Spectroscopic ellipsometry studies of as-grown and annealed CdS:O thin films

PACS 68.37.Hk

Spectroscopic ellipsometry studies of as-grown and annealed CdS:O thin films

Khuraman N. Khalilova

Institute of Physics, Azerbaijan National Academy of Sciences,
AZ 1143, H.Javid ave.33, Baku


ABSTRACT

We present the results of spectroscopic ellipsometry studies on both as-grown and annealed CdS thin films. CdS:O thin films deposited at O/Ar partial pressures of 3 and 5% are vacuum annealed at temperatures 250, 300, and 350oC. As-grown and annealed films are then examined at room temperature by using M-2000 spectroscopic rotating compensator ellipsometer.
Key words: Spectroscopic ellipsometry, thin films, solar cells.

INTRODUCTION
Cadmium sulfide (CdS) thin films are widely used as window layers in thin film solar cells based on cadmium telluride (CdTe), copper indium diselenide (CIS), and copper indium gallium diselenide (CIGS) [1,2]. It is straightforward that one can increase light harvesting in solar cells by extending transparency of the CdS:O window layer into UV spectral region. CdS deposition in argon/oxygen atmosphere has been reported [3] as a suitable technology to obtain CdS thin films with increased band gap. However, comprehensive optical data verifying such a conclusion have not been presented then.
Spectroscopic ellipsometry (SE) is commonly recognized as a powerful, reliable and non-destructive technique to obtain dielectric function and optical constants of thin films.
In [4] we determined dielectric function and optical constants of CdS thin films deposited on soda-lime glass substrates in argon/oxygen atmosphere by rf magnetron sputtering. However, the films we studied were not subjected to after-growth thermal treatment.
We are studying both annealed and as-grown CdS:O thin films by using spectroscopic ellipsometry. The data acquisition, together with the subsequent linear regression analysis to restore dielectric function has been performed over the photon energy range 1-7eV.


EXPERIMENTAL PROCEDURE
Previously, similar films deposited on soda-lime glass (SLG) at 0% and 5% values of O/Ar ratios during 26-30 min were examined by spectroscopic ellipsometry (SE) [5]. For implementation of current experiment, CdS:O thin films deposited at O/Ar partial pressures of 3 and 5% were vacuum annealed at temperatures of 250, 300, and 350oC. As-grown and annealed films were then examined ellipsometrically at room temperature. SE was performed with the aid of a spectroscopic rotating compensator ellipsometer (M-2000DI J.A. Woollam Co.,Inc.) over the photon energy range 1–7 eV with a step of 0.02 eV at the angle of incidence of 800.
We have measured and modeled ellipsometric parameters ψ and Δ, determined from the main ellipsometry equation as

,

where, Rp and Rs are the complex amplitude Fresnel reflection coefficients, for s and p polarized light.
In the performed linear regression analysis (LRA) the following parameters were the variables d (thickness of CdS:O film), 1 and 2 (real and imaginary parts of dielectric function of CdS:O, respectively).

RESULTS AND DISCUSSION
The best fit to the experimental ellipsometric data was obtained within an optical model based on Gaussian and PSemi-Tri type oscillators. The data fitting was performed to minimize the mean square error (MSE) between the measured and calculated parameters of ψ and Δ. The fitting results are shown in Figs.1 and 2. The best-fit-based dielectric function of as-grown and annealed samples is shown in Figs.3 and 4, respectively. We have shown that if CdS:O(5%) films are applied to window layer of solar cells, the amount of the solar spectral irradiance reaching the active layer is increased compared with CdS film. The results suggest that conversion efficiency of compound semiconductors solar cell can become evaluated.
References:

[1] K. Ramanathan, G. Teeter, J.C. Keane, and R. Noufi, Thin Solid Films 480-481 (2005) 499.
[2] T. L.Chu, Shirley S.Chu, G. Chen, J. Britt, C. Ferekides, and C. Q. Wu, J. Appl. Phys., 3865 (1992 ,71.
[3] X. Wu, Sol. Energy 803 (2004) 77.
[4] Y. Shim, J. Sakamoto, A. Suzuki, K. Khalilova etl. Jpn. J. Appl. Phys. 05FC14 (2011), 50.
[5] R. M. A. Azzam and N. M. Bashara: Ellipsometry and Polarized Light (Elsevier, Amsterdam, 1986) Chap. 3.

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