Surface studies of the nucleation and growth of hydrogenated amorphous silicon thin films

Hydrogenated amorphous silicon (a-Si:H) thin films are among the most promising candidates for large area semiconductor applications, such as photovoltaic solar cells and active matrix flat panel displays. Although the electronic properties are not as good as single crystal materials, continuous films can be deposited on a variety of substrates over very large areas.; The biggest problem with a-Si:H photovoltaic cells at this time is known as the Stabler-Wronski effect. The material is metastable, and long term exposure to light creates defects in the material. These defects can be removed by annealing the films. The creation and annealing of the defects is correlated with the migration of H atoms, and the defects are associated with dangling Si bonds. There is substantial evidence that the mobile H species are associated with clusters of H atoms in small voids in the amorphous network.; In this thesis study, a scanning tunneling microscope (STM) has been used to study the topology of the surfaces of a series of thin a-Si:H films. The substrates were atomically flat oxide free single-crystals. Films are grown in the laboratory with no air exposure between growth and measurement, or at other laboratories and transported under vacuum with minimal air exposure. The rms roughness and lateral correlation lengths of the surfaces are measured. Results are contrasted with those of in-situ ellipsometry, Monte Carlo calculations, and the dynamic scaling model. In addition, the topographs reveal structures of larger size and lower density on the film surface. These structures may be nanoparticulates deposited from the plasma during film growth or at the termination of the discharge.; This work suggests that the structural voids in the a-Si:H network may be dominantly the result of nanoparticles depositing in the films during growth. It is likely that if films could be produced without the incorporation of these nanoparticles, they could have superior electronic quality.