Trap Level Spectroscopy in Amorphous Selenium-Based Semiconductors

Abstract

An informative and relatively simple spectroscopic technique, namely measurement of thermally stimulated depolarisation currents, is considered in this chapter for the study of defect states in the mobility gap of amorphous Se-based semiconductors.

Thermally stimulated conductivity (TSC) and thermally stimulated depolarisation currents (TSDCs) are well-known techniques for obtaining data on the trapping levels of crystalline semiconductors, and the techniques have also been successfully applied to amorphous semiconductors [6–18]. TSDC provides researchers today with an active arena of technological as well as fundamental study. On the fundamental front, TSDC provides a powerful framework for understanding the bandgap structure and properties of amorphous materials. The main attraction of TSDC as an experimental method for the study of defects in high-resistance solids was, for many years, their apparent simplicity. Trapping levels in the bandgap determine the fundamental electronic properties of both phases. In conventional TSC measurements difficulties arise if the thermal excitation of equilibrium carriers becomes comparable with the excitation of trapped nonequilibrium carriers. In this situation the TSC signal appears in the best case only as a shoulder on the dark current–temperature curve. One of the main difficulties in observing TSC in amorphous semiconductors is the small magnitude of the TSC currents [11, 14–18]. In most chalcogenide materials – and this may be considered a universal property – the TSC measurements yield no peak. As for the amorphous ones, it is important to note that presently no universal method is known to detect the entire spectrum of trapping levels in the mobility gap. This is why investigators employ several complementary methods. Among these, those that are convenient for the study of shallow and deep trapping levels, respectively, should be distinguished. For the former, nonisothermal relaxation techniques and time-of-flight measurements seem to be “suitable,” whereas for the latter xerographic spectroscopy is most frequently employed. Each method has its advantages and disadvantages and often it may be useful to apply several of the methods listed above to the same specimen. TSDCs allow a relaxation processes attributed to relatively shallow trapping levels to be studied. The disadvantage in measuring TSDCs is the fact that the signals detected are very small and, sometimes, can be observed only in a relatively narrow temperature interval. At the same time, the advantage is that the TSDC method is inherently more sensitive than other methods and the resolution is usually much better. In addition, these measurements may be classified as nondestructive.