The role of grain boundaries in the magnetoresistance ͑MR͒ properties of the manganites has been investigated by comparing the properties of epitaxial and polycrystalline films of La 0.67 D 0.33 MnO 3Ϫ␦ (DϭCa, Sr, or vacancies͒. While the MR in the epitaxial films is strongly peaked near the ferromagnetic transition temperature and is very small at low temperatures, the polycrystalline films show large MR over a wide temperature range down to 5 K. The results are explained in terms of switching of magnetic domains in the grains and disorder-induced canting of Mn spins in the grain-boundary region. ͓S0163-1829͑96͒51746-7͔

We report on the fabrication of a new class of trilayer epitaxial thin film devices based on the doped perovskite manganates La–Ca–Mn–O and La–Sr–Mn–O. We show that large resistance changes, up to a factor of 2, can be induced by a moderate applied magnetic field below 200 Oe in these trilayers supporting current-perpendicular-to-plane transport. These results show that low-field spin-dependent transport in manganates can be accomplished, the magnitude of which is suitable for magnetoresistive field sensors.

We report significant enhancements of magnetoresistance in granular (La0.67Ca0.33MnO3)x/(SrTiO3)1−x. The system exhibits a conduction threshold at x=xc∼60%, around which magnetoresistance versus x has a maximum. The composition xc at which maximum enhancement in magnetoresistance is observed is the same at high (around 5 T) and at low (a few hundred Oersted) fields. The enhancement is consistent with the disorder-driven amplification of spin-dependent transport at the structural boundaries of the mixture.

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Insulated-gate field-effect transistors (IGFETs) (Fig. 1) comprising pentacene as the semiconducting layer have been fabricated and studied previously.[1±5] The characteristics and performance of pentacene IGFETs make them competitive for applications requiring large area coverage, mechanical flexibility, and low overall cost. Such applications include thin-film transistor (TFT) switching devices for active matrix liquid-crystal displays (AMLCDs), where hydrogenated amorphous silicon (a-Si:H) TFTs are currently used, active matrix organic light-emitting diode displays (AMOLEDs), and low end data storage (e.g., smart cards and identification tags). Pentacene TFTs have produced the highest field effect mobility values reported for organic IGFETs. A mobility of 0.62 cm 2 V ±1 s ±1 and a current on/off ratio of 10 8 have been reported for devices comprising vapor-deposited purified pentacene films grown at substrate temperatures of approximately 120 C. [3,5] These values of mobility and on/off ratio are similar to the ones reported for a-Si:H TFTs. However, an operating voltage swing from ±100 to +100 V is required in order to achieve such high performance from pentacene devices. This is too high compared to a-Si:H and for any practical applications in general. [6] This shortcoming is also demonstrated by comparing the subthreshold slopes, s, of a-Si:H devices (about 0.5 V/decade) and those of pentacene-based IGFETs (5±12 V/decade). The best subthreshold slope value for pentacene IGFETs reported to date is s^1.6 V/decade. [5] This was achieved by the use of a self-assembled monolayer (SAM) of octadecyltrichlorosilane deposited between the SiO 2 insulator and pentacene. Still, the required voltage span had to be larger than 100 V for high mobility and high current on/off ratio to be achieved in addition to low s. Recently, a mobility of more than 1 cm 2 V ±1 s ±1 was reported for a pentacene-based device that also required high operating voltages (about ±80 V).[7]A dependence of the calculated mobility of pentacene IGFETs on gate voltage has been observed earlier. [2,8] In the present paper, we have used our understanding of the gate voltage dependence of mobility in pentacene devices [8] to demonstrate high-performance pentacene IGFETs exhibiting mobility similar to (a-Si:H) TFT, good current modulation, and excellent subthreshold slopes at operating voltages of about 5 V. For this purpose we used a relatively high dielectric constant e metal oxide film as the gate insulator, specifically barium strontium titanate (BST), deposited by means of chemical solution processing. The typical IGFET configuration is depicted in Figure 1. Pentacene films were deposited using vapor deposition. As-received pentacene (97+ % FLUKA Chemical Co.) was used. It has been shown previously that both mobility and current modulation (I on /I off ) increase when pentacene is purified before deposition.[5] Thus we expect that our results could be improved substantially by using purified pentacene. Further details on the vacuum chamber configu...

Epitaxial thin films of lanthanum-deficient LaxMnO3−δ (0.67≤x≤1) have been grown on (100) SrTiO3 substrates by pulsed laser deposition. The as-deposited films exhibit a ferromagnetic transition at temperatures ranging from 115 to 240 K, with the transition temperature (Tc) increasing with higher La deficiency. A sharp drop in resistivity and negative magnetoresistance is observed close to Tc, a behavior similar to that observed in divalent substituted La1−xMxMnO3−δ (M=Ba, Sr, Ca, Pb) films. Postannealing the films in O2 reduces the resistivity and raises the Tc to values close to room temperature. A magnetoresistance value of 130% (Δρ/ρH) has been obtained at 300 K at 4 T for a post-annealed film with x=0.75.

dielectrics for future stackedcapacitor DRAM Thin films of barium-strontium titanate (Ba,Sr)TiO 3 (BSTO) have been investigated for use as a capacitor dielectric for future generations of dynamic random-access memory (DRAM). This paper describes progress made in the preparation of BSTO films by liquid-source metal-organic chemical vapor deposition (LS-MOCVD) and the issues related to integrating films of BSTO into a DRAM capacitor. Films of BSTO deposited on planar Pt electrodes meet the electrical requirements needed for future DRAM. The specific capacitance and charge loss are found to be strongly dependent on the details of the BSTO deposition, the choice of the lower electrode structure, the microstructure of the BSTO, the post-electrode thermal treatments, BSTO dopants, and thin-film stress. Films of BSTO deposited on patterned Pt electrodes with a feature size of 0.2 m are found to have degraded properties compared to films on large planar structures, but functional bits have been achieved on a DRAM test site at 0.20-m ground rules. Mechanisms influencing specific capacitance and charge loss of BSTO films are described, as are the requirements for the electrode and barrier materials used in stacked-capacitor structures, with emphasis given to the properties of the Pt/TaSi(N) electrode/barrier system. Major problems requiring additional investigation are outlined.

We report spin-dependent perpendicular transport in the magnetic trilayer junction structure La0.67Sr0.33MnO3/SrTiO3/La0.67Sr0.33MnO3. Large (factor of 5) changes of magnetoresistance induced by a field of ∼200 Oe are observed at 4.2 K. Junction I–V characteristics at low temperatures are consistent with a metal–insulator–metal tunneling process with a large spin-polarization factor of 0.81 for the conduction electrons. Above 100 K, a variable range-hopping conduction shunts out the magnetoresistance contribution. This second conduction channel comes from the impurity states within SrTiO3 barrier and therefore is not an intrinsic limit to the magnetoresistance performance of the device at high temperatures.