Frequency and dc bias voltage dependent dielectric properties and electrical conductivity of BaTiO3SrTiO3/(SiO2)x nanocomposites

Publisher Summary This chapter describes the reprogrammable nonvolatile semiconductor memory devices. In the early period of development of the digital computer, the ferrite material was accepted as a gift of nature because it made the storing of large amounts of information possible and eased the construction of one of the essential parts in the computer; the memory. The requirement of high speed is met by semiconductor memories, but the low cost requirement is only in the small ones. Hence, at the moment big computers have a multilevel memory structure, in which the part (level) nearest to the CPU is made of semiconductors and the other levels are magnetic systems, such as disc and tape. Ovonics have not yet found application because of their limited number of write–erase operations, and the noncompatibility of the technology of devices and surrounding circuitry. For good charge retention in the dielectric, the dielectric layers between the storage side in the dielectric (traps, metal, or polycrystalline Si) and the outside world (gate or Si) must have high resistance.

Nonvolatile Semiconductor Memories

A capacitor structure for semiconductor devices (1, 2), utilizing a dual or duplex dielectric wherein one dielectric layer (3) is comprised of silicon nitride or aluminum oxide and a second dielectric layer (4) is formed of Ta₂O₅, HfO₂, TiO₂, PbTiO₃, BaTiO₃, CaTiO₃ or SrTiO₃. One electrode (2) is preferably selected from conductive polycrystalline silicon, Ta, and Hf. The other electrode (5) is preferably selected from Al or Au based metals.

Capacitor structure with dual dielectrics

Lead Zirconate Titanate (PZT) thin films have been sputter deposited on p–Si. The patterned PZT films were annealed in a furnace as well as in a rapid thermal annealer. The high frequency characterization of metal–PZT– p– Si capacitors show good Capacitance–Voltage characteristics. The effective dielectric constant of PZT on Si was found to be very low and depends on the thickness of the PZT film and the annealing conditions. The high frequency C–V shows hysteresis which is due to the injection of charges across the interface.

Characteristics of Metal/PZT/p—Si MIS Capacitors

The requirements for stable gate insulators and for dielectrics in MIOS non-volatile memory structures are formulated with the silicon FET technology in mind. A review of the literature on insulating layers pertinent to these applications is presented. It is shown that in addition to SiO2, Si3N4 and Al2O3 only a small number of binary compounds deserves further consideration. To optimize dielectric behavior vitreous materials are be preferred. Since the properties of insulators are so strongly dependent on their crystallographic and chemical constitution the methods of film preparation determine to a considerable extend their characteristics.

Potential of alternate dielectrics for single and composite film FET gate applications

Two MIS versions have been fabricated employing rf‐sputtered dual insulator structures comprised of 1000 A HfO2‐20 A SiO2 and 1000 A SrTiO3‐20 A SiO2. The high‐dielectric‐constant insulators in combination with a 20‐A layer of silicon dioxide permit the transfer of charge by tunneling into traps at the dual‐insulator interface to occur at considerably lower voltages than comparable structures employing silicon nitride or aluminum oxide. It was found that trap density and, hence, the degree of flat‐band voltage shift could be altered by sputtering 50‐A layers of selected materials at the insulator‐SiO2 interface. The devices employing hafnium dioxide show promise in an application as a nonvolatile electrically alterable memory element. Although the strontium titanate devices exhibit a low threshold voltage for onset of charge transfer, the charge retention characteristics are poor.

A. J. Shuskus

Papers

Charge storage characteristics of MIS structures employing dual‐insulator composites of HfO2–SiO2 and SrTiO3–SiO2