the eyes.

Figure 5

(Color online) Transmission coefficient spectra of ${\text{Pb}}_{1-x}{\text{Eu}}_x\text{Te}/{\text{BaF}}_2$ epitaxial layers with different concentrations $x$ and thicknesses $d$: $x=0$, $d=0.94\mu \text{m}$; $x=0.076$, $d=0.67\mu \text{m}$; $x=0.25$, $d=4.33\mu \text{m}$; and $x=0.37$, $d=3.74\mu \text{m}$. $T=5\text{K}$.

Figure 6

(Color online) Spectra of refraction index $n$ and absorption coefficient alpha of two epitaxial layers ${\text{Pb}}_{1-x}{\text{Eu}}_x\text{Te}$ with concentrations $x=0.076$ (solid lines) and $x=0.06$ (dashed lines), at three different temperatures. Solid and dashed lines correspond to layers grown on ${\text{BaF}}_2$ and silicon substrates, respectively.