Raman Spectroscopy of Optical Properties In Cds Thin Films

Properties of CdS thin films were investigated applying atomic force microscopy (AFM) and Raman spectroscopy. CdS thin films were prepared by using thermal evaporation technique under base pressure 2 x 10 torr. The quality of these films was investigated by AFM spectroscopy. We apply Raman scattering to investigate optical properties of CdS thin films, and reveal existence of surface optical phonon (SOP) mode at 297 cm. Effective permittivity of mixture were modeled by Maxwell – Garnet approximation.


Introduction
Thin film polycrystalline semiconductors have attracted great interest in an expanding variety of applications in various electronic and optoelectronic devices.Thin films now occupy a prominent place in basic research and solid state technology.The technological interest in polycrystalline based devices is mainly caused by their very low production costs.
Among the II -VI semiconductors, CdS polycrystalline thin film is a representative material.Cadmium sulphide (CdS) is a very useful optoelectronic [1,2], piezo -electronic [3] and semiconducting material.It has a wide direct band gap (2.42 eV) so has been used as a window material together with several semiconductors such as CdTe, Cu 2 S and CuInSe 2 [4].
The deposition of CdS films has been explored by different techniques: sputtering, thermal evaporation, chemical bath deposition, and molecular beam epitaxy [5 -9] in each of these methods polycrystalline, uniform and hard films are obtained, and their electrical properties are very sensitive to the method of preparation.
In the case of crystal with relatively small dimension, in the frequency range between bulk longitudinal optical phonon frequency (ω LO ) and transversal optical phonon frequency (ω TO ), a new mode known as a surface phonon mode appears.
It is well established that for the case of real crystal, when their dimension is relatively small, surface modes and effects of dimension will be also manifested in addition to the normal mode of infinite lattice.When dimension become extremely small, only the surface mode persist [10].
Surface modes play an important role in deciding the different physical properties of nanocrystals.For a plane wave propagating in the x -direction in a bulk crystal, the temporal and spatial variation of the wave is described by the factor exp[i(kx − ωt)], where the wavevector k = ( ) and ε(ω) is the dielectric constant of the crystal.In the frequency range between bulk longitudinal (ω LO ) and transverse optical mode frequency (ω TO ), ε(ω) has negative value, and accordingly k is imaginary.Therefore, in this frequency range the wave decays exponentially in the medium, i.e. it cannot propagate in bulk crystals and only surface modes exist [11].
In this work we report experimental studies of CdS thin films evaporated by thermal evaporation technique properties.Thickness of the films we analyzed was 1.6, 1.8, 2.0 and 2.2 μm.Samples characterization was performed using atomic force microscopy (AFM) while optical properties were analyzed using Raman spectra measurements.

Samples preparation and characterization
CdS powder was purchased from Sigma -Aldrich Company with high purity.CdS thin films of different thicknesses were deposited onto highly pre-cleaned glass substrates using the thermal evaporation at room temperature.A high vacuum coating unit (Edwards, E -306 A) was used under base pressure 2 x 10 -5 torr.The distance between the evaporation source (molybdenum boat) and the substrate holder was about 21 cm to avoid the heating flow from the heating source to the substrates.The rate of deposition was 10 nm/s and the film thickness was controlled using a quartz crystal thickness monitor (FTM4, Edwards).
The morphology of CdS thin films was investigated by atomic force microscopy (AFM).An atomic force microscopy (AFM) was used to determine the general cell wall structure together with the assembly of particular components into the wall structure as a whole.
AFM images for investigated CdS samples were presented in Fig. 1.For all samples hillocks having the height at about 30 nm were observed (Tab.I).  1, the employed surfaces at micro scale are relatively smooth and uniform (with the exception of a few scratches).AFM images showed that all CdS samples present well defined nanosized grains, having relatively small roughness values, ranging from 3.84 nm to 5.84 nm, as shown in Tab.I.

Results and discussion
The micro -Raman spectra were taken in the backscattering configuration and analyzed by Jobin Yvon T64000 spectrometer, equipped with nitrogen cooled chargecoupled -device detector.As an excitation source we used the 514.5 nm line of an Ar -iron laser.The measurements were performed at different laser power.Spectra were taken at room temperature.
Cadmium sulfide has wurtzite crystal structure.It has C 6v symmetry with 4 atoms per unit cell.Group theory predicts, at zone center that of 9 optical branches there is one A 1 and one doubly degenerate E 1 , which are both Raman and infrared active, two doubly degenerate E 2 branches which are Raman active only, and two inactive B 1 branches [12].As opposite to E 2 phonons, both A 1 and E 1 are polar modes and split into transverse (TO) and longitudinal (LO) phonons with different frequencies due to macroscopic electric fields associated with LO phonons.As we have emphasized, dimensions of our samples belongs to nanoscale.As it has been mentioned many times [13][14][15][16][17][18] reduction of the particles dimensions to nanoscale results in breakdown of phonon momentum selection rule and allows phonons with l ≠ 0 to contribute to Raman scattering.Hence some new forbidden vibration modes (acoustic modes -in low frequency region and surface optical modes -in high frequency region) will emerge due to imperfections, impurity, valence band mixing and/or nonspherical geometry of the nanostructures.
The typical Raman spectra of the bulk CdS crystal is presented in Fig. 2. The modes we observe are characteristic for wurtzite structure CdS [19][20][21].Those are the mode at 212 cm -1 with B 2 symmetry, mode at 234 cm -1 which is transversal mode with A 1 symmetry, E 1 symmetry mode at 245 cm -1 and mode at 252 cm -1 with E 2 symmetry.Dominant structure in these spectra is longitudinal mode at 305 cm -1 and its first overtone at 611 cm -1 (Inset in Fig. 2), which is consistent with the fact that the one of the striking features of the Raman spectra CdS is the remarkable overtones series of the longitudinal optical phonons [10].
Surface phonon modes are observed for particles sizes smaller then the wavelength of exciting laser light inside the particles.Usually these modes of small particles appear in polar crystals [28].The dielectric function for the case of polar semiinsulating semiconductor: describe its optical properties in the IR region.Here, ω TO and ω LO are the frequencies of the transverse and longitudinal optical bulk phonons, respectively; ε ∞ is the dielectric constant at high frequencies, and γ is the damping constant.The bulk phonons in small particles have properties similar to those of the corresponding phonons in infinite crystals; however their wave functions are adapted to the geometry of small particle.When visible light interacting with semiconducting nanoparticles (characteristic size L, dielectric function ε 2 ) which are distributed in a medium with the dielectric constant ε 1 in the limit λ >> L, the heterogeneous composite can be treated as a homogeneous medium, and so -called effective medium theory applies.
There are many mixing models for the effective dielectric permittivity of such a mixture [29].Since all our samples are well defined and separated nanosized grains we decided to use Maxwell -Garnet model for present case.
For the spherical inclusions case, the prediction of the effective permittivity of mixture ε eff according to the Maxwell -Garnet mixing rule is [30,31]: Here, spheres of permittivity ε 2 are located randomly in homogeneous environment ε 1 and occupy a volume fraction f.
In the area of interest for the appearance of surface optical phonons, we have two phonons ω A 1TO = 233.5 cm -1 , ω A 1LO = 305 cm -1 and ω E 1TO = 242 cm -1 , ω E 1LO = 308 cm -1 [12].Low mobility and low free carriers concentration allow us to neglect the plasmon -phonon interaction influence.Our nanoparticles are randomly distributed in space, and accordingly, to the incident ligh.As one can see, the E 1 symmetry phonon is registered in the Raman spectra, while there is no A 1 symmetry phonon, so we concluded that A 1 symmetry phonon participate in the creating SOP.
In order to demonstrate the influence of various parameters on the SOP, the appearance of Raman line for the dielectric function (equation ( 1)) with n = 1 and phonon with A 1 symetry, whereby the nanoparticles are situated in air (ε 1 = 1) is shown on Fig. 4. In this case Raman intensity due to the excitation of dominant extraordinary phonons is given with: Such calculation predicts appearance of one asymmetric peak with wavenumbers below ω A 1 (LO) in the area of Maxwell -Garnet formula applicability.Therefore, the difference in the intensity and line shape of simulated SOP modes is mainly the results of variation in main volume fraction and damping rate, as demonstrated in Fig. 4. In our case, the position of the SOP mode maximum directly follows the change of filling factor (Fig. 4a).As one can see from Fig. 4b, changes in damping leads only to a change in mode shape while its position remains unchanged.Raman spectra usually are analyzed with the help of Lorentzian and Gaussian curvers [32].In this work we have assumed that all phonon lines are of Lorentzian type.SOP lines are calculated by Eq.(2 -3), with ε 1 = 1 and n = 1 in Eq.1 (A 1 phonon).
Main volume fractions f obtained as the best fit parameter estimation, are presented in Tab.II.

Fig. 1 .
Fig. 1.AFM surface image of CdS thin films of different thicknesses on glass substrate: a) d = 1.6 μm, b) d = 1.8 μm, c) d = 2.0 μm and d) d = 2.2 μm.b) Surface topology and values of average roughness (R a ) and root mean squared (RMS) roughness was analyzed.According to obtained results which are presented in table1, the employed surfaces at micro scale are relatively smooth and uniform (with the exception of a few scratches).AFM images showed that all CdS samples present well defined nanosized grains, having relatively small roughness values, ranging from 3.84 nm to 5.84 nm, as shown in Tab.I.