Effect of Deposition Working Power on Physical Properties of RF-Sputtered CdTe Thin Films for Photovoltaic Applications

Abstract

The main objective of this study was to determine the variation in the properties of cadmium telluride (CdTe) thin films deposited on a p-type Si substrate by the radio frequency magnetron sputtering technique at four different working powers (70 W, 80 W, 90 W, and 100 W). The substrate temperature, working pressure, and deposition time during the deposition process were kept constant at 220 °C, 0.46 Pa, and 30 min, respectively. To study the structural, morphological, and optical properties of the CdTe films grown under the mentioned experimental conditions, X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), and optical spectroscopy were used. For a better analysis of the films' structural and optical properties, a group of films were deposited onto optical glass substrates under similar deposition conditions. The electrical characterisation of Ag/CdTe/Al "sandwich" structures was also performed using current-voltage characteristics in the dark at different temperatures. The electrical measurements allowed the identification of charge transport mechanisms through the structure. New relevant information released by the present study points towards 90 W RF power as the optimum for obtaining a high crystallinity of ~1 μm nanostructured thin films deposited onto p-Si and optical glass substrates with optical and electrical properties that are suitable for use as absorber layers. The obtained high-quality CdTe nanostructured thin films are perfectly suitable for use as absorbers in CdTe thin-film photovoltaic cells.

Keywords: RF–magnetron sputtering (RF–MS); cadmium telluride (CdTe) thin films; current–voltage measurements; physical properties.

Figure 1

Schematic representation of the Ag/CdTe/Al “sandwich” structure.

Figure 2

GIXRD patterns of CdTe nanostructured thin films deposited at different RF powers.

Figure 3

Deconvolution using Voigt profiles of the (111) diffraction peak corresponding to (a) CdTe_70W; (b) CdTe_80W; (c) CdTe_90W; and (d) CdTe_100W deposited by RF–magnetron sputtering on a p-Si substrate. The peak XRD profiles were recorded in Bragg–Brentano theta–theta geometry. The experimental data have been processed with the Voigt function, and the associated residual is shown at the bottom of each graph.

Figure 4

Cross-section scanning electron microscope (SEM) micrographs of the CdTe nanostructured thin films sputtered on p-Si substrate for 30 min, at the following working RF powers: (a) 70 W; (b) 80 W; (c) 90 W; and (d) 100 W. Corresponding estimated thickness SEM measurements for the CdTe nanostructured thin films are indicated.

Figure 5

Variation in CdTe nanostructured thin films’ thickness as a function of RF plasma power, under the manufacturing sputtering conditions specified in the text.

Figure 6

The 2D and corresponding 3D atomic force microscope (AFM) images of the CdTe nanostructured thin films sputtered at various RF powers for the scanned areas of 10 × 10 μm² (a), 5 × 5 μm² (b), and 1 × 1 μm² (c).

Figure 7

Root mean square (RMS) roughness as a function of the different RF powers for the sputtered CdTe nanostructured thin films, at the three scanned areas of 10 × 10 μm² (blue triangles), 5 × 5 μm² (red circles), and 1 × 1 μm² (black squares).

Figure 8

Optical transmission spectra of CdTe nanostructured thin films deposited on optical glass substrates.

Figure 9

Determination of thicknesses corresponding to CdTe nanostructured thin films sputtered onto optical glass substrates at the RF plasma power of 70 W (a), 80 W (b), 90 W (c), and 100 W (d).

Figure 10

(a–d) Plots of (αhυ)² vs. hυ for CdTe nanostructured thin films deposited on optical glass at different RF powers.

Figure 11

Current density–voltage (J-V) dark ambipolar characteristic recorded at a temperature of 316 K, for the Ag/CdTe/Al “sandwich” structure.

Figure 12

Logarithmic fit of ln J versus ln U for the Ag/CdTe/Al structure at forward bias, emphasizing the two linear regions corresponding to different conduction mechanisms through the structure, separated by the transition voltage.

Figure 13

Logarithmic J_S as a function of the U^1/2 characteristic at reverse bias that allows the determination of the depletion layer $w$.

Figure 14

(a) Logarithmic J_S vs. U^1/2 characteristics recorded at four temperatures, 316K, 324K, 343K, and 364K, that allowed the evaluation of the Schottky barrier at the Al/CdTe interface as $\mathsf{\Phi }=0.47 \mathrm{e}\mathrm{V}$ by using the displayed (b) dependence of $\ln \left(\frac{{\mathrm{J}}_{\mathrm{s}0}}{{\mathrm{T}}^2}\right)$ $\mathrm{v}\mathrm{s}.$ $\left(\frac{{10}^3}{\mathrm{T}}\right)$.