Hydrogen-related defects in hydrogenated amorphous semiconductors.

Abstract

One of the key steps in the formation of glow-discharge-deposited (GD) a-Si:H or a-Ge:H films by plasma deposition from the gas phase is the elimination of excess hydrogen from the growth surface which is necessary for the cross linking of the Si or Ge network and the reduction of the defect density associated with the hydrogen-rich surface layer. The high defect density (\ensuremath{\sim}${10}^{18}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) in a growing surface layer can, depending on preparation conditions, be either reduced (to \ensuremath{\sim}${10}^{16}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}3}$) or be trapped in the bulk upon subsequent growth, as evidenced by a great deal of data. However, little is known about its origin and implication. We have investigated the change in electronic structure related with this process using UHV-evaporated a-Ge as a model system, subjected to thermal hydrogenation, plasma hydrogenation, and various annealing cycles. The density of occupied states in the pseudogap of the a-Ge(:H) surface (probing depth \ensuremath{\sim}50 \AA{}) was determined with total-yield photoelectron spectroscopy. In this way, effects of thermal annealing, hydrogenation, and ion bombarding on the near-surface defect density could be studied. We identify in room-temperature (RT) hydrogenated a-Ge:H another defect at about ${\mathit{E}}_{\mathit{v}}$+0.45 eV in addition to the dangling-bond defect.This defect exists at the initial stage of hydrogen incorporation, decreases upon \ensuremath{\sim}250 \ifmmode^\circ\else\textdegree\fi{}C annealing, and is restored upon RT rehydrogenation. Therefore we suspect that this defect is hydrogen induced and concomitant with the formation of unexpected bondings [both multiply bonded X${\mathrm{H}}_{\mathit{x}}$ (X=Si or Ge and x=2 and 3) and polyhydride (X${\mathrm{H}}_{2}$${)}_{\mathit{n}}$ configurations] favored at RT hydrogenation. As a possible candidate we suggest the Ge-H-Ge three-center bond in which one electron is placed in a nonbonding orbital that gives rise to the paramagnetic state in the gap of a-Ge:H observed here. This defect also accounts for the large defect density at the growing surface in the optimized plasma chemical-vapor-deposition process, where the special bonding configurations mentioned above are the predominant species. The formation and annealing of this defect will be discussed.