E. P. Kvam

Critical current density and microstructure of melt-processed YBa2Cu3Ox with PtO2 additions

Abstract YBa 2 Cu 3 O x (123) with excess Y 2 BaCuO 5 (211) in the molar ratio of 5:1 (123/211) were processed using the SLMG (“solid-liquid melt growth”) technique. The effect of PtO 2 on the microstructure and properties of samples with coarse (∼ 1–10 μm) and fine 211 precipitates (on the order of 100 nm) was studied. PtO 2 leads to a decrease in the total volume of the coarser 211 precipitates and segregation of these 211 particles, forming “tracks” of the precipitates within a domain. Also, the morphology of the coarse 211 phase changed to a long acicular shape with PtO 2 additions, as compared with approximately spherical particles in undoped samples. TEM studies revealed the presence of 211 particles of ∼ 100 nm size in both doped and undoped samples with a density of 10 13 / cm 2 along with the coarser 1–10 μm 211 particles. The defect density was found to be higher in the case of PtO 2 doped samples. The intragrain J c measured on powdered textured specimens was found to be enhanced by the PtO 2 additions, especially at low temperatures, despite the fact that the coarser 211 precipitates were inhomogeneously distributed. Magnetization measurements on textured domains also showed an improvement of J c ( H ‖ c at 1 T) with PtO 2 doping, corroborating the measurements on the powdered samples.

Surface orientation and stacking fault generation in strained epitaxial growth

It has been observed that partial dislocation glide and stacking fault introduction occur in diamond cubic materials during tensile mismatched growth on (001) and compressive mismatched growth on (110) and (111). For reversed sense of mismatch, however, only full lattice dislocations are observed for strain relief. The general criteria are presented for when a partial misfit dislocation is possible as a function of growth surface orientation. It is shown that, for zero stacking fault energy, the slip regime (dislocation type) expected during (001) growth will hold for any growth orientation (hkl) for which 0≤h≤k≤l/2, and the opposite regime should occur for (hkl) when l≤k/2≤l. Effects of heterointerfacial line tension and stacking fault energy are also considered.

Residual strains in cubic silicon carbide measured by Raman spectroscopy correlated with x-ray diffraction and transmission electron microscopy

Cubic (3C) silicon carbide (SiC) epilayers grown on Si substrates by chemical vapor deposition, characterized using transmission electron microscopy (TEM), high-resolution x-ray diffraction (HRXRD), and Raman spectroscopy, reveal the presence of biaxial in-plane strain. Defect (stacking faults, twins, dislocations) distributions revealed by TEM are correlated with peak widths obtained from HRXRD measurements and Raman shifts of the zone center longitudinal optical phonon line. TEM showed defect densities decreasing with increasing distance from SiC∕Si interface as the lattice mismatch stress is relaxed. Structural defect densities show the most significant reduction within the first 2μm of the epilayer. TEM observations were correlated with a monotonic decrease in HRXRD peak width (full width at half maximum) from 780arcsec (1.5μm thick epilayer) to 350arcsec (10μm thick epilayer). Raman spectroscopy indicates that the residual biaxial in-plane strain decreases with increasing epilayer thickness initially...

Reduced dislocation density in Ge/Si epilayers

Mismatched epilayers, for which the coherency strain has been relieved by misfit dislocation introduction, typically exhibit high epithreading dislocation densities. When the misfit is substantial, as for Ge or GaAs grown by molecular beam epitaxy onto (001) Si substrates, the density is regularly over 109 cm−2. We have grown Ge on Si (111) and (001) with epithreading dislocation densities in the 106 cm−2 range by use of chemical vapor deposition. This is because longer, and thus fewer, misfit dislocations appear for strain relief. Potential explanations for this are postulated. The most likely reason is that thermally activated dislocation glide is much faster at chemical vapor deposition growth temperatures.

Effects of processing parameters on the levitation force of melt-processed YBa{sub 2}Cu{sub 3}O{sub {ital x}}

Melt processing of YBa{sub 2}Cu{sub 3}O{sub {ital x}} (Y-123) with a NdBa{sub 2}Cu{sub 3}O{sub {ital x}}-123 single-crystal seed has been used to grow large single-grain Y-123 samples. Processing parameters have been varied to observe the resulting differences in the levitation force of the samples. Increased hold time above the peritectic temperature and increased undercooling temperature have been observed to be beneficial in enhancing the levitation force of the samples. The levitation force measurements have been correlated with magnetization measurements on the crushed domains.

Evidence for misfit dislocation-related carrier accumulation at the InAs/GaP heterointerface

The electrical properties of the mismatched interface between InAs and GaP have been investigated. High-resolution transmission electron microscopy images show the presence of strain relieving, 90° misfit dislocations at this interface. Hall measurements and electrochemical capacitance–voltage profiling indicate the presence of a high-density sheet of carriers (electrons and holes) at the interface. A linkage is drawn between interfacial carriers and misfit dislocations. A model based on Fermi-level pinning in InAs at the interface by misfit dislocations is proposed to account for the observed electrical behavior.

Behavior of a new ordered structural dopant source in InAs/(001) GaP heterostructures

We report the characteristics of molecular-beam epitaxy grown InAs on highly lattice mismatched (001) GaP substrates. Strain relaxation in this system occurs at low thickness by the generation of a periodic two-dimensional square grid network of 90° misfit dislocations at the heterointerface. The very high interface dislocation density (∼1013 intersections/cm2) exerts a unique influence on the electronic properties of the system. An extended defect structure at the intersection of 90° misfit dislocations is proposed to act as an ordered structural donor source. Hall effect measurements indicate that this source is fully ionized with a constant sheet carrier concentration of 1013 cm−2, irrespective of the InAs layer thickness, and exhibits no freeze out at low temperatures. We have also demonstrated that the electron mobility increases significantly with InAs layer thickness, reaching values in excess of 10 000 cm2/V s in nominally undoped layers. The high threading dislocation density (∼1010 cm−2) in the ...

Pinning potential of stacking fault-related dislocations in YBa2Cu3O7

Abstract The pinning potential of the 1 6 〈031〉 type dislocation has been calculated. This dislocation bounds the excess CuO layer defect commonly observed in YBa2Cu3O7. It was found that there is a strong pinning anisotropy between edge dislocations lying along [100] and [010]. For the former, a repulsive potential of about 30 meV is found, while the latter exhibits an attractive interaction of less than 1 3 this magnitude. This potential is short-range (about 2ξ), indicating that these dislocations would act as distributed point-like pinning sites.

Interactions of dislocations and antiphase (inversion) domain boundaries in III-V/IV heteroepitaxy

The potential for interactions of misfit strain relieving dislocations with inversion-type antiphase domain boundaries is considered for III–V on group IV growth. The specific cases of GaAs grown on Si (001) and vicinal (001) substrates are examined. It is shown that threading dislocation densities for these growths should be unacceptably high due to obstruction of threading dislocation motion by the antiphase domain boundaries, even when the boundaries are eliminated shortly after the inception of GaAs layer growth. Cutting of the antiphase domain boundaries by mobile dislocations would require creation of new highenergy surface area. It is suggested that more successful routes would be growth of a strain-relieving buffer or growth on a {hhk} surface.

Crystalline linkage and defect structures in bulk zone melt textured YBa2Cu3O7 observed by transmission electron microscopy

The microstructural features of zone melt textured wires of YBa2Cu3O7 (123) with and without additions of Y2BaCuO5 have been examined by transmission electron microscopy. The 123 phase is observed to form well aligned platelets, which are of limited extent along [001], but extremely long in the perpendicular directions. Although platelet boundaries are wetted by a second phase, junctions are commonly observed to interconnect the plates. This suggests the possibility that the wires are actually highly defective single crystals. High densities of stacking faults are also observed, particularly at Y2BaCuO5/YBa2Cu3O7 interfaces and at dislocation arrays. It is suggested that these result from a drive for phase conversion to YBa2Cu4O8, and that they may be associated with flux pinning.