Terahertz time-domain spectroscopy of zone-folded acoustic phonons in 4H and 6H silicon carbide

Abstract

We investigate the dielectric properties of the 4H and 6H polytypes of silicon carbide in the 0.1-19 THz range, below the fundamental transverse-optical phonons. Folding of the Brillouin zone due to the specific superlattice structure of the two polytypes leads to activation of acoustic phonon modes. We use a combination of ultrabroadband terahertz time-domain spectroscopy and simulations based on density-functional perturbation theory to observe and characterize these modes, including band splitting due to the dissimilar carbon and silicon sublattices of the structures, and an indirect measurement of the anisotropic sound velocities in the two polytypes.

Phase Retrieval in Terahertz Time-Domain Measurements: a “how to” Tutorial

“…Intrinsic (undoped) SiC has high transparency in the THz region below the TO frequency, and has a strong phonon-induced dispersion of the refractive index across the 0-20 THz region [26,27]. The dielectric function of 4H SiC can be expressed as a sum of two Lorentz oscillator terms, that describes the TO phonon, and a weak term (WTA) that describes a transverse acoustic (TA) phonon, which is infraredactive due to polytype-specific folding of the Brillouin zone [28]. The experiment and the results are described in detail in Ref.…”

“…The experiment and the results are described in detail in Ref. [28]. Important to note here is that the spectroscopy was performed with large-aperture samples of SiC (10 cm diameter) placed in the parallel section of the THz beam in the spectrometer, in order to obey the plane-wave approximation behind Eqs.…”

Subcycle Nonlinear Response of Doped 4H Silicon Carbide Revealed by Two-Dimensional Terahertz Spectroscopy

“…In this manner, the detected signals are directly comparable in amplitude. The refractive index of undoped 4H-SiC is approximately 3.12 in the 0-2 THz range [12], yielding a field transmission of 0.735 due to Fresnel reflection losses. The low-field peak THz transmission of only 0.143 at room temperature is due to freecarrier absorption (FCA) of the significant density of thermally activated free carriers from the N dopant states.…”

“…Based on the amplitude (Figure 1(b)) and phase (not shown) of the sample and reference spectra, we extract the permittivity of the sample [42] as shown in Figure 1(c), where the blue and red curves represent the real and imaginary part of the permittivity, respectively. For comparison, we have included corresponding data for undoped 4H SiC [12], represented by dotted curves. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 7 undoped 4H-SiC [12].…”

“…For comparison, we have included corresponding data for undoped 4H SiC [12], represented by dotted curves. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 7 undoped 4H-SiC [12]. (d) The extracted conductivity of charge carriers in the co-doped 4H-SiC sample.…”