S. Sinharoy

PULSED LASER DEPOSITION AND FERROELECTRIC CHARACTERIZATION OF BISMUTH TITANATE FILMS

Stoichiometric films of bismuth titanate, Bi4Ti3O12, have been grown for the first time by the technique of pulsed excimer laser deposition. Ferroelectric films were obtained at temperatures as low as 500 °C on Si(100), MgO(110), and Pt‐coated Si(100) substrates. Hysteresis measurements using a Pt‐coated Si sample yielded a saturation polarization value of about 28 μC/cm2, consistent with a randomly oriented titanate film structure. A preliminary metal‐insulator‐semiconductor sandwich structure of the form Bi4Ti3O12‐CaF2(100 A)‐Si was grown and used to examine polarization induced memory switching effects.

Surface stability of NbN single-crystal films

The surfaces of NbN(100) and NbN(111) single‐crystal films were analyzed as a function of annealing temperature in ultrahigh vacuum by low‐energy electron diffraction and x‐ray photoelectron spectroscopy. The NbN(100) surface changed from a (1×1) to a ((2)1/2×(2)1/2)45° structure at 900 °C accompanied by a loss of nitrogen. The NbN(111) surface, initially richer in nitrogen, transformed from a (1×1) to a (2×2) structure at 800 °C with an even greater loss of nitrogen from the surface. Increasing the carbon content of the films increased the temperatures at which reconstructions were observed. Structural models of the surfaces are presented that are consistent with these observations. At higher temperatures (up to 1100 °C), additional nitrogen was lost from films of both orientations. Measurements of superconducting transition temperatures and energy gaps indicated that the entire film was nitrogen deficient after annealing, with the greatest nitrogen loss at the surface.

Growth and the microstructural and ferroelectric characterization of oriented BaMgF/sub 4/ thin films

The growth of ferroelectric BaMgF/sub 4/ thin films on Si

Epitaxial growth of LaF3 on GaAs(111)

We report the first epitaxial growth of lanthanum trifluoride on the (111) surface of gallium arsenide. Smooth, crack‐free, and high‐crystalline quality films of thickness up to 200 nm were grown at 500 °C on a GaAs(111) surface that was cleaned by ion heat treatment and post‐anneal. The film surfaces were examined in situ by low‐energy electron diffraction (LEED) and ex situ by reflection high‐energy electron diffraction (RHEED) and Nomarski optical microscopy. Capacitance‐voltage (C‐V) and current‐voltage (I‐V) measurements showed a breakdown strength of 2×106 V/cm and no significant flat‐band shift due to insulator charges.

Surface morphology of epitaxial CaF2 films on GaAs(100)

The surface morphology of CaF2 films grown epitaxially on (100) GaAs substrates in an ultrahigh vacuum system is examined using reflection high‐energy electron diffraction, scanning electron microscopy, and replica transmission electron microscopy. Proper substrate surface preparation and film deposition techniques are critical for obtaining epitaxial films which are crack free and smooth. We have demonstrated that group II‐A fluoride films which have a barely perceptible surface texture can be grown onto (100) oriented substrates.

Pulsed Laser Deposition (PLD) of Oriented Bismuth Titanate Films for Integrated Electronic Applications

Abstract : In this paper we describe recent successes of growth of epitaxial bismuth titanate (BTO) films by pulsed laser deposition (PLD) suitable for electro-optic and electrical switching device structures, and fabrication of an improved gate structure for a ferroelectric memory FET (FEMFET). TEM and x-ray results indicate that excellent crystalline quality BTO films were achieved on LaAlO3. Polarization switching was demonstrated for BTO capacitors with epitaxial superconducting YBaCu3Oy as the lower electrode. Using an SiO2 buffer layer, a BTO/Si structure was fabricated and direct charge modulation in the Si by polarization reversal in the BTO was demonstrated.

Growth and characterization of epitaxial silicon on heteroepitaxial CaF2/Si(111) structures

We report the epitaxial growth of silicon on a CaF2/Si(111) heteroepitaxial structure. The results show that contrary to previous reports, the room‐temperature predeposition of a very thin layer of silicon does not significantly affect the problem of calcium migration to the top surface of the silicon film, although it appears to improve the surface morphology of the film. Planar and cross‐sectional transmission electron microscope and x‐ray diffraction studies have shown that the silicon film, although single crystalline, is highly defective, the main defects being twins on both the inclined {111} planes and the parallel (111) planes.

Growth and characterization of lattice‐matched CaxSr1−xF2 on GaAs(100)

The epitaxial growth and characterization of the structural, morphological, interfacial, and electrical properties of lattice‐matched Ca0.43Sr0.57F2 on GaAs(100) are reported here. The ∼200 nm films were grown at 500 °C on the GaAs(100) substrates that had been cleaned by annealing at 600 °C. Nomarski optical microscopy, transmission electron microscopy (TEM), scanning electron microscopy, and reflection high‐energy electron diffraction results showed that the films were smooth and crackfree, with good crystalline quality. TEM showed evidence of two general types of extended defects in the film which occur at or near the film/substrate interface region. Film resistivity was found to be (3.6±0.75)×1013 Ω cm, and the breakdown field strength was (5.8±1.4)×105 V/cm. However, the breakdown in this case represented a nondestructive conduction process, as opposed to the catastrophic and irreversible change observed in previous studies. The dielectric constant of the films was measured to be 7.43±0.22. Capacitan...

UHV Processing of Ferroelectric Barium Magnesium Fluoride Films and Devices

Barium magnesium fluoride (BaMgF4) has recently emerged as a strong candidate for application as the gate dielectric in ferroelectric random access memory (FERRAM) devices with nondestructive readout (NDRO). In earlier papers we reported the successful growth of oriented BaMgF4 films on Si(100) and other substrates in a ultrahigh vacuum (UHV) system, as well as the results of the structural and electrical characterization of these ferroelectric films. In the present paper, we review some of the earlier results, and also examine the effect of variations in the growth temperature and various post-growth anneals on the stoichiometry, crystallinity, orientation, and electrical characteristics of the BaMgF4 films. Initial attempts at integrating the ferroelectric field-effect transistor (FEMFET) with the standard CMOS VLSIC processing, as well as the effect of adding a thin capping layer of SiO2 on the BaMgF4 will also be described.

BaMgF4 thin film development and processing for ferroelectric FETS

Abstract A ferroelectric memory field-effect transistor (FEMFET) where a ferroelectric thin film is incorporated directly into the gate structure of the transistor is attractive, because it provides not only nonvolatility, but also nondestructive readout (NDRO). At Westinghouse, we are currently developing a FEMFET using thin film barium magnesium fluoride (BaMgF4), a ferroelectric material that was discovered in 1969, but was not fabricated in thin film form until 1989. The BaMgF4 films are grown by evaporation in an ultrahigh vacuum (UHV) chamber on clean Si(100). The natural tendency of these films to grow with the ferroelectric a-axis in the Si(100) plane has been overcome to obtain more random orientation with larger reversible polarization perpendicular to the film. A capping layer (SiO2) has been found to be essential for process integrability of these BaMgF4 films. Ti-W metallization produced only a slight reduction in the capacitance-voltage (C-V) memory window. Switching speed of these films has...