Properties of a-Si:H and a-(Si,Ge):h films grown using combined hot wire–ECR plasma processes

Abstract

It has been suggested previously that ion bombardment with low energy ions is necessary in order to obtain the growth of high quality a-Si:H materials. Ion bombardment is presumed to play a major role both in the removal of excess hydrogen and in assisting in the surface movement of heavy radicals such as germyl during the growth of amorphous films. We report on an unambiguous experiment which shows that one needs low energy ion bombardment to promote the growth of high quality amorphous materials. The experiment consists of growing a-Si:H and a-(Si,Ge):H using hot wire deposition, while simultaneously subjecting the growing film to low energy (∼10 eV) H and He ions generated in a low-pressure (electron cyclotron resonance) ECR reactor. Experiments show that the primary mode of film growth is from the catalytic decomposition of silane and germane from the hot wire, with less than 10% growth resulting from plasma decomposition. A systematic set of measurements of film properties made at different substrate temperatures and ion bombardment conditions reveals that ion bombardment results in a decrease of H concentration and in the ratio of dihydride to monohydride bonded silicon and germanium. The presence of ions resulted in a significant decrease of the Urbach energy of valence band tails to below 50 meV. Photoconductivity improves by orders of magnitude, as does the photosensitivity ratio.