Glow-discharge amorphous silicon: Growth process and structure

Hydrogenated amorphous silicon (a-Si:H) is the first “structure-sensitive” amorphous semiconductor, and its conduction type is controlled by impurity doping using the glow-discharge technique. However, in contrast to crystalline counterparts, the network structure of amorphous materials takes a wide variety depending on their growth process, and therefore, electronic properties are not unique even in undoped (intrinsic) a-Si:H. In this paper, we are concerned with the growth process of a-Si:H films via the glow-discharge decomposition of SiH₄, and its relationship to structural, optical and electronic properties of the deposited films. Emphasis is placed on the understanding of the film growth mechanism as well as the microscopic characterization of the film structures. New plasma diagnostic tools such as optical emission spectroscopy and ion mass spectrometry are employed for describing the SiH₄-glow-discharge plasma, and dominant species responsible for the film deposition is suggested. Structural characterization of a-Si:H includes TEM observation (morphology), infrared absorption (bonded hydrogen), ¹H NMR (spatial distribution of hydrogens), Raman-scattering spectroscopy (local structural order) and ESR (defect density), being discussed in relation with optical and electronic properties. Hydrogenated amorphous SiGe and SiC alloys as well as amorphous superlattice structures are also described as recent important topics.