Substitutional doping of hydrogenated amorphous silicon (a-Si:H) and germanium (a-Ge:H) with phosphorus, arsenic, and boron has been investigated, with use of electron-spin-resonance techniques, optical absorption, and transport measurements. Doping-induced changes in the density of shallow states and deep defects (dangling bonds) are compared for the different doping-host systems. Hyperfine spectra of neutral donor levels are observed in spin resonance and used to deduce a microscopic picture of the underlying donor wave functions. Based on the dependence of the occupancy of deep and shallow states on doping levels and temperature, a detailed model for the electronic density of states in n-type a-Si:H and a-Ge:H below the conduction-band mobility edge is obtained. Furthermore, similar studies in nominally compensated a-Si:H are used to discuss the location of boron acceptor states in this material as well as questions concerning light-induced creation of metastable dangling bonds. Experimental evidence for the existence of exchange-coupled electron-hole pairs in compensated a-Si:H is presented. For an investigation of the doping process, the incorporation of the various dopant atoms from the deposition gas phase into the amorphous film has been studied by secondary-ion mass spectroscopy. The concentration of electronically active dopants in the deposited film is related to the total concentrations of dopants in the solid or the deposition gas phase for a calculation of the corresponding doping efficiencies. The results are discussed in the context of previous doping models based on the octet rule for chemical valences and of a new model concerning charge-induced structural transitions between weak bonds and dangling bonds.

• Received 1 October 1986

DOI:https://doi.org/10.1103/PhysRevB.35.5666