T. Morrow

Superconducting YBa2Cu3O7 thin films on MgO by KrF laser ablation: Optimization of deposition parameters

The optimization of interrelated deposition parameters during deposition of in situ YBa2Cu3O7 thin films on MgO 〈001〉 substrates by KrF laser ablation was systematically studied in a single experimental chamber. The optimum condition was found to be a substrate temperature of 720 °C and a target‐substrate distance of 5 cm in an oxygen partial pressure of 100 mTorr. These conditions produced films with Tc = 87 K. The presence of YO in the plasma plume was found to be important in producing good quality films. The films were characterized by resistance‐temperature measurements, energy dispersive x‐ray analyses, scanning electron microscopy, and x‐ray‐diffraction measurements, and the physical reasons underlying film quality degradation at parameter values away from optimal are discussed.

EMISSION STUDIES OF THE PLUME PRODUCED DURING YBA2CU3O7 FILM PRODUCTION BY LASER-ABLATION

The optical plume emissions produced on excimer laser ablation of a YBa2Cu3O7 target are reported and identified with the various atomic, ionic, and molecular species present. The spatial and temporal distribution of these emissions were studied as a function of the laser fluence and oxygen pressure. At the laser fluences used (4–6 J/cm2) some target material is ablated or evaporated directly in molecular form. In addition efficient formation of molecular oxides is observed at the contact front of the expanding plume with the surrounding oxygen atmosphere. The intensity and spatial distribution of oxide emission in the visible plume therefore provides a sensitive diagnostic for optimization of substrate location and deposition conditions.

Langmuir probe measurements of plasma parameters in the late stages of a laser ablated plume

A simple Langmuir probe technique has been used to measure the electron density, electron temperature, and plasma potential in the late stages (>5 μs) of a laser ablated plasma plume. In the plasma, formed following 248 nm laser irradiation of a copper target, in vacuum at a laser fluence of 2.5 J cm−2, electron densities of ∼1018 m−3 and temperatures of ∼0.5 eV were measured. These values are comparable with those reported previously using Faraday cup detectors and optical emission spectroscopy, respectively.

Wavelength dependent refractive effects and Stark broadening in laser‐induced YBa2Cu3Ox plasma plumes

Spectral lines observed at short delay times and close to the target during laser ablation of YBa2Cu3Ox show strongly distorted line shapes arising partly from optical refraction by the large density gradients within the expanding plume. Spatially and temporally resolved linewidths, corrected for refraction effects, are reported for the Ba(I) 553.5 nm absorption transition and Stark widths, deduced from these corrected linewidths, indicate that the maximum electron number density at 1.0 mm from target is ∼1×1019 cm−3.

Optical absorption spectroscopy study of the role of plasma chemistry in YBa2Cu3O7 pulsed laser deposition

Time‐resolved optical absorption spectroscopy techniques were used to study Ba, metastable Ba+, and YO absorptions in the laser‐produced plasma plume from a YBa2Cu3O7 target. Results obtained indicate an initial explosive removal of material from the target surface followed by a subsequent evaporation process. Some YO is ejected from the target in molecular form, particularly at laser fluence <6 J/cm2, whilst additional YO is formed in subsequent reactions of Y with oxygen at the plasma plume edges. The formation of metastable Ba+ (52D5/2) is also observed in the outer reactive layers of the plasma plume.

The Langmuir probe as a diagnostic of the electron component within low temperature laser ablated plasma plumes

A Langmuir probe has been used as a diagnostic of the temporally evolving electron component within a laser ablated Cu plasma expanding into vacuum, for an incident laser power density on target similar to that used for the pulsed laser deposition of thin films. Electron temperature data were obtained from the retarding region of the probe current/voltage (I/V) characteristic, which was also used to calculate an associated electron number density. Additionally, electron number density data were obtained from the saturation electron current region of the probe (I/V) characteristic. Electron number density data, extracted by the two different techniques, were observed to show the same temporal form, with measured absolute values agreeing to within a factor of 2. The Langmuir probe, in the saturation current region, has been shown for the first time to be a convenient diagnostic of the electron component within relatively low temperature laser ablated plasma plumes.

A QUANTITATIVE INVESTIGATION OF EMISSION FROM LOW TEMPERATURE LASER-INDUCED YBA2CU3OX PLASMA PLUMES

Quantitative emission spectroscopy has been used to study the plume formed following laser ablation of YBCO in an oxygen atmosphere. Excited state population distributions, determined from emission line intensity ratios, are used to investigate spatiotemporal variations in the local Boltzmann temperatures for both neutral and ionic species within the expanding plume. Temperatures, obtained from emission line intensity ratios of both Cu(I) and Y(I), decrease slowly and nonadiabatically in the range 1.0–0.5 eV during plume expansion. Higher initial Boltzmann temperatures of ∼3 eV are however obtained from the emission line intensity ratios of fast ions, which dominate the composition of the highly luminous expanding front of the plume. Quantitative comparison of emission intensities for the different neutral species present indicate that the neutral composition in the luminous region of the expanding plume is increasingly dominated by Cu(I) which has a faster expansion velocity and lower oxidation rate than...

Three-dimensional number density mapping in the plume of a low-temperature laser-ablated magnesium plasma

Simultaneous optical absorption and laser-induced fluorescence measurements have been used to map the three-dimensional number densities of ground-state ions and neutrals within a low-temperature KrF laser-produced magnesium plasma expanding into vacuum. Data is reported for the symmetry plane of the plasma, which includes the laser interaction point at a delay of I μs after the 30 ns KrF laser ablation pulse and for a laser fluence of 2 J cm -2 on target. The number density distributions of ion and neutral species within this plane indicate that two distinct regions exist within the plume; one is a fast component containing ions and neutrals at maximum densities of ∼ 3 x 10 13 cm -3 and ∼ 4 X 10 12 cm -3 . respectively and the second is a high-density region containing slow neutral species, at densities up to ∼ 1 x 10 15 cm -3 .

THE ROLE OF GAS-PHASE OXIDATION AND COMBINATION DURING LASER DEPOSITION OF YBA2CU3O7-X IN AMBIENT OXYGEN

Spectroscopic absorption and emission measurements have been used to study laser deposition of YBCO films. They show that >95% of the monatomic Y and Ba initially ablated from the target undergo gas-phase chemical combination before film deposition. In contrast, considerable monatomic Cu persists into the deposition region. In this region, equilibrated gas temperatures are of the order of 2700 K. It is suggested that this high temperature facilitates film crystallization and epitaxial growth. The survival of monatomic Cu in the plume to the site of deposition is a manifestation of its endothermic reaction with O2.

Diagnostic of rf discharge plasma by Thomson scattering with gated intensified charge coupled device detectors

A Thomson scattering diagnostic, using an intensified gated charged coupled device detector and a high-repetition rate yttrium aluminum garnet laser, was utilized to measure electron parameters in a radio-frequency discharge. Both inductively and capacitively coupled plasmas in argon and argon–oxygen mixtures were studied, with electron densities as low as ∼5×109 cm−3. Different modes of detection (direct accumulation and photon counting) have been compared. The photon counting technique has been used to provide better accuracy at low light level (i.e., low electron densities). The results are compared with Langmuir probe measurements performed under the same plasma conditions, and good agreement is found between the two diagnostics.