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Study of Cu-doped SnO thin films prepared by reactive co-sputtering with facing targets of Sn and Cu
Study of Cu-doped SnO thin films prepared by reactive co-sputtering with facing targets of Sn and Cu Jeung Sun Ahn, Ramchandra Pode, Kwang Bae Lee PII: DOI: Reference:
S0040-6090(16)30093-1 doi: 10.1016/j.tsf.2016.04.024 TSF 35157
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
4 November 2015 5 April 2016 17 April 2016
Please cite this article as: Jeung Sun Ahn, Ramchandra Pode, Kwang Bae Lee, Study of Cu-doped SnO thin films prepared by reactive co-sputtering with facing targets of Sn and Cu, Thin Solid Films (2016), doi: 10.1016/j.tsf.2016.04.024
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Department of Physics, Kyung Hee University, Dongdaemun-gu, Seoul 02453, Korea
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Jeung Sun Ahna, Ramchandra Podea, Kwang Bae Leeb,*
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Study of Cu-doped SnO thin films prepared by reactive co-sputtering with facing targets of Sn and Cu
Department of Applied Physics & Electronics, Sangji University, Wonju, Gangwondo 26339,
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Korea
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The properties of Cu-doped SnO thin films with various Cu contents (Sn1-xCuxO) prepared by reactive co-sputtering using facing targets of Sn and Cu are investigated. The
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XRD patterns show the tetragonal SnO phases including the mixed phase of Cu2O (200) and
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metallic Cu (111). The XRD phase ratio of Cu2O/Cu increases from 0 to 41/59 for x = 3 to 22
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at%, which is compatible with the values of the content ratio of Cu1+/Cu0 estimated from the XPS peaks. The decrease of the resistivity and the optical transmission with increasing Cu content in Sn1-xCuxO thin films up to 17 at% are mainly attributed to the Cu phase. Such
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behavior of results displays the n-type conduction, whereas the Sn1-xCuxO thin film at x = 22 at% shows the p-type conduction with Hall mobility of 1.1 cm2/Vs and the resistivity of 0.45 cm.
*corresponding author: [email protected]
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ACCEPTED MANUSCRIPT Keywords: Cu-doped SnO thin films
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Reactive co-sputtering
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Facing targets
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Ratio of Cu2O/Cu phase
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Optical band gap Hall mobility
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p-type conduction
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ACCEPTED MANUSCRIPT 1. Introduction
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Transparent oxide semiconductors (TOS) have recently attracted much attention owing
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to applications in optoelectronic devices, solar cells, flat panel displays, and gas sensors [1-4].
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Well known and widely-used TOS, such as ZnO [5], and amorphous InGaZnO [6], are n-type semiconductors due to the existence of oxygen vacancies and/or metal interstitials [6].
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However, the transparent tin oxide semiconductor shows a typical p-type of behavior. In 2008, Ogo et al. [7] reported the possibility for high hole mobility in SnO thin films grown
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epitaxially on (001) yttria-stabilized zirconia substrates. Consequently, much interest in ptype SnO thin films for realizing low-power and high-performance complementary oxide-thin
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film transistor (TFT) circuits has been generated. Many researches employed the sputtering
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methods to prepare the improved p-type SnO thin films, mainly for the TFTs applications. In
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sputtering technique, the optimization of the deposition conditions of reactive sputtering, such as the sputtering target composition [8], the sputtering power [9], and the partial pressure of O2 and Ar [10], etc., have been investigated. Different forms of tin oxide
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semiconductors such as p-type SnO, p-type SnO2 can be achieved by doping p-type elements like Al, Sb, In and Ga as the acceptor impurity which have lower valence cation in n-type SnO2 [11-14]. For the preparation of doped p-type SnO2 thin films, the most commonly used method is the sol-gel technique. However, this synthesis method is marred with the production of poor quality and limited thickness films; the results were not as per expectation and not satisfactory. Compared to the doped p-type SnO2, meagre information on the doping of p-type SnO is available. Earlier, the improvement of the electrical transport properties of p-type SnO thin films by 5 at% Y-doping [15] was reported wherein the film was deposited on the SiO2/Si wafers employing the electron beam evaporation by mixing the SnO2 source 3
ACCEPTED MANUSCRIPT with Y2O3. Nitrogen-doping (5 at%) in SnO thin films was realized via reactive rf sputtering with an Sn target [16]. However, the nitrogen doping creates donor defects in SnO thin films
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causing lower electrical conductivity.
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In this work, we study the properties of SnO thin films by Cu-doping in order to
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investigate the possibility of the dopant Cu as an acceptor impurity in SnO. The Cu-doped SnO thin films with various Cu contents were prepared by reactive co-sputtering using two
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different facing targets of Sn and Cu. The employed method is a little different from the conventional facing target sputtering (FTS) method [17], but ensures all the advantages of the
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FTS method such as higher plasma density, less damage by energetic secondary electron and negative oxygen ions, and less rise in substrate temperature. We investigate the possibility of
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the formation of Cu2O in SnO films obtained using the reactive co-sputtering method, since
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the Cu2O is very well known p-type TOS [18].
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ACCEPTED MANUSCRIPT 2. Experimental details
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2.1. Fabrication of Cu-doped SnO thin films
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The deposition system for Cu-doped SnO thin films consisted of two horizontally parallel targets 100 mm apart and the substrate located parallel to targets plane at a perpendicular
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distance of 50 mm from the common axis of the targets, as illustrated in Fig. 1. Metallic-Sn and -Cu (purity 99.99% and 99.995%, respectively) with 2 inch in diameter were used as
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sputtering targets. Pre-cleaned soda lime glass substrate with dimensions of 0.65 mm in thickness and 20 20 mm2 in area, attached to the substrate holder, was rotated with the
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speed of 15 rpm. The rf power of Sn-target was fixed at 100 W. The dc power of Cu-target
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was varied in the relatively small range from 0 to 20 W in order to have the Cu-doped SnO
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thin films with varying content of Cu. The deposition time was fixed at 5 min for all samples. As a result, the Cu-doped SnO thin films exhibit a tendency to increase in the thickness from about 190 to 270 nm with increasing the dc power of Cu target from 0 to 20 W. The ratio of
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the partial pressure of O2 was 6.0 % in the working pressure of 0.133 Pa. All the samples were deposited at room temperature and followed by a postannealing treatment at 300 oC for 1 hr in air.
2.2. Characterizations of Cu-doped SnO thin films
The crystalline nature of thin films was analyzed using the X-ray diffractometer (XRD; Rigaku, D/Max 2200), and the surface morphologies were observed by an atomic force microscope (AFM; Park, NX20). The chemical binding states as well as the compositional 5
ACCEPTED MANUSCRIPT analysis were investigated using X-ray photoelectron spectroscopy (XPS; Thermo Electron, K-Alpha), which was measured for etched films using 2 keV-Ar+ sputtering for 2 min, in
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which the maximum spot size of about 1000 m was used to minimize the sputtering damage of the binding states. The etched depth was estimated to be about 10 nm in thickness. The
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electrical resistivity, carrier mobility and concentration were measured at room temperature employing the Hall effect measurement systems by the van der Pauw method (Ecopia HMS-
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3000). The thickness and the optical constants, such as extinction coefficient (k) and refractive index, were determined using the reflectometry analysis (Kmac, ST2000DLX-n),
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based on the Cauchy’s equation for dispersion. The optical transmittance spectra were
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investigated by a UV-Vis-near IR spectrophotometer (Kmac, SV2100).
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ACCEPTED MANUSCRIPT 3. Results and discussion
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The XRD patterns of Cu-doped SnO films with varying dc power of Cu target are shown
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in Fig. 2(a). All patterns shows the tetragonal SnO with a preferred orientation of (101)
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(according to JCPDS card # 06-0395), except XRD patterns for the dc power of 0 and 20 W showing a preferred orientation of (110). Whereas for 25W of dc power, the amorphous
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phase of Cu-doped SnO films was observed. Such a distinctive behavior of preferential orientation may be attributed to the difference in lattice mismatches and preferential
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crystallite orientation. The crystallinity is somewhat enhanced by Cu-doping as observed in increasing the peaks intensities, see Fig. 2(a). In order to show the influence of Cu-doping on
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the crystallite structure of SnO films for the dc power of Cu target of up to 20 W, the XRD
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patterns in a narrower 2θ range from 41o to 45o are shown in Fig. 2(b). The peaks at 2θ values
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of 43.3o and 41.9o are assigned to Cu (111) and Cu2O (200) (according to JCPDS card # 659743 and 65-3288), respectively. As the dc power of Cu target increases, these peaks intensities improve and become prominent for 20W. The ratio of the integrated intensities of
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these peaks, denoted as I(Cu2O/Cu), was estimated to be about 0/100, 16/84, 35/65, and 41/59 for the dc power of 6, 10, 15, and 20 W, respectively. Evidently, the content of Cu in the Cu-doped SnO thin films increases by increasing the dc power of Cu target. The metallic Cu phase is formed preferentially for the lower Cu content (i.e. lower dc power of Cu target), whereas the Cu2O phase is produced at the relatively higher dc power. Also seen in Fig. 2(b), the 2θ value of Cu2O (200) peaks (41.9o) is lower than the reported in standard spectra (42.40o). Such difference in 2θ value is due to the increase of the interplanar spacing of Cu2O (200). No shift in SnO XRD peaks positions by Cu-doping (as shown in Fig. 2(a)) would be 7
ACCEPTED MANUSCRIPT attributed to the mixed phases of Cu and Cu2O in SnO, rather than the substitution of Cu for Sn. This result is probably attributed to the injection of Cu particles with relatively low
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kinetic energy due to the 90o-off-axis sputtering into a SnO thin film by our FTS method.
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Figure 3 shows the narrow-scan XPS spectra of Cu 2p3/2 core level of Cu-doped SnO
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films with varying the dc power of Cu target. The peaks of Cu 2p3/2 are found to be at around 932.5 eV and almost no contribution Cu2+ peak is centered at the binding energy of 933.7 eV.
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The peaks of Cu 2p3/2 shift by small value from 932.8 to 932.4 eV with increasing the dc power of Cu target due to the variation of the content ratio of Cuo and Cu1+. Whereas, all Sn
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3d5/2 peaks represent almost one Gaussian distribution with the constant core level of Sn2+ peaking at 486.6 eV for the various dc power of Cu target as displayed in the inset of Fig. 3.
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These results are consistent with the XRD results as presented in Fig. 2, wherein the
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formation of both the mixed Cu and Cu2O phases are demonstrated instead of the substitution
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of Cu for Sn. However, the accurate ratio of Cuo and Cu1+ in XPS spectra of Cu 2p3/2 for all films is difficult to estimate due to the close core levels of these two peaks. Hence, the Gaussian fitting was performed typically only for a Cu 2p3/2 peak for a dc power of 20 W,
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where the binding energy values of Cuo and Cu1+ core levels is used as 932.8 and 932. 5 eV, respectively, as shown in Fig. 4. From the integrated intensities of these deconvoluted curves, the atomic ratio of Cu1+/Cuo in Cu was estimated to be 43/57, which is in good agreement with the estimated value from the XRD results, as shown in Fig. 2(b), within the limit of error of 2%. The atomic ratio of Cu, as well as of Sn and O, in Cu-doped SnO films, is found from the XPS analysis, as shown in Fig. 5. The atomic ratio of Cu increased nearly linearly up to 11.4% with increasing to the dc power of Cu target of 20 W, while the atomic ratio of O was constant in almost 48.2 0.5%. Hence, the resultant atomic ratio of Cu can be approximately 8
ACCEPTED MANUSCRIPT denoted as x in Sn1-xCuxO, which is also shown in Fig. 4, and the maximum x is 22 at% for the dc power of Cu target of 20 W.
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Figures 6(a) to 6(c) show the surface morphology of Cu-doped SnO thin films for the dc
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power of (a) 0, (b) 10, and (c) 20 W. As seen from Fig 6(a), an undoped SnO film shows the
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continuously distributed surface with the grains of the hillock-shaped, wherein the biggest one is about 0.2-m-width with height of 200 nm. The smooth surface with continuous
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hillocks is observed for the dc power of 10 W, as seen from Fig. 6(b), whereas the lumpshaped grains are distributed discontinuously on the smooth surface for the dc power of 20 W,
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wherein the biggest one is about 0.5-m-width with height of 150 nm, as shown from Fig. 6(c). However, the significant reduction of rms roughness is found to be from about 21 nm
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for an undoped SnO film to 7 ~ 8 nm for Cu-doped SnO films with the dc power of 10 ~ 20
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W, respectively.
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Figure 7 shows the plot of (hν)2 versus the photon energy hν for various value of x in Sn1-xCuxO thin films, where is the absorption coefficient extracted from the reflectometry analysis using = 4k/ (where is the wavelength of light). The direct optical bandgaps
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(Eopt) of Sn1-xCuxO thin films are determined by linearly extrapolating the (hν)2 curve to the horizontal axis, see Fig. 7. The Eopt for a SnO thin film of x = 0 is 2.44 eV, which is somewhat lower than the earlier reported value of ~2.7 eV [7]. The lower optical bandgap value in our sample is considered to be due to the different deposition method as well as the substrate type. As the x increases, the optical bandgap decreases continuously to a value of 2.02 eV for x = 17 at% and is almost constant to x = 22 at%. The optical transmittance spectra of Sn1-xCuxO thin films are also shown in the inset of Fig. 7. The decrease in Eopt is thought to be attributed to the increase mainly of the Cu2O phase in the films since the Eopt of 9
ACCEPTED MANUSCRIPT Cu2O thin films is reported to be about ~2.1 eV [19], lower than that of SnO. Whereas, the remarkable decrease in optical transmission with increasing x is interpreted to be due to the
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increase mainly of Cu phase, since the metallic Cu film has low transmission for the overall
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wavelength range. Additionally with increasing the dc power of Cu target, the increase of
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thickness of the samples is also contributing to the decrease in the transmission. There might be two distinct absorption edges in the Cu-doped SnO films having mixed phases of Cu and
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Cu2O in SnO, rather than the substitution of Cu for Sn, one for Cu2O and one for SnO. However, those could not be observed in Fig. 7, which is presumably due to the low Cu 2O
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phase of less than 10% and the low transmittance (less than 10%), as shown in the inset of Fig. 7, even for the maximum content of x = 22 at% in Sn1-xCuxO. Such optical properties are
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also related to the grain shape (or size), as typically shown in Fig. 6, since an increase in
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edge was lowered [20].
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grain size was known to deteriorate the optical transmission and also the optical absorption
Finally, the electrical properties, such as resistivity , carrier concentration n, and Hall mobility H as a function of x in Sn1-xCuxO thin films are shown in Fig. 8. Hall effect
xCuxO
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measurements show the negative Hall voltages, confirming the n-type conduction in Sn1thin films with x value less than 17 at%, even with x = 0, where the maximum H of
3.6 cm2/Vs is for x = 17 at%. Whereas the H of less than 1.0 cm2/Vs was observed for x = 0 ~ 7 at%. However, the p-type conduction was ambiguously obtained only for x = 22 at% with the H value of 1.1 cm2/Vs, which is similar to values reported in p-type conductive oxides such as NiO [21], ZnO [22], and Cu2O [23]. The value of decreases continuously from 1.2 102 to 0.37 cm with increasing value of x up to 17 at% and later slightly increased to a value of 0.45 cm for x = 22 at%, showing the same behavior as that of Eopt, as displayed in 10
ACCEPTED MANUSCRIPT Fig. 7. The values of carrier concentration n vary from 3 1018 to 5 1018 cm-3 for x = 0 ~ 17 at%, indicating the n-type conduction due to the relatively large metallic-Cu phases (see Fig.
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2(b)) or the Cuo bonds as shown in Fig. 3 in Cu-doped SnO thin films. Whereas, the value of
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n (1.2 1019 cm-3) for x = 22 at% is somewhat larger, showing the p-type conduction due to
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the high density of defects formed in SnO heavily doped with Cu films. Indeed, from these results it is argued that the mixed Cu2O phase in SnO provides p-type conduction, due the
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proper ratio of Cu2O to Cu of 41/59 for x = 22 at%, as evaluated from Fig. 2(b) or from Fig. 3.
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ACCEPTED MANUSCRIPT 4. Conclusion
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We have investigated the properties of Cu-doped SnO thin films with varying Cu
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contents (Sn1-xCuxO) fabricated by reactive co-sputtering using two facing targets of Sn and
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Cu. The XRD patterns showed the tetragonal SnO phases including the mixed phase of Cu2O (200) and metallic Cu (111) up to the Cu atomic content value of x = 22 at%. The XRD peaks
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of Cu2O (200) and Cu (111) corroborated that the metallic Cu phase is formed preferentially for lower Cu contents and the Cu2O phase is produced at the relatively higher dc power of Cu
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target. The phase ratio of Cu2O/Cu increases from 0 to 41/59 for x = 3 to 22 at%, determined from the XRD peak analysis. The Cu2O/Cu ratio value for x =22 at% is compatible to the
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values of the content of Cu1+/Cu0, estimated from the deconvolution of a Cu 2p3/2 XPS peak.
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The decrease of the resistivity and the optical transmission with increasing x in Sn1-xCuxO
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thin films up to 17 at% show the n-type conduction, resulting mainly due to the increase of Cu phase. However, for x = 22 at% the proper ratio of Cu2O phases to Cu phase is believed to provide the p-type conduction with a mobility of 1.1 cm2/Vs and the resistivity of 0.45 cm.
xCuxO)
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In summary, the proper ratio of Cu2O/Cu is vital to realize the p-type Cu-doped SnO (Sn1thin films which have wide application prospects in transparent optoelectronic and
electro-optic devices.
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ACCEPTED MANUSCRIPT Acknowledgment
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This work was supported by a grant from Kyung Hee University and was partly supported by
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the Sangji University 2014 Project.
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ACCEPTED MANUSCRIPT Figure captions Fig. 1. Arrangement of substrate and targets for preparing Cu-doped SnO thin films.
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and (b) XRD patterns in a narrower 2θ range from 41o to 45o.
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Fig. 2. XRD patterns of Cu-doped SnO thin films with (a) the various dc power of Cu target
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Fig. 3. Cu 2p3/2 core level XPS spectra of Cu-doped SnO thin films and Sn 3d5/2 core level XPS spectra (inset) with the various dc power of Cu target.
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Fig. 4. Curve fitting of a Cu 2p3/2 core level XPS spectrum for dc power of 20 W. Fig. 5. Plot of the atomic contents of O, Sn, and Cu and x in Sn1-xCuxO vs. the dc power of
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Cu target.
Fig. 6. Typical surface AFM morphologies (5 m 5 m) for (a) the dc power of Cu target
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of (a) 0 , (b) 10, and (c) 20 W.
thin films.
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Fig. 7. Plot of (hν)2 versus hν and the transmittance spectra (inset) for various x in Sn1-
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Fig. 8. Plot of resistivity , carrier concentration n, and Hall mobility H vs. x in Sn1-xCuxO.
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Research Highlights
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►Cu-doped SnO films were prepared by reactive co-sputtering with facing targets. ►Mixed phase of Cu2O and Cu in tetragonal SnO phases was found. 2 ►The Sn1-xCuxO film at x = 22 at% shows the p-type with a mobility of 1.1 cm /Vs. ►Proper ratio of Cu2O/Cu is imperative to realize the p-type conduction.
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S0040-6090(16)30093-1 doi: 10.1016/j.tsf.2016.04.024 TSF 35157
To appear in:
Thin Solid Films
Received date: Revised date: Accepted date:
4 November 2015 5 April 2016 17 April 2016
Please cite this article as: Jeung Sun Ahn, Ramchandra Pode, Kwang Bae Lee, Study of Cu-doped SnO thin films prepared by reactive co-sputtering with facing targets of Sn and Cu, Thin Solid Films (2016), doi: 10.1016/j.tsf.2016.04.024
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Department of Physics, Kyung Hee University, Dongdaemun-gu, Seoul 02453, Korea
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Jeung Sun Ahna, Ramchandra Podea, Kwang Bae Leeb,*
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Study of Cu-doped SnO thin films prepared by reactive co-sputtering with facing targets of Sn and Cu
Department of Applied Physics & Electronics, Sangji University, Wonju, Gangwondo 26339,
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Korea
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The properties of Cu-doped SnO thin films with various Cu contents (Sn1-xCuxO) prepared by reactive co-sputtering using facing targets of Sn and Cu are investigated. The
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XRD patterns show the tetragonal SnO phases including the mixed phase of Cu2O (200) and
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metallic Cu (111). The XRD phase ratio of Cu2O/Cu increases from 0 to 41/59 for x = 3 to 22
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at%, which is compatible with the values of the content ratio of Cu1+/Cu0 estimated from the XPS peaks. The decrease of the resistivity and the optical transmission with increasing Cu content in Sn1-xCuxO thin films up to 17 at% are mainly attributed to the Cu phase. Such
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behavior of results displays the n-type conduction, whereas the Sn1-xCuxO thin film at x = 22 at% shows the p-type conduction with Hall mobility of 1.1 cm2/Vs and the resistivity of 0.45 cm.
*corresponding author: [email protected]
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ACCEPTED MANUSCRIPT Keywords: Cu-doped SnO thin films
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Reactive co-sputtering
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Facing targets
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Ratio of Cu2O/Cu phase
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Optical band gap Hall mobility
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p-type conduction
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ACCEPTED MANUSCRIPT 1. Introduction
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Transparent oxide semiconductors (TOS) have recently attracted much attention owing
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to applications in optoelectronic devices, solar cells, flat panel displays, and gas sensors [1-4].
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Well known and widely-used TOS, such as ZnO [5], and amorphous InGaZnO [6], are n-type semiconductors due to the existence of oxygen vacancies and/or metal interstitials [6].
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However, the transparent tin oxide semiconductor shows a typical p-type of behavior. In 2008, Ogo et al. [7] reported the possibility for high hole mobility in SnO thin films grown
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epitaxially on (001) yttria-stabilized zirconia substrates. Consequently, much interest in ptype SnO thin films for realizing low-power and high-performance complementary oxide-thin
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film transistor (TFT) circuits has been generated. Many researches employed the sputtering
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methods to prepare the improved p-type SnO thin films, mainly for the TFTs applications. In
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sputtering technique, the optimization of the deposition conditions of reactive sputtering, such as the sputtering target composition [8], the sputtering power [9], and the partial pressure of O2 and Ar [10], etc., have been investigated. Different forms of tin oxide
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semiconductors such as p-type SnO, p-type SnO2 can be achieved by doping p-type elements like Al, Sb, In and Ga as the acceptor impurity which have lower valence cation in n-type SnO2 [11-14]. For the preparation of doped p-type SnO2 thin films, the most commonly used method is the sol-gel technique. However, this synthesis method is marred with the production of poor quality and limited thickness films; the results were not as per expectation and not satisfactory. Compared to the doped p-type SnO2, meagre information on the doping of p-type SnO is available. Earlier, the improvement of the electrical transport properties of p-type SnO thin films by 5 at% Y-doping [15] was reported wherein the film was deposited on the SiO2/Si wafers employing the electron beam evaporation by mixing the SnO2 source 3
ACCEPTED MANUSCRIPT with Y2O3. Nitrogen-doping (5 at%) in SnO thin films was realized via reactive rf sputtering with an Sn target [16]. However, the nitrogen doping creates donor defects in SnO thin films
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causing lower electrical conductivity.
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In this work, we study the properties of SnO thin films by Cu-doping in order to
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investigate the possibility of the dopant Cu as an acceptor impurity in SnO. The Cu-doped SnO thin films with various Cu contents were prepared by reactive co-sputtering using two
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different facing targets of Sn and Cu. The employed method is a little different from the conventional facing target sputtering (FTS) method [17], but ensures all the advantages of the
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FTS method such as higher plasma density, less damage by energetic secondary electron and negative oxygen ions, and less rise in substrate temperature. We investigate the possibility of
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the formation of Cu2O in SnO films obtained using the reactive co-sputtering method, since
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the Cu2O is very well known p-type TOS [18].
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ACCEPTED MANUSCRIPT 2. Experimental details
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2.1. Fabrication of Cu-doped SnO thin films
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The deposition system for Cu-doped SnO thin films consisted of two horizontally parallel targets 100 mm apart and the substrate located parallel to targets plane at a perpendicular
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distance of 50 mm from the common axis of the targets, as illustrated in Fig. 1. Metallic-Sn and -Cu (purity 99.99% and 99.995%, respectively) with 2 inch in diameter were used as
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sputtering targets. Pre-cleaned soda lime glass substrate with dimensions of 0.65 mm in thickness and 20 20 mm2 in area, attached to the substrate holder, was rotated with the
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speed of 15 rpm. The rf power of Sn-target was fixed at 100 W. The dc power of Cu-target
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was varied in the relatively small range from 0 to 20 W in order to have the Cu-doped SnO
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thin films with varying content of Cu. The deposition time was fixed at 5 min for all samples. As a result, the Cu-doped SnO thin films exhibit a tendency to increase in the thickness from about 190 to 270 nm with increasing the dc power of Cu target from 0 to 20 W. The ratio of
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the partial pressure of O2 was 6.0 % in the working pressure of 0.133 Pa. All the samples were deposited at room temperature and followed by a postannealing treatment at 300 oC for 1 hr in air.
2.2. Characterizations of Cu-doped SnO thin films
The crystalline nature of thin films was analyzed using the X-ray diffractometer (XRD; Rigaku, D/Max 2200), and the surface morphologies were observed by an atomic force microscope (AFM; Park, NX20). The chemical binding states as well as the compositional 5
ACCEPTED MANUSCRIPT analysis were investigated using X-ray photoelectron spectroscopy (XPS; Thermo Electron, K-Alpha), which was measured for etched films using 2 keV-Ar+ sputtering for 2 min, in
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which the maximum spot size of about 1000 m was used to minimize the sputtering damage of the binding states. The etched depth was estimated to be about 10 nm in thickness. The
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electrical resistivity, carrier mobility and concentration were measured at room temperature employing the Hall effect measurement systems by the van der Pauw method (Ecopia HMS-
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3000). The thickness and the optical constants, such as extinction coefficient (k) and refractive index, were determined using the reflectometry analysis (Kmac, ST2000DLX-n),
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based on the Cauchy’s equation for dispersion. The optical transmittance spectra were
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investigated by a UV-Vis-near IR spectrophotometer (Kmac, SV2100).
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ACCEPTED MANUSCRIPT 3. Results and discussion
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The XRD patterns of Cu-doped SnO films with varying dc power of Cu target are shown
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in Fig. 2(a). All patterns shows the tetragonal SnO with a preferred orientation of (101)
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(according to JCPDS card # 06-0395), except XRD patterns for the dc power of 0 and 20 W showing a preferred orientation of (110). Whereas for 25W of dc power, the amorphous
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phase of Cu-doped SnO films was observed. Such a distinctive behavior of preferential orientation may be attributed to the difference in lattice mismatches and preferential
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crystallite orientation. The crystallinity is somewhat enhanced by Cu-doping as observed in increasing the peaks intensities, see Fig. 2(a). In order to show the influence of Cu-doping on
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the crystallite structure of SnO films for the dc power of Cu target of up to 20 W, the XRD
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patterns in a narrower 2θ range from 41o to 45o are shown in Fig. 2(b). The peaks at 2θ values
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of 43.3o and 41.9o are assigned to Cu (111) and Cu2O (200) (according to JCPDS card # 659743 and 65-3288), respectively. As the dc power of Cu target increases, these peaks intensities improve and become prominent for 20W. The ratio of the integrated intensities of
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these peaks, denoted as I(Cu2O/Cu), was estimated to be about 0/100, 16/84, 35/65, and 41/59 for the dc power of 6, 10, 15, and 20 W, respectively. Evidently, the content of Cu in the Cu-doped SnO thin films increases by increasing the dc power of Cu target. The metallic Cu phase is formed preferentially for the lower Cu content (i.e. lower dc power of Cu target), whereas the Cu2O phase is produced at the relatively higher dc power. Also seen in Fig. 2(b), the 2θ value of Cu2O (200) peaks (41.9o) is lower than the reported in standard spectra (42.40o). Such difference in 2θ value is due to the increase of the interplanar spacing of Cu2O (200). No shift in SnO XRD peaks positions by Cu-doping (as shown in Fig. 2(a)) would be 7
ACCEPTED MANUSCRIPT attributed to the mixed phases of Cu and Cu2O in SnO, rather than the substitution of Cu for Sn. This result is probably attributed to the injection of Cu particles with relatively low
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kinetic energy due to the 90o-off-axis sputtering into a SnO thin film by our FTS method.
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Figure 3 shows the narrow-scan XPS spectra of Cu 2p3/2 core level of Cu-doped SnO
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films with varying the dc power of Cu target. The peaks of Cu 2p3/2 are found to be at around 932.5 eV and almost no contribution Cu2+ peak is centered at the binding energy of 933.7 eV.
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The peaks of Cu 2p3/2 shift by small value from 932.8 to 932.4 eV with increasing the dc power of Cu target due to the variation of the content ratio of Cuo and Cu1+. Whereas, all Sn
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3d5/2 peaks represent almost one Gaussian distribution with the constant core level of Sn2+ peaking at 486.6 eV for the various dc power of Cu target as displayed in the inset of Fig. 3.
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These results are consistent with the XRD results as presented in Fig. 2, wherein the
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formation of both the mixed Cu and Cu2O phases are demonstrated instead of the substitution
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of Cu for Sn. However, the accurate ratio of Cuo and Cu1+ in XPS spectra of Cu 2p3/2 for all films is difficult to estimate due to the close core levels of these two peaks. Hence, the Gaussian fitting was performed typically only for a Cu 2p3/2 peak for a dc power of 20 W,
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where the binding energy values of Cuo and Cu1+ core levels is used as 932.8 and 932. 5 eV, respectively, as shown in Fig. 4. From the integrated intensities of these deconvoluted curves, the atomic ratio of Cu1+/Cuo in Cu was estimated to be 43/57, which is in good agreement with the estimated value from the XRD results, as shown in Fig. 2(b), within the limit of error of 2%. The atomic ratio of Cu, as well as of Sn and O, in Cu-doped SnO films, is found from the XPS analysis, as shown in Fig. 5. The atomic ratio of Cu increased nearly linearly up to 11.4% with increasing to the dc power of Cu target of 20 W, while the atomic ratio of O was constant in almost 48.2 0.5%. Hence, the resultant atomic ratio of Cu can be approximately 8
ACCEPTED MANUSCRIPT denoted as x in Sn1-xCuxO, which is also shown in Fig. 4, and the maximum x is 22 at% for the dc power of Cu target of 20 W.
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Figures 6(a) to 6(c) show the surface morphology of Cu-doped SnO thin films for the dc
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power of (a) 0, (b) 10, and (c) 20 W. As seen from Fig 6(a), an undoped SnO film shows the
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continuously distributed surface with the grains of the hillock-shaped, wherein the biggest one is about 0.2-m-width with height of 200 nm. The smooth surface with continuous
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hillocks is observed for the dc power of 10 W, as seen from Fig. 6(b), whereas the lumpshaped grains are distributed discontinuously on the smooth surface for the dc power of 20 W,
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wherein the biggest one is about 0.5-m-width with height of 150 nm, as shown from Fig. 6(c). However, the significant reduction of rms roughness is found to be from about 21 nm
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for an undoped SnO film to 7 ~ 8 nm for Cu-doped SnO films with the dc power of 10 ~ 20
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W, respectively.
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Figure 7 shows the plot of (hν)2 versus the photon energy hν for various value of x in Sn1-xCuxO thin films, where is the absorption coefficient extracted from the reflectometry analysis using = 4k/ (where is the wavelength of light). The direct optical bandgaps
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(Eopt) of Sn1-xCuxO thin films are determined by linearly extrapolating the (hν)2 curve to the horizontal axis, see Fig. 7. The Eopt for a SnO thin film of x = 0 is 2.44 eV, which is somewhat lower than the earlier reported value of ~2.7 eV [7]. The lower optical bandgap value in our sample is considered to be due to the different deposition method as well as the substrate type. As the x increases, the optical bandgap decreases continuously to a value of 2.02 eV for x = 17 at% and is almost constant to x = 22 at%. The optical transmittance spectra of Sn1-xCuxO thin films are also shown in the inset of Fig. 7. The decrease in Eopt is thought to be attributed to the increase mainly of the Cu2O phase in the films since the Eopt of 9
ACCEPTED MANUSCRIPT Cu2O thin films is reported to be about ~2.1 eV [19], lower than that of SnO. Whereas, the remarkable decrease in optical transmission with increasing x is interpreted to be due to the
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increase mainly of Cu phase, since the metallic Cu film has low transmission for the overall
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wavelength range. Additionally with increasing the dc power of Cu target, the increase of
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thickness of the samples is also contributing to the decrease in the transmission. There might be two distinct absorption edges in the Cu-doped SnO films having mixed phases of Cu and
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Cu2O in SnO, rather than the substitution of Cu for Sn, one for Cu2O and one for SnO. However, those could not be observed in Fig. 7, which is presumably due to the low Cu 2O
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phase of less than 10% and the low transmittance (less than 10%), as shown in the inset of Fig. 7, even for the maximum content of x = 22 at% in Sn1-xCuxO. Such optical properties are
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also related to the grain shape (or size), as typically shown in Fig. 6, since an increase in
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edge was lowered [20].
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grain size was known to deteriorate the optical transmission and also the optical absorption
Finally, the electrical properties, such as resistivity , carrier concentration n, and Hall mobility H as a function of x in Sn1-xCuxO thin films are shown in Fig. 8. Hall effect
xCuxO
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measurements show the negative Hall voltages, confirming the n-type conduction in Sn1thin films with x value less than 17 at%, even with x = 0, where the maximum H of
3.6 cm2/Vs is for x = 17 at%. Whereas the H of less than 1.0 cm2/Vs was observed for x = 0 ~ 7 at%. However, the p-type conduction was ambiguously obtained only for x = 22 at% with the H value of 1.1 cm2/Vs, which is similar to values reported in p-type conductive oxides such as NiO [21], ZnO [22], and Cu2O [23]. The value of decreases continuously from 1.2 102 to 0.37 cm with increasing value of x up to 17 at% and later slightly increased to a value of 0.45 cm for x = 22 at%, showing the same behavior as that of Eopt, as displayed in 10
ACCEPTED MANUSCRIPT Fig. 7. The values of carrier concentration n vary from 3 1018 to 5 1018 cm-3 for x = 0 ~ 17 at%, indicating the n-type conduction due to the relatively large metallic-Cu phases (see Fig.
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2(b)) or the Cuo bonds as shown in Fig. 3 in Cu-doped SnO thin films. Whereas, the value of
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n (1.2 1019 cm-3) for x = 22 at% is somewhat larger, showing the p-type conduction due to
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the high density of defects formed in SnO heavily doped with Cu films. Indeed, from these results it is argued that the mixed Cu2O phase in SnO provides p-type conduction, due the
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proper ratio of Cu2O to Cu of 41/59 for x = 22 at%, as evaluated from Fig. 2(b) or from Fig. 3.
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ACCEPTED MANUSCRIPT 4. Conclusion
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We have investigated the properties of Cu-doped SnO thin films with varying Cu
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contents (Sn1-xCuxO) fabricated by reactive co-sputtering using two facing targets of Sn and
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Cu. The XRD patterns showed the tetragonal SnO phases including the mixed phase of Cu2O (200) and metallic Cu (111) up to the Cu atomic content value of x = 22 at%. The XRD peaks
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of Cu2O (200) and Cu (111) corroborated that the metallic Cu phase is formed preferentially for lower Cu contents and the Cu2O phase is produced at the relatively higher dc power of Cu
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target. The phase ratio of Cu2O/Cu increases from 0 to 41/59 for x = 3 to 22 at%, determined from the XRD peak analysis. The Cu2O/Cu ratio value for x =22 at% is compatible to the
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values of the content of Cu1+/Cu0, estimated from the deconvolution of a Cu 2p3/2 XPS peak.
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The decrease of the resistivity and the optical transmission with increasing x in Sn1-xCuxO
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thin films up to 17 at% show the n-type conduction, resulting mainly due to the increase of Cu phase. However, for x = 22 at% the proper ratio of Cu2O phases to Cu phase is believed to provide the p-type conduction with a mobility of 1.1 cm2/Vs and the resistivity of 0.45 cm.
xCuxO)
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In summary, the proper ratio of Cu2O/Cu is vital to realize the p-type Cu-doped SnO (Sn1thin films which have wide application prospects in transparent optoelectronic and
electro-optic devices.
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ACCEPTED MANUSCRIPT Acknowledgment
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This work was supported by a grant from Kyung Hee University and was partly supported by
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the Sangji University 2014 Project.
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ACCEPTED MANUSCRIPT Figure captions Fig. 1. Arrangement of substrate and targets for preparing Cu-doped SnO thin films.
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and (b) XRD patterns in a narrower 2θ range from 41o to 45o.
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Fig. 2. XRD patterns of Cu-doped SnO thin films with (a) the various dc power of Cu target
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Fig. 3. Cu 2p3/2 core level XPS spectra of Cu-doped SnO thin films and Sn 3d5/2 core level XPS spectra (inset) with the various dc power of Cu target.
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Fig. 4. Curve fitting of a Cu 2p3/2 core level XPS spectrum for dc power of 20 W. Fig. 5. Plot of the atomic contents of O, Sn, and Cu and x in Sn1-xCuxO vs. the dc power of
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Cu target.
Fig. 6. Typical surface AFM morphologies (5 m 5 m) for (a) the dc power of Cu target
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of (a) 0 , (b) 10, and (c) 20 W.
thin films.
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Fig. 7. Plot of (hν)2 versus hν and the transmittance spectra (inset) for various x in Sn1-
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Fig. 8. Plot of resistivity , carrier concentration n, and Hall mobility H vs. x in Sn1-xCuxO.
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Cu (111)
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Research Highlights
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►Cu-doped SnO films were prepared by reactive co-sputtering with facing targets. ►Mixed phase of Cu2O and Cu in tetragonal SnO phases was found. 2 ►The Sn1-xCuxO film at x = 22 at% shows the p-type with a mobility of 1.1 cm /Vs. ►Proper ratio of Cu2O/Cu is imperative to realize the p-type conduction.
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