The role of ion-beam cleaning in the growth of strained-layer epitaxial thin transition metal films
- PDF / 1,329,581 Bytes
- 10 Pages / 595.44 x 841.68 pts Page_size
- 117 Downloads / 171 Views
J. Talvacchio Westinghouse Research and Developement Center, Pittsburgh, Pennsylvania 15235 (Received 10 November 1986; accepted 6 June 1987) Low-energy ion-beam cleaning of the substrates prior to a deposition greatly enhances the quality of ultrathin ( < 100 A) refractory superconducting (Nb, V) films. Using this technique Nb films as thin as 7 A have been grown, from which good tunnel junctions have been fabricated. Both the native films and the tunnel junctions are sturdy and can be thermally recycled without any degradation. In-situ surface study along with transmission electron microscopy (TEM) results suggest the removal of the carbon atoms from the surface of the substrate without an apparent surface damage as the causes of the improvement. The TEM results indicate that the Nb films grow perfectly lattice matched to the sapphire substrate when the substrate is ion-beam cleaned. This strained-layer epitaxy is observed up to 40 A, the maximum thickness investigated through TEM.
I. INTRODUCTION Very thin quasi-two-dimensional films are of interest for their rich physics as well as their possible applications in microelectronics. However, until recently studies on ultrathin metallic films have been limited to a few cases of quench-condensed films, since only they stayed continuous and homogeneous at a thickness of a few monolayers. Thin Nb films have been prepared in various ways and studied extensively for their superconducting properties.1^* For superconductors, the superconducting properties together with the normal state transport behavior provide natural and convenient means to probe the quality of a film. For example, the width of the transition temperature is a good indicator of how homogeneous a film is on the scale of the superconducting coherence length. Most ultrathin ( < 100 A) Nb films reported in the literature have depressed Tc from their bulk value with the transition width getting as wide as ~ 2 K. Even in our case the films prepared without ionbeam cleaning had wide transition temperatures. In addition, good tunnel junctions could not be fabricated on such films. In this paper we report a deposition procedure that yields high quality and homogeneous ultrathin transition metal films (Nb and V) at a 100 °C substrate temperature. The relatively low substrate temperature is desirable for many applications because the chemical
a)
b)
Present address: I. B. M. Thomas J. Watson Research Center, Yorktown Heights, New York 10598. Also at Bell Communications Laboratory, Red Bank, New Jersey 07001-7020.
446
J. Mater. Res. 2 (4), Jul/Aug 1987
http://journals.cambridge.org
reaction between the film and the substrate or subsequently deposited material is decreased. II. SAMPLE PREPARATION The samples are prepared by e-beam evaporating Nb (or V) onto \X\va. polished single-crystal sapphire substrates oriented in the (ll02) or R plane. The substrates are cleaned thoroughly with a combination of H2SO4:H2O2, deionized water, and methanol before they are put inside the evaporator. The evaporator is describ
Data Loading...
