Masaki Aoki

Properties of ZnTe-ZnSe and -ZnS superlattices prepared by hot wall epitaxy

Abstract ZnTe-ZnSe and -ZnS superlattices were prepared on GaAs(100) substrates by hot wall epitaxy. Their high-energy electron and θ-2θ X-ray diffraction and photoluminescence at 77 and 300 K were measured. Though the lattice mismatches between the materials are 7% (ZnTe-ZnSe) and 13% (ZnTe-ZnS), strong photoluminescence associated with band edges was observed, i.e. a superlattice consisting of a thin ZnTe layer and a thick ZnSe layer showed strong luminescence associated with band edges and weak luminescence associated with deep level defects. The ZnTe (20 A)-ZnS (50 A) superlattice showed strong but rather broad luminescence near 2.6 eV. Variations of the luminescence photon energy of the ZnTe-ZnSe superlattice with the thickness of the ZnTe or the ZnSe layer can be explained qualitatively by strain effects due to the lattice mismatch on the band gap of ZnTe and on the band offsets of the superlattice. The experimental results show the superlattices are of type II, where the conduction and valence band edges of ZnTe are above the corresponding edges of ZnSe or ZnS.

Semimetallic Hall properties of PbTe‐SnTe superlattice

The PbTe‐SnTe superlattice was expected to be a type‐II superlattice where the valence band edge of SnTe is higher than the conduction band edge of PbTe. To ascertain the type‐II structure, we prepared the PbTe‐SnTe superlattice by hot wall epitaxy, and performed its Hall measurement. Magnetic‐field‐dependent and relatively small Hall coefficients were obtained for the superlattices, which show coexistence of free electrons and holes in the superlattice. Hall coefficients of the superlattices increased with annealing time owing to the gradual disappearance of the coexistence. Diffusion of Sn was studied using x‐ray diffraction analysis.

Superconducting behavior in PbTeSnTe superlattices

Abstract Transport measurements show that PbTeSnTe superlattices (SL) undergo superconducting transitions below 7 K which depend upon the structure of the SL. In the superlattices grown on the KCl (100) plane, a characteristic transition temperature linearly decreases with decreasing nominal layer thickness d1 of PbTe at a fixed layer thickness d 2 = 50 A of SnTe, or with increasing sheet resistance per SL period. The rate of breaking the superconducting behavior by magnetic fields is largest for the perpendicular configuration to the layers. Typically in the SL (200-50 A) at 1.5 K, critical magnetic fields Hc obey the Tinkham formula. The SLs on the BaF2 (111) plane also show superconductivity with lower transition temperatures than the former SLs. A possible origin of the superconductivity is discussed.

Sn diffusion effects on x‐ray diffraction patterns of Pb1−xSnxTe–PbSeyTe1−y superlattices

Three kinds of superlattices consisting of good lattice‐matched and ‐mismatched materials (Pb0.76Sn0.24Te‐PbSe0.10Te0.90, Pb0.76Sn0.24Te‐PbTe, and Pb0.76Sn0.24Te‐PbSe0.18Te0.82 superlattices) were prepared by a hot‐wall epitaxy and their high‐angle x‐ray diffractions were measured. Sn‐diffusion effects on the satellite structure are studied and it was found that Sn diffusion increases lattice distortion for lattice‐matched Pb0.76Sn0.24Te‐PbSe0.10Te0.90 superlattice and for lattice‐mismatched Pb0.76Sn0.24Te‐PbSe0.18Te0.82 superlattice, on the other hand, decreases it for Pb0.76Sn0.24Te‐PbTe superlattice. And it was also found that very large diffusion of Sn occurs during growth even when the substrate temperature is 250 °C.

Resonant Raman scattering in ZnTe‐ZnSe strained layer superlattices

Resonant Raman scattering and photoluminescence spectra have been measured in ZnTe‐ZnSe superlattice. Strong out‐going and in‐coming resonant enhancements have been observed for multilongitudinal optic phonon bands of the ZnTe‐like mode close to the photoluminescence band, which is assigned to direct recombination of excitons. The calculated exciton energies taking into account the effects of quantum well and strain field agree well with photoluminescence energies in superlattices with different thicknesses of the ZnTe layer.

Properties of Photoluminescence from ZnTe-ZnSe Superlattices

Excitation spectra and temperature dependence measurements of the photoluminescence(PL) were carried out for the various kinds of the ZnTe–ZnSe superlattices (SLs) prepared by hot wall epitaxy. First observation of strong absorption band by about 0.2 eV above the PL emission band and observed activation energy (0.2 eV) of the thermal quenching which are independent of the SL structures suggest that the PL is related to the radiative recombinations between band edge and localized centers, i.e. Free-to-Bound transitions.

Intrinsic Gettering of Iron Impurities in Silicon Wafers

We present a new experimental approach to using intrinsic gettering to remove Fe impurities. We annealed samples, followed by quenching, and measured the Fe concentration near the surface using deep-level transient spectroscopy. The supersaturation of Fe impurities is necessary for the intrinsic gettering of Fe. However, for a higher supersaturation, Fe impurities precipitate faster than gettering. The optimum degree of supersaturation is one order of magnitude. Gettering is limited by the reaction of Fe with the oxygen precipitates in the defect region, rather than by Fe diffusion to the defect region.

Enhanced Thermal Stability in Perpendicular Top-Pinned Magnetic Tunnel Junction With Synthetic Antiferromagnetic Free Layers

Synthetic antiferromagnetic (SAF) free layers consisting of [Co (0.3 nm)/Pd (0.7 nm)]n/Ru/Ta/CoFeB were investigated for use in the free layers of top-pinned magnetic tunnel junctions (MTJs). We found that interlayer exchange coupling properly provides thermal stability when the saturation magnetization of the CoPd layer is less than that of the CoFeB layer. Furthermore, we demonstrated that the thermal stability factor of the top-pinned MTJ with [Co (0.3 nm)/Pd (0.7 nm)]n=1/ Co (0.3 nm)/Ru/Ta/CoFeB- SAF free layers was improved without increasing the intrinsic switching current.

Novel Circuitry Configuration with Paired-Cell Erase Operation for High-Density 90-nm Embedded Resistive Random Access Memory

We propose a novel circuitry configuration for high-density 90-nm embedded resistive random access memory (ReRAM). The memory cells are operated at 2 V, and a small memory cell size of 6F2 consisting of a 1.2-V standard transistor and a resistive junction (1T–1R) is designed, where F is the feature size. The unique circuitry configuration is that each pair of source-lines connects to each source-line selective gate. Therefore, erasing is done by a pair of cells in turn in the whole sector, while the reading or programming is done by a random accessing operation. We simulated the ReRAM circuit for read and write operations with SPICE. As a result, we found that 5-ns high-speed read access was obtained in the 256-word lines (WLs) × 256-bit lines (BLs) and that the SET/RESET operation was stable.

Distribution of Fe in an intrinsic gettered silicon wafer after annealing at supersaturation temperature

We studied the distribution of Fe in thermal equilibrium after annealing at supersaturation temperature. Analyses by deep-level transient spectroscopy (DLTS) showed that the Fe concentrations in the denuded zone and in the bulk region of a Czochralski (CZ) intrinsic gettered wafer are the same when depletion of Fe from the denuded zone stops. This Fe concentration corresponds to the solid solubility of Fe in the wafer at the annealing temperature used. This result indicates that there is no difference in Fe solid solubility between the bulk region and the denuded zone of the wafer. Results of transmission electron microscopy (TEM)-energy dispersive X-ray (EDX) analyses suggest that supersaturated Fe in an intrinsic gettered wafer precipitates preferentially with oxygen precipitates in the bulk region, while supersaturated Fe in an as-grown wafer precipitates in the near-surface region.