Gap states

Fig. 5. A schematic band structure and molecular orbital diagram for a conjugated polymer containing no mid-gap states.

Very useful information concerning the surface of emersed electrodes, however, can be deduced from UPS spectra directly, like the electronic density of states at the Fermi level, the position of the valence band with respect to the Fermi level or possible band gap states. The valence band of UPD metals might help to explain the respective optical data (see Sections 3.2.1 and 3.2.5). 

In the early 1970s, Spear and coworkers (Spear, 1974 Le Comber et al., 1974), although unaware of the presence of hydrogen, demonstrated a substantial reduction in the density of gap states (with a corresponding improvement in the electronic transport properties) in amorphous silicon films that were deposited from the decomposition of silane (SiH4) in an rf glow discharge. 

In addition to the generation of platelets, hydrogenation of silicon also induces electronic deep levels in the band gap. As in the case of platelet formation, these defects are considered to be unrelated to either plasma or radiation damage because they can be introduced with a remote hydrogen plasma. Comparison of depth distributions and annealing kinetics of the platelets and gap states has been used to a limited extent to probe the relationship among these manifestations of H-induced defects. 

Fig. 13.2 Upper panel showing field lines from a conductive grain boundary inside a solid, under field emission. Lower panel shows the band bending and charging of gap states at the grain boundary.

As shown in Fig. 2- 3, localized electron levels arise (A and C in the figure) near the band edges at relatively high state densities tailing into the band gap these are called diffuse band tail states. Further, localized electron levels may occur due to dangling bonds and impurities (B in the figure) in the band gap, which are called gap states. 

Fig. 2-33. Electron energy and state density in amorphous semiconductors A and C = diffuse band tail states B = gap states, cmc = mobility edge level for electrons MV = mobility edge level for holes ...

The gap states in amorphous materials are known to result in charged defects, transport occurring through the hopping of bipolarons. In chalcogenide glasses, the bipolarons correspond to over-coordinated (Cj) and under-coordinated (Cj") centres. 

Mobility measurements by the TOP methods considered in Chapters 3 and 4 are particularly important, but they cannot give information about the whole spectrum of states in the mobility gap of amorphous chalcogenides. Therefore, in addition to TOP, XTOP, IPTOP, TSC, and TSDC, other complimentary techniques that probe the gap states are needed. Xerographic techniques that were initially developed to characterize properties of electrophotographic (xerographic) receptors [1] seemed to be informative, suitable, and widely applicable for the study of amorphous thin films and photoconductive insulator thin films [2],... 

After charging, the decay of the open-circuit surface potential is measured. From these measurements, important information can be extracted. In the past few decades, the xerographic probe technique has become a very popular and unique means to characterize electronic gap states. In particular, a map of states near mid-gap is determined by a time-resolved analysis of the xerographic surface potential. 

The preceding changes in the photocurrent of amorphous chalcogenides may be related to specific changes in electronic gap states, which act as trapping and recombination centers and, therefore, limit the photoconductivity. 

Spectroscopic Studies of Gap States and Laser-Induced Structural Transformations in Se-Based As-Free Amorphous Semiconductors... 

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