Journal of Materials Science: Materials in Electronics

, Volume 23, Issue 1, pp 174–179

Dielectric property and electrical conduction mechanism of ZrO2–TiO2 composite thin films

Authors

  • Ming Dong
    • State Key Laboratory of Electrical Insulation and Power EquipmentSchool of Electrical Engineering, Xi’an Jiaotong University
    • Faculty of Physics and Electronic TechnologyHubei University
  • Liangping Shen
    • Faculty of Physics and Electronic TechnologyHubei University
    • College of Electrical and Electronic EngineeringHuazhong University of Science and Technology
  • Yun Ye
    • Faculty of Physics and Electronic TechnologyHubei University
  • Cong Ye
    • Faculty of Physics and Electronic TechnologyHubei University
  • Yi Wang
    • Faculty of Physics and Electronic TechnologyHubei University
  • Jun Zhang
    • Faculty of Physics and Electronic TechnologyHubei University
  • Yong Jiang
    • School of Materials Science and EngineeringUniversity of Science and Technology Beijing
Article

DOI: 10.1007/s10854-011-0378-x

Cite this article as:
Dong, M., Wang, H., Shen, L. et al. J Mater Sci: Mater Electron (2012) 23: 174. doi:10.1007/s10854-011-0378-x

Abstract

ZrO2–TiO2 composite films were fabricated by radio frequency magnetron sputtering and post annealing in O2. It was found the films remained amorphous below the annealing temperature of 500 °C. The as-deposited ZrO2–TiO2 film has a high dielectric constant of 22, and which increases to 34 after annealing at 400 °C. At low electric field, the dominant conduction mechanisms are Schottky emission for both the as-deposited and the annealed thin films. At high electric field, the conduction mechanism changes to space-charge-limited current and then changes to Poole–Frenkel (PF) emission after annealing at 400 °C.

1 Introduction

As very large scale integration technology continues to be scaled down to the nanometer region, SiO2 gate dielectrics reaches its limit. It will require alternative gate dielectrics instead of conventional silicon dioxide or oxynitrides [1]. The recent searches for high dielectric constant (k) materials focus on Hf-based oxides [27] or Zr-based oxides [812] due to its relatively high permittivity, large band gap, good thermal and chemical stabilities. However, ZrO2 has its intrinsic drawbacks. Due to the low crystallization temperature of ZrO2 (~300 °C), crystalline ZrO2 thin film will cause high leakage current when used as dielectric layer [13]. Also, the dielectric constant of amorphous ZrO2 is about 10–20, although it is much higher than SiO2, the value of k will not meet the demand of the future integrated circuit [8].

Adding SiO2 and Al2O3 to ZrO2 or HfO2 thin films can get relative high dielectric constant than SiO2, and also help to stabilize them in an amorphous structure during high temperature annealing. However, compared to ZrO2 or HfO2, the overall dielectric constant will be reduced [10, 14]. TiO2 is a high-k material with very high permittivity about 80. In order to improve the permittivity of ZrO2, the feasible way is to fabricate ZrO2–TiO2 composite films. Meanwhile, as a composite thin film, the addition of TiO2 can improve the crystallization temperature [15]. There are some reports about TiO2 admixing with HfO2 can obtain high dielectric constant (~30), remarkable thermal stability and also the leakage current can be well controled [1619]. Nevertheless, there has been few report about ZrO2–TiO2 composite films until now.

In this work, ZrO2–TiO2 composite films were fabricated by radio frequency (rf) magnetron sputtering and post annealing in O2 atmosphere at different temperatures. The structure and electric properties of composite films were analyzed. Pt/ZrO2–TiO2/p-Si Metal–oxide–semiconductor (MOS) capacitors were fabricated in this experiment to explore the conduction mechanism of the ZrO2–TiO2 composite films. Conduction mechanism can reflect the charge transfer and energy band in the films, and reveal the origin of the leakage current.

2 Experiment

In this experiment, (100) p-type silicon wafers were used as substrates. ZrO2–TiO2 composite films were deposited by rf-magnetron sputtering in argon ambient at room temperature using ZrO2 and TiO2 ceramic targets (99.99% purity). The chamber was pumped down to base pressure of 5 × 10−4 Pa, and the total pressure during deposition was 0.5 Pa. A layer of ZrO2 film was deposited first, and then TiO2 was deposited on ZrO2 film. We controlled the sputtering speed of each target to get an atom ratio of Zr : Ti ≈ 1:1 (measured by X-ray fluorescence spectrometer). Subsequently, the films were annealed at 300 °C, 400 °C and 500 °C in O2 atmosphere for 2 h.

The structural characteristics of the films were investigated by X-ray diffraction (XRD, Bruker D8) and transmission electron microscopy (TEM, FEI Tecnai G20). The TEM samples were prepared by careful mechanical grinding and polishing followed by low-angle Ar-ion milling till the film thickness lower than 100 nm (showing in red color while observed by using an optical microscope). MOS capacitors were fabricated by sputtering a Pt-top electrode through a shadow mask with an area of 1.96 × 10−3 cm2. The back side of the wafer was HF cleaned and Pt thin film was deposited by sputtering. The MOS capacitors were electrically characterized using Radiant Precision Premier (Radiant Technology, USA) to obtain current–voltage (I–V) curves. Capacitance–voltage (C–V) measurements were performed by a precision LCR meter (Agilent 4294A).

3 Results and discussion

Figure 1 shows the X-ray diffraction patterns of the as-deposited and annealed ZrO2–TiO2 films at various temperatures. It can be seen the as-deposited film and the samples after annealing at 300 °C and 400 °C are shown to be amorphous. For the sample after annealing at 500 °C, a very weak diffraction band centered at 39.3° can be indexed to (200) plane of rutile TiO2 (JCPDS 89-4920), which shows the appearance of a crystalline structure of the films at the annealing temperature higher than 400 °C. These can be seen more clearly in Fig. 2 for the enlarged patterns of 400 °C and 500 °C annealed samples. For 400 °C annealed sample, there is not any diffraction peaks, however, for 500 °C annealed sample, a weak band around 39.3° tends to appear. Because for a dielectric thin film a crystalline structure will cause a high leakage current [12]. To obtain a dielectric thin film with excellent performance in the MOS integrated circuit, here, we only discuss non-crystalline ZrO2–TiO2 thin films.
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Fig. 1

X-ray diffraction patterns of ZrO2–TiO2 thin film (a) as-deposited, (b) annealed at 300 °C, (c) annealed at 400 °C, (d) annealed at 500 °C

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Fig. 2

The enlarged XRD patterns of 400 °C and 500 °C annealed samples

The detail structures were studied by TEM. The micrograph of the as-deposited sample is shown in Fig. 3a. The physical thickness of the as-deposited ZrO2–TiO2 thin film was measured to be 56 nm. Figure 3b shows the TEM cross-sectional image of 400 °C annealed ZrO2–TiO2 thin film. There is an obvious interfacial layer about 3 nm which we consider to be SiO2 appear at the ZrO2/Si interface. It is believed that the interface layers play an important role on the electrical properties, including the dielectric constant and the leakage currents. The inset graph in Fig. 3b presents the electron diffraction (ED) pattern of the thin film. The ED pattern shows a few very weak halo-like diffraction spots, indicating the amorphous-like structure for the film after annealing at 400 °C.
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Fig. 3

Cross-sectional TEM images of ZrO2–TiO2 thin film (a) as-deposited, (b) annealed at 400 °C. The inset graph in (b) presents the ED pattern of the annealed thin film

C–V characteristics of the MOS capacitor consisted of Pt/ZrO2–TiO2/p-Si was measured at high-frequency (1 MHz). Figure 4 shows the C–V curves for ZrO2–TiO2 thin films. From Fig. 4, it can be seen with increasing the annealed temperature, the saturated capacitance increased. According to the saturated capacitance, we can get the effective dielectric constant of the thin films by the formula.
$$ C_{o} = {\frac{{K\varepsilon_{o} A}}{d}} $$
where Co is the saturated capacitance, K, εo, A and d stand for the effective dielectric constant of the oxide, the permittivity of free space, the area of the capacitor, and is the thickness of the dielectric, respectively. The calculated dielectric constants for three samples are shown in Table 1. Obviously, the dielectric constants of composite thin films are higher than ZrO2 due to the addition of TiO2. Furthermore, with increasing the annealing temperature the dielectric constant increases. The as-deposited ZrO2–TiO2 composite film has a high dielectric constant of 22, and which increases to 34 after annealing at 400 °C. Normally, for a composite thin film the dielectric constant is mainly dependent on the component of the film and the microstructure including the crystalline property, interface, surface roughness and various vacancies and defects in the film, etc. [1821]. For the ZrO2–TiO2 composite films, the dielectric constant increases with increasing the annealing temperature, and although the content of TiO2 is the same for each film, the films have different microstructures. The interface will be thicker with increasing the annealing temperature, which would make the total dielectric constant of thin film decrease. While the oxygen vacancies and defects in the films will be compensated after annealing for 2 h in O2, and that would make the dielectric constant increase. There is enough time for the compensation of oxygen vacancies, so the influence of the oxygen vacancies and defects in the thin film is beyond that of the interface. The same phenomenon of the dielectric constant varied with the annealing temperature can be seen in Al2O3-HfO2 films [21].
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Fig. 4

High-frequency (1 MHz) capacitance–voltage curves for ZrO2–TiO2 thin film (a) as-deposited, (b) annealed at 300 °C, (c) annealed at 400 °C

Table 1

Electrical characteristics calculated from C–V data, k is the dielectric constant of the films, EOT is the effective oxide thickness of the films, Vfb is flat band voltage, Doc is the density of effective oxide charges

Sample

K

EOT (nm)

Vfb (V)

Df (cm−2)

Dit (cm−2)

As-deposited

22

9.8

−1.19

3.55 × 1012

7.51 × 1011

300 °C (O2 annealed)

30

7.3

−1.17

2.12 × 1012

1.03 × 1012

400 °C (O2 annealed)

34

6.3

−1.37

2 × 1012

1.21 × 1012

From the high frequency C–V curves, we can obtain the flat band voltage (Vfb). Vfb is primarily affected by fixed charges (Qf) in the ZrO2–TiO2 gate dielectrics and the interface traps (Qit) at the ZrO2/Si interface. The calculated values are shown in Table 1. And the interface traps density is calculated by the Terman method [22]. From the table, with increasing the annealing temperature, Vfb shifts to the negative direction, indicating more positive effective charges are generated in the oxide films or at interface with the Si [23, 24]. Figure 4 also presents the hysteresis, which is mainly decided by the mobile and trapped charges at the interface [25]. It can be observed the sample with higher annealing temperature has a larger hysteresis loop and the shift of Vfb (ΔVfb) of the loop increases, which suggests more interface states in the film, and that is related to the conduction mechanism of leakage current.

Figure 5 shows the J-V characteristics of the as-deposited and annealed samples with both gate electron injection (negative Vg) and substrate electron injection (positive Vg). The asymmetry in the J-V curve is attributed to the difference in the material properties and conduction mechanisms across the Pt/thin film and thin film/Si interfaces [26]. As shown in Fig. 5, the films annealed in O2 exhibit larger leakage current densities than as-deposited, which can be confirmed by the TEM images. After annealing in O2 atmosphere, the interfaces present in the thin film, which suggests the existence of numerous interface traps in the thin film as shown in Table 1.
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Fig. 5

Current-voltage curves for the as-deposited and annealed ZrO2–TiO2 thin film (a) as-deposited, (b) annealed at 300 °C, (c) annealed at 400 °C

To analyze the leakage current conduction mechanisms of the ZrO2–TiO2 thin film, many conduction mechanisms have been investigated and evaluated. The theory of the charge conduction mechanisms has been extensively reported [2630]. The voltage dependences for these mechanisms are:
$$ Schottky\; emission: \quad J = AT\exp \left[ {{\frac{{ - q\left( {\Upphi_{B} - \sqrt {{\raise0.7ex\hbox{${qE}$} \!\mathord{\left/ {\vphantom {{qE} {4\pi \varepsilon }}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{${4\pi \varepsilon }$}}} } \right)}}{kT}}} \right] $$
$$ Poole-Frenkel\; emission: \quad J = BE\exp \left[ {{\frac{{ - q\left( {\Upphi_{t} - \sqrt {{\raise0.7ex\hbox{${qE}$} \!\mathord{\left/ {\vphantom {{qE} {\pi \varepsilon }}}\right.\kern-\nulldelimiterspace} \!\lower0.7ex\hbox{${\pi \varepsilon }$}}} } \right)}}{kT}}} \right] $$
$$ Space-charge-limited current: \quad J = {\frac{{9\varepsilon \mu V^{2} }}{{8d^{3} }}} $$

In brief, for direct tunneling, E1/2 is proportional to the lnJ, while for Poole–Frenkel emission, E1/2 is proportional to ln(J/E), and for space-charge-limited current, E2 is proportional to J. Where A and B are constants, ε and E are the dielectric constant and electric field, q is the electronic charge, k is Boltzmann constant, and T is the temperature in kelvins, μ is the electronic mobility, d is the thickness of the thin film, φB is the barrier height seen by the injecting electrons for the Schottky emission mechanism, while φt is the barrier seen by the trapped electrons for the PF emission mechanism (equivalent to the depth of the potential well at the trapping site). For the gate electron injection, the conduction mechanism of all the samples is Ohmic current (J∝E). It indicates that the top Pt electrodes have a perfect contact with the ZrO2–TiO2 films.

Figure 6 shows the calculated (fitted) J–E characteristics of as-deposited ZrO2–TiO2 thin film with substrate electron injection. Fig. 6a plots lnJ versus the square root of the applied electric field (E1/2) in low electric field. A good linear fitting indicates Schottky emission; a thermionic emission dominates the conduction mechanism while the applied electric field is lower than 0.31 MV/cm. The graph of J versus E2 is given in Fig. 6b. The linear trend exhibits space-charge-limited current in high electric field (E > 0.31 MV/cm). More electrons inject into insulator as the electric field increasing that induces the conversion of conduction mechanisms. The similar phenomenon can be found in film annealed in O2 at 300 °C as shown in Fig. 7. But when the annealed temperature is 400 °C, the conduction mechanism presents PF emission in high electric field (E > 0.31 MV/cm) as show in Fig. 8. In the case of PF emission, the conduction is decided by charged traps in the insulator thin films with high trap concentration [31]. And for the 400 °C sample, it exists the most trap and interfacial densities in thin film, which are shown in Table 1. That may induce the transfer of the conduction mechanism from space-charge-limited current to PF emission in high electric field.
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Fig. 6

Conduction mechanism fitting of the as-deposited ZrO2–TiO2 thin film with substrate electron injection (a) the curve of ln J versus E1/2 in low field (E < 0.31 MV/cm), (b) the curve of J versus E2 in high field (E > 0.31 MV/cm)

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Fig. 7

Conduction mechanism fitting of ZrO2–TiO2 thin film after annealing at 300 °C in O2 with substrate electron injection (a) the curve of ln J versus E1/2 in low field (E < 0.38 MV/cm), (b) the curve of J versus E2 in high field (E > 0.38 MV/cm)

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Fig. 8

Conduction mechanism fitting of ZrO2–TiO2 thin film after annealing at 400 °C in O2 with substrate electron injection (a) the curve of ln J versus E1/2 in low field (E < 0.31 MV/cm), (b) the curve of ln (J/E) versus E1/2 in high field (E > 0.31 MV/cm)

4 Conclusions

The structure and electrical properties of ZrO2–TiO2 thin films fabricated by radio frequency magnetron sputtering and subsequent post annealing in O2 atmosphere at different temperatures were studied. The film remained amorphous below the annealing temperature of 500 °C. It was found that annealing in O2 led to an increase of the dielectric constant, which increases to about 34 when the film is annealed at 400 °C. For the gate electron injection, the conduction mechanism of all the samples is Ohmic current, indicating the top Pt electrodes have a perfect contact with the films. For the substrate electron injection, a thermionic Schottky emission dominates the conduction mechanism in low electric field. With increasing of the electric field, more electrons inject into insulator that causes the conversion of conduction mechanisms to space-charge-limited current for the samples after annealing at or below 300 °C. The enhanced traps and interfacial densities in the 400 °C annealed film lead to the conduction mechanism of PF emission in high electric field.

Acknowledgments

This work is supported in partial by the National Nature Science Foundation of China (No. 51072049), MOST of China (No.2007CB936202), STD and ED of Hubei Province (Grant Nos. 2009CDA035, 2008BAB010, and Z20091001).

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