The effects of zinc substitution on the electrical properties of MgNb2O6 thin films

ABSTRACT The effects of zinc substitution on the electrical properties of sol-gel derived MgNb2O6 (Mg1-xZnxNb2O6; MZxNO) thin films were investigated. Accordingly, the experimental results revealed that the optimal optical and electrical properties of the devices can be obtained for specimen with x = 0.2. The dielectric constant, average transparency and optical band gap of the devices at 400°C annealing are 21.2 (@1 MHz), ~80% and 4.86 eV, respectively. The results indicated that the electrical properties of the sol-gel derived MgNb2O6 thin films are tuneable based upon the variation of zinc content. More importantly, the fabrication temperature of the MZ0.2NO thin films is 100°C lower than the bending point of the glass substrate making it suitable for practical applications. With the increase of the applied electric field, the leakage conduction mechanisms of the Al/MZxNO/ITO devices are mainly controlled first by ohmic conduction at low electric field, changes to space-charge-limited conduction (SCLC) and then FN tunneling at high electric field. Moreover, the critical electric field strength of the conduction mechanisms is also a function of the zinc content.


Introduction
Transparent oxide film is an indispensable part of the development of transparent electronics and has been applied for several applications [1,2]. However, the unavoidable high-temperature crystallization process may lead to a rough surface and higher leakage current in the films. An alternation of the crystalline films by using an amorphous material is one of the choices for novel transparent electronic applications that may offer several advantages, including lower processing temperatures and leakage current, better surface uniformity, and superior mechanical flexibility [3,4]. Among these dielectric materials, MgNb 2 O 6 (MNO) is known as a popular microwave dielectric [5], optoelectronic and photoactive material due to the excellent dielectric properties, good thermal stability and wide optical band gap. These superior properties also make MNO a potential candidate to replace the dielectric layer in the transparent electronics or multifunctional optoelectronics.
In our previous work [6], highly transparent amorphous-MNO thin films on indium tin oxide (ITO)/glass substrates were fabricated by the sol-gel method, and the effects of annealing temperature and atmospheres (N 2 , air and O 2 ) on the electrical and optical properties of these films were also discussed [6,7]. The results indicated that the amorphous MNO thin films featured a high dielectric constant, low leakage current and good transparency after 500°C annealing. In addition, the electrical properties of amorphous MNO thin films can be easily tuned via the O 2 /N 2 ratio during annealing, which is related to the ratio of Nb 4+ /Nb 5+ content in the films. The percentage of Nb 4+ content increased when the films were annealed in the oxygen-deficient conditions, which could lead to semiconducting films; in contrast, annealing in an oxygen-rich environment could lead to dielectric films. Moreover, the conduction mechanisms of MNO/ITO heterojunctions were also transferred by annealing. The experimental results suggest that amorphous MNO films have considerable potential for use in multifunctional optoelectronics, due to their smooth surfaces, flexible electrical properties and good transparency.
In comparison with crystalline MNO [8][9][10], the amorphous MNO is more promising for low-temperature applications. However, it required a 500°C annealing which is close to the bending point of the glass. Metallic oxides are commonly used as a dopant to reduce the fabrication temperature of the devices [11]. Since the radius of Zn 2+ and Mg 2+ are similar, ZnO was selected as a dopant to lower the fabrication temperature of the sample. Moreover, the effect of the Zn substitution on the electrical and optical properties of the amorphous MNO thin films was also investigated in this study. The molar ratio of the (Mg+Zn) and Nb precursors was fixed at 1:2, with acetylacetone (C 5 H 8 O 2 , 99%, Alfa Aesar) used as the stabilizing reagent. Then, the precursor sol was spin-coated on the glass and ITO/glass substrates. The detail of the deposition procedure is shown in our previous work [6,7]. Finally, the Mg 1-x Zn x Nb 2 O 6 (MZ x NO) films were annealed at 400°C and 500° C for 30 min in air. The heating rate was fixed at 2°C/ min, and the final thickness of the films was about 60 nm after annealing.

Materials and methods
The crystalline structure was analyzed by grazingincidence X-ray diffraction (GIXRD, Rigaku D/Max III, V) using Cu Kα radiation at 40 kV and 100 mA; the surface morphology was observed using field emission scanning electron microscopy (FESEM, HITACHI SU-8000) in SEI mode. In order to identify the chemical states and composition of the thin films, X-ray photoelectron spectroscopy (XPS, ULVAC-PHI PHI5000; C1s as a reference at 284.8 eV) with Al Kα monochromatized source was used in this study. In order to estimate the brief energy band diagram of the device, the absorbance spectra were directly obtained by a UV-visible spectrophotometer and the valence band maximum of the samples were measured by UV photoelectron spectra (UPS) which was carried out with the He I line (21.2 eV). To minimize the influence of surface contamination, all samples were degassed in an entry chamber for 30 min and transferred to the main chamber sputtered with low dosage Ar + ions to remove the surface contamination. The main chamber pressure was then maintained in the range 10 −8 Pa.
The optical transmittance spectra of the MZ x NO/ quartz structure were measured using a UV-visible recording spectrophotometer (UV-vis, HITACHI U4100) from 200 to 800 nm. To form the Al/MZ x NO/ ITO/glass MIM structure, 200 nm Al electrodes with a diameter of 1 mm were deposited by E-beam evaporation. The capacitance-voltage (C-V) and currentvoltage (I-V) measurements of the MIM capacitor were obtained using an Agilent E4980A LCR meter and Agilent E5270B Parameter Analyzer. Figure 1 shows the GIXRD patterns of the MZ x NO samples annealed at 400°C and 500°C. All samples revealed amorphous-like patterns after 400°C annealing. As the annealing temperature increased to 500°C, clear diffraction peaks identified as columbite-M 2+ Nb 2 O 6 (M = Mg or Zn) phase were observed. Based on the findings of our previous study [6,7], the crystalline temperature of the MNO films is over 600°C. These results indicated that the crystalline temperature of the magnesium-based columbite thin films could be effectively reduced by zinc substitution. The average grain size was estimated by using the cross line method and listed in Table 1. A slight grain growth could be observed in the 500°C-annealed films with the x content increasing due to the improvement in the crystallinity. Figure 2(a) and (b) illustrate the plane view of FESEM figures of the MZ x NO samples annealed at 400°C and 500°C, respectively. According to the FESEM results, MZ x NO films exhibited a smooth and uniform surface morphology when the annealing temperature was lower than 500°C. The current-voltage (I-V) characteristics of the Al/MZ x NO/ITO/glass MIM capacitors are shown in Figure 3. With the increase of Zn content, the leakage current increased from 3 × 10 −7 to 1 × 10 −4 A and from 8 × 10 −8 to 4 × 10 −3 A at 1 V for samples annealed at 400°C and 500°C, respectively. Moreover, the leakage current also increased with the increase of annealing temperature due to a higher film density as shown in Figure 2. The 500°C-annealed film revealed a higher density leading to a higher leakage current. The I-V curves revealed almost symmetric behavior within −3 V~+3 V range may be owing to that the work function of Al and ITO are similar. In order to obtain a high quality amorphous transparent dielectric film for transparent optoelectronics, the chemical, optical and electric properties of 400° C-annealed samples are discussed below. XPS was used to confirm the chemical bonding states of the films. The characteristic spectra of the 400°C-annealed films (x = 0, 0.2, 0.6, 1) are shown in Figure 4. The binding energy for Mg 2p and Zn 2p revealed divalent oxidation states, which are very similar to those found in our previous work [6,7]. The O 1s and Nb 3d spectra of the films could be decomposed clear shoulders (non-lattice oxygen and Nb 4+ ). The binding energy for the lattice oxygen signal is at 529.6 eV, which indicates the metallic-oxygen bonding while the non-lattice oxygen signal is at 531.02 eV [12]. The Nb 3d spectra showed two main peaks for Nb 3d 3/2 and Nb 3d 5/2 , which can both be assigned to a pentavalent oxidation state. The energy difference between Nb 3d 5/2 and Nb 3d 3/2 is 2.72 eV in all cases. According to the literatures [13,14], the presence of Nb 4+ within MgNb 2 O 6 films may act as an n-type dopant which may highly affect the electric properties of the films. The area ratio of the decomposed peaks of   lattice oxygen (O lattice ), non-lattice oxygen (O non-lattice ), Nb 5+ and Nb 4+ are shown in Table 2. However, no obvious change in the Nb 4+ ratio of the MZ x NO films at various x contents. Therefore, the effect of Nb 4+ ratio of films can be excluded from the dominant factor of the variation of the electrical properties. The transmittance spectra (    and tanδ-f values are given to allow a direct comparison, where C (100 kHz) is the capacitance measured at 100 kHz. A large difference in capacitance and dielectric loss can be found between 1 and 10 kHz, especially when the zinc content becomes significant. In XPS results, no obvious variation in the Nb 4+ /Nb 5+ ratio could be found in these samples. One possible reason for the high dielectric constant and leakage current in the samples with high Zn content may be due to the high ionic polarization of Zn or Zn interstitial [13]. However, the film density increased (porosity decreased) with the increase of Zn content as can be seen in Figure 2(a). Therefore, the high dielectric constant and leakage current of the samples was more likely attributed to the density of the films. According to the past research [6], the fluctuation in the leakage current of the samples produced under various conditions may be due to the variation of the conduction mechanisms, which is related to the Nb 4+ (or oxygen vacancy) concentration. The re-plotted I-V     curves shown in Figure 8 revealed that the calculated K values deviate from the expected ones (K ~ 4) [6]. Therefore, SE and PFE should be excluded from discussions of the conduction mechanism of the MZ x NO samples in this work. Interface-limited Fowler-Nordheim (FN) tunneling and space-charge-limited conduction (SCLC) are also considered. Figure 9 shows the double logarithmic I-V curves for the MZ x NO samples. The typical I-V characteristics for the SCLC in the double-log I-V plot can be divided into three regions. When the applied voltage is low, the slope of the double-log I-V curve is near 1, which implies that the current depends on the applied voltage and this behavior is like the Ohmic conduction mechanism (I / V). Next, the slope of the double-log I-V curve is about 2. It is assumed that the injected electrons are partly trapped by the thin film, while the others not trapped contribute to the current. This behavior is similar to traps-filled limitation (I / V 2 ). Not until all existing trapping centers in the thin film are occupied, the subsequently injected electrons are free to move in the thin film. Finally, if the current is fully controlled by the space charges, which were built up by the injection electrons and limit the further injection electrons, the current is proportional to the voltage square [15].

Results and discussion
The ln J=E 2 À � À 1=E relation for the MZ x NO samples is shown in Figure 10. FN tunneling is the quantum effect by which the electrons do not overcome the barrier, but flow through. A roughly straight line fitting with a negative slope in the high electric region shown in the MZ x NO (x = 0-0.6) samples suggested that when a high electric field is applied, the dominant leakage mechanism of the MZ x NO samples is FN tunneling.
Considering the positive voltage portion of I-V curve, the conduction mechanisms of bottom injection (MZ x NO/ITO interface) of the MZ x NO samples are summarized in Table 3. The results revealed that all of the MZ x NO samples followed an ohmic behavior under a low electric field suggesting that the barrier height between ITO and MZ x NO is small enough for the electrons to pass through directly.  Figure 11. The valence band spectra of the 400°C-annealed MZxNO samples. Moreover, the samples showed an SCLC behavior under strong injection conditions (high electric field) as the density of trapped charges in the MZ x NO films became significant [16]. These results can be partially confirmed by the above mentioned XPS data: the Nb 4+ existed in the MZ x NO films, and the injected electrons were then trapped by the oxygen vacancies, leading to the SCLC. When the electric field is high enough, the MZ x NO conduction band crosses the position of the ITO Fermi level and then tunneling occurs [14]. To further understand the leakage mechanisms of the MZ x NO samples, the valence band spectra ( Figure 11) Figure 12, where E C MZNO , E V MZNO , and E C ITO , E V ITO are the conduction band minimal and valence band maximum values of the MZ x NO and ITO, respectively. When a positive bias applied on the top electrode is large enough, the MZ x NO conduction band crosses the position of the ITO Fermi level and tunneling occurs. The calculated values of the band diagram of the MZ x NO samples are shown in Table 4. From the data, E C MZNO -E F value of the MZ x NO samples increased with the increase of x value. This result also provided a well explanation of the increase of onset tunneling field (EFN) as the x value increased.

Conclusion
In summary, the optical band gaps of the MZ x NO films decreased from 4.69 to 4.95 eV as the x value varied from 1 to 0. Also, the leakage current densities (J), dielectric constant and tanδ of the samples are increased as the x value increased. The fluctuation in the leakage current of the samples is Zn content related. All of the MZ x NO samples followed an ohmic behavior under a low electric field suggesting that the barrier height between the ITO and the MZ x NO is small enough for the electrons to directly pass through. On the other hand, the FN tunneling occurs when the samples were subject to a high applied bias. The experimental results also revealed that the onset tunneling field (EFN) is a function of the Zn content.