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Effects of oxygen partial pressure on the characteristics of magnetron-sputtered ZnMgBeO thin films
Applied Surface Science 355 (2015) 582–586
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effects of oxygen partial pressure on the characteristics of magnetron-sputtered ZnMgBeO thin films Hoang Ba Cuong, Byung-Teak Lee ∗ Photonic and Electronic Thin Film Laboratory, Department of Materials Science and Engineering, Chonnam National University, 300 Yongbong-dong, Gwangju 500-757, Republic of Korea
a r t i c l e
i n f o
Article history: Received 10 March 2015 Received in revised form 8 June 2015 Accepted 8 July 2015 Available online 15 July 2015 Keywords: ZnO ZnMgBeO UV detector Oxygen effects
a b s t r a c t Effects of oxygen partial pressure within the Ar process plasma on the optical, structural, and electrical properties of magnetron-sputtered ZnMgBeO films were investigated in detail. It was observed that the optical energy bandgap (Eg ) values of the ZnMgBeO films substantially decrease with the oxygen addition, from 5.3 to 4.3 eV as the oxygen partial pressure increases from zero to one. The full-widthat-half-maximum (FWHM) values of the (0 0 0 2) XRD peaks drastically decrease with the addition of a small amount of oxygen but then increase with further oxygen addition. All the films had very high sheet resistance, 1.3–1.4 G/䊐. It was also observed that the concentration of Zn within the films significantly increased with the oxygen addition, which was proposed to be mainly responsible for the observed decrease in Eg . It was also proposed that the FWHM change due to the oxygen addition may be attributable to three factors, film composition, grain size, and point defect density, as confirmed by results of TEM and XPS investigations. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Zinc magnesium beryllium oxide (ZnMgBeO) is being investigated as a promising material for ultraviolet (UV)-range optoelectronics, such as the solar-blind ultraviolet (UV) photodetectors [1,2], because the bandgap (Eg ) values can be modulated continuously, from 3.7 to 5.1 eV, by changing the concentration of MgO and BeO [1,2]. Wurtzite ZnMgO films have been studied intensively for the UV application but it is generally known to be difficult to obtain Eg values higher than ∼4 eV due to the phase segregation occurring at higher Mg concentrations [3], although it was recently reported that ZnMgO films with 4.55 eV Eg can be grown utilizing the two-step growth and the molecular beam epitaxial technique [4]. In deposition process of ZnO-based thin films, addition of O2 gas to the reaction plasma is one of the most important factors that influence electrical and optical properties of the films. It has been reported that the bandgap energy values, the residual stress, and the carrier concentration are critically affected by the oxygen stoichiometry [5,6]. It was also reported in the case of GaZnO and GaAlZnO that structural properties are improved and the carrier
∗ Corresponding author. Tel.: +82 104655538. E-mail address: [email protected] (B.-T. Lee). http://dx.doi.org/10.1016/j.apsusc.2015.07.051 0169-4332/© 2015 Elsevier B.V. All rights reserved.
concentration reduced by introducing oxygen to the Ar process plasma during the sputtering [6,7]. It is therefore very interesting and essential to study and understand effects of the oxygen on the film microstructure and properties, to obtain high-quality ZnMgBeO films with proper characteristics. In this work, effects of the O2 addition to the Ar process plasma on the characteristics of the reactive magnetron-sputtered ZnMgBeO films were studied in detail. The optical, structural, and electrical properties were investigated and possible mechanisms were proposed.
2. Experimental details The ZnMgBeO films, 250–300-nm thick, were grown on quartz substrates using a conventional RF magnetron-sputtering system. The 2-in. ceramic Zn0.59 Mg0.21 Be0.2 O target was made from high purity powders ZnO, MgO, and BeO. The sputter chamber was initially evacuated down to ∼10−6 Torr, and then the chamber pressure was maintained at 10−2 Torr during the deposition process. The growth temperature was 200 ◦ C, and the RF power was 75 W. During the sputtering process, high purity Ar and O2 gases were introduced into the sputter chamber and their mass flow rates were adjusted so that the O2 /Ar + O2 ratio ([O2 ]) ranged from 0.0 to 1.0. Before film growth, targets were pre-sputtered for 10 min to remove possible surface contamination.
H.B. Cuong, B.-T. Lee / Applied Surface Science 355 (2015) 582–586
Table 1 Composition of the Znx Mgy Bez O thin films sputtered from the Zn0.59 Mg0.21 Be0.2 O target at various oxygen partial pressures, [O2 ] = O2 /(Ar + O2 ).
80 Ar 5% 10% 30% 50% 70% O2
60 40 20
300
400
Transmittance (cm)
Transmittance (%)
100 (a)
0 200
100
[O2 ]
xZn
yMg
zBe
80
0 0.05 0.1 0.3 0.5 0.7 1.0
0.31 0.51 0.55 0.57 0.58 0.57 0.54
0.36 0.25 0.23 0.22 0.21 0.21 0.24
0.33 0.24 0.22 0.21 0.21 0.22 0.23
60 40 20 0 220
240
260
280
300
Wavelength (nm)
500
600
700
800
(αhv)^2 (a.u.)
(b)
(c) Ar 5% 10% 30% 50% 70% O2 4
Experiment 6 Vegard's law
5
4 5
Bandgap (eV)
0.0
0.4
[O2]
0.8
Optical bandgap (eV)
Wavelength (nm)
3
583
on the Eg change of the films would not be significant. It has been reported that the carrier concentration must be higher than about 1018 cm−3 to effectively change the Eg of ZnO-based materials [10]. Chemical compositions of the films were measured using the ICP-OES method and results are shown in Table 1. It is clearly observed that the concentration of Zn (xZn ) substantially increases with the addition of the oxygen to the Ar plasma, from 0.32 at [O2 ] = 0.0–0.55 at [O2 ] = 0.1 and then remain almost unchanged with further increase in the oxygen partial pressure within the plasma. Accordingly, the concentration of Mg (yMg ) and Be (zBe ) significantly decreases with the oxygen addition. It has been proposed that effects of the composition on the Eg MgO
Fig. 1. Results of the optical characterization of ZnMgBeO films sputtered at various [O2 ]. The insets show plots of (˛hv)2 against hv and variation in the measured optical bandgap and the estimated values, as a function of the [O2 ].
The crystal structures of the deposited thin films were examined by high-resolution X-ray diffraction (XRD, X’pert PRO, Philips, Eindhoven, Netherlands) operated at 40 kV and 30 mA using Nifiltered Cu K␣ radiation and a transmission electron microscope (TEM, JEOLJEM) operating at 200 kV. Compositions of the films were measured using inductively coupled plasma optical emission spectroscopy (ICP-OES, Perkin Elmer OPTIMA 4300 DV). The optical transmittance of the deposited films was obtained using a UV-visible spectroscope (Cary 100, Varian) and the electrical properties were using a four-point probe system (Changmin Tech, CMT-SR2000). The chemical binding energies of the ZnMgBeO films were studied by the X-ray photoelectron spectroscopy (XPS, VG Multilab 2000) at room temperature. 3. Results and discussion Fig. 1 shows the transmittance spectra of ZnMgBeO films sputter grown in the Ar + O2 plasma with various [O2 ]. It is observed in Fig. 1a that the average transmittance in the visible wavelength region is more than 85% for all samples and the cut-off wavelength increases with increasing O2 partial pressure, from 234 nm for the pure Ar plasma to 289 nm for the pure oxygen plasma. This indicates that the optical band-gap energy (Eg ) decreases from 5.3 to 4.3 eV, as the [O2 ] increased from zero to 1.0 as shown in Fig. 1b and c. It has been reported that the Eg of semiconductor materials are mostly determined by the composition (Vegard’s rule) and the free carrier concentration (Burstein-Moss effect) [8]. Results of the fourpoint probe measurement of the ZnMgBeO films indicated that all the films have very high sheet resistance, 1.3–1.4 G/䊐. The Hall measurement was not possible to apply, due to the high resistivity. Considering that a Zn0.92 Mg0.05 Be0.03 O film with a sheet resistance of ∼1 M/䊐 showed a carrier concentration of 3 × 1016 cm−3 and a mobility of 4.4 cm2 /V s in our previous report [9], it is believed that the free carrier concentrations of the ZnMgBeO films studied in this work must have been very low, probably 1013 –1014 cm−3 . This indicates that the contribution of the Burstein–Moss (B–M) effect
of Znx Mgy Bez O films can be described as Eg = xEgZnO + yEg + zEgBeO − bZnMgO y(1 − y) − bZnBeO z(1 − z) − byz yz, where bZnMgO , bZnBeO denote the bowing parameters of ZnMgO and ZnBeO, respectively, and the byz can be obtained using the equation byz = bZnBeO − bZnMgO − bMgBeO [11]. The Eg values were estimated using previously proposed bowing parameter values of 8.2116, 1.4172, and 5.4772 eV for the bZnBeO , the bZnMgO , and the bMgBeO , respectively [11], and the film composition measured in this work (Table 1). The results of the calculation are shown in Fig. 1c. It must be mentioned that the estimated Eg values in Fig. 1b may not be exactly accurate due to the uncertainty in the bowing parameters utilized (it is known that the bowing parameter values change with the film composition). It is clearly noticeable in Fig. 1c, however, that the estimated Eg values do reflect the change of the measured Eg values as a function of the [O2 ] very well. This observation, along with the fact that the carrier concentrations of the films are very small, indicates that the observed O2 effects on Eg are mainly due to the composition change. It is believed that the increase in the Zn concentration due to the oxygen addition is mainly because the re-evaporation of Zn from the growing film is suppressed. It has been reported that metallic Zn phase forms during the sputtering under the oxygen deficient ambient, which readily re-evaporates from the growing films due to its low binding energy [12,13]. Also, the MgO phase, with lower formation enthalpy, was reported to form prior to the ZnO phase if the oxygen supply is not enough [12]. At the oxygen-rich environment, formation of the ZnO phase would increase, and the Zn concentrations within the films also increase. Fig. 2a shows XRD patterns of ZnMgBeO films grown at various oxygen partial pressures. It is noticeable that the c-axis lattice parameter (d(0 0 0 1)) increases from 0.483 to 0.495 nm and the (0 0 0 2) FWHM drastically decreases from 0.54o to 0.3o with addition of small amount of oxygen to the Ar process plasma. With increasing oxygen partial pressure, the FWHM continuously increases to 0.7◦ for [O2 ] = 1, as summarized in Fig. 2b. The d(0 0 0 1) remains almost unchanged until [O2 ] = 0.5 and then increases to 0.513 nm at [O2 ] = 1 (Fig. 2b). It has been suggested that a few factors are operating to affect crystalline quality (represented by the FWHM values) of the ZnOrelated films, such as the film composition, the grain size, and the density of the point defects [5,6]. Concentrations of Mg and Be
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ZnMgBeO (0002)
XRD Intensity (a.u.)
(a)
O2 Ar+70%O2 Ar+50%O2 Ar+30%O2 Ar+10%O2 Ar+5%O2 Ar
40
45
2-theta
50
55
(b)
60
0.52
o
FWHM ( )
0.8
35
0.6
0.48
0.4
0.44 0.40
0.2 0.0
0.2
0.4
0.6
[O2]
0.8
1.0
c-axis lattice constant (nm)
30
Fig. 2. Results of X-ray characterization of the ZnMgBeO films, (a) XRD patterns and (b) the variation of the c-axis parameter and the FWHM of (0 0 0 2) peak as a function of the oxygen concentration.
Fig. 3. Plan-view TEM images of the ZnMgBeO films sputtered at various [O2 ], (a) [O2 ] = 0.0, (b) [O2 ] = 0.3, and (c) [O2 ] = 1.0.
reported that a typical O 1s XPS peak is composed of three distinct Lorentzian–Gaussian profiles, centered at 530.2, 531.2, and 532.4 eV [15]. It was proposed that the high energy peak at 532.4 eV may be attributable to the presence of loosely bound oxygen on the
(a)
Relative Intensity
within the ZnMgBeO films decrease with increasing [O2 ], as shown in Table 1, which would results in smaller FWHM of the XRD peaks. Notice that the ionic radii of Mg (0.057 nm) and Be (0.027 nm) are smaller than that of Zn (0.06 nm). It is believed that this factor would be one of the possible mechanisms responsible for the initial decrease in the FWHM due to the O2 addition (Fig. 2b). It has been reported that the grain size of GaZnO thin films are affected by oxygen addition to the process plasma [5]. It was proposed that, in an oxygen-rich environment, the excess oxygen segregated at the grain boundaries and inhibited the grain growth, resulting in grains with reduced size [5]. It is also suspected that the number and the energy of the ions impinging onto the substrate decrease with addition of oxygen to Ar plasma, which would reduce the nucleation of the growing films and therefore increase the grain size [14]. It is reminded that the ionization energy of oxygen (47.76 eV) is higher than that of Ar (15.76 eV) and the ion density and the ion energy of the Ar plasma are much higher than those of the oxygen plasma [14]. Some of the films were studied using the TEM, in order to directly observe microstructures of the ZnMgBeO films. It is clearly seen in Fig. 3, plan-view TEM images of the ZnMgBeO films that the grain size of films initially increases with addition of O2 , from 22 nm at [O2 ] = 0.0 (Fig. 3a) to 30 nm at [O2 ] = 0.3 (Fig. 3b), and then decrease with further O2 addition, to 20 nm at [O2 ] = 1.0 (Fig. 3c). From the Scherrer equation, the FWHM of the XRD peaks due to the grain size are estimated to be 15 nm for [O2 ] = 0.0, 25 nm for [O2 ] = 0.3, and 12 nm for [O2 ] = 1.0, which follows a trend consistent with that of the FWHM change. Although the calculated FWHM values are slightly smaller than the observed ones, these results clearly indicate that the grain size change is one of the mechanisms responsible for the observed FWHM change. The XPS investigation of the films was performed to study behaviour of the point defects within the ZnMgBeO films and the O 1s peaks of the XPS spectra of films sputtered at (a) [O2 ] = 0.0, (b) [O2 ] = 0.3 and (c) [O2 ] = 1.0 are shown in Fig. 4. It has been
O 1s
(b)
O 1s
(c)
O 1s
538
536
534
532
530
528
526
524
Binding energy (eV) Fig. 4. The XPS O 1s peaks of the ZnMgBeO films, sputtered in (a) pure Ar plasma, (b) Ar plasma with 30% O2 , and (c) pure O2 plasma.
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surface of the films [16,17]. The low binding energy peak at 530.2 eV is attributable to the O2− ions binding with Zn2+ ions in ZnO and the medium energy peak at 531.2 eV is due to oxygen vacancies (O2− ions in oxygen-deficient regions) [15,16]. It is observed in Fig. 4 that the medium energy peak, related to the oxygen vacancy, is comparatively strong in the films sputtered in pure Ar plasma, Fig. 4a, and diminishes in the cases of the films sputtered in Ar/O2 plasma, Fig. 4b and c. This result indicates that a high density of oxygen vacancies exists within the ZnMgBeO films grown in pure Ar plasma, whose density will be significantly reduced by the oxygen addition. In fact, it has been previously reported that oxygen ambient during the film growth plays a major role in reducing the defects due to oxygen deficiency, based upon results of the Raman observation [18] and the optical absorption measurement [19]. It is believed that this phenomenon would be partly responsible for the initial decrease in the FWHM values by the oxygen addition (Fig. 2). It has been also reported that defects related to the excess oxygen, such as oxygen interstitial, Zn vacancy, and their complexes, formed in the case of the various ZnO-related films grown at the oxygen-rich conditions [6,18]. It is believed that the FWHM increase observed for the samples with high [O2 ] is critically related to the formation of the defects related to the excess oxygen, which would degrade the crystallinity of the films and widen the X-ray peaks. Unfortunately, the XPS spectra obtained in this work did not show firm evidence on the formation of excess-oxygen defects (Fig. 4b and c do not show any critical difference) as the defects related to the excess oxygen are not well represented by specific XPS peak(s) [15–17]. Summarizing the discussions above, it is believed that all of the proposed three factors, reduced Mg/Be concentration, increased grain size, and reduced density of defects related to the oxygen deficiency, contributed to the initial decrease in the FWHM of the XRD (0 0 0 2) peaks (Fig. 2). It is also suggested that, with further increase in the oxygen composition in the Ar plasma, the grain size decreases and the density of defects (associated with excess oxygen) increases, resulting in the increased FWHM. As to the c-axis lattice parameter change due to the oxygen addition, it is believed that the initial increase in the d(0 0 0 1), from 0.483 nm at [O2 ] = 0 to 0.498 nm at [O2 ] = 0.1, is mainly due to the film composition change. It is reminded that the c-axis lattice constants d(0 0 0 1) of BeO and MgO, 0.43 nm [20] and 0.41 nm [21], respectively, are smaller than that of ZnO (0.52 nm [22]), and the lattice will expand as the concentrations of BeO and MgO decrease (Table 1). In the case of the films with [O2 ] > 0.5, where the compositional change is not significant, it is suspected that the presence of point defects related to the excess oxygen played a significant role in the lattice constant increase. In fact, it has been reported that the lattice expansion was observed in the case of the doped ZnO films grown at oxygen-rich environment, which was partly attributed to the presence of oxygen interstitials [23,24]. Quantitative estimation of the changes in the XRD FWHM and the lattice change requires detailed information on the nature and the amount of the associated point defects. Further investigation on the electrical as well as the structural properties is in progress to clarify the defect dynamics related to the stoichiometry of the ZnO-based thin films and the results will be reported in a future publication.
4. Summary Effects of oxygen partial pressure within the process plasma on the optical, structural and electrical properties of magnetron sputtered ZnMgBeO films were investigated in detail. It was observed that the Eg values of the ZnMgBeO films substantially decrease with
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the oxygen addition, from 5.3 to 4.3 eV as the [O2 ] increases from zero to one. The FWHM values of the (0002) XRD peaks drastically decreased from ∼0.55◦ to ∼0.3◦ with addition of small amount of oxygen to the Ar process plasma and then increase to ∼0.6◦ with further increase in the oxygen partial pressure. Results of the fourpoint probe measurement indicated that all the films have very high sheet resistance, 1.3–1.4 G/䊐. It was also observed that the concentration of Zn within the films significantly increased with the oxygen addition. It was proposed that the observed decrease in the Eg is mainly due to the film composition change and the FWHM change due to the oxygen addition may be attributed mainly to three factors: film composition, grain size, and point defect density. It was observed from the TEM investigation that the grain size increases with initial oxygen addition but then decrease with further addition. High density of oxygen vacancies exists in the ZnMgBeO films grown in pure Ar plasma, which is significantly reduced in the case of the Ar + O2 plasma, as confirmed by the XPS measurement. Acknowledgments This research was supported by the Basic Science Research Program (2013R1A1A2008793) through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning. Authors thank the Korea Basic Science Institute for their assistance in the materials characterization. References [1] C. Yang, X.M. Li, Y.F. Gu, W.D. Yu, X.D. Gao, Y.W. Zhang, ZnO based oxide system with continuous bandgap modulation from 3.7 to 4.9 eV, Appl. Phys. Lett. 93 (2008) 112114. [2] M. Toporkov, V. Avrutin, S. Okur, N. Izyumskaya, D. Demchenko, J. Volk, D.J. Smith, H. Morkoc¸, Ü. Özgür, Enhancement of Be and Mg incorporation in wurtzite quaternary BeMgZnO alloys with up to 5.1 eV optical bandgap, J. Cryst. Growth 402 (2014) 60–64. [3] P. Kumar, J.P. Singh, Y. Kumar, A. Gaur, H.K. Malik, K. Asokan, Investigation of phase segregation in Zn1−x Mgx O systems, Curr. Appl. Phys. 12 (2012) 1166–1172. [4] X. Du, Z. Mei, Z. Liu, Y. Guo, T. Zhang, Y. Hou, Z. Zhang, Q. Xue, A.Y. Kuznetsov, Controlled growth of high-quality ZnO-based films and fabrication of visible-blind and solar-blind ultra-violet detectors, Adv. Mater. 21 (2009) 4625–4630. [5] B. Zhou, A.V. Rogachev, Z. Liu, D.G. Piliptsou, H. Ji, X. Jiang, Effects of oxygen/argon ratio and annealing on structural and optical properties of ZnO thin films, Appl. Surf. Sci. 258 (2012) 5759–5764. [6] H. Makino, H. Song, T. Yamamoto, Influences of oxygen gas flow rate on electrical properties of Ga-doped ZnO thin films deposited on glass and sapphire substrates, Thin Solid Films 559 (2014) 78–82. [7] H. Jeong Soo, M. Nobuhiro, K. Kyung Hwan, Investigation of the effect of oxygen gas on properties of GAZO thin films fabricated by facing targets sputtering system, Semicond. Sci. Technol. 29 (2014) 075007. [8] B.E. Sernelius, K.F. Berggren, Z.C. Jin, I. Hamberg, C.G. Granqvist, Band-gap tailoring of ZnO by means of heavy Al doping, Phys. Rev. B 37 (1988) 10244–10248. [9] H.B. Cuong, C.-S. Lee, B.-T. Lee, Effects of Ga concentration, process conditions, and substrate materials on properties of ZnMgBeGaO ultraviolet-range transparent conducting films, Thin Solid Films 573 (2014) 95–99. [10] J.G. Lu, S. Fujita, T. Kawaharamura, H. Nishinaka, Y. Kamada, T. Ohshima, Z.Z. Ye, Y.J. Zeng, Y.Z. Zhang, L.P. Zhu, H.P. He, B.H. Zhao, Carrier concentration dependence of band gap shift in n-type ZnO:Al films, J. Appl. Phys. 101 (2007) 083705. [11] H.L. Shi, Y. Duan, Band-gap bowing and p-type doping of (Zn, Mg, Be)O wide-gap semiconductor alloys: a first-principles study, Eur. Phys. J. B 66 (2008) 439–444. [12] C.X. Cong, B. Yao, Y.P. Xie, G.Z. Xing, B.H. Li, X.H. Wang, Z.P. Wei, Z.Z. Zhang, Y.M. Lv, D.Z. Shen, X.W. Fan, Nitrogen partial pressure-dependent Mg concentration, structure, and optical properties of Mgx Zn1−x O film grown by magnetron sputtering, J. Mater. Res. 22 (2007) 2936–2942. [13] F.E. Dart, Evaporation of zinc and zinc oxide under electron bombardment, Phys. Rev. 78 (1950) 761–764. [14] I.T. Tang, Y.C. Wang, W.C. Hwang, C.C. Hwang, N.C. Wu, M.-P. Houng, Y.-H. Wang, Investigation of piezoelectric ZnO film deposited on diamond like carbon coated onto Si substrate under different sputtering conditions, J. Cryst. Growth 252 (2003) 190–198.
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Contents lists available at ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Effects of oxygen partial pressure on the characteristics of magnetron-sputtered ZnMgBeO thin films Hoang Ba Cuong, Byung-Teak Lee ∗ Photonic and Electronic Thin Film Laboratory, Department of Materials Science and Engineering, Chonnam National University, 300 Yongbong-dong, Gwangju 500-757, Republic of Korea
a r t i c l e
i n f o
Article history: Received 10 March 2015 Received in revised form 8 June 2015 Accepted 8 July 2015 Available online 15 July 2015 Keywords: ZnO ZnMgBeO UV detector Oxygen effects
a b s t r a c t Effects of oxygen partial pressure within the Ar process plasma on the optical, structural, and electrical properties of magnetron-sputtered ZnMgBeO films were investigated in detail. It was observed that the optical energy bandgap (Eg ) values of the ZnMgBeO films substantially decrease with the oxygen addition, from 5.3 to 4.3 eV as the oxygen partial pressure increases from zero to one. The full-widthat-half-maximum (FWHM) values of the (0 0 0 2) XRD peaks drastically decrease with the addition of a small amount of oxygen but then increase with further oxygen addition. All the films had very high sheet resistance, 1.3–1.4 G/䊐. It was also observed that the concentration of Zn within the films significantly increased with the oxygen addition, which was proposed to be mainly responsible for the observed decrease in Eg . It was also proposed that the FWHM change due to the oxygen addition may be attributable to three factors, film composition, grain size, and point defect density, as confirmed by results of TEM and XPS investigations. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Zinc magnesium beryllium oxide (ZnMgBeO) is being investigated as a promising material for ultraviolet (UV)-range optoelectronics, such as the solar-blind ultraviolet (UV) photodetectors [1,2], because the bandgap (Eg ) values can be modulated continuously, from 3.7 to 5.1 eV, by changing the concentration of MgO and BeO [1,2]. Wurtzite ZnMgO films have been studied intensively for the UV application but it is generally known to be difficult to obtain Eg values higher than ∼4 eV due to the phase segregation occurring at higher Mg concentrations [3], although it was recently reported that ZnMgO films with 4.55 eV Eg can be grown utilizing the two-step growth and the molecular beam epitaxial technique [4]. In deposition process of ZnO-based thin films, addition of O2 gas to the reaction plasma is one of the most important factors that influence electrical and optical properties of the films. It has been reported that the bandgap energy values, the residual stress, and the carrier concentration are critically affected by the oxygen stoichiometry [5,6]. It was also reported in the case of GaZnO and GaAlZnO that structural properties are improved and the carrier
∗ Corresponding author. Tel.: +82 104655538. E-mail address: [email protected] (B.-T. Lee). http://dx.doi.org/10.1016/j.apsusc.2015.07.051 0169-4332/© 2015 Elsevier B.V. All rights reserved.
concentration reduced by introducing oxygen to the Ar process plasma during the sputtering [6,7]. It is therefore very interesting and essential to study and understand effects of the oxygen on the film microstructure and properties, to obtain high-quality ZnMgBeO films with proper characteristics. In this work, effects of the O2 addition to the Ar process plasma on the characteristics of the reactive magnetron-sputtered ZnMgBeO films were studied in detail. The optical, structural, and electrical properties were investigated and possible mechanisms were proposed.
2. Experimental details The ZnMgBeO films, 250–300-nm thick, were grown on quartz substrates using a conventional RF magnetron-sputtering system. The 2-in. ceramic Zn0.59 Mg0.21 Be0.2 O target was made from high purity powders ZnO, MgO, and BeO. The sputter chamber was initially evacuated down to ∼10−6 Torr, and then the chamber pressure was maintained at 10−2 Torr during the deposition process. The growth temperature was 200 ◦ C, and the RF power was 75 W. During the sputtering process, high purity Ar and O2 gases were introduced into the sputter chamber and their mass flow rates were adjusted so that the O2 /Ar + O2 ratio ([O2 ]) ranged from 0.0 to 1.0. Before film growth, targets were pre-sputtered for 10 min to remove possible surface contamination.
H.B. Cuong, B.-T. Lee / Applied Surface Science 355 (2015) 582–586
Table 1 Composition of the Znx Mgy Bez O thin films sputtered from the Zn0.59 Mg0.21 Be0.2 O target at various oxygen partial pressures, [O2 ] = O2 /(Ar + O2 ).
80 Ar 5% 10% 30% 50% 70% O2
60 40 20
300
400
Transmittance (cm)
Transmittance (%)
100 (a)
0 200
100
[O2 ]
xZn
yMg
zBe
80
0 0.05 0.1 0.3 0.5 0.7 1.0
0.31 0.51 0.55 0.57 0.58 0.57 0.54
0.36 0.25 0.23 0.22 0.21 0.21 0.24
0.33 0.24 0.22 0.21 0.21 0.22 0.23
60 40 20 0 220
240
260
280
300
Wavelength (nm)
500
600
700
800
(αhv)^2 (a.u.)
(b)
(c) Ar 5% 10% 30% 50% 70% O2 4
Experiment 6 Vegard's law
5
4 5
Bandgap (eV)
0.0
0.4
[O2]
0.8
Optical bandgap (eV)
Wavelength (nm)
3
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on the Eg change of the films would not be significant. It has been reported that the carrier concentration must be higher than about 1018 cm−3 to effectively change the Eg of ZnO-based materials [10]. Chemical compositions of the films were measured using the ICP-OES method and results are shown in Table 1. It is clearly observed that the concentration of Zn (xZn ) substantially increases with the addition of the oxygen to the Ar plasma, from 0.32 at [O2 ] = 0.0–0.55 at [O2 ] = 0.1 and then remain almost unchanged with further increase in the oxygen partial pressure within the plasma. Accordingly, the concentration of Mg (yMg ) and Be (zBe ) significantly decreases with the oxygen addition. It has been proposed that effects of the composition on the Eg MgO
Fig. 1. Results of the optical characterization of ZnMgBeO films sputtered at various [O2 ]. The insets show plots of (˛hv)2 against hv and variation in the measured optical bandgap and the estimated values, as a function of the [O2 ].
The crystal structures of the deposited thin films were examined by high-resolution X-ray diffraction (XRD, X’pert PRO, Philips, Eindhoven, Netherlands) operated at 40 kV and 30 mA using Nifiltered Cu K␣ radiation and a transmission electron microscope (TEM, JEOLJEM) operating at 200 kV. Compositions of the films were measured using inductively coupled plasma optical emission spectroscopy (ICP-OES, Perkin Elmer OPTIMA 4300 DV). The optical transmittance of the deposited films was obtained using a UV-visible spectroscope (Cary 100, Varian) and the electrical properties were using a four-point probe system (Changmin Tech, CMT-SR2000). The chemical binding energies of the ZnMgBeO films were studied by the X-ray photoelectron spectroscopy (XPS, VG Multilab 2000) at room temperature. 3. Results and discussion Fig. 1 shows the transmittance spectra of ZnMgBeO films sputter grown in the Ar + O2 plasma with various [O2 ]. It is observed in Fig. 1a that the average transmittance in the visible wavelength region is more than 85% for all samples and the cut-off wavelength increases with increasing O2 partial pressure, from 234 nm for the pure Ar plasma to 289 nm for the pure oxygen plasma. This indicates that the optical band-gap energy (Eg ) decreases from 5.3 to 4.3 eV, as the [O2 ] increased from zero to 1.0 as shown in Fig. 1b and c. It has been reported that the Eg of semiconductor materials are mostly determined by the composition (Vegard’s rule) and the free carrier concentration (Burstein-Moss effect) [8]. Results of the fourpoint probe measurement of the ZnMgBeO films indicated that all the films have very high sheet resistance, 1.3–1.4 G/䊐. The Hall measurement was not possible to apply, due to the high resistivity. Considering that a Zn0.92 Mg0.05 Be0.03 O film with a sheet resistance of ∼1 M/䊐 showed a carrier concentration of 3 × 1016 cm−3 and a mobility of 4.4 cm2 /V s in our previous report [9], it is believed that the free carrier concentrations of the ZnMgBeO films studied in this work must have been very low, probably 1013 –1014 cm−3 . This indicates that the contribution of the Burstein–Moss (B–M) effect
of Znx Mgy Bez O films can be described as Eg = xEgZnO + yEg + zEgBeO − bZnMgO y(1 − y) − bZnBeO z(1 − z) − byz yz, where bZnMgO , bZnBeO denote the bowing parameters of ZnMgO and ZnBeO, respectively, and the byz can be obtained using the equation byz = bZnBeO − bZnMgO − bMgBeO [11]. The Eg values were estimated using previously proposed bowing parameter values of 8.2116, 1.4172, and 5.4772 eV for the bZnBeO , the bZnMgO , and the bMgBeO , respectively [11], and the film composition measured in this work (Table 1). The results of the calculation are shown in Fig. 1c. It must be mentioned that the estimated Eg values in Fig. 1b may not be exactly accurate due to the uncertainty in the bowing parameters utilized (it is known that the bowing parameter values change with the film composition). It is clearly noticeable in Fig. 1c, however, that the estimated Eg values do reflect the change of the measured Eg values as a function of the [O2 ] very well. This observation, along with the fact that the carrier concentrations of the films are very small, indicates that the observed O2 effects on Eg are mainly due to the composition change. It is believed that the increase in the Zn concentration due to the oxygen addition is mainly because the re-evaporation of Zn from the growing film is suppressed. It has been reported that metallic Zn phase forms during the sputtering under the oxygen deficient ambient, which readily re-evaporates from the growing films due to its low binding energy [12,13]. Also, the MgO phase, with lower formation enthalpy, was reported to form prior to the ZnO phase if the oxygen supply is not enough [12]. At the oxygen-rich environment, formation of the ZnO phase would increase, and the Zn concentrations within the films also increase. Fig. 2a shows XRD patterns of ZnMgBeO films grown at various oxygen partial pressures. It is noticeable that the c-axis lattice parameter (d(0 0 0 1)) increases from 0.483 to 0.495 nm and the (0 0 0 2) FWHM drastically decreases from 0.54o to 0.3o with addition of small amount of oxygen to the Ar process plasma. With increasing oxygen partial pressure, the FWHM continuously increases to 0.7◦ for [O2 ] = 1, as summarized in Fig. 2b. The d(0 0 0 1) remains almost unchanged until [O2 ] = 0.5 and then increases to 0.513 nm at [O2 ] = 1 (Fig. 2b). It has been suggested that a few factors are operating to affect crystalline quality (represented by the FWHM values) of the ZnOrelated films, such as the film composition, the grain size, and the density of the point defects [5,6]. Concentrations of Mg and Be
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ZnMgBeO (0002)
XRD Intensity (a.u.)
(a)
O2 Ar+70%O2 Ar+50%O2 Ar+30%O2 Ar+10%O2 Ar+5%O2 Ar
40
45
2-theta
50
55
(b)
60
0.52
o
FWHM ( )
0.8
35
0.6
0.48
0.4
0.44 0.40
0.2 0.0
0.2
0.4
0.6
[O2]
0.8
1.0
c-axis lattice constant (nm)
30
Fig. 2. Results of X-ray characterization of the ZnMgBeO films, (a) XRD patterns and (b) the variation of the c-axis parameter and the FWHM of (0 0 0 2) peak as a function of the oxygen concentration.
Fig. 3. Plan-view TEM images of the ZnMgBeO films sputtered at various [O2 ], (a) [O2 ] = 0.0, (b) [O2 ] = 0.3, and (c) [O2 ] = 1.0.
reported that a typical O 1s XPS peak is composed of three distinct Lorentzian–Gaussian profiles, centered at 530.2, 531.2, and 532.4 eV [15]. It was proposed that the high energy peak at 532.4 eV may be attributable to the presence of loosely bound oxygen on the
(a)
Relative Intensity
within the ZnMgBeO films decrease with increasing [O2 ], as shown in Table 1, which would results in smaller FWHM of the XRD peaks. Notice that the ionic radii of Mg (0.057 nm) and Be (0.027 nm) are smaller than that of Zn (0.06 nm). It is believed that this factor would be one of the possible mechanisms responsible for the initial decrease in the FWHM due to the O2 addition (Fig. 2b). It has been reported that the grain size of GaZnO thin films are affected by oxygen addition to the process plasma [5]. It was proposed that, in an oxygen-rich environment, the excess oxygen segregated at the grain boundaries and inhibited the grain growth, resulting in grains with reduced size [5]. It is also suspected that the number and the energy of the ions impinging onto the substrate decrease with addition of oxygen to Ar plasma, which would reduce the nucleation of the growing films and therefore increase the grain size [14]. It is reminded that the ionization energy of oxygen (47.76 eV) is higher than that of Ar (15.76 eV) and the ion density and the ion energy of the Ar plasma are much higher than those of the oxygen plasma [14]. Some of the films were studied using the TEM, in order to directly observe microstructures of the ZnMgBeO films. It is clearly seen in Fig. 3, plan-view TEM images of the ZnMgBeO films that the grain size of films initially increases with addition of O2 , from 22 nm at [O2 ] = 0.0 (Fig. 3a) to 30 nm at [O2 ] = 0.3 (Fig. 3b), and then decrease with further O2 addition, to 20 nm at [O2 ] = 1.0 (Fig. 3c). From the Scherrer equation, the FWHM of the XRD peaks due to the grain size are estimated to be 15 nm for [O2 ] = 0.0, 25 nm for [O2 ] = 0.3, and 12 nm for [O2 ] = 1.0, which follows a trend consistent with that of the FWHM change. Although the calculated FWHM values are slightly smaller than the observed ones, these results clearly indicate that the grain size change is one of the mechanisms responsible for the observed FWHM change. The XPS investigation of the films was performed to study behaviour of the point defects within the ZnMgBeO films and the O 1s peaks of the XPS spectra of films sputtered at (a) [O2 ] = 0.0, (b) [O2 ] = 0.3 and (c) [O2 ] = 1.0 are shown in Fig. 4. It has been
O 1s
(b)
O 1s
(c)
O 1s
538
536
534
532
530
528
526
524
Binding energy (eV) Fig. 4. The XPS O 1s peaks of the ZnMgBeO films, sputtered in (a) pure Ar plasma, (b) Ar plasma with 30% O2 , and (c) pure O2 plasma.
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surface of the films [16,17]. The low binding energy peak at 530.2 eV is attributable to the O2− ions binding with Zn2+ ions in ZnO and the medium energy peak at 531.2 eV is due to oxygen vacancies (O2− ions in oxygen-deficient regions) [15,16]. It is observed in Fig. 4 that the medium energy peak, related to the oxygen vacancy, is comparatively strong in the films sputtered in pure Ar plasma, Fig. 4a, and diminishes in the cases of the films sputtered in Ar/O2 plasma, Fig. 4b and c. This result indicates that a high density of oxygen vacancies exists within the ZnMgBeO films grown in pure Ar plasma, whose density will be significantly reduced by the oxygen addition. In fact, it has been previously reported that oxygen ambient during the film growth plays a major role in reducing the defects due to oxygen deficiency, based upon results of the Raman observation [18] and the optical absorption measurement [19]. It is believed that this phenomenon would be partly responsible for the initial decrease in the FWHM values by the oxygen addition (Fig. 2). It has been also reported that defects related to the excess oxygen, such as oxygen interstitial, Zn vacancy, and their complexes, formed in the case of the various ZnO-related films grown at the oxygen-rich conditions [6,18]. It is believed that the FWHM increase observed for the samples with high [O2 ] is critically related to the formation of the defects related to the excess oxygen, which would degrade the crystallinity of the films and widen the X-ray peaks. Unfortunately, the XPS spectra obtained in this work did not show firm evidence on the formation of excess-oxygen defects (Fig. 4b and c do not show any critical difference) as the defects related to the excess oxygen are not well represented by specific XPS peak(s) [15–17]. Summarizing the discussions above, it is believed that all of the proposed three factors, reduced Mg/Be concentration, increased grain size, and reduced density of defects related to the oxygen deficiency, contributed to the initial decrease in the FWHM of the XRD (0 0 0 2) peaks (Fig. 2). It is also suggested that, with further increase in the oxygen composition in the Ar plasma, the grain size decreases and the density of defects (associated with excess oxygen) increases, resulting in the increased FWHM. As to the c-axis lattice parameter change due to the oxygen addition, it is believed that the initial increase in the d(0 0 0 1), from 0.483 nm at [O2 ] = 0 to 0.498 nm at [O2 ] = 0.1, is mainly due to the film composition change. It is reminded that the c-axis lattice constants d(0 0 0 1) of BeO and MgO, 0.43 nm [20] and 0.41 nm [21], respectively, are smaller than that of ZnO (0.52 nm [22]), and the lattice will expand as the concentrations of BeO and MgO decrease (Table 1). In the case of the films with [O2 ] > 0.5, where the compositional change is not significant, it is suspected that the presence of point defects related to the excess oxygen played a significant role in the lattice constant increase. In fact, it has been reported that the lattice expansion was observed in the case of the doped ZnO films grown at oxygen-rich environment, which was partly attributed to the presence of oxygen interstitials [23,24]. Quantitative estimation of the changes in the XRD FWHM and the lattice change requires detailed information on the nature and the amount of the associated point defects. Further investigation on the electrical as well as the structural properties is in progress to clarify the defect dynamics related to the stoichiometry of the ZnO-based thin films and the results will be reported in a future publication.
4. Summary Effects of oxygen partial pressure within the process plasma on the optical, structural and electrical properties of magnetron sputtered ZnMgBeO films were investigated in detail. It was observed that the Eg values of the ZnMgBeO films substantially decrease with
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the oxygen addition, from 5.3 to 4.3 eV as the [O2 ] increases from zero to one. The FWHM values of the (0002) XRD peaks drastically decreased from ∼0.55◦ to ∼0.3◦ with addition of small amount of oxygen to the Ar process plasma and then increase to ∼0.6◦ with further increase in the oxygen partial pressure. Results of the fourpoint probe measurement indicated that all the films have very high sheet resistance, 1.3–1.4 G/䊐. It was also observed that the concentration of Zn within the films significantly increased with the oxygen addition. It was proposed that the observed decrease in the Eg is mainly due to the film composition change and the FWHM change due to the oxygen addition may be attributed mainly to three factors: film composition, grain size, and point defect density. It was observed from the TEM investigation that the grain size increases with initial oxygen addition but then decrease with further addition. High density of oxygen vacancies exists in the ZnMgBeO films grown in pure Ar plasma, which is significantly reduced in the case of the Ar + O2 plasma, as confirmed by the XPS measurement. Acknowledgments This research was supported by the Basic Science Research Program (2013R1A1A2008793) through the National Research Foundation of Korea (NRF), funded by the Ministry of Science, ICT and Future Planning. Authors thank the Korea Basic Science Institute for their assistance in the materials characterization. References [1] C. Yang, X.M. Li, Y.F. Gu, W.D. Yu, X.D. Gao, Y.W. Zhang, ZnO based oxide system with continuous bandgap modulation from 3.7 to 4.9 eV, Appl. Phys. Lett. 93 (2008) 112114. [2] M. Toporkov, V. Avrutin, S. Okur, N. Izyumskaya, D. Demchenko, J. Volk, D.J. Smith, H. Morkoc¸, Ü. Özgür, Enhancement of Be and Mg incorporation in wurtzite quaternary BeMgZnO alloys with up to 5.1 eV optical bandgap, J. Cryst. Growth 402 (2014) 60–64. [3] P. Kumar, J.P. Singh, Y. Kumar, A. Gaur, H.K. Malik, K. Asokan, Investigation of phase segregation in Zn1−x Mgx O systems, Curr. Appl. Phys. 12 (2012) 1166–1172. [4] X. Du, Z. Mei, Z. Liu, Y. Guo, T. Zhang, Y. Hou, Z. Zhang, Q. Xue, A.Y. Kuznetsov, Controlled growth of high-quality ZnO-based films and fabrication of visible-blind and solar-blind ultra-violet detectors, Adv. Mater. 21 (2009) 4625–4630. [5] B. Zhou, A.V. Rogachev, Z. Liu, D.G. Piliptsou, H. Ji, X. Jiang, Effects of oxygen/argon ratio and annealing on structural and optical properties of ZnO thin films, Appl. Surf. Sci. 258 (2012) 5759–5764. [6] H. Makino, H. Song, T. Yamamoto, Influences of oxygen gas flow rate on electrical properties of Ga-doped ZnO thin films deposited on glass and sapphire substrates, Thin Solid Films 559 (2014) 78–82. [7] H. Jeong Soo, M. Nobuhiro, K. Kyung Hwan, Investigation of the effect of oxygen gas on properties of GAZO thin films fabricated by facing targets sputtering system, Semicond. Sci. Technol. 29 (2014) 075007. [8] B.E. Sernelius, K.F. Berggren, Z.C. Jin, I. Hamberg, C.G. Granqvist, Band-gap tailoring of ZnO by means of heavy Al doping, Phys. Rev. B 37 (1988) 10244–10248. [9] H.B. Cuong, C.-S. Lee, B.-T. Lee, Effects of Ga concentration, process conditions, and substrate materials on properties of ZnMgBeGaO ultraviolet-range transparent conducting films, Thin Solid Films 573 (2014) 95–99. [10] J.G. Lu, S. Fujita, T. Kawaharamura, H. Nishinaka, Y. Kamada, T. Ohshima, Z.Z. Ye, Y.J. Zeng, Y.Z. Zhang, L.P. Zhu, H.P. He, B.H. Zhao, Carrier concentration dependence of band gap shift in n-type ZnO:Al films, J. Appl. Phys. 101 (2007) 083705. [11] H.L. Shi, Y. Duan, Band-gap bowing and p-type doping of (Zn, Mg, Be)O wide-gap semiconductor alloys: a first-principles study, Eur. Phys. J. B 66 (2008) 439–444. [12] C.X. Cong, B. Yao, Y.P. Xie, G.Z. Xing, B.H. Li, X.H. Wang, Z.P. Wei, Z.Z. Zhang, Y.M. Lv, D.Z. Shen, X.W. Fan, Nitrogen partial pressure-dependent Mg concentration, structure, and optical properties of Mgx Zn1−x O film grown by magnetron sputtering, J. Mater. Res. 22 (2007) 2936–2942. [13] F.E. Dart, Evaporation of zinc and zinc oxide under electron bombardment, Phys. Rev. 78 (1950) 761–764. [14] I.T. Tang, Y.C. Wang, W.C. Hwang, C.C. Hwang, N.C. Wu, M.-P. Houng, Y.-H. Wang, Investigation of piezoelectric ZnO film deposited on diamond like carbon coated onto Si substrate under different sputtering conditions, J. Cryst. Growth 252 (2003) 190–198.
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