OPTICAL PROPERTIES OF SILICON NANOWIRES

Silicon nanowires are a promising material for modern micro- and nanoelectronics. The purpose of this work is to study the optical reflection and transmission spectra in the spectral range (0.2÷25) ·10-6 m of silicon nanowires of different heights to obtain data about the energy band structure. On the surface of a p+ - type single crystal silicon plate oriented (100) (with a thickness of 510 ± 20 μm and a specific resistance of 0.01 Ω·cm) grown silicon nanowires (NWs). The manufactured samples of silicon nanowires had a porosity of 60%. The length of silicon nanowires lNW was 5.5 μm; 20 μm; 50 microns. The oxide coating of the silicon surface is an amorphous film whose thickness ranges from 0.5 nm to 7 nm. The thickness of the SiOx transition layer between the single crystal silicon and silicon dioxide is 0.5 nm–0.7 nm. Since the reflection coefficient R as a function of the electromagnetic wavelength λ R = f(λ) is related to the transmission coefficient T = f(λ) and the absorption D = f(λ) by the ratio R(λ) + T(λ) + D (λ) = 1, then the optical absorption spectra D(λ) = 1 – [R(λ) + T(λ)] were also studied in this paper. Based on the Heisenberg uncertainty principle for energies E and time t (∆E·∆t ≥ ћ), the relaxation effects in the absorption of electromagnetic waves by a crystal are described by the broadening parameter Г = ћ/τ (broadening of the electronic transition E0 is related to the lifetime of free charge carriers through their interaction with fluctuations of the crystal lattice, impurities, defects, including those of a surface nature), where τ is the energy relaxation time of photogenerated charge carriers. In the optical reflection spectra of silicon nanowires in the spectral range of wavelengths 200 nm ÷ 1700 nm, two energy peaks are observed at the energies of 0.862 eV and 1.046 eV, and in the optical transmission spectra, energy peaks appeared at the energies of 0.854 eV and 1.048 eV. The optical absorption spectra D(λ) are fully correlated with the reflection and transmission spectra. From the spectra of optical reflection and optical transmission for silicon nanowires with these lengths, the energy broadening of the optical spectra of the materials equal to 0.184 eV and the energy relaxation time of photogenerated charge carriers τ equal to 3.577∙10-15 s were determined. Experimental data and calculations indicate a change in the energy band structure of silicon nanowires, compared to a p-Si (100) single crystal, which can be explained by the quantum-dimensional effect that occurs in the studied objects.