TEM
of microstructures and lattice bending formed in thin
Sb
film with thickness gradient
Authors: Kolosov Vladimir , Yushkov Anton , Veretennikov Lev , Pologov Ilya 
Published: December 2016

Thin‐film Sb‐based phase‐change materials (PCMs) are widely used for memory devices that utilize amorphous‐crystalline transitions in local areas. Microstructures of Sb thin films are of special interest since Sb films are known for fast crystallization rates [1], including “explosive” crystallization (more known and studied for Si and Ge films). In this paper we study the role of film thickness and use bend‐contour technique [2] for crystal misorientation studies [3] (supported by local thickness estimates). It was out of focus for several dozens of papers devoted to the Sb crystallized thin films.

Sb thin films were evaporated in vacuum on mica substrates covered by thin evaporated carbon film to provide initial amorphous structure. Since thin‐film crystallization strongly depends upon thickness we use masks to obtain strong thickness gradients (about 40 nm per film length 1 mm). The film separated from the substrate and placed on TEM grid are studied using TEM, SAD, STEM, SEM, EDX in JEM‐2100 (80Kv and 200 Kv). Bend contours are indexed with the help of bright and dark fields and indexed electron diffraction patterns. Measured distances between bend contours and ZAPs are used for estimates of lattice bending, while measurements of fine structure of some strong contours in dark fields add estimates of film thickness.

The final microstructures give indications of initial amorphous structure. The crystallization starts at the thicker area and stops at the thinner areas,
Fig. 1 a
, where labyrinthine and islands microstructures are observed. SADs of these areas demonstrate superposition of amorphous hallo and single crystalline orientation. The spot pattern usually does not change while moving aperture several microns around. The amorphous hallo outside crystallization front does not include any crystal spots. Going from thinner area to the thicker one,
Fig. 1 c‐d‐e‐f
, along the thickness gradient one can trace in TEM the increase of density of amorphous islands finally resulting in labyrinthine microstructure. In parallel the spread of island sizes decreases from 0‐20nm to around 40 nm and their density increases twice from ~250/µm
2
.

More or less prominent bend contours (
Fig. 1 b
) or similar weak contrast (
Fig. 1 a
) are observed everywhere in crystalized areas. Similar but not analyzed bend contour patterns were published earlier in some TEM studies of Sb films (see e.g. [1, 4]). Going from thinner area to the thicker one in the areas of entire film along the thickness gradient,
Fig. 1 g‐h‐i‐j
, we calculate essential decrease of lattice planes bending (around axes lying in the film plane) in the range from most strong values, 120 degrees per µm, to ~ 10 degrees per µm. We suppose that this lattice bending can be attributed to the “transrotational” structure revealed and proved earlier for other thin‐film crystals of different chemical nature and preparation conditions [3]. Both maximal values and film thickness dependence of transrotation correspond to those studied earlier for some other substances [5]. The microstructure texture reminding parquet drawing is also observed (in the regions of intermediate thickness) with subgrain sizes: width 0,1 ‐ 1 µm, length 2‐10 µm and above. The lattice orientation texture is also revealed with preferred orientations [0 0 1], [‐1 1 1], [1 2 ‐1]. There are other details of lattice orientation texture observed along the crystal grains and thickness gradient.

The nature of transrotation, unusual phenomenon for crystal growth in amorphous films, is discussed.