Structural and Electrical Properties of Titanium-Nickel Films Deposited Onto Silicon Substrates
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STRUCTURAL AND ELECTRICAL PROPERTIES OF TITANIUM-NICKEL FILMS DEPOSITED ONTO SILICON SUBSTRATES KATHLEEN R. COLLEN,* ARTHUR B. ELLIS,* J. D. BUSCH **and A. D.JOHNSON** *University Wisconsin-Madison, Department of Chemistry, Madison, WI 53706 **TiNi Alloy ofCo., Oakland, CA 94608
ABSTRACT Thin films of the shape memory alloy NiTi have been sputter-deposited onto ptype silicon substrates. Films that are initially amorphous may be crystallized by vacuum annealing. The crystalline films exhibit the B2->B19° phase change associated with the shape memory effect while remaining in contact with the silicon substrate. Transition temperatures were determined by resistance measurements and x-ray diffraction. The NiTi - Si contacts are diodes, as evidenced by their current-voltage characteristics; however, the effect of the phase change on the barrier height could not be determined. INTRODUCTION The intermetallic nickel-titanium alloy is the most common material exhibiting the shape memory effect [1]. A thermoelastic martensitic phase transformation provides the mechanism for the shape memory. In NiTi, the high temperature phase (austenite) has the CsCI, or B2, structure. An intermediate rhombohedral structure, the R - phase, has been identified. The low temperature phase (martensite) is monoclinic, and may exist in any one of twenty-four geometrically-related variants. Deforming martensitic NiTi interconverts these variants to whichever ones most reduce the internal energy. Upon heating, the NiTi must recover the high temperature structure: The only low energy pathway to the CsCI structure exactly retraces the atomic movements responsible for the deformation, and this causes the shape memory. There has been growing interest in the fabrication of free-standing shape memory alloy films [2], and also in films that remain bound to a substrate [3]. The films prepared to date can be resistively heated to produce a mechanical response. They have been incorporated into the control element in microvalves and may prove useful as microactuators. To our knowledge, semiconductor-metal structures have not previously been fabricated from NiTi films. Diodes constructed from such structures could in principle exhibit current-voltage properties reflecting the phase of the NiTi overlayer. In this paper we lay the groundwork for such a study by demonstrating that crystalline NiTi films can be deposited onto p-Si, yielding structures that exhibit rectifying behavior. EXPERIMENTAL NiTi films were prepared by standard d.c. magnetron sputtering from a composite NiTi target. Polished (100) p-type silicon substrates were chemically etched with a 10% solution of HF and rinsed with methanol prior to deposition. No intermediate or adhesion layers were deposited between the silicon and the NiTi. The films were annealed in sealed glass ampules, at 540 0 C under vacuum (10-3 torr), to initiate crystallization. Higher temperature annealing was not used because it is reported to cause precipitation within the metal film and reaction between the film and the
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