EffectsofZrionssubstitutiononmicrostructuresandelectricalpropertiesofBi315Nd085Ti3O12films.doc

精品论文effects of zr ions substitution on microstructures and electrical properties of bi3.15nd0.85ti3o12 filmsliao hui a,b, zhong xiangli a,b, liao min a,b, wang jinbin a,b, zhouyichun a,ba key laboratory of low dimensional materials & application technology of ministry ofeducation, xiangtan university, hunan (411105)b faculty of materials, optoelectronics and physics , xiangtan university, hunan (411105)e-mail: abstractthin films of bi3.15nd0.85ti3o12 (bnt) and bi3.15nd0.85ti3-xzrxo12 (bntzx， x=0.1 and 0.2) werefabricated on pt/tio2/sio2/si(100) substrates by a chemical solution deposition (csd) technique. structures and electrical properties of the films were studied as a function of zr ions composition. experimental results indicate that a small amount of zr ions substitution in bnt can improve electrical properties and surface morphology of the film. the double remanent polarization (2pr) androot-mean-square (rms) roughness of bntz0.1 film are 52.7 c/cm2 and 9.461 nm, respectively.keywords: ferroelectric thin films; electrical properties; chemical solution deposition1. introductionwith growing interest in ferroelectric material for non-volatile ferroelectric random access memories (nvferams) applications, many efforts have been done to research and develop these ferroelectric films 1. bismuth layer-structured ferroelectrics (blsfs) have been recognized as the promising film materials for lead-free ferroelectric oxides for nvferams application. as one of few blsfs, bi4ti3o12 (bit) is currently regarded as one of the most promising candidatematerials 2 for nvferams due to its large spontaneous polarization (ps) along the a axis (50c/cm2), low processing temperature, high curie temperature, and lead-free chemical composition properties 3. however, polycrystalline bit film has a reduced remanent polarization (pr) of approximate 10 c/cm2 and it suffers from severe polarization suppression after 106 read/write switching cycles 4. many researchers have tried to achieve enhanced ferroelectric property of thebit film using various techniques. for example, property-design concept by the substitution of each crystal site in the pseudoperovskite of the bit thin film, i.e., site-engineering concept, was proposed by funakubo 5. and the fatigue-free films with excellent ferroelectric properties obtained by the substitution of the a-site ions in bit film by rare earth ions have been received great attention 6-9. noted that chon et al. reported nd3+ substituted bit bi3.15nd0.85ti3o12 (bnt)has a larger 2pr than that of other a-site substituted bit 6-9. according to the reported results,however, for the a-site substituted bit film, the coercive field (2ec) usually becomes larger than that of pure bit film. recent studies revealed that b-site substitution in bit film by some ions such as zr4+, mo6+, and w6+ could effectively decrease 2ec10, 11. these imply the fabrication of both a- and b-site cosubstituted bit for the balance of 2pr and 2ec. in former work our team has found that zr4+ substitution in bnt film can improve ferroelectric properties and decrease the leakage current effectively 12. in order to further optimize the process, thin films of bnt and bi3.15nd0.85ti3-xzrxo12 (bntzx, x=0.1 and 0.2) were prepared and the effect of the zr ions content about their structural, morphological and electrical properties were investigated in this paper.2. experimentthe bnt and bntzx ferroelectric thin films were fabricated on pt/tio2/sio2/si(100) substrates by a chemical solution deposition (csd) technique. bismuth nitrate, neodymium acetate, zirconium propoxide, and titanium butoxide were used as precursor materials. glacial- 6 -acetic acid and 2-methoxyethanol were used as solvent. 10% excess of bismuth nitrate was added to compensate for possible bismuth loss during the high temperature process. the precursor solutions were spin coated at a rate of 4000 rpm for 40 s, followed by a drying process at 180 c for 5 minutes and a pyrolysis process at 400 c for 5 min. these processes were repeated six times. the resultant films were annealed for 10 min in air by a rapid thermal annealing process at 700 c. the structures of the films were characterized by x-ray diffraction (xrd) using a d8 advance x-ray diffractometer with cu k radiation. the surface morphology and the root-mean-square (rms) roughness of the films were evaluated using atomic force microscope (afm). in order to measure the electrical properties, pt dot electrodes with diameter of 200 m were deposited on the top surface of the films by dc sputtering through a shadow mask. the ferroelectric properties and leakage current behaviors of the films were measured using a radiant technologies precision workstation ferroelectric test system.3. results and discussionsfigure 1 shows the xrd patterns of bnt and bntzx films on pt/ti/sio2/si (100) substrates. all the peaks are indexed according to the standard powder diffraction data of bit. the films were entirely crystallized into layered perovskite structure and no secondary phase formed. the position of diffraction peaks of bnt and bntzx is in agreement with that of standard diffractionpeaks of bit indicating that a-site nd3+ and b-site zr4+ substitutions do not affect the layeredstructure.fig. 1 x-ray diffraction patterns of the bntzx (x=0, 0.1, and 0.2) filmsthe surface morphology and roughness of the films were investigated using afm as shown in fig 2. afm micrographs on the 10.0 m10.0 m scale of bntzx (x=0, 0.1, and 0.2) thin films evidence that the bntzx (x=0, 0.1, and 0.2) films are composed of fine grains of about 150-250 nm in diameter and the films surfaces are dense and crack free. fig 2. shows that the root-mean-square (rms) roughness of the bnt, bntz0.1 and bntz0.2 thin films are 9.765, 9.461 and 10.972 nm, respectively. the surface roughness of these films shows a dependence of zr ions content. this is due to its more stable valence compared to ti4+ ions. the substitution of ti4+ ions by zr4+ ions can decrease the concentration of oxygen vacancies, so it can make more nucleation cores which can get a smaller root-mean-square (rms) roughness value 13. however, excessive zr4+ ions doping can destroy the lattice structure partially and then make the surface of the filmrougher.fig. 2 afm micrographs of the bntzx (x=0, 0.1, and 0.2) films: (a) bnt, (b) bntz0.1, (c) bntz0.2. (d) root-mean-square (rms) roughness value of the bntzx (x=0, 0.1, and 0.2) filmsthe magnitude of dc leakage current is usually one of the most concerned factors for nvferam application of ferroelectric thin films due to its direct relation to power consumption and function failure of devices 14. figure 3 shows leakage current properties as a function of dc voltage for bntzx (x=0, 0.1, and 0.2) films. from fig. 3, it can be seen that the leakage currentsof the bntz0.1 and bntz0.2 films are lower than 10-7 a at the lower voltage region than 4 v andthe leakage current of the bntz0.2 film is smaller nearly four orders of magnitude than that of bnt film at high voltage region (higher than 4 v). in order to analyze the leakage conduction mechanisms of these films, the log (i)/log (v) curves of the bntzx (x=0, 0.1, and 0.2) films are plotted and fitted in fig. 4. the leakage curves are divided into several regions with different slopes and the slopes () (= log (i)/log (v) of each region are fitted as shown in fig. 4. from fig.4, we can see that leakage curves of the bnt, bntz0.1 and bntz0.2 films are divided into 3 regions, 2 regions and 1 region, respectively. all i-v curves can be modeled in terms of sclc 15,16. these i-v curves clearly exhibits different slope region. in region the slope is close to about1.0 from 0 to 2.69 v for the bntz0.1 film, 0-5 v for the bntz0.2 film, but the slope of the bntfilm is 3.14943, which is larger than these of the bntz0.1 and bntz0.2 films. it is followed by a transition region (region ) with a larger slope: 1.96-2.45 v for the bnt film, 3.06-5.00 v for thebntz0.1 film. and the slopes are 21.768 and 9.771 for the bnt and bntz0.1 film, respectively. at region (the voltage higher than 2.45 v) the slope of the bnt film becomes 4.7249. theseslopes can be understood as follow: region i follows ohms law for the ratio of log (i)/log (v) near1.0; region ii corresponds to accomplishment of trap-filling process, and displays a sharp increase of the slopes of leakage curve; region iii corresponds to trap-free square law with self-blockage of charge carriers 16-18.the leakage current of the bnt film decreases with the increase of the zr4+ ions content, andthe leakage current of the bntz0.2 film is smaller nearly four orders of magnitude than that of bnt film at high voltage region (higher than 3 v). moreover number of regions divided in each leakage curve of the bntzx (x=0, 0.1, and 0.2) films are different. changes of the leakage curves can be all attributed to the decrease of the ti3+ ions. it can be explained as follows. the conduction mechanism of bit based films is related to the hopping of electrons between ti4+ ionsand ti3+ ions when oxygen vacancies are present in the lattice, which act as a “bridge” betweenti4+ ions and ti3+ ions and play an important role in the electronic conduction. the valence of zr4+ ions is more stable than that of ti4+ ions. the conduction by the electron hopping between ti4+ ions and ti3+ ions can be depressed by the substitution of zr4+ ions for ti4+ ions because zr4+ ions would block the path between two adjacent ti ions and enlarge hopping distance. therefore, the leakage current will reduce with the substitution of ti4+ ions by zr4+ ions.fig. 3 leakage current characteristics of the bntzx (x=0, 0.1, and 0.2) filmsfig. 4 the plots of log (i) vs log (v) of the bntzx (x=0, 0.1, and 0.2) filmsfig. 5 shows the typical ferroelectric polarization-electric field (p-e) hysteresis loops of thebntzx (x=0, 0.1, and 0.2) films. the bntz0。1 film has the largest 2pr of 52.7 c/cm2, and the2pr values are 25.2 and 9.3 c/cm2 for bnt and bntz0.2 films. the 2pr increases with the increase of zr content in the bntzx films (x=0.0-0.1), which might be attributed to the latticedistortion induced by zr doping 19 and more smooth surface as shown in fig. 3. however, when the zr content increase to 0.20, the 2pr value decreases sharply, which may be caused by the reason that excessive doping can destroy the lattice structure partially and induce more rough film surface.fig. 5 p-e hysteresis loops of the bntzx (x=0, 0.1, and 0.2) films4. conclusionsin summary, bnt and bntzx (x = 0.1, and 0.2) thin films were fabricated on pt/ti/sio2/si(100) substrates by csd technique. the leakage current decreases with the increase of the zr+4 ions content and at high voltage region (higher than 3 v) the leakage current of thebntz0.2 film is smaller nearly four orders of magnitude than that of the bnt film. and the results also demonstrate that appropriate zr4+ ions substitution for bnt can improve hysteresis characteristics and surface morphology. the 2pr and rms roughness of bntz0.1 film are 52.7c/cm2 and 9.461 nm, respectively.acknowledgethis work was financially supported by the national natural science foundation of china (nos. 10525211, 10802072 and 50772093), the cultivation fund of the key scientific and technical innovation project, ministry of education of china (no. 076044), and the specialized research fund for the doctoral program of higher education (no. 20070530010).references1 x. h. wang and h. ishiwara, appl. phys. lett. 82, 2479 (2003). 2 a. i. kingon, nature (london). 401, 658 (1999).3 w. li, j. gu, c. h