Growth mechanisms and near-interface structure in relation to orientation of MoS 2 sputtered thin films

  • PDF / 2,719,287 Bytes
  • 7 Pages / 576 x 792 pts Page_size
  • 12 Downloads / 234 Views

DOWNLOAD

REPORT


The growth of sputter-deposited MoS 2 thin films is investigated by high-resolution transmission electron microscopy. Pure high-temperature grown films are compared with H2O-contaminated films and amorphous annealed films. In the first case, the films are oriented. They have a first interface layer with crystallites having their (002) planes parallel with the substrate. The subsequent growth leads to the already described lamellar structure, with flakes perpendicular to the substrate. This structure can be explained in terms of a local branching process during crystal growth. The orientation relations between the crystallites in the parallel layer and the lamellae are determined. The local structure at the root of the lamellae, as well as at the interface, is investigated by image calculation. Water contamination in the plasma is shown to result in an amorphization of the interfacial region, followed by lamellar growth. Amorphous films annealed under vacuum do not show a lamellar structure, but have isotropic crystallization. In each of these cases, the mechanism determining the film structure is different.

I. INTRODUCTION

II. EXPERIMENTAL

The orientation of basal planes in lubricating MoS 2 thin films is the subject of many recent studies. Usually, sputtered MoS 2 thin films are known to have a lamellar hexagonal structure, with basal (002) planes perpendicular to the substrate.1 3 With different methods, some authors succeeded in producing films with basal planes parallel with the substrate.4"9 The advantages of such films are a better stability with respect to oxidation and an orientation of the basal planes directly in the sliding direction of the substrate, resulting in an increased wear life.9 Basal plane orientation is also important in thin films of other similar layered materials for various applications. A recent example is TiS2 thin films, whose charge accumulation properties in Li batteries depend on crystalline orientation.10 Many authors use chemical characterization to investigate the mechanisms determining this orientation. The most promising explanation, proposed by Buck, relates to the water partial pressure during deposition.5"7 Recently, a more complex situation has been described: the morphology results from the succession of both orientations during the growth of the film.11 The aim of this paper is to provide a precise structural description of the interfacial region. A powerful means for this purpose is high-resolution transmission electron microscopy (HRTEM), with the beam parallel (cross section) or perpendicular (top view) to the interface. Structure models can be tested using computer image simulation and comparing results with experimental images.

MoS 2 thin films are deposited by RF-magnetron sputtering. Substrates are Si[100], for cross-sectional TEM observation, and amorphous C films on Au grids for top-view TEM observation. Si substrates are covered by a thin amorphous layer of native SiO 2 . The deposition parameters are substrate temperature from room temperature to 500 °C, Ar

Data Loading...