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Synthesis and characterization of multi-element oxynitride semiconductor film prepared by reactive sputtering deposition
Applied Surface Science 263 (2012) 58–61
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis and characterization of multi-element oxynitride semiconductor film prepared by reactive sputtering deposition Ruei-Sung Yu a , Rong-Hsin Huang b , Chih-Ming Lee b , Fuh-Sheng Shieu b,∗ a b
Department of Photonics and Communication Engineering, Asia University, 500, Lioufeng Rd., Wufeng, Taichung 413, Taiwan Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan
a r t i c l e
i n f o
Article history: Received 16 March 2012 Received in revised form 25 August 2012 Accepted 26 August 2012 Available online 20 September 2012 Keywords: Semiconductor Structure Multi-element Thin film Oxynitride
a b s t r a c t This study concerns the use of reactive magnetron sputtering to prepare (TiVCrZrTa)-based oxide and oxynitride films. (TiVCrZrTa)1−x Ox and (TiVCrZrTa)1−x−y Ny Ox films were prepared, and were found to be amorphous and free of multi-phase structure. Cations and anions in such structures were arranged in a random homogeneous dispersion. The introduction of nitrogen atoms into (TiVCrZrTa)1−x Ox yields (TiVCrZrTa)1−x−y Ny Ox , which has a reduced oxidation state and thus, an increased number of the valence electrons. The (TiVCrZrTa)1−x−y Ny Ox film is an n-type semiconductor, with an indirect band gap of 1.95 eV, and a carrier concentration (N) and conductivity () of 1.01 × 1019 cm−3 and 2.75 × 10−2 ( cm)−1 , respectively. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Binary semiconductor compounds, such as n-type In2 O3 , ZnO and TiO2 , have been recognized as important semiconductor materials, and are widely utilized in flat panel displays, solar cells, and many other semiconductor products [1,2]. Ternary materials, such as copper-based oxides including p-type CuAlO2 , CuCrO2 , CuInO2 , and CuGaO2 , have also been well studied [3–5]. However, to this day, there have been few studies into multi-element semiconductor materials composed of more than six elements. In an effort to address this, we propose a synthesis method in this study motivated by the high-entropy effect theory of multi-element materials that has been discussed in many previous reports [6–14]. The theory addresses the following issues: (a) forming simple body centered cubic (BCC) and/or face centered cubic (FCC) solid solution crystal structure(s); (b) easy formation of amorphous materials; achieving the advantageous material properties of (c) high hardness; and (d) thermal stability and corrosion resistance. There are numerous potential multi-element semiconductors that could be developed. In this paper, we describe the study of novel ceramic materials with their constituent materials selected from TiN, VN, CrN, ZrN and TaN, which are all known to be nitride coating films, as well as the elements Ti, V, Cr, Zr and Ta, giving films of the
∗ Corresponding author. Tel.: +886 4 22854563; fax: +886 4 22857017. E-mail address: [email protected] (F.-S. Shieu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.109
form (TiVCrZrTa)1−x Ox and (TiVCrZrTa)1−x−y Ny Ox . Through experiment, the multi-element oxynitride material was found to exhibit semiconducting behavior.
2. Experimental 2.1. Synthesis method Clean glass substrates were prepared by sequential ultrasonic vibration in baths of hot degreasing solvent, acid solution, and deionized water. Deposition of the films was carried out by reactive magnetron sputtering in a home-built system at around room temperature. The base pressure and deposition pressure in the vacuum chamber were 4.0 × 10−4 and 4.0 × 10−1 Pa, respectively. The sputtering target material was prepared by melting a mix of the metallic starting materials under vacuum in equi-atomic proportions, which resulted in the composition Ti0.2 V0.2 Cr0.2 Zr0.2 Ta0.2 . The deposition time for each film specimen was 20 min. A 200 W direct current was applied during film deposition. The target diameter was 10.16 cm and the area of the cathode was 81.03 cm2 . The power density was 2.47 W/cm2 . During multi-element oxide film deposition, the O2 /(O2 + Ar) gas mass flow ratio was maintained at 15%. During deposition of multi-element oxynitride film, the O2 /(O2 + N2 + Ar) and N2 /(O2 + N2 + Ar) gas mass flow ratios were 2.0% and 50%, respectively. Target-to-substrate distance was kept constant at 70 mm for all depositions.
R.-S. Yu et al. / Applied Surface Science 263 (2012) 58–61
Fig. 1. The X-ray diffraction (TiVCrZrTa)1−x−y Ny Ox thin films.
spectra
of
the
(TiVCrZrTa)1−x Ox
and
2.2. Characterization The structural and semiconductor properties of the films were characterized by X-ray diffraction (XRD, MAC SCIENCE MPXIII, BRUKER), high resolution transmission electron microscope (HRTEM, JEOL JEM-2010), X-ray photoelectron spectroscopy (XPS, ULVAC-PHI 5000 VersaProbe), UV-vis spectrophotometer (HITACHI U3010), and Hall effect measurement (ACCENT HL5500PC).
3. Results and discussion 3.1. X-ray diffraction analysis Fig. 1 shows the X-ray diffraction patterns of the multi-element oxide and oxynitride films deposited onto glass. No diffraction
59
peaks indicative of crystal structures were detected, suggesting that both films were amorphous. According to the laws of thermodynamics, a multi-element material with atoms in a homogeneous random arrangement tends to form a solid-solution phase, which maximizes the entropy and minimizes the free energy of the material [8,15]. A review of related literature [11] shows that the multi-element nitride film (AlMoNbSiTaTiVZr)50 N50 is amorphous, but not of a multi-phase structure. In the amorphous phase atoms are in a homogeneous random arrangement. Due to the amorphous characteristics displayed by the (AlMoNbSiTaTiVZr)50 N50 film, we selected this material as a representative case for the aforementioned fundamental theories of multi-element materials. The XRD and HRTEM (see discussion below) analysis conducted in this study demonstrated that multi-element materials are amorphous and free from multi-phase structure, in agreement with previous reports. To sum up the aforementioned analyses, the multi-element oxide and oxynitride materials examined in this study show solid-solution amorphous characteristics. In the three-dimensional structure, anions (O and N) act as cores and are surrounded by cations (Ti, V, Zr, Cr and Ta) in a homogeneous random arrangement.
3.2. X-ray photoelectron spectroscopy analysis X-ray photoelectron spectroscopy (XPS) measurements were performed to evaluate the chemical composition and chemical state of the films. The results of the chemical composition analysis of (TiVCrZrTa)1−x Ox and (TiVCrZrTa)1−x−y Ny Ox films are shown in Table 1. The two films both have cation (Ti, V, Zr, Cr and Ta) to anion (O and N) ratios close to 2:3. According to these results, the (TiVCrZrTa)1−x Ox and (TiVCrZrTa)1−x−y Ny Ox films can be expressed as (TiVCrZrTa)0.415 O0.585 and (TiVCrZrTa)0.408 N0.243 O0.349 , respectively. Oxygen has a relatively strong reactivity, when compared to nitrogen [16]. Oxygen thus has a greater tendency to react with
Fig. 2. XPS diagrams of the (TiVCrZrTa)1−x Ox and (TiVCrZrTa)1−x−y Ny Ox films containing (a) Ti, (b) V, (c) Cr, and (d) O elements, respectively.
60
R.-S. Yu et al. / Applied Surface Science 263 (2012) 58–61
Table 1 Chemical composition analysis of (TiVCrZrTa)1−x Ox and (TiVCrZrTa)1−x−y Ny Ox films obtained by the X-ray photoelectron spectroscopy (XPS). Specimens
(TiVCrZrTa)1−x Ox (TiVCrZrTa)1−x−y Ny Ox
XPS atomic concentration (%) Ti
V
Cr
Zr
Ta
O
N
3.77 3.58
17.85 8.62
7.36 10.86
5.47 8.56
7.03 9.20
58.52 34.92
– 24.26
metallic species during film deposition. Residual oxygen in the vacuum chamber contributes to the formation of oxynitride materials [17]. For this reason, the multi-element oxynitride film contains 34.92 at.% oxygen, even though the oxygen mass flow was heavily decreased during the deposition process. There is a literature report of sputtering deposition with no mass flow of oxygen during the deposition of TiNx Oy films [17], in which the oxygen originated from the PET substrate or residual gas in the chamber. According to theories of material sciences, the amorphous character of the film means cations (Ti, V, Zr, Cr and Ta) and anions (O and N) tend to remain close to one another, resulting in atomic bonding. Fig. 2 shows XPS spectra of the (TiVCrZrTa)1−x Ox and (TiVCrZrTa)1−x−y Ny Ox films containing Ti, V, Cr and O, respectively. Fig. 2(a)–(c) illustrates that the introduction of nitrogen atoms into (TiVCrZrTa)1−x Ox to form (TiVCrZrTa)1−x−y Ny Ox can reduce the binding energy of the Ti, V, and Cr cations in the film, which translates to increased numbers of valence electrons or a decreased oxidation state, in turn causing higher conductivity. For instance, in Fig. 2(b) the electron binding energy of V 2p3/2 decreases from 514.6 to 512.1 eV, in response to the oxynitride. A decrease in binding energy is also observed in the Zr and Ta spectra (not shown). In other words, it is possible to alter the electrical properties of the (TiVCrZrTa)1−x Ox film by adjusting the quantity of nitrogen introduced. Fig. 2(d) indicates that the binding energy of O 1s was unchanged in the oxide and oxynitride films. That is to say, the introduction of nitrogen does not influence the binding energy between oxygen and nitrogen. Since both films are amorphous, the cations and anions in the material tend to stay close to one another. 3.3. Semiconductor properties analysis A correlation between the band gaps and electrical properties of the films is listed in Table 2. An amorphous material shows an indirect transition on the electronic band diagram [18], hence the two films are both amorphous and do not possess a direct band gap. The value of the band gap can be estimated using the standard relation [19], (˛hv)
1/n
= A(hv − Eg )
(1)
where ˛ is the absorption coefficient, h is the photon energy, A is a constant and Eg is the band gap. The value of n for an indirect
allowed transition is 2. Plots of (˛h)1/2 versus h were made, from which the values of the indirect band gap were derived. As shown in Fig. 3(a), the (TiVCrZrTa)1−x Ox film exhibited a relatively large indirect band gap of 2.38 eV, and displayed insulating properties. As illustrated in Fig. 3(b), upon the introduction of nitrogen atoms, causing the formation of (TiVCrZrTa)1−x−y Ny Ox , the material changed to show semiconductor characteristics, and its indirect band gap was lowered to 1.95 eV. It has been reported that the indirect band gap of multi-element (ZnSnCuTiNb)1−x Ox semiconductor films are 1.69 and 2.26 eV, respectively [20]. These discrepancies may be attributed to differences in the chemical composition of the material. Hall effect measurements were employed to determine the electrical properties of the films. The (TiVCrZrTa)1−x Ox film had a resistance larger than the upper measurable limit of the Hall effect measurement system. With the introduction of nitrogen atoms, the (TiVCrZrTa)1−x−y Ny Ox formed a semiconductor with a carrier concentration (N) and conductivity value () of 1.01 × 1019 cm−3 and 2.75 × 10−2 ( cm)−1 , respectively. The (TiVCrZrTa)1−x−y Ny Ox film is amorphous with atoms irregularly arrayed, thus it is impossible to describe the conductivity mechanism by means of a nonstoichiometric defect equation. Since oxygen and nitrogen are both anions, some oxygen atoms are replaced by nitrogen during the addition of nitrogen into the multi-element oxide. Thus, in a three-dimensional structure, oxygen and nitrogen anions act as cores, surrounded by cations in a homogeneous random arrangement. The relatively weak electronegativity of nitrogen makes it a less potent oxidizer than oxygen, and thus the replacing of oxygen with nitrogen atoms leaves some cations with untransferred valence electrons, which may act as carriers. The conductivity mechanism of the multi-element oxynitride is thus subject to the existence of outerorbital valence electrons in the cations. This was confirmed by the measurement of a negative Hall coefficient (Rh), indicating that electrons are the main carriers and that the (TiVCrZrTa)1−x−y Ny Ox film is an n-type semiconductor. A further discussion, with related studies reported about the structural and optoelectronic properties of the ZrNx Oy and TiNx Oy films [21–23]. Structurally, (TiVCrZrTa)1−x−y Ny Ox is an amorphous solid, unlike crystalline ZrNx Oy and TiNx Oy . The ZrNx Oy film showed the sheet resistance and band gap of 1.1 × 103 /sq and 3.04 eV, respectively [21]. For other relevant oxynitride studies, the ZrNx Oy
Fig. 3. Indirect band gaps of (a) the (TiVCrZrTa)1−x Ox and (b) the (TiVCrZrTa)1−x−y Ny Ox thin films.
R.-S. Yu et al. / Applied Surface Science 263 (2012) 58–61
61
Table 2 Band gaps and electrical properties of the (TiVCrZrTa)1−x Ox and (TiVCrZrTa)1−x−y Ny Ox films, where gap is the indirect band gap, N is the carrier concentration, Rh is the Hall coefficient, is the carrier mobility and is the conductivity. Specimens
Gap (eV)
N (cm−3 )
Rh (m2 /C)
(cm2 /V s)
( cm)−1
(TiVCrZrTa)1−x Ox (TiVCrZrTa)1−x−y Ny Ox
2.38 1.95
– 1.01 × 1019
– −1.89
– 0.017
– 2.75 × 10−2
Fig. 4. HRTEM images of the (TiVCrZrTa)1−x−y Ny Ox film, including the (a) cross-sectional image, (b) nanometerscale image, and (c) SAD pattern.
films have band gaps between 1.96 and 2.26 eV [22], and the TiNx Oy have values between 2.11 and 3.34 eV [23]. The variations within the same kind of oxynitride films are attributed to the different crystal structures and chemical compositions. The multielement oxynitride band gap of 1.95 eV is very close to the literature reported value of 1.96 eV (ZrNx Oy ). 3.4. High resolution transmission electron microscope (HRTEM) analysis To further investigate the film, an examination using crosssectional HRTEM analysis was performed. Based on the measured electrical properties, we selected the (TiVCrZrTa)1−x−y Ny Ox film for its semiconductor properties. Fig. 4(a) shows a cross-sectional HRTEM image of the (TiVCrZrTa)1−x−y Ny Ox specimen. The three layers – Pt film, the oxynitride film, and glass – are shown. In preparing the material for HRTEM analysis, Pt film was deposited as the top layer in order to protect the oxynitride film during the cutting of by focused ion beam (FIB). The oxynitride film shows a smooth crosssection and is without notable contrast, indicating that the material is amorphous. In Fig. 4(b), the bright field nanometer scale image of the oxynitride film looks disordered, without a lattice structure. In Fig. 4(c), the broad ring pattern in the selected area diffraction (SAD) likewise indicates that the film was amorphous. This result is also consistent with that of the XRD. 4. Conclusion In this study, the structural and semiconductor properties of (TiVCrZrTa)-based oxide and oxynitride films were examined. The results revealed that (TiVCrZrTa)1−x Ox and (TiVCrZrTa)1−x−y Ny Ox are amorphous. In the three-dimensional structure, anions (O and N) act as cores surrounded by cations (Ti, V, Zr, Cr and Ta) in a random and homogeneous dispersion. The (TiVCrZrTa)1−x Ox film is nonconducting, with an indirect band gap of 2.38 eV. The introduction of nitrogen to the multi-element oxide film increases the contribution of conducting electrons from the outer orbital electrons of the cations. The resulting (TiVCrZrTa)1−x−y Ny Ox film is an n-type semiconductor. The indirect band gap and conductivity of the multi-element oxynitride film were 1.95 eV and 2.75 × 10−2 ( cm)−1 , respectively. The adjustability of the elemental composition in the multi-element material facilitates
further development of new oxynitride semiconductors. Based on these results, multi-element oxynitride films could be a candidate material for use in semiconductor technologies. Acknowledgements The authors would like to thank the National Science Council of Taiwan for financial support of this research under Contract No. NSC 97 – 2811 – E – 005 – 005. References [1] S. Flickyngerova, K. Shtereva, V. Stenova, D. Hasko, I. Novotny, V. Tvarozek, P. Sutta, E. Vavrinsky, Appl. Surf. Sci. 254 (2008) 3643. [2] C.M. Maghanga, J. Jensen, G.A. Niklasson, C.G. Granqvist, M. Mwamburi, Sol. Energy Mater. Sol. Cells 94 (2010) 75. [3] H. Kawazoe, H. Yanagi, K. Ueda, H. Hosono, MRS Bull. 25 (2000) 28. [4] J. Tate, H.L. Ju, J.C. Moon, A. Zakutayev, A.P. Richard, J. Russell, D.H. McIntyre, Phys. Rev. B 80 (2009) 165206–165211. [5] C. Yaicle, A. Blacklocks, A.V. Chadwick, J. Perrière, A. Rougier, Appl. Surf. Sci. 254 (2007) 1343. [6] S.Y. Chang, M.K. Chen, D.S. Chen, J. Electrochem. Soc. 156 (2009) G37. [7] S. Varalakshmi, M. Kamaraj, B.S. Murty, J. Alloys Compd. 460 (2008) 253. [8] C.Z. Yao, P. Zhang, M. Liu, G.R. Li, J.Q. Ye, P. Liu, Y.X. Tong, Electrochim. Acta 53 (2008) 8359. [9] C.W. Tsai, Y.L. Chen, M.H. Tsai, J.W. Yeh, T.T. Shun, S.K. Chen, J. Alloys Compd. 486 (2009) 427. [10] O.N. Senkov, J.M. Scott, S.V. Senkova, D.B. Miracle, C.F. Woodward, J. Alloys Compd. 509 (2011) 6043. [11] M.H. Tsai, C.W. Wang, C.H. Lai, J.W. Yeh, J.Y. Gan, Appl. Phys. Lett. 92 (2008) 052109–52111. [12] T.K. Chen, M.S. Wong, T.T. Shun, J.W. Yeh, Surf. Coat. Technol. 200 (2005) 1361. [13] B. Ren, Z.X. Liu, L. Shi, B. Cai, M.X. Wang, Appl. Surf. Sci. 257 (2011) 7172. [14] Y.L. Chen, Y.H. Hu, C.W. Tsai, C.A. Hsieh, S.W. Kao, J.W. Yeh, T.S. Chin, S.K. Chen, J. Alloys Compd. 477 (2009) 696. [15] C.P. Lee, Y.Y. Chen, C.Y. Hsu, J.W. Yeh, H.C. Shih, J. Electrochem. Soc. 154 (2007) C424. [16] N. Martin, O. Banakh, A.M.E. Santo, S. Springer, R. Sanjinés, J. Takadoum, F. Lévy, Appl. Surf. Sci. 185 (2001) 123. [17] M.C. Lin, L.S. Chang, H.C. Lin, Surf. Coat. Technol. 202 (2008) 5440. [18] N.F. Mott, E.A. Davis, Electronic Processes in Non-crystalline Materials, Oxford, New York, 1979, pp. 288. [19] J.I. Pankove, Optical Processes in Semiconductors, Prentice-Hall, Inc., Englewood Cliffs, NJ, 1971, pp. 34. [20] R.S. Yu, C.J. Huang, R.H. Huang, C.H. Sun, F.S. Shieu, Appl. Surf. Sci. 257 (2011) 6073. [21] M. Laurikaitis, J. Dudonis, D. Milˇcius, Thin Solid Films 516 (2008) 1549. [22] M.H. Chan, P.L. Wu, F.H. Lu, Thin Solid Films 518 (2010) 7300. [23] S.K. Rawal, A.K. Chawla, V. Chawla, R. Jayaganthan, R. Chandra, Appl. Surf. Sci. 256 (2010) 4129.
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Synthesis and characterization of multi-element oxynitride semiconductor film prepared by reactive sputtering deposition Ruei-Sung Yu a , Rong-Hsin Huang b , Chih-Ming Lee b , Fuh-Sheng Shieu b,∗ a b
Department of Photonics and Communication Engineering, Asia University, 500, Lioufeng Rd., Wufeng, Taichung 413, Taiwan Department of Materials Science and Engineering, National Chung Hsing University, Taichung 402, Taiwan
a r t i c l e
i n f o
Article history: Received 16 March 2012 Received in revised form 25 August 2012 Accepted 26 August 2012 Available online 20 September 2012 Keywords: Semiconductor Structure Multi-element Thin film Oxynitride
a b s t r a c t This study concerns the use of reactive magnetron sputtering to prepare (TiVCrZrTa)-based oxide and oxynitride films. (TiVCrZrTa)1−x Ox and (TiVCrZrTa)1−x−y Ny Ox films were prepared, and were found to be amorphous and free of multi-phase structure. Cations and anions in such structures were arranged in a random homogeneous dispersion. The introduction of nitrogen atoms into (TiVCrZrTa)1−x Ox yields (TiVCrZrTa)1−x−y Ny Ox , which has a reduced oxidation state and thus, an increased number of the valence electrons. The (TiVCrZrTa)1−x−y Ny Ox film is an n-type semiconductor, with an indirect band gap of 1.95 eV, and a carrier concentration (N) and conductivity () of 1.01 × 1019 cm−3 and 2.75 × 10−2 ( cm)−1 , respectively. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Binary semiconductor compounds, such as n-type In2 O3 , ZnO and TiO2 , have been recognized as important semiconductor materials, and are widely utilized in flat panel displays, solar cells, and many other semiconductor products [1,2]. Ternary materials, such as copper-based oxides including p-type CuAlO2 , CuCrO2 , CuInO2 , and CuGaO2 , have also been well studied [3–5]. However, to this day, there have been few studies into multi-element semiconductor materials composed of more than six elements. In an effort to address this, we propose a synthesis method in this study motivated by the high-entropy effect theory of multi-element materials that has been discussed in many previous reports [6–14]. The theory addresses the following issues: (a) forming simple body centered cubic (BCC) and/or face centered cubic (FCC) solid solution crystal structure(s); (b) easy formation of amorphous materials; achieving the advantageous material properties of (c) high hardness; and (d) thermal stability and corrosion resistance. There are numerous potential multi-element semiconductors that could be developed. In this paper, we describe the study of novel ceramic materials with their constituent materials selected from TiN, VN, CrN, ZrN and TaN, which are all known to be nitride coating films, as well as the elements Ti, V, Cr, Zr and Ta, giving films of the
∗ Corresponding author. Tel.: +886 4 22854563; fax: +886 4 22857017. E-mail address: [email protected] (F.-S. Shieu). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2012.08.109
form (TiVCrZrTa)1−x Ox and (TiVCrZrTa)1−x−y Ny Ox . Through experiment, the multi-element oxynitride material was found to exhibit semiconducting behavior.
2. Experimental 2.1. Synthesis method Clean glass substrates were prepared by sequential ultrasonic vibration in baths of hot degreasing solvent, acid solution, and deionized water. Deposition of the films was carried out by reactive magnetron sputtering in a home-built system at around room temperature. The base pressure and deposition pressure in the vacuum chamber were 4.0 × 10−4 and 4.0 × 10−1 Pa, respectively. The sputtering target material was prepared by melting a mix of the metallic starting materials under vacuum in equi-atomic proportions, which resulted in the composition Ti0.2 V0.2 Cr0.2 Zr0.2 Ta0.2 . The deposition time for each film specimen was 20 min. A 200 W direct current was applied during film deposition. The target diameter was 10.16 cm and the area of the cathode was 81.03 cm2 . The power density was 2.47 W/cm2 . During multi-element oxide film deposition, the O2 /(O2 + Ar) gas mass flow ratio was maintained at 15%. During deposition of multi-element oxynitride film, the O2 /(O2 + N2 + Ar) and N2 /(O2 + N2 + Ar) gas mass flow ratios were 2.0% and 50%, respectively. Target-to-substrate distance was kept constant at 70 mm for all depositions.
R.-S. Yu et al. / Applied Surface Science 263 (2012) 58–61
Fig. 1. The X-ray diffraction (TiVCrZrTa)1−x−y Ny Ox thin films.
spectra
of
the
(TiVCrZrTa)1−x Ox
and
2.2. Characterization The structural and semiconductor properties of the films were characterized by X-ray diffraction (XRD, MAC SCIENCE MPXIII, BRUKER), high resolution transmission electron microscope (HRTEM, JEOL JEM-2010), X-ray photoelectron spectroscopy (XPS, ULVAC-PHI 5000 VersaProbe), UV-vis spectrophotometer (HITACHI U3010), and Hall effect measurement (ACCENT HL5500PC).
3. Results and discussion 3.1. X-ray diffraction analysis Fig. 1 shows the X-ray diffraction patterns of the multi-element oxide and oxynitride films deposited onto glass. No diffraction
59
peaks indicative of crystal structures were detected, suggesting that both films were amorphous. According to the laws of thermodynamics, a multi-element material with atoms in a homogeneous random arrangement tends to form a solid-solution phase, which maximizes the entropy and minimizes the free energy of the material [8,15]. A review of related literature [11] shows that the multi-element nitride film (AlMoNbSiTaTiVZr)50 N50 is amorphous, but not of a multi-phase structure. In the amorphous phase atoms are in a homogeneous random arrangement. Due to the amorphous characteristics displayed by the (AlMoNbSiTaTiVZr)50 N50 film, we selected this material as a representative case for the aforementioned fundamental theories of multi-element materials. The XRD and HRTEM (see discussion below) analysis conducted in this study demonstrated that multi-element materials are amorphous and free from multi-phase structure, in agreement with previous reports. To sum up the aforementioned analyses, the multi-element oxide and oxynitride materials examined in this study show solid-solution amorphous characteristics. In the three-dimensional structure, anions (O and N) act as cores and are surrounded by cations (Ti, V, Zr, Cr and Ta) in a homogeneous random arrangement.
3.2. X-ray photoelectron spectroscopy analysis X-ray photoelectron spectroscopy (XPS) measurements were performed to evaluate the chemical composition and chemical state of the films. The results of the chemical composition analysis of (TiVCrZrTa)1−x Ox and (TiVCrZrTa)1−x−y Ny Ox films are shown in Table 1. The two films both have cation (Ti, V, Zr, Cr and Ta) to anion (O and N) ratios close to 2:3. According to these results, the (TiVCrZrTa)1−x Ox and (TiVCrZrTa)1−x−y Ny Ox films can be expressed as (TiVCrZrTa)0.415 O0.585 and (TiVCrZrTa)0.408 N0.243 O0.349 , respectively. Oxygen has a relatively strong reactivity, when compared to nitrogen [16]. Oxygen thus has a greater tendency to react with
Fig. 2. XPS diagrams of the (TiVCrZrTa)1−x Ox and (TiVCrZrTa)1−x−y Ny Ox films containing (a) Ti, (b) V, (c) Cr, and (d) O elements, respectively.
60
R.-S. Yu et al. / Applied Surface Science 263 (2012) 58–61
Table 1 Chemical composition analysis of (TiVCrZrTa)1−x Ox and (TiVCrZrTa)1−x−y Ny Ox films obtained by the X-ray photoelectron spectroscopy (XPS). Specimens
(TiVCrZrTa)1−x Ox (TiVCrZrTa)1−x−y Ny Ox
XPS atomic concentration (%) Ti
V
Cr
Zr
Ta
O
N
3.77 3.58
17.85 8.62
7.36 10.86
5.47 8.56
7.03 9.20
58.52 34.92
– 24.26
metallic species during film deposition. Residual oxygen in the vacuum chamber contributes to the formation of oxynitride materials [17]. For this reason, the multi-element oxynitride film contains 34.92 at.% oxygen, even though the oxygen mass flow was heavily decreased during the deposition process. There is a literature report of sputtering deposition with no mass flow of oxygen during the deposition of TiNx Oy films [17], in which the oxygen originated from the PET substrate or residual gas in the chamber. According to theories of material sciences, the amorphous character of the film means cations (Ti, V, Zr, Cr and Ta) and anions (O and N) tend to remain close to one another, resulting in atomic bonding. Fig. 2 shows XPS spectra of the (TiVCrZrTa)1−x Ox and (TiVCrZrTa)1−x−y Ny Ox films containing Ti, V, Cr and O, respectively. Fig. 2(a)–(c) illustrates that the introduction of nitrogen atoms into (TiVCrZrTa)1−x Ox to form (TiVCrZrTa)1−x−y Ny Ox can reduce the binding energy of the Ti, V, and Cr cations in the film, which translates to increased numbers of valence electrons or a decreased oxidation state, in turn causing higher conductivity. For instance, in Fig. 2(b) the electron binding energy of V 2p3/2 decreases from 514.6 to 512.1 eV, in response to the oxynitride. A decrease in binding energy is also observed in the Zr and Ta spectra (not shown). In other words, it is possible to alter the electrical properties of the (TiVCrZrTa)1−x Ox film by adjusting the quantity of nitrogen introduced. Fig. 2(d) indicates that the binding energy of O 1s was unchanged in the oxide and oxynitride films. That is to say, the introduction of nitrogen does not influence the binding energy between oxygen and nitrogen. Since both films are amorphous, the cations and anions in the material tend to stay close to one another. 3.3. Semiconductor properties analysis A correlation between the band gaps and electrical properties of the films is listed in Table 2. An amorphous material shows an indirect transition on the electronic band diagram [18], hence the two films are both amorphous and do not possess a direct band gap. The value of the band gap can be estimated using the standard relation [19], (˛hv)
1/n
= A(hv − Eg )
(1)
where ˛ is the absorption coefficient, h is the photon energy, A is a constant and Eg is the band gap. The value of n for an indirect
allowed transition is 2. Plots of (˛h)1/2 versus h were made, from which the values of the indirect band gap were derived. As shown in Fig. 3(a), the (TiVCrZrTa)1−x Ox film exhibited a relatively large indirect band gap of 2.38 eV, and displayed insulating properties. As illustrated in Fig. 3(b), upon the introduction of nitrogen atoms, causing the formation of (TiVCrZrTa)1−x−y Ny Ox , the material changed to show semiconductor characteristics, and its indirect band gap was lowered to 1.95 eV. It has been reported that the indirect band gap of multi-element (ZnSnCuTiNb)1−x Ox semiconductor films are 1.69 and 2.26 eV, respectively [20]. These discrepancies may be attributed to differences in the chemical composition of the material. Hall effect measurements were employed to determine the electrical properties of the films. The (TiVCrZrTa)1−x Ox film had a resistance larger than the upper measurable limit of the Hall effect measurement system. With the introduction of nitrogen atoms, the (TiVCrZrTa)1−x−y Ny Ox formed a semiconductor with a carrier concentration (N) and conductivity value () of 1.01 × 1019 cm−3 and 2.75 × 10−2 ( cm)−1 , respectively. The (TiVCrZrTa)1−x−y Ny Ox film is amorphous with atoms irregularly arrayed, thus it is impossible to describe the conductivity mechanism by means of a nonstoichiometric defect equation. Since oxygen and nitrogen are both anions, some oxygen atoms are replaced by nitrogen during the addition of nitrogen into the multi-element oxide. Thus, in a three-dimensional structure, oxygen and nitrogen anions act as cores, surrounded by cations in a homogeneous random arrangement. The relatively weak electronegativity of nitrogen makes it a less potent oxidizer than oxygen, and thus the replacing of oxygen with nitrogen atoms leaves some cations with untransferred valence electrons, which may act as carriers. The conductivity mechanism of the multi-element oxynitride is thus subject to the existence of outerorbital valence electrons in the cations. This was confirmed by the measurement of a negative Hall coefficient (Rh), indicating that electrons are the main carriers and that the (TiVCrZrTa)1−x−y Ny Ox film is an n-type semiconductor. A further discussion, with related studies reported about the structural and optoelectronic properties of the ZrNx Oy and TiNx Oy films [21–23]. Structurally, (TiVCrZrTa)1−x−y Ny Ox is an amorphous solid, unlike crystalline ZrNx Oy and TiNx Oy . The ZrNx Oy film showed the sheet resistance and band gap of 1.1 × 103 /sq and 3.04 eV, respectively [21]. For other relevant oxynitride studies, the ZrNx Oy
Fig. 3. Indirect band gaps of (a) the (TiVCrZrTa)1−x Ox and (b) the (TiVCrZrTa)1−x−y Ny Ox thin films.
R.-S. Yu et al. / Applied Surface Science 263 (2012) 58–61
61
Table 2 Band gaps and electrical properties of the (TiVCrZrTa)1−x Ox and (TiVCrZrTa)1−x−y Ny Ox films, where gap is the indirect band gap, N is the carrier concentration, Rh is the Hall coefficient, is the carrier mobility and is the conductivity. Specimens
Gap (eV)
N (cm−3 )
Rh (m2 /C)
(cm2 /V s)
( cm)−1
(TiVCrZrTa)1−x Ox (TiVCrZrTa)1−x−y Ny Ox
2.38 1.95
– 1.01 × 1019
– −1.89
– 0.017
– 2.75 × 10−2
Fig. 4. HRTEM images of the (TiVCrZrTa)1−x−y Ny Ox film, including the (a) cross-sectional image, (b) nanometerscale image, and (c) SAD pattern.
films have band gaps between 1.96 and 2.26 eV [22], and the TiNx Oy have values between 2.11 and 3.34 eV [23]. The variations within the same kind of oxynitride films are attributed to the different crystal structures and chemical compositions. The multielement oxynitride band gap of 1.95 eV is very close to the literature reported value of 1.96 eV (ZrNx Oy ). 3.4. High resolution transmission electron microscope (HRTEM) analysis To further investigate the film, an examination using crosssectional HRTEM analysis was performed. Based on the measured electrical properties, we selected the (TiVCrZrTa)1−x−y Ny Ox film for its semiconductor properties. Fig. 4(a) shows a cross-sectional HRTEM image of the (TiVCrZrTa)1−x−y Ny Ox specimen. The three layers – Pt film, the oxynitride film, and glass – are shown. In preparing the material for HRTEM analysis, Pt film was deposited as the top layer in order to protect the oxynitride film during the cutting of by focused ion beam (FIB). The oxynitride film shows a smooth crosssection and is without notable contrast, indicating that the material is amorphous. In Fig. 4(b), the bright field nanometer scale image of the oxynitride film looks disordered, without a lattice structure. In Fig. 4(c), the broad ring pattern in the selected area diffraction (SAD) likewise indicates that the film was amorphous. This result is also consistent with that of the XRD. 4. Conclusion In this study, the structural and semiconductor properties of (TiVCrZrTa)-based oxide and oxynitride films were examined. The results revealed that (TiVCrZrTa)1−x Ox and (TiVCrZrTa)1−x−y Ny Ox are amorphous. In the three-dimensional structure, anions (O and N) act as cores surrounded by cations (Ti, V, Zr, Cr and Ta) in a random and homogeneous dispersion. The (TiVCrZrTa)1−x Ox film is nonconducting, with an indirect band gap of 2.38 eV. The introduction of nitrogen to the multi-element oxide film increases the contribution of conducting electrons from the outer orbital electrons of the cations. The resulting (TiVCrZrTa)1−x−y Ny Ox film is an n-type semiconductor. The indirect band gap and conductivity of the multi-element oxynitride film were 1.95 eV and 2.75 × 10−2 ( cm)−1 , respectively. The adjustability of the elemental composition in the multi-element material facilitates
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