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A comparative study on Zn0.8Cd0.2O films deposited on different substrates
Applied Surface Science 252 (2006) 5051–5056 www.elsevier.com/locate/apsusc
A comparative study on Zn0.8Cd0.2O films deposited on different substrates D.W. Ma a,b,*, Z.Z. Ye b, Y.S. Yang a a b
RCDAMP, Department of Physics, Pusan National University, Pusan 609-735, South Korea State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China Received 4 July 2005; received in revised form 20 July 2005; accepted 20 July 2005 Available online 12 September 2005
Abstract Highly (0 0 2)-oriented Zn0.8Cd0.2O crystal films were prepared on different substrates, namely, glass, Si(1 1 1) and aAl2O3(0 0 1) wafers by the dc reactive magnetron sputtering technique. The Zn0.8Cd0.2O/a-Al2O3 film has the best crystal quality with a FWHM of (0 0 2) peak of 0.37008, an average grain size of about 200 nm and a root-mean-square surface roughness of about 70 nm; yet the Zn0.8Cd0.2O/glass holds the worst crystal quality with a much larger FWHM of 0.62818. SIMS depth profile shows that the Zn and O compositions change little along the film depth direction; the Cd incorporation also almost holds the line towards the top surface other than an accumulation at the interface between the film and the substrate. The Cd content in the film is nearly consistent with that in target. # 2005 Elsevier B.V. All rights reserved. PACS: 78.66.Hf; 81.15.Cd Keywords: Zn1 xCdxO films; Substrate; dc sputtering
1. Introduction Recently, II–VI compound semiconductor ZnO and its alloys have attracted great interest because of its promising applications in optoelectronics typically short wavelength light-emitters and detectors. As an analogy to GaN, ZnO has a wide direct band-gap of 3.37 eV, a hexagonal wurtzite structure and an even larger excitonic binding energy of 60 meV indicative * Corresponding author. E-mail address: [email protected] (D.W. Ma).
of the existence and extreme stability of excitons at RT and/or even higher temperatures as compared to 25 meV of GaN [1,2]. To design ZnO-based devices, one of the crucial issues is the realization of band gap engineering. CdO is also a II–IV compound semiconductor with a direct band-gap of 2.3 eV and a cubic structure. Therefore, Zn1 xCdxO alloy films can be prepared and used for the active layers of Zn1 xCdxO/ZnO heterostructures because of its wavelength tunability and narrow bandgaps. In addition, considering the similarity in the radii ˚ ) and Cd2+ (0.97 A ˚ ), the incorporated of Zn2+ (0.74 A
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.07.062
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Cd may easily substitute the Zn lattice points up to a certain amount. However, few reports of Zn1 xCdxO alloy were referred to so far, and the crystal quality of the reported Zn1 xCdxO was not satisfactory due to the coexistence of multiphase or the polycrystalline state without preferred orientations [3–8]. We have successfully prepared highly (0 0 2)preferred Zn1 xCdxO alloy crystal films by the dc reactive sputtering method [9]. To optimize the crystal quality of the films, different wafers such as glass, Si and a-Al2O3 were chosen as substrates and also Zn0.8Cd0.2O film was taken as an example to show the effects of the substrates on this film in this paper.
2. Experimental details ZnCdO films were deposited on glass, Si(1 1 1) and a-Al2O3(0 0 1) substrates by the dc reactive magnetron sputtering method. A Zn–Cd alloy metal (99.999% pure) with a molar ratio of Zn/Cd = 4/1 was used as the sputtering target. The deposition chamber was evacuated to a base pressure of 1 10 3 Pa by a combination of mechanical-pump and oil-diffusion-pump. High purities (99.995% pure) of argon and oxygen were used as sputtering and reactive gases. The specific flux of argon to oxygen was controlled at a ratio of Ar/O2 1/4. The sputtering pressure, the total pressure of oxygen and argon maintained 4 Pa. The distance from the target to substrate was 60 mm. The sputtering current and sputtering voltage were 200 mA and 200 V, respectively. The substrate temperature was fixed at 375 8C. The sputtering time was 30 min for each sample. Before depositing each film, the Zn–Cd alloy target was presputtered in the argon–oxygen atmosphere for about 10 min. The crystal structures of the samples were investigated by the X-ray diffraction (XRD) method, where a Cu Ka (l = 0.154056 nm) source was used. The transmittance measurements were performed by a Cary-100-Bio UV–vis spectrophotometer. The surface morphology was examined by using an atomic force microscopy (AFM). The X-ray photoelectron spectroscopy (XPS) measurements were performed with an apparatus equipped with an Omicron EAC2000-125 hemispherical analyzer and a monochromatic Mg Ka (1253.6 eV) X-ray source. The secondary ions mass
spectrometry (SIMS) depth profile was performed with a CAMECA IMS-THREEF mass spectrometer.
3. Results and discussion Fig. 1 shows the XRD spectra of Zn0.8Cd0.2O films deposited on glass, Si(1 1 1) and a-Al2O3(0 0 1) substrates at a fixed substrate temperature of 375 8C. For all samples, only one diffraction peak around 34.38 appears, suggesting highly (0 0 2)preferred, i.e., c-preferred orientations [10]. It should be pointed out here that for the Zn0.8Cd0.2O/a-Al2O3 sample, the peak at about 41.938 arises from the (0 0 6) plane of the substrate. As far as the diffraction intensity of the film is concerned, the ones on Si(1 1 1) and a-Al2O3(0 0 1) substrates are obviously much larger than that on glass, which indicates much better crystal quality for the Zn0.8Cd0.2O/Si(1 1 1) and Zn0.8Cd0.2O/a-Al2O3(0 0 1) films. To further evaluate the crystal quality of the films, some parameters including the diffraction angle (2u) and the full width at half maximum (FWHM) of the (0 0 2) peak, the interplanar spacing d of (0 0 2) planes and the lattice parameter c of the Zn0.8Cd0.2O films on different substrates are listed in Table 1. For the Zn0.8Cd0.2O/a-Al2O3 film, its FWHM holds a minimum value of 0.37008, and the diffraction angle 2u of the (0 0 2) peak possesses a maximum value of
Fig. 1. XRD spectra of Zn0.8Cd0.2O films on different substrates.
D.W. Ma et al. / Applied Surface Science 252 (2006) 5051–5056
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Table 1 The diffraction angle (2u) and the FWHM of (0 0 2) peak, the interplanar spacing d of (0 0 2) planes and the lattice parameter c of Zn0.8Cd0.2O films on different substrates Substrate
Glass
Si(1 1 1)
a-Al2O3(0 0 1)
2u (8) FWHM (8) d (nm) c (nm)
34.27 0.6281 0.2897 0.5794
34.31 0.5307 0.2894 0.5788
34.39 0.3700 0.2887 0.5774
34.398; its resulting spacing d therefore has a minimum value of 0.2887 nm and the lattice parameter c the value of 0.5774 nm in terms of the Bragg law and the hexagonal equation, which indicates a more close-packed structure. Also the grain size, bigger than that either on the glass or the Si substrate, can be roughly evaluated according to the Scherrer’s formula. All of the above reveal that the Zn0.8Cd0.2O/a-Al2O3 film has a better crystal quality, even better than that on the Si substrate. It is known that the axes c of Zn0.8Cd0.2O crystal films are perpendicular to their corresponding substrates, i.e., the out-of-plane orientation relationships between Zn0.8Cd0.2O and a-Al2O3(0 0 1), Si(1 1 1) are considered as Zn0.8Cd0.2O[0 0 2]//Al2O3[0 0 1] and Zn0.8Cd0.2O[0 0 2]//Si[1 1 1], respectively. Therefore, to compare the lattice-to-lattice matching between the films and their substrates simply needs a comparison of their lattice parameters a. The aAl2O3 possesses a hexagonal wurtzite structure with lattice constants of a = 0.4758 nm and c = 1.299 nm, and the Si exhibits a cubic diamond structure with a lattice constant of a = 0.54301 nm; evidently the lattice parameter a of the Zn0.8Cd0.2O is more close to that of the a-Al2O3 substrate than that of the Si substrate, thus, a-Al2O3 is a better lattice-matching substrate for growing Zn0.8Cd0.2O than Si. On the other hand, both Zn0.8Cd0.2O and a-Al2O3 hold the same structure of hexagonal wurtzite, whereas, Si has a cubic structure. Therefore, the Zn0.8Cd0.2O/aAl2O3 film has a better crystal quality than the Zn0.8Cd0.2O/Si. Also it is unsurprising that the Zn0.8Cd0.2O/glass film has the poorest crystal quality in view of the amorphous state of the glass substrate. Fig. 2 displays the transmittance spectra of the Zn0.8Cd0.2O films on glass and a-Al2O3 substrates. Sharp absorption phenomena are observed around the cut-off wavelengths, which implies that the films have
Fig. 2. Transmittance spectra of Zn0.8Cd0.2O films on glass and aAl2O3 substrates.
direct energy band-gaps. The optical band-gapenergies obtained from the transmittance spectra are found to be 3.19 eV for the Zn0.8Cd0.2O/glass film and 3.22 eV for the Zn0.8Cd0.2O/a-Al2O3 film. The difference of the band-gap-energies can be associated with the lattice parameter c, which is smaller for the Zn0.8Cd0.2O/a-Al2O3 film than for the Zn0.8Cd0.2O/ glass film (as can be seen from Table 1), thus the ions coalesce more tightly in the Zn0.8Cd0.2O/a-Al2O3 film and the band-gap-energy evaluated inclines to a bigger value [11]. The high transmittance is a measure of the ability of the film surface roughness to reduce the diffuse reflectivity [12]. The Zn0.8Cd0.2O/glass film has a poorer crystal quality, and the average size of the crystalline grains is smaller, therefore the film surface seems to be flat. Moreover, a relatively structural homogeneity caused by the predominance of amorphous phase in the Zn0.8Cd0.2O/glass film also contributes to the high transmittance [12]. The interference fringes emphasize the surface uniformity with small crystalline size [13]. However, for the Zn0.8Cd0.2O/a-Al2O3 film, the interference fringe disappears owing to a coarse surface. Fig. 3 shows the AFM images of the Zn0.8Cd0.2O films on three different substrates. The Zn0.8Cd0.2O/a-Al2O3 film has an average grain size of about 200 nm and a rootmean-square (RMS) surface roughness of about 70 nm, and the grains are more close-packed; the Zn0.8Cd0.2O/Si film holds a grain size around 120– 130 nm and a RMS surface roughness of about 14 nm; the Zn0.8Cd0.2O/glass film has an average grain size of 100 nm or so and a RMS surface roughness of 12 nm;
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Fig. 3. AFM images of Zn0.8Cd0.2O films deposited on three kinds of substrates: (a) a-Al2O3, (b) Si(1 1 1) and (c) glass.
D.W. Ma et al. / Applied Surface Science 252 (2006) 5051–5056
additionally the grain boundaries are unconspicuous, which is likely due to the formations of non-crystals. The AFM results are consistent with the above discussions, and also it is concluded that the Zn0.8Cd0.2O/a-Al2O3 film has the best crystal quality, whereas the Zn0.8Cd0.2O/glass film has the worst. The XPS measurements were carried out to determine the compositions of the films and analyze the chemical states of Cd, Zn and O. Fig. 4 shows the XPS survey spectra (0–1150 eV) of the Zn0.8Cd0.2O/ Si(1 1 1) film. The peaks located around 1021, 530.8, and 405.5 eV correspond to the electronic states of Zn 2p3/2, O 1s and Cd 3d5/2, respectively. The C 1s core level at approximately 284.9 eV arises from the contamination of the oil-diffusion-pump. The Cd content in the film is nearly consistent with that in target, as was fully discussed elsewhere [10]. The SIMS measurement was performed in order to check the composition and in-depth homogeneity of the sample. Fig. 5 shows the SIMS depth profile of the Zn0.8Cd0.2O/Si film. There is a sharp interface between the film and the substrate, which indicates a very small mutual-diffusion phenomenon because of the low substrate temperature while depositing the film; the Zn and O compositions change little along the film depth direction; the Cd incorporation also almost holds the line towards the top surface other than an accumulation at the interface between the film and the substrate, maybe due to a certain kind of
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Fig. 5. SIMS depth profile of Zn0.8Cd0.2O/Si film.
segregation effect, or due to a relatively smaller binding energy for Cd than for Zn, therefore, the Cd content is easily sputtered at the beginning of the film deposition, and the Cd content is seemingly enriched at the interface. With the process going, the sputtering rate of the component Cd comes to an equilibrium state.
4. Conclusions Highly (0 0 2)-oriented Zn0.8Cd0.2O crystal films were prepared on different substrates, namely, glass, Si(1 1 1) and a-Al2O3(0 0 1) wafers by the dc reactive magnetron sputtering technique. XRD and AFM measurements indicate that the Zn0.8Cd0.2O/aAl2O3 film has the best crystal quality, and the Zn0.8Cd0.2O/glass film has the worst, the Zn0.8Cd0.2O/ Si film midst. SIMS depth profile suggests that the Zn and O compositions change little along the film depth direction, and the Cd incorporation also almost holds the line towards the top surface except for an accumulation at the interface. The Zn1 xCdxO alloy films may have great potential in the applications of short wavelength optoelectronic devices.
Acknowledgement Fig. 4. XPS survey spectrum of Zn0.8Cd0.2O/Si film (the peaks with binding energies of 200–400 eV all ascribe to ZnLMM peaks other than the C 1s peak).
This work was supported by Natural Science Foundation of the Education Ministry of Zhejiang Province of China under Grant No. G20408 and the
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Korean Research Foundation under Grant KRF-2004005-C0041.
References [1] D.C. Look, D.C. Reynolds, J.R. Sizelove, R.L. Jones, C.W. Litton, G. Cantwell, W.C. Harsch, Solid State Commun. 105 (1998) 399. [2] Z.K. Tang, G.K.L. Wong, P. Yu, M. Kawasaki, A. Ohtomo, H. Koinuma, Y. Segawa, Appl. Phys. Lett. 72 (1998) 3270. [3] H. Tabet-Derraz, N. Benramdance, D. Nacer, A. Bouzidi, M. Medles, Solid Energy Mater. Solid C 73 (2002) 249. [4] O. Vigil, L. Vaollant, F. Cruz, G. Santana, A. Moroles-Acevedo, G. Contreras-Puente, Thin Solid Films 361–362 (2000) 53. [5] Y.S. Choi, C.G. Lee, S.M. Cho, Thin Solid Films 289 (1996) 153. [6] G. Torres-Delgado, C.I. Zuniga-Romero, O. JimenezSandoval, R. Castanedo-Perez, B. Chao, S. Jimenez-Sandoval, Adv. Funct. Mater. 12 (2002) 129.
[7] K. Sakurai, T. Takagi, T. Kubo, D. Kajita, T. Tanabe, H. Takasu, S. Fujita, S. Fujita, J. Cryst. Growth 237–239 (2002) 514. [8] T. Makino, Y. Segawa, M. Kawasaki, A. Ohtomo, R. Shiroki, K. Tamura, T. Yasuda, H. Koinuma, Appl. Phys. Lett. 78 (2001) 1237. [9] Z.Z. Ye, D.W. Ma, J.H. He, J.Y. Huang, B.H. Zhao, X.D. Luo, Z.Y. Xu, J. Cryst. Growth 256 (2003) 78. [10] D.W. Ma, Z.Z. Ye, H.M. Lu, J.Y. Huang, B.H. Zhao, L.P. Zhu, H.J. Zhang, P.M. He, Thin Solid Films 461 (2004) 250. [11] O. Vigil, F. Cruz, A. Morales-Acevedo, G. Contreras-Puente, L. Vaillant, G. Santana, Mater. Chem. Phys. 68 (2001) 249. [12] K. Ramamoorthy, C. Sanjeeviraja, M. Jayachandran, K. Sankaranarayanan, Pijush Bhattacharya, L.M. Kukreja, J. Cryst. Growth 226 (2001) 281. [13] R. Groenen, J.L. Linden, H.R.M. van Lierop, D.C. Schram, A.D. Kuypers, M.C.M. van de Sanden, Appl. Surf. Sci. 173 (2001) 40.
A comparative study on Zn0.8Cd0.2O films deposited on different substrates D.W. Ma a,b,*, Z.Z. Ye b, Y.S. Yang a a b
RCDAMP, Department of Physics, Pusan National University, Pusan 609-735, South Korea State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China Received 4 July 2005; received in revised form 20 July 2005; accepted 20 July 2005 Available online 12 September 2005
Abstract Highly (0 0 2)-oriented Zn0.8Cd0.2O crystal films were prepared on different substrates, namely, glass, Si(1 1 1) and aAl2O3(0 0 1) wafers by the dc reactive magnetron sputtering technique. The Zn0.8Cd0.2O/a-Al2O3 film has the best crystal quality with a FWHM of (0 0 2) peak of 0.37008, an average grain size of about 200 nm and a root-mean-square surface roughness of about 70 nm; yet the Zn0.8Cd0.2O/glass holds the worst crystal quality with a much larger FWHM of 0.62818. SIMS depth profile shows that the Zn and O compositions change little along the film depth direction; the Cd incorporation also almost holds the line towards the top surface other than an accumulation at the interface between the film and the substrate. The Cd content in the film is nearly consistent with that in target. # 2005 Elsevier B.V. All rights reserved. PACS: 78.66.Hf; 81.15.Cd Keywords: Zn1 xCdxO films; Substrate; dc sputtering
1. Introduction Recently, II–VI compound semiconductor ZnO and its alloys have attracted great interest because of its promising applications in optoelectronics typically short wavelength light-emitters and detectors. As an analogy to GaN, ZnO has a wide direct band-gap of 3.37 eV, a hexagonal wurtzite structure and an even larger excitonic binding energy of 60 meV indicative * Corresponding author. E-mail address: [email protected] (D.W. Ma).
of the existence and extreme stability of excitons at RT and/or even higher temperatures as compared to 25 meV of GaN [1,2]. To design ZnO-based devices, one of the crucial issues is the realization of band gap engineering. CdO is also a II–IV compound semiconductor with a direct band-gap of 2.3 eV and a cubic structure. Therefore, Zn1 xCdxO alloy films can be prepared and used for the active layers of Zn1 xCdxO/ZnO heterostructures because of its wavelength tunability and narrow bandgaps. In addition, considering the similarity in the radii ˚ ) and Cd2+ (0.97 A ˚ ), the incorporated of Zn2+ (0.74 A
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.07.062
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D.W. Ma et al. / Applied Surface Science 252 (2006) 5051–5056
Cd may easily substitute the Zn lattice points up to a certain amount. However, few reports of Zn1 xCdxO alloy were referred to so far, and the crystal quality of the reported Zn1 xCdxO was not satisfactory due to the coexistence of multiphase or the polycrystalline state without preferred orientations [3–8]. We have successfully prepared highly (0 0 2)preferred Zn1 xCdxO alloy crystal films by the dc reactive sputtering method [9]. To optimize the crystal quality of the films, different wafers such as glass, Si and a-Al2O3 were chosen as substrates and also Zn0.8Cd0.2O film was taken as an example to show the effects of the substrates on this film in this paper.
2. Experimental details ZnCdO films were deposited on glass, Si(1 1 1) and a-Al2O3(0 0 1) substrates by the dc reactive magnetron sputtering method. A Zn–Cd alloy metal (99.999% pure) with a molar ratio of Zn/Cd = 4/1 was used as the sputtering target. The deposition chamber was evacuated to a base pressure of 1 10 3 Pa by a combination of mechanical-pump and oil-diffusion-pump. High purities (99.995% pure) of argon and oxygen were used as sputtering and reactive gases. The specific flux of argon to oxygen was controlled at a ratio of Ar/O2 1/4. The sputtering pressure, the total pressure of oxygen and argon maintained 4 Pa. The distance from the target to substrate was 60 mm. The sputtering current and sputtering voltage were 200 mA and 200 V, respectively. The substrate temperature was fixed at 375 8C. The sputtering time was 30 min for each sample. Before depositing each film, the Zn–Cd alloy target was presputtered in the argon–oxygen atmosphere for about 10 min. The crystal structures of the samples were investigated by the X-ray diffraction (XRD) method, where a Cu Ka (l = 0.154056 nm) source was used. The transmittance measurements were performed by a Cary-100-Bio UV–vis spectrophotometer. The surface morphology was examined by using an atomic force microscopy (AFM). The X-ray photoelectron spectroscopy (XPS) measurements were performed with an apparatus equipped with an Omicron EAC2000-125 hemispherical analyzer and a monochromatic Mg Ka (1253.6 eV) X-ray source. The secondary ions mass
spectrometry (SIMS) depth profile was performed with a CAMECA IMS-THREEF mass spectrometer.
3. Results and discussion Fig. 1 shows the XRD spectra of Zn0.8Cd0.2O films deposited on glass, Si(1 1 1) and a-Al2O3(0 0 1) substrates at a fixed substrate temperature of 375 8C. For all samples, only one diffraction peak around 34.38 appears, suggesting highly (0 0 2)preferred, i.e., c-preferred orientations [10]. It should be pointed out here that for the Zn0.8Cd0.2O/a-Al2O3 sample, the peak at about 41.938 arises from the (0 0 6) plane of the substrate. As far as the diffraction intensity of the film is concerned, the ones on Si(1 1 1) and a-Al2O3(0 0 1) substrates are obviously much larger than that on glass, which indicates much better crystal quality for the Zn0.8Cd0.2O/Si(1 1 1) and Zn0.8Cd0.2O/a-Al2O3(0 0 1) films. To further evaluate the crystal quality of the films, some parameters including the diffraction angle (2u) and the full width at half maximum (FWHM) of the (0 0 2) peak, the interplanar spacing d of (0 0 2) planes and the lattice parameter c of the Zn0.8Cd0.2O films on different substrates are listed in Table 1. For the Zn0.8Cd0.2O/a-Al2O3 film, its FWHM holds a minimum value of 0.37008, and the diffraction angle 2u of the (0 0 2) peak possesses a maximum value of
Fig. 1. XRD spectra of Zn0.8Cd0.2O films on different substrates.
D.W. Ma et al. / Applied Surface Science 252 (2006) 5051–5056
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Table 1 The diffraction angle (2u) and the FWHM of (0 0 2) peak, the interplanar spacing d of (0 0 2) planes and the lattice parameter c of Zn0.8Cd0.2O films on different substrates Substrate
Glass
Si(1 1 1)
a-Al2O3(0 0 1)
2u (8) FWHM (8) d (nm) c (nm)
34.27 0.6281 0.2897 0.5794
34.31 0.5307 0.2894 0.5788
34.39 0.3700 0.2887 0.5774
34.398; its resulting spacing d therefore has a minimum value of 0.2887 nm and the lattice parameter c the value of 0.5774 nm in terms of the Bragg law and the hexagonal equation, which indicates a more close-packed structure. Also the grain size, bigger than that either on the glass or the Si substrate, can be roughly evaluated according to the Scherrer’s formula. All of the above reveal that the Zn0.8Cd0.2O/a-Al2O3 film has a better crystal quality, even better than that on the Si substrate. It is known that the axes c of Zn0.8Cd0.2O crystal films are perpendicular to their corresponding substrates, i.e., the out-of-plane orientation relationships between Zn0.8Cd0.2O and a-Al2O3(0 0 1), Si(1 1 1) are considered as Zn0.8Cd0.2O[0 0 2]//Al2O3[0 0 1] and Zn0.8Cd0.2O[0 0 2]//Si[1 1 1], respectively. Therefore, to compare the lattice-to-lattice matching between the films and their substrates simply needs a comparison of their lattice parameters a. The aAl2O3 possesses a hexagonal wurtzite structure with lattice constants of a = 0.4758 nm and c = 1.299 nm, and the Si exhibits a cubic diamond structure with a lattice constant of a = 0.54301 nm; evidently the lattice parameter a of the Zn0.8Cd0.2O is more close to that of the a-Al2O3 substrate than that of the Si substrate, thus, a-Al2O3 is a better lattice-matching substrate for growing Zn0.8Cd0.2O than Si. On the other hand, both Zn0.8Cd0.2O and a-Al2O3 hold the same structure of hexagonal wurtzite, whereas, Si has a cubic structure. Therefore, the Zn0.8Cd0.2O/aAl2O3 film has a better crystal quality than the Zn0.8Cd0.2O/Si. Also it is unsurprising that the Zn0.8Cd0.2O/glass film has the poorest crystal quality in view of the amorphous state of the glass substrate. Fig. 2 displays the transmittance spectra of the Zn0.8Cd0.2O films on glass and a-Al2O3 substrates. Sharp absorption phenomena are observed around the cut-off wavelengths, which implies that the films have
Fig. 2. Transmittance spectra of Zn0.8Cd0.2O films on glass and aAl2O3 substrates.
direct energy band-gaps. The optical band-gapenergies obtained from the transmittance spectra are found to be 3.19 eV for the Zn0.8Cd0.2O/glass film and 3.22 eV for the Zn0.8Cd0.2O/a-Al2O3 film. The difference of the band-gap-energies can be associated with the lattice parameter c, which is smaller for the Zn0.8Cd0.2O/a-Al2O3 film than for the Zn0.8Cd0.2O/ glass film (as can be seen from Table 1), thus the ions coalesce more tightly in the Zn0.8Cd0.2O/a-Al2O3 film and the band-gap-energy evaluated inclines to a bigger value [11]. The high transmittance is a measure of the ability of the film surface roughness to reduce the diffuse reflectivity [12]. The Zn0.8Cd0.2O/glass film has a poorer crystal quality, and the average size of the crystalline grains is smaller, therefore the film surface seems to be flat. Moreover, a relatively structural homogeneity caused by the predominance of amorphous phase in the Zn0.8Cd0.2O/glass film also contributes to the high transmittance [12]. The interference fringes emphasize the surface uniformity with small crystalline size [13]. However, for the Zn0.8Cd0.2O/a-Al2O3 film, the interference fringe disappears owing to a coarse surface. Fig. 3 shows the AFM images of the Zn0.8Cd0.2O films on three different substrates. The Zn0.8Cd0.2O/a-Al2O3 film has an average grain size of about 200 nm and a rootmean-square (RMS) surface roughness of about 70 nm, and the grains are more close-packed; the Zn0.8Cd0.2O/Si film holds a grain size around 120– 130 nm and a RMS surface roughness of about 14 nm; the Zn0.8Cd0.2O/glass film has an average grain size of 100 nm or so and a RMS surface roughness of 12 nm;
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Fig. 3. AFM images of Zn0.8Cd0.2O films deposited on three kinds of substrates: (a) a-Al2O3, (b) Si(1 1 1) and (c) glass.
D.W. Ma et al. / Applied Surface Science 252 (2006) 5051–5056
additionally the grain boundaries are unconspicuous, which is likely due to the formations of non-crystals. The AFM results are consistent with the above discussions, and also it is concluded that the Zn0.8Cd0.2O/a-Al2O3 film has the best crystal quality, whereas the Zn0.8Cd0.2O/glass film has the worst. The XPS measurements were carried out to determine the compositions of the films and analyze the chemical states of Cd, Zn and O. Fig. 4 shows the XPS survey spectra (0–1150 eV) of the Zn0.8Cd0.2O/ Si(1 1 1) film. The peaks located around 1021, 530.8, and 405.5 eV correspond to the electronic states of Zn 2p3/2, O 1s and Cd 3d5/2, respectively. The C 1s core level at approximately 284.9 eV arises from the contamination of the oil-diffusion-pump. The Cd content in the film is nearly consistent with that in target, as was fully discussed elsewhere [10]. The SIMS measurement was performed in order to check the composition and in-depth homogeneity of the sample. Fig. 5 shows the SIMS depth profile of the Zn0.8Cd0.2O/Si film. There is a sharp interface between the film and the substrate, which indicates a very small mutual-diffusion phenomenon because of the low substrate temperature while depositing the film; the Zn and O compositions change little along the film depth direction; the Cd incorporation also almost holds the line towards the top surface other than an accumulation at the interface between the film and the substrate, maybe due to a certain kind of
5055
Fig. 5. SIMS depth profile of Zn0.8Cd0.2O/Si film.
segregation effect, or due to a relatively smaller binding energy for Cd than for Zn, therefore, the Cd content is easily sputtered at the beginning of the film deposition, and the Cd content is seemingly enriched at the interface. With the process going, the sputtering rate of the component Cd comes to an equilibrium state.
4. Conclusions Highly (0 0 2)-oriented Zn0.8Cd0.2O crystal films were prepared on different substrates, namely, glass, Si(1 1 1) and a-Al2O3(0 0 1) wafers by the dc reactive magnetron sputtering technique. XRD and AFM measurements indicate that the Zn0.8Cd0.2O/aAl2O3 film has the best crystal quality, and the Zn0.8Cd0.2O/glass film has the worst, the Zn0.8Cd0.2O/ Si film midst. SIMS depth profile suggests that the Zn and O compositions change little along the film depth direction, and the Cd incorporation also almost holds the line towards the top surface except for an accumulation at the interface. The Zn1 xCdxO alloy films may have great potential in the applications of short wavelength optoelectronic devices.
Acknowledgement Fig. 4. XPS survey spectrum of Zn0.8Cd0.2O/Si film (the peaks with binding energies of 200–400 eV all ascribe to ZnLMM peaks other than the C 1s peak).
This work was supported by Natural Science Foundation of the Education Ministry of Zhejiang Province of China under Grant No. G20408 and the
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Korean Research Foundation under Grant KRF-2004005-C0041.
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