K. T. Short

Mesotaxy: Single‐crystal growth of buried CoSi2 layers

Buried single‐crystal CoSi2 layers in silicon have been formed by high dose implantation of cobalt followed by annealing. These layers grow in both the (100) and (111) orientations—those in (111) have better crystallinity, but those in (100) are of higher electrical quality. Electrical transport measurements on the layers give values for the resistance ratios and superconducting critical temperatures that are better than the best films grown by conventional techniques and comparable to bulk CoSi2.

In situ epitaxial growth of Y1Ba2Cu3O7−x films by molecular beam epitaxy with an activated oxygen source

Highly oriented, epitaxial Y1Ba2Cu3O7−x thin films were prepared on MgO(100) by molecular beam epitaxy at a substrate temperature of 550–600 °C. The in situ growth was achieved by incorporating reactive oxygen species produced by a remote microwave plasma in a flow‐tube reactor. The epitaxial (001) orientation is demonstrated by x‐ray diffraction and ion channeling. In situ reflection high‐energy electron diffraction showed that a layer‐by‐layer growth has produced a well‐ordered, atomically smooth surface in the as‐grown tetragonal phase of an oxygen stoichiometry of 6.2–6.3. A 500 °C anneal in 1 atm of O2 converted the oxygen content to 6.7 to 6.8. Typical superconducting transport properties of an Y1Ba2Cu3O7−x film 1000 A thick are ρ(300 K)=325 μΩ cm, ρ(300 K)/ρ(100 K)=2.4, Tc(onset)=92 K, and Tc(R=0)=82 K. The transport Jc at 75 K is 1×105 A/cm2, and increases to 1×106 A/cm2 at 70 K.

Interpretation of dislocation propagation velocities in strained GexSi1−x/Si(100) heterostructures by the diffusive kink pair model

Experimental measurements of misfit dislocation velocities, obtained from in situ transmission electron microscope observations of strained GexSi1−x/Si(100) heterostructures, are compared with predictions of the diffusive double‐kink (or ‘‘kink pair’’) model of dislocation propagation. Good agreement is observed between experiment and theory for buried strained layers with applied stresses in the range of hundreds of MPa. For very thin uncapped strained layers, the diffusive double‐kink model does not describe experimental data well. In these structures better agreement between experiment and theory is obtained if we model single‐kink nucleation at the epilayer free surface. We compare our experimental data to those of other groups, and show how our modeling can reconcile apparently disparate trends deduced by these other groups.

Enhanced strain relaxation in Si/GexSi1−x/Si heterostructures via point‐defect concentrations introduced by ion implantation

It is shown that strain relaxation during annealing of Si/GexSi1−x/Si heterostructures is significantly enhanced if the strained GexSi1−x layers are implanted with p (B) or n (As) type dopants below the amorphization dose. Comparison of strain relaxation during in situ annealing studies in a transmission electron microscope between unimplanted and implanted structures reveals that the latter show residual strains substantialy below those for unimplanted structures. We propose that this enhanced relaxation is caused by increased dislocation nucleation probabilities due to the high point‐defect concentrations arising from implantation.

Controllable reduction of critical currents in YBa2Cu3O7−δ films

The critical currents in high quality thin films of the high Tc superconductor, YBa2Cu3O7−δ, can be controllably reduced by orders of magnitude using ion irradiation. This reduction in critical current occurs without substantial decrease in Tc or increase in room‐temperature resistivity. Using this technique, we have fabricated weak links that exhibit an ac Josephson effect at 77 K.The critical currents in high quality thin films of the high Tc superconductor, YBa2Cu3O7−δ, can be controllably reduced by orders of magnitude using ion irradiation. This reduction in critical current occurs without substantial decrease in Tc or increase in room‐temperature resistivity. Using this technique, we have fabricated weak links that exhibit an ac Josephson effect at 77 K.

Mechanisms of buried oxide formation by ion implantation

We have studied the process of buried oxide formation as a function of implantation and annealing conditions. Concentrating on substoichiometric implants (<1×1018 O/cm2), we varied the implantation energies from 100 keV to 1 MeV. Some apparent precipitation of SiO2 similar to that observed in Czochralski‐grown silicon occurs on implantation. This means that formation of the buried oxide layer and perfection of the overlying crystalline Si layer depend more strongly on the substrate temperature during the implant than on the annealing temperature.

Ion implantation damage and annealing in InAs, GaSb, and GaP

The characteristics of ion implantation induced damage in InAs, GaSb, and GaP, and its removal by rapid thermal annealing have been investigated by Rutherford backscattering and transmission electron microscopy. There is relatively poor regrowth of these materials if they were amorphized during the implantation, leaving significant densities of dislocation loops, microtwins, and in the case of GaSb, polycrystalline material. For implant doses below the amorphization threshold, rapid annealing produces good recovery of the lattice disorder, with backscattering yields similar to unimplanted material. The redistribution of the implanted acceptor Mg is quite marked in all three semiconductors, whereas the donor Si shows no measurable motion after annealing of InAs or GaP. In GaSb, however, where it appears to predominantly occupy the group III site, it shows redistribution similar to that of Mg.

Preparation of superconducting thin films of calcium strontium bismuth copper oxides by coevaporation

Superconducting films of Ca‐Sr‐Bi‐Cu oxides have been prepared by coevaporation of CaF2, SrF2, Bi, and Cu, followed by post‐oxidation in wet O2. The films were characterized by four‐probe resistivity measurements, Rutherford backscattering, transmission electron microscopy, x‐ray diffraction, and Hall measurements. Zero resistance was achieved at ∼80 K, although evidence of traces of superconductivity at higher temperatures was seen in resistivity and Hall data. The critical current at 4.2 K was 1.0×106 A cm−2. The films were epitaxial on 〈100〉 and 〈110〉 SrTiO3 substrates. The electrical and structural properties of the films were insensitive to film composition over a wide range of stoichiometries.

High sensitivity optical image processing device based on CdZnTe/ZnTe multiple quantum well structures

We present results on the operation of a high sensitivity semi‐insulating multiple quantum well device for optical image processing. This device operates in the visible spectrum using II‐VI CdZnTe/ZnTe multiple quantum well structures. Incident light creates charge carriers that screen an applied ac electric field modulating the absorption and refractive index of the structure through the quantum confined Stark effect. In this way, an incident intensity pattern is recorded as an absorption and refractive index variation. The semi‐insulating nature of the material eliminates the need for pixelation. In a wave‐mixing experiment, a peak diffraction efficiency of 0.25% was observed from 2.25 μm active layer of the device. Two‐beam‐coupling gain coefficients of ∼500 cm−1 at wavelengths longer than the exciton absorption peak should be possible.

Parameters for in situ growth of high Tc superconducting thin films using an oxygen plasma source

Superconducting thin films of Dy‐Ba‐Cu‐O have been grown on 3 in. sapphire wafers with a molecular beam deposition process. Dissociated oxygen from a glow discharge source was used to improve the oxygen incorporation. This allows growth on a relatively low‐temperature substrate kept below 600 °C followed by an in situ anneal below 400 °C. Thin films of Dy‐Ba‐Cu‐O which were fully superconducting at 40 K have been fabricated by this in situ growth process.