B. W. Hussey

Superconducting oxide films with high transition temperature prepared from metal trifluoroacetate precursors

Superconducting thin films of Y‐Ba‐Cu oxide have been prepared on yttria‐stabilized zirconia substrates using metal trifluoroacetate spin‐on precursors. The films exhibit an extremely sharp resistive transition with zero resistance at temperatures as high as 94 K. The superconducting phase is formed by a three‐step process: (a) decomposition of the spun‐on trifluoroacetate film to the fluorides, (b) conversion of the fluorides to oxides by reacting with water vapor, and (c) annealing followed by slow cooling in oxygen. The properties of the films depend on the amount of conversion of the fluorides by reaction with water. Films which show the presence of some unreacted barium fluoride have strong c‐axis normal preferred orientation, with a sharp resistive transition. When all the barium fluoride is converted, the film is more randomly oriented and exhibits a broader transition to zero resistance.

Direct imaging of the fragments produced during excimer laser ablation of YBa2Cu3O7−δ

The evolution dynamics of the fragments produced during KrF excimer laser (248 nm, 25 ns) ablation of YBa2Cu3O7−δ has been observed by ultrafast photography using a synchronized dye laser beam (∼1 ns) to probe above the target surface. The images show that fragment removal is initiated near the beginning of the laser pulse (≳10 ns), continues for a maximum of a few hundred nanoseconds, and has an expansion front velocity suggesting a target temperature varying from about 1500 to 4000 K or greater. The shock wave formed due to interaction of the ablated fragments with background oxygen gas has also been photographed, and its temporal evolution as a function of oxygen pressure has been shown to agree better with a planar than spherical model. The overall picture is that of an ablation process showing pronounced but understandable gas‐dynamic effects.

Laser deposition of YBa2Cu3O7−δ films using a pulsed oxygen source

Thin films of YBa2Cu3O7−δ (YBCO) have been grown by pulsed laser deposition in a low‐pressure background (10−4–10−3 Torr) by using a pulsed, high intensity jet of O2 or N2O for oxidation. The oxygen source is provided by a pulsed molecular beam valve, and the opening of the valve and the triggering of the laser are synchronized with appropriate delay so that the supersonic gas jet and the ablated fragments arrive at the substrate at the same time. This provides the necessary oxygen to form the YBCO phase while maintaining a low oxygen background. The YBCO phase is not formed if the oxygen pulse is provided either before or much after the arrival of the ablation fragments at the substrate. The ability to grow superconducting films at low background pressures should allow usage of in situ analysis techniques, such as reflection high‐energy electron diffraction, during pulsed laser deposition.

Layer-by-layer deposition of La1.85Sr0.15CuOx films by pulsed laser ablation

Reflection high‐energy electron diffraction (RHEED) has been used to monitor the growth of La1.85Sr0.15CuOx (LSCO) thin films on (100) SrTiO3 substrates by pulsed laser deposition. The films are grown using a combination of pulsed molecular oxygen and a continuous source of atomic oxygen, with the average background pressure maintained as low as 1 mTorr. The RHEED pattern is sharp and streaky, and the intensity of the specular beam oscillates during the deposition, indicating a two‐dimensional layer‐by‐layer epitaxial growth. The film thickness measured by x‐ray small‐angle interference is consistent with the thickness determined by the RHEED oscillation period with a growth unit of half a unit cell. Thin films of YBa2Cu3O7−δ (YBCO) with good RHEED oscillations have also been grown under similar oxygenation conditions. The low‐pressure‐grown LSCO and YBCO films are superconducting, with zero‐resistance temperatures of 15 and 80 K, respectively.

Defect formation caused by a transient decrease in the ambient oxygen concentration during growth of YBa2Cu3O7−δ films

Thin films of YBa2Cu3O7−δ have been grown in an oxygen atmosphere by pulsed laser deposition using two synchronized lasers, separated by a variable delay (1 μs–10 ms). The ablated fragments from the first laser lead to formation of a blast wave in O2, leaving behind a rarefied ambient. If the second laser is triggered before the O2 pressure returns to equilibrium, the resulting films show a decrease in transition temperature with an expanded c‐lattice parameter caused by defects in the nonchain sites of YBa2Cu3O7−δ. This demonstrates the necessity of maintaining a sufficiently high concentration of oxygen within the time period that the fragments travel and deposit on the substrate.

Temperature distribution during heating using a high repetition rate pulsed laser

A general equation has been derived for computing the temperature distribution produced in a substrate heated by a high repetition rate pulsed laser. The theoretical predictions have been compared with experimental results obtained for etching of Mn‐Zn ferrite in KOH solution using a copper vapor laser. The effects of the laser power and substrate scan speed on the temperature distribution have been investigated, and the predicted melt‐zone boundaries have been compared with the experimentally observed width, depth, and shape of the etched grooves.

Effect of different oxidizing gases on the in-situ growth of YBa2Cu3O7−δ films by pulsed laser deposition

Abstract The effectiveness of oxygen (O 2 ), nitrous oxide (N 2 O), and nitrogen dioxide (NO 2 ) as oxidizing agents during in-situ growth of YBa 2 Cu 3 O 7−δ (YBCO) films on (100) SrTiO 3 substrates by pulsed laser deposition has been studied as a function of deposition temperature (700–800°C), and laser wavelength (193,248 and 355 nm), for a wide range of oxidizer gas pressure (0.1–200 mTorr). In general, the superconducting transition temperature of the films has been found to increase with increasing oxidant pressure, with zero-resistance temperature ≈90 K only obtained in films prepared in a relatively high pressure (150–200 mTorr) of oxidizer gas. At lower pressures, the transition temperature while being depressed is quite sensitive to the nature of the oxidant, the laser wavelength and the deposition temperature. Nevertheless, independent of the oxygen source or other growth parameters, an almost linear decrease in transition temperature with a corresponding increase in the c -axis lattice parameter has been observed for all the film. YBCO films have also been deposited in a low pressure background (≤ 1 mTorr) using a combination of atomic oxygen and pulsed molecular oxygen. The results are discussed in terms of the oxygen requirement for kinetic and thermodynamic stability of YBCO during growth of the film by pulsed laser deposition.

Layer-by-layer growth of cuprate thin films by pulsed laser deposition

Abstract The pulsed laser deposition technique has been used to deposit, in a layer-by-layer growth mode, superconducting cuprate thin films on (100) SrTiO 3 substrates. The films are deposited using a combination of pulsed O 2 jet and a continuous source of atomic O, with the average background pressure maintained as low as 1 mTorr. This permits the in-situ monitoring of the growth process using reflection high-energy electron diffraction (RHEED). Before growth of the cuprate films, a thin SrTiO 3 buffer layer is homoepitaxially deposited on the substrate to provide an atomically smooth surface. On a smooth surface, the growth of SrTiO 3 occurs in a layer-by-layer mode as indicated by the intensity oscillations of the specular spot in the RHEED pattern. Subsequent layer-by-layer growth of a variety of cuprate films, including YBa 2 Cu 3 O 7− δ (YBCO), La 1.85 Sr 0.15 CuO 4 (LSCO), Nd 1.85 Ce 0.15 CuO 4 (NCCO), and SrCuO 2 (SCO), has been achieved under appropriate deposition conditions.

Laser‐assisted etching of YBa2Cu3O7−δ

Superconducting YBa2Cu3O7−δ has been etched using a focused cw Ar+ laser beam in KOH solution. The results indicate clean and rapid removal of reaction products leaving high aspect ratio grooves. Etch rates are investigated under several conditions; etch geometry is discussed as a function of laser power density and scan speed. The surface reaction with KOH is thermally activated, and the residual material is identified by energy dispersive x‐ray spectroscopy, x‐ray powder diffraction, and chemical analysis.

Role of bubbles in laser-assisted wet etching

Evidence is presented to explain laser‐based aqueous salt etching in terms of vapor bubble growth conditions at the irradiated surface of a substrate. The effect of pulse and frequency modulation of a cw laser is investigated to determine growth behavior for adhering bubbles in pure water. The role of salt in solution (such as KOH, NaOH, or CaCl2) is discussed in terms of its function as a diffusion barrier for growing vapor bubbles. Etching data are presented for the case of a Cu‐vapor laser applied to a Mn‐Zn‐ferrite substrate, comparing results with and without KOH in the etching solution.