Selenium, pure amorphous

Let us first consider the carrier drift in pure amorphous selenium. Both the electron and hole drift mobility can be measured in a-Se by the TOF technique outhned earlier. At temperatures above 200 K, a well-defined transit pulse is observed. The transient... 

Figure 7.9 Diffraction efficiency versus energy density for pure amorphous selenium in the low-energy-density region. The lines are eye-guides.

It was pointed out earlier that photosensitivity may be realized not only by the inclusion of dyes in a polymer matrix, but also by means of multilayered system production. For TPA dispersed in a polycarbonate, sensitized by a thin layer of vacuum-deposited amorphous selenium, the quantum yield was equal to 0.7 at the electric field strength 6 x 10s V cm-1 [308]. The photoinjection efficiency of holes into polymer was equal to the efficiency in pure selenium. The spectra of the quantum efficiency is shown in Fig. 54. 

Dark, amorphous Se is obtained by treating red Se with boiling water. It is also formed by reduction of heated selenous acid with hydrazinlum hydroxide (see Very Pure Selenium above). The grayish-black powder is thoroughly washed with warm water and dried in vacuum over CaClg. 

Pure selenium, like sulfur, is allotropic, and exists in three forms a gray hexagonal form, a red monoclinic form, and a vitreous amorphous form. All forms can exist at room temperature and one atmosphere, but only the hexagonal form is thermodynamically stable [53]. 

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