Discrete defect levels in the amorphous selenium bandgap

Determination of Optical Properties of Undoped Amorphous Selenium (a-Se) Films by Dielectric Modeling of Their Normal-Incidence Transmittance Spectra

“…Therefore, in principle, two independent optical measurements are necessary at each spectral wavelength to solve for the unknowns and but, in practice, the true geometric thickness of the film is not known in prior and is thus to be determined at the same time-that is, three unknowns to be retrieved from two independent optical measurements at the same spectral wavelength, which demands adequate model suppositions or an otherwise optical analytical approach (Tan, 2006;Tan et al, 2007;Tan et al, 2006;Vautier et al, 1988;Bettstelleret al, 1993;Mulama et al, 2014;Benkhedir, 2006;Solieman et al, 2014;Adachi & Kao1980;Kotkata & Abdel-Wahab, 1990;Kotkata et al, 2009;Navarrete et al, 1990;Solieman & Abu-Sehly, 2010;Nagels et al, 1995;Tichy et al, 1996;Nagels et al, 1997;Tauc, 1972;Tauc, 1979;Heavens, 1991;Azzam & Bashara, 1987;Ward, 1994;Dragoman & Dragoman, 2002;Stenzel, 2005;Joo et al, 1999;Dobrowolski et al, 1983;Klein et al, 1990;Kukinyi et al, 1996;Chiao et al, 1995;Case, 1983;Peng & Desu, 1994;Birgin et al, 1999;Mulato et al, 2000;Erarslan & Gungor, 2010;Ventura et al, 2005;Manifacier et al, 1976;Swanepoel, 1983;Minkov, 1990;El-Naggar et al, 2009;…”

“…The actual functional form of the DOS in band tails is somewhat difficult to specify adequately and is still controversial, but the transition in DOS function between the band region and tail region is a smooth one The energies and denote, respectively, the sharp edges of conduction and valence bands in the absence of structural disorder, while and are the broadening spreads of their respective tails into the semiconductor's bandgap that increase with increasing in its structural disorder. The energy terms 2 ⁄ and 2 ⁄ signify the transition points between the square-root and linear-exponential distribution of energy states within the bands and into the bandgap region, with their DOS functions g and g , with their first derivatives being continuous at these points (Benkhedir, 2006;Theiss, 2012;O'Leary et al, 1997;Dias da Silva et al, 2004). Let all band-tail energy states to be localized and all band energy states to be extended, the energy levels 2 ⁄ and 2 ⁄ represent the so-called mobility edges of the conduction and valence bands, respectively, with the energy difference 2 ⁄ is the mobility-edge bandgap energy, where is the Tauc absorption edge or Tauc bandgap energy in the disorderless limit-that is, when → 0 and → 0.…”

“…Figure 1. Schematic diagram of the density-of-states DOS in parabolic valence and conduction energy bands and their linear-exponential energy distribution tail states for an undoped disordered semiconductor (Benkhedir, 2006;Theiss, 2012) Optical absorptions in disordered amorphous semiconductor with tails of localized energy states in its bandgap can result from four types of radiation-induced electronic transitions as depicted in Figure (2 …”

“…Thus, it is only possible to exploit inherent physical properties of as-deposited undoped a-Se films at low ambient temperatures, after ageing them in dark and humidity-free environment to reach structural relaxation. Extrinsic doping increases glass transformation and crystallization temperatures of a-Se films, often referred to as stabilized a-Se when slightly doped with arsenic (As), to limit deterioration of their performance with temperature and time when are incorporated in solid-state devices (Tan, 2006;Tan et al, 2007;Tan et al, 2006;Vautier et al, 1988;Bettstelleret al, 1993;Mulama et al, 2014;Benkhedir, 2006;Solieman et al, 2014). Reduction of time-ageing and glass transformation of undoped a-Se films, and hence virtuous performance in devices integrating them, can also be realized by growing and/or annealing them at temperatures around 70 , but their electrical resistivity is significantly reduced (Abdul-Gader et al, 1998;Abdul-Gader & Nigmatullin, 2001;Al-Hamarneh et al, 2009).…”

“…The diversity of these findings may be related to different film thicknesses, to dissimilar methods and preparation conditions used in film deposition, and to variability of experimental techniques used to characterize optical response of studied films. Discrepancies in reported optical properties of undoped a-Se films may also be linked to techniques employing different theoretical formulations for analyzing measured optical data, and to the interband transition mechanisms and band-energy structure models used for interpreting the optical absorption phenomena operative in them (Mott & Davis, 1979;Tan et al, 2006;Vautier et al, 1988;Bettstelleret al, 1993;Mulama et al, 2014;Benkhedir, 2006;Adachi & Kao1980;Kotkata & Abdel-Wahab, 1990;Kotkata et al, 2009;Navarrete et al, 1990;Solieman & Abu-Sehly, 2010;Nagels et al, 1995;Tichy et al, 1996;Nagels et al, 1997;.…”