Bi 2 SiO 5 Doping Concentration Effects on the Electrical Properties of SrBi 2 Ta 2 O 9 Films
MING LI,1YANG ZHANG,1YAYUN SHAO,1MIN ZENG,1ZHANG ZHANG,1 XINGSEN GAO,1XUBING LU,1,3,4,5J.-M. LIU,2and HIROSHI ISHIWARA3
1.—South China Academy of Advanced Optoelectronics, Institute of Advanced Materials, South China Normal University, Guangzhou 510006, China. 2.—Laboratory of Solid State Microstruc- tures, Nanjing University, Nanjing 210093, China. 3.—Department of Electronics and Applied Physics, Tokyo Institute of Technology, Yokohama, Kanagawa 226-8502, Japan. 4.—e-mail:
[email protected]. 5.—e-mail: [email protected]
In this paper, we investigated the microstructure and electrical properties of Bi2SiO5(BSO) doped SrBi2Ta2O9(SBT) films deposited by chemical solution deposition. X-ray diffraction observation indicated that the crystalline struc- tures of all the BSO-doped SBT films are nearly the same as those of a pure SBT film. Through BSO doping, the 2Pr and 2Ec values of SBT films were changed from 15.3lC/cm2 and 138 kV/cm of pure SBT to 1.45lC/cm2 and 74 kV/cm of 10 wt.% BSO-doped SBT. The dielectric constant at 1 MHz for SBT varied from 199 of pure SBT to 96 of 10 wt.% BSO-doped SBT. The doped SBT films exhibited higher leakage current than that of non-doped SBT films.
Nevertheless, all the doped SBT films still had small dielectric loss and low leakage current. Our present work will provide useful insights into the BSO doping effects to the SBT films, and it will be helpful for the material design in the future nonvolatile ferroelectric memories.
Key words: SrBi2Ta2O9film, Bi2SiO5 film, ferroelectric film, memory, low voltage operation
INTRODUCTION
Ferroelectric random access memory (FeRAM) has been widely studied for decades, because it has many advantages over the conventional flash memories such as higher operation speed, lower power consumption, and better radiation resistance, etc.1–3 There are two types of FeRAM.4 One is the 1T1C type or capacitor type, in which the cell structure is very similar to that of dynamic RAM (DRAM) and the polarization reversal current is detected in the readout operation. The other is the 1T type or transistor type, in which the gate dielectric film of a metal oxide semiconductor field- effect-transistor (MOSFET) is replaced with a fer- roelectric film and the semiconductor surface charge induced by the polarization of the ferroelectric film
is read out nondestructively as a drain current.
Because of the difference of the operation principle, requirements for the ferroelectric films are different between the two types of FeRAM.1,2,4,5For example, a large Pr value is always needed for 1T1C-type FeRAMs to meet the continuous scaling require- ments for higher density products. Recent develop- ment on the high polarization ferroelectric materials has reported a Pr value as large as 100lC/cm2in a Mn-doped BiFeO3 film,6 while a relatively small polarization value is needed for 1T-type FeRAMs.2,4,5 Nevertheless, the same requirement for a low crys- tallization temperature of ferroelectric film is needed for both of the two types of FeRAMs.1,2,4,5
SrBi2Ta2O9 (SBT) is one of the most promising ferroelectric materials for nonvolatile FeRAMs applications due to its excellent fatigue-free nature and other electric properties.7However, it requires high processing temperatures (typically‡750C),8 which is not favorable for its applications in both 1T1C- and 1T-type FeRAMs. Bismuth silicate
(Received November 28, 2013; accepted May 16, 2014;
published online June 14, 2014)
2014 TMS
3625
(Bi2SiO5) crystal consists of a bismuth-layer struc- ture and a silicon dioxide layer.9 It has been dem- onstrated to have a low crystallization temperature of 400C and good insulating properties.10 It has also been demonstrated that the crystallization temperature of usual ferroelectric materials such as PbZr1 xTixO3(PZT), SBT and Bi4Ti3O12 can be low- ered by 150–200C through BSO doping into these original materials.11 Wang et al.12 also reported that the SBT thin films can be synthesized at low temperatures ranging from 575C to 675C by using BSO as a seed layer. In addition, Kijima and Ishiwara11 reported that well-saturated ferroelec- tric polarization can be observed at 0.5 V for BSO- doped SBT films with 13 nm thickness. These results demonstrated that BSO doping can effec- tively reduce the high crystallization temperature of SBT film while keeping good film quality.
Although the BSO doping effects on lowering the crystallization temperature have been well demon- strated, there is a lack of some basic knowledge on the doping effects on its ferroelectric, dielectric and leakage properties. In this paper, we carried out a further investigation on the effects of BSO doping concentration on the crystal structure and electrical properties of SBT films. Since the lowering effects of crystallization temperature by BSO doping have been well demonstrated in the above-mentioned work, we focused in the present work on the doping concentration effects on the electrical properties of SBT films while fixing the annealing temperature at a specified value.
EXPERIMENTAL
The BSO-doped SBT thin films were formed by sol–gel spin-coating on Pt/Ti/SiO2/Si (100) sub- strates. The Pt bottom electrode with 200 nm thickness was prepared by RF sputtering on SiO2/Si substrate. Commercially available (Toshima) sol–
gel solutions of BSO (Bi2O3:SiO2= 1:1) and SBT (8 wt.% precursor solution with 20% Sr-deficient and 10% Bi-excess composition) were mixed together to obtain the stoichiometric (SBT)1 x(BSO)x(x = 0.0–
0.10) solution. The coated film was dried at 240C for 5 min in air in order to remove the organic materials and successively fired by using a rapid thermal annealing (RTA) furnace at 400C for 10 min in O2 flow. These processes were repeated several times until the total thickness was approx- imately 250 nm. For measurements of the electrical properties, dot-shaped Pt top electrodes with an area of 3.14 910 4cm2 were fabricated on BSO- doped SBT/Pt structures at room temperature by electron beam evaporation through a shadow mask.
After Pt top electrode deposition, the BSO-doped SBT films were annealed by using a RTA furnace at 750C for 60 min in O2flow.
The polarization–voltage (P–V) characteristics of the ferroelectric films were measured using a RT66a
standard ferroelectric test system (Radiant Tech- nologies) at room temperature. The current–voltage (I–V) characteristics of the films were investigated by an Agilent 4156C high-precision semiconductor parameter analyzer. The dielectric properties of the films were measured by an Agilent 4194A imped- ance analyzer at 0 V bias and 50 mV oscillation voltage. Crystallinity of the films was studied by a multi-purpose x-ray diffraction meter (MXRD) (X’Pert–Pro MPD; Philips).
RESULTS AND DISCUSSIONS
The crystal structures of the non-doped SBT film and SBT films doped with BSO in different con- centrations were investigated by x-ray diffraction measurements. As shown in Fig.1, all the SBT films exhibit polycrystalline structure, and strong peaks are from the (115) and (200) planes. Through BSO doping, the intensity of (115) peaks remains nearly unchanged while the (200) peaks were suppressed.
For 2.5 wt.%- and 10 wt.%-doped SBT films, a very weak (111) peak can be observed. Based on the XRD patterns, it can be concluded that no secondary phase was introduced into the SBT films after BSO doping and that the doped SBT films were more (115) oriented. Also, doping of small amounts of BSO did not change the crystal structure, nor con- tribute significantly to lowering of the crystalliza- tion temperature of SBT film, although BSO has a low crystallization temperature of 400C.
Figure2a shows the typical P–E characteristics measured at 1 kHz. It can be clearly seen that the P–E hysteresis loops of all the SBT films can be well saturated under a 400-kV/cm electric field. One noticeable feature for the BSO-doped SBT films is that the hysteresis loop is tilted and less square-like than that of the non-doped SBT film. The Si doping into SBT films will induce some Si atom interstitial defects.13 The tilting of the P–E hysteresis loop is
Fig. 1. XRD patterns of pure SBT and different BSO concentration doped SBT films (Color online).
assumed to be due to the defects-induced domain pinning effects.14 Therefore, replacing the Si atom in the Ta site to form a SiO6 octahedron will be likely to reduce the Si atom interstitial defects and finally reduce the tilting of the P–E hysteresis loop of the BSO-doped SBT films.
The impact of the BSO doping concentration and external electric field on the Pr values is shown in Fig.2b. Under a 400-kV/cm electric field, the 2Pr value changes from 15.3lC/cm2 of non-doped SBT to 1.5lC/cm2 of 10 wt.% BSO-doped SBT, which indicates that BSO doping can effectively tune the polarization in a wide range between 15.3lC/cm2 and 1.5lC/cm2. According to Wu et al.’s15 reports, the ferroelectric phase of SBT films mainly originate from the (115) and (200) peaks. From the XRD results shown in Fig.1, the intensity of the (115) peaks slightly increases while that of the (200) peaks decreases. The decrease in the remanent polarization of BSO-doped SBT films implies that the (200) plane dominates the ferroelectric polari- zation of the SBT films. Figure2c shows that the coercive field also decreases with the increase of the BSO doping concentration. The 2Ec values can be tuned from 138 kV/cm of pure SBT to 74 kV/cm of 10 wt.% BSO-doped SBT. The ferroelectric behav- iors of SBT film have been found to be closely related to the distortion and tilting of the TaO6
octahedron.13 In BSO-doped SBT film, the addition of Si atom into SBT film will decrease the tilting angle in the a–c plane of the TaO6 octahedron,13 which is also assumed to be one of the possible mechanisms responsible for the decrease of both the polarization and the coercive field in our BSO-doped SBT films.
Figure3 shows the impact of the BSO doping concentration on the dielectric properties of SBT film. The dielectric constant of SBT films doped with different BSO concentrations exhibits good fre- quency stability in the measurement range between 1 kHz and 1 MHz, as shown in Fig.3a. It is clear that the dielectric constant of SBT films decreases with the increase of the BSO doping concentration.
The dielectric constant measured at 1 MHz changes from 199 of non-doped SBT to 96 of 10 wt.% BSO- doped SBT (Fig.3b). All the films exhibit a dielectric loss smaller than 0.1 in the measured frequency range (Fig.3c). Except for non-doped SBT film, the dielectric loss of doped SBT films decreases with the increase of the doping concentration.
According to the results from Figs.2 and 3, the BSO doping will reduce the polarization value and dielectric constant of the SBT films. This doping effect is unfavorable for its application in the 1T1C- type FeRAMs. However, it may find applications in the low voltage operation of a MFIS (metal-ferro- electric-insulator-semiconductor) structure based on 1T-type FeRAMs. The reasons can be explained as follows. First, the charge density necessary for operation of FETs is lower than 1013 electrons/cm2 (=1.6lC/cm2), which is a sufficient amount for the
remanent polarization of the BSO-doped SBT to induce. Furthermore, according to the charge matching principle between the ferroelectric film and insulator layers in the MFIS structure,2we can get:CFVF =CIVI, whereCF,VF,CI, andVIrepresent the ferroelectric capacitance, voltage drop across the ferroelectric layer, dielectric layer capacitance, and voltage drop across the dielectric layer, respectively.
A reduced dielectric constant (or ferroelectric polarization) of the ferroelectric layer will reduce
Fig. 2. Ferroelectric properties of SBT films with different BSO doping concentration. (a) P–E characteristics; (b) impact of the BSO doping concentration on the 2Pr values; (c) impact of BSO doping concentration on the 2Ec values (Color online).
the ferroelectric capacitance CF. Consequently VF will be increased in cases where the dielectric layer (CI,VI) remains unchanged. Therefore, a higherVF
can be applied under the same gate voltage, and finally the operation voltage can be decreased.
Based on this principle, several kinds of ferroelectric materials such as Sr2(Ta1 x,Nbx)2O7,16 Pb5Ge3O11,17 and YMnO318
with low dielectric constants have already been studied for low voltage operations of MFIS devices. However, the advantages of their low dielectric constants are not well demonstrated due
to some other material properties such as a high crystallization temperature. Although further study is necessary, the BSO-doped SBT films with lower crystallization temperature and low dielectric con- stant may find applications in low voltage opera- tions of MFIS devices.
Figure4 shows the leakage current characteris- tics of the doped and non-doped SBT films, all of which exhibit small leakage current at the level of 10 4A/cm2 under a 500-kV/cm electric field. A noticeable feature shown in the figure is that the non-doped SBT film exhibits the lowest leakage current among all of these films. The reported band gap of BSO film is 4.89 eV,19 which is higher than that of SBT (4.2 eV).20 Therefore, it is difficult to explain this result from the viewpoint of the energy band. Our speculation on the high leakage current in the BSO-doped SBT films is as follows. It has been proved that the Bi atoms in BSO will always occupy the Bi or Sr sites in SBT.21,22 However, the Si atom position in BSO-doped SBT film is still under dispute. One hypothesis is that the Si atom occupies the Ta site as suggested by Kijima and Ishiwara.11 Another hypothesis is that Si atom cannot replace the Ta atom to construct the SiO6
octahedron since 6-coordinated Si rarely occurs in natural or synthesized compounds at ambient con- ditions.13 The Si atoms may exist as the interstitial defects, which will lead to the increase of the leak- age current. The high oxygen pressure annealing work done by Kijima et al.23 indirectly indicates that the second hypothesis may be more reasonable.
They demonstrate in their work that the BSO-doped SBT films annealed at high oxygen pressure (9.9 atm) show much lower leakage current than that of the film without high oxygen pressure annealing. The significant reduction of the leakage current could be due to the change of the film structure under high pressure annealing. The high oxygen pressure annealing may be favorable for the formation of the SiO6 octahedron and reduce the amount of the Si atoms in the interstitial space.23 Therefore, the leakage of the BSO-doped SBT film was reduced. Based on these reported results, the higher leakage current observed in the BSO-doped SBT film in our work could be mainly due to the defects induced by interstitial Si atoms.
Another noticeable feature shown in Fig.4is that the leakage current decreases with the increase of the BSO doping concentration for the four doped SBT films. This result cannot be explained by the simple interstitial Si atom model proposed above. It is believed that the leakage current of the SBT films is affected by more complex mechanisms. In our previous work, interfacial quality between the metal electrodes (include top and bottom electrodes) and the ferroelectric film was found to affect the leakage current of SBT films.5 Recently, ferroelec- tric polarization has been demonstrated to affect the bulk charge transport in the ferroelectric film.24In our work, BSO has a lower crystallization temperature
Fig. 3. Dielectric properties of SBT films with different BSO doping concentration. (a) Frequency dependence of dielectric constant;
(b) impact of the BSO doping concentration on their dielectric constants; (c) impact of BSO doping concentration on the their dielectric losses (Color online).
than that of SBT film. Under the same annealing processes, the BSO-doped SBT films may have more amorphous characteristics. Therefore, SBT films with higher BSO doping concentration are expected to have smaller gain sizes, more flat surfaces, and smaller pinholes, which can provide better metal/
ferroelectric interface quality. The reduced polari- zation value of the higher BSO doping concentration SBT films will have less scattering for bulk charge transport in the film. The complete mechanism explaining the decreased leakage current with the increase of the BSO doping concentration for the four doped SBT films is still not clear. It is assumed to be that competition occurs among interstitial Si atoms, interface quality, and ferroelectric polariza- tion-induced bulk charge scattering, etc. Further work will be carried out to clarify the complex mechanisms in affecting the leakage current of the doped SBT films.
CONCLUSIONS
In summary, SBT films with different BSO doping concentrations have been fabricated by chemical solution deposition. XRD measurements indicate that no secondary phase exists for the doped SBT films, which indicates negligible doping effects on the crystalline structure change. The ferroelectric polarization, coercive field, and dielectric constant of the SBT films were found to be significantly reduced through the BSO doping. All the doped SBT films
exhibit a small leakage current, although the leak- age current level was a little higher in the doped films than that in the non-doped film. The reduced ferroelectric polarization and dielectric constant may provide possible applications for the low voltage operation of the future MFIS FETs used in 1T-type FeRAMs.
ACKNOWLEDGEMENT
This work was supported by the National Natural Science Foundation of China (51332006, 61271127).
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Fig. 4. Leakage current characteristics of SBT films with different BSO doping concentration (Color online).
