Dielectric property and electrical conduction mechanism of ZrO2–TiO2 composite thin films
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- Dong, M., Wang, H., Shen, L. et al. J Mater Sci: Mater Electron (2012) 23: 174. doi:10.1007/s10854-011-0378-x
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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.
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 . The recent searches for high dielectric constant (k) materials focus on Hf-based oxides [2–7] or Zr-based oxides [8–12] 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 . 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 .
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 . 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 [16–19]. 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.
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
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
3.55 × 1012
7.51 × 1011
300 °C (O2 annealed)
2.12 × 1012
1.03 × 1012
400 °C (O2 annealed)
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 . 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 . 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.
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.
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.
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).