Optical constants of Er2O3-Al2O3 films studied by spectroscopic ellipsometry

JOURNAL OF RARE EARTHS, Vol. 29, No. 10, Oct. 2011, P. 958

Optical constants of Er2O3-Al2O3 films studied by spectroscopic ellipsometry ZHU Yanyan (ᴅ➩㡇)1, FANG Zebo (ᮍ⋑⊶)2, XU Run (ᕤ䯄)3 (1. Department of Mathematics and Physics, Shanghai University of Electric Power, Shanghai 200090, China; 2. Department of Physics, Shaoxing University, Shaoxing 312000, China; 3. School of Materials Science and Engineering, Shanghai University, Shanghai 200072, China) Received 10 April 2011; revised 31 May 2011

Abstract: Er2O3-Al2O3 film was deposited on the Si(001) substrate by radio frequency magnetron technique at room temperature. The sample was annealed at 450, 600 and 750 ºC for 30 min in O2 ambience, respectively. The optical constants were studied by spectroscopic ellipsometry for both the as-deposited and the annealed samples. The proper values of refractive index indicated that it could be a useful material for solar cells. Keywords: optical properties; rare earth oxides; solar cells

Rare earth (RE) oxides are a kind of interesting materials due to their excellent chemical, thermal, optical, and electrical properties[1]. Among others, Er2O3 is more interesting because of its versatility and multifunctionality which has superior properties such as showing high k value, high chemical and thermal stability in contact with Si, high mechanical strength, substantial hardness and being highly transparent in the working spectral range of visible light[2]. Therefore, Er2O3 has various applicatoins in optical and electronic devices. Specifically, Er2O3 is an alternative to replace SiO2 as the dielectric film to suppress the gate leakage current with scaling of complementary metal oxide semiconductor (CMOS) devices[3]. In addition, Er2O3 films can be used as anti-reflection (AR) coatings in Si solar cells[4,5]. As the alternative material of AR coatings to be used in photovoltaic cells, it has to fulfill different requirements including the absence of scattering centers to avoid optical losses, proper refractive index, low reflectivity and high optical transparency. It has been observed that the properties can be improved and tuned when Al2O3 is added to some materials such as silica[5,6]. Thus, it is supposed to improve the optical properties such as refractive index and reflectivity by alloying of Er2O3 and Al2O3 to form Er2O3-Al2O3 (ErAlO) films. Up to now, there has been no report about the growth of ErAlO films on Si substrates. In this work, ErAlO films were prepared by radio frequency (RF) magnetron sputtering on Si (001) substrates. Eellipsometry measurements were used to obtain the optical constants. All the results indicated that it could be a useful material for solar cells.

Sample was grown on 1.5 in. p-type polished Si (001) wafer with resistivity of 2–10 ȍ·cm. ErAlO film with thickness of 73 nm was deposited by radio frequency (RF) magnetron sputtering with an (Er2O3)0.7(Al2O3)0.3 composite ceramic target at room temperature. The base pressure of the sputter chamber was about 4.0×10í4 Pa and the deposition was done in an argon gas atmosphere with 1.33 Pa. The substrate was parallel to the target surface and rotating during the growth process to obtain uniform film. After growth, the sample was annealed at 450, 600 and 750 ºC for 30 min in 1.01×105 Pa O2 ambience, respectively. The optical constants of the films were measured in the wavelength range from 400 to 1000 nm by a WVASE Spectroscopic Ellipsometry (SE) from J. A. Woollam Inc. The ellipsometric measurements were performed using a variable angle. In general, we would like to look at angles of incidence where we have maximum difference in reflectivity between p and s polarization states. Angles near Brewster’s angle are optimal. Brewster’s angle is determined by the index of the top surface, and is generally 55–60 degrees for low index dielectrics, and near 75 degrees for semiconductors. Therefore, the ellipsometric spectra were recorded at angles of incidence near the Brewster angle of Si. For the fitting of the measured ellipsometric spectra and for the simulation of in situ real-time ellipsometric parameters ȥ and ǻ, the WVASE32 program was employed.

1 Experimental

Spectroscopic ellipsometry is an optical technique which measures the changes of the polarization state of a polarized

2 Results and discussion 2.1 Ellipsometric parameters ȥ and ǻ

Foundation item: Project supported by the National Natural Science Foundation of China (11004130 and 60806031), and the Key Fundamental Project of Shanghai (10JC1405900) Corresponding author: ZHU Yanyan (E-mail: [email protected]; Tel.: +86-21-65430410) DOI: 10.1016/S1002-0721(10)60578-9

ZHU Yanyan et al., Optical constants of Er2O3-Al2O3 films studied by spectroscopic ellipsometry

light beam after reflection from the sample under study. These changes are measured as the ellipsometric parameters ȥ and ǻ which are related to the ratio of the effective reflection coefficients and for p-and s-polarized light: < rp > S= = tan(ȥ )exp(iǻ) (1) < rs > where ȡ is the complex reflectance ratio. and are functions of the complex refractive indices (Fresnel coefficients) of the films, the substrate and their thicknesses. In order to extract useful information about a sample, the experimental data are compared with data generated using a model which describes the structure of the sample and its optical response. The unknown parameters in the model are adjusted in order to get the best match between the model and the experimental data. The Cauchy model was chosen for ¿tting the dispersive optical constants to the ellipsometric data for the transparent ErAlO films. Figs. 1(a) and (b) show the measured and modelled values of the ellipsometric parameters Ȍ and ¨ for the as-deposited film. The excellent agreement between them indicates that it is not necessary to model inhomogeneity (gradient) in refractive index of the ErAlO layer, as has been required by other researchers[7]. However, differences are found for the data of Ȍ and ¨ with different incident angles of 70° and 75°, as shown in Fig. 1. It reveals that the as-deposited film and the films annealed in O2 have an interface for the following reasons. The optical properties can vary as a function of depth because the p- and s-components reflect differently from the interface depending on the angle of incidence. As the angle of incidence (to the interface normal) increases, steadily increases from a small value but starts out at a small value and then decreases until it reaches zero at the Brewster angle, șB, before increasing

Fig. 1 Measured and modelled values for Ȍ (a) and ¨ (b) for the asdeposited ErAlO film on Si substrate

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again[8]. The results from Fig. 1 are consistent with our previous study about the structures of ErAlO films, which confirmed that the film contain an interfacial layer of SiOx and ErOx (ErO or any other nonstoichiometric phase)[3]. At the initial growth stage, oxygen atoms react with both Si, Er and Al atoms, thus it is natural to form an ErAlO layer lack of oxygen (ErOx) at the interface. Obviously, the data of Ȍ and ¨ obtained from incident angles of 70° comes mainly from the ErAlO layer. Figs. 2 (a) and (b) show the results for Ȍ and ¨ data as a function of incident wavelength for incident angles of 70°. No oscillations is observed for either the as-deposited films or the annealed films, indicating that the film thicknesses are small and uniform[6]. 2.2 Refractive index n and the extinction coefficient k Fig. 3(a) displays the refractive index n as a function of incident wavelength obtained from the ErAlO film on Si (001) substrate by SE. For the annealed samples, the data n decreases and then increases with annealing temperatures increasing. This trend is inconsistent with other optical materials such as TiO2 who exhibit a trend of increasing with higher annealing temperatures[6]. It may be attributed to the variation in the deposition method and annealing processes because thedifferent stoichiometries and structures of the sample influence greatly on the optical properties. As the extinction coefficient k, the data is maintained at very low values which can be negligible, as shown in Fig. 3(b), indicating that the films are all transparent for the wavelength from 400 to 1700 nm, due to the fact that the extinction coefficient k is related to the decay, or damping of

Fig. 2 ¨ (a) and Ȍ (b) obtained with incident angles of 70° for the asdeposited samples of ErAlO on Si substrates

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JOURNAL OF RARE EARTHS, Vol. 29, No. 10, Oct. 2011

and ambient medium, the minimum reÀectance at 600 nm will be obtained. To achieve zero reÀectance at one wavelength, the value of nAR should be

nAR

n0 nsi

(2)

and the ¿lm thickness (dAR) must meet the quarter-wave optical thickness requirement d AR

O0 4nAR

(3)

This indicates that an AR coating for a silicon solar cell in air should have a refractive index of 1.985 at about 600 nm and a thickness of 75.6 nm. Therefore, the ErAlO film need more design to obtain an optimal data for a solar cell with air surrounding medium.

3 Conclusions

Fig. 3 Measured values of the refractive index n (a) and the extinction coefficient k (b) as a function of incident wavelength by SE

the oscillation amplitude of the incident electric field. A trend of decreasing in the value of k with increasing annealing temperatures is observed in Fig. 3(b), showing that the annealing process can improve the transparency of the film and then improve transmission. Although there is small discrepancy in both curves k and n, no peaks are observed in the whole spectral range of SE for the annealed samples indicating that the direct energy gap of this ErAlO film must be out of the incident photon energy of our SE measurement because the peaks in curves of n and k versus photon energy correspond to a direct photoexitation process of electrons excited from the valence band to the conduction one. Clearly, the band gap for the annealed film will be larger than that of the as-deposited one. It may be due to the less state in the forbidden band after being annealed. The semiconducting materials used in solar cells generally exhibit high refractive indices. For example, silicon has a refractive index of nsi=3.939 at 600 nm. This refractive index is much greater than air, which has a constant refractive index of n0=1.0. Therefore, 35.4% of the light is reÀected from an air:silicon interface in the ¿rst bounce. A single layer AR coating is the minimum requirement for any solar cell produced today so that light is well absorbed by semiconducting materials such as silicon. Furthermore, a single-layer AR coating should exhibit a minimum reÀectance at about 600 nm in order to take full advantage of the peak in working spectral range of the Si solar cells (550í750 nm). If an optimum-thickness AR coating is inserted between the silicon

ErAlO films were deposited on Si (001) substrates by radio frequency magnetron technique. The study on optical properties showed proper values of refractive index and a lower extinction coefficient for the ErAlO film, indicating that it could be a useful material in solar cells.

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