How to grow single-crystalline and epitaxial NiTi films in (100)- and (111)-orientation

Abstract

Understanding the martensitic microstructure in NiTi thin films helps to optimize their properties for applications in microsystems. Epitaxial and single-crystalline films can serve as model systems to understand the microstructure, as well as to exploit the anisotropic mechanical properties of NiTi. Here, we analyze the growth of NiTi on single-crystalline MgO(100) and Al2O3(0001) substrates and optimize film and buffer deposition conditions to achieve epitaxial films in (100)- and (111)-orientation. On MgO(100), we compare the transformation behavior and crystal quality of (100)-oriented NiTi films on different buffer layers. We demonstrate that a vanadium buffer layer helps to decrease the low-angle grain boundary density in the NiTi film, which inhibits undesired growth twins and leads to higher transformation temperatures. On Al2O3(0001), we analyze the orientation of a chromium buffer layer and find that it grows (111)-oriented only in a narrow temperature range around 500 °C. By depositing the Cr buffer below the NiTi film, we can prepare (111)-oriented, epitaxial films with transformation temperatures above room temperature. Transmission electron microscopy (TEM) confirms a martensitic microstructure with Guinier Preston (GP)-zone precipitates at room temperature. We identify the deposition conditions to approach the ideal single crystalline state, which is beneficial for the analysis of the martensitic microstructure and anisotropic mechanical properties in different film orientations.

Integration of Multifunctional Epitaxial (Magnetic) Shape Memory Films in Silicon Microtechnology

“…For optimum properties, NiTi requires different deposition and heat treatment conditions compared to Ni-Mn-Ga, as described in the Experimental Section. However, we could directly use the growth parameters optimized for MgO substrates [40] for the growth on STO/SOI, which illustrates that our approach requires a minimum adaption of existing processes. The 500 nm thick film, depicted in 2e.…”

“…The epitaxial NiTi film was grown by sputter deposition, as described in. [40] A 50 nm thick Cr buffer layer was deposited on the STO/SOI substrate at a deposition temperature of 500 °C, followed by the 500 nm thick NiTi film deposited at 525 °C. The 4″ NiTi target had a composition of Ni 48.6 Ti 51.4 .…”

Thermal Stability of Cu-Al-Ni Shape Memory Alloy Thin Films Obtained by Nanometer Multilayer Deposition

“…Most of the works on this topic have been carried out on TiNi SMA [ 17 ], and thin films of this alloy have been produced by a variety of techniques. DC sputtering deposition [ 19 , 20 , 21 , 22 ] and RF magnetron sputtering [ 23 , 24 , 25 ] have been the most used techniques, and were recently reviewed [ 26 , 27 ]. However, other techniques are also used, such as co-evaporation [ 28 ], pulsed laser deposition [ 29 ] and e-beam evaporation [ 30 ].…”