Raman Spectroscopy of Optical Properties In Cds Thin Films

Abstract

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^sup -5^ 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^sup -1^. Effective permittivity of mixture were modeled by Maxwell - Garnet approximation.

Headnote

Abstract:

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^sup -5^ 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^sup -1^. Effective permittivity of mixture were modeled by Maxwell - Garnet approximation.

Keywords: Thin films, Crystal growth, Phonons, Atomic force microscopy, Raman spectroscopy

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(ProQuest: ... denotes formulae omitted.)

1. 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, Cu2S and CuInSe2 [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 (ωLO) and transversal optical phonon frequency (ωTO), 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 = (ωc )[varepsilon](ω) and [varepsilon](ω) is the dielectric constant of the crystal. In the frequency range between bulk longitudinal (ωLO) and transverse optical mode frequency (ωTO), [varepsilon](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.

2. 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).

Surface topology and values of average roughness (Ra) and root mean squared (RMS) roughness was analyzed. According to obtained results which are presented in table 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.

3. Results and discussion

The micro - Raman spectra were taken in the backscattering configuration and analyzed by Jobin Yvon T64000 spectrometer, equipped with nitrogen cooled charge - coupled - 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 C6v symmetry with 4 atoms per unit cell. Group theory predicts, at zone center that of 9 optical branches there is one A1 and one doubly degenerate E1, which are both Raman and infrared active, two doubly degenerate E2 branches which are Raman active only, and two inactive B1 branches [12]. As opposite to E2 phonons, both A1 and E1 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 [13-18] 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 [19-21]. Those are the mode at 212 cm-1 with B2 symmetry, mode at 234 cm-1 which is transversal mode with A1 symmetry, E1 symmetry mode at 245 cm-1 and mode at 252 cm-1 with E2 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].

Raman spectra of CdS thin films (d = 1.6, 1.8, 2.0 and 2.2 μm) at room temperature are presented in Fig. 3. The observed Raman spectra for all those samples besides characteristic CdS modes and their multiphonon combinations, show the LO frequency shift from 305 cm-1 to 297 cm-1, and 611 cm-1 to 594 cm-1. The LO phonon shift of CdS crystal is attributed to the surface optical phonon (SOP) mode effect [10, 22-27].

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:

... (1)

describe its optical properties in the IR region. Here, ωTO and ωLO are the frequencies of the transverse and longitudinal optical bulk phonons, respectively; [varepsilon]∞ 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 [varepsilon]2) which are distributed in a medium with the dielectric constant [varepsilon]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 [varepsilon]eff according to the Maxwell - Garnet mixing rule is [30, 31]:

... (2)

Here, spheres of permittivity [varepsilon]2 are located randomly in homogeneous environment [varepsilon]1 and occupy a volume fraction f.

In the area of interest for the appearance of surface optical phonons, we have two phonons ωA1TO = 233.5 cm-1, ωA1LO = 305 cm-1 and ωE1TO = 242 cm-1, ωE1LO = 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 E1 symmetry phonon is registered in the Raman spectra, while there is no A1 symmetry phonon, so we concluded that A1 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 A1 symetry, whereby the nanoparticles are situated in air ([varepsilon]1 = 1) is shown on Fig. 4. In this case Raman intensity due to the excitation of dominant extraordinary phonons is given with:

... (3)

Such calculation predicts appearance of one asymmetric peak with wavenumbers below ωA1(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 [varepsilon]1 = 1 and n = 1 in Eq.1 (A1 phonon).

Main volume fractions f obtained as the best fit parameter estimation, are presented in Tab. II.

4. Conclusions

In this paper, we present results of investigation CdS thin films with different thicknesses. We determined that all samples surfaces are relatively smooth and uniform, having well defined nanosized grains with relatively small roughness values. Raman spectra measurements reveal, besides characteristic CdS modes and their multiphonon combinations, existence of surface optical phonon modes. We threat the CdS thin film as a mixture of homogenous spherical inclusion in air modeled by Maxwell - Garnet formula. Besides characteristic cadmium sulphide modes we registered the existence of SOP modes.

Acknowledgements

This research was financially supported by the Serbian Ministry of Education, Science and Technological Development (Project 45003).

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