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.

Amorphous and nano-crystalline germanium is of potential interest for a wide range of electronic, optical, opto-electronic and photovoltaic applications. In this work the influence of deposition temperature on hydrogenated germanium (Ge:H) films was characterized, using over 200 Ge:H and over 70 SiGe:H films. The demonstrated temperature-induced densification of Ge:H films resulted in more stable films with a lower bandgap energy and dark conductivity and higher activation energy.