Structural and electrical characterization of epitaxial, large area ferroelectric films of Ba 2 Bi 4 Ti 5 O 18 grown by pulsed excimer laser ablation

Transcription

1 JOURNAL OF APPLIED PHYSICS VOLUME 87, NUMBER 6 15 MARCH 2000 Structural and electrical characterization of epitaxial, large area ferroelectric films of Ba 2 Bi 4 Ti 5 O 18 grown by pulsed excimer laser ablation A. R. James, a) A. Pignolet, D. Hesse, and U. Gösele Max Planck Institute of Microstructure Physics, Weinberg 2, D Halle, Germany Received 8 September 1999; accepted for publication 7 December 1999 Excimer laser ablation was used to deposit epitaxial thin films of Ba 2 Bi 4 Ti 5 O 18 having mostly c-axis oriented grains. The ferroelectric layer was deposited on an epitaxial conducting oxide electrode layer of LaNiO 3, the electrode layer in turn being grown on an epitaxial buffer layer stack on single crystalline 100 -oriented silicon wafers of 3 in. diameter. X-ray diffraction measurements indicate a strong c-axis orientation of the ferroelectric layer. Scanning electron microscopy and atomic force microscopy were used to observe the microstructure of the films and these revealed the homogenous quality of the film with good uniformity over the wafers. Well-defined, saturated ferroelectric hysteresis loops were observed in the compound with values of 1.3 and 0.6 C/cm 2 as saturation and remnant polarization values, respectively, and a coercive field of 47 kv/cm. The results of I V measurements and complex impedance spectroscopy measurements are presented and a defect formula is proposed for the compound American Institute of Physics. S I. INTRODUCTION Recent years have seen extensive research being directed towards the integration of ferroelectric materials into semiconductor technology. Ferroelectric capacitors have already been successfully integrated into silicon-based integrated circuits to be used as nonvolatile memories. 1,2 The ferroelectric field-effect transistor in which the ferroelectric oxide plays the role of a gate is by far the simplest architecture for a ferroelectric nonvolatile memory. The development of sophisticated film-synthesis methods providing high quality films, along with ongoing efforts to find new or alternative solutions for computer memories, make ferroelectric memories viable alternatives to presently existing technologies. Owing to their hysteretic behavior, faster switching speeds, and lower operating voltages, ferroelectric thin films are finding applications in new generation data storage systems. Ferroelectric lead zirconate titanate PZT has been by far the most extensively studied material in this respect, especially near the morphotropic phase boundary. 3 Despite the fact that films of PZT show high remnant polarization and excellent square hysteresis loops, they suffer from severe fatigue problems. Fatigue, a phenomenon which leads to the decrease in switchable polarization reversals, is attributed to the trapping of vacancies at the trap sites and their subsequent diffusion towards the electrodes, where they form a nonswitching layer caused by the low conductivity of such films. This feature is especially undesirable when taking into account the possibility of using a material as a nonvolatile random access memory device, wherein each read-write operation involves a switching of polarization. The switchable polarization in PZT is commonly suppressed after the ferroelectric capacitor has been subjected to repeated polarization a Present address: Dr. A.J. James, 212 Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802; electronic mail arj3@psu.edu reversals typically of the order of 10 8 cycles or more. In view of the aforementioned problems, alternative materials were sought after among which, bismuth-layer structured ferroelectrics BLSFs show promising behavior. SrBi 2 Ta 2 O 9 is one such BLSF which shows excellent fatigue endurance and it has been shown that this due the easy release of trapped oxygen vacancies. 4,5 BLSFs are characterized by the general formula (Bi 2 O 2 ) 2 (A n 1 B n O 3n 1 ) 2 where n is the number of pseudoperovskite-like oxygen octahedra sandwiched between the Bi 2 O 2 layers, and the high fatigue resistance of these oxides seems to be related to their crystal structure. The authors have been involved in the growth of epitaxial films belonging to these Aurivillius phases. Films with varying number of layers from 2 to 4 have been grown, viz. SrBi 2 Ta 2 O 9, Bi 4 Ti 3 O 12, BaBi 4 Ti 4 O 15 and these have been studied with a view to correlate the microstructure with the electrical properties of the films so grown. 6,7 In the present study, a new five-layered compound Ba 2 Bi 4 Ti 5 O 18 hereafter referred to as B2BiT has been synthesized using pulsed laser deposition PLD on large area wafers of Si, 3 in. in diameter; details of the experimental set up are given in earlier articles. 8,9 This article in the first part reports in detail the method of synthesis and in the second the results of extensive microstructure and electrical property measurements. Impedance spectroscopy measurements were performed in order to observe dielectric relaxations in the sample. II. EXPERIMENT Epitaxial films of B2BiT were grown by PLD using a KrF excimer laser ( 248 nm). These films were deposited on Si 100 wafers using a stack of epitaxial buffer layers YSZ/CeO 2 /LNO, YSZ-yttria stabilized zirconia, i.e., ZrO 2 with 5 10 mole % of Y 2 O 3, LNO LaNiO 3 ). The YSZ and CeO 2 layers were deposited at a frequency of 10 Hz and an energy of 450 mj, and the LNO layer at a frequency of 5 Hz, and 300 mj LNO, a conductive oxide, was used as a bottom /2000/87(6)/2825/5/$ American Institute of Physics

2 2826 J. Appl. Phys., Vol. 87, No. 6, 15 March 2000 James et al. electrode. Epitaxial layers of B2BiT were grown at a temperature of 675 C in an oxygen ambient of 100 mtorr. The sample was cooled to room temperature in the presence of oxygen. Both the target and substrate were rotated using an off-axis deposition technique 8 during the deposition process, resulting in homogenous films. The thickness of each of the layers was measured using a Tencor P-2 long scan profilometer and it was found that the ferroelectric layer had a thickness of about 300 nm. X-ray diffraction was used to verify the formation of the crystalline phase of the films as well as to have an insight into the quality of epitaxy of the films so grown. This was done by using a Philips X Pert MRD four circle diffractometer. Basically, 2 scans were made between 10 and 70, at a scanning rate of 1 /min, and pole figure scans scan performed. The microstructure of the films was probed by scanning electron microscopy SEM using a JEOL 6400 scanning electron microscope and scanning probe microscopy Dimension In order to study the ferroelectric nature of the films, a RT 66A hysteresis loop tracer was used to obtain hysteresis loops in the virtual ground mode. Time dependent dielectric breakdown and I V characteristics of these films were also studied. This was done using a Keithley high-impedance multimeter Model 6517 interfaced to a computer through an IEEE-488 interface bus. These measurements were carried out using a planar capacitor structure configuration. Impedance data were acquired with an HP 4192 A impedance analyser. Complex impedance spectroscopy CIS was used in order to gain a deeper insight into the electrical characteristics of the films under investigation. Some theoretical aspects related to CIS are discussed in subsequent sections of this article. III. RESULTS AND DISCUSSION The quality of out-of-plane orientation of different layers forming the heterostructures, namely YSZ, CeO 2, and LNO were optimized by analyzing the width of selected peaks in the 2 scan x-ray measurement. Figure 1 a shows the x-ray 2 diffractogram of the film with the respective Miller indices. It was found that the B2BiT film had excellent c-axis orientation. A similar orientation of other BLSF films was found in an earlier work. 8 The buffer layers showed high epitaxial quality, too. Figure 1 b shows an x-ray diffraction pole figure scan of an epitaxial Ba 2 Bi 4 Ti 5 O 18 film performed with the B2BiT reflection ( ). The positions of the four large peaks ( 62 ; 0, 90, 180, 270 ) clearly demonstrate the perfect in-plane alignment of the c-oriented Ba 2 Bi 4 Ti 5 O 18 film. The peak in the middle ( 0) corresponds to Ag 111 ( ) and originates from the 111 -oriented grains of the silver electrode used previously for some electrical measurements. Also, the inner set of four very weak peaks at 35 stems from the silver electrode, viz. from its 110 -oriented grains. The outer set of four very weak peaks at 51.6 indicates the presence of a very low amount of a-oriented Ba 2 Bi 4 Ti 5 O 18 regions in the films. It has to be stressed that these peaks are not seen, if the FIG. 1. a X-ray diffraction pattern 2 scan of the film deposited on an epitaxial LNO electrode layer, on an epitaxial CeO 2 /YSZ buffer layer stack onasi 100 substrate. The spectrum shows (00l) peaks of B2BiT revealing the good epitaxial quality out-of-plane orientation of the film. Indices prefixed with L, C, or Y correspond to YSZ, CeO 2, and LNO, respectively. Those without any prefix correspond to B2BiT peaks. b X-ray diffraction pole figure scan of an epitaxial Ba 2 Bi 4 Ti 5 O 18 film performed with the Ba2BiT peak ( ). Count rate given in logarithmic scale. ranges from zero midpoint to 90 edge of the figure. count rate is shown in linear scale, so that the share of a orientation is very small indeed. Transmission electron microscopy and atomic force microscopy AFM investigations are in progress to identify the a-oriented grains. As the scanning electron micrograph of Fig. 2 shows, the films consist of more or less rectangular platelets of about 0.5 m in lateral size. The edges of most of these platelets are oriented along two mutually perpendicular directions. From the figure it is also seen that the films are uniform over large areas and have a rather flat surface. Long narrow cracks seen as dark lines in the figure extend along the two directions given by the edges of most of the platelets. On the surface of the film, a few particles show up white areas on the figure, which have diameters between 0.5 and 1 m. AFM images were taken on different parts of the samples. Figures 3 a and 3 b show a rather homogenous

3 J. Appl. Phys., Vol. 87, No. 6, 15 March 2000 James et al FIG. 2. Scanning electron micrograph showing rectangular platelets, edges of the platelets are seen oriented along two mutually perpendicular directions. FIG. 4. Macroscopic ferroelectric hysteresis loop of an epitaxially grown film on an epitaxial heterostructure of LNO/CeO 2 /YSZ/Si(100). topography of the films at different magnifications. The film consists of grains platelets with rather uniform morphology. Both SEM and AFM images obviously point to a rather good in-plane epitaxy, with the edges of the platelets mostly being well oriented. FIG. 3. a and b Atomic force microscope images at different magnifications indicating the uniform platelet morphology of the films. In order to establish the presence of ferroelectricity, ferroelectric hysteresis loop measurements were carried out on the films using a MFM capacitor geometry with a contact area of cm 2. A typical hysteresis loop traced is shown in Fig. 4. The values of P s and P r were found to be 1.3 and 0.6 C/cm 2, respectively, and the coercive field was found to be 47 kv/cm, which compare rather well with tabulated values. 10 Having established ferroelectricity in the compound on the basis of the observation of the hysteresis loops, further measurements were carried out, in order to elicit more information regarding the electrical properties of this sample. For many applications involving ferroelectrics, it is desirable that the current flowing through the gate insulating film is as low as possible. A high leakage current would mean that the data stored in the capacitor cell are easily lost. It is therefore desirable to understand the mechanism underlying the conduction process and if possible, further control the current conduction. As electronic devices become more and more miniaturized, the electric field across these devices becomes increasingly high. Therefore, it is also important to understand the high field conduction through these devices. In general, the current through a film is related to the voltage through the equation I V, 1 where is an exponent. If is 1, then the behavior may be ascribed to ohmic conduction. Figure 5 shows an I V curve obtained on the sample at room temperature. Basically, the experiment consisted in noting the steady-state values of current at each voltage of measurement, i.e., each reading of I was recorded after waiting for a period of 300 s, after which, no significant decay in its value was noted. The conduction to a first approximation may be regarded as ohmic in nature. However, further studies are underway using different field and temperature regimes in order to ascertain the exact conduction mechanism. Frequency explicit and complex plane plots have been utilized for a long time to explain the dielectric behavior and electrical conductivity of a wide range of solid-state materials. Since dielectric measurements are generally concerned

4 2828 J. Appl. Phys., Vol. 87, No. 6, 15 March 2000 James et al. Y t/a, 2 where is the real part of ac conductivity, Y is the real part of the complex admittance, t is the thickness, and A is the area of the film. It can be seen that there is an increase in the values of conductivity with increasing frequency, however, there is not much difference in the values of conductivity as a function of changing voltage. The dependence of conductivity on frequency can be expressed as a power law 11 in the form of the following equation: FIG. 5. I V curve obtained with a delay time of 300 s at room temperature. with the movement of charges, and measuring instruments cannot discriminate between the true dielectric response which does not contain the dc contribution, and the effective dielectric response, dielectric spectroscopy is very often used as a tool to resolve the dc conduction and dielectric phenomena which are essentially distinct and, for the most part, independent processes. 11 CIS offers a large variety of options to present data depending upon the nature of the particular response function chosen. For example, in the present case it was found to be advantageous to plot the frequency dependent conductivity ( ) instead of the dielectric loss. Figure 6 shows a plot of the variation of the real part of ac conductivity at room temperature. Basically, the real part of ac conductivity is computed from the complex admittance, using the formula a n 0, where a is a constant and n is a number actually the slope of the plot of conductivity versus log frequency. The values of ac conductivity were found to be quite high for the present films and were always higher than those for dc conduction dc conductivity was found to be always below the nanoampere range. This is obvious from the earlier formula in which ( ), the ac conductivity, is shown to contain inherently the dc contribution of 0. We propose a defect formula for the compound Ba 2 Bi 4 Ti 5 O 18 in this article on the basis of which we try to explain such values of conductivity observed in this and other analogous compounds. Basically, Ba 2 Bi 4 Ti 5 O 18 can be written as Bi 2 O 3 5 Bi 2/5 Ba 2/5 1/5 TiO 3, where represents an unoccupied site in the A position. Representation of the compound in this manner shows in an explicit fashion that there is an unoccupied site in the A position of the perovskite layer. There is a sharing/ substitution in the A site and, basically, a part of trivalent Bi is being substituted by divalent Ba. Such an aliovalent substitution in which substitutional cations have a lesser charge than the host ion they replace, and are commonly called acceptor impurities results in a point defect in the form of a vacancy. In order to compensate for this charge imbalance, there is a creation of oxygen vacancies generally denoted by the term V 0 ). This is because represents a deficiency of positive charge, and hence, carries an effective negative defect charge, which requires the presence of a positively charged compensating defect, such as an oxygen vacancy or a hole and it is well known that the oxygen vacancies are highly mobile species in a given lattice on account of hopping of charges. 12 It may be pointed out that these vacancies V 0 are generally formed in addition to the ones created thermally, and they are regarded as being responsible for the excellent fatigue resistance of BLSFs in comparison to other well-known systems like PZT, in terms of the easy release of trapped charges. 4,5 The frequency dependent properties of a material can be described by any one or more of four basic immittance functions which are the complex impedance (Z*), the complex admittance (Y*), the complex electric modulus (M*) and the complex permittivity ( *). All these immittance functions are related to each other as follows: Z* Z jz 1/j C 0 *, Y* Y jy j C 0 *, M* M jm 1/ *, 6 7 FIG. 6. ac conductivity vs frequency plot for different voltages. * j 8 where is the angular frequency ( 2 f ), C 0 being the geometrical capacitance and j ( 1). The particular immittance function that is selected by the user is basically

5 J. Appl. Phys., Vol. 87, No. 6, 15 March 2000 James et al FIG. 7. (Y / ) vs frequency plot at room temperature. dependent on the nature of the material being studied, i.e., whether it is an insulator, semiconductor or a conductor. A strongly dispersive behavior at low frequencies in dielectric thin films would generally mean an interfacial effect in the form of a capacitive layer near the electrode which is due to the formation of a Schottky barrier, the latter being less conducting than the bulk material. However, Fig. 7 which is a plot between (Y / ) and the log of frequency does not show a strongly dispersive behavior at low frequencies and, hence, it may be concluded that the Maxwell Wagner interfacial polarization effects are not observable here. However, above 1 MHz, a distinct relaxation process with a maximum near 10 MHz, is observed at all voltages of measurement indicating a strong relaxation even at room temperature. In general whether a relaxation is observable within the experimentally available frequencies is dependent on the strength of the corresponding relaxations in a material and the values of the distribution parameters. Hence, after a careful observation of the curves using various formalisms, the admittance formalism was selected on account of the fact that the resolution of curves was most explicit in this case. IV. CONCLUSIONS Pulsed excimer laser ablation was used to deposit large area 3 in. wafers epitaxial heterostructures comprising an epitaxial bismuth-layer structured ferroelectric overlayer. Lattice matched epitaxial buffer layers were used in order to serve as a template to promote the epitaxial growth of the subsequent upper layers, viz. the epitaxial conductive oxide layer LNO serving the purpose of the bottom electrode for the ferroelectric film. This was done by optimizing the growth conditions for each layer so as to obtain epitaxial layers at each stage. The quality of epitaxy was studied using various techniques such as x-ray diffraction, SEM, and AFM. The ferroelectric layer showed excellent c-axis orientation. The ferroelectric properties of the epitaxially grown Ba 2 Bi 4 Ti 5 O 18 films were studied and a ferroelectric hysteresis recorded. I V measurements indicated that the films were highly resistive and did not break down until 20 V at room temperature. CIS was used as a tool to further characterize these films. With the help of CIS, relaxations in the sample could be observed even at room temperature. A defect formula has been proposed for the present compound on the basis of which the better fatigue resistance characteristics of such films belonging to the family of BLSFs can be explained. ACKNOWLEDGMENTS The authors are grateful to Dr. M. Alexe, Dr. S. Senz, and Dr. C. Schaefer for their help and acknowledge S. Reichelt for her technical contributions during the stage of film deposition. 1 J. F. Scott and C. A. Paz de Araujo, Science 246, T. Sumi et al., Integr. Ferroelectr. 6, H. Hu and S. B. Krupanidhi, J. Mater. Res. 9, T. C. Chen, C. L. Thio, and S. B. Desu, J. Mater. Res. 12, A. R. James, S. Balaji, and S. B. Krupanidhi, Mater. Sci. Eng., B 64, K. M. Satyalakshmi, M. Alexe, A. Pignolet, N. D. Zakharov, C. Harnagea, S. Senz, and D. Hesse, Appl. Phys. Lett. 74, A. Pignolet, K. M. Satyalakshmi, M. Alexe, N. D. Zakharov, C. Harnagea, S. Senz, D. Hesse, and U. Gösele, Integrated Ferroelectrics Proc. ISIF, 1999, Integr. Ferroelectr. 26, A. Pignolet, C. Curran, M. Alexe, S. Senz, D. Hesse, and U. Gösele, Integr. Ferroelectr. 21, A. Pignolet, M. Alexe, K. M. Satyalakshmi, S. Senz, D. Hesse, and U. Gösele, Ferroelectrics 225, Landolt-Börnstein NSIII/16a Spriner, Berlin, 1981, p A. K. Jonscher, Dielectric Relaxation in Solids Chelsea Dielectrics, London, R. Waser and D. M. Smyth, in Ferroelectric Thin Films: Synthesis and Basic Properties, edited by C. Paz de Araujo, J. F. Scott, and G. W. Taylor Gordon and Breach, New York, 1996.