Free carrier optical absorption

Reference 59 provides a comprehensive explanation of the optical spectra and extracted bandgaps. The direct bandgap of ca. 2.36 eV is compared to the literature value of ca. 2.2 eV and explained by size quantization in the fairly small (20 nm) crystals. An indirect bandgap of 1.9 eV was measured (literature value < 1.4 eV), but it was stressed that this provided an upper limit only, since the absorption in this region was dominated by free-carrier absorption, which masked the indirect absorption. Annealing decreased the conductivity and the free-carrier absorption and changed the indirect gap to > 1.3 eV. 

Bandgap measurements for Cu sulphides and selenides are complicated by the fact that these semiconductors are normally degenerate, with high free-carrier absorption in the near-infrared and possible Moss-Burstein shifts (due to saturation of the top of the valence band by holes) in the optical gap. It is quite possible that variations in bandgaps in these materials are due to differences in stoichiometry, phase, and doping rather than to any quantum size effect. Only studies where crystal size can be estimated and are possibly in the quantum size range are given here. 

The problem of whether optical absorption in metals can show exciton lines is closely related to the considerations of the last section, and a short discussion will be given here. Experimentally, the addition of free carriers to a non-metal leads first to the broadening and eventually the disappearance of exciton lines. Figure 2.10 shows the results of Wilson and Yoffe (1969) on WSe2 doped with NbSe2, the Nb atom containing one fewer d-electron than the W atom. Somewhat similar results on Mg-Bi are due to Slowik and Brown (1972). 

LP-CVD ZnO Optical total and diffuse transmittance spectra (TT and DT spectra) of a temperature series of undoped LP-CVD ZnO films are shown in Fig. 6.25 TT does not vary strongly with substrate temperature. Indeed, as Fig. 6.25 is related to a series of undoped samples, the values of carrier density N are too low to produce an observable free carrier absorption effect... 

The transmittance and reflectance spectra of an undoped AP-CVD ZnO film and of a doped AP-CVD ZnO Al film are shown in Fig. 6.40. While the transmittance of the undoped film stays over 80% along the whole visible range, the transmittance of the doped film displays a pronounced drop in the near-infrared wavelength range. The drop corresponds to a minimum in the reflectance curve, as well as to a maximum (peak) in the absorbance curve. This occurs close to the so-called plasma frequency. These effects are due to free carrier absorption. When N is increased, the plasma frequency is shifted towards shorter wavelengths, and the drop in optical transmittance becomes more pronounced. This is illustrated for the case of LP-CVD ZnO.B films in... 

According to the Burstein-Moss effect [106,107], the optical band gap increases with carrier concentration n. This effect can be observed at the short wavelength turn-on of transmission. Between 400 and 600 nm all films show very high transmission of similar values above 82% in average. The differences above 600 nm can be attributed to free carrier absorption, resulting in lower transmission for highly doped films [108]. 

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