ABSTRACT
The theoretical investigations on the PVC have been performed by Lukács et al. [2010] and Lukács and Kugler [2011].
(3) Photo-induced defect creation (PDC)The microscopic nature of defects is generally elucidated from magnetic resonance techniques such as ESR, ENDOR, ODMR, ODENDOR, and so on, similar to those in a-Si:H. No ESR signals are normally observed in the dark in a-Chs., but ESR signals under illumination, that is, light-induced ESR (LESR) signals have been observed, as shown in Fig. 5.4 for three kinds of a-Chs, that is, a-As2S3, a-As2Se3, and amorphous selenium (a-Se). The ESR spectra consist of two types of LESR centers classified by thermal stability, that is, the type I center is less thermally stable (completely annealed at ~190 K), whereas the type II center is more thermally stable (annealed at room temperature), as shown by their thermal annealing measurements in Fig. 5.2. Each center has two components due to electrons and holes. The type I and II centers constitute approximately 15% and 85% of the induced spins, respectively, after prolonged illumination at high intensities (≥100 mW/cm2) at low temperatures. For the origins of two types of LESR centers
Figure 5.4 Optically induced ESR spectra obtained near 4.2 K in chalcogenide glasses. Dashed line superimposed in Se curve is a computer simulation. [Reproduced from Bishop et al., Phys. Rev. B, 15, 2278 (1977) by permission of American Physical Society.]
in a-As2S3, Hautala et al. [1988] suggested that the type I centers are an electron predominantly localized on a s-p hybridized orbital of a twofold-coordinated arsenic atom (AsI) (more delocalized than AsII) and a hole on a nonbonding 3p orbital of a probably singly coordinated sulfur atom (SI), while the type II centers are an electron on a nonbonding 4p orbital of a twofold-coordinated arsenic atom (AsII) and a hole predominantly localized on a nonbonding 3p orbital of a sulfur atom (SII). These AsII and SII are created by the breaking of As-As bonds and S-S bonds, respectively. This is based on their result that the relative spin density of the AsII center decreases rapidly with increasing arsenic content x, whereas that of the SII center increases with increasing x. These models of centers stand on a model of D+–D-pairs proposed by Biegelsen and Street, in which D+ and D-designate positively charged defects and negatively charged defects, respectively. D+and D-correspond to C 3+ and C 1+in a-Se, respectively, as shown in the atomic configuration in Fig. 5.5, in which the superscript and
Figure 5.5 (a) Formation of charged defects (valence alternation pairs:
VAP) in a-Se. (b) Configuration coordinate diagram for the formation of a D+( C 3+) – D-( C 1-) pair. The overall energy is lowered by the effective correlation energy Ueff. [Reproduced from Singh and Shimakawa, Advances in Amorphous Semiconductors (2003) by permission of Taylor & Francis.]
the subscript designate the charge state of defects and their coordination number, respectively. These defects are stabilized by the effective correlation energy Ueff, as shown in Fig. 5.5(b). Such a stabilization is caused by strong electron-phonon interaction [Street and Mott, 1975]. The D+–D-pair is called the valence-alternation pair [Kastner et al., 1976]. At high-intensity illumination above 100 mW/cm2, the inducing spin density does not saturate with increasing illumination time and exceeds 1020 cm-3. At pre-existing defects, D+ and D-limit the inducing spin density, ~1017 cm-3; this means that more defects are created by prolonged illumination. These defects are mostly annealed out at Tg.Elliott and Shimakawa proposed a mechanism for breaking of bonds to create light-induced defects: for example, for a-As2S3, As-S bonds are broken and, as a consequence, the type I LESR centers are an electron at A s 1+(2S) and a hole at C 1-(As), in which the symbol in the parentheses denotes the neighboring atom to the defect. The type II LESR centers are an electron at A s 2+(As, S) and a hole at C 1-(S), as illustrated in Figs. 5.6(a)–(c). As shown in Figs. 5.6(b) and (c) for n-type II center, there is a wrong bond, either an As-As bond or a S-S bond neighboring to the As-S bond to be broken by illumination. From the result, the model is qualitatively consistent with the result of the relative spin density of the type I and type II centers as a function of arsenic content measured by Hautala et al. [1988]. Further separation of conjugate pairs of defects takes place through subsequent bond-switching reactions, as illustrated in Fig. 5.7. This identification is based on the theoretical calculation of Vanderbilt and Joannopoulos [1980, 1981].
