Browsing by Author "Joshi, Pooran C."

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    Chemically modified Ta₂O₅ thin films for dynamic random access memory (DRAM) applications
    Desu, Chandra S. (Virginia Tech, 1998-07-17)
    Increasing demand for high-density memories has necessitated the search for new materials with higher dielectric constants to satisfy the minimum charge storage density requirements. Several materials such as Ta₂O₅, BST¹, BBT² are being investigated to replace the currently used Si based oxide/nitride dielectrics. Among the materials under investigation, Ta₂O₅ is one of the most promising, especially from the fab compatibility point of view. Ta₂O₅ thin films offer a six-fold increase in dielectric constant compared to conventional dielectrics. However, the significant improvement in dielectric constant is offset by higher leakage currents compared to conventional dielectrics. Improvement in both, dielectric and insulating properties is required for the successful integration of Ta₂O₅ thin films into devices. In the current research work, it was demonstrated that by chemically modifying the tantalum pentoxide matrix, significant improvements in its electrical properties can be achieved which would enable the fabrication of a reliable high-density memory device. In the present work, the effects of Al addition on Ta₂O₅ thin films were systematically studied. The structural and electrical properties of these chemically modified thin films were investigated in detail to establish their potential for device applications. The effects on dielectric and insulating characteristics due to incorporation of Al in Ta₂O₅ matrix were studied in capacitor configuration. A metallorganic solution decomposition (MOSD) technique was used to deposit thin films onto Pt coated Si(100) substrates. The capacitors were fabricated by sputter depositing Pt electrodes on the top surface of the films. The dielectric and insulating properties of pure and modified Ta₂O₅ thin films and their dependence on film composition, processing temperature, and the thickness were discussed and an attempt was made to provide theoretical understanding for the experimental observations. The dielectric and insulating properties of Ta₂O₅ were found to be significantly modified by addition of Al. It was observed that Al addition has decreased the leakage currents approximately by an order of magnitude and improved thermal and bias stability characteristics of Ta₂O₅ capacitors. For example, the leakage currents in crystalline pure Ta₂O₅ thin films were found to be 4.5 x 10⁷ A/cm² in a 1MV/cm dc field which decreased to 3.4 x 10⁸ A/cm² for 10% Al modified Ta₂O₅ thin films. A typical dielectric constant of 42.5 was obtained for 10% Al modified Ta₂O₅ thin films. This is significantly higher compared to the commonly reported dielectric constant of 25 to 35 for Ta₂O₅ thin films. This enhancement was attributed to strong (100) orientation exhibited by both pure and modified Ta₂O₅ thin films. The high dielectric constant, low dielectric loss, low leakage currents and low temperature coefficient of capacitance suggest the suitability of Al modified Ta₂O₅ as a capacitor dielectric for future generation DRAM applications. ¹Barium strontium titanate, ²Barium bismuth tantalate
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    Highly oriented ferroelectric CaBi2Nb2O9 thin films deposited on Si(100) by pulsed laser deposition
    Desu, Seshu B.; Cho, H. S.; Joshi, Pooran C. (AIP Publishing, 1997-03-01)
    We report the successful deposition of highly c-axis oriented CaBi2Nb2O9 (CBN) thin films directly on p-type Si(100) substrates by pulsed laser deposition. The CBN thin films exhibited good structural, dielectric, and CBN/Si interface characteristics. The electrical measurements were conducted on CBN thin films in a metal-ferroelectric-semiconductor (MFS) capacitor configuration. The typical measured small signal dielectric constant and the dissipation factor at a frequency of 100 kHz were 80 and 0.051, respectively. The leakage current of the MFS capacitor structure was governed by the Schottky barrier conduction mechanism and the leakage current density was lower than 10(-7)A/cm(2) at an applied electric field of 100 kV/cm. The capacitance-voltage measurements on MFS capacitors established good ferroelectric polarization switching characteristics. (C) 1997 American Institute of Physics.
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    Infrared activity in the Aurivillius layered ferroelectric SrBi2Ta2O9
    Moret, M. P.; Zallen, Richard H.; Newnham, R. E.; Joshi, Pooran C.; Desu, Seshu B. (American Physical Society, 1998-03-01)
    Experimental studies were carried out on infrared-active phonons in the Aurivillius ferroelectric SrBi2Ta2O9 (SBT), using reflectivity measurements (down to 200 cm(-1)) and transmission measurements (down to 100 cm(-1)) on crystals, pellets, and thin films. An analysis was done of the contrasting consequences of the competing orthorhombic and pseudotetragonal symmetries in SBT. Reflectivity results for light polarized in the ab plane show that the anisotropy in this plane is small in the frequency range from 300 to 1200 cm(-1),, indicating the influence of the approximate tetragonal symmetry. Dielectric dispersion properties were derived for this polarization (E perpendicular to c) in this frequency range. The transverse-optical (TO) and longitudinal-optical (LO) frequencies corresponding to the dominant E perpendicular to c band are 613 and 773 cm(-1), respectively. This strong, high-frequency band arises from a mode dominated by the motion of the oxygen sublattice; its TO and LO frequencies yield an oxygen-ion Szigeti effective charge of -1.5e. Frequency estimates for the TO (LO) pairs of other strong bands are 188(330) and 334(451) cm(-1) for E perpendicular to c, and 610(675) and 780(815) cm(-1) for E parallel to c. In addition to the main infrared bands, the main Raman bands of SBT are also reported.
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    Low temperature processed 0.7SrBi(2)Ta(2)O(9)-0.3Bi(3)TaTiO(9) thin films fabricated on multilayer electrode-barrier structure for high-density ferroelectric memories
    Ryu, S. O.; Joshi, Pooran C.; Desu, Seshu B. (AIP Publishing, 1999-10-01)
    Thin films of solid-solution material 0.7SrBi(2)Ta(2)O(9)-0.3Bi(3)TaTiO(9) (0.7SBT-0.3BTT) were fabricated on n(+)-polycrystalline (n(+)-poly) Si substrates by a metalorganic solution deposition technique at a low processing temperature of 650 degrees C using a Pt-Rh/Pt-Rh-O-x electrode-barrier structure. The Pt-Rh/Pt-Rh-O-x structure was deposited using an in situ reactive radio frequency sputtering process. The electrodes had a smooth and fine-grained microstructure and were excellent diffusion barriers between the 0.7SBT-0.3BTT thin film and Si substrate. The ferroelectric (0.7SBT-0.3BTT) test capacitors using these electrode-barrier grown directly on Si showed good ferroelectric hysteresis properties, measured through n(+)-poly Si substrate, with 2P(r) and E-c values of 11.5 mu C/cm(2) and 80 kV/cm, respectively, at an applied electric field of 200 kV/cm. The films exhibited good fatigue characteristics (< 10% decay) under bipolar stressing up to 10(11) switching cycles and the leakage current density was lower 10(-7) A/cm(2) at an applied electric field of 200 kV/cm. The good ferroelectric properties of 0.7SBT-0.3BTT solid-solution thin films at a low processing temperature of 650 degrees C and excellent electrode-diffusion barrier properties of a Pt-Rh/Pt-Rh-O-x structure are encouraging for the realization of high-density nonvolatile ferroelectric random access memories on silicon substrates. (C) 1999 American Institute of Physics. [S0003-6951(99)01240-1].
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    Process-Induced Degradation during the Integration of Pb(Zr/x Ti/1-x)O3 Ferroelectric Capacitors
    Lee, June Key (Virginia Tech, 1999-09-10)
    Three types of major process-induced damage which hampers the realization of FRAM (ferroelectric random access memory) device are investigated; dry etching induced damage, hydrogen-induced degradation, and stress effect. Since ferroelectric capacitors utilize the movement of body-centered atoms in perovskite structure, Ti or Zr in the case of Pb(Zr/x Ti/1-x)O³ (PZT), the movement can be suppressed or inhibited by many factors such as space charges, defects, chemical reactions, and stress of stacked layers. Unlike conventional silicon processes, the integration of ferroelectric capacitor module requires high density plasma to pattern their shapes because of a low volatility of etched byproducts, therefore the degradation of ferroelectric capacitor performance could occur by the collision of high energetic particles. The damage of PZT thin film due to dry etching process was characterized in terms of the microstructure and electrical properties. The damaged layer seems to be amorphous and the thickness is about 10 nm. The existence of such a layer in Pt/ PZT/Pt ferroelectric capacitor tends to increase the coercive voltage and the leakage current. The damaged layer was not fully reverted to perovskite phase by the thermal annealing, even at PZT formation temperature. For the elimination of this damaged layer, a novel wet cleaning solution was designed. Scanning electron microscopy (SEM) pictures clearly show that treatment with the cleaning solution completely removed the etching damaged layer. With the cleaning solution, a sidewall cleaning process and a surface cleaning process were proposed to eliminate non-ferroelectric phases such as pyrochlore, PbO, and etching damaged layer. After removing the non-ferroelectric phases, ferroelectric properties such as remnant polarization, coercive voltage, and leakage current were remarkably improved. In addition, the wet cleaned ferroelectric capacitors yielded superior endurance against hydrogen-induced damage compared to those of the non-cleaned capacitors. Several parameters such as Zr/Ti compositional ratios, excess amounts of Pb, the domain poling state, and electrode structures (Pt/PZT/Pt and Ir/IrO₂/PZT/Pt/IrO₂) were investigated in terms of hydrogen-induce degradation. It was found that the hydrogen-induce degradation is enhanced when PZT films have high compositions of Ti and Pb, and can be suppressed by domain poling prior to the hydrogen anneal. From the SIMS analysis and hysteresis loop shifts, it can be concluded that the hydrogen damage occurs mainly at the PZT/electrode interface and results in the development of negative charge buildup. To reduce the hydrogen-induced damage, an electron cyclotron resonance (ECR) oxygen plasma treatment of the Pt/PZT/Pt capacitor was attempted. It was found that oxygen plasma treatment modifies the surface of Pt electrodes. Surface modification alleviates catalytic activity of Pt electrodes, thereby significantly improving ferroelectric properties such as remnant polarization and leakage current. It seems that highly reactive oxygen radicals in ECR plasma play an important role in suppressing the catalytic activity of Pt electrodes. The cause of the blister formation on the PECVD (plasma enhanced chemical vapor deposition) SiO2/Pt/PZT/Pt capacitor was studied by means of annealing in various ambient. The blisters were observed at a temperature of 325°C in an O₂ atmosphere, while in a N2 and an Ar atmosphere blisters were not produced even at 500°C. Hydrogen evolution analysis from PECVD SiO2 layer showed a sharp peak near 320°C. The results indicate that the accumulation of water vapor pressure, developed via a chemical reaction between oxygen and hydrogen could be the dominant factor for blister formation in PECVD SiO₂/Pt/PZT/Pt capacitors. The effect of stress was investigated with two different interlayer dielectric (ILD) materials, ECR CVD Oxide and PECVD TEOS Oxide (PE-TEOS). Since the stress of PZT capacitor strongly depends on the ILD deposition temperature, the PZT capacitor with PE-TEOS showed more compressive stress than that with ECR oxide, which results in severe remnant polarization (Pr) degradation of PZT capacitor with PE-TEOS. This large stress effect of PE-TEOS was confirmed by measuring d-spacing values of (111) PZT films with XRD technique. These results suggest that the low ILD deposition temperature is a key parameter for achieving an ILD integration with a minimal Pr degradation.
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    Properties of Ba(Mg1/3Ta2/3)O-3 thin films prepared by metalorganic solution deposition technique for microwave applications
    Joshi, Pooran C.; Desu, Seshu B. (AIP Publishing, 1998-08-01)
    We report on the properties of Ba(Mg1/3Ta2/3)O-3 thin films prepared by the metalorganic solution deposition technique. Bulk Ba(Mg1/3Ta2/3)O-3 ceramics have shown excellent dielectric properties at microwave frequencies; however, the high sintering temperature of bulk material is the major obstacle in their use as dielectric resonators to miniaturize microwave circuits. It was possible to obtain an ordered-perovskitz phase of 0.3-mu m-thick Ba(Mg1/3Ta2/3)O-3 films with trigonal symmetry at an annealing temperature of 700 degrees C, which is much lower than the bulk sintering temperatures. The electrical measurements were conducted on Pt/Ba(Mg1/3Ta2/3)O-3/Pt capacitors. The typical measured small signal dielectric constant and dissipation factor at 100 kHz were 22.2 and 0.009, respectively. The dielectric constant of thin films was comparable to the typical values (epsilon(r) similar to 23.5-25) reported for bulk ceramics. The temperature coefficient of capacitance was -145 ppm/degrees C in the measured temperature range of 25-125 degrees C. The leakage current density was lower than 10(-7) A/cm(2) at an applied electric field of 0.5 MV/cm. The high dielectric constant, which is comparable to bulk, low dielectric loss, and good temperature and bias stability suggest the suitability of Ba(Mg1/3Ta2/3)O-3 thin films for microwave communications and integrated capacitor applications. (C) 1998 American Institute of Physics. [S0003-6951(98)01931-7].
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    Properties of SrBi2Ta2O9 ferroelectric thin films prepared by a modified metalorganic solution deposition technique
    Joshi, Pooran C.; Ryu, S. O.; Zhang, X.; Desu, Seshu B. (AIP Publishing, 1997-03-01)
    Polycrystalline SrBi2Ti2O9 thin films having a layered-perovskite structure were fabricated by a modified metalorganic solution deposition technique using room temperature processed alkoxide-carboxylate precursor solution. It was possible to obtain a complete perovskite phase at an annealing temperature of 650 degrees C and no pyrochlore phase was observed even up to 600 degrees C. In addition, the SrBi2Ta2O9 thin films annealed at 750 degrees C exhibited better structural, dielectric, and ferroelectric properties than those reported by previous techniques. The effects of postdeposition annealing on the structural, dielectric, and ferroelectric properties were analyzed. The electrical measurements were conducted on Pt/SrBi2Ta2O9/Pt capacitors. The typical measured small signal dielectric constant and dissipation factor at 100 kHz were 330 and 0.023 and the remanent polarization and the coercive field were 8.6 mu C/cm(2) and 23 kV/cm, respectively, for 0.25-mu m-thick films annealed at 750 degrees C. The leakage current density was lower than 10(-8) A/cm(2) at an applied electric Weld of 150 kV/cm. The films showed good switching endurance under bipolar stressing at least up to 10(10) switching cycles. (C) 1997 American Institute of Physics.
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    Structural and electrical characteristics of rapid thermally processed ferroelectric Bi4Ti3O12 thin films prepared by metalorganic solution deposition technique
    Joshi, Pooran C.; Desu, Seshu B. (American Institute of Physics, 1996-08-15)
    Polycrystalline Bi4Ti3O12 thin films having layered perovskite structure were fabricated by metalorganic solution deposition technique on both Pt-coated Si and bare Si substrates at a temperature as low as 500 degrees C. The effects of post-deposition rapid thermal annealing on the structural and electrical properties were analyzed. The electrical measurements were conducted on metal-ferroelectric-metal capacitors. The typical measured small signal dielectric constant and dissipation factor at 100 kHz were 184 and 0.018 and the remanent polarization and the coercive field were 4.4 mu C/cm(2) and 84 kV/cm, respectively. The films exhibited high resistivity in the range 10(8)-10(12) Omega cm for films annealed at temperatures of 500-700 degrees C for 10 s. The I-V characteristics were found to be Ohmic at low fields and space-charge-limited at high fields. A V-3/2 dependence of the current was observed in the space-charge region. This could be explained by assuming the mobility to be field dependent since in thin films the electric fields are invariably high even at low applied voltages. (C) 1996 American Institute of Physics.
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    Structural and electrical properties of crystalline (1-x)Ta2O5-xAl(2)O(3) thin films fabricated by metalorganic solution deposition technique
    Joshi, Pooran C.; Stowell, S.; Desu, Seshu B. (AIP Publishing, 1997-09-01)
    Polycrystalline (1-x)Ta2O5-xAl(2)O(3) thin films were fabricated by metalorganic solution deposition technique on Pt-coated Si substrate at a temperature of 750 degrees C. Thin films with 0.9Ta(2)O(5)-0.1Al(2)O(3) composition exhibited improved dielectric and insulating properties compared to Ta2O5 thin films. The measured small signal dielectric constant and dissipation factor at 100 kHz were 42.8 and 0.005, respectively. The temperature coefficient of capacitance was 20 ppm/degrees C in the measured temperature range of 25-125 degrees C. The leakage current density was lower than 6 x 10(-8) A/cm(2) up to an applied electric field of 1 MV/cm. A charge storage density of 18.9 fC/mu m(2) was obtained at an applied electric field of 0.5 MV/cm. The high dielectric constant, low dielectric loss, low leakage current density. and good temperature and bias stability suggest (1-x)Ta2O5-xAl(2)O(3) thin films to be a suitable dielectric layer in integrated electronic devices in place of conventional dielectrics such as SiO2 or Si3N4. (C) 1997 American Institute of Physics.
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    Synthesis and Characterization of Ferroelectric (1-x)SrBi2Ta2O9-xBi3TaTiO9 thin films for Non-volatile memory Applications
    Ryu, Sang-Ouk (Virginia Tech, 1999-05-03)
    The (1-x)SrBi2Ta2O9-xBi3TaTiO9 thin films fabricated by modified metalorganic solution deposition technique showed much improved properties compared to SrBi2Ta2O9: a leading candidate material for memory applications. A pyrochlore free crystalline phase was obtained at a low annealing temperature of 600 oC and grain size was found to be considerably increased for the (1-x)SrBi2Ta2O9-xBi3TaTiO9 compositions. The film properties were found to be strongly dependent on the composition and annealing temperatures. The measured dielectric constant of the thin films was in the range 180-225 for films with 10-50 mol % of Bi3TaTiO9 content in the solid solution. Ferroelectric properties of (1-x)SrBi2Ta2O9-xBi3TaTiO9 thin films were significantly improved compared to SrBi2Ta2O9. For example, the observed 2Pr and Ec values for films with 0.7SrBi2Ta2O9-0.3Bi3TaTiO9 composition, annealed at 650 oC, were 12.4 micro C/cm2 and 80 kV/cm, respectively. The solid solution thin films showed less than 5 % decay of the polarization charge after 10^10 switching cycles and good memory retention characteristics after about 10^6 s of memory retention. The size and temperature effect of 0.7SrBi2Ta2O9-0.3Bi3TaTiO9 thin films were studied by determining how the ferroelectric properties vary with film thickness and temperature. It was found that the ferroelectric properties were determined by the grain size, and not by the thickness of the film in our studied thickness range of 80-350 nm. A 80 nm thick film showed good ferroelectric properties similar to the 350 nm thick film. Thermal stability of the 0.7SrBi2Ta2O9-0.3Bi3TaTiO9 thin film was found to be much better compared to the SrBi2Ta2O9 and Pb(Zr,Ti)O9 thin films due to its higher Curie temperature and lower Schottky activation energy according to temperature changes. Also, 0.7SrBi2Ta2O9-0.3Bi3TaTiO9 thin films has shown good ferroelectric properties on multilayer system such as PtRh/PtRhOx/poly-Si suggest their suitability for high density FRAM applications.
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    Thin films of layered-structure (1-x)SrBi2Ta2O9-xBi(3)Ti(Ta1-yNby)O-9 solid solution for ferroelectric random access memory devices
    Desu, Seshu B.; Joshi, Pooran C.; Zhang, X.; Ryu, S. O. (AIP Publishing, 1997-08-01)
    We report on the thin films of solid-solution material (1-x)SrBi2Ta2O9-xBi(3)Ti(Ta1-yNby)O-9 fabricated by a modified metalorganic solution deposition technique for ferroelectric random access memory devices. Using the modified technique, it was possible to obtain the pyrochlore free crystalline thin films at an annealing temperature as low as 600 degrees C. The solid-solution of layered perovskite materials helped us to significantly improve the ferroelectric properties, higher P-r and higher T-c, compared to SrBi2Ta2O9; a leading candidate material for memory applications. For example, the films with 0.7 SrBi2Ta2O9-0.3Bi(3)TiTaO(9) composition and annealed in the temperature range 650-750 degrees C exhibited 2 P-r and E-c values in the range 12.4-27.8 mu C/cm(2) and 68-80 kV/cm, respectively. The leakage current density was lower than 10(-8) A/cm(2) at an applied electric field of 200 kV/cm. The films exhibited good fatigue characteristics under bipolar stressing. (C) 1997 American Institute of Physics.