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Effect of sputtering power on the properties of Cd1 − xZnxTe films deposited by radio frequency magnetron sputtering
Thin Solid Films 519 (2011) 4158–4161
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Effect of sputtering power on the properties of Cd1 − xZnxTe films deposited by radio frequency magnetron sputtering Dongmei Zeng a,b,⁎, Wanqi Jie b, Hai Zhou a, Yingge Yang a a b
Department of Materials Science and Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, PR China State Key Laboratory of Solidification Processing, Northwestern Polytechnical, University, Xi'an 710072, PR China
a r t i c l e
i n f o
Article history: Received 11 June 2010 Received in revised form 24 January 2011 Accepted 28 January 2011 Available online 5 March 2011 Keywords: Cd1–xZnxTe films Thin films Radio-Frequency Magnetron Sputtering Sputtering power
a b s t r a c t Cd1 − xZnxTe films were prepared by radio frequency (r.f.) magnetron sputtering from Cd0.9Zn0.1Te slices target with different sputtering power (60–120 W). The effects of sputtering power on the properties of Cd1 − xZnxTe films were studied using X-ray diffraction (XRD), energy dispersive X-ray (EDX), atomic force microscopy (AFM), ultraviolet spectrophotometer and Hall effect measurements. The composition of the deposited films was determined by EDX. The Cd content was found always to be higher than the Te content, regardless of sputtering power. This behavior may be explained by the preferential sputtering of cadmium atoms in the target. XRD studies suggest that ZnTe secondary phases were coexisted in Cd1 − xZnxTe films. The origin of the secondary phase is ascribed to the lowest sticking coefficient of Zn atom. AFM micrographs show that the grain size increases with the sputtering power. The optical transmission data indicate that shallow absorption edge occurs in the range of 750–850 nm, and the sputtering power does not have a clear effect on the optical absorption coefficient. In Hall Effect measurements, the sheet resistivities of the deposited films are 1.988 × 108, 8.134 × 107, 8.088 × 107 and 3.069 × 107 Ω/sq, respectively, which increase with the increasing of sputtering power. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Polycrystalline Cd1 − xZnxTe films have received much attention because they exhibit high quantum efficiency and very low leakage currents and have great potential applications in photoelectronic devices such as solar cells, electro-optical modulators, photoconductors, light emitting diodes, X-ray and gamma ray detectors etc. [1–4] Polycrystalline Cd1 − xZnxTe films have been grown by several methods such as radio frequency (r.f.) magnetron sputtering, closed space sublimation, electrodeposition and metal-organic chemical vapor deposition [5–8]. Among the methods utilized for deposition of Cd1 − xZnxTe films, the sputtering technique offers in principle a simple and flexible control of the film stoichiometry over a large scale at relatively low cost, thus having a high potential for industrial applications. Cd1 − xZnxTe films grown by r.f. magnetron sputtering were obtained from the targets made either of mixed powders or alloys of CdTe and ZnTe [9–14]. As an alternative choice, we have grown polycrystalline Cd1 − xZnxTe films from CdZnTe slices target. In this paper, we focus on the preparation and characterization of polycrystalline Cd1 − xZnxTe films for X-ray detectors, where attention was paid to the influence of the growth parameters on the physical properties of the prepared ⁎ Corresponding author at: Department of Materials Science and Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, PR China. Tel./fax: +86 010 81292214. E-mail address: [email protected] (D. Zeng). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.01.378
materials. In our previous paper, we have reported the effects of the deposition temperature on structure and physical properties of Cd1 − xZnxTe films which were prepared by r.f. magnetron sputtering using Cd0.9Zn0.1Te crystals target [15]. Here, we investigate the influence of the sputtering power on the structural, optical, as well as electrical properties of the sputtered Cd1 − xZnxTe films, in order to master the characteristics of Cd1 − xZnxTe films prepared by r.f. sputtering.
2. Experimental details Radio frequency magnetron sputtering system was used to deposit Cd1 − xZnxTe films on glass substrates. The sputtering target was cut from Cd0.9Zn0.1Te ingot with diameter of a 60 mm, which was grown by the modified vertical Bridgman method in the State Key Laboratory of Solidification Processing. The details of the crystal growth procedure can be seen in some references [16–18]. Glass substrate was placed parallel to the target surface with a vertical distance of 4 cm. The glass substrate was cleaned with trichloroethylene and acetone to remove grease and organic contaminations. Considering the tolerable temperature of glass slide and melting point of elemental cadmium, the substrate temperature was set at 300 °C. The sputtering chamber was evacuated to a pressure less than 8.0 × 10− 4 Pa. Argon (99.999%) was admitted under control through a needle valve. The sputtering pressure was fixed at 1.5 Pa and the deposition time was 1 h. The sputtering power was varied from 60 to 120 W. The
D. Zeng et al. / Thin Solid Films 519 (2011) 4158–4161
deposition process has good reproducibility, so, we selected four different representative films deposited at different sputtering power. The structure of the deposited Cd1 − xZnxTe films was studied using X-ray diffraction (XRD). A Shimadzu XRD 7000 maxima powder diffraction was used at the conditions "tube voltage 40 KV, current 30 mA and wavelength Cu Kα1, λ = 0.154178 nm ". The films morphology was investigated by atomic force microscopy (AFM). A Shimedozu SPM9500-J3 using a Si3N4 cantilever tip, operated in contact mode, with the cantilever frequency of 24 kHz, and force constant of 0.15 N/m was performed. The scans were performed in air. The images were acquired at 256 × 256 pixels. Quantitative elemental analysis on the films was carried out by a scanning electron microscope (Shimadzu, superscan SSX-550) equipped with energy dispersive X-ray (EDX) analysis. EDX resolution is 10 eV/Ch, and the acceleration voltage is 15 kV. The film thickness was determined by a step profiler (Profilometer XP-1; AMBIOS). At room temperature, the linear transmission spectra of the films were recorded on UV-3501 S spectrophotometer in the 200–1100 nm region. The electrical properties were measured by Hall effect measurements at room temperature (ACCENT HL5500PC). The vacuum evaporated Au electrode formed the top metal contact. 3. Results and discussion 3.1. Deposition rate and film composition Fig. 1 shows the variations of the deposition rate of the films as a function of the sputtering power. The deposition rate increases with the increasing of the sputtering power. It can be reasoned that higher sputtering power means more argon ions in the plasma and the bombardment on the target increased accordingly. Table 1 shows the compositions of the deposited films under different sputtering power. The composition of the crystal target was 50 at.% Te, 48 at.% Cd, and 2 at.% Zn. It has been found that the Cd concentration was higher than that of Te in all films. This behavior may be explained by the preferential sputtering of Cd atoms in the Cd0.9Zn0.1Te target. From the factor of energy transfer given by Winters and Sigmund [19], 2
γ = 4M1 ⋅M2 = ðM1 + M2 Þ
ð1Þ
where M1 is the incident ion mass (Ar+) and M2 is the target atom mass, we find: γAr-Zn = 0.94, γAr-Cd =0.78 and γAr-Te =0.73. There was a difference in the energy transfer efficiency among the target atoms. Considering that the zinc concentration in the crystal target is very low, the Zn atomic energy transfer efficiency was ignored here and was supposed to have little effect on the composition of the films. The factor of energy transfer of Cd (γAr-Cd = 0.78) under argon plasma is higher than that of the Te atoms (γAr-Te =0.73), thus caused Cd excess in the films.
Fig. 1. The effect of sputtering power on the deposition rate of Cd1 − xZnxTe films.
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Table 1 The composition of the films deposited with different sputtering power determined by EDX. Sputtering power (W)
Cadmium (at.%)
Zinc (at.%)
Tellurium (at.%)
60 80 100 120
49 48 47 48
10 10 10 11
42 42 43 41
Note: The relatively error of EDX is 1%.
3.2. Structural properties Fig. 2 shows the XRD patterns of the films obtained with the sputtering powers of 60, 80, 100 and 120 W. All films show polycrystalline and preferred [111] orientation. The diffraction peaks appeared at 2θ = 23.85°, 39.56°, and 46.78° have been identified as the (111), (220) and (311) diffraction peaks of Cd1 − xZnxTe. Beside the (111), (220) and (311) diffraction lines of Cd1 − xZnxTe in the samples, two small peak also appear at 25.69° and 42.98° which could be attributed to (111) and (220) plane of the binary ZnTe. It seems that ZnTe phase was coexisted with the ternary Cd1 − xZnxTe. It is reasonable to explain this phenomenon with the desorption of Zn. The Zn, because it has a lowest sticking coefficient than Cd and Te, could be physically adsorbed on the surface of the films. During the deposition process, Zn on the films surface desorbs in the binary of ZnTe alloy with the decrease of the temperature [20]. According to the XRD patterns of Cd1 − xZnxTe films prepared at different sputtering powers (Fig. 2), the (111) diffraction peak (2θ) of Cd1 − xZnxTe films prepared at 60, 80, 100 and 120 W is appeared at 24.02°, 23.98°, 23.91° and 23.85°, respectively. The (111) peak of Cd1 − xZnxTe shifts towards the lower angle with the increasing of sputtering powers. The full width at half maximum (FWHM) of diffraction peak is the width (in degrees) at half the maximum peak intensity. The sharp diffraction line of (111) plane indicates a good crystalline quality. To illustrate the polycrystalline systematically, we plotted the FWHM of the (111) reflections versus various sputtering powers in Fig. 3. The FWHM of the (111) plane of Cd1 − xZnxTe films decreases with the increasing of the sputtering power. The high power sputtered films have high thickness. In the samples which are sufficiently thick, there is much more matter which can diffract X-rays and the peak is more intense. This means that the crystallinity is improved when the film thickness increases. On the other hand, the increase of sputtering power can lead to the increase of the kinetic energy of the sputtered particles, which causes the increase of the particles' mobility in the surface of the film. The particles under large driving force can migrate to more suitable lattice sites and adjust their own bond direction and length to obtain optimum bonding to the adjacent ones, which are
Fig. 2. X-ray diffraction patterns for Cd1 − xZnxTe films deposited at different sputtering power. (a) 60 W; (b) 80 W; (c) 100 W; (d) 120 W.
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The films deposited at low sputtering power (60 W) showed pebblelike crystal grains with the mean diameter of 130 nm. When the sputtering power increases from 80 to 100 W, there is no big difference in the grain size and the morphology of the films. As shown in Fig. 4b and c, the morphology of the grains is found to be polygonal shape; the grain size of the films was about 230 nm. The films deposited at the sputtering power of 120 W have an irregularly prismatic structure with the average grain size of 300 nm. On the whole, the crystallite sizes increase with the increasing of the sputtering power because the sputtered particles can obtain more energy prior to collision with the substrates. 3.4. Optical properties Fig. 3. Variation of the FWHM of Cd1 − xZnxTe (111) peak as a function of the sputtering power.
helpful for nucleation and growth, and consequently improved the crystallinity.
Fig. 5(a) shows the transmission spectra for the Cd1 − xZnxTe films grown at various sputtering powers from 60 to 120 W. All spectra of the films show a well-defined absorption edge in the range of 750– 850 nm. The transmittance data were used to calculate absorption coefficients of the Cd1 − xZnxTe films at different wavelengths. The absorption coefficient,α, is given by the relation [21]:
3.3. Surface morphology
I = I0 expð−α⋅dÞ
Fig. 4 shows the surface morphologies of the Cd1 − xZnxTe samples deposited at sputtering power of 60, 80, 100 and 120 W, respectively.
where I is the intensity of transmitted light, I0 is the intensity of incident light, and d is the thickness of Cd1 − xZnxTe films. The
Fig. 4. AFM micrographs (2.5 μm × 2.5 μm) for the Cd1 − xZnxTe films deposited at sputtering power of 60 W (a), 80W (b), 100W (c) and 120W (d).
ð2Þ
D. Zeng et al. / Thin Solid Films 519 (2011) 4158–4161
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Fig. 6. The sheet resistivities, mobility and carrier concentrations as a function of sputtering power.
4. Conclusions
Fig. 5. (a). Transmittance spectra of films deposited at different sputtering powers. (b). The optical absorption coefficient of four films deposited at different sputtering powers.
thicknesses of the films deposited with different sputtering power are 530, 783, 823 and 956 nm, respectively. As the transmittance is defined as I/I0, we can obtain α from Eq. (2), as shown in Fig. 5(b). There are no clear relations between the optical absorption coefficient and the effect of sputtering power. 3.5. Electrical properties The electrical properties of Cd1 − xZnxTe films were determined by Hall effect measurements. Fig. 6 shows the variations of sheet resistivity, carrier concentration and mobility of Cd1 − xZnxTe films with respect to the sputtering power. The carrier types of the four samples are p-type. The sheet resistivities of the deposited films were 1.988 × 108, 8.134 × 107, 8.088 × 107 and 3.069 × 107 Ω/sq, respectively. As the sputtering power increases from 60 to 120 W, the sheet resistivity decreased and the carrier concentration and mobility both increased. The increase of conductivity can be attributed to the improvement of crystallinity. When the sputtering power increases from 60 to 120 W, the crystallite size increases and the crystallinity is improved. A larger crystallite size results in a lower density of grain boundaries, which behave as traps for free carriers and barriers for carrier transport. Hence, an increase in grain size can cause a decrease in grain-boundary scattering and an increase of carrier lifetime [22,23] and consequently leads to an increase of conductivity due to the increase in both carrier concentration and Hall mobility. With the improvement of crystallinity, the concentration of electrically active donor sites is improved [24], which can also increase the carrier concentration.
Cd1 − xZnxTe films were obtained by r.f. magnetron sputtering with different sputtering powers (60–120 W). As the sputtering power increases, the deposition rate of the films also increases. The Cd concentration in the films was higher than that of Te regardless of sputtering power. The crystallinity of the films increases as the deposition power increases. For the films prepared at the sputtering power higher than 80 W, ZnTe secondary phases were observed. The average grain size increased with the increase of the sputtering power. The films exhibit shallow absorption edge in the transmission spectra, and the sputtering power seems to have no clear effect on the optical absorption coefficient. By increasing the sputtering power, the sheet resistivity of the films decreased, due to the larger crystallite size and the improvement of crystallinity. Acknowledgement This work was supported by the fund of the State Key Laboratory of Solidification Processing in NWPU (Grant no.SKLSP201005) and National Natural Science Foundation of China (51002012). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
T.L. Chu, S.S. Chu, C. Ferekides, J. Britt, J. Appl. Phys. 71 (11) (1992) 5635. M. Singh, E. Mumcuoglu, IEEE Trans. Nucl. Sci. 45 (1998) 1158. R.B. James, R.C. Schiratio (Eds.), SPIE Proc., vol. 3768, Denver Colorado, July 1999. M.J.A. van Pamelen, C. Budtz-Jorgensen, Nucl. Instrum. Methods 403 (1998) 390. Anu Bansal, P. Rajaram, Mater. Lett. 59 (2005) 3666. Bulent M. Basol, Vijay K. Kapur, Michael L. Ferris, J. Appl. Phys. 66 (1989) 1816. T.L. Chu, S.S. Chu, C. Ferekides, J. Britt, J. Appl. Phy. 71 (1992) 5623. K. Prasada Rao, O.Md. Hussain, K.T.R. Reddy, P.S. Reddy, S. Uthanna, B.S. Naidu, P. Jayarama Reddy, J. Alloys Compd. 218 (1995) 86. C.S. Ferekides, R. Mamazza, U. Balasubramanian, D.L. Morel, Thin Solid Films 480–481 (2005) 471. H. Neff, K.Y. Lay, M.S. Su, P. Lange, K.J. Bachmann, Surf. Sci. 189–190 (1987) 661. S.H. Lee, A. Gupta, S. Wang, Alvin D. Compaan, Brian E. McCandless, Sol. Energy Mater. Sol. Cells 86 (2005) 551. A. Arbaoui, A. Outzourhit, N. Achargui, H. Bellakhder, E.L. Ameziane, J.C. Bernede, Sol. Energy Mater. Sol. Cells 90 (2006) 1364. P. Banerjee, R. Ganguly, B. Ghosh, Appl. Surf. Sci. 256 (2009) 213. M. Becerril, H. Silva-L'opez, O. Zelaya-Angel, Rev. Mex. Fis. 50 (2004) 588. D. Zeng, W. Jie, H. Zhou, Y. Yang, Nucl. Instrum. Methods Phys. Res. A 614 (2010) 68. D. Zeng, W. Jie, T. Wang, W. Li, J. Zhan, Mater. Sci. Eng., B 142 (2007) 144. G. Li, W. Jie, Z. Gu, H. Hua, J. Cryst. Growth 263 (2004) 332. B. Li, J. Zhu, X. Zhang, J. Chu, J. Cryst. Growth 181 (1997) 204. H.F. Winters, P. Sigmund, J. Appl. Phys. 45 (1974) 4760. A. Arbaoui, A. Outzourhit, N. Achargui, H. Bellakhder, E.L. Ameziane, J.C. Bernede, Sol. Energy Mater. Sol. Cells 90 (2006) 1364. R. Swanepoel, J. Phys. E: Sci. Instrum. 16 (1983) 214. H. Kim, J.S. Horwitz, G. Kushto, A. Piqué, Z.H. Kafafi, C.M. Gilmore, D.B. Chrisey, J. Appl. Phys. 88 (2000) 6021. X.T. Hao, J. Ma, D.H. Zhang, T.L. Yang, H.L. Ma, Y.G. Yang, C.F. Cheng, J. Huang, Appl. Surf. Sci. 183 (2001) 137. Y.S. Jung, J.Y. Seo, D.W. Lee, D.Y. Jeon, Thin Solid Films 445 (1) (2003) 63.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Effect of sputtering power on the properties of Cd1 − xZnxTe films deposited by radio frequency magnetron sputtering Dongmei Zeng a,b,⁎, Wanqi Jie b, Hai Zhou a, Yingge Yang a a b
Department of Materials Science and Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, PR China State Key Laboratory of Solidification Processing, Northwestern Polytechnical, University, Xi'an 710072, PR China
a r t i c l e
i n f o
Article history: Received 11 June 2010 Received in revised form 24 January 2011 Accepted 28 January 2011 Available online 5 March 2011 Keywords: Cd1–xZnxTe films Thin films Radio-Frequency Magnetron Sputtering Sputtering power
a b s t r a c t Cd1 − xZnxTe films were prepared by radio frequency (r.f.) magnetron sputtering from Cd0.9Zn0.1Te slices target with different sputtering power (60–120 W). The effects of sputtering power on the properties of Cd1 − xZnxTe films were studied using X-ray diffraction (XRD), energy dispersive X-ray (EDX), atomic force microscopy (AFM), ultraviolet spectrophotometer and Hall effect measurements. The composition of the deposited films was determined by EDX. The Cd content was found always to be higher than the Te content, regardless of sputtering power. This behavior may be explained by the preferential sputtering of cadmium atoms in the target. XRD studies suggest that ZnTe secondary phases were coexisted in Cd1 − xZnxTe films. The origin of the secondary phase is ascribed to the lowest sticking coefficient of Zn atom. AFM micrographs show that the grain size increases with the sputtering power. The optical transmission data indicate that shallow absorption edge occurs in the range of 750–850 nm, and the sputtering power does not have a clear effect on the optical absorption coefficient. In Hall Effect measurements, the sheet resistivities of the deposited films are 1.988 × 108, 8.134 × 107, 8.088 × 107 and 3.069 × 107 Ω/sq, respectively, which increase with the increasing of sputtering power. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Polycrystalline Cd1 − xZnxTe films have received much attention because they exhibit high quantum efficiency and very low leakage currents and have great potential applications in photoelectronic devices such as solar cells, electro-optical modulators, photoconductors, light emitting diodes, X-ray and gamma ray detectors etc. [1–4] Polycrystalline Cd1 − xZnxTe films have been grown by several methods such as radio frequency (r.f.) magnetron sputtering, closed space sublimation, electrodeposition and metal-organic chemical vapor deposition [5–8]. Among the methods utilized for deposition of Cd1 − xZnxTe films, the sputtering technique offers in principle a simple and flexible control of the film stoichiometry over a large scale at relatively low cost, thus having a high potential for industrial applications. Cd1 − xZnxTe films grown by r.f. magnetron sputtering were obtained from the targets made either of mixed powders or alloys of CdTe and ZnTe [9–14]. As an alternative choice, we have grown polycrystalline Cd1 − xZnxTe films from CdZnTe slices target. In this paper, we focus on the preparation and characterization of polycrystalline Cd1 − xZnxTe films for X-ray detectors, where attention was paid to the influence of the growth parameters on the physical properties of the prepared ⁎ Corresponding author at: Department of Materials Science and Engineering, Beijing Institute of Petrochemical Technology, Beijing 102617, PR China. Tel./fax: +86 010 81292214. E-mail address: [email protected] (D. Zeng). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.01.378
materials. In our previous paper, we have reported the effects of the deposition temperature on structure and physical properties of Cd1 − xZnxTe films which were prepared by r.f. magnetron sputtering using Cd0.9Zn0.1Te crystals target [15]. Here, we investigate the influence of the sputtering power on the structural, optical, as well as electrical properties of the sputtered Cd1 − xZnxTe films, in order to master the characteristics of Cd1 − xZnxTe films prepared by r.f. sputtering.
2. Experimental details Radio frequency magnetron sputtering system was used to deposit Cd1 − xZnxTe films on glass substrates. The sputtering target was cut from Cd0.9Zn0.1Te ingot with diameter of a 60 mm, which was grown by the modified vertical Bridgman method in the State Key Laboratory of Solidification Processing. The details of the crystal growth procedure can be seen in some references [16–18]. Glass substrate was placed parallel to the target surface with a vertical distance of 4 cm. The glass substrate was cleaned with trichloroethylene and acetone to remove grease and organic contaminations. Considering the tolerable temperature of glass slide and melting point of elemental cadmium, the substrate temperature was set at 300 °C. The sputtering chamber was evacuated to a pressure less than 8.0 × 10− 4 Pa. Argon (99.999%) was admitted under control through a needle valve. The sputtering pressure was fixed at 1.5 Pa and the deposition time was 1 h. The sputtering power was varied from 60 to 120 W. The
D. Zeng et al. / Thin Solid Films 519 (2011) 4158–4161
deposition process has good reproducibility, so, we selected four different representative films deposited at different sputtering power. The structure of the deposited Cd1 − xZnxTe films was studied using X-ray diffraction (XRD). A Shimadzu XRD 7000 maxima powder diffraction was used at the conditions "tube voltage 40 KV, current 30 mA and wavelength Cu Kα1, λ = 0.154178 nm ". The films morphology was investigated by atomic force microscopy (AFM). A Shimedozu SPM9500-J3 using a Si3N4 cantilever tip, operated in contact mode, with the cantilever frequency of 24 kHz, and force constant of 0.15 N/m was performed. The scans were performed in air. The images were acquired at 256 × 256 pixels. Quantitative elemental analysis on the films was carried out by a scanning electron microscope (Shimadzu, superscan SSX-550) equipped with energy dispersive X-ray (EDX) analysis. EDX resolution is 10 eV/Ch, and the acceleration voltage is 15 kV. The film thickness was determined by a step profiler (Profilometer XP-1; AMBIOS). At room temperature, the linear transmission spectra of the films were recorded on UV-3501 S spectrophotometer in the 200–1100 nm region. The electrical properties were measured by Hall effect measurements at room temperature (ACCENT HL5500PC). The vacuum evaporated Au electrode formed the top metal contact. 3. Results and discussion 3.1. Deposition rate and film composition Fig. 1 shows the variations of the deposition rate of the films as a function of the sputtering power. The deposition rate increases with the increasing of the sputtering power. It can be reasoned that higher sputtering power means more argon ions in the plasma and the bombardment on the target increased accordingly. Table 1 shows the compositions of the deposited films under different sputtering power. The composition of the crystal target was 50 at.% Te, 48 at.% Cd, and 2 at.% Zn. It has been found that the Cd concentration was higher than that of Te in all films. This behavior may be explained by the preferential sputtering of Cd atoms in the Cd0.9Zn0.1Te target. From the factor of energy transfer given by Winters and Sigmund [19], 2
γ = 4M1 ⋅M2 = ðM1 + M2 Þ
ð1Þ
where M1 is the incident ion mass (Ar+) and M2 is the target atom mass, we find: γAr-Zn = 0.94, γAr-Cd =0.78 and γAr-Te =0.73. There was a difference in the energy transfer efficiency among the target atoms. Considering that the zinc concentration in the crystal target is very low, the Zn atomic energy transfer efficiency was ignored here and was supposed to have little effect on the composition of the films. The factor of energy transfer of Cd (γAr-Cd = 0.78) under argon plasma is higher than that of the Te atoms (γAr-Te =0.73), thus caused Cd excess in the films.
Fig. 1. The effect of sputtering power on the deposition rate of Cd1 − xZnxTe films.
4159
Table 1 The composition of the films deposited with different sputtering power determined by EDX. Sputtering power (W)
Cadmium (at.%)
Zinc (at.%)
Tellurium (at.%)
60 80 100 120
49 48 47 48
10 10 10 11
42 42 43 41
Note: The relatively error of EDX is 1%.
3.2. Structural properties Fig. 2 shows the XRD patterns of the films obtained with the sputtering powers of 60, 80, 100 and 120 W. All films show polycrystalline and preferred [111] orientation. The diffraction peaks appeared at 2θ = 23.85°, 39.56°, and 46.78° have been identified as the (111), (220) and (311) diffraction peaks of Cd1 − xZnxTe. Beside the (111), (220) and (311) diffraction lines of Cd1 − xZnxTe in the samples, two small peak also appear at 25.69° and 42.98° which could be attributed to (111) and (220) plane of the binary ZnTe. It seems that ZnTe phase was coexisted with the ternary Cd1 − xZnxTe. It is reasonable to explain this phenomenon with the desorption of Zn. The Zn, because it has a lowest sticking coefficient than Cd and Te, could be physically adsorbed on the surface of the films. During the deposition process, Zn on the films surface desorbs in the binary of ZnTe alloy with the decrease of the temperature [20]. According to the XRD patterns of Cd1 − xZnxTe films prepared at different sputtering powers (Fig. 2), the (111) diffraction peak (2θ) of Cd1 − xZnxTe films prepared at 60, 80, 100 and 120 W is appeared at 24.02°, 23.98°, 23.91° and 23.85°, respectively. The (111) peak of Cd1 − xZnxTe shifts towards the lower angle with the increasing of sputtering powers. The full width at half maximum (FWHM) of diffraction peak is the width (in degrees) at half the maximum peak intensity. The sharp diffraction line of (111) plane indicates a good crystalline quality. To illustrate the polycrystalline systematically, we plotted the FWHM of the (111) reflections versus various sputtering powers in Fig. 3. The FWHM of the (111) plane of Cd1 − xZnxTe films decreases with the increasing of the sputtering power. The high power sputtered films have high thickness. In the samples which are sufficiently thick, there is much more matter which can diffract X-rays and the peak is more intense. This means that the crystallinity is improved when the film thickness increases. On the other hand, the increase of sputtering power can lead to the increase of the kinetic energy of the sputtered particles, which causes the increase of the particles' mobility in the surface of the film. The particles under large driving force can migrate to more suitable lattice sites and adjust their own bond direction and length to obtain optimum bonding to the adjacent ones, which are
Fig. 2. X-ray diffraction patterns for Cd1 − xZnxTe films deposited at different sputtering power. (a) 60 W; (b) 80 W; (c) 100 W; (d) 120 W.
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The films deposited at low sputtering power (60 W) showed pebblelike crystal grains with the mean diameter of 130 nm. When the sputtering power increases from 80 to 100 W, there is no big difference in the grain size and the morphology of the films. As shown in Fig. 4b and c, the morphology of the grains is found to be polygonal shape; the grain size of the films was about 230 nm. The films deposited at the sputtering power of 120 W have an irregularly prismatic structure with the average grain size of 300 nm. On the whole, the crystallite sizes increase with the increasing of the sputtering power because the sputtered particles can obtain more energy prior to collision with the substrates. 3.4. Optical properties Fig. 3. Variation of the FWHM of Cd1 − xZnxTe (111) peak as a function of the sputtering power.
helpful for nucleation and growth, and consequently improved the crystallinity.
Fig. 5(a) shows the transmission spectra for the Cd1 − xZnxTe films grown at various sputtering powers from 60 to 120 W. All spectra of the films show a well-defined absorption edge in the range of 750– 850 nm. The transmittance data were used to calculate absorption coefficients of the Cd1 − xZnxTe films at different wavelengths. The absorption coefficient,α, is given by the relation [21]:
3.3. Surface morphology
I = I0 expð−α⋅dÞ
Fig. 4 shows the surface morphologies of the Cd1 − xZnxTe samples deposited at sputtering power of 60, 80, 100 and 120 W, respectively.
where I is the intensity of transmitted light, I0 is the intensity of incident light, and d is the thickness of Cd1 − xZnxTe films. The
Fig. 4. AFM micrographs (2.5 μm × 2.5 μm) for the Cd1 − xZnxTe films deposited at sputtering power of 60 W (a), 80W (b), 100W (c) and 120W (d).
ð2Þ
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Fig. 6. The sheet resistivities, mobility and carrier concentrations as a function of sputtering power.
4. Conclusions
Fig. 5. (a). Transmittance spectra of films deposited at different sputtering powers. (b). The optical absorption coefficient of four films deposited at different sputtering powers.
thicknesses of the films deposited with different sputtering power are 530, 783, 823 and 956 nm, respectively. As the transmittance is defined as I/I0, we can obtain α from Eq. (2), as shown in Fig. 5(b). There are no clear relations between the optical absorption coefficient and the effect of sputtering power. 3.5. Electrical properties The electrical properties of Cd1 − xZnxTe films were determined by Hall effect measurements. Fig. 6 shows the variations of sheet resistivity, carrier concentration and mobility of Cd1 − xZnxTe films with respect to the sputtering power. The carrier types of the four samples are p-type. The sheet resistivities of the deposited films were 1.988 × 108, 8.134 × 107, 8.088 × 107 and 3.069 × 107 Ω/sq, respectively. As the sputtering power increases from 60 to 120 W, the sheet resistivity decreased and the carrier concentration and mobility both increased. The increase of conductivity can be attributed to the improvement of crystallinity. When the sputtering power increases from 60 to 120 W, the crystallite size increases and the crystallinity is improved. A larger crystallite size results in a lower density of grain boundaries, which behave as traps for free carriers and barriers for carrier transport. Hence, an increase in grain size can cause a decrease in grain-boundary scattering and an increase of carrier lifetime [22,23] and consequently leads to an increase of conductivity due to the increase in both carrier concentration and Hall mobility. With the improvement of crystallinity, the concentration of electrically active donor sites is improved [24], which can also increase the carrier concentration.
Cd1 − xZnxTe films were obtained by r.f. magnetron sputtering with different sputtering powers (60–120 W). As the sputtering power increases, the deposition rate of the films also increases. The Cd concentration in the films was higher than that of Te regardless of sputtering power. The crystallinity of the films increases as the deposition power increases. For the films prepared at the sputtering power higher than 80 W, ZnTe secondary phases were observed. The average grain size increased with the increase of the sputtering power. The films exhibit shallow absorption edge in the transmission spectra, and the sputtering power seems to have no clear effect on the optical absorption coefficient. By increasing the sputtering power, the sheet resistivity of the films decreased, due to the larger crystallite size and the improvement of crystallinity. Acknowledgement This work was supported by the fund of the State Key Laboratory of Solidification Processing in NWPU (Grant no.SKLSP201005) and National Natural Science Foundation of China (51002012). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
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