Hot-electron degradation in hydrogenated amorphous-silicon-nitride thin-film diodes

Two series of thin-film diodes (TFDs), used as switching elements in active -matrix liquid-crystal displays, have been prepared with various amorphous- silicon-nitride (a-SiNxHy) thicknesses. In a first series, with thin top me tal contacts, it was observed by photon-emission spectroscopic analysis tha t both the effective electron temperature and the number of hot electrons i ncreased as the nitride thickness (at constant field) or the electric field across the TFD (at constant nitride thickness) increased. A further analys is revealed that for the thicker samples, the electrons became progressivel y hotter on moving from the cathode to the anode. In a second series, the d rift of the TFD current-voltage characteristic under dc-driving conditions has been monitored as a function of nitride thickness at various fields. Th e anodic drift, resulting from defect-state creation in the anodic sample r egion, and the field and thickness dependence of the hot-electron intensity , show very similar trends. Therefore, it is concluded that the anodic drif t is driven by hot-electron-induced defect-state creation. Also, it was fou nd that optical phonon scattering effectively limits the electron temperatu re up to a field of around 1.5 MV/cm, while for larger field strengths impa ct ionization appears a probable additional energy-loss mechanism. For the anodic drift in a TFD with an a-SiNxHy layer thickness of 104 nm, a compara ble critical-field strength of around 1.5 MV/cm was determined, below which an efficient electron-phonon scattering mechanism strongly limits the prod uction of hot electrons. For a decreasing amorphous-layer thickness, the cr itical-field strength increases. As a result, an improved lifetime of a-SiN xHy TFDs in their application has been obtained using a small amorphous-lay er thickness. (C) 2001 American Institute of Physics.