MIOMD - XVI 2023 Session MIOMD-ThA1: Materials for Mid-Infrared

The compatibility with the main-stream silicon technology makes silicene and a few layers of silicon very promising for VLSI beyond 3nm technology node. Similar as its carbon counterpart graphene, energy bandgap (EB) opening presents the most critical demand for potential applications in digital electronics. Based on density function theory, EB opening in bi-layer silicon structures (BLSi) has been obtained under biaxial in-plane strain in our previous works [1, 2]. In this work, we performed a theoretical study of the optical properties of the strained BLSi in mid-IR. It turns out that by applying the in-plane tensile strain, the optical properties of the BLSi can be tuned in a wide range over the entire mid-IR bandwidth.

Our previous works show that buckle-free bilayer silicon structure (Fig.1) can be obtained once the biaxial in-plan tensile strain exceeds 11.2% [1, 2]. As the in-plane strain continuously increases to 16.4%, a direct EB (E_{g_D}) is formed at G point in addition to the in-direct EBs (E_{g_ID}) (Fig.2), which promotes direct band transition at IR. To verify it, various optical properties including permittivity, refraction index, and optical conductivity of the BLSi have been calculated versus photonic energy at different strain levels. Figure 3 plots the complex in-plane permittivity of the BLSi under an in-plane strain of 14.8%. The observed absorption (P1 marked in Fig. 3) evidences the direct-band transition. By varying the applied strain, such featured absorptions can be tuned in the range between 0.154-1.056 eV, which covers the entire mid-IR (See supplementary pages). In addition to the featured absorptions, its real part of the permittivity in Mid-IR (<0.2 eV) is of a factor 3 greater than the fully relaxed BLSi, leading to two times enhancement of reflectance. We believe that the buckle-free planar BLSI might open new opportunities of applications at IR.

While polar dielectric materials provide natural low-loss infrared hyperbolic resonances through the excitation of phonon polaritons, the operational bandwidth of these materials is limited to a few hundred wavenumbers (cm-1) or tenths of electronvolts. Also, integrating these materials with large-scale infrared optoelectronic devices presents many challenges. In this work, we implemented an ultrawide low-loss Type I hyperbolic metamaterial covering a spectral bandwidth of 2000 cm-1 for wavelengths above 4.7 µm. We produced the hyperbolic metamaterial with a stack of intercalated heavily-doped InAs and undoped InAs epilayers grown by molecular beam epitaxy. The InAs epilayer was heavily doped with Tellurium to obtain electron concentrations of ~8×1019 cm-3. The Type II hyperbolicity of these stacks was determined through infrared ellipsometry obtaining effective optical constants for the stacks. These materials were then dry etched to form one-dimensional (1D) square gratings with periods and linewidths ranging from 1 to 5 µm. The measured effective optical constants measured through ellipsometry were used to model the grating’s optical response by finite element electromagnetic calculations (COMSOL). The models agree with measurements, showing the formation of hyperbolic plasmon polaritons at the same frequencies where experimental features were observed. This work demonstrates that high subdiffractional light confinement can be achieved with a III-V metamaterial that can be integrated with III-V semiconductor infrared devices such as photodetectors and emitters at a large scale.

This material is based upon work supported by the Office of the Undersecretary of Defense for Research and Engineering Basic Research Office STTR under Contract No. W911NF-21-P-0024. Disclaimer: The content of the information does not necessarily reflect the position or the policy of the Government, and no official endorsement should be inferred.