A. H. Dayem

Reproducible technique for fabrication of thin films of high transition temperature superconductors

We report on a new process to make films of Y1Ba2Cu3O7 using coevaporation of Y, Cu, and BaF2 on SrTiO3 substrates. The films have high transition temperatures (up to 91 K for a full resistive transition), high critical current densities (106 A/cm2 at 81 K), and a reduced sensitivity to fabrication and environmental conditions. Because of the lower reactivity of the films, we have been able to pattern them in both the pre‐annealed and post‐annealed states using conventional positive photoresist technology.

High quality narrow GaInAs/InP quantum wells grown by atmospheric organometallic vapor phase epitaxy

A series of GaInAs/InP quantum wells from 10 to 135 A has been grown by atmospheric organometallic vapor phase epitaxy using pressure balancing techniques. These wells exhibit strong exciton peaks at 4 K and have quantized energy shifts of up to 326 meV. These energy shifts are compared with two simple finite well models (Kronig–Penney and envelope function approximation) using a conduction‐band offset of Vc≊40% ΔEg(GaInAs) and are in close agreement with the latter model. The full width half‐maximum linewidths indicate an average interface roughness of ≊1 monolayer.

Growth of Cd1−xZnxTe by molecular beam epitaxy

Cd1−xZnxTe has been grown on GaAs substrates for compositions from x=0 to x=1. Binaries are shown to be of high quality, but x‐ray rocking curve half‐widths are extremely broad for most ternary compositions. Attempts to modify the interface yield only modest and uneven improvement in rocking curve half‐widths. The poor quality appears to be due to a phase separation.

Chemical beam epitaxial growth of extremely high quality InGaAs on InP

Full widths at half‐maximum intensity of the (004) Bragg reflecton peak as small as 24 arcs are obtained from InGaAs epilayers 4–6 μm thick. Such linewidth is the narrowest reported thus far for an InGaAs epilayer grown by any vapor phase technique reported in literature. Such extreme compositon uniformity is also supported by results from Auger depth profiles and 2 K photoluminescence measurements. Very intense efficient luminescence peaks due to excitonic transitions with linewidths (FWHM) as narrow as 1.2 meV are obtained. This again represents the narrowest linewidth ever reported for InGaAs grown by any technique. In fact, such a linewidth represents the narrowest linewidth ever measured for any alloy semiconductor. Further, the photoluminescence spectra reveal the donor‐to‐acceptor pair recombination is nearly absent. This indicates that the InGaAs is of very high purity. Hall measurements of 2–5‐μm‐thick epilayers grown directly on InP substrates have mobilities of 10 000–12 000 and 40 000–57 000 c...

Doping studies using thermal beams in chemical‐beam epitaxy

The use of chemical‐beam expitaxy instead of chemical vapor deposition allows the incorporation of thermal atomic beams for doping. Dopant profile measurements by a differential capacitance‐voltage technique and secondary ion mass spectroscopy technique show that there is no surface segregation with Si dopants in InP at all growth temperatures (<600 °C). On the other hand, Sn and to a lesser extent Be show surface segregation which can be reduced by growing the InP at a lower temperature (<550 °C). From this study, it is seen that Si should be preferred as the n‐type dopant in producing InP/InGaAs high electron mobility transistors.

High resolution electron beam lithography on CaF2

We have fabricated 30‐nm lines on 200‐nm centers in CaF2 using a scanning transmission electron microscope. The lines were written by electron beam radiolysis of a fine grain polycrystalline CaF2 film and reaction to CaO followed by development in H2O.

Thin‐film CaF2 inorganic electron resist and optical‐read storage medium

Polished silicon substrates were e‐gun deposited with 100‐nm films of CaF2 in a molecular beam epitaxy vacuum station. Measurements of this material system reveal a factor of 35 reduction in reflectivity for films exposed to electron irradiation. Exposure is accomplished by simply viewing the desired area using a 3‐kV Auger electron microprobe. It is further reported that this exposed inorganic material can be developed (selectively washed away) in H2O. In addition, both over and under exposures have been observed. Optimum electron exposure dose has been determined to be 0.1–0.2 C/cm2.

Epitaxial growth of high Tc superconducting Nb3Ge on Nb3Ir

Nb3Ir polycrystalline films with the A15 structure deposited on sapphire were used as substrates for the epitaxial growth of Nb3Ge because of the favorable lattice parameter match. The experimental results clearly show that epitaxial growth indeed occurs and helps to extend the range of homogeneity of the A15 phase up to 26.3 at.% Ge as compared with the thermodynamic equilibrium boundary at 19 at.% Ge. Furthermore, epitaxy results in a considerable rise in the superconducting transition temperature for Ge‐rich samples together with a reduction in the transition width.

Laser crystallization of Si films on glass

By using glass substrates which approximate the thermal expansion of a deposited Si film, thermal stress cracking during laser crystallization is eliminated. Results on three types of transparent silicate glass are reported. Chemical vapor deposition is used to coat these substrates first with a buffer layer of SiO2 or Si3N4 and then with a Si film. When the Si films are melted with an argon ion laser beam, large crystal grains are produced. Secondary ion mass spectroscopy is used to determine the effectiveness of the buffer layer in isolating the Si from impurities in the glass.

Epitaxial growth of high T/sub c/ superconducting Nb/sub 3/Ge on Nb/sub 3/Ir

Nb3Ir polycrystalline films with the A15 structure deposited on sapphire were used as substrates for the epitaxial growth of Nb3Ge because of the favorable lattice parameter match. The experimental results clearly show that epitaxial growth indeed occurs and helps to extend the range of homogeneity of the A15 phase up to 26.3 at.% Ge as compared with the thermodynamic equilibrium boundary at 19 at.% Ge. Furthermore, epitaxy results in a considerable rise in the superconducting transition temperature for Ge‐rich samples together with a reduction in the transition width.