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In this letter, magnetization reversal is investigated in (Ga,Mn)As thin films using the magneto-optical Kerr effect. A Stoner–Wohlfart model, modified to take into account the existence of magnetic reversal discontinuities associated with nucleation and propagation phenomena allows us to estimate most of the characteristic constants. These results demonstrate a reversal behavior analogous to that observed in metallic magnetic layers (coherent rotation followed by a nucleation propagation process). The dynamic study at T=20 K shows a strong increase of the coercivity with the increase of dH/dt. This effect is related to the random distribution of Mn magnetic ions in the lattice.

Continuous‐wave (CW) terahertz (THz) generation has been successfully achieved by low‐temperature grown (LTG) In1−xGaxAs (x = 0.47) epitaxial layer as a THz emitter and a photomixer module with dual‐wavelength laser as an optical beat source. The reflectance decay spectroscopy revealed a carrier lifetime of ∼ 2.5 ps in LTG‐InGaAs layer grown at 220 °C and subsequently in‐situ annealed at 550 °C for 10 min. The THz ouput characteristics are demonstrated in the range of 0.3–0.8 THz.

We develop highly‐resistive (i.e. low‐carrier‐density) and ultrafast Fe‐ion implanted InGaAs layers for the applications of THz photomixer and photoconductive switch. The measured Hall mobility, sheet resistance, carrier density, and carrier lifetime of the optimized 1.2‐μm‐thick Fe‐implanted InGaAs layer are 3.4×102 cm2/Vs, 0.24 MΩ, 6.5×1014 cm−3, and 0.13ps, respectively.

A significant charge transfer, which differs from tunneling, over thick AlxGa1−xAs barrier in GaAs/AlxGa1−xAs asymmetric double quantum wells is studied by cw photoluminescence excitation (PLE) and time‐resolved photoluminescence. It is found that 300‐A‐thick Al0.3Ga0.7As barrier is universally ‘‘leaky’’ with transport time of ∼300 ps, while AlAs and AlAs/GaAs digital alloy barriers with same thickness are not. Aided by a model calculation, we suggest that the intrinsic inhomogeneities in the alloy, which recent x‐ray and scanning tunneling microscope studies revealed, may be responsible.

We study coherent phonon oscillations in GaAs/AlxGa1−xAs MQW’s with varying x and fixed well and barrier widths of 100A. We have observed large plasmon-phonon coupling in x=0.2 MQW, although it is generally believed that the 2D nature of plasmon makes the plasmon-phonon coupling very difficult. One possible explanation for this phenomenon is the bulk-like, growth direction plasmon oscillations mediated by the anomalously large well-to-well coupling for small x (x

The effect of argon ion bombardment on the structure and properties of thick copper films was studied. Primary deposition variables were ion flux, ion energy, substrate temperature, and substrate type. The effects of ion bombardment are profoundly different at high energy (600 eV) as opposed to low energy (62 eV). Trends in crystallographic texture, microhardness, crystallite size, and resistivity are significantly different at different ion energies on all substrates examined. At high energy, the substrate dependence of properties is small while a large dependence is seen at lower energy. This work demonstrates the independence of ion energy and ion flux as control parameters in property modification using ion‐assisted deposition processes.

In situ measurements during MeV ion bombardment at room temperature are used to study the superlinear increase of resistivity with fluence which occurs in irradiated ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{x}}$. The functional form of the resistivity increase is found to be exactly exponential for O and As ion irradiation, confirming previous results of lower precision. For low ion damage rates we find deviations from the exponential dependence at low doses. We show that these are caused by an unidentified phase transformation, occurring during the irradiation, which lowers the resistivity of the samples. Annealing of resistivity increments caused by brief irradiations occurs at room temperature, indicating that radiation-induced defects are mobile. The rate of annealing is strongly sample dependent, but in all cases the transformation induced by a small dose of irradiation causes a reduction in the resistivity of the sample. We suggest that the reduction in resistivity may be due to ordering on the oxygen sublattice.

Higher nitrides of Ti, Zr, and Hf have been prepared by reactive rf sputtering of Ti, Zr, and Hf metals in a pure nitrogen atmosphere. The electrical and optical properties of these films are highly dependent on two deposition parameters, namely, the substrate temperature and the target voltage. We find that at each particular target voltage, there exists a critical temperature below which the higher nitride phase can be deposited and above which the phase cannot form. This suggests that the higher nitride phase (which is speculated to be an ordered defect structure) is metastable and is not energetically favored at high processing temperature or at high input power. At 2000 V target voltage, this critical temperature is determined to be approximately 200 °C for Hf–N, 25 °C for Zr–N, and is unknown for Ti–N system. The optical band gap of the ZrNx films decreases from 5.31 to 1.66 eV as the substrate temperature varies from −196 to 750 °C. Thermal analyses, using modulated beam mass spectrometry for monit...

High quality La1.8Sr0.2CuO4 and YBa2Cu3O7 superconducting thin films, with zero resistance at 88 K, have been made by dual‐ion‐beam sputtering of metal and oxide targets at elevated temperatures. The films are about 1.0 μm thick and are single phase after annealing. The substrates investigated are Nd‐YAP, MgO, SrF2, Si, CaF2, ZrO2‐9% Y2O3, BaF2, Al2O3, and SrTiO3. Characterization of the films was carried out using Rutherford backscattering spectroscopy, resistivity measurements, transmission electron microscopy, x‐ray diffraction, and secondary ion mass spectroscopy. Substrate/film interaction was observed in every case. This generally involves diffusion of the substrate into the film, which is accompanied by, for example, the replacement of Ba by Sr in the YBa2Cu2O7 structure, in the case of SrTiO3 substrate. The best substrates were those that did not significantly diffuse into the film and which did not react chemically with the film. In general, the superconducting transition temperature is found to ...

Superconducting thin films of the Bi–Sr–Ca–Cu–O system (2 and 10 kÅ) have been successfully prepared by multilayer deposition from elemental metal sources and annealed in N2/O2 mixtures. The onset of the superconducting transition is greater than 80 K with zero resistance at 74 K. The films contain primarily a Bi2(SrCaBi)3Cu2O8 + δ phase with a high degrees of (001) preferred orientation.

We report on the structure and superconducting properties of a single phase polycrystalline composition given by Bi2.2Sr1.7Ca1.2Cu2O8+δ as well as measurements on single crystals. X‐ray and electron diffraction analyses of both single‐crystal and single phase materials show an incommensurately modulated orthorhombic superlattice, derivable from a pseudotetragonal substructure. The various compositions have the common feature that the major portion of the superconductivity disappears above 85 K but there is a small tail in the diamagnetic susceptibility extending to 110 K, above which the sample becomes paramagnetic. Single crystals, grown by the flux method, exhibit the same incommensurately modulated pseudotetragonal structure as the ceramic, but show a higher diamagnetic shielding and in some cases also have tails. The critical currents in the (ab) plane are only about one‐fifth of the value found for YBa2Cu3O7 crystals and the anisotropy is smaller. These reduced values may be an intrinsic property of ...

The effect of ion bombardment on film stress was studied by evaporating Nb thin films in the presence of a controlled ion beam from a Kaufman ion source. Films were deposited at 4 A/s by electron‐beam evaporation with ion bombardment over the energy range 100–800 eV, at ion current densities of 0.001 to 1.1 mA/cm2. Film stress was measured by an x‐ray bending‐beam technique on Si substrates. Stress values were found to depend strongly on both ion flux and sample temperature. For films deposited at 400 °C, film stress is tensile in the absence of ion bombardment. Increasing argon ion flux causes a change toward compressive stress, in some cases passing through zero. For films deposited at room temperature, film stress is compressive in the absence of ion bombardment, due to incorporation of oxygen. Argon ion bombardment causes a change toward tensile stress, correlating with an improvement in film purity by preferential resputtering of oxygen. With 100 eV argon ions, an ion‐to‐atom ratio of only 3–10 % is ...

A study of massive chromium electrodeposited from aqueous chromic-acid electrolytes at elevated temperatures—the relative effect of the catalytic action of sulfate, fluoride, and silicofluoride ions on mechanical properties was investigated with a subsequent determination of optimum compositions.

MoN in the NaCl structure has been predicted to be a superconductor with a T/sub c/ of 29.4 K. Because of the metastable nature of the structure, however, this high superconducting temperature has not been achieved experimentally. In this work we investigate the combined ZrN-MoN system, assuming an ordered MoZr/sub 3/N/sub 4/ structure which is related to the stable defect Zr/sub 3/N/sub 4/ structure. Using augmented-spherical-wave band-structure calculations, we can predict an increase of the density of state at E/sub F/ (one important factor for a high-T/sub c/ material) by about a factor of 2 when 25% of the Zr is substituted by Mo.

It was found that the increase in room-temperature resistivity caused by ion bombardment damage in YBa2Cu3O7−x could be completely recovered by annealing if the damage dose was small. A small resistivity increment (~ 0.2%), caused by a very small ion dose, annealed at room temperature. For a larger ion dose, sufficient to double the resistivity, complete recovery of room-temperature resistivity was produced by annealing at 500 °C in O2, but recovery after still larger doses was incomplete.

Cu/BaO/Y2O3/Ag layer structures were deposited on SiO2 substrates at 600 °C and post‐annealed in oxygen at different temperatures. Superconducting oxides with a thickness of 5000 A were formed at 800 °C, with an onset of 92 K and a zero resistance at 70 K. The films contain mainly the YBa2Cu3O7 phase, with the 103 and 020 diffraction peaks being the major ones observed. The high deposition temperature used is shown to help the formation of the YBa2Cu3O7 phase, leading to the observed superconductivity using SiO2 substrates.

High Tc Y‐Ba‐Cu‐O films have been prepared by dc magnetron sputtering of metal alloy targets. To circumvent the negative ion effect, two alloy targets, YCu and BaCu, are sputtered in an argon atmosphere with an oxygen spray near the substrate. Films deposited on sapphire with onsets at 92 K and a 6° transition width (10–90%) have been achieved using this technique. These films have been successfully patterned with the technique of laser ablation.

We have investigated reactively sputtered films of RuO2 for possible application in very large scale integrated circuits. Sputtering yields stoichiometric ruthenium dioxide in a large window of oxygen pressures and the films are reasonably low stressed in the 10−9 dyn cm−2 range. The resistivity of as deposited films is 40 μΩ cm. The films are excellent barriers against interdiffusion of Si and Al.

Preferred orientation and the microstructure of polycrystalline YBa2Cu3O7 films, both of which strongly depend on the substrate used, have been shown to be important factors in determining the critical current density. A film grown on cubic ZrO2 has a random polycrystalline structure with a Jc of 2.4×104A/cm2 at 5K. In contrast, a film with a [001] texture (grown on BaF2) has significantly higher Jc of 7.47×104A/cm2. The characteristic differences in the temperature dependence of Jc in these two types of films are discussed.

We report the deposition of crystalline as‐deposited ion beam sputtered Y1Ba2Cu3Ox thin films with orientations of (100), (001), and (110)/(103) and a method for controlling the orientation of the films. Films deposited on Al2O3 Yttria (9%) stabilized ZrO2 typically have a near random orientation, while the films on MgO show a high degree of orientation. In addition, post deposition heat treatments at 500 C were found to reduce the room temperature resisitivity significantly, although no superconducting material, maintaining a specular surface. RBS analysis shows a variation in the composition as a function of substrate position on the platen, which indicates a re‐sputtering mechanism due to highly energetic neutrals from the target surface.

Superconducting YBa2Cu3O7 films have been deposited by the plasma spray deposition process. The sequence of processing steps, including powder preparation plasma deposition, and post‐deposition treatment were studied to optimize Tc and minimize process complexity. The as deposited film can be varied in a controlled way from poorly crystalline to the desired orthorhombic YBa2Cu3O7 phase. Plasma temperature, residence time in the plasma, and substrate temperature were used to control the phase of the as‐deposited film. With high temperature (930°C) followed by low temperature (470°C) anneals, well crystallized YBa2Cu3O7 films were grown on a variety of substrates. Depending on substrate interactions, completion temperatures range from 84 K to

The interaction between the substrate and YBa2Cu3O7 films made by ion beam sputtering and e‐beam evaporation is found to significantly influence the superconducting transition temperature, Tc. The substrates investigated are Al2O3, MgO, SrTiO3, ZrO2‐9% Y2O3 (YSZ), and BaF2. In the case of Al2O3, a spinel is formed at the interface and Al is found to be present in solution in the YBa2Cu3O7. This reaction significantly depresses Tc. For SrTiO3, both Sr and Ti are found in the film without severely depressing the Tc; this, however, depends on the annealing conditions. Although under similar conditions little interaction was observed for the MgO substrates, a moderate reduction in Tc was observed. An interaction zone of BaZrO3 perovskite is formed at the interface between the YSZ substate and the 123 structure. In addition, Zr is found in solution in the 123 film. The presence of Zr has little effect on the Tc. The films were characterized using Rutherford backscattering spectroscopy (RBS), transmission elect...

We report on the structure and characterization of the major phase in the Bi-Sr-Ca-Cu-O system that becomes superconducting below 80 K. A superlattice modulation in the structure that is incommensurate with a tetragonal perovskite-based subcell is observed using electron diffraction. High-resolution imaging indicates that the modulation occurs primarily in bismuth-rich layers in the structure.

Ion-assisted deposition allows for independent control of ion energy, ion flux, and adatom flux incident on the growing film. We describe herein general trends in property and structure modification of thin film metals using this technique.

Thin film superconductors of the YBaCuO family have been fabricated using layer structures of Cu/BaO/Y2O3 deposited by electron-beam evaporation. Using layer thicknesses off the stoichiometric YBa2Cu3O7, we have formed superconducting films with zero resistance above 77°K using MgO and SrTiO3 substrates. The films were found to be composed of both the superconducting YBa2Cu3O7 phase, and the non-superconducting green Y2BaCuO7 phase. Dependence of transition temperatures on the annealing temperatures and the substrates has also been studied.

The modification of film stress in evaporated tungsten was studied as a function of deposition environment. Using concurrent ion bombardment of the growing film, the stress was seen to vary systematically with ion energy, ion flux, and substrate temperature. The qualitative behavior fits the model of stress modification developed for niobium films. X-ray diffraction was used to study the structure of the films, and a clear correlation between crystallographic texture and film stress is found. The original structure/impurity model for film stress modification due to ion bombardment has been modified to account for the relationship between film stress and texture.

Films of the high‐temperature superconductor in the Bi‐Sr‐Ca‐Cu‐O system were deposited by pulsed laser evaporation from a ceramic disk. Films deposited onto (100) MgO show x‐ray diffraction patterns characteristic of c‐axis textured Bi2(Sr,Ca,Bi)3Cu2Ox phase following annealing for 10 min at 850 °C in 20% O2‐80% N2. These textured films have zero resistance at 78 K and show resistance drops near 110 K. The effects of the annealing environment on the electrical and structural properties are presented.