Revealing the crystal orientation of ultrathin films

Nanocrystalline films display distinct chemical and physical properties that hold significant promise for the design of novel nanostructures. Among these structures are ultrathin surface coatings with unique antifriction, anticorrosion, electrochromic, antistatic, and even ‘smart’ properties that can detect changes in the environment or facilitate chemical reactions. Research and development of these films, however, requires precise imaging of their surface texture (i.e., crystal orientations).

The morphology and texture of a film directly controls many important physical characteristics, including optical, magnetic, mechanical, electrical, and electrochemical behaviors. As such, the crystal-orientation distribution of nanocrystalline grains is one of the most fascinating and challenging topics in the study of thin-film growth.¹ Tracking the surface-texture evolution of the material provides insight into how the structure evolves over time and helps scientists understand the mechanism of nanostructure formation, thereby allowing engineers to design and recreate uniquely functional nanomaterials.

The most common technique used to study the texture orientation of a polycrystalline or nanocrystalline film is the x-ray pole-figure measurement.² The pole figure is a stereographic projection of a specimen that represents the variation in pole density distribution of a specific family of crystal planes. The pole figure can unambiguously determine both the in-plane and out-of-plane texture of the film. However, x-rays penetrate deep (several microns) into the film, and the obtained texture is an average of the entire thickness of the film. As the texture of a film often changes during film growth, information on the basic mechanisms is often lost. For this reason, it is important to develop a technique that is surface or growth-front sensitive to allow acquisition of the complete evolution of the texture during film growth in situ.

At the Rensselaer Polytechnic Institute, we recently developed a new reflection high-energy electron diffraction (RHEED) surface pole-figure technique that can quantitatively reveal the surface texture of nanocrystalline films.^3–5 In RHEED, the electron beam strikes the surface at a grazing angle, and the electron's escape length is only a few nanometers. The polar angle intensity profile of a particular family of crystal planes is first constructed from a single RHEED pattern containing rings or arcs at a particular azimuthal (in-plane) angle. The combination of intensity profiles from multiple RHEED patterns at different azimuthal angles provides a surface pole figure that covers the entire reciprocal space above the sample surface. This surface pole-figure technique represents a significant advance beyond the recently employed in-plane rocking-curve method, and makes it possible to clearly display the complete information about the azimuthal angle orientation of textured thin films.⁶ Figure 1 shows a RHEED surface pole figure (top) from a ruthenium nanorod film and a schematic (bottom) of RHEED from a nanostructured film. The pole figure consists of concentrated rings that indicate the nanorods have a fiber texture with random in-plane orientation.³

Figure 1. (top) Reflection high-energy electron diffraction (RHEED) surface pole figure consisting of a ring structure from a ruthenium nanorod film revealing a fiber texture with random in-plane crystal orientation.³(bottom) A schematic diagram showing RHEED from a nanostructured film.

The creation of surface pole figures is particularly important for revealing the nature of the growth of nanostructures such as nanodots, nanorods, and nanoblades, because their texture orientation can change dramatically over time. When films are grown on a noncrystalline surface, they often exhibit a multigrain structure with a preferred out-of-plane orientation that has a minimum surface energy called a ‘fiber texture.’ Typically, the pole figure of a fiber texture consists of a circular ring structure, indicating that there is no preferred orientation in the in-plane direction. However, if the deposition flux enters the surface at an oblique angle with respect to the surface normal, then the film may not possess a fiber texture. Due to atomic shadowing and anisotropic surface diffusion effects, the film can also exhibit a preferred orientation in the in-plane direction even when deposited on a noncrystalline substrate. This is called a ‘biaxial texture.’ Unlike the ring structure of a fiber texture, the pole figure of a biaxial texture contains a number of localized poles with high intensity, indicating a strong in-plane preferred crystal orientation.

Figure 2 shows an example of the constructed RHEED surface pole figures from magnesium nanoblades grown on an amorphous substrate at deposition times of 0.5min (i.e., ~22nm thickness) and 34.7min (i.e., ~1.49μm thickness) during oblique-angle deposition at an incidence angle of 75^° with respect to the surface normal.⁴ During deposition, the magnesium nanoblade orientation changes from a more or less completely random orientation at ~22nm thickness—Figure 2(a)—with a uniform pole intensity distribution, to a biaxial orientation at ~1.49μm thickness—Figure 2(b)—where the intensity distribution becomes localized at specific pole positions. This example clearly demonstrates the feasibility of the RHEED surface pole-figure technique as a tool for studying growth-front texture evolution.

Figure 2. RHEED surface pole-figure measurements during the growth of magnesium nanoblades using an oblique incident vapor flux at deposition times of (a) 0.5min (~22nm thick) and (b) 34.7min (~1.49μm thick). The poles developed as the film grew. (c) A side view of a scanning electron microscopy image of magnesium nanoblades viewed from the direction parallel to the incident flux direction that is out of the page. The scale bar is 200nm. (Reproduced with permission.⁴)

This method will provide new insight into how these kinds of structures evolve over time and help scientists to understand the mechanism of ultrathin film formation. With this information, engineers will be able to design nanomaterials optimized for specific applications, in particular, catalysis and energy-conversion devices.