Physical Properties of CVD-Derived Nb3Ge Films

A compilation of optimum physical properties of CVD-derived Nb3Ge films (i.e., T, 7c, and //c2 values of 22.5 K, 1.8 x 10 Acm and 27.3 T) is listed in Table 2-4. Evaluated variables for Nb3Ge film growth include substrate temperature, the gaseous metal chloride hydrogen mole ratio in the carrier gas stream, NbCU (g) to GeCb (g) mole ratio, and the reactor viscosity (Reynolds number). 

The Tc of CVD-derived Nb Ge films has been correlated with a parameter which involves a function of mole ratio, dilution, and reactor Reynolds number [52], Samples with high Tc ( 17 K) were produced over a wide range of flow conditions and mole ratios. 

There also exists a relationship between u-axis length and superconducting transition temperature in Nb-based A15 materials [58]. Noolandi and Testardi compiled experimental data which showed a direct correlation between an increase in -axis spacing (Aa/oo) and degradation in (-A7].). According to this empirical relationship, thin film materials with optimum values have a smaller a-axis lattice spacing than bulk A15 samples or thin film samples with non-optimum 7].s. That is, as the a-axis decreases from its equilibrium value (e.g., 5.167 A for NbiGe), a concomitant increase in 

is observed in NbjX materials where X = Ge, Sn, Ga, and Al. This effect has been observed for CVD-derived Nb Ge by numerous research groups, with an optimum a-axis spacing 5.138 Oo 5.142 A affording values in excess of 21 K [52, 56, 57, 59]. Also, a correlation between the lattice constant and Nb Ge ratio has been reported for CVD-derived Nb Ge films on Nb Ir lattice-matched substrates [47]. 

The effects of grain size development upon critical current density have been evaluated and compared for CVD-derived versus sputtered Nb Ge films [60]. A columnar structure is observed for Nb.iGe thick films (18-22 pm) grown by CVD with an average grain size greater than 1 pm and values up to 21.5 K. By way of comparison, sputtered Nb Ge films exhibit average grain sizes of about 0.1 pm and Tc values up to 22.7 

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