WHY ARE HIGH-T-C SUPERCONDUCTORS, HTSC, DEPOSITED BY 248-NM LASERS AT400-MW CM(2)/

HTSC are commonly deposited by pulsed laser deposition. The laser 'ope rating point' is usually near the above values. It is interesting to e xplore the underlying physical processes which make these values near optimum. The critical point in understanding the operating point is wh ether the stoichiometry of the (source) target is retained upon deposi tion. This question usually reverts to a question of retaining CuO as a diatomic (dissociation energy D-0 = 2.8 eV) as compared to the much more stable YO (D-0 = 7.3 eV) or BaO (D-0 = 5.8 eV). High temperatures obviously serve to dissociate these diatomics, with CuO being the mos t susceptible. First, consider the (very important) irradiance level o f I approximate to 400 MW/cm(2). This value is at the lower limit of i ntense plume ionization, above which the ablation enters the 'plasma c ontrolled' situation. This latter occurs when inverse Bremsstrahlung a bsorption by free electrons (10 to 20 mu m) above the target surface c ontrol the laser transmission and ablation dynamics. While this descri ption is only firmly established at I greater than or equal to 1 GW/cm (2), computer modeling [2] confirms an extrapolation down to 500 MW/cm (2). The important predictions of plasma controlled etching are: The t otal material transfer goes as area, A(1/2) (and not A(-1/2)). The rea son is higher fluences waste energy in heating the free electron densi ty above the target instead of heating the target surface directly. On the other hand, one wishes to work adjacent to this border to maximiz e the temperature, T, and (exponential in T) vapor pressure. Avoiding intense plasma heating has additional benefits for maintaining the Cu as CuO molecules. Laser-induced fluorescence measurements indicate tha t the reaction Cu + N2O --> CuO + NO requires > 10 mu s and is not a s imple collisional process, but rather an attainment [4-8] of quasi-equ ilibrium at T less than or similar to 2000 K. Combining this observati on with the fact that typically < 10(-6) cm(3) or typically similar to 10(16) atoms are removed per pulse, a static gas fill of similar to 0 .2 Torr is necessary to decelerate the expanding plume. The initial (t ypically 3-10 eV) beam energies are transformed into a gas temperature less than or similar to 2500 K when stopped by and mixed with the oxi dizing gas. For present gas densities this temperature is near the max imum [7] for the non-dissociation of CuO (as based on thermodynamics a nd densities of 3 x 10(17) Cu/cm(3)). T is rapidly elevated for too in tense plasmas; i.e. for I > 1 GW/cm(2). Hence, increased irradiance pl aces one in the (undesirable) region of CuO decomposition. Furthermore , large I increases the pressure impulse on the target, which may be t he critical quantity leading to particulate emission. One can now visu alize the advantage of 248 nm versus longer wavelengths. At longer lam bda the inverse Bremsstrahlung heating of free electrons is elevated b y a factor greater than or equal to lambda(2), again unfortunately ele vating T-plume. At shorter wavelength (193 nm), elevated photodissocia tion eliminates nearby all the initial CuO; i.e. an undersirable situa tion. In summary, one can now make quantitative statements which point to similar laser lambda and I values as found empirically.