Abstract

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⁺ (5²D_5/2) is also observed in the outer reactive layers of the plasma plume.

Abstract

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.

Abstract

The plasma produced during laser ablation deposition of thin film YBCO has been studied by optical emission spectroscopy.  There is evidence of increased YO band emission in the range 590-625 nm as the ambient oxygen gas pressure confining the plume is increased in the range 30 200 m Torr.  Temporal profiles show that close to the target the plume is insensitive to ambient oxygen pressure.  It is deduced that the optical emission here is excited by electron impact excitation.  Further away from the target there is evidence that two distinct processes are at work.  One is again electron excitation; the emission from this process decreases with distance because the expanding plume cools and collisions become less frequent in the expanding gas.. The second is driven by oxidation of atomic species expelled at high speeds from the target.  The main region of this activity is in the plume sheath where a shock front ensures heating of ambient O₂ and reaction of monatomic plasma species to form oxide in an exothermic reaction.  Spatial mapping of the emission demonstrates clearly how increasing oxygen gas pressure confines the plasma and enhances the emission intensity from the molecular YO species ejected from the target in a smaller region close to the target.  Ba' is observed as a dominant species only very close to (within I mm of) the target.  Absorption spectra have been taken in an attempt to examine ground state and cool species in the plume.  They reveal the quite surprising result that YO persists in the chamber for periods up to I msec.  This suggests an explanation for the recent report of off-axis laser deposition in terms of simple condensation.  Previously, quasi-ballistic transfer of material from target to substrate has been considered the only significant process.

Abstract

Pulsed laser deposition (PLD) of a ZnS:Mn hot pressed target using a KrF laser, has produced a high rate deposition method for growing luminescent thin films.  Good stoichiometric quality and typical luminescent crystal structures have been observed with a predominant hexagonal phase and little evidence of the cubic phase.  'Re luminescent characteristics were determined by CL and PL excitation and stable electroluminescence was observed under pulsed de conditions with a minimum brightness of 150cd/m².  PLD film characteristics are compared with those observed in RF sputtered samples.

Abstract

Josephson junctions are prepared in a multilayer process using laser deposited films and patterning with conventional photolithography and argon-ion etching.  The effect of microwave radiation at 12.3GHz applied to ramp type Josephson junctions was observed.  The current-voltage characteristics of these junctions at 8.8K follow the resistively shunted junction model with excess current.  In addition to the usual Shapiro steps half-integral constant-voltage steps are observed in the presence of an additional static magnetic field.  The dependence of the widths of the Shapiro steps and of the half-integral constant-voltage steps on induced microwave current and magnetic field is investigated.