Effect of substrate temperature on the microstructure of thin niobium films

Abstract

Niobium films with constant thickness have been deposited on sapphire (1120) by electron-beam evaporation at different substrate temperatures ( 150 °C ~< T s ~< 750 °C). The samples were characterized by X-ray diffraction and resistivity measurements. X-ray reflectivity shows that all films are covered with an oxide layer of about 20 A in ambient atmosphere. The (110) texture at high Ts decreases towards lower Ts, accompanied by an increasing surface roughness. Below T s = 350 °C the grain size in the growth direction becomes smaller than the film thickness and a relaxation of intrinsic stress is observed. A transition from a columnar growth structure to a fine grained microstructure is inferred. The change of the microstructure with T s is probably due to the temperature dependent grain boundary mobility during the deposition process.

Crossover from dirty to clean superconducting limit in dc magnetron-sputtered thin Nb films

“…We did not succeed to reproduce the sharp minima in the reflectivity data. This difficulty has also been reported by other authors [12] and is commonly attributed to the presence of inhomogeneous strain. This background component is a characteristic of rotational disorder [28] and accounts for the orientational distribution of epitaxial crystallites (mosaicity).…”

“…While the thickness determines chiefly the number and the position of oscillations, the roughness causes the damping of the oscillations with increasing angle. The formation of an oxide layer has not been taken into account in the simulation as no over-modulation due to the very thin oxide layer has been detected (as peculiar to thinner films [12,29]). We did not succeed to reproduce the sharp minima in the reflectivity data.…”

“…In particular, the influence of the lattice parameter a, grain size D, and surface roughness Δd on the critical temperature T c and the normal-state resistivity ρ n has been a matter of thorough experimental investigations on nano-granular disordered [7][8][9], high-purity [10], and dirty [11] Nb films. Different growth techniques have been used in these works [3][4][5][6][7][8][9][10][11][12][13][14][15]. For the growth of high-purity thin Nb films revealing outstanding structural quality, the molecular beam epitaxy (MBE) method has been shown to be the most efficient as compared to others [12][13][14], see also [15] for a topical review.…”

“…Different growth techniques have been used in these works [3][4][5][6][7][8][9][10][11][12][13][14][15]. For the growth of high-purity thin Nb films revealing outstanding structural quality, the molecular beam epitaxy (MBE) method has been shown to be the most efficient as compared to others [12][13][14], see also [15] for a topical review. At the same time, much less attention has been paid to optimize the growth conditions with emphasis on the tunability of the normal-state and superconducting properties of the films prepared by means of sputter deposition, in particular, by dc magnetron sputtering.…”

“…The correlation between the structural and superconducting properties in thin films of Nb represents a widely studied problem [3][4][5][6][7][8][9][10][11][12][13][14][15]. In particular, the influence of the lattice parameter a, grain size D, and surface roughness Δd on the critical temperature T c and the normal-state resistivity ρ n has been a matter of thorough experimental investigations on nano-granular disordered [7][8][9], high-purity [10], and dirty [11] Nb films.…”