Semiconductors based on group IV elements are widely used in the fields of micro-electronics, optics and photonics. The group IV alloys are processed using plasma enhanced chemical vapor deposition and its opto-electrical properties are a result of the material composition and structure. Infrared and Raman spectroscopy are complementary and powerful tools for providing these essential material characteristics. In this work, the vibrational modes present in hydrogenated, oxygenated and carbonated group IV alloys are investigated in a unique range of amorphous and nano-crystalline Si_X≥0Ge_1−X:H films and their alloys with C, O and Sn. Measurements are performed both post-deposition and following extended exposure to the ambient and de-ionized water. This comprehensive review is of value in the fields of material science and engineering as a single work of reference for group IV peak identification. Additionally, the effect of electrical screening, the influence of the dielectric medium on the peak frequency of a vibrational mode, is illustrated using the experimentally observed frequency shifts of X-O and X-H (X = C, Si, Ge) vibrational modes. All experimentally observed center frequencies of silicon hydride stretching modes in silicon solids and corresponding silicon-hydride configurations are identified using a straightforward Lorentz-Lorenz model approximation and considering all potential hydrogenated volume deficiencies within a tetrahedrally coordinated amorphous and nanocrystalline lattice. It shows that the stretching mode signature can reveal detailed information on the volume deficiencies in IV-valence alloys.

In this paper the opto-electrical nature of hydrogenated group IV alloys with optical bandgap energies ranging from 1.0 eV up to 2.3 eV are studied. The fundamental physical principles that determine the relation between the bandgap and the structural characteristics such as material density, elemental composition, void fraction and crystalline phase fraction are revealed. Next, the fundamental physical principles that determine the relation between the bandgap and electrical properties such as the dark conductivity, activation energy, and photoresponse are discussed. The unique wide range of IV valence alloys helps to understand the nature of amorphous (a-) and nanocrystalline (nc-) hydrogenated (:H) germanium films with respect to the intrinsicity, chemical stability, and photoresponse. These insights resulted in the discovery of i) a processing window that results in chemically stable Ge:H films with the lowest reported dark conductivity values down to 4.6·10^-4 (Ω ·cm)^-1 for chemical vapor deposited Ge:H films, and ii) O, C and Sn alloying approaches to improve the photoresponse and chemical stability of the a/nc-Ge:H alloys.

Low-cost multijunction photovoltaic devices are the next step in the solar energy revolution. Adding a bottom junction with a low bandgap energy material through plasma enhanced chemical vapor deposition (PECVD) processing could potentially provide a low-cost boost in conversion efficiency. A logical candidate for this low bandgap material is germanium. In this work we investigate the growth of PECVD processed hydrogenated amorphous/nano-crystalline germanium (a/nc-Ge:H), by characterizing over 100 samples, processed with a wide range of deposition pressures, powers, temperatures and GeH₄ dilution in hydrogen, using elemental analysis, vibrational analysis and analysis of the opto-electrical properties. We have identified a small processing window in which nc-Ge:H films are processed reproducibly. We also report on the strong correlation between the refractive index of the films and the presence- and extent of post-deposition oxidation. Notably, the oxidation generally increased the photoresponse of the films, as it results in a decrease of room temperature σ_d by 1-3 orders of magnitude. However, oxidation results in an increase of the bandgap energy and therefore impedes the development of a low bandgap material. The lowest E₀₄ we report is about 1.1eV, with an E_Tauc of 0.9eV and an σ_ph/σ_d of 3.4.