D. J. Werder

Anisotropic etching of SiO2 in low‐frequency CF4/O2 and NF3/Ar plasmas

Anisotropic etching of SiO2 films is reported in low frequency (∼100 kHz), moderate‐pressure (0.35 Torr) CF4/O2 and NF3/Ar plasmas. Rates up to 2000 A/min were achieved with high selectivity over GaAs and InP substrates. The etching mechanism was studied with optical spectroscopy and downstream chemical titrations. Anisotropy is attributed to ion‐enhanced reactivity of fluorine atoms with SiO2 at rates up to two hundred times larger than purely chemical etching by fluorine atoms. Damage and product sputter desorption models of this process were evaluated. These two models are nearly mathematically equivalent at steady state, and show that the effectiveness of ions in etching by enhanced reaction is roughly 15 times that in physical sputtering under these conditions.

Role of strained layer superlattices in misfit dislocation reduction in growth of epitaxial Ge0.5Si0.5 alloys on Si(100) substrates

We investigate the role of strained layer superlattices in threading dislocation reduction in the growth of Ge0.5Si0.5 alloys on Si(100) substrates by molecular‐beam epitaxy. Several superlattice structures are studied with zero, negative, and positive net strains with respect to a Ge0.5Si0.5 buffer layer. Control samples consisting of uniform strained layers are also grown for each superlattice structure. Transmission electron microscopy analysis of defect densities is found to be hampered by defect losses from thin foils, particularly in the plan view geometry. It is found that although strained interfaces are effective at deflecting threading dislocations into the interfacial plane, little surface threading dislocation density reduction is observed as a function of the presence of the superlattices for dislocation densities of the order 108 cm−2. This observation may be understood in terms of threading dislocation propagation at strained interfacial planes, and a simple predictive model is developed fo...

Superlattice modulation and superconductivity in the electron-doped Nd2CuO4−xFx and Nd2−xCexCuO4 systems

Abstract We have observed a periodic superlattice modulation in Nd 2 CuO 4 and in the “electron-doped” Nd 2 CuO 4−x F x and Nd 2−x Ce x− CuO 4 by electron diffraction and microscopy. The superlattice modulation characterized by a wave vector q=( 1 4 1 4 ,0) parallel to the Cu-O plane was found in both superconducting and non-superconducting samples, but most of the samples examined were not homogeneous with respect to the superlattice. The orthorhombic superlattice phase is found to exist in four possible variants with its c-axis parallel to the four equivalent [111] zone-axes in the underlying tetragonal lattice. Samples quenched from high temperature invariably showed no superlattice or superconductivity; bulk superconductivity and regions of superlattice were observed on annealing in the range 250–750°C. We believe that oxygen migration is responsible for this disproportionation into superlattice and nonsuperlattice regions.

Advanced dielectrics for gate oxide, DRAM and RF capacitors

A new class of high dielectric constant materials is presented based on binary and ternary amorphous metal oxides. These films are compatible with low temperature processing and can be used with conventional metal electrodes like TiN. Metal alloys of both tantalum oxide and titanium oxide will be discussed with dielectric constants as high as 62 and leakage currents of less than 10/sup -8/ A/cm/sup 2/ at 1 MV/cm.

Single domain high Tc La2−xSrxCuO4 films with tilted CuO2 planes

The two fold degeneracy inherent to epitaxial growth of high Tc films on (101) perovskite substrates has been removed successfully. This is demonstrated by single domain, (103) oriented La2−xSrxCuO4 films of 0.04≤x≤0.34 on vicinal (101) SrTiO3 substrates using 90° off‐axis sputtering. (101) substrates that have the surface normal rotated about [010] by 0.5°–3.5° produced essentially single crystal films with the c‐axis direction determined by the sense of the miscut. Misoriented antidomains (103)’ have been eliminated effectively to a percentage less than 1 part in 104. The mechanism for symmetry breaking is understood on the basis of a surface step model in which the energetics promoting single domain growth is derived from nucleation and epitaxy on the (001) face found at surface steps of vicinal substrates. Furthermore, the incommensuration of the c axis interplanar spacings with the a, b, plane lattice parameters in the La2−xSrxCuO4 structure provides a natural selection of (103) domains over (103)’ d...

Magnetic properties of epitaxial ferrite multilayer films

We have fabricated and studied the structure and magnetic properties of high quality single crystalline (Mn,Zn)Fe2O4, NiFe2O4, and CoFe2O4 films. Although (Mn,Zn)Fe2O4 and NiFe2O4 films grown directly on SrTiO3 and MgAl2O4 show mediocre structural and magnetic properties, these same films grown on SrTiO3 and MgAl2O4 buffered with CoCr2O4 or NiMn2O4 exhibit excellent crystallinity and bulk saturation magnetization values, thus indicating the importance of lattice match and structural similarity between the film and the underlying layer. X ray, Rutherford backscattering spectroscopy, atomic force microscopy, and transmission electron microscopy analysis provide a consistent picture of the structural properties of these ferrite films.

Low temperature microstructure and phase transitions in La2−xSrxCuO4 and La2−xBaxCuO4

Abstract We have studied microstructural phenomena and structural phase transitions of La2−xSrxCuO4 (with 0≤x≤0.15) and La2−xBaxCuO4 (with x=0.125) at low temperatures by convergent beam electron diffraction and electron microscopy. Microtwin density in the orthorhombic phase is found to increase with increasing dopant concentration. At the lowest temperature (∼10 K) in the present experiment, La2−xSrxCuO4 remains orthorhombic and La2−xBaxCuO4 exhibits a fine mixture of orthorhombic and tetragonal phases with a typical domain size ≤300 A. The new phase transition does not alter the twin domain configurations that already exists in the orthorhombic phase. A new strain-relieving mechanism for the new phase transition with small variations of lattice parameters among the orthorhombic and tetragonal domains is observed. Convergent beam electron diffraction studies indicate that the samples are relatively homogeneous and free of distinct second phases, and variation of short range (∼ lattice parameter) lattice ordering, however, is commonly observed on a scale of ∼ 100 A.

Ba2YCu3O7−δ crystal surface layers: Orthorhombic splitting, dislocations, and chemical etching

Abstract We find by transmission electron diffraction that the orthorhombic splitting of the upper surface layers ( 2 YCu 3 O 7− δ is reduced, differing by 10 to 30 percent from the bulk value. We also find by transmission electron microscopy that in general the surfaces are of inferior quality and, thus, not representative of the bulk. These results have important consequences for those experiments that probe only the upper surface layers. By etching with Br/ethanol and HClO 4 /NaClO 4 the poor quality surfaces are removed.

Microstructural changes induced by lattice instabilities in La2−xBaxCuO4(x=0.125) and Nd2NiO4

Abstract We have studied the microstructural changes associated with the structural phase transition from a low temperature orthorhombic phase to a low-temperature tetragonal phase in metallic La 2− x Ba x CuO 4 and insulating Nd 2 NiO 4 . Employing electron microscopy, diffraction and dark field imaging the transition was found to be similar in both systems. Below the transition temperature a fine mixture exists of both tetragonal and orthorhombic domains. Details of microstructural changes of defect networks are compound specific. A simple model is also proposed to understand the microstructural changes observed in these two systems.

Epitaxial Ferroelectric Heterostructures of Isotropic Metallc Oxide (SrRuO 3 ) and Pb(Zr 0.52 Ti 0.48 )O 3

We have fabricated epitaxial ferroelectric heterostructures of isotropic metallic oxide (SrRuO 3) and ferroelectric thin films [SrRuO 3/Pb(Zro. 52Tio.48)0 3 /SrRuO 3] on (100) SrTiO 3 and YSZ buffer layered Si substrates by 900 off-axis sputtering. These heterostructures have high crystalline quality and coherent interfaces as revealed by X-ray diffraction, Rutherford backscattering spectroscopy and cross-sectional transmission electron microscopy. The ferroelectric layers exhibit superior fatigue characteristics over 1010 cycles with large remnant polarization. Ferroelectrics hold potential for both dynamic and permanent data storage in digital memory systems[l]. Conventionally, to build these memory devices, polycrystalline ferroelectric thin films are grown on a Pt or Al base electrode. The high angle grain boundaries which occur in the ferroelectric thin film layer are detrimental to device performance because they cause aging and fatigue due to charge segregation and decay at the grain boundaries. Furthermore, the undesirable nature of the interfaces between ferroelectric layers and electrodes resulting from non-epitaxial growth leads to degraded performance characteristics [2]. Ramesh et al. reported that epitaxial thin film heterostructures of ferroelectric materials and certain cuprate superconductors grown by the laser ablation technique have outstanding fatigue properties[2, 3]. Unfortunately, the cuprate superconductors are not fully chemically and thermally stable, which puts some constraints on their processing and usage. Furthermore, the high Tc superconducting cuprates typically have relatively poor crystalline quality and rough surfaces. These drawbacks may limit the application of such device structures for non-volatile memory applications. We recently reported the synthesis and properties of single crystal epitaxial thin films of the isotopic metallic oxide SrRuO3 on miscut (100) SrTiO3 substrates [4]. SrRuO 3 is a pseudo-cubic perovskite with psuedo-cubic lattice parameter of 3.93A [5, 6]. (Note: the aand c-lattice parameters of PZT are 4.036A and 4.146A, respectively). The lattice mismatches of SrRuO3 and ferroelectric Pb(Zr0.52Ti0.48)0 3 (PZT) on the 10011 surface is fairly small (-2.7%), which allows us to grow high quality epitaxial ferroelectric heterostructures with SrRuO 3 electrodes. Unlike most oxide superconductors, SrRuO 3 is stable up to 1200 K in oxidizing or inert gas atmospheres [7]. The resistivities of these films are isotropic and low (-340 pfl-cm at room temperature), and the temperature dependence (dpldT) shows good metallic behavior, which is important for electrode applications. All of these observations suggest that SrRuO3 might be an ideal electrode material for epitaxial ferroelectric device structures. Mat. Res. Soc. Symp. Proc. Vol. 310. 01993 Materials Research Society