Biaxial Texture Evolution of Nanostructured Films under Dynamic Shadowing Effect and Applications.

摘要

Texture formation and evolution in polycrystalline films are quite complicated, and they still remain as challenging subjects. Oblique angle deposition is an effective way to control the texture due to the shadowing effect introduced by oblique incident flux. A new dynamic oblique angle sputter deposition technique, called flipping rotation, was developed. In this rotation mode, the substrate is arranged to rotate continuously at a fixed speed around an axis lying within and parallel to the substrate. The incident flux is always perpendicular to the rotational axis and the flux incident angle relative to the substrate normal changes continuously.;To study the texture formation and evolution of Mo and W films grown by DC magnetron sputter depositions, three film categories were prepared: (1) normal incidence deposition without the shadowing effect, (2) stationary oblique angle deposition at various fixed flux incident angles with static shadowing effect, and (3) convention rotation and flipping rotation deposition with dynamic shadowing effect.;Under the normal incidence deposition, ultrathin (2.5 nm) to thin (100 nm) Mo films have been deposited on SO2 membranes on transmission electron microscopy (TEM) grids. These samples can be directly compared with the films grown on glass or native oxide covered Si substrates. The result of a fiber texture with the [110] out-of-plane direction implies that the growth has gone through a recrystallization process that selects the minimum surface energy plane parallel to the substrate. This is in contrast to the conventional understanding of the selection of out-of-plane orientation, which is the fastest growth direction [100] at room temperature based on the low Mo homologous temperature (room temperature/melting temperature) of ~0.1.;Under stationary oblique angle deposition, Mo thin films in the range of 175 nm to 1300 nm were observed to undergo a dramatic change in crystal texture orientation from a (110)[11¯¯0] biaxial texture that has the minimum energy plane (110) parallel to the substrate surface at low oblique angle deposition (0<&agr;≤45°) to the (111)[112¯] biaxial texture at high oblique angle deposition (&agr;≥60°) due to the large tilting of the fastest growth [001] direction towards the flux. We believe that the biaxial textures formation under different flux incident angles have different out-of-plane orientation selection mechanisms, i.e., minimum surface energy plane vs. fastest growth direction. However, the in-plane orientation selection could still be explained by the largest capture area model where the in-plane texture favors the orientations along the largest capture area or length facing the flux.;Under dynamic shadowing effect using the newly developed dynamic flipping rotation of the substrate, both Mo and W thin films in the range of 550 nm to 650 nm grown on amorphous substrates have (110)[11¯0] biaxial textures with a body center cubic (BCC) structure characterized by a reflection high-energy electron diffraction pole figure technique developed in house. Depending on the rotational speed in the flipping rotation, the biaxial textures can have various morphologies, such as vertical, S-shape, or C-shape nanocolumns, as observed by scanning electron microscopy. The possible growth mechanisms in the formation of various morphologies due to different degrees of shadowing effect were suggested. This is in contrast with the films grown by conventional rotation, which usually have fiber textures with different out-of-plane orientation, [111] for Mo, or even a different phase, A15 for W.;The biaxial Mo(110) and W(110) thin films were used as buffer layers to grow semiconductor films of GaN for the LED applications and CdTe for the thin film photovoltaic applications. Detailed X-ray pole figure analyses show the heteroepitaxial growth of GaN/Mo, GaN/W, and CdTe/Mo. A prototype biaxial CdTe film based Schottky junction solar cell on biaxial Mo film has been fabricated and characterized. Although the open circuit voltage is small for the prototype device, the chance for successful improvements is high. The promising optical and electrical properties of these epitaxial films may offer a potential alternative strategy for the growth of high quality functional semiconductors on amorphous substrates using biaxial metal buffer layers. (Abstract shortened by UMI.).