Origin of characteristic grain-subgrain structure of tin-doped indium oxide films

Abstract

The microstructures of polycrystalline and heteroepitaxial tin-doped indium oxide (ITO) thin films prepared by sputtering and electron beam (EB) evaporation were investigated by electron microscopy, X-ray diffraction and reflection high-energy electron diffraction. The sputter-deposited ITO film on a glass substrate revealed a polycrystalline structure of grain size 400–800 nm. Within each grain, there were oriented subgrain regions 10–40 nm in size. This is a so-called “grain-subgrain structure” characteristic of sputter-deposited polycrystalline ITO films. These grains in sputter-deposited ITO films (“grain-subgrain structure”) were classified into three groups according to the morphology of the subgrains. The grains consisted of square, triangular and rectangular-shaped subgrains respectively. These three kinds of grain were oriented with the <400≫, <222≫ and <440≫ axes normal to the substrate surface respectively. It was also revealed that the film thickness of these three kinds of grain decreased in the order of the grains consisting of <400≫-, <222≫- and <440≫-oriented subgrains. However, heteroepitaxial ITO films sputtered on yttria-stabilized zirconia (YSZ) substrates, and EB-evaporated ITO films on both glass and YSZ substrates showed different structures. By comparing the microstructures of polycrystalline films with those of heteroepitaxial films, the origin of the above-mentioned “grain-subgrain structure” (variation of shapes and thickness of the subgrains) was described on the basis of the crystalline-plane-dependent resputtering rate of ITO films during sputter deposition. The resputtering rate during sputter deposition of ITO films is considered to be dependent on the crystalline planes of ITO and increases in the order of the (400), (222) and (440) planes.