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Methane as a novel doping precursor for deposition of highly conductive ZnO thin films by magnetron sputtering
Vacuum 174 (2020) 109199
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Methane as a novel doping precursor for deposition of highly conductive ZnO thin films by magnetron sputtering A.V. Vasin a, b, *, A.V. Rusavsky a, b, E.G. Bortchagovsky a, Y.V. Gomeniuk a, A.S. Nikolenko a, V. V. Strelchuk a, R. Yatskiv c, S. Tiagulskyi c, S. Prucnal d, W. Skorupa d, A.N. Nazarov a, b a
Lashkaryov Institute of Semiconductor Physics NAS of Ukraine, Kyiv, Ukraine National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”, Kyiv, Ukraine Institute of Photonics and Electronics of the Czech Academy of Sciences Chabersk� a 1014/57, 182 51, Prague, Czech Republic d Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, Dresden, Germany b c
A R T I C L E I N F O
A B S T R A C T
Keywords: ZnO thin films RF magnetron sputtering Doping effects of methane Powder target
ZnO thin films were deposited by RF-magnetron sputtering of ZnO powder target using pure argon and argon with methane as reactive gas. It is found that growth morphology and electronic properties of the films are strongly affected by adding of methane to argon during the deposition process. Adding of methane resulted in a high energy shift of near band edge ultraviolet photoluminescence band and quenching of deep level emission in the visible spectral range. The strongest effect of methane has been found for electrical resistivity that reduced by 3 orders of magnitude in comparison with films deposited in pure argon. Unexpectedly, the analysis of the chemical composition showed no carbon incorporated from methane. Therefore, modification effects were assigned to hydrogen incorporation. However, the direct comparison of resistivity of the films deposited using methane and molecular hydrogen as doping precursors has demonstrated that doping efficiency of the methane is about an order of magnitude larger than that of molecular hydrogen under similar deposition conditions. This advantage of the methane is discussed and assigned to specific surface chemistry of Zn–O–C–H system that enhances the formation of shallow donor defects during plasma assisted deposition process.
1. Introduction Transparent conducting oxides (TCOs) are essential components in modern optoelectronics as basic material for transparent conductive windows [1]. Sn-doped In2O3 (ITO) is the most widely used TCO in current industries and scientific experiments, as it has superior combi nation of electrical conductivity and optical transparency. Significant disadvantage of ITO is a high and permanently growing cost due to very limited natural resources of indium. One of low-cost alternatives to ITO is ZnO. Zinc oxide is a direct and wide band gap (3.37 eV) semiconductor that can be easily doped into n-type. While the growth of large scale ZnO crystals is still an unsolved technological problem, the polycrystalline thin films can be produced at low temperature by various deposition techniques such as laser deposition [2], plasma enhanced chemical vapor deposition [3], cathodic vacuum arc technique [4], atomic layer deposition [5] and magnetron sputtering [6,7]. Among deposition pro cesses, RF magnetron sputtering technique is one of the most effective and developed for large scale production. Pure ZnO commonly exhibits
n-type conductivity due to intrinsic structural defects like vacancies, interstitials and antisites [8–10]. Further increase in n-type conductivity can be effectively achieved by extrinsic dopants like aluminum, gallium, hydrogen, or their combinations [8,9]. Optimization of functional properties and further development of cost efficiency of conductive ZnO films is of great interest for many application fields. In the present manuscript it is reported for the first time the remarkable n-type doping efficiency of methane as reactive gas during RF-magnetron sputtering deposition. 2. Experimental procedure Zinc oxide thin films were deposited on silicon and oxidized silicon wafers by RF (13.56 MHz) magnetron sputtering. The compacted ZnO powder (99.9% purity, grain size about 5 μm) was used as a sputtering target. It has been demonstrated recently that ZnO powder target can be successfully used as cost effective alternative to sintered ceramic target [11, 12]. A powder target provides several advantages in regard to large scale
* Corresponding author. Lashkaryov Institute of Semiconductor Physics NAS of Ukraine, prospect Nauki 41, Kyiv, 03028, Ukraine. E-mail address: [email protected] (A.V. Vasin). https://doi.org/10.1016/j.vacuum.2020.109199 Received 19 November 2019; Received in revised form 13 January 2020; Accepted 14 January 2020 Available online 17 January 2020 0042-207X/© 2020 Elsevier Ltd. All rights reserved.
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production: low cost, no limits of the size/shape, and almost 100% ef ficiency of target material utilization. Our sputtering target has been fabricated by “compacting” of ZnO powder into 6–7 mm thick layer loaded in 160 mm diameter stainless plate fixed at the cathode. Before deposition of experimental samples the target was degassed in Ar plasma for several hours. Taking into account that in case of powder target the effect of residual contaminations of water vapor is a critical issue, we have per formed several experiments with sequential deposition processes to check this effect. The idea of the experiment was as follows. The substrate holder of the deposition system is equipped with revolving system that allows changing of one substrate holder with another. Two identical substrates were put on the two holders and after completing one deposition process the first substrate holder was replaced by the 2-nd one without stopping of discharge. Then the deposition process was repeated at the same conditions for the same time. With such processing, the only difference in the deposition conditions of two thin film series is expected to be the time of plasma treatment of ZnO target and chamber surfaces: the longer plasma treatment the smaller residual water vapor contaminations. It has been found that structural, optical and electrical properties of the both ZnO series are identical regardless the type of deposition process. Therefore we conclude that residual contaminations, even if present, do not have a critical effect on the phenomena discussed in the manuscript. The sputtering chamber was evacuated to a base pressure of 3 � 10 4 Pa followed by introducing working gas that was pure argon gas (9 sccm) or argon (9 sccm)/methane (2 sccm) mixture keeping working pressure at 10 1 Pa. Standard silicon wafers and wafers with thermally grown silicon oxide layer (200 nm) were used as substrates for structural and electrical characterizations. During the sputtering process, the sub strate holder was heated by halogen lamps up to 200 � C (�10 � C). Target-tosubstrates distance was 70 mm, and RF discharge power was 190 W. The thickness of the films examined in the present research was about 250 nm. Surface and cross section morphology was examined by scanning electron microscope Tescan Lyra 3 GM using the in-beam SE detector. Composition of the films was analyzed by Auger electron spectroscopy using Microlab 310F (Fisons) with field-emission cathode and hemi spherical sector analyser. Micro-Raman and photoluminescence mea surements were carried out at room temperature in backscattering configuration using T-64000 (Horiba Jobin Yvon), equipped with an electrically cooled CCD detector. For electrical measurements the ZnO films were deposited on oxidized Si wafer, and the Ni strip contacts (1062 μm � 287 μm) have been deposited by DC magnetron sputtering. The distance between strips varied from 90 to 920 μm. Resistivity of the films was estimated from measurements of the resistance between contacts separated by different distances using the transfer length method (TLM) [13]. The measurements were performed in the dark using semiconductor parameter analyzer Agilent 4156C. Carrier concentration and mobility were measured using ezHEMS Hall effect measurement system (Nano Magnetics Instruments, UK).
Fig. 1. Profile of zinc, oxygen (a) and carbon (b) distribution in the ZnO films measured by Auger electron spectroscopy.
films deposited using Ar and Ar/CH4. The surface of the sample depos ited in argon depicted an irregular surface morphology structure with shapeless grains of about 50 nm and less (Fig. 2a and b). The adding of methane to argon results in the significant increase of the size of grains up to 200 nm (Fig. 2c and d). The cross sections of the doped and undoped ZnO films shows that the thicknesses of the films are very similar indicating that the difference in the morphology is not due to the “thickness effect” [14]. Typical Raman scattering spectra of undoped and doped ZnO films on Si wafers are presented in Fig. 3. The main effect of the adding of the methane is appearance of the peak at 275 cm 1 that cannot be assigned to any of ZnO intrinsic vibration modes. However, it is known that doped zinc oxide exhibits anomalous Raman mode near 275 cm 1 that has been assigned to the local vibration of Zn interstitial clusters composed of few atoms [15]. Room temperature PL spectra of the film deposited in Ar gas exhibited typical multiband emission with the dominant ultraviolet band at 3.2 eV and the broad visible band centered at about 2.0 eV originated from band-edge emission (BEE) and deep level emission (DLE) respectively (Fig. 4). The optical band gap and corresponding BEE are managed basically by variation of relative contribution of free exciton emission and phonon replicas, which depend on the defect structure and growth conditions. Identification of contributions in DLE is more sofisticated problem. In spite of several decades of researches the structural identification of DLE spectral features is still debated. Yellow-
3. Results and discussion Examination of chemical composition of the films by Auger electron spectroscopy (AES) revealed that Zn/O ratio is almost the same in both undoped and doped samples with a small excess of zinc over oxygen (52 at.%/48 at.%) regardless of working gas (Fig. 1a). Some carbon was found on the surface and under the surface thin layer of the films, but surprisingly carbon appeared below AES detection limits inside the film (Fig. 1b). Surface carbon originates most likely from ambient contami nation because carbon poisoning of the surface layer in the case of ZnO film deposited in pure Ar is even stronger in comparison with the sample deposited using methane (Fig. 1b). Rutherford backscattering mea surements (results not presented here) confirmed the absence of carbon in the samples deposited using methane. This unexpected result will be discussed in the next paragraphs. Fig. 2 illustrates the surface and cross section morphology of ZnO 2
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Fig. 2. SEM images of the surface and corresponding cross section of ZnO films deposited using pure Ar (a,b) and in (Ar þ CH4) (b,c) working gas.
Fig. 3. Raman scattering spectra of ZnO films deposited using Ar (spectrum 2) and Ar þ CH4 working gas (spectrum 3). Spectrum 1 is Raman scattering of clean Si wafer-substrate for reference (excitation by 488 nm).
Fig. 4. Room temperature PL spectra of ZnO films deposited using Ar and Ar þ CH4 working gas (excitation by 325 nm).
orange emission was commonly assigned to deep band gap levels asso ciated with intrinsic structural defects like oxygen vacancies and
interstitials [16–19] as well as with extrinsic contaminations [20]. More recent research argued that DLE in hydrothermally grown ZnO crystals is contributed by Li contaminations, VZn–H complexes, and oxygen vacancies 3
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[21]. Violet/blue emission is often assigned to shallow band gap levels associated with zinc interstitials [22,23]. Discussion of the nature of contributions in DLE is still ongoing in research community, however in the present report we confine general identification of the nature of this band as radiative recombination through deep band gap levels. Adding of methane to Ar resulted in (1) quenching of DLE band, (2) the increase of relative contribution of violet/blue emission, and (3) obvious narrowing and high energy shift of the BEE band. From this observation it can be concluded that adding of methane increases the concentration of optically active shallow levels and strongly reduces the concentration of deep levels in the band gap. Another strong effect of methane adding was observed in electrical properties of the films manifested as the increase of conductivity by more than 3 orders of magnitude (Fig. 5a). Moreover, the mechanism of charge transport, which was examined by the temperature dependence of the conductivity, appeared to be quite different in undoped and methane doped films. The ZnO film deposited using pure Ar gas shows the increase of the conductivity with the increase of the temperature. Such dependence is best fitted by T1/4 law (Fig. 5b) that corresponds to the mechanism of Mott variable-range hopping of the electrons near Fermi level. From such interpretation it can be concluded that planar conductivity in the undoped films is determined mainly by amorphous structural component presumably in inter-grain space. In contrast, the highly conductive film deposited using Ar þ CH4 gas mixture exhibited the small decrease of conductivity with the increase of the temperature which can be assigned to phonon scattering of free electrons i.e. mechanism of “metallic” conductivity (Fig. 5c). Such interpretation suggests that methane doping results in Mott’s semiconductor-metal transition due to high concentration of free electron in the doped film [24]. However, the doping effect appeared to be unstable at the temper ature higher than deposition temperature. Fig. 6 demonstrates evolution of the sheet resistance of the doped film as a function of the temperature of annealing (20 min in air). It can be seen that the sheet resistance of the film is stable with increasing of annealing temperature up to 200 � C (the temperature of deposition process) followed by the strong increase. At the first sight the doping mechanism of the methane is ambiguous. Preliminarily, it was expected that carbon incorporation may cause doping effects. However, as it was noted above, the AES analysis of the doped ZnO films found no detectable carbon. It is suggested that such specific effect of the “rejection” of carbon by growing ZnO film can be explained by specific chemical affinity of hydrocarbons and zinc oxide. It is known that ZnO surface is photocatalytically active to organic molecules under UV radiation [25]. In frames of such consideration, methane molecules and methyl radicals being absorbed on highly active ZnO surface can be decomposed into volatile carbon oxide and water molecules under intense UV radiation of gas discharge plasma ZnOðsÞ þ CHn hv ZnOðsÞ þ CO þ H2 O !
(1)
Carbon oxidation process can also be caused by highly reactive atomic oxygen sputtered from ZnO target O þ CHn → CO þ 2H2
(2)
Fig. 5. (a) I-T curves for undoped and doped ZnO films; (b) I-T curve repre sented in Mott coordinates for undoped film; (c). I-T curve of methane doped film.
Both of these mechanisms restrict carbon incorporation. The absence of carbon in the film suggests that carbon is not involved directly in the doping mechanism. Another candidate for extrinsic doping agent is atomic hydrogen, which is expected as byproduct of plasma decompo sition/oxidation of methane molecules. It is well known that hydrogen is an effective n-type dopant in ZnO [26–30]. Hydrogen hypothesis is well consistent with observed effects. The low temperature instability of the doping is inherent to hydrogen doping. Furthermore, quenching of DLE in PL spectra and enhancement of grain size of the ZnO films (Figs. 1 and 4) are also well consistent with hydrogen hypothesis because similar effects were observed in hydrogenated ZnO films [31–34]. Hydrogen commonly acts as an amphoteric impurity in
semiconductors i.e. exhibits charge state opposite to conductivity type of the host semiconductor, so that incorporation of hydrogen reduces conductivity. In ZnO, however, interstitial hydrogen occurs exclusively in the positive charge state, i.e. it always acts as a donor [9,28]. The hydrogen interstitial has low formation energies in ZnO and, therefore, it can be easily incorporated during magnetron sputtering processes. Thus, it can be concluded that hydrogen incorporation is the main source of the efficient doping. The specific resistance of the undoped and doped ZnO films deposited on oxidized silicon wafer was measured to be about to 1.5 Ohm � cm and 10 3 Ohm � cm respectively. The obtained 4
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Table 1 Bulk concentration and Hall mobility of charge carriers in ZnO:H films deposited using methane and hydrogen as reactive gas. Reactive gas concentration, vol% CH4 5 20 H2 5 20
Carrier concentration, cm 3
Hall mobility, cm2/ Vs
1.2 � 1020 0.9 � 1020
6.8 0.5
2.4 � 1019 1.03 � 1019
0.3 1.0
4. Summary ZnO thin films were deposited by RF magnetron sputtering of ZnO powder target using Ar and Ar þ CH4 as working gases. The effect of methane as reactive gas has been examined. It has been demonstrated that optical and electronic properties of ZnO films are effectively modified by methane. Conductivity of ZnO film was increased by 3 or ders of magnitude by methane adding. No carbon was found in doped films, that has been assigned to photocatalytic oxidation of hydroge nated carbons on the zinc oxide surface under ultraviolet radiation of plasma. It is suggested and argued that dominant doping effect is due to incorporation of hydrogen. However, it has been demonstrated directly that doping efficiency of the methane is one order of magnitude larger than that of pure molecular hydrogen under similar deposition condi tions. As a preliminary hypothesis it is suggested that methane causes not only hydrogen incorporation but also enhances formation of shallow donors associated with intrinsic defects. More detailed identification of the methane doping effect in ZnO needs further investigations that are in progress nowadays.
Fig. 6. Evolution of sheet resistance of methane doped ZnO film with annealing temperature.
value of specific resistance of the methane doped films is close to the best results for highly conductive ZnO materials doped by hydrogen [5,26, 27], or Al and hydrogen [35], or Ga and hydrogen [36]. Finally, a direct comparison of the doping effect of methane and molecular hydrogen as doping agents at similar deposition conditions has been performed. Bulk concentration of charge carriers and Hall mobility of the carriers in the ZnO:H films deposited with different reactive gas dilution are presented in Table 1. The highest carrier concentration and Hall mobility was observed in the sample deposited using lowest concentration of methane. Increase of the methane concentration resulted in decrease both carrier concentration and mobility. In case of molecular hydrogen, carrier concentration is about one order of magnitude lower and decreased with increase of reactive gas concentration while mobility increased. This exper iment demonstrates that doping efficiency of the methane is significantly higher than that of molecular hydrogen under identical deposition con ditions. At present time it is hard to suggest a reliable and detailed model for developed doping ability of methane. One of realistic scenario is that the doping effect of methane is contributed by two factors: (1) extrinsic doping by incorporations of hydrogen into the structure of ZnO, and (2) formation of additional electrically active intrinsic defects due to spe cific surface chemistry of Zn–O–C–H system during plasma enhanced deposition process. It should be noted that Raman scattering of the methane doped sample (Fig. 3) points out the presence of Zn interstitial (Zni) complexes in the doped ZnO film. It is known that Zni is an effective shallow donor in ZnO [8,37]. It is commonly believed that contribution of interstitials in n-type conductivity is minor because of high formation energy in n-type ZnO under equilibrium conditions [37]. However it is reasonable to suggest that formation of interstitials can be enhanced under non-equilibrium condensation due to plasma effect, for example involving reduction of zinc by carbon ZnO þ C → Zn þ CO
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The work was supported by Ministry of Education and Science of Ukraine (Project 2211-F) and by the Czech Science Foundation (Project 19-02804S). The authors would like to thank Dr. Stefan Facsko for his assistance in measurements of the chemical composition of ZnO films at the Ion Beam Center of the Helmholtz-Zentrum Dresden-Rossendorf, a member of the Helmholtz Association. References [1] S.C. Dixon, D.O. Scanlon, C.J. Carmalta, I.P. Parkin, n-Type doped transparent conducting binary oxides: an overview, J. Mater. Chem. C 4 (2016) 6946–6961, https://doi.org/10.1039/C6TC01881E. [2] S.L. Gupta, R.K. Thareja, ZnO thin film deposition using colliding plasma plumes and single plasma plume: structural and optical properties, J. Appl. Phys. 114 (2013), 224903, https://doi.org/10.1063/1.4846115. [3] P.K. Shishodia, H.J. Kim, A. Wakahara, A. Yoshida, G. Shishodia, R.M. Mehra, Plasma enhanced chemical vapor deposition of ZnO thin films, J. Non-Cryst. Solids 352 (2006) 2343–2346, https://doi.org/10.1016/j.jnoncrysol.2006.01.086. [4] M. Kumar, S.-Y. Choi, Fabrication of As-doped p-type ZnO thin films using As2O3 as doping source material by E-beam evaporation, Appl. Surf. Sci. 255 (2008) 2173–2175, https://doi.org/10.1016/j.apsusc.2008.07.054. [5] T. Tynell, M. Karppinen, Atomic layer deposition of ZnO: a review, Semicond. Sci. Technol. 29 (2014), 043001, https://doi.org/10.1088/0268-1242/29/4/043001. [6] D. Gaspar, L. Pereira, K. Gehrke, B. Galler, E. Fortunato, R. Martins, High mobility hydrogenated zinc oxide thin films, Sol. Energy Mater. Sol. Cells 163 (2017) 255–262, https://doi.org/10.1016/j.solmat.2017.01.030. [7] K. Ellmer, T. Welze, Reactive magnetron sputtering of transparent conductive oxide thin films: role of energetic particle (ion) bombardment 27 (2012) 765–779, https://doi.org/10.1557/jmr.2011.428.
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Methane as a novel doping precursor for deposition of highly conductive ZnO thin films by magnetron sputtering A.V. Vasin a, b, *, A.V. Rusavsky a, b, E.G. Bortchagovsky a, Y.V. Gomeniuk a, A.S. Nikolenko a, V. V. Strelchuk a, R. Yatskiv c, S. Tiagulskyi c, S. Prucnal d, W. Skorupa d, A.N. Nazarov a, b a
Lashkaryov Institute of Semiconductor Physics NAS of Ukraine, Kyiv, Ukraine National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”, Kyiv, Ukraine Institute of Photonics and Electronics of the Czech Academy of Sciences Chabersk� a 1014/57, 182 51, Prague, Czech Republic d Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, Dresden, Germany b c
A R T I C L E I N F O
A B S T R A C T
Keywords: ZnO thin films RF magnetron sputtering Doping effects of methane Powder target
ZnO thin films were deposited by RF-magnetron sputtering of ZnO powder target using pure argon and argon with methane as reactive gas. It is found that growth morphology and electronic properties of the films are strongly affected by adding of methane to argon during the deposition process. Adding of methane resulted in a high energy shift of near band edge ultraviolet photoluminescence band and quenching of deep level emission in the visible spectral range. The strongest effect of methane has been found for electrical resistivity that reduced by 3 orders of magnitude in comparison with films deposited in pure argon. Unexpectedly, the analysis of the chemical composition showed no carbon incorporated from methane. Therefore, modification effects were assigned to hydrogen incorporation. However, the direct comparison of resistivity of the films deposited using methane and molecular hydrogen as doping precursors has demonstrated that doping efficiency of the methane is about an order of magnitude larger than that of molecular hydrogen under similar deposition conditions. This advantage of the methane is discussed and assigned to specific surface chemistry of Zn–O–C–H system that enhances the formation of shallow donor defects during plasma assisted deposition process.
1. Introduction Transparent conducting oxides (TCOs) are essential components in modern optoelectronics as basic material for transparent conductive windows [1]. Sn-doped In2O3 (ITO) is the most widely used TCO in current industries and scientific experiments, as it has superior combi nation of electrical conductivity and optical transparency. Significant disadvantage of ITO is a high and permanently growing cost due to very limited natural resources of indium. One of low-cost alternatives to ITO is ZnO. Zinc oxide is a direct and wide band gap (3.37 eV) semiconductor that can be easily doped into n-type. While the growth of large scale ZnO crystals is still an unsolved technological problem, the polycrystalline thin films can be produced at low temperature by various deposition techniques such as laser deposition [2], plasma enhanced chemical vapor deposition [3], cathodic vacuum arc technique [4], atomic layer deposition [5] and magnetron sputtering [6,7]. Among deposition pro cesses, RF magnetron sputtering technique is one of the most effective and developed for large scale production. Pure ZnO commonly exhibits
n-type conductivity due to intrinsic structural defects like vacancies, interstitials and antisites [8–10]. Further increase in n-type conductivity can be effectively achieved by extrinsic dopants like aluminum, gallium, hydrogen, or their combinations [8,9]. Optimization of functional properties and further development of cost efficiency of conductive ZnO films is of great interest for many application fields. In the present manuscript it is reported for the first time the remarkable n-type doping efficiency of methane as reactive gas during RF-magnetron sputtering deposition. 2. Experimental procedure Zinc oxide thin films were deposited on silicon and oxidized silicon wafers by RF (13.56 MHz) magnetron sputtering. The compacted ZnO powder (99.9% purity, grain size about 5 μm) was used as a sputtering target. It has been demonstrated recently that ZnO powder target can be successfully used as cost effective alternative to sintered ceramic target [11, 12]. A powder target provides several advantages in regard to large scale
* Corresponding author. Lashkaryov Institute of Semiconductor Physics NAS of Ukraine, prospect Nauki 41, Kyiv, 03028, Ukraine. E-mail address: [email protected] (A.V. Vasin). https://doi.org/10.1016/j.vacuum.2020.109199 Received 19 November 2019; Received in revised form 13 January 2020; Accepted 14 January 2020 Available online 17 January 2020 0042-207X/© 2020 Elsevier Ltd. All rights reserved.
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production: low cost, no limits of the size/shape, and almost 100% ef ficiency of target material utilization. Our sputtering target has been fabricated by “compacting” of ZnO powder into 6–7 mm thick layer loaded in 160 mm diameter stainless plate fixed at the cathode. Before deposition of experimental samples the target was degassed in Ar plasma for several hours. Taking into account that in case of powder target the effect of residual contaminations of water vapor is a critical issue, we have per formed several experiments with sequential deposition processes to check this effect. The idea of the experiment was as follows. The substrate holder of the deposition system is equipped with revolving system that allows changing of one substrate holder with another. Two identical substrates were put on the two holders and after completing one deposition process the first substrate holder was replaced by the 2-nd one without stopping of discharge. Then the deposition process was repeated at the same conditions for the same time. With such processing, the only difference in the deposition conditions of two thin film series is expected to be the time of plasma treatment of ZnO target and chamber surfaces: the longer plasma treatment the smaller residual water vapor contaminations. It has been found that structural, optical and electrical properties of the both ZnO series are identical regardless the type of deposition process. Therefore we conclude that residual contaminations, even if present, do not have a critical effect on the phenomena discussed in the manuscript. The sputtering chamber was evacuated to a base pressure of 3 � 10 4 Pa followed by introducing working gas that was pure argon gas (9 sccm) or argon (9 sccm)/methane (2 sccm) mixture keeping working pressure at 10 1 Pa. Standard silicon wafers and wafers with thermally grown silicon oxide layer (200 nm) were used as substrates for structural and electrical characterizations. During the sputtering process, the sub strate holder was heated by halogen lamps up to 200 � C (�10 � C). Target-tosubstrates distance was 70 mm, and RF discharge power was 190 W. The thickness of the films examined in the present research was about 250 nm. Surface and cross section morphology was examined by scanning electron microscope Tescan Lyra 3 GM using the in-beam SE detector. Composition of the films was analyzed by Auger electron spectroscopy using Microlab 310F (Fisons) with field-emission cathode and hemi spherical sector analyser. Micro-Raman and photoluminescence mea surements were carried out at room temperature in backscattering configuration using T-64000 (Horiba Jobin Yvon), equipped with an electrically cooled CCD detector. For electrical measurements the ZnO films were deposited on oxidized Si wafer, and the Ni strip contacts (1062 μm � 287 μm) have been deposited by DC magnetron sputtering. The distance between strips varied from 90 to 920 μm. Resistivity of the films was estimated from measurements of the resistance between contacts separated by different distances using the transfer length method (TLM) [13]. The measurements were performed in the dark using semiconductor parameter analyzer Agilent 4156C. Carrier concentration and mobility were measured using ezHEMS Hall effect measurement system (Nano Magnetics Instruments, UK).
Fig. 1. Profile of zinc, oxygen (a) and carbon (b) distribution in the ZnO films measured by Auger electron spectroscopy.
films deposited using Ar and Ar/CH4. The surface of the sample depos ited in argon depicted an irregular surface morphology structure with shapeless grains of about 50 nm and less (Fig. 2a and b). The adding of methane to argon results in the significant increase of the size of grains up to 200 nm (Fig. 2c and d). The cross sections of the doped and undoped ZnO films shows that the thicknesses of the films are very similar indicating that the difference in the morphology is not due to the “thickness effect” [14]. Typical Raman scattering spectra of undoped and doped ZnO films on Si wafers are presented in Fig. 3. The main effect of the adding of the methane is appearance of the peak at 275 cm 1 that cannot be assigned to any of ZnO intrinsic vibration modes. However, it is known that doped zinc oxide exhibits anomalous Raman mode near 275 cm 1 that has been assigned to the local vibration of Zn interstitial clusters composed of few atoms [15]. Room temperature PL spectra of the film deposited in Ar gas exhibited typical multiband emission with the dominant ultraviolet band at 3.2 eV and the broad visible band centered at about 2.0 eV originated from band-edge emission (BEE) and deep level emission (DLE) respectively (Fig. 4). The optical band gap and corresponding BEE are managed basically by variation of relative contribution of free exciton emission and phonon replicas, which depend on the defect structure and growth conditions. Identification of contributions in DLE is more sofisticated problem. In spite of several decades of researches the structural identification of DLE spectral features is still debated. Yellow-
3. Results and discussion Examination of chemical composition of the films by Auger electron spectroscopy (AES) revealed that Zn/O ratio is almost the same in both undoped and doped samples with a small excess of zinc over oxygen (52 at.%/48 at.%) regardless of working gas (Fig. 1a). Some carbon was found on the surface and under the surface thin layer of the films, but surprisingly carbon appeared below AES detection limits inside the film (Fig. 1b). Surface carbon originates most likely from ambient contami nation because carbon poisoning of the surface layer in the case of ZnO film deposited in pure Ar is even stronger in comparison with the sample deposited using methane (Fig. 1b). Rutherford backscattering mea surements (results not presented here) confirmed the absence of carbon in the samples deposited using methane. This unexpected result will be discussed in the next paragraphs. Fig. 2 illustrates the surface and cross section morphology of ZnO 2
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Fig. 2. SEM images of the surface and corresponding cross section of ZnO films deposited using pure Ar (a,b) and in (Ar þ CH4) (b,c) working gas.
Fig. 3. Raman scattering spectra of ZnO films deposited using Ar (spectrum 2) and Ar þ CH4 working gas (spectrum 3). Spectrum 1 is Raman scattering of clean Si wafer-substrate for reference (excitation by 488 nm).
Fig. 4. Room temperature PL spectra of ZnO films deposited using Ar and Ar þ CH4 working gas (excitation by 325 nm).
orange emission was commonly assigned to deep band gap levels asso ciated with intrinsic structural defects like oxygen vacancies and
interstitials [16–19] as well as with extrinsic contaminations [20]. More recent research argued that DLE in hydrothermally grown ZnO crystals is contributed by Li contaminations, VZn–H complexes, and oxygen vacancies 3
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[21]. Violet/blue emission is often assigned to shallow band gap levels associated with zinc interstitials [22,23]. Discussion of the nature of contributions in DLE is still ongoing in research community, however in the present report we confine general identification of the nature of this band as radiative recombination through deep band gap levels. Adding of methane to Ar resulted in (1) quenching of DLE band, (2) the increase of relative contribution of violet/blue emission, and (3) obvious narrowing and high energy shift of the BEE band. From this observation it can be concluded that adding of methane increases the concentration of optically active shallow levels and strongly reduces the concentration of deep levels in the band gap. Another strong effect of methane adding was observed in electrical properties of the films manifested as the increase of conductivity by more than 3 orders of magnitude (Fig. 5a). Moreover, the mechanism of charge transport, which was examined by the temperature dependence of the conductivity, appeared to be quite different in undoped and methane doped films. The ZnO film deposited using pure Ar gas shows the increase of the conductivity with the increase of the temperature. Such dependence is best fitted by T1/4 law (Fig. 5b) that corresponds to the mechanism of Mott variable-range hopping of the electrons near Fermi level. From such interpretation it can be concluded that planar conductivity in the undoped films is determined mainly by amorphous structural component presumably in inter-grain space. In contrast, the highly conductive film deposited using Ar þ CH4 gas mixture exhibited the small decrease of conductivity with the increase of the temperature which can be assigned to phonon scattering of free electrons i.e. mechanism of “metallic” conductivity (Fig. 5c). Such interpretation suggests that methane doping results in Mott’s semiconductor-metal transition due to high concentration of free electron in the doped film [24]. However, the doping effect appeared to be unstable at the temper ature higher than deposition temperature. Fig. 6 demonstrates evolution of the sheet resistance of the doped film as a function of the temperature of annealing (20 min in air). It can be seen that the sheet resistance of the film is stable with increasing of annealing temperature up to 200 � C (the temperature of deposition process) followed by the strong increase. At the first sight the doping mechanism of the methane is ambiguous. Preliminarily, it was expected that carbon incorporation may cause doping effects. However, as it was noted above, the AES analysis of the doped ZnO films found no detectable carbon. It is suggested that such specific effect of the “rejection” of carbon by growing ZnO film can be explained by specific chemical affinity of hydrocarbons and zinc oxide. It is known that ZnO surface is photocatalytically active to organic molecules under UV radiation [25]. In frames of such consideration, methane molecules and methyl radicals being absorbed on highly active ZnO surface can be decomposed into volatile carbon oxide and water molecules under intense UV radiation of gas discharge plasma ZnOðsÞ þ CHn hv ZnOðsÞ þ CO þ H2 O !
(1)
Carbon oxidation process can also be caused by highly reactive atomic oxygen sputtered from ZnO target O þ CHn → CO þ 2H2
(2)
Fig. 5. (a) I-T curves for undoped and doped ZnO films; (b) I-T curve repre sented in Mott coordinates for undoped film; (c). I-T curve of methane doped film.
Both of these mechanisms restrict carbon incorporation. The absence of carbon in the film suggests that carbon is not involved directly in the doping mechanism. Another candidate for extrinsic doping agent is atomic hydrogen, which is expected as byproduct of plasma decompo sition/oxidation of methane molecules. It is well known that hydrogen is an effective n-type dopant in ZnO [26–30]. Hydrogen hypothesis is well consistent with observed effects. The low temperature instability of the doping is inherent to hydrogen doping. Furthermore, quenching of DLE in PL spectra and enhancement of grain size of the ZnO films (Figs. 1 and 4) are also well consistent with hydrogen hypothesis because similar effects were observed in hydrogenated ZnO films [31–34]. Hydrogen commonly acts as an amphoteric impurity in
semiconductors i.e. exhibits charge state opposite to conductivity type of the host semiconductor, so that incorporation of hydrogen reduces conductivity. In ZnO, however, interstitial hydrogen occurs exclusively in the positive charge state, i.e. it always acts as a donor [9,28]. The hydrogen interstitial has low formation energies in ZnO and, therefore, it can be easily incorporated during magnetron sputtering processes. Thus, it can be concluded that hydrogen incorporation is the main source of the efficient doping. The specific resistance of the undoped and doped ZnO films deposited on oxidized silicon wafer was measured to be about to 1.5 Ohm � cm and 10 3 Ohm � cm respectively. The obtained 4
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Table 1 Bulk concentration and Hall mobility of charge carriers in ZnO:H films deposited using methane and hydrogen as reactive gas. Reactive gas concentration, vol% CH4 5 20 H2 5 20
Carrier concentration, cm 3
Hall mobility, cm2/ Vs
1.2 � 1020 0.9 � 1020
6.8 0.5
2.4 � 1019 1.03 � 1019
0.3 1.0
4. Summary ZnO thin films were deposited by RF magnetron sputtering of ZnO powder target using Ar and Ar þ CH4 as working gases. The effect of methane as reactive gas has been examined. It has been demonstrated that optical and electronic properties of ZnO films are effectively modified by methane. Conductivity of ZnO film was increased by 3 or ders of magnitude by methane adding. No carbon was found in doped films, that has been assigned to photocatalytic oxidation of hydroge nated carbons on the zinc oxide surface under ultraviolet radiation of plasma. It is suggested and argued that dominant doping effect is due to incorporation of hydrogen. However, it has been demonstrated directly that doping efficiency of the methane is one order of magnitude larger than that of pure molecular hydrogen under similar deposition condi tions. As a preliminary hypothesis it is suggested that methane causes not only hydrogen incorporation but also enhances formation of shallow donors associated with intrinsic defects. More detailed identification of the methane doping effect in ZnO needs further investigations that are in progress nowadays.
Fig. 6. Evolution of sheet resistance of methane doped ZnO film with annealing temperature.
value of specific resistance of the methane doped films is close to the best results for highly conductive ZnO materials doped by hydrogen [5,26, 27], or Al and hydrogen [35], or Ga and hydrogen [36]. Finally, a direct comparison of the doping effect of methane and molecular hydrogen as doping agents at similar deposition conditions has been performed. Bulk concentration of charge carriers and Hall mobility of the carriers in the ZnO:H films deposited with different reactive gas dilution are presented in Table 1. The highest carrier concentration and Hall mobility was observed in the sample deposited using lowest concentration of methane. Increase of the methane concentration resulted in decrease both carrier concentration and mobility. In case of molecular hydrogen, carrier concentration is about one order of magnitude lower and decreased with increase of reactive gas concentration while mobility increased. This exper iment demonstrates that doping efficiency of the methane is significantly higher than that of molecular hydrogen under identical deposition con ditions. At present time it is hard to suggest a reliable and detailed model for developed doping ability of methane. One of realistic scenario is that the doping effect of methane is contributed by two factors: (1) extrinsic doping by incorporations of hydrogen into the structure of ZnO, and (2) formation of additional electrically active intrinsic defects due to spe cific surface chemistry of Zn–O–C–H system during plasma enhanced deposition process. It should be noted that Raman scattering of the methane doped sample (Fig. 3) points out the presence of Zn interstitial (Zni) complexes in the doped ZnO film. It is known that Zni is an effective shallow donor in ZnO [8,37]. It is commonly believed that contribution of interstitials in n-type conductivity is minor because of high formation energy in n-type ZnO under equilibrium conditions [37]. However it is reasonable to suggest that formation of interstitials can be enhanced under non-equilibrium condensation due to plasma effect, for example involving reduction of zinc by carbon ZnO þ C → Zn þ CO
Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments The work was supported by Ministry of Education and Science of Ukraine (Project 2211-F) and by the Czech Science Foundation (Project 19-02804S). The authors would like to thank Dr. Stefan Facsko for his assistance in measurements of the chemical composition of ZnO films at the Ion Beam Center of the Helmholtz-Zentrum Dresden-Rossendorf, a member of the Helmholtz Association. References [1] S.C. Dixon, D.O. Scanlon, C.J. Carmalta, I.P. Parkin, n-Type doped transparent conducting binary oxides: an overview, J. Mater. Chem. C 4 (2016) 6946–6961, https://doi.org/10.1039/C6TC01881E. [2] S.L. Gupta, R.K. Thareja, ZnO thin film deposition using colliding plasma plumes and single plasma plume: structural and optical properties, J. Appl. Phys. 114 (2013), 224903, https://doi.org/10.1063/1.4846115. [3] P.K. Shishodia, H.J. Kim, A. Wakahara, A. Yoshida, G. Shishodia, R.M. Mehra, Plasma enhanced chemical vapor deposition of ZnO thin films, J. Non-Cryst. Solids 352 (2006) 2343–2346, https://doi.org/10.1016/j.jnoncrysol.2006.01.086. [4] M. Kumar, S.-Y. Choi, Fabrication of As-doped p-type ZnO thin films using As2O3 as doping source material by E-beam evaporation, Appl. Surf. Sci. 255 (2008) 2173–2175, https://doi.org/10.1016/j.apsusc.2008.07.054. [5] T. Tynell, M. Karppinen, Atomic layer deposition of ZnO: a review, Semicond. Sci. Technol. 29 (2014), 043001, https://doi.org/10.1088/0268-1242/29/4/043001. [6] D. Gaspar, L. Pereira, K. Gehrke, B. Galler, E. Fortunato, R. Martins, High mobility hydrogenated zinc oxide thin films, Sol. Energy Mater. Sol. Cells 163 (2017) 255–262, https://doi.org/10.1016/j.solmat.2017.01.030. [7] K. Ellmer, T. Welze, Reactive magnetron sputtering of transparent conductive oxide thin films: role of energetic particle (ion) bombardment 27 (2012) 765–779, https://doi.org/10.1557/jmr.2011.428.
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