Uong Chon

Fatigue-free samarium-modified bismuth titanate (Bi4−xSmxTi3O12) film capacitors having large spontaneous polarizations

Fatigue-free and highly c-axis oriented Bi3.15Sm0.85Ti3O12 (BSmT) thin films were grown on Pt/TiO2/SiO2/Si(100) substrates using the method of metalorganic sol decomposition. The BSmT film capacitor with a top Pt electrode showed significantly improved values of the remanent polarization (2Pr) and the nonvolatile charge as compared to those of the Bi4−xLaxTi3O12 (x=0.75) film capacitor, recently known as the most promising candidate for nonvolatile memories. The 2Pr value of the BSmT capacitor was 49 μC/cm2 at an applied voltage of 10 V while the net nonvolatile switching charge was as high as 20 μC/cm2 and remained essentially constant up to 4.5×1010 read/write switching cycles at a frequency of 1 MHz. In addition to these, the capacitor demonstrated excellent charge–retention characteristics with its sensing margin of 17 μC/cm2 and a strong resistance against the imprinting failure.

Fatigue-free behavior of highly oriented Bi3.25La0.75Ti3O12 thin films grown on Pt/Ti/SiO2/Si(100) by metalorganic solution decomposition

Fatigue-free and highly c-axis oriented Bi3.25La0.75Ti3O12 (BLT) thin films were grown on Pt/Ti/SiO2/Si(100) substrates using metalorganic solution decomposition. Films annealed above 500 °C were characterized by strong c-axis preferential growth with an in-plane alignment of grains. The BLT film capacitors with a Pt top electrode showed excellent ferroelectric properties. The remanent polarization (2Pr) and the coercive field (Ec) were in the range of 26–28 μC/cm2and 50–75 kV/cm, respectively. More importantly, the BLT capacitors did not show any significant fatigue up to 3.5×1010 read/write switching cycles at a frequency of 1 MHz.

Ferroelectric properties and crystal structure of praseodymium-modified bismuth titanate

Fatigue-free and highly c-axis oriented Bi4−xPrxTi3O12 (BPrT, x=0.85) thin films were grown on Pt/TiO2/SiO2/Si (100) substrates using the method of metalorganic sol decomposition. The BPrT film capacitor with a top Pt electrode showed significantly improved values of the remanent polarization (Pr) and the nonvolatile charge as compared to those of the Bi4−xLaxTi3O12 (x=0.75) film capacitor, recently known as a promising candidate for nonvolatile memories. The 2Pr value of the BPrT capacitor was 40 μC/cm2 at an applied voltage of 10 V while the net nonvolatile charge was as high as 20 μC/cm2 and remained essentially constant up to 4.5×1010 read/write switching cycles at a frequency of 1 MHz. In addition to these, the capacitor showed excellent charge-retention characteristics with its sensing margin of 16 μC/cm2 and a strong resistance against the imprinting failure.

Formation and characteristics of highly c -axis-oriented Bi 3.25 La 0.75 Ti 3 O 12 thin films on SiO 2 /Si(100) and Pt/Ti/SiO 2 /Si(100) substrates

Highly c -axis-oriented Bi 3.25 La 0.75 Ti 3 O 12 (BLT) films with a homogeneous in-plane orientation were successfully grown on SiO 2 /Si(100) and Pt/Ti/SiO 2 /Si(100) substrates by a sol-coating route. The substitution of lanthanum ions for bismuth ions in the layered perovskite suppressed the formation of pyrochlore phase and enhanced the c -axis-oriented growth. The c -axis-oriented BLT film fabricated on a Pt/Ti/SiO 2 /Si(100) substrate showed fatigue-free characteristics with a large remanent polarization of 26–28 μC/cm 2 and the coercive field of 50–75 kV/cm. These features significantly enhance the potential value of the BLT film for the applications to high-density ferroelectric random-access memories devices. In addition, the c -axis-oriented BLT film, with a homogeneous in-plane orientation on an amorphous surface, can be used as a suitable template material for applications to various electro-magneto-optic devices.

Degradation mechanism of ferroelectric properties in Pt/Bi4-xLaxTi3O12/Pt capacitors during forming gas annealing

Degradation mechanism of ferroelectric properties in the Pt/Bi4-xLaxTi3O12/Pt (Pt/BLT/Pt) capacitors during forming gas annealing (FGA) was systematically investigated by examining ferroelectric responses and spatial distributions of relevant species using secondary ion mass spectrometry. It was shown that the degradation of ferroelectric properties during FGA was not originated from the oxygen loss induced by a reducing atmosphere but was mainly caused by protons catalytically dissociated from molecular hydrogen (H2) by the top Pt electrode. The following sequential mechanism has been identified from the present study: (i) the adsorption and dissociation of H2 to produce protons and electrons by the top Pt electrode, (ii) the columnar penetration of protons into the BLT film, accelerated by the region of negatively charged Bi-site vacancies near the bottom electrode, and (iii) the decomposition of perovskite phase after FGA at 400 °C.

Recovery process of degraded ferroelectric properties in the forming-gas-annealed Pt/Bi4-xLaxTi3O12/Pt capacitor

The recovery of ferroelectric properties in the forming-gas-annealed Pt/Bi4−xLaxTi3O12/Pt (Pt/BLT/Pt) capacitor was studied by examining changes in ferroelectric responses, phase evolution, and spatial distributions of relevant species during the recovery annealing. The degraded ferroelectric properties were practically restored to their original values after the recovery annealing at 600 °C for 10 min in an O2 atmosphere. The following recovery process has been delineated from the present study: (i) the removal of impregnated protons from the degraded capacitor due to the chemical potential difference of protons between the forming-gas-annealed capacitor and the contacting atmosphere, and (ii) the restoration of perovskite BLT phase with the help of replenishment of the Bi and oxygen losses via diffusion from the neighboring intact region to the Bi-depleted columnar region located beneath the top Pt electrode.

Ferroelectric properties of highly c-axis oriented Bi4−xLaxTi3O12 film-based capacitors

Abstract Highly c -axis oriented lanthanum-modified bismuth titanate (Bi 4− x La x Ti 3 O 12 ) films having a variety of lanthanum (La) contents were grown on Pt/TiO 2 /SiO 2 /Si(100) substrates using metal-organic sol deposition and subsequent annealing at 650 °C for 1 h. After systematically examining the ferroelectric properties of Bi 4− x La x Ti 3 O 12 films as a function of the La-content, it was concluded that the film with x =0.85 had the largest remanent polarization in the direction parallel to the c -axis. The Pt/Bi 3.15 La 0.85 Ti 3 O 12 /Pt capacitor showed a well-saturated polarization–electric field (P–E) switching curve with the switching remanent polarization (2 P r ) value of 33 μC/cm 2 and the coercive field ( E c ) of 68 kV/cm at an applied voltage of 10 V. More importantly, the capacitor exhibited fatigue-free behavior up to 6.5×10 10 read/write switching cycles at a frequency of 1 MHz. The capacitor also demonstrated an excellent charge-retaining ability and a strong resistance against the imprinting failure.

Lead-Free Layered Perovskite Film Capacitor for Ferroelectric Random Access Memory

Lead-free Bi 3.15 Sm 0.85 Ti 3 O 12 (BSmT) thin films were grown on Pt/TiO 2 /SiO 2 /Si(100) substrates using the method of metal-organic sol decomposition. The BSmT film capacitor with a top Pt electrode showed significantly improved values of the remanent polarization (2P r ) and the nonvolatile charge as compared to those of the Bi 4-x La x Ti 3 O 12 (BLT; x=0.75) film capacitor, recently known as the most promising candidate for nonvolatile memories. 2P r value of the BSmT capacitor was 52 μC/cm 2 at an applied voltage of 12 V while the net nonvolatile switching charge was as high as 20 μC/cm 2 and remained essentially constant up to 4.5×10 10 read/write switching cycles at a frequency of 1 MHz. In addition to these, the capacitor demonstrated excellent charge-retention characteristics with its sensing margin of 17 μC/cm 2 and a strong resistance against the imprinting failure.