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Influence of target power on properties of CuxO thin films prepared by reactive radio frequency magnetron sputtering
TSF-34355; No of Pages 6 Thin Solid Films xxx (2015) xxx–xxx
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Influence of target power on properties of CuxO thin films prepared by reactive radio frequency magnetron sputtering J. Gan ⁎, V. Venkatachalapathy, B.G. Svensson, E.V. Monakhov University of Oslo, Department of Physics/Center for Materials Science and Nanotechnology, P.O. Box 1048 Blindern, N-0316 Oslo, Norway
a r t i c l e
i n f o
Article history: Received 24 October 2014 Received in revised form 19 May 2015 Accepted 19 May 2015 Available online xxxx Keywords: Solar cells Reactive sputtering Oxide semiconductors Cuprous oxide Thin films Target power
a b s t r a c t CuxO thin films have been deposited by reactive radio frequency magnetron sputtering at different target powers Ptar (140–190 W) by fixing other process parameters: oxygen mass flow, argon mass flow and substrate temperature. Follow-up characterization (structural, electrical and optical) results reveal that the target power has a strong influence on both composition and functional properties of the resulting CuxO films and particularly, the films tend to enter a Cu-rich phase by increasing the target power. Furthermore, the films prepared at the highest power (190 W) exhibit single phase Cu2O and demonstrate superior electrical properties and high growth rate. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Recently, p-n junctions entirely based on metal-oxide semiconductors, have received enormously growing attention and rapidly been developed into an emerging research field. Oxides being chemically stable, non-toxic and earth-abundant, enable manufacturing in a largescale at low costs. This is especially suitable for the next-generation of semiconductor based solar cells, and at the same time they are very environment-friendly [1]. Oxide based hetero-junction thin film solar cells, specifically with intrinsic p-type Cu2O (band gap EgCu2 O ≈2:1 eV) as the absorber layer, show a theoretical efficiency up to 18% [2], while currently reported experimental values remain between 2–4% [3,4]. For further understanding and improvement, both the thin film properties (transport, optical, structural) and interface defects should be investigated. Cu2O (cuprous oxide) can be fabricated by several different techniques such as thermal oxidation [5], direct current (dc) [6,7] or radio frequency (rf) [8] reactive sputtering, molecular beam epitaxy [9] and electrodeposition [10]. Among these different techniques, reactive rf magnetron sputtering has a good stoichiometry control of Cu2O because there are several different growth parameters that can be varied in order to form either Cu-rich or O-rich phases. Besides, it produces films with high uniformity at high growth rate. Fig. 1, shows the unit cell of Cu2O, which has a cubic structure consisting of 4 Cu-atoms and 2 O-atoms in each cell. The lattice constant is determined to be 4.2696 Å [11]. ⁎ Corresponding author. E-mail address: [email protected] (J. Gan).
Using reactive magnetron sputtering, copper oxides with different phases can be deposited simultaneously (e.g., Cu2O, CuO and Cu4O3) depending on growth conditions (Cu-rich/O-rich). Therefore, a strict control of the processing parameters including: oxygen mass flow Q(O2), argon mass flow Q(Ar), target power Ptar and substrate temperature Tsub is crucial to sustain a stoichiometry-stable Cu2O thin film as desired [12], preventing phase transition to the more O-rich phases. In comparison to the thermal oxidation process, where in the phase diagram [13] merely the oxygen partial pressure p(O2) and the Tsub matter, in the reactive sputtering process there are two additional growth parameters namely Ptar and Q(Ar) worthy of attention. The partial pressure of the reactive gas p(O2) determines the composition of the thermo-dynamically stable CuxO phase of the films, and this holds irrespective of the growth technique used [12,13]. Deuermeier et al. [14] compared Cu2O films that were rf magnetron reactively sputtered at different oxygen pressures, for their use in solar cells. In comparison to deposition with 4–6% of O2 content (defined as mass flow ratio of pure O2 to the mixture of O2 and Ar), deposition with 9% O2 content changed the peak shape and position of the main Cu 2p3/2 emission signal in X-ray Photoelectron Spectroscopy spectra and strong satellite occurred, indicating a noticeable amount of Cu(II) in the film. In deposition by sputtering, the stoichiometry of Cu2O is retained by the ratio of the partial pressure of the reactive gas over the flux of Ar, Φ(Ar), that erodes the target surface [12]. At a fixed Q(Ar) inlet, Φ(Ar) depends on the sheath potential/target potential [15], which is correlated to the target power [16]. Once the partial pressure of Ar and O2 are set, the flux of the non-reactive gas and consequently the balance of Cu versus O atoms accumulated on the substrate will be controlled by the target
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Please cite this article as: J. Gan, et al., Influence of target power on properties of CuxO thin films prepared by reactive radio frequency magnetron sputtering, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.05.029
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Fig. 1. The unit cell of Cu2O (cuprous oxide, Cu(I) oxide). The space group for its lattice structure is O4h ; Pn3m.
power during the reactive sputtering process. In a previous study Kyounga Lim et al. [17] briefly compared properties of copper oxide films sputtered at target powers of 200, 300 and 400 W, but with limited systematic results on the properties of the sputtered films. In the present work, a systematic study on the influence of the target power on the electrical, optical and structural properties of CuxO thin film has been undertaken. The results show that the films tend to enter a Cu-rich phase (Cu2O) by increasing the target power and the one fabricated at the highest power (190 W) exhibits single phase Cu2O and demonstrates superior electrical properties plus high growth rate. 2. Experimental details CuxO thin films were fabricated on a quartz substrate in the clean room of MiNaLab at University of Oslo employing a commercial DC/RF Magnetron Sputter system (Semicore Triaxis). Quartz substrates (double-sided polished) were laser-cut into sizes of 1 × 1 and 1 × 2 cm2 and treated in consecutive ultra-sonic baths using acetone, isopropanol and deionized water; 10 min each. Subsequently, the substrates were dried with a nitrogen flow and loaded into the growth chamber. The target used was a circular copper plate with dimensions of 7.62 cm in diameter and 0.51 cm in thickness. The films were synthesized at different Ptar from 140 to 190 W with a step of 10 W while all the other sputtering parameters were kept constant yielding stable operational conditions: Q(Ar) = 20.0 sccm, Q(O2) = 3.0 sccm and Tsub = 400 °C.
The base pressure was below 4.0 × 10−4 Pa at the substrate temperature of 400 °C, while the total pressure during the deposition was stable in the range of 0.73–0.75 Pa after the igniter was turned on. The targetsubstrate distance was measured to be 7.2 cm with the target surface being parallel to the substrate. Additionally, the sample stage was rotated at a constant speed of 12 rpm during deposition in order to attain a good film uniformity. After 20 min of sputtering, the O2 flow was turned off immediately before closing the target shutter, which suppresses oxidation of Cu2O being sensitive to O2 at elevated temperature. The films were characterized by: (1) X-ray Diffraction (XRD), (Bruker AXS D8 Discover, Cu Kα X-rays) analysis to determine the crystal structures (including the crystal orientation and the grain size) in normal θ-2θ scanning mode, (2) UV–Visible spectrophotometer measurements in the spectral range of 290–1500 nm, (Shimadzu SolidSpe-3700 DUV) to determine the optical band gap, (3) room temperature Hall effect measurements using the van-der Pauw configuration (LakeShore 7604) to determine carrier mobility μ, and carrier concentration N, (4) surface profilometry to determine the film thickness and growth rate (Veeco Dektak 8 Stylus profilometer).
3. Results and discussion Fig. 2 shows optical images of 6 thin film CuxO samples deposited on quartz substrates with varying sputtering power (Ptar = 140–190 W). Each type of film was sputtered on 2 pieces of substrates (1 × 2 and 1 × 1 cm2 in sizes) and the color of the CuxO films turns from dark/ black into light orange with increasing power. The different phases of CuxO were determined by comparing the experimental XRD peak pattern with the standard Powder Diffraction (PDF) cards (ICDD patterns: 01-071-3645 or space group O4h ; Pn3m for Cu2O, 00-080-1916 or space group C2/c for CuO and 01-071-6397 or space group I41/amd for Cu4O3). Fig. 3(a) shows overview spectra of the CuxO thin films in the range 26–80° 2θ, while Fig. 3(b) and (c) separate Fig. 3(a) into two magnified parts. At low target power, Ptar = 140 W (pattern shown in black), there exist several CuxO phases with different orientations (all are marked in the figures): CuO (111), Cu2O (111), and Cu4O3 (202)/(213)/(224)/(325), and Cu4O3 (202) and Cu2O (111) share a wide peak-packet (Fig. 3(b)). Increasing the power from 140 to 170 W, the diffraction peaks of the CuO and Cu4O3 phases disappear; further, the Cu2O phase along the crystal orientations (220) and (311) start to emerge and concurrently, the intensity of the dominant (111) diffraction peak increases with a decrease in FWHM (Full Width Half Maximum) from 1.15 to 0.62. This clearly demonstrates that an increase in the target power promotes the Cu-rich phases of CuxO, and when Ptar is increased up to 190 W, the intensity of the Cu2O(200) peak becomes comparable to that of the Cu2O(111) peak (Fig. 3(b)).
Fig. 2. Optical images of the samples (CuxO thin films on quart substrates) prepared at different rf target powers ((a) to (f) from 140 to 190 W).
Please cite this article as: J. Gan, et al., Influence of target power on properties of CuxO thin films prepared by reactive radio frequency magnetron sputtering, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.05.029
J. Gan et al. / Thin Solid Films xxx (2015) xxx–xxx
The promotion of the Cu-rich phases by a high Ptar can be understood by considering the film growth rate, R. In a first approximation, R is linearly proportional to the Ar+ ion flux and described by the following equation [12]: R ¼ C 1 ð J=eÞ½SN θ1 þ SM ð1−θ1 Þ
ð1Þ
where C1 is a constant converting R to units of nm/min used in this work, J is the flux of sputtering argon ions that impinge on the target surface, e is the elementary charge, SN and SM are the sputtering yield of the ‘compound’ CuxO and the pure metal Cu, respectively, from the target during the Ar ion bombardment, i.e., the average number of ejected species per incoming Ar ion, and θ1 is the portion of the sputtered target area that has reacted with the O2 gas. In Eq. (1), it is assumed that the sputtered species are 100% transported to the substrate without any losses and the equation gives a rather simplified but illustrative description of the relation between the growth rate and the ion flux. J increases with Ptar while θ1 decreases since the removal rate of O atoms adsorbed on the target surface increases with J while the adsorption rate remains constant since p(O2) is kept fixed. Hence, the steady state is changed such that θ1 decreases leading to an increase in the sputtering of metallic Cu, and the steady-state of the CuxO film growth will shift from a Cu-poor to a Cu-rich phase (Cu2O) with increasing Ptar. Thermodynamically, Cu2O is also favored relative to the CuO/Cu4O3 phases, under O-poor (or Cu-rich) conditions [18], corroborating the trend to form Cu2O under Cu-rich sputtering conditions. In this context, it can also be mentioned that sputtering with closed shutter to the substrate was performed for ~5 min prior to the actual film deposition (so-called pre-sputtering) and no influence of the deposition time on the resulting film stoichiometry has been found, i.e., steadystate conditions prevail. The size Dp of the film grains was deduced from Scherrer's formula: Dp ¼
0:94λ β = cosθ 1
3
above 170 W, while at lower target powers, a mixture of the phases CuO, Cu2O and Cu4O3 occurs. The Hall mobility at room temperature increases from μ = 4.3 to 9.3 cm2·V−1·s− 1 between 180 and 190 W, while below 170 W μ is too low for a reliable determination (1 cm2·V− 1·s− 1) and thus not considered. Meyer et al. [24] studied the carrier concentration (N) of different phases of CuxO as a function of oxygen flow during rf sputtering. N of CuxO increased from 1015 to 1018 cm− 3 with an increase of the
ð2Þ
2
where λ is the X-ray wavelength (0.1543 nm), β = is the FWHM of a peak of a particular orientation in the XRD pattern, and θ is the corresponding orientation peak position in radians. In Fig. 4, Dp obtained from the Cu2O (111) peak is depicted and exhibits an increase from 10 to 30 nm with increasing Ptar. Similarly, Dp from the Cu2O (200) peak increases from 14 to 20 nm as Ptar is increased from 160 to 190 W. Ponyatovskii et al. [19] studied the effect of high external pressure on the structure of Cu2O and revealed a phase transition to CuO accompanied by a decrease in grain size. Machon et al. [20] also obtained similar results and attributed these effects under high pressure to a considerable increase in the microstrain within the film. In contrast, we observe the opposite effects for both phase transition and Dp and therefore, a decrease in the compressive microstrain of the Cu2O films with increasing Ptar is anticipated. In fact, the lattice parameter a is noticed to increase from 4.251 ± 0.004 to 4.261 ± 0.004 Å when Ptar is increased from 170 to 190 W and approaches the relaxed value of the cell (4.26849 Å, reference of powder diffraction from XRD card or 4.2696 Å in Fig. 1 for Cu2O). Further, the promotion of the (200) orientation relative to the (111) one, Fig. 3(b), at Cu-rich conditions (Ptar N 170 W) may be attributed to the surface/strain energy balance [21,22]. Table 1 shows the extracted values for majority carrier mobility, carrier concentration and the resistivity of the sputtered CuxO films. The films indicate n-type conductivity for Ptar b 170 W, although the electrical measurement results are not reliable due to too low values in carrier mobility. For Ptar N 170 W, in contrast, the films demonstrate a clear p-type conductivity. Lu et al. [23] have reported a corresponding change in the conductivity from n-type to p-type (and then back to n-type), by varying p(O2) and attributed this to mixing of the Cu2O and CuO phases. The observed evolution of electrical properties with increasing Ptar correlates well with the XRD results; Cu2O prevails 1
2
Fig. 3. (a) 2θ-θ XRD patterns for CuxO films on quartz substrates (the y-axis was evenly offset for visibility of each pattern and the background from the substrate (SiO2) has been calibrated); (b) direct comparison between the XRD patterns in the 2θ range 34–44° with no Y-offset; (c) enlarged XRD patterns in the 2θ range 43–82° with an Y-offset for each pattern of an even 50. The peak at around 64° 2θ, matched with *, arises from the quartz substrate and it is not related to the CuxO films.
Please cite this article as: J. Gan, et al., Influence of target power on properties of CuxO thin films prepared by reactive radio frequency magnetron sputtering, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.05.029
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Fig. 4. Grain sizes deduced from Scherrer's equation for Cu2O with (111) or (200) crystal orientations (based on FWHM values determined from XRD patterns in Fig. 3(b)) as a function of target power during sputtering.
oxygen flow and ultimately reached 1019 to 1020 cm−3 for Cu4O3 and CuO under truly O-rich conditions. Similarly, for Ptar b 170 W the CuO and Cu4O3 phases dominate electrically yielding electron concentrations of 1019–1020 cm−3. For Ptar = 180 W and above, the Cu2O phase determines the conductivity and N decreases to the 1015 cm−3 range, representing the hole concentration. The electrical properties are closely linked with the structural properties of the films, and Jeong et al. [25] concluded the hole mobility in Cu2O is limited by grain boundary scattering (the larger the grain size, the higher hole mobility). Indeed, our mobility data in Table 1 for Ptar = 180 and 190 W, i.e., the films dominated by the Cu2O phase, are fully consistent with that grain boundary scattering is limiting the hole mobility. Fig. 5(a) shows the transmittance of the CuxO films in the wavelength range of 290–1500 nm. All the films display interference fringes, caused by interference between the incoming light and that reflected at the interface between the films and the quartz substrates. In fact, this interference demonstrates a sharp interface between the substrate and the CuxO films. All the films (sputtered at different powers) show zero transmittance for λ b 500 nm while for λ = 567 nm, equivalent to EgCu2 O ¼ 2:17 eV and indicated by the dashed line in Fig. 5(a), photons start to be transmitted through films sputtered with Ptar equal to 180 and 190 W. Hence, these films exhibit a larger band gap than the others and have an average transmittance of 65% above 567 nm (~ 80% in the peaks and ~50% in the valleys). At low target powers, the average transmittance decreases to 55%, reflecting the absorption coefficients of the films with mixed phases of CuO, Cu4O3 and Cu2O.
Table 1 Results from Hall effect measurements performed at room temperature on CuxO films sputtered on quartz substrates. For the films deposited with a power above 170 W, the phase Cu2O prevails strongly. Target power (W)
Majority carrier mobility μ (cm2·V−1·s−1)
Carrier concentration NA (cm−3)
Resistivity R (ohm·cm)
Type
140 150 160 170 180 190
b0.1a b0.1a b0.1a ~0.5 4.4 9.3
2.1 × 1019 1.9 × 1020 4.5 × 1017 2.4 × 1016 1.4 × 1015 1.3 × 1015
134 131 291 633 1015 532
– – – p p p
a Represents that the mobility values measured for films with Ptar b 170 W were too small to be reliable and should thus not considered in detail.
Fig. 5. (a) Transmittance spectra for CuxO thin films (thickness ~500–800 nm) prepared with conditions of Ptar = 140–190 W, Q(O2) = 3.0 sccm, Q(Ar) = 20.0 sccm, and Tsub = 400 °C; (b) Tauc plot for the CuxO thin films, converted from the transmittance spectra in (a), the optical band gap values for each type of films are estimated from extrapolation to the abscissa (dashed lines) and are listed in the inset table.
A Tauc plot analysis was made to determine the optical band gap of the films employing the relation, 1=n
ðhνα Þ
¼ A hν−Eg
ð3Þ
where h is Planck's constant, ν is the photon frequency, α is the absorption coefficient, and A is a proportional constant. The value of the exponent n in Eq. (3) denotes the nature of the transition (n = 1/2 for direct allowed transitions, n = 3/2 for direct forbidden transition, n = 2 for indirect allowed transition and n = 3 for indirect forbidden transition). Here, it can be mentioned that Malerba et al. [26] have argued that a fit of the absorption coefficient with any expression of the form (hν − Eg)n and n ≠ 1 is not a reliable way to determine absolute values EgCu2 O , as in the energy range 2.1–2.3 eV α follows a nearly linear trend. Despite the argument in [26], n = 1/2 was used in the present work to determine Eg, but for relative comparison mainly. Eg (i.e., the extrapolation of the absorption slope to the abscissa) increases from ~ 2.15 to ~2.48 eV (Fig. 5(b)) with increasing Ptar from 140 to 190 W where it should be pointed out that the optical bandgap extraction involves excitonic features [11], i.e., a direct comparison xO with the absolute values of ECu is not valid. Maruyama et al. [27] g have demonstrated that increasing the fraction of Cu2+ on the expense of Cu+ decreases the bandgap from 2.5 eV to 1.8 eV, implying a larger
Please cite this article as: J. Gan, et al., Influence of target power on properties of CuxO thin films prepared by reactive radio frequency magnetron sputtering, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.05.029
J. Gan et al. / Thin Solid Films xxx (2015) xxx–xxx
fraction of CuO while increasing the p(O2). This is fully consistent with our results since a larger Ptar yields a larger fraction of Cu2O and moreover, the absorption edge becomes steeper at high Ptar because of the dominant single-phase Cu2O (Fig. 5(b)). The films prepared at low target powers do not demonstrate a sharp edge due to mixed phases of CuO, Cu4O3 and Cu2O and their optical images (see Fig. 2(a)–(c)) are dark/black and less transparent. Finally, in Fig. 6, the growth rate at the different target powers is compared based on the film thickness measurements, with all the films being sputtered for an equivalent period of time (20 min for a thickness of 500–800 nm under all power conditions). The growth rate increases roughly linearly with the target power, from ~ 29 to 43 nm/min. This agrees well with observations made by other authors [16,28] showing that an increase in Ptar gives a higher erosion rate of the Cu target, provided that the oxidation environment remains fixed for all powers. Hence, the growth rate is limited by the supply of Cu atoms and the O-rich phases dominate at low Ptar. However, with increasing Ptar not only the growth rate is enhanced but also the formation of the Cu-rich Cu2O phase which eventually becomes the prevailing one, as evidenced by the XRD-data. In fact, the linear relation in Fig. 6 can be inferred from Eq. (1) by realizing that the flux of sputtering Ar ions, J, is proportional to the plasma discharge current and the Ar ion energy to the discharge voltage when keeping the pressure constant (Ptar = discharge current × discharge voltage). Further, at the ion energies used for sputtering (≤1 keV) elastic collisions dominate strongly and the sputtering yields, SN and SM are proportional to the energy [29]. Accordingly, the product J × SN and J × SM in Eq. (1) becomes directly proportional to Ptar and if the atomic density of the different compounds deposited are similar, which holds within ~ 20% for CuO, Cu4O3 and Cu2O, the conversion constant C1 in Eq. (1) will not change with Ptar, leading to a linear relationship between the growth rate R and Ptar.
4. Conclusions In summary, the target power plays a decisive role in keeping a stable stoichiometry of Cu2O films prepared by reactive RF magnetron sputtering. By increasing Ptar, the growth rate of the CuxO films increases where the O-rich phases (CuO and Cu4O3) gradually disappear while the p-type Cu2O single phase is promoted. Concurrently, the Cu2O grain size becomes larger for both the (111) and (200) orientations and the hole mobility increases, indicating that grain boundary scattering is a limiting mechanism.
Fig. 6. The growth rate of CuxO thin films (with error bars) as a function of the rf target power (140–190 W with a step of 10 W) for Q(O2) = 3.0 sccm, Q(Ar) = 20.0 sccm, and Tsub = 400 °C. The growth rate shows an approximate linear increase with the target power (the black dashed line).
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The films grown with Ptar = 190 W have superior properties under the current process parameters (Q(O2)/Q(Ar) = 3.0 sccm/20.0 sccm, Tsub = 400 °C), but a further increase in Ptar may lead to even better film quality, i.e., prevalence of the Cu2O phase with larger grains, higher hole mobility, sharper band edges and improved structural quality. A hole mobility of 9–10 cm2·V− 1·s− 1 is demonstrated at room temperature for Cu2O films with an average grain size of ~30 nm and ~ 20 nm in the (111) and (200) directions, respectively. Further, the optical band gap increases with Ptar and at 190 W, where the Cu2O phase dominates, a value of ~ 2.45–2.50 eV is obtained at 295 K. The transmittance of these 190 W films with a thickness of ~ 830 nm approaches 85% for λ N 567 nm. Acknowledgements Fruitful discussions with Dr. Ramon Schifano during the initial stage of this work are gratefully acknowledged. 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Please cite this article as: J. Gan, et al., Influence of target power on properties of CuxO thin films prepared by reactive radio frequency magnetron sputtering, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.05.029
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Influence of target power on properties of CuxO thin films prepared by reactive radio frequency magnetron sputtering J. Gan ⁎, V. Venkatachalapathy, B.G. Svensson, E.V. Monakhov University of Oslo, Department of Physics/Center for Materials Science and Nanotechnology, P.O. Box 1048 Blindern, N-0316 Oslo, Norway
a r t i c l e
i n f o
Article history: Received 24 October 2014 Received in revised form 19 May 2015 Accepted 19 May 2015 Available online xxxx Keywords: Solar cells Reactive sputtering Oxide semiconductors Cuprous oxide Thin films Target power
a b s t r a c t CuxO thin films have been deposited by reactive radio frequency magnetron sputtering at different target powers Ptar (140–190 W) by fixing other process parameters: oxygen mass flow, argon mass flow and substrate temperature. Follow-up characterization (structural, electrical and optical) results reveal that the target power has a strong influence on both composition and functional properties of the resulting CuxO films and particularly, the films tend to enter a Cu-rich phase by increasing the target power. Furthermore, the films prepared at the highest power (190 W) exhibit single phase Cu2O and demonstrate superior electrical properties and high growth rate. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Recently, p-n junctions entirely based on metal-oxide semiconductors, have received enormously growing attention and rapidly been developed into an emerging research field. Oxides being chemically stable, non-toxic and earth-abundant, enable manufacturing in a largescale at low costs. This is especially suitable for the next-generation of semiconductor based solar cells, and at the same time they are very environment-friendly [1]. Oxide based hetero-junction thin film solar cells, specifically with intrinsic p-type Cu2O (band gap EgCu2 O ≈2:1 eV) as the absorber layer, show a theoretical efficiency up to 18% [2], while currently reported experimental values remain between 2–4% [3,4]. For further understanding and improvement, both the thin film properties (transport, optical, structural) and interface defects should be investigated. Cu2O (cuprous oxide) can be fabricated by several different techniques such as thermal oxidation [5], direct current (dc) [6,7] or radio frequency (rf) [8] reactive sputtering, molecular beam epitaxy [9] and electrodeposition [10]. Among these different techniques, reactive rf magnetron sputtering has a good stoichiometry control of Cu2O because there are several different growth parameters that can be varied in order to form either Cu-rich or O-rich phases. Besides, it produces films with high uniformity at high growth rate. Fig. 1, shows the unit cell of Cu2O, which has a cubic structure consisting of 4 Cu-atoms and 2 O-atoms in each cell. The lattice constant is determined to be 4.2696 Å [11]. ⁎ Corresponding author. E-mail address: [email protected] (J. Gan).
Using reactive magnetron sputtering, copper oxides with different phases can be deposited simultaneously (e.g., Cu2O, CuO and Cu4O3) depending on growth conditions (Cu-rich/O-rich). Therefore, a strict control of the processing parameters including: oxygen mass flow Q(O2), argon mass flow Q(Ar), target power Ptar and substrate temperature Tsub is crucial to sustain a stoichiometry-stable Cu2O thin film as desired [12], preventing phase transition to the more O-rich phases. In comparison to the thermal oxidation process, where in the phase diagram [13] merely the oxygen partial pressure p(O2) and the Tsub matter, in the reactive sputtering process there are two additional growth parameters namely Ptar and Q(Ar) worthy of attention. The partial pressure of the reactive gas p(O2) determines the composition of the thermo-dynamically stable CuxO phase of the films, and this holds irrespective of the growth technique used [12,13]. Deuermeier et al. [14] compared Cu2O films that were rf magnetron reactively sputtered at different oxygen pressures, for their use in solar cells. In comparison to deposition with 4–6% of O2 content (defined as mass flow ratio of pure O2 to the mixture of O2 and Ar), deposition with 9% O2 content changed the peak shape and position of the main Cu 2p3/2 emission signal in X-ray Photoelectron Spectroscopy spectra and strong satellite occurred, indicating a noticeable amount of Cu(II) in the film. In deposition by sputtering, the stoichiometry of Cu2O is retained by the ratio of the partial pressure of the reactive gas over the flux of Ar, Φ(Ar), that erodes the target surface [12]. At a fixed Q(Ar) inlet, Φ(Ar) depends on the sheath potential/target potential [15], which is correlated to the target power [16]. Once the partial pressure of Ar and O2 are set, the flux of the non-reactive gas and consequently the balance of Cu versus O atoms accumulated on the substrate will be controlled by the target
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Please cite this article as: J. Gan, et al., Influence of target power on properties of CuxO thin films prepared by reactive radio frequency magnetron sputtering, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.05.029
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Fig. 1. The unit cell of Cu2O (cuprous oxide, Cu(I) oxide). The space group for its lattice structure is O4h ; Pn3m.
power during the reactive sputtering process. In a previous study Kyounga Lim et al. [17] briefly compared properties of copper oxide films sputtered at target powers of 200, 300 and 400 W, but with limited systematic results on the properties of the sputtered films. In the present work, a systematic study on the influence of the target power on the electrical, optical and structural properties of CuxO thin film has been undertaken. The results show that the films tend to enter a Cu-rich phase (Cu2O) by increasing the target power and the one fabricated at the highest power (190 W) exhibits single phase Cu2O and demonstrates superior electrical properties plus high growth rate. 2. Experimental details CuxO thin films were fabricated on a quartz substrate in the clean room of MiNaLab at University of Oslo employing a commercial DC/RF Magnetron Sputter system (Semicore Triaxis). Quartz substrates (double-sided polished) were laser-cut into sizes of 1 × 1 and 1 × 2 cm2 and treated in consecutive ultra-sonic baths using acetone, isopropanol and deionized water; 10 min each. Subsequently, the substrates were dried with a nitrogen flow and loaded into the growth chamber. The target used was a circular copper plate with dimensions of 7.62 cm in diameter and 0.51 cm in thickness. The films were synthesized at different Ptar from 140 to 190 W with a step of 10 W while all the other sputtering parameters were kept constant yielding stable operational conditions: Q(Ar) = 20.0 sccm, Q(O2) = 3.0 sccm and Tsub = 400 °C.
The base pressure was below 4.0 × 10−4 Pa at the substrate temperature of 400 °C, while the total pressure during the deposition was stable in the range of 0.73–0.75 Pa after the igniter was turned on. The targetsubstrate distance was measured to be 7.2 cm with the target surface being parallel to the substrate. Additionally, the sample stage was rotated at a constant speed of 12 rpm during deposition in order to attain a good film uniformity. After 20 min of sputtering, the O2 flow was turned off immediately before closing the target shutter, which suppresses oxidation of Cu2O being sensitive to O2 at elevated temperature. The films were characterized by: (1) X-ray Diffraction (XRD), (Bruker AXS D8 Discover, Cu Kα X-rays) analysis to determine the crystal structures (including the crystal orientation and the grain size) in normal θ-2θ scanning mode, (2) UV–Visible spectrophotometer measurements in the spectral range of 290–1500 nm, (Shimadzu SolidSpe-3700 DUV) to determine the optical band gap, (3) room temperature Hall effect measurements using the van-der Pauw configuration (LakeShore 7604) to determine carrier mobility μ, and carrier concentration N, (4) surface profilometry to determine the film thickness and growth rate (Veeco Dektak 8 Stylus profilometer).
3. Results and discussion Fig. 2 shows optical images of 6 thin film CuxO samples deposited on quartz substrates with varying sputtering power (Ptar = 140–190 W). Each type of film was sputtered on 2 pieces of substrates (1 × 2 and 1 × 1 cm2 in sizes) and the color of the CuxO films turns from dark/ black into light orange with increasing power. The different phases of CuxO were determined by comparing the experimental XRD peak pattern with the standard Powder Diffraction (PDF) cards (ICDD patterns: 01-071-3645 or space group O4h ; Pn3m for Cu2O, 00-080-1916 or space group C2/c for CuO and 01-071-6397 or space group I41/amd for Cu4O3). Fig. 3(a) shows overview spectra of the CuxO thin films in the range 26–80° 2θ, while Fig. 3(b) and (c) separate Fig. 3(a) into two magnified parts. At low target power, Ptar = 140 W (pattern shown in black), there exist several CuxO phases with different orientations (all are marked in the figures): CuO (111), Cu2O (111), and Cu4O3 (202)/(213)/(224)/(325), and Cu4O3 (202) and Cu2O (111) share a wide peak-packet (Fig. 3(b)). Increasing the power from 140 to 170 W, the diffraction peaks of the CuO and Cu4O3 phases disappear; further, the Cu2O phase along the crystal orientations (220) and (311) start to emerge and concurrently, the intensity of the dominant (111) diffraction peak increases with a decrease in FWHM (Full Width Half Maximum) from 1.15 to 0.62. This clearly demonstrates that an increase in the target power promotes the Cu-rich phases of CuxO, and when Ptar is increased up to 190 W, the intensity of the Cu2O(200) peak becomes comparable to that of the Cu2O(111) peak (Fig. 3(b)).
Fig. 2. Optical images of the samples (CuxO thin films on quart substrates) prepared at different rf target powers ((a) to (f) from 140 to 190 W).
Please cite this article as: J. Gan, et al., Influence of target power on properties of CuxO thin films prepared by reactive radio frequency magnetron sputtering, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.05.029
J. Gan et al. / Thin Solid Films xxx (2015) xxx–xxx
The promotion of the Cu-rich phases by a high Ptar can be understood by considering the film growth rate, R. In a first approximation, R is linearly proportional to the Ar+ ion flux and described by the following equation [12]: R ¼ C 1 ð J=eÞ½SN θ1 þ SM ð1−θ1 Þ
ð1Þ
where C1 is a constant converting R to units of nm/min used in this work, J is the flux of sputtering argon ions that impinge on the target surface, e is the elementary charge, SN and SM are the sputtering yield of the ‘compound’ CuxO and the pure metal Cu, respectively, from the target during the Ar ion bombardment, i.e., the average number of ejected species per incoming Ar ion, and θ1 is the portion of the sputtered target area that has reacted with the O2 gas. In Eq. (1), it is assumed that the sputtered species are 100% transported to the substrate without any losses and the equation gives a rather simplified but illustrative description of the relation between the growth rate and the ion flux. J increases with Ptar while θ1 decreases since the removal rate of O atoms adsorbed on the target surface increases with J while the adsorption rate remains constant since p(O2) is kept fixed. Hence, the steady state is changed such that θ1 decreases leading to an increase in the sputtering of metallic Cu, and the steady-state of the CuxO film growth will shift from a Cu-poor to a Cu-rich phase (Cu2O) with increasing Ptar. Thermodynamically, Cu2O is also favored relative to the CuO/Cu4O3 phases, under O-poor (or Cu-rich) conditions [18], corroborating the trend to form Cu2O under Cu-rich sputtering conditions. In this context, it can also be mentioned that sputtering with closed shutter to the substrate was performed for ~5 min prior to the actual film deposition (so-called pre-sputtering) and no influence of the deposition time on the resulting film stoichiometry has been found, i.e., steadystate conditions prevail. The size Dp of the film grains was deduced from Scherrer's formula: Dp ¼
0:94λ β = cosθ 1
3
above 170 W, while at lower target powers, a mixture of the phases CuO, Cu2O and Cu4O3 occurs. The Hall mobility at room temperature increases from μ = 4.3 to 9.3 cm2·V−1·s− 1 between 180 and 190 W, while below 170 W μ is too low for a reliable determination (1 cm2·V− 1·s− 1) and thus not considered. Meyer et al. [24] studied the carrier concentration (N) of different phases of CuxO as a function of oxygen flow during rf sputtering. N of CuxO increased from 1015 to 1018 cm− 3 with an increase of the
ð2Þ
2
where λ is the X-ray wavelength (0.1543 nm), β = is the FWHM of a peak of a particular orientation in the XRD pattern, and θ is the corresponding orientation peak position in radians. In Fig. 4, Dp obtained from the Cu2O (111) peak is depicted and exhibits an increase from 10 to 30 nm with increasing Ptar. Similarly, Dp from the Cu2O (200) peak increases from 14 to 20 nm as Ptar is increased from 160 to 190 W. Ponyatovskii et al. [19] studied the effect of high external pressure on the structure of Cu2O and revealed a phase transition to CuO accompanied by a decrease in grain size. Machon et al. [20] also obtained similar results and attributed these effects under high pressure to a considerable increase in the microstrain within the film. In contrast, we observe the opposite effects for both phase transition and Dp and therefore, a decrease in the compressive microstrain of the Cu2O films with increasing Ptar is anticipated. In fact, the lattice parameter a is noticed to increase from 4.251 ± 0.004 to 4.261 ± 0.004 Å when Ptar is increased from 170 to 190 W and approaches the relaxed value of the cell (4.26849 Å, reference of powder diffraction from XRD card or 4.2696 Å in Fig. 1 for Cu2O). Further, the promotion of the (200) orientation relative to the (111) one, Fig. 3(b), at Cu-rich conditions (Ptar N 170 W) may be attributed to the surface/strain energy balance [21,22]. Table 1 shows the extracted values for majority carrier mobility, carrier concentration and the resistivity of the sputtered CuxO films. The films indicate n-type conductivity for Ptar b 170 W, although the electrical measurement results are not reliable due to too low values in carrier mobility. For Ptar N 170 W, in contrast, the films demonstrate a clear p-type conductivity. Lu et al. [23] have reported a corresponding change in the conductivity from n-type to p-type (and then back to n-type), by varying p(O2) and attributed this to mixing of the Cu2O and CuO phases. The observed evolution of electrical properties with increasing Ptar correlates well with the XRD results; Cu2O prevails 1
2
Fig. 3. (a) 2θ-θ XRD patterns for CuxO films on quartz substrates (the y-axis was evenly offset for visibility of each pattern and the background from the substrate (SiO2) has been calibrated); (b) direct comparison between the XRD patterns in the 2θ range 34–44° with no Y-offset; (c) enlarged XRD patterns in the 2θ range 43–82° with an Y-offset for each pattern of an even 50. The peak at around 64° 2θ, matched with *, arises from the quartz substrate and it is not related to the CuxO films.
Please cite this article as: J. Gan, et al., Influence of target power on properties of CuxO thin films prepared by reactive radio frequency magnetron sputtering, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.05.029
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Fig. 4. Grain sizes deduced from Scherrer's equation for Cu2O with (111) or (200) crystal orientations (based on FWHM values determined from XRD patterns in Fig. 3(b)) as a function of target power during sputtering.
oxygen flow and ultimately reached 1019 to 1020 cm−3 for Cu4O3 and CuO under truly O-rich conditions. Similarly, for Ptar b 170 W the CuO and Cu4O3 phases dominate electrically yielding electron concentrations of 1019–1020 cm−3. For Ptar = 180 W and above, the Cu2O phase determines the conductivity and N decreases to the 1015 cm−3 range, representing the hole concentration. The electrical properties are closely linked with the structural properties of the films, and Jeong et al. [25] concluded the hole mobility in Cu2O is limited by grain boundary scattering (the larger the grain size, the higher hole mobility). Indeed, our mobility data in Table 1 for Ptar = 180 and 190 W, i.e., the films dominated by the Cu2O phase, are fully consistent with that grain boundary scattering is limiting the hole mobility. Fig. 5(a) shows the transmittance of the CuxO films in the wavelength range of 290–1500 nm. All the films display interference fringes, caused by interference between the incoming light and that reflected at the interface between the films and the quartz substrates. In fact, this interference demonstrates a sharp interface between the substrate and the CuxO films. All the films (sputtered at different powers) show zero transmittance for λ b 500 nm while for λ = 567 nm, equivalent to EgCu2 O ¼ 2:17 eV and indicated by the dashed line in Fig. 5(a), photons start to be transmitted through films sputtered with Ptar equal to 180 and 190 W. Hence, these films exhibit a larger band gap than the others and have an average transmittance of 65% above 567 nm (~ 80% in the peaks and ~50% in the valleys). At low target powers, the average transmittance decreases to 55%, reflecting the absorption coefficients of the films with mixed phases of CuO, Cu4O3 and Cu2O.
Table 1 Results from Hall effect measurements performed at room temperature on CuxO films sputtered on quartz substrates. For the films deposited with a power above 170 W, the phase Cu2O prevails strongly. Target power (W)
Majority carrier mobility μ (cm2·V−1·s−1)
Carrier concentration NA (cm−3)
Resistivity R (ohm·cm)
Type
140 150 160 170 180 190
b0.1a b0.1a b0.1a ~0.5 4.4 9.3
2.1 × 1019 1.9 × 1020 4.5 × 1017 2.4 × 1016 1.4 × 1015 1.3 × 1015
134 131 291 633 1015 532
– – – p p p
a Represents that the mobility values measured for films with Ptar b 170 W were too small to be reliable and should thus not considered in detail.
Fig. 5. (a) Transmittance spectra for CuxO thin films (thickness ~500–800 nm) prepared with conditions of Ptar = 140–190 W, Q(O2) = 3.0 sccm, Q(Ar) = 20.0 sccm, and Tsub = 400 °C; (b) Tauc plot for the CuxO thin films, converted from the transmittance spectra in (a), the optical band gap values for each type of films are estimated from extrapolation to the abscissa (dashed lines) and are listed in the inset table.
A Tauc plot analysis was made to determine the optical band gap of the films employing the relation, 1=n
ðhνα Þ
¼ A hν−Eg
ð3Þ
where h is Planck's constant, ν is the photon frequency, α is the absorption coefficient, and A is a proportional constant. The value of the exponent n in Eq. (3) denotes the nature of the transition (n = 1/2 for direct allowed transitions, n = 3/2 for direct forbidden transition, n = 2 for indirect allowed transition and n = 3 for indirect forbidden transition). Here, it can be mentioned that Malerba et al. [26] have argued that a fit of the absorption coefficient with any expression of the form (hν − Eg)n and n ≠ 1 is not a reliable way to determine absolute values EgCu2 O , as in the energy range 2.1–2.3 eV α follows a nearly linear trend. Despite the argument in [26], n = 1/2 was used in the present work to determine Eg, but for relative comparison mainly. Eg (i.e., the extrapolation of the absorption slope to the abscissa) increases from ~ 2.15 to ~2.48 eV (Fig. 5(b)) with increasing Ptar from 140 to 190 W where it should be pointed out that the optical bandgap extraction involves excitonic features [11], i.e., a direct comparison xO with the absolute values of ECu is not valid. Maruyama et al. [27] g have demonstrated that increasing the fraction of Cu2+ on the expense of Cu+ decreases the bandgap from 2.5 eV to 1.8 eV, implying a larger
Please cite this article as: J. Gan, et al., Influence of target power on properties of CuxO thin films prepared by reactive radio frequency magnetron sputtering, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.05.029
J. Gan et al. / Thin Solid Films xxx (2015) xxx–xxx
fraction of CuO while increasing the p(O2). This is fully consistent with our results since a larger Ptar yields a larger fraction of Cu2O and moreover, the absorption edge becomes steeper at high Ptar because of the dominant single-phase Cu2O (Fig. 5(b)). The films prepared at low target powers do not demonstrate a sharp edge due to mixed phases of CuO, Cu4O3 and Cu2O and their optical images (see Fig. 2(a)–(c)) are dark/black and less transparent. Finally, in Fig. 6, the growth rate at the different target powers is compared based on the film thickness measurements, with all the films being sputtered for an equivalent period of time (20 min for a thickness of 500–800 nm under all power conditions). The growth rate increases roughly linearly with the target power, from ~ 29 to 43 nm/min. This agrees well with observations made by other authors [16,28] showing that an increase in Ptar gives a higher erosion rate of the Cu target, provided that the oxidation environment remains fixed for all powers. Hence, the growth rate is limited by the supply of Cu atoms and the O-rich phases dominate at low Ptar. However, with increasing Ptar not only the growth rate is enhanced but also the formation of the Cu-rich Cu2O phase which eventually becomes the prevailing one, as evidenced by the XRD-data. In fact, the linear relation in Fig. 6 can be inferred from Eq. (1) by realizing that the flux of sputtering Ar ions, J, is proportional to the plasma discharge current and the Ar ion energy to the discharge voltage when keeping the pressure constant (Ptar = discharge current × discharge voltage). Further, at the ion energies used for sputtering (≤1 keV) elastic collisions dominate strongly and the sputtering yields, SN and SM are proportional to the energy [29]. Accordingly, the product J × SN and J × SM in Eq. (1) becomes directly proportional to Ptar and if the atomic density of the different compounds deposited are similar, which holds within ~ 20% for CuO, Cu4O3 and Cu2O, the conversion constant C1 in Eq. (1) will not change with Ptar, leading to a linear relationship between the growth rate R and Ptar.
4. Conclusions In summary, the target power plays a decisive role in keeping a stable stoichiometry of Cu2O films prepared by reactive RF magnetron sputtering. By increasing Ptar, the growth rate of the CuxO films increases where the O-rich phases (CuO and Cu4O3) gradually disappear while the p-type Cu2O single phase is promoted. Concurrently, the Cu2O grain size becomes larger for both the (111) and (200) orientations and the hole mobility increases, indicating that grain boundary scattering is a limiting mechanism.
Fig. 6. The growth rate of CuxO thin films (with error bars) as a function of the rf target power (140–190 W with a step of 10 W) for Q(O2) = 3.0 sccm, Q(Ar) = 20.0 sccm, and Tsub = 400 °C. The growth rate shows an approximate linear increase with the target power (the black dashed line).
5
The films grown with Ptar = 190 W have superior properties under the current process parameters (Q(O2)/Q(Ar) = 3.0 sccm/20.0 sccm, Tsub = 400 °C), but a further increase in Ptar may lead to even better film quality, i.e., prevalence of the Cu2O phase with larger grains, higher hole mobility, sharper band edges and improved structural quality. A hole mobility of 9–10 cm2·V− 1·s− 1 is demonstrated at room temperature for Cu2O films with an average grain size of ~30 nm and ~ 20 nm in the (111) and (200) directions, respectively. Further, the optical band gap increases with Ptar and at 190 W, where the Cu2O phase dominates, a value of ~ 2.45–2.50 eV is obtained at 295 K. The transmittance of these 190 W films with a thickness of ~ 830 nm approaches 85% for λ N 567 nm. Acknowledgements Fruitful discussions with Dr. Ramon Schifano during the initial stage of this work are gratefully acknowledged. 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Please cite this article as: J. Gan, et al., Influence of target power on properties of CuxO thin films prepared by reactive radio frequency magnetron sputtering, Thin Solid Films (2015), http://dx.doi.org/10.1016/j.tsf.2015.05.029