M. H. Francombe

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.

Exchange interactions in quantum well subbands

It is shown that exchange interactions in the two‐dimensional electron gas in quantum wells could cause observable effects on subband energies and intersubband transition energies. In the case of doped quantum wells, the intrasubband exchange interaction can produce an energy shift which is substantially larger than the direct Coulomb energy shift. Theoretical estimates of such shifts are compared with experimental measurements of the infrared photoconductivity of multiple quantum well AlGaAs/GaAs structures with wells doped at about 1018 cm−3.

Optical switching characteristics of epitaxial bismuth titanate films for matrix-addressed displays

Abstract Electro-optic switching properties of (010) epitaxial Bi4Ti3O12 films grown on (110) MgAl2O4 substrates have been investigated. The measurements were carried out on large-area films in situ on the spinel substrate using an inter-digitated electrode array. A contrast ratio higher than 7:1 was obtained. Switching times of less than 5 μsec were measured at fields >60 kV/cm. Due to substrate clamping the field needed for switching the epitaxial film was about seven times higher than that for bulk single crystal at a same switching speed. The films exhibit an apparent switching threshold and grey scale capability. Finally, an experimental X-Y matrix-addressed display has been designed and evaluated.

Ferroelectric films and their device applications

Abstract Recent reseach on ferroelectric films is discussed, with emphasis on preparative problems, optimization of film structure and of electrical properties and application in devices. Rapid progress has been made in each of these research areas and this fact, together with the emergence of certain new materials such as polyvinylidene fluoride (PVF 2 ), is increasing the variety of possible applications to solid state devices. Typical examples considered here include microwave capacitors, thermistor bolometers, pyroelectrics, piezoelectric transducers, memories and optical display devices. The present state of the art for these applications and materials development needed for further progress are reviewed.

Indium antimonide‐bismuth compositions grown by molecular beam epitaxy

Thin films of InSb‐InBi solid solutions have been prepared by molecular beam epitaxy. Using in situ reflection electron diffraction, conditions for epitaxial growth of stoichiometric layers were established on (001) and (110) surfaces of both InSb and GaAs wafers. Bi is shown to modify the diffraction patterns of (001) InSb from C(8×2) and (√2×√2) 45° to (1×3). Surface residence times of Bi were found indefinitely long (≳10 min) at temperatures ?420 °C. Bi incorporation into InSb during growth by molecular beam epitaxy is strongly dependent on Bi surface concentration, and influenced by substrate temperature and surface nonstoichiometry. Secondary ion mass spectrometry depth profiling and 2.5 MeV ion dechanneling spectra showed that ∼3% Bi can be incorporated substitutionally in Sb sites, under In rich growth conditions (largest available concentration of VSb sites). Increased Bi surface accumulation and interstitial incorporation are observed under Sb‐rich surface conditions and as the relative flux of B...

Domain structure and polarization reversal in films of ferroelectric bismuth titanate

A preliminary study has been made of the structure, optical properties and polarization reversal behavior of epitaxial bismuth titanate films grown by sputtering on MgO substrates. As grown, the films are twinned on a coarse scale, with alternate regions oriented with the monoclinic (100) and (010) plane parallel to the (110) MgO surface. Optical birefringence measurements on these regions yielded Δna and Δnb values as a function of temperature which were in general agreement with those measured for bulk crystals. By annealing film sections detached from the substrate, a single orientation of the type (010) was obtained, and regions of such samples could be poled to a single-domain condition. Hysteresis data, with the field applied along the a and c axes, for single domain regions yielded spontaneous polarization results comparable to those reported previously for bulk crystals, but indicated somewhat higher coercive fields. Similar measurements with the field along the [101] axis showed that it is possib...

Electro‐optic contrast observations in single‐domain epitaxial films of bismuth titanate

Electro‐optic switching has successfully been demonstrated on untwinned (010) epitaxial bismuth titanate films grown on (110) MgAl2O4 substrates. This result is achieved on large‐area films using an interdigitated electrode array structure which can be used both for electrical poling and subsequent electro‐optic address. A contrast ratio higher than 7:1 between the on and off states is obtained with a 10‐μm‐thick film. A gradual variation of the contrast ratio with switching field is observed, which gives the film a capability for grey scale.

Thin film and bulk structures of phases in the system gold-aluminum

Abstract Novel epitaxial film techniques have been employed to grow homogeneous products of the compounds AuAl, Au2Al, Au5Al2 and Au4Al, and using electron and X-ray diffraction methods the crystal symmetries and lattice parameters of the first three phases have for the first time been determined. Comparison of the results with X-ray data for bulk samples show that except for Au2Al the structures of the thin film phases are the same as those assumed by bulk crystals of the same nominal composition. Interdiffusion between epitaxial Al/Au couples has been examined and it is shown that the initial epitaxial reaction product adopts a f.c.c. cubic structure (a = 6.4 A ) not previously observed in studies of the AuAl phase diagram. This structure appears to be a metastable Au-rich phase which on further reaction with Al transforms mainly into epitaxial AuAl2. High-temperature electron diffraction studies have also been made in order to establish the orientation relationships involved in the β ⇄ β′ transformation for Au4Al.

GaAs/AlGaAs superlattice miniband detector with 14.5 μm peak response

Extended long wavelength infrared detection with a miniband‐type AlGaAs/GaAs superlattice structure is reported. The experimental response band of the detector is peaked near 14.5 μm in good agreement with the theoretical response, provided that electron‐electron interactions are taken into account. The detector operates at a low bias voltage, which could lead to important advantages in application to IR focal plane arrays.

Long-wavelength infrared detection in a Kastalsky-type superlattice structure

The first successful demonstration of long‐wavelength infrared (LWIR) detection with a Kastalsky‐type AlGaAs/GaAs superlattice structure is reported. The experimental response band of the detector is centered near 10 μm in very good agreement with the theoretical response band provided that electron‐electron interactions are taken into account. The detector operates at significantly lower bias voltage than photoconductive multiple quantum well LWIR detectors. This could lead to important advantages in applications to photovoltaic detector arrays. The response at 83 K is about 50% of the response at 24 K. Optimization of the response, operating temperature, or bias voltage has not been carried out.