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Electrical and optical properties of gallium-doped zinc oxide thin films prepared by Ion-Beam-Assisted Deposition
Vacuum 118 (2015) 43e47
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Electrical and optical properties of gallium-doped zinc oxide thin films prepared by Ion-Beam-Assisted Deposition J.H. Hsieh a, b, *, C.K. Chang a, b, H.H. Hsieh c, Y.J. Cho a, b, J. Lin a a
Dept. of Materials Engineering, Ming Chi University of Technology, Taishan, New Taipei City, Taiwan Center for Thin Film Technologies and Applications, Taishan, New Taipei City, Taiwan c Green Energy and Environment Research Lab., ITRI, Hinchu 31040, Taiwan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 June 2014 Received in revised form 9 February 2015 Accepted 27 February 2015 Available online 7 March 2015
Gallium doped zinc oxide films were prepared by Ion-Beam-Assisted Deposition. The effects of argon and oxygen bombardment flux on electrical and optical properties were studied according to the existence of excited species. The prepared films were polycrystalline in nature with c-axis perpendicular to the substrate. The resistivity of the film decreased to 1.35 103 U-cm, and the optical transparency increased up to 80%, as the discharge current in ion gun was increased. The variation of discharge current mostly affected the grain size and crystallinity, which in turn affected the carrier mobility. Too much argon bombardment may disrupt grain growth, which resulted in higher resistivity. An optical emission spectrometer (OES) was used to examine the optical emission spectra of the ion beam, mainly for the excited oxygen and argon species. It was found the O*/Ar* ratio peaked with the increase of discharge current set in ion gun, then, decreased. This trend was similar to that of the electrical properties. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Coatings IBAD O/Ar ion ratio Optical properties Mobility Resistivity
1. Introduction Many optoelectronic devices require highly transparent and conductive oxide (TCO) thin films. These devices include light emitting diodes, flat panel displays, and many others. Tin-doped indium oxide (ITO) is by far the most popular TCO material in many applications. ITO has many desirable properties such as low electrical resistivity, high optical transmittance in the visible wavelength range, ease of wide-area deposition and subsequent etching, etc. [1]. However, due to indium shortage, poor mechanical flexibility [2], and instability in some environment [3], it has become urgent to develop a suitable substitute for ITO. Impurity doped zinc oxide (ZnO) is regarded as an ideal material to serve for this purpose, because it is nontoxic, cheap and stable in various environment [4e6]. Up to date, the most common dopants are shown to be In, B, Ga and Al. Among these dopants, Ga is less reactive and more resistive to oxidation. Its atomic size is similar to the size of Zn, resulting in large solubility. This also makes the deformation of the ZnO lattice small even with high doping
concentration of Ga. Therefore, gallium doped zinc oxide (GZO) has gained great attention in the past years. It is one of the most suitable TCO materials to replace ITO in optoelectronic devices [7e9]. Many previous studies have reported GZO thin films prepared by various thin film techniques. These techniques include metal organic chemical vapor deposition, evaporation, magnetron sputtering, solegel, plasma-assisted molecular beam epitaxy and more [7]. However, reports on the deposition of GZO thin films by IonBeam-Assisted Deposition (IBAD) is rare. In this study, an IBAD technique was used to deposit GZO film. The ion beam was generated by an end-hall ion gun applying argon and oxygen gases with fixed ratio. During the deposition, the discharge current (proportional to ion flux) was varied, which caused the change of plasma species that include excited Ar and O particles. Ar bombardment on the deposited films may result in denser but disrupted structure, while oxygen bombardment may induce chemical oxidation and perhaps grain growth due to the energy released during oxidation reaction.
2. Experimental methods * Corresponding author. 84 Gung Juan Road, Taishan, New Taipei City, 24301, Taiwan . E-mail address: [email protected] (J.H. Hsieh). http://dx.doi.org/10.1016/j.vacuum.2015.02.034 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
In this study, gallium doped zinc oxide films were deposited on glass substrates (Corning 1737) by Ion-Beam-Assisted Deposition
44
J.H. Hsieh et al. / Vacuum 118 (2015) 43e47 Table 1 Process parameters for IBAD process. Deposition rates (nm/sec) Workng pressure (torr) Background pressure (torr) Substrate temp. (C) Ar flow rate (sccm) Oxygen flow rate (sccm) Ion gun (End Hall) Anode potential (V) Anode current (A) Deposition time (sec)
0.1 1.4 104 6 106 R.T. 3 1 180 0, 0.2, 0.35, 0.5 1800
system equipped with an end-Hall ion gun, at room temperature (RT). The glass substrates were cleaned in 5% KOH solution, deionized water, acetone, ethanol, deionized water in orderly sequence, then, blown dry. The ion gun mainly consisted of an annular anode and a cylindrical hollow cathode enclosed by magnetic poles and an inner shield. The magnetic field in the discharge channel was produced by SmCo permanent magnetic poles. The open exit with a diameter of 30 mm was employed to extract the ion beam. A tungsten wire, serving as the neutralizer, was attached 1 cm outside the gun exit. The beam current was about one-third of the discharge current (anode current). During this study, the anode voltage was maintained at 180 V, while the discharge current was varied from 0 to 0.5 A. By doing this, the effects of argon and oxygen
Fig. 1. X-ray diffraction pattern for various GZO samples deposited with various discharge current.
Fig. 2. Cross-sectional SEM micrographs of the films deposited with various discharge current: (a) without ion beam, (b) 0.2 A, (c) 0.35 A, (d) 0.5 A.
J.H. Hsieh et al. / Vacuum 118 (2015) 43e47
45
microscope and a UVeVis spectrophotometer (JASCO, V-570). Hall measurements were performed using Van der Pauw technique at room temperature. 3. Results and discussion 3.1. Structural analysis
Fig. 3. Optical transmission for the prepared GZO films, as a function of discharge (anode) current.
ion bombardment rate on electrical and optical properties could be investigated. The flow ratio of Ar to O2 was fixed at 3 to 1. The evaporation source was the granule of ZnO with 2 wt.% G2O3. The electron-beam evaporation rate was set at 0.1 nm/s. The process parameters are presented in Table 1. For the analysis of excited oxygen and argon atoms in the ion beam, an optical emission spectrometer (OES, PLASUS EmiCon) was used. The spectrum resolution of this OES was 0.1 nm. This OES was used mainly to evaluate the intensity of Ar* and O* emissions. Several wavelengths corresponding to atomic transitions in plasma containing argon and oxygen were normally used to analyze the plasma emission spectra. The most significant oxygen lines under our experimental conditions were the 777.4 nm and 845 nm lines. These lines corresponded to the de-excitation of oxygen atom in the state 5P (O*), whose creation was predicted by the following ways e þ O2 / e þ O*þ O and e þ O / e þ O*, i.e. dissociative excitation or direct impact excitation of oxygen atom, respectively [10]. The most outstanding argon peak in Ar/O2 mixture was 750.4 nm line. The structural, electrical and optical properties of the thin films were examined using XRD (Philips PW 1830), a four-point probe system (Jeihan HT-100), a field-emission scanning electron
The X-ray diffraction patterns of the prepared films are shown in Fig. 1. It can be observed that a preferred orientation in c-axis is developed when the ion flux is increased. According to Ma et al., (002) has the fastest growth rate during deposition [11]. It is not surprised that the developed film has such preferred orientation. More importantly the film deposited with discharge current set at 0.35 A shows the sharpest GZO(002) peak, which implies that this film has the largest grain size or better crystallinity. The films deposited without (discharge current ¼ 0) or with little ion bombardment (discharge current ¼ 0.2 A) show a mixture of GZO and Zn phases. This implies that the oxidation process as well as the ion-induced annealing are closely related to the amount of the active species (i.e., Ar*, O*). Further explanation will be addressed in Section 3.3. Fig. 2 shows the cross-sectional SEM micrographs of these films. It can be seen that the film structure become denser with the increase of discharge current. As the discharge current increases, the columnar structure appears. This normally means the formation of preferred orientation. 3.2. Optical and electrical properties The resistivity of the film decreased to 1.35 103 U-cm, and the optical transparency increased up to 85%, as the ion bombardment flux was increased. The carrier concentration and mobility were measured by a Hall-effect measurement system. Fig. 3 shows the optical transmission of these GZO films deposited with various ion bombardment fluxes. As observed, the optical transmission increases with the increase of ion bombardment. The bombarding ions may bring energy into the growing films, which would cause, so called, thin film annealing [12]. The annealed films would have less defects and higher optical transmission. When the ion-gun discharge current was equal or greater than 0.35 A, the films had
Fig. 4. The variation of resistivity, as a function of discharge (anode) current.
46
J.H. Hsieh et al. / Vacuum 118 (2015) 43e47
optical band gaps ranging within 3.40e3.50 eV Fig. 4 shows the variation of resistivity, as a function of discharge current. To further understand the conducting mechanisms, a Hall measurement system was used to measure carrier concentration and mobility. Fig. 5a shows the carrier concentration vs. discharge current, while Fig. 5b shows the carrier mobility vs. discharge current. Comparing Figs. 4 and 5, it is obvious that the increase of carrier mobility of these films is the dominating factor for the improvement of the films' conductivity. The film deposited with discharge current set at 0.35 A has the highest carrier mobility. This result is consistent with that obtained from the XRD studies, as shown in Fig. 1. According to Fig. 1, the film deposited with discharge current at 0.35 A has the largest grain size and better crystallinity. It is reported that a film with larger grain size would have higher carrier mobility due to less barriers, such as grain boundaries, dislocations, or voids [11]. 3.3. Ion beam characteristics The oxygen and argon species in the ion beam was characterized using an OES. This system can detect the optical emission lines caused by the decay of the excited argon and oxygen atoms. The most significant oxygen lines are the 777.8 nm and 844.8 nm lines. Besides direct excitation by electron collision, the fraction of O atoms with respect to O2 molecules may be enhanced in oxygen/ argon plasma due to the quenching reaction between Ar* metastables and O2 molecules, i.e. Ar* þ O2 ¼ Ar þ O þ O. Regarding the formation of Ar* metastables (IAr(750.4)), it is generally thought that direct electron-impact excitation from the ground state is the dominating reaction. Therefore, the kinetic energy of electron (i.e. electron temperature) as well as the electron density is the most critical in this matter. To estimate the atomic oxygen concentration in an oxygen/argon mixed plasma, it is useful to plot the intensity ratio of emission-lines IO(777.8) nm to IAr(750.4) nm, which is affected by the change of discharge conditions. Fig. 6 shows one of the OES patterns. Fig. 7 shows the variation of the relative emission intensities of IO(844.8)/IAr(750.4), as a function of discharge current. The intensity ratio reaches its maximum when the discharge is controlled at 0.35 A. Apparently, under this condition, the deposited film may be oxidized and crystallized faster than those deposited with other conditions. This result can be attributable to high oxidation potential of oxygen radicals, either in ground or excitation state. In more details, it is well known that more energy is needed to either excite (>11.7 eV) or ionize (>15.76) Ar atoms by the electron-impact mechanism, than to dissociate O2 (>4.5 eV) and form oxygen radicals [13,14]. Under this situation, the generation rate of O* may be higher than Ar*, which may be the case when the discharge current was at 0.35 A. As a result, ZnO phase can be more easily formed. Furthermore, when ZnO phase is formed, certain energy may be released due to the following reaction [15]: Zn þ O (radicals / ZnO (DH ¼ 83.2 kcal/mol)
Fig. 5. (a) Carrier concentrations, and (b) carrier mobility of the prepared GZO films, as a function of discharge (anode) current.
sudden drop of O*/Ar* ratio is not caused by the decrease of O*. Rather, it is due to the sudden increase of Ar* intensity. It is not clear why a sudden increase of Ar* metastables occurred during the experiment. One possible reason could be due to that large amount of electrons are created at 0.5 A, which results in dramatic increase in electron-Ar atom collisions. 4. Conclusions According to the study, it is found an optimal ion bombardment flux should be used to develop GZO film with high transmission, low electrical resistivity, and with C-axis orientation. It is found the carrier concentration can reach 5 1020 cm3, while the mobility
(1)
The total energy released will be proportionally increased with the increase of oxygen radicals, or, in other words, oxidation reactions. This energy may eventually cause certain annealing phenomenon on ZnO phase. This explains why, at 3.5 A, the grain size is larger than those of other films deposited at different discharge current. These results are consistent with those obtained from XRD and Hall-effect studies. However, when the discharge current reached 0.5 A, the O*/Ar* decreased. This implies that the deposited film, in this case, could be bombarded by more Ar ions, which can actually disrupt the grain growth and create smaller grains, although the film may have less line and point defects due to local annealing on film surface. According to Fig. 7, it is observed that the
Fig. 6. The OES spectrum pattern taken from the ion beam under 0.35 A discharge current.
J.H. Hsieh et al. / Vacuum 118 (2015) 43e47
47
Fig. 7. The variation of the relative emission intensities of IO(777.8)/IAr(750.4), as a function of discharge current. The emission intensities of IO(777.8), IO(844.8), and IAr(750.4), as a function of discharge current are also plotted.
can reach 10 cm2/VeS. It is confirmed that the increase of carrier mobility of these films is the dominating factor for the improved conductivity of these GZO films prepared by IBAD. This improved property is closely related to IO* to IAr* ratio in the ion beam. Higher IO* to IAr* ratio can provide film with larger grain size and better crystallinity. References [1] Canhola P, Martins N, Raniero P, Pereira S, Fortunato SE, Ferreira I, et al. Thin Solid Films 2005;487:271e6. [2] Chen Z, Cotterell B, Wang W, Guenther E, Chua SA. Thin Solid Films 2001;394: 201e5. [3] Major S, Kumar S, Bhatnagar M, Chopra KL. Appl Phys Lett 1986;49:394e6. [4] Crossay A, Buecheler S, Kranz SL, Perrenoud J, Fella CM, Romanyuk YE, et al. Sol Energy Mater Sol C 2012;101:283e8. [5] Inamdar D, Agashe C, Kadam P, Mahamuni S. Thin Solid Films 2012;520: 3871e7.
[6] Chen X, Lin Q, Ni J, Zhang D, Sun J, Zhao Y, et al. Thin Solid Films 2011;520: 1263e7. [7] Fortunato E, Raniero L, Silva L, Goncalves A, Pimentel A, Barquinha P, et al. Sol Energy Mater Sol C 2008;92:1605e10. [8] Liu YY, Yang SY, Wei GX, Song HS, Cheng CF, Xue CS, et al. Surf Coat Tech 2011;205:3530. [9] Zhang ZY, Bao CG, Li Q, Ma SQ, Hou SZ. J Mater Sci e Mater Electron 2012;23: 376. ka A. Vacuum 1997;48:689e92. [10] Hrachov a V, Kao [11] Ma QB, Ye ZZ, He HP, Hu SH, Wang JR, Zhu LP, et al. J Cryst Growth 2007;304: 64e8. [12] Hsieh JH, Li C, Wu W, Hochman RF. Thin Solid Films 2003;424:103e6. [13] Fuller NCM, Malyshev MV, Donnelly VM, Herman IP. Plasma Sources Sci Technol 2000;9:116e27. [14] Godyak VA, Piejak RB, Alexandrovich BM. Plasma Sources Sci Technol 2002;11:525e43. [15] Kubaschewski O, Evans EL, Alcock CB. Metallurgical thermochemistry. 4th ed. Oxford, London, UK: Pergamon Press; 1967.
Contents lists available at ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Electrical and optical properties of gallium-doped zinc oxide thin films prepared by Ion-Beam-Assisted Deposition J.H. Hsieh a, b, *, C.K. Chang a, b, H.H. Hsieh c, Y.J. Cho a, b, J. Lin a a
Dept. of Materials Engineering, Ming Chi University of Technology, Taishan, New Taipei City, Taiwan Center for Thin Film Technologies and Applications, Taishan, New Taipei City, Taiwan c Green Energy and Environment Research Lab., ITRI, Hinchu 31040, Taiwan b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 30 June 2014 Received in revised form 9 February 2015 Accepted 27 February 2015 Available online 7 March 2015
Gallium doped zinc oxide films were prepared by Ion-Beam-Assisted Deposition. The effects of argon and oxygen bombardment flux on electrical and optical properties were studied according to the existence of excited species. The prepared films were polycrystalline in nature with c-axis perpendicular to the substrate. The resistivity of the film decreased to 1.35 103 U-cm, and the optical transparency increased up to 80%, as the discharge current in ion gun was increased. The variation of discharge current mostly affected the grain size and crystallinity, which in turn affected the carrier mobility. Too much argon bombardment may disrupt grain growth, which resulted in higher resistivity. An optical emission spectrometer (OES) was used to examine the optical emission spectra of the ion beam, mainly for the excited oxygen and argon species. It was found the O*/Ar* ratio peaked with the increase of discharge current set in ion gun, then, decreased. This trend was similar to that of the electrical properties. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Coatings IBAD O/Ar ion ratio Optical properties Mobility Resistivity
1. Introduction Many optoelectronic devices require highly transparent and conductive oxide (TCO) thin films. These devices include light emitting diodes, flat panel displays, and many others. Tin-doped indium oxide (ITO) is by far the most popular TCO material in many applications. ITO has many desirable properties such as low electrical resistivity, high optical transmittance in the visible wavelength range, ease of wide-area deposition and subsequent etching, etc. [1]. However, due to indium shortage, poor mechanical flexibility [2], and instability in some environment [3], it has become urgent to develop a suitable substitute for ITO. Impurity doped zinc oxide (ZnO) is regarded as an ideal material to serve for this purpose, because it is nontoxic, cheap and stable in various environment [4e6]. Up to date, the most common dopants are shown to be In, B, Ga and Al. Among these dopants, Ga is less reactive and more resistive to oxidation. Its atomic size is similar to the size of Zn, resulting in large solubility. This also makes the deformation of the ZnO lattice small even with high doping
concentration of Ga. Therefore, gallium doped zinc oxide (GZO) has gained great attention in the past years. It is one of the most suitable TCO materials to replace ITO in optoelectronic devices [7e9]. Many previous studies have reported GZO thin films prepared by various thin film techniques. These techniques include metal organic chemical vapor deposition, evaporation, magnetron sputtering, solegel, plasma-assisted molecular beam epitaxy and more [7]. However, reports on the deposition of GZO thin films by IonBeam-Assisted Deposition (IBAD) is rare. In this study, an IBAD technique was used to deposit GZO film. The ion beam was generated by an end-hall ion gun applying argon and oxygen gases with fixed ratio. During the deposition, the discharge current (proportional to ion flux) was varied, which caused the change of plasma species that include excited Ar and O particles. Ar bombardment on the deposited films may result in denser but disrupted structure, while oxygen bombardment may induce chemical oxidation and perhaps grain growth due to the energy released during oxidation reaction.
2. Experimental methods * Corresponding author. 84 Gung Juan Road, Taishan, New Taipei City, 24301, Taiwan . E-mail address: [email protected] (J.H. Hsieh). http://dx.doi.org/10.1016/j.vacuum.2015.02.034 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
In this study, gallium doped zinc oxide films were deposited on glass substrates (Corning 1737) by Ion-Beam-Assisted Deposition
44
J.H. Hsieh et al. / Vacuum 118 (2015) 43e47 Table 1 Process parameters for IBAD process. Deposition rates (nm/sec) Workng pressure (torr) Background pressure (torr) Substrate temp. (C) Ar flow rate (sccm) Oxygen flow rate (sccm) Ion gun (End Hall) Anode potential (V) Anode current (A) Deposition time (sec)
0.1 1.4 104 6 106 R.T. 3 1 180 0, 0.2, 0.35, 0.5 1800
system equipped with an end-Hall ion gun, at room temperature (RT). The glass substrates were cleaned in 5% KOH solution, deionized water, acetone, ethanol, deionized water in orderly sequence, then, blown dry. The ion gun mainly consisted of an annular anode and a cylindrical hollow cathode enclosed by magnetic poles and an inner shield. The magnetic field in the discharge channel was produced by SmCo permanent magnetic poles. The open exit with a diameter of 30 mm was employed to extract the ion beam. A tungsten wire, serving as the neutralizer, was attached 1 cm outside the gun exit. The beam current was about one-third of the discharge current (anode current). During this study, the anode voltage was maintained at 180 V, while the discharge current was varied from 0 to 0.5 A. By doing this, the effects of argon and oxygen
Fig. 1. X-ray diffraction pattern for various GZO samples deposited with various discharge current.
Fig. 2. Cross-sectional SEM micrographs of the films deposited with various discharge current: (a) without ion beam, (b) 0.2 A, (c) 0.35 A, (d) 0.5 A.
J.H. Hsieh et al. / Vacuum 118 (2015) 43e47
45
microscope and a UVeVis spectrophotometer (JASCO, V-570). Hall measurements were performed using Van der Pauw technique at room temperature. 3. Results and discussion 3.1. Structural analysis
Fig. 3. Optical transmission for the prepared GZO films, as a function of discharge (anode) current.
ion bombardment rate on electrical and optical properties could be investigated. The flow ratio of Ar to O2 was fixed at 3 to 1. The evaporation source was the granule of ZnO with 2 wt.% G2O3. The electron-beam evaporation rate was set at 0.1 nm/s. The process parameters are presented in Table 1. For the analysis of excited oxygen and argon atoms in the ion beam, an optical emission spectrometer (OES, PLASUS EmiCon) was used. The spectrum resolution of this OES was 0.1 nm. This OES was used mainly to evaluate the intensity of Ar* and O* emissions. Several wavelengths corresponding to atomic transitions in plasma containing argon and oxygen were normally used to analyze the plasma emission spectra. The most significant oxygen lines under our experimental conditions were the 777.4 nm and 845 nm lines. These lines corresponded to the de-excitation of oxygen atom in the state 5P (O*), whose creation was predicted by the following ways e þ O2 / e þ O*þ O and e þ O / e þ O*, i.e. dissociative excitation or direct impact excitation of oxygen atom, respectively [10]. The most outstanding argon peak in Ar/O2 mixture was 750.4 nm line. The structural, electrical and optical properties of the thin films were examined using XRD (Philips PW 1830), a four-point probe system (Jeihan HT-100), a field-emission scanning electron
The X-ray diffraction patterns of the prepared films are shown in Fig. 1. It can be observed that a preferred orientation in c-axis is developed when the ion flux is increased. According to Ma et al., (002) has the fastest growth rate during deposition [11]. It is not surprised that the developed film has such preferred orientation. More importantly the film deposited with discharge current set at 0.35 A shows the sharpest GZO(002) peak, which implies that this film has the largest grain size or better crystallinity. The films deposited without (discharge current ¼ 0) or with little ion bombardment (discharge current ¼ 0.2 A) show a mixture of GZO and Zn phases. This implies that the oxidation process as well as the ion-induced annealing are closely related to the amount of the active species (i.e., Ar*, O*). Further explanation will be addressed in Section 3.3. Fig. 2 shows the cross-sectional SEM micrographs of these films. It can be seen that the film structure become denser with the increase of discharge current. As the discharge current increases, the columnar structure appears. This normally means the formation of preferred orientation. 3.2. Optical and electrical properties The resistivity of the film decreased to 1.35 103 U-cm, and the optical transparency increased up to 85%, as the ion bombardment flux was increased. The carrier concentration and mobility were measured by a Hall-effect measurement system. Fig. 3 shows the optical transmission of these GZO films deposited with various ion bombardment fluxes. As observed, the optical transmission increases with the increase of ion bombardment. The bombarding ions may bring energy into the growing films, which would cause, so called, thin film annealing [12]. The annealed films would have less defects and higher optical transmission. When the ion-gun discharge current was equal or greater than 0.35 A, the films had
Fig. 4. The variation of resistivity, as a function of discharge (anode) current.
46
J.H. Hsieh et al. / Vacuum 118 (2015) 43e47
optical band gaps ranging within 3.40e3.50 eV Fig. 4 shows the variation of resistivity, as a function of discharge current. To further understand the conducting mechanisms, a Hall measurement system was used to measure carrier concentration and mobility. Fig. 5a shows the carrier concentration vs. discharge current, while Fig. 5b shows the carrier mobility vs. discharge current. Comparing Figs. 4 and 5, it is obvious that the increase of carrier mobility of these films is the dominating factor for the improvement of the films' conductivity. The film deposited with discharge current set at 0.35 A has the highest carrier mobility. This result is consistent with that obtained from the XRD studies, as shown in Fig. 1. According to Fig. 1, the film deposited with discharge current at 0.35 A has the largest grain size and better crystallinity. It is reported that a film with larger grain size would have higher carrier mobility due to less barriers, such as grain boundaries, dislocations, or voids [11]. 3.3. Ion beam characteristics The oxygen and argon species in the ion beam was characterized using an OES. This system can detect the optical emission lines caused by the decay of the excited argon and oxygen atoms. The most significant oxygen lines are the 777.8 nm and 844.8 nm lines. Besides direct excitation by electron collision, the fraction of O atoms with respect to O2 molecules may be enhanced in oxygen/ argon plasma due to the quenching reaction between Ar* metastables and O2 molecules, i.e. Ar* þ O2 ¼ Ar þ O þ O. Regarding the formation of Ar* metastables (IAr(750.4)), it is generally thought that direct electron-impact excitation from the ground state is the dominating reaction. Therefore, the kinetic energy of electron (i.e. electron temperature) as well as the electron density is the most critical in this matter. To estimate the atomic oxygen concentration in an oxygen/argon mixed plasma, it is useful to plot the intensity ratio of emission-lines IO(777.8) nm to IAr(750.4) nm, which is affected by the change of discharge conditions. Fig. 6 shows one of the OES patterns. Fig. 7 shows the variation of the relative emission intensities of IO(844.8)/IAr(750.4), as a function of discharge current. The intensity ratio reaches its maximum when the discharge is controlled at 0.35 A. Apparently, under this condition, the deposited film may be oxidized and crystallized faster than those deposited with other conditions. This result can be attributable to high oxidation potential of oxygen radicals, either in ground or excitation state. In more details, it is well known that more energy is needed to either excite (>11.7 eV) or ionize (>15.76) Ar atoms by the electron-impact mechanism, than to dissociate O2 (>4.5 eV) and form oxygen radicals [13,14]. Under this situation, the generation rate of O* may be higher than Ar*, which may be the case when the discharge current was at 0.35 A. As a result, ZnO phase can be more easily formed. Furthermore, when ZnO phase is formed, certain energy may be released due to the following reaction [15]: Zn þ O (radicals / ZnO (DH ¼ 83.2 kcal/mol)
Fig. 5. (a) Carrier concentrations, and (b) carrier mobility of the prepared GZO films, as a function of discharge (anode) current.
sudden drop of O*/Ar* ratio is not caused by the decrease of O*. Rather, it is due to the sudden increase of Ar* intensity. It is not clear why a sudden increase of Ar* metastables occurred during the experiment. One possible reason could be due to that large amount of electrons are created at 0.5 A, which results in dramatic increase in electron-Ar atom collisions. 4. Conclusions According to the study, it is found an optimal ion bombardment flux should be used to develop GZO film with high transmission, low electrical resistivity, and with C-axis orientation. It is found the carrier concentration can reach 5 1020 cm3, while the mobility
(1)
The total energy released will be proportionally increased with the increase of oxygen radicals, or, in other words, oxidation reactions. This energy may eventually cause certain annealing phenomenon on ZnO phase. This explains why, at 3.5 A, the grain size is larger than those of other films deposited at different discharge current. These results are consistent with those obtained from XRD and Hall-effect studies. However, when the discharge current reached 0.5 A, the O*/Ar* decreased. This implies that the deposited film, in this case, could be bombarded by more Ar ions, which can actually disrupt the grain growth and create smaller grains, although the film may have less line and point defects due to local annealing on film surface. According to Fig. 7, it is observed that the
Fig. 6. The OES spectrum pattern taken from the ion beam under 0.35 A discharge current.
J.H. Hsieh et al. / Vacuum 118 (2015) 43e47
47
Fig. 7. The variation of the relative emission intensities of IO(777.8)/IAr(750.4), as a function of discharge current. The emission intensities of IO(777.8), IO(844.8), and IAr(750.4), as a function of discharge current are also plotted.
can reach 10 cm2/VeS. It is confirmed that the increase of carrier mobility of these films is the dominating factor for the improved conductivity of these GZO films prepared by IBAD. This improved property is closely related to IO* to IAr* ratio in the ion beam. Higher IO* to IAr* ratio can provide film with larger grain size and better crystallinity. References [1] Canhola P, Martins N, Raniero P, Pereira S, Fortunato SE, Ferreira I, et al. Thin Solid Films 2005;487:271e6. [2] Chen Z, Cotterell B, Wang W, Guenther E, Chua SA. Thin Solid Films 2001;394: 201e5. [3] Major S, Kumar S, Bhatnagar M, Chopra KL. Appl Phys Lett 1986;49:394e6. [4] Crossay A, Buecheler S, Kranz SL, Perrenoud J, Fella CM, Romanyuk YE, et al. Sol Energy Mater Sol C 2012;101:283e8. [5] Inamdar D, Agashe C, Kadam P, Mahamuni S. Thin Solid Films 2012;520: 3871e7.
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