Low Temperature Fluorinated Silicon FILMS Synthesis
Authors: Milovzorov Dmitry Evgenyevich 
Published: February 2020

Second-harmonic generation (SHG), photoluminescence (PL) and Fourier-transformed infrared (FT IR) spectra from silicon films with different average size of nanocrystals were analized to develop new possible material for active channel in nonlinear optical devices. Silicon films were prepared with dominant concentrations of silicon bonding with oxygen or fluorine atoms. Also, the volume fraction of nanocrystals was varied from 30% to 70%. It is seen the spectral peak with energy 3.26 eV is related to defects appeared in interface area silicon-silicon dioxide. For films with small silicon crystals (less than 20 nm) the nonlinear optical response contains two spectral peaks. The second peak is caused by optical response from nanocrystal grain boundary that contains oxygen or fluorine atoms incorporated in silicon as dipoles inside film. The optical nonlinear switch device based on the nonlinear optical response of SiOxmedia inside film was proposed. Also, the silicon film with quartz micro-clusters was investigated as material for making the nonlinear optical transmitter device. The structural properties of silicon films can be easily illustrated by using the films morphology which was determined by using atomic-force microscopy. Figure 1 shows the atomic-force microscopic photos for Sample 1, Sample 2 and Sample3, respectively. It is seen, that the nanocrystal grains are in all types of investigated silicon films. The lateral distribution of nanocrystals with dominant orientation (111) are homogeneous for Sample 1 (see Fig. 1 a)), but for the  Sample 2 is unhomogeneous due to appeared conglomerates which are created from the amorphous phase separately (see Fig. 1 b)). The Sample 3 has the triangular shape of nanocrystals (111) which are distributed in submicron crystalline (111) domens. It is clear, that there is great difference in morphological properties of three types of films. The HF etching the silicon surface is efficient because at Td=100oC the surface diffusion and desorption from the surface are low. By adding the SiF4 we change the H2 :SiF4 ratio and some amount of fluorine atoms are included into H2SiF6 assembly creation. Because, the etching process is low at [SiF4] =0.25 sccm and [SiF4] =0.375 sccm. After this values of silicon tetra-fluoride flow rate the H2:SiF4 ratio is changed to destruct intensive creation of molecular assemblies. This stage at [SiF4] =0.5 sccm the HF significant etches the silicon and causes the decreasing the grain sizes. It is assumed that there are two possibilities of fluorine removing: the first is hydrogen incorporation with fluorine, and, the second, the H2SiF6 assemble is creation by 300oC. The SHG is forbidden for centrosymmetric crystal such as bulk silicon because the sum dipole moment is zero, but it is possible due to the surface breaking symmetry and quadruple terms contributions. The opposite situation is for nanostructured oxidized (or fluorinated) silicon films for which the surface area of great quantity of nanocrystals is significant, and the breaking of symmetry is permanent and lateral isotropic. The spectral characteristic of optical nonlinear response from the thin silicon film strictly depends on the spectral properties of polarization and susceptibility tensors  χ(2)
distortion
and χ(2)
Si-O.F
  are the nonlinear susceptibilities produced by nonzero dipole moment caused by shape distortion from the spherical shape of nanocrystals and due to the existing of dangling bonds caused by fluorine or oxygen incorporation in grain boundary region, respectively. By these values of densities the dipole moments of such nanoscaled ellipsoidal dipoles covered by surface charges can be estimated as PSiO
=0.04 Debye and PSiH
=0.015 Debye. Therefore, the concentration of nanocrystals mainly plays a great role in PL and SHG signal intensity of radiation due to their great surface area covered by silicon bonds with contaminants (atoms of O, H, F) in grain boundary but not as a separate nanoscaled dipoles. By increasing the nanocrystal quantity inside the silicon film the ratio S/V grows significant. The size effect in PL from nanocrystalline silicon film can be explained by means of statistical method. We suggested that the ƒ(x) is distribution function of crystallites in their sizes, θ(x) is a quantum efficiency of nanocrystalls. It is supposed, that the large quantity of nanocrystals is Gauss distributed in their sizes. The Nnc-Si
value is proportional to the square under the both functions ƒ(x) and θ(x) near the zero values. Accordingly, the quantity of emitted hydrogenized nanocrystals by band-to-band radiative transitions is important to esimate the PL linear optical response. For nanocrystalline silicon films for Pin

àSout
optical scheme χ(2)
xxx component of susceptibility tensor is detected. By using  optical scheme Pin

àPout 
SHG spectra contains isotropic and anisotropic component of susceptibility tensor χ(2)
zzz, χ(2)
zxx,χ(2)
xxz
, and χ(2)
xxx.

Figure 1