Thickness dependence of dielectric properties in bismuth layer-structured dielectrics

Abstract

1-4 However, the significant drop in the dielectric constant with decreased film thickness, widely known as "size effect," has been reported to become serious. [1][2][3][4] The size effect in thin film dielectrics has made it difficult to design their performance characteristics. There has therefore been a strong motivation to discover size-effect-free highly insulating dielectric materials with a high dielectric constant. In this letter, we propose thin films with c-axis-oriented bismuth layer-structured dielectrics ͑BLDs͒ as candidates for size-effect-free materials.BLDs have a natural superlattice structure along the c axis consisting of two kinds of two-dimensional nanolayers, i.e., a bismuth oxide ͑Bi 2 O 2 ͒ 2+ sheet and a pseudoperovskite block generally described as ͑A m−1 B m O 3m+1 ͒ 2− , where m is the number of BO 6 octahedra in a pseudoperovskite block. Interestingly, BLDs with even m exhibit no ferroelectricity along the c axis. 5,6 In our previous study, we found that c-axis-oriented epitaxial SrBi 2 Ta 2 O 9 ͑SBT, m =2͒ films exhibited no degradation in dielectric constant down to 20 nm in thickness, corresponding to eight unit cells.7 However, their dielectric constant of 55 was obviously not enough to replace the conventional perovskite-structured oxides. We therefore expanded our concept to a higher m of 4, i.e., SrBi 4 Ti 4 O 15 ͑SBTi͒ and CaBi 4 Ti 4 O 15 ͑CBTi͒, to obtain higher dielectric constant and to deepen the understanding of the basic characteristics of c-axis-oriented BLD films. As a result, we found the design concept of their properties and revealed that thin films of c-axis-oriented BLDs are promising candidates for high-density capacitor application. c-axis-oriented epitaxial SBTi and CBTi films were grown at a substrate temperature of 700°C by metal organic chemical vapor deposition. ͑001͒ c -oriented epitaxial SrRuO 3 ͑SRO͒ films having an atomically flat surface grown on ͑001͒STO single crystals were used as substrates. 8 The constituent phase and the orientation were identified by conventional x-ray diffraction ͑XRD͒. The crystal structure was analyzed in detail by XRD using synchrotron radiation ͑SPring-8, BL13XU͒. 9 The dielectric and insulation properties of the films were measured after formation of the top electrodes of 100 m Pt on the film surface followed by postannealing in an electrical furnace at 400°C for 30 min under 1 atm flowing oxygen ambience.a͒ Electronic

Degradation-free dielectric property using bismuth layer-structured dielectrics having natural superlattice structure

“…10) In addition, they also have little temperature dependency: less than 200 ppm/K (within the range −50 to 150°C). It is thus possible to make films with zero electric field dependence of εr by making solid-solution films of SBTi and CBTi.…”

Dielectric properties of BaTiO₃–Bi(Mg_1/2Ti_1/2)O₃ films with preferential crystal orientation

“…These values are significantly smaller than that of BT film, meaning more stable TCC behavior, and then comparable with those of conventional high-temperature materials and advanced thin-film capacitors consisting of bismuthlayer-structured dielectrics (Ca,Sr)Bi 4 Ti 4 O 15 [¾ r = ³200, «TCC« = ³250 ppm/K]. 43)46) The (111)-oriented BTBMT film exhibited slightly higher «TCC« value compared with that of the (100)-oriented film. The larger dependency of dielectric permittivity can arise from the crystal orientation along direction of polar axis, as confirmed in previous works of single crystal BaTiO 3 and bismuth-layer-structured dielectrics.…”

(Invited) New Dielectric Nanomaterials Fabricated from Nanosheet Technique

“…This relationship in perovskite nanosheets is a peculiar feature related to their larger atomic polarizability, and the high-k properties are materials properties inherent to perovskite nanosheets. as well as various perovskite thin films for comparison (13)(14)(15)(16). In the ultrathin region (< , 45 (3) 3-8 (2012) 20 nm), the ε r values of Nb-based perovskite nanosheets are larger than those of the other perovskites.…”