La-doped (sm) Bi <sub> 2 </ Sub> Ti <sub> 2 </ Sub> O <sub> 7 </ Sub> Thin Films And Their Performance

With the quick development of microelectronics, the size of semiconductor devices is becoming smaller and smaller according to the 'Moore Law'. SiO₂, as the present gate dielectric, can't satisfy this trend in the coming several years. Thus the research of high-k gate dielectric materials for next generation MOSFET (metal-oxide-semiconductor field effect transistor) applications has become a focus in the field of present microelectronic materials. All of the High-k materials must meet a set of criteria to perform as successful gate dielectric. A summary of the appropriate materials properties for the selection of materials for gate dielectric applications: (1) high permittivity and barrier height; (2) thermodynamic stability on Si; (3) film morphology; (4) good interface quality; (5) gate compatibility; (6) process compatibility; (7) good reliability, etc. A material, which satisfies all of these considerations, has yet to be determined, but the material of Bi₂Ti₂O₇ has been identified as a promising candidate.In the bismuth titanate series, there are several phases, including Bi₂Ti₄O₁₁, Bi₂Ti₂O₇, Bi₄Ti₃O₁₂, Bi₈TiO_<sub>14 and Bi₁₂TiO₂₀, et al. Among them, Bi₄Ti₃O₁₂ has been more studied than others. However, the phase of Bi₂Ti₂O₇ in the bismuth titanate series has been studied a few. Polycrystalline thin films of composition Bi₂Ti₂O₇ have high permittivity and low leakage current density. High permittivity dielectrics are widely investigated as alternative gate insulating layers in advanced MOS (metal-oxide-semiconductor) transistors. It also been used successfully as a buffer layer to improve the electrical properties of ferroelectric PZT, PST and Bi₄Ti₃O₁₂ thin films. Though it has the above advantages, Bi₂Ti₂O₇ thin films can be easily transformed into other phases, and its phase becomes unstable at high temperature. It is accepted that the chemical stability would be heightened after substituting Bi atoms with some La atoms during the preparation of ceramics. Many references have reported the preparation and properties of Lanthanum-substituted Bi₄Ti₃O₁₂ thin films.However, we have not seen a report about La-modified Bi2Ti2O7 thin films. In view of above depiction, a small amount of impurity is important to tailor physical properties and microstructures of Bi2H2O7 materials. In this paper, we report the preparation, phase stability and electrical properties of La(Sm)-doped Bi2Ti2(>7 thin films.There are many methods to prepare thin films, such as metal organic chemical vapor deposition, sol-gel, pulsed laser deposition, metal-organic decomposition method, molecular beam epitaxy and chemical solution decomposition (CSD), etc. With the development and improvement of fabrication methods, ferroelectric thin films have received great attention for the applications in many electronic or electro-optic devices. Compared with other techniques, CSD method has the advantages of simplifying process, better homogeneity, lower cost and easy control of stoichiometry.In this paper, we reported the preparation of synthesis of Bi2Ti2O7 thin films with Lanthanum (or samarium)-substitutions on Si [100] substrates and studied phase stability and electrical properties. The dissertation mainly includes:The (Bio.8Lao.2)2Ti207 thin films were successfully prepared on [100] Si substrates by chemical solution decomposition technique. The influence of precursor solution's concentration on crystalline temperature, crystal grain size, structure and electrical properties of thin films have been studied. It is found that the higher the precursor solution's concentration, the more instability of thepyrochlore structure. The sample of 0.2mol/Lhas transformed completely from pyrochlore phase into perovskite phase annealed at 800°C. However, the pyrochlore phase of 0.1 mol/L was still stable at the annealing temperature of 850°C. The SEM micrographs showed that the lower concentration sample was easier to gain large grain size. The average grain size of 0.1 mol/L sample wasabout 150 nm. We also found that the concentration of precursor solution had an obvious effect on leakage current and dielectric constant. At the same conditions, the higher the concentration, the bigger leakage current and the lower dielectricconstant. As to the film of 0.1 mol/L annealed at 800°C, the leakage current was 0.21 X 10"10A (bias voltage was 2V) and the dielectric constant was 136(bias voltage was -4V). At the same conditions, the values of the O.lmol/L sample were 0.82 X 10"10A and 88, respectively. The above results showed that the O.lmol/L films had good insulating properties and high dielectric constant, whichindicated that the film is suitable for new insulating gate materials in dynamic random access memory (DRAM).Under the condition of same concentration of precursor solution, we prepared La-doped (Bii.ILax)2Ti2O7(x=0.5, 0.2, 0.1, 0.075, 0.05, 0.03) (BLTO) thin films with different La contents by CSD method. All the XRD patterns of the samples showed that the films were polycrystalline films. The intensities of the peaks decreased with the increasing of La contents. Some electrical properties were also studied. The general trend of the changes of leakage current was decreased with the increasing of x. The leakage current density of BLTO (x=0.2) was the lowest. Atthe applied electric field of 100 kV/cm, the value of leakage current density is 0.47X \0'% AJcm2 at 700°C. The case of dielectric constant as a function of La contentis complicated. When x=0.03, 0.075, 0.2, dielectric constants were increased with the temperature increasing. But when x=0.1, the rule of change was reverse. In view of dielectric constant and dielectric loss, we think that the film of x=0.2 has relative low leakage current and high dielectric constant, which is considered to be adequate for a DRAM.As to the film of (BiagLao^^Oz [BLTO(0.2)], we have studied more about it. XRD patterns indicated that BLTO(0.2) was effective for the chemical stability of the Bi2Ti2C>7 phase at a higher annealing temperature. A possible explanation of the chemical stability has been sought from the viewpoint of the charge compensation. The oxygen ions near the Bi^are unstable due to the volatility of the Bi ions. We prepared BLTO(0.2) films, where some of the Bi ions in the Bi2Ti2O7 films are substituted with La ions in order to eliminate the oxygen vacancies andobtain charge neutrality. The grain size of the BLTO(0.2) film was calculated by Scherrer formula and SEM micrograph. The value was about 33.75 nm and 25-40 ran, respectively.XPS studies were carried out for BLTO(0.2) and Bi2Ti207 thin films. Compared with Bi2Ti2O7thin film, it could be observed clearly that the additional peak intensity of BLTO was lower than that of Bi2Ti207. This difference may be understood in terms of the finding that the stronger additional peak intensity is, the more Bi-0 bonding breakage and oxygen dissociation there are, and the more oxygen vacancies there are. This finding supports our assumption about phase stability of BLTO thin films against oxygen vacancies after substituting Bi atoms with some La atoms. The BLTO(0.2) powders annealed at different temperatures were characterized by IR spectrum. The result showed that we could gain steady pyrochlore phase of BLTO(0.2) at 750°C. The band gap of BLTO(0.2) material was estimated to about 3.14 eV from the onset of UV-vis diffuse reflectance spectrum, which is higher than pure Bi2Ti207. The band gap absorption edge of BLTO(0.2) is determined to be 395 nm.The BLTO(0.2) film annealed at 800°C showed good insulating property and high resistance to breakdown. The leakage current density versus applied voltage with different film thickness showed that the films with lower thickness had good electric properties. Under following conditions as 750°C annealing temperature, 100 kHz frequency and -3V applied bias voltage, the value of dielectric constant,loss factor and fixed charge density is 278,0.07 and 4.24 X 101 * cm², respectively.The BLTO(0.2) film had not only the higher dielectric constant, but also the lower loss factor at the function of frequency between 2 100 kHz.Sm-doped Bi2Ti207 (BSTO) thin films had also prepared under the same qualifications with BLTO(0.2). XRD patterns showed that during the forming of (Bio.8Smo.2)2Ti207 phase, some (BiasSmo^TiaOn unstable phases were appeared. Therefore the chemical stability of BLTO was steadier than BSTO. The BSTO thin film's surface micrograph was studied by SEM. The film was free of microcracksand had a dense, smooth specular appearance. The size of the crystal grains was about 30nm. The band gap energy value for BSTO was 3.02eV observed by a UV-vis recording spectre-photometer, which was lower than that of BLTO. At the annealing temperature between 500 and 700°C, the leakage current density of BSTO film increased with the annealing temperature increasing. But at 800°C, the leakage current density decreased to the lowest. Dielectric constant as a function of annealing temperature was different from the leakage current. At the annealing temperature 600°C and 700°C, the dielectric constant was 208 and 207, respectively. However at 500°C and 800°C, the value was 146 and 168, respectively. The content above indicated that BSTO not only had good insulating properties and resistance to breakdown, but also had high dielectric constant, although BSTO has more volatile than BLTO.In summary, according to the above results, we think that the La (Sm)-doped Bi2Ti207 thin films were successfully prepared by chemical solution decomposition technique. These films have steadier chemical stability, lower leakage current and higher dielectric constant, which are considered to be as a High-k candidate material and play an imporment part as for a DRAM.