Tae Geun Seong

Resistance Random Access Memory Based on a Thin Film of CdS Nanocrystals Prepared via Colloidal Synthesis

We demonstrate that resistance random access memory (RRAM) can be fabricated based on CdS-nanocrystal thin films. A simple drop-drying of the CdS-nanocrystal solution leads to the formation of uniform thin films with controlled thickness. RRAMs with a Ag/Al(2) O(3) /CdS/Pt structure show bipolar switching behavior, with average values of the set voltage (V(Set) ) and reset voltage (V(Reset) ) of 0.15 V and -0.19 V, respectively. The RRAM characteristics are critically influenced by the thickness of the Al(2) O(3) barrier layer, which prevents significant migration of Ag into the CdS layer as revealed by Auger electron spectroscopy (AES). Interestingly, RRAM without an Al(2) O(3) layer (i.e., Ag/CdS/Pt structure) also shows bipolar switching behavior, but the polarity is opposite to that of RRAM with the Al(2) O(3) layer (i.e., Ag/Al(2) O(3) /CdS/Pt structure). The operation of both kinds of devices can be explained by the conventional conductive bridging mechanism. Additionally, we fabricated RRAM devices on Kapton film for potential applications in flexible electronics, and the performance of this RRAM device was comparable to that of RRAMs fabricated on hard silicon substrates. Our results show a new possibility of using chalcogenide nanocrystals for RRAM applications.

Bipolar Resistive Switching Behavior of a Pt/NiO/TiN Device for Nonvolatile Memory Applications

Bipolar resistive switching behavior was observed in a Pt/NiO/TiN device. The device exhibited switching behavior that was stable over 100 cycles and did not degrade after 104 s. An electroforming process was required to obtain these bipolar resistive switching properties, and the conduction behavior of the low resistance state followed Ohms law, indicating that conductive filaments formed during the electroforming process. The conductive filaments consisted of oxygen vacancies and the Pt electrode behaved as an oxygen reservoir. The bipolar resistive switching of the Pt/NiO/TiN device was explained by the generation and annihilation of oxygen vacancies in the filaments.

Resistive switching properties of amorphous Pr0.7Ca0.3MnO3 films grown on indium tin oxide/glass substrate using pulsed laser deposition method

Amorphous Pr0.7Ca0.3MnO3 (APCMO) films, which were grown on indium tin oxide (ITO)/glass at room temperature (RT), were n-type materials. The APCMO/ITO/glass device exhibited an average transparency of 77% in the visible range with a maximum transparency of 84% at a wavelength of 530 nm. The Pt/APCMO/ITO device showed stable bipolar resistive switching behavior over 200 cycles that did not degrade after 105 s at RT. The resistance of the APCMO film decreased in both low- and high-resistance states with increasing device area. The resistive switching behavior of the Pt/APCMO/ITO device can be explained by the trap-charged space-charge-limited current mechanism.

Effect of Oxygen Pressure on the Electrical Properties of

Bi₅Nb₃O₁₅ (B₅N₃) films grown under a low oxygen partial pressure (OP) of 1.7 mtorr showed a high leakage current density of 0.1 A/cm² at 1.0 MV/cm. However, the leakage current density decreased with increasing OP to a minimum of 5.8 times 10^-9 A/cm² for the film grown under 5.1 mtorr due to the decreased number of oxygen vacancies. This film also showed an improved breakdown field of 2.2 MV/cm and a large capacitance density of 24.9 fF /mum². The electrical properties of the film, however, deteriorated with a further increase in OP, which is probably due to the formation of oxygen interstitial ions. Therefore, superior electrical properties for the B₅N₃ film can be obtained by careful control of OP.

\hbox{Bi}_{5} \hbox{Nb}_{3}\hbox{O}_{15}

A composite material with anisotropic microstructure was fabricated by DC electric field. Alumina was used as filler for thermal conductivity and polysiloxane resin was used as matrix. According to the alumina morphology and loading amount, various anisotropic microstructures were assembled. The thermal properties of the composite material were investigated parallel to the direction of the applied electric field accordingly. For plate-like alumina with 20 vol% loading, a thermal conductivity of 0.44 W/mK was achieved. With spherically shaped alumina, with an equal loading of 20 vol%, a higher thermal conductivity of 0.46 W/mK was achieved. The increase in the thermal conductivity of the fabricated alumina composites with anisotropic microstructure was attributed to increased filler-to-filler connectivity.

Films Grown by RF Magnetron Sputtering

Buckling was observed in Bi₅Nb₃O₁₅ (BiNbO) films grown on TiN/SiO₂/Si at 300°C but not in films grown at room temperature and annealed at 350 °C. The 45-nm-thick films showed a high capacitance density and a low dissipation factor of 8.81 fF/¿m² and 0.97% at 100 kHz, respectively, with a low leakage current density of 3.46 nA/cm² at 2 V. The quadratic and linear voltage coefficients of capacitance of this film were 846 ppm/V² and 137 ppm/V, respectively, with a low temperature coefficient of capacitance of 226 ppm/°C at 100 kHz. This suggests that a BiNbO film grown on a TiN/SiO₂/Si substrate is a good candidate material for high-performance metal-insulator-metal capacitors.

Electric field assembled anisotropic alumina composite for thermal dissipation applications

The amorphous Bi(5)Nb(3)O(15) film grown at room temperature under an oxygen-plasma sputtering ambient (BNRT-O(2) film) has a hydrophobic surface with a surface energy of 35.6 mJ m(-2), which is close to that of the orthorhombic pentacene (38 mJ m(-2)), resulting in the formation of a good pentacene layer without the introduction of an additional polymer layer. This film was very flexible, maintaining a high capacitance of 145 nF cm(-2) during and after 10(5) bending cycles with a small curvature radius of 7.5 mm. This film was optically transparent. Furthermore, the flexible, pentacene-based, organic thin-film transistors (OTFTs) fabricated on the poly(ether sulfone) substrate at room temperature using a BNRT-O(2) film as a gate insulator exhibited a promising device performance with a high field effect mobility of 0.5 cm(2) V(-1) s(-1), an on/off current modulation of 10(5), and a small subthreshold slope of 0.2 V decade(-1) under a low operating voltage of -5 V. This device also maintained a high carrier mobility of 0.45 cm(2) V(-1 )s(-1) during the bending with a small curvature radius of 9 mm. Therefore, the BNRT-O(2) film is considered a promising material for the gate insulator of the flexible, pentacene-based OTFT.

Electrical properties of Bi5Nb3O15 thin film grown on TiN/SiO2/Si at room temperature for metal - Insulator - Metal capacitors

Amorphous Bi5Nb3O15 (BNO) films were grown at room temperature (RT) on a Cu/Ti/SiO2 /Si substrate using radio frequency magnetron sputtering. All the films were well formed on the Cu electrode with a sharp interface between the film and the electrode. The dielectric constant of the amorphous BNO film grown under 25 W was 46, with a low dissipation factor of 2.7% at 100 kHz. This film exhibited a low leakage current density of 5.5 × 10-8 A/cm2 at 4.5 V and a large breakdown voltage of 7.2 V. However, the electrical properties deteriorated as the sputtering power and the growth temperature increased due to the increased surface roughness; this was because a film with a rough surface generally has a larger surface area, and there can be electric field intensification at surface asperity, which degrade the electrical properties of the film. In addition, the electrical properties were not influenced by the oxygen partial pressure (OPP) because the variation of OPP during the growth of the films did not affect their surface roughness. The amorphous BNO film grown on the Cu/Ti/SiO2/Si substrate at RT under 25 W may be a good candidate material for an embedded capacitor.

A Flexible Amorphous Bi5Nb3O15 Film for the Gate Insulator of the Low-Voltage Operating Pentacene Thin-Film Transistor Fabricated at Room Temperature

NiO films were grown on a Pt substrate by radio frequency (RF) magnetron sputtering using a NiO ceramic target. A crystalline NiO phase with the [111] preferred orientation was formed for the films grown above 100 °C. Resistance switching behavior was not observed in the NiO films annealed in the air or in ambient O2 after film deposition. However, the NiO films annealed in ambient N2 exhibited resistance switching properties. The stability of the switching voltage was considerably influenced by the oxygen to argon ratio during film growth. In particular, the NiO film grown under an 8.0 mTorr oxygen partial pressure exhibited stabilized switching voltages (Vset~1.45±0.20 V and Vreset~0.62±0.09 V). Therefore, the control of the ambient gas pressure during the growth and annealing of the NiO films was important for obtaining good resistance switching properties.

Microstructure and Electrical Properties of Amorphous Bi 5 Nb 3 O 15 Films Grown on Cu/Ti/SiO 2 /Si Substrates Using RF Magnetron Sputtering

With the addition of TiO2, the dielectric constant (?r) of a Bi5Nb3O15(B5N3) film was slightly increased and the leakage current decreased, probably due to the increased dipole moment and the decreased number of free electrons in the film, respectively. The energy density of the laser beam considerably influenced the structure and electrical properties of the 1.0?mol% TiO2-doped B5N3 (TBN) films. At low beam energy densities (?2.0?J?cm?2), Bi3NbO7 and Bi8Nb18O57 phases with a porous microstructure were formed and a poor interface was also formed between the film and the electrode. However, for the TBN film grown at 200??C at a high beam energy density of 4.0?J?cm?2, a dense Bi3NbO7 phase was formed with a sharp interface. The ?r value of this TBN film was very high, approximately 115, with a low leakage current density of 1.4 ? 10?8?A?cm?2 at 0.5?MV?cm?1 and a high breakdown field of 0.55?MV?cm?1. This improvement in the ?r value and the electrical properties was explained by the formation of a dense Bi3NbO7 phase with a (1?1?1) preferred orientation, Ti doping and a sharp interface, indicating that the TBN film is a good candidate material for embedded capacitors.