Effect of substrate temperature on transparent conducting Al and F co-doped ZnO thin films prepared by rf magnetron sputtering
Graphical abstract
Introduction
Transparent conducting oxide (TCO) thin films are promising for applications in various optoelectronic devices, such as flat panel display, photovoltaic devices, light-emitting diodes, and waveguides [1], [2], [3], [4], [5]. Sn-doped In2O3 (ITO) films with relatively low resistivity in the order of 10−4–10−5 Ω-cm are the most commonly used TCO films in optoelectronic devices. However, an increasing demand for scarce indium mineral causes an expensive material cost in the past decade. Zinc oxide (ZnO) has been recognized as promising substitute material of ITO in terms of its direct and wide band gap (3.37 eV), high abundance, low-price, non-toxicity and durability against hydrogen plasma in solar cell fabrication process [3], [6]. The n-type conductivity of unintentionally doped ZnO films, attributing to intrinsic defects (such as oxygen vacancies and zinc interstitials) and/or others to noncontrollable hydrogen impurities introduced during growth [7], [8], [9], is still not enough high. Thus, various impurities have been used to dope into ZnO matrix to enhance the conductivity. ZnO thin films doped with group III elements such as B, Al, Ga, and In have been widely studied as TCO thin films due to superior optoelectronic properties and low material cost as well as non-toxicity [4], [5], [6], [10], [11], [12], [13], [14]. Wang et al. have reported a resistivity of 2.39 × 10−3 Ω-cm in the 450 °C-annealed ZnO:B (BZO) films [5]. Kumar et al. have reported a resistivity as low as 4.89 × 10−4 Ω-cm along the carrier concentration 5.6 × 1020 cm−3 in their 200 nm-thick ZnO:Al (AZO) films [11]. Zhu et al. have reported that the resistivity of the ZnO:Ga (GZO) thin films ranges from 4.0 × 10−1 to 8.08 × 10−4 Ω-cm dependent on annealing temperature [12]. Previous literatures [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14] indicated that the resistivity of the sputtered cation-doped ZnO thin films was in the order of 10−1 to 10−4 Ω-cm, which were higher than that of ITO ones [15]. Although it has been reported that resistivities as low as 8.5 × 10−5 Ω-cm and 8.1 × 10−5 Ω-cm were achieved for AZO [16] and GZO [17] thin films prepared by pulsed laser deposition. However, large-area deposition with high deposition rates is still challenging for pulsed laser deposition process.
Except these cation dopants, anion dopants such as F and Cl have been doped into ZnO films because they can substitute for oxygen in ZnO lattice to provide an extra conducting electron [18], [19], [20], [21], [22], [23], [24], [25], [26]. The ionic radius of F− (131 pm) is close to that of O2− (138 pm), and thus F is a suitable anion dopant to substitute O without introducing a large lattice distortion in ZnO crystal. A resistivity of 1.5 × 10−3 Ω-cm has been observed for the vacuum-annealed ZnO:F (FZO) thin films containing 2 at% F [18]. Our previous report showed a resistivity of 9.29 × 10−4 Ω-cm and an average visible transmittance of above 90% for sputtered FZO thin films with 1.5 wt% ZnF2 in the sputtering target [19]. A theoretical understanding indicates that the substitution of oxygen by fluorine perturbs the valence band only, thereby leaving the conduction band relatively free from scattering, which could reduce light absorption and enhance carrier mobility [27]. Liang et al. [24] and Xu et al. [20] have reported the FZO thin films with considerably high Hall mobility of above 45 cm2/V-s. However, the resistivity of the sputtered single dopant doped ZnO thin films is still inferior to that of the ITO thin film [18], [19], [26].
In the past decade, researches have used co-doping strategy to further improve the specific properties of ZnO thin films [28], [29], [30], [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42]. For example, group III element (Al, Ga, or In) and Mg co-doped ZnO thin films show a widened band gap and a good conductivity [28], [29], [30]. Al–Ni and Al–Co co-doped ZnO thin films have ferromagnetism [31], [32]. Al and other group III element (B, Ga, or In) co-doped ZnO thin films exhibit lower resistivities than that of the AZO film [33], [34], [35]. Cation–anion co-doping of ZnO is less focused but worth exploring owing to the capability of tailoring electrical and optical properties of ZnO. N and a group III element (B, Al, or Ga) co-doped ZnO thin films have a p-type conductivity [36], [37], [38]. F and a group III or IV element (Al, Ga, or Sn) co-doped ZnO thin films have been studied [39], [40], [41]. Kim et al. [40] adopted two ceramic targets, i.e. ZnO:Al2O3 (3 wt%) and ZnO:ZnF2 (0–10 wt%), to co-sputter Al and F doped ZnO (AFZO) thin films with varying fluorine content. Their results indicated that AZO thin films co-doped with a small amount of fluorine achieved a relatively low electrical resistivity of 5.9 × 10−4 Ω-cm. Shi et al. [41] developed high-quality Ga and F co-doped ZnO thin films by using mid-frequency sputtering and a low resistivity of 6.4 × 10−4 Ω-cm and a high visible transmittance of above 90% were obtained. Still there are not so many researches on this topic to clarify the detailed mechanism, especially co-dopants acting on the physical characteristics of ZnO thin films.
In this paper, a single ceramic ZnO target containing a small amount of Al2O3 and ZnF2 was used to fabricate AFZO thin films by rf magnetron sputtering in pure Ar atmosphere at substrate temperatures (Tsub) of room temperature (RT), 100 °C, and 200 °C. The structural, morphological, electrical, chemical, and optical properties of AFZO thin films are investigated with various Tsub.
Section snippets
Experimental details
The composite ceramic target consists of 97.5 wt% ZnO (99.999%), 1.0 wt% aluminum oxide (Al2O3) (99.999%), and 1.5 wt% zinc fluoride (ZnF2) (99.995%). First, these three types of the powders were mixed, dried and ground. Next, the mixed powders were pressed into a pellet using a steel die. After debindering, the pellet was calcined at 600 °C for 1 h and sintered at 1060 °C for 3 h to form a ceramic target with 2.5 in. in diameter and 5 mm in thickness.
Glass substrates (Corning Eagle XG) were cleaned
Structural and morphological properties of AFZO thin films
Fig. 1 shows the θ–2θ scan XRD patterns of AFZO thin films with different Tsub. It was observed that a strong (0 0 2) diffraction peak appeared at the 2θ of around 34.5° due to its lowest surface free energy in this direction. It indicates that all the AFZO films have a wurtzite structure with a preferentially c-axis orientation. No Al2O3 or ZnF2 phase was found in the XRD spectra. In addition, the peak intensity of the AFZO thin film increased with the increasing Tsub, indicating that the AFZO
Conclusions
Al and F co-doped ZnO thin films were deposited on glass substrates at RT, 100 °C, and 200 °C using a 1.5 wt% ZnF2 and 1 wt% Al2O3 co-doped ZnO target by rf magnetron sputtering. All the AFZO thin films exhibited a typical wurtzite structure with a strong (0 0 2) preferred orientation regardless of substrate temperature. As the substrate temperature increased from RT to 200 °C, the grain size increased from 24.6 to 34.7 nm, the surface RMS roughness decreased from 4.25 to 2.90 nm, and the residual film
Acknowledgement
The authors would like to thank Ministry of Science and Technology, Taiwan under the Grant MOST 103-2221-E-005-040-MY2 for financial support.
References (73)
- et al.
Surface textured MF-sputtered ZnO films for microcrystalline silicon-based thin-film solar cells
Sol. Energy Mater. Sol. Cells
(2006) - et al.
Transparent Al-doped ZnO anodes in organic light-emitting diodes investigated using a hole-only device
Appl. Surf. Sci.
(2012) - et al.
Electrical and structural properties of annealed ZnO:B thin films
J. Alloy. Compd.
(2015) - et al.
Effects of H2 plasma treatment on properties of ZnO:Al thin films prepared by RF magnetron sputtering
Surf. Coat. Technol.
(2011) - et al.
Optimization of process parameters for the electrical properties in Ga-doped ZnO thin films prepared by r.f. magnetron sputtering
Appl. Surf. Sci.
(2014) - et al.
Wide-spectrum Mg and Ga co-doped ZnO–TCO thin films with introduced hydrogen grown by magnetron sputtering at room temperature
Appl. Surf. Sci.
(2014) - et al.
Low resistivity transparent conducting Al-doped ZnO films prepared by pulsed laser deposition
Thin Solid Films
(2003) - et al.
Effects of substrate temperature on the properties of Ga-doped ZnO by pulsed laser deposition
Thin Solid Films
(2006) - et al.
Properties of fluorine doped ZnO thin films deposited by magnetron sputtering
Sol. Energy Mater. Sol. Cells
(2008) - et al.
Influence of RF power on physical properties of ZnO:ZnF2 thin films by RF magnetron sputtering
Superlattices Microstruct.
(2015)
Anion-controlled passivation effect of the atomic layer deposited ZnO films by F substitution to O-related defects on the electronic band structure for transparent contact layer of solar cell applications
Sol. Energy Mater. Sol. Cells
Properties of fluorine and tin co-doped ZnO thin films deposited by sol–gel method
J. Alloy. Compd.
Effect of chlorine doping on electrical and optical properties of ZnO thin films
Thin Solid Films
Fluorine doped ZnO thin films by RF magnetron sputtering
Thin Solid Films
Wide-spectrum Mg and Ga co-doped ZnO TCO thin films for solar cells grown via magnetron sputtering with H2 introduction
Appl. Surf. Sci.
Comparative study of quaternary Mg and group III element co-doped ZnO thin films with transparent conductive characteristics
Thin Solid Films
Development of transparent conductive Mg and Ga co-doped ZnO thin films: Effect of Mg concentration
Surf. Coat. Technol.
Al and Ni co-doped ZnO films with room temperature ferromagnetism, low resistivity and high transparence
Mater. Chem. Phys.
Comparative study of the sintering process and thin film sputtering of AZO, GZO and AGZO ceramics targets
Ceram. Int.
Low temperature rf-sputtered In and Al co-doped ZnO thin films deposited on flexible PET substrate
Ceram. Int.
Impact of rapid thermal annealing on structural, optical and electrical properties of DC sputtered doped and co-doped ZnO thin film
Appl. Surf. Sci.
Boron and nitrogen co-doped ZnO thin films for opto-electronic applications
Ceram. Int.
Electrical and optical properties of Al–N co-doped p-type zinc oxide films
J. Cryst. Growth
Characterization of gallium–nitrogen co-doped zinc oxide thin films prepared by RF diode sputtering
Vacuum
An insight into doping mechanism in Sn–F co-doped transparent conducting ZnO films by correlating structural, electrical and optical properties
J. Alloy. Compd.
Growth of high-quality Ga–F codoped ZnO thin films by mid-frequency sputtering
Ceram. Int.
Investigation of electronic and optical properties in Al-Ga codoped ZnO thin films
Curr. Appl. Phys.
Effect of substrate temperature on the structural and optical properties of ZnO and Al-doped ZnO thin films prepared by dc magnetron sputtering
Opt. Commun.
Effects of substrate temperature on the efficiency of hydrogen incorporation on the properties of Al-doped ZnO films
Superlattices Microstruct.
Effects of deposition temperature on the properties of Ga-doped ZnO thin films
J. Sci. Innovation
Effect of substrate temperature on the structural electrical, and optical properties of GZO/ZnO films deposited by radio frequency magnetron sputtering
Ceram. Int.
Influence of buffer layer thickness on the structure and optical properties of ZnO thin films
Appl. Surf. Sci.
Physical and structural properties of ZnO sputtered films
Mater. Lett.
Room temperature preparation of high performance AZO films by MF sputtering
Ceram. Int.
Stress-related effects in thin films
Thin Solid Films
Comparative study on structure and internal stress in tin-doped indium oxide and indium-zinc oxide films deposited by r.f. magnetron sputtering
Thin Solid Films
Cited by (134)
Synthesis and characterization of Al-doped ZnO and Al/F co-doped ZnO thin films prepared by atomic layer deposition
2023, Materials Science and Engineering: B