Bandgap indirect

Diamond, however, is not the universal semiconductor panacea it is an indirect bandgap semiconductor and does not lase. In addition, present semiconductor materials, such as silicon and gallium arsenide, are solidly entrenched with a well-established technology, and competing with them will not be an easy task. CVD diamond will also compete with silicon carbide, which has also an excellent potential as a high-performance semiconductor material and is considerably easier and cheaper to produce. 

Because of its indirect bandgap, bulk crystalline silicon shows only a very weak PL signal at 1100 nm, as shown for RT and 77 K in Fig. 7.9. Therefore optoelectronic applications of bulk silicon are so far limited to devices that convert light to electricity, such as solar cells or photodetectors. The observation of red PL from PS layers at room temperature in 1990 [Cal] initiated vigorous research in this field, because efficient EL, the conversion of electricity into light, seemed to be within reach. Soon it was found that in addition to the red band, luminescence in the IR as well as in the blue-green region can be observed from PS. 

While silicon is not the ideal solar cell material, it currently dominates the solar PV market due to its prevalence in the microelectronics industry. Crystalline silicon (c-Si) is an inorganic semiconductor, in which the valence-band maximum and conduction-band minimum are not directly aligned in Uspace, making c-Si an indirect bandgap material. The indirect nature of the bandgap in c-Si means that a considerable change in momentum is required for the promotion of an electron from... 

The industrial application of SiC began with the blue light emitting diode (LED), which was very weak due to the indirect bandgap of SiC but was the only commercial blue electroluminescent light source at the time (the late 1980s). The SiC blue LED was soon surpassed in intensity by the gallium nitride (GaN)-based... 

The photoluminescence (PL) spectrum in Figure 1.7 shows a number of lines related to nitrogen-bound excitons and free excitons. SiC has an indirect bandgap, thus the exciton-related luminescence is often assisted by a phonon. Bound exciton luminescence without phonon assistance can, however, occur because conservation in momentum can be accomplished with the help of the core or the nucleus of the nitrogen atom. That is why the zero phonon lines of the nitrogen atom are seen, denoted and Q , in the spectrum but not the zero phonon line of the free exciton. 

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. 

The SnS had an indirect bandgap of 1.0-1.3 eV and was p-type. It was more difficult to estimate the bandgaps of the other films due to their mixed nature. However, approximate bandgaps of 1.8 eV (Sn2Ss) and 2.4 eV (SnS2) could be estimated from the optical spectra. 

X-ray diffraction showed the film to be CuiO, with no detectable amount of CuO and with a crystal size, estimated from the peak widths, of 20 nm. Optical transmission measurements of the films gave a value of (indirect) bandgap of 2.28 eV (literature room-temperature bandgap 2.1 eV but is rather variable). 

In Ref. 69, various compositions were obtained by varying the Cuithiosul-phate ratio. Only CuS gave an XRD pattern that allowed an estimation of crystal size other compositions and phases (Cui sS, CU14S, CuiS) showed no XRD patterns. The (indirect) bandgaps found for these films were 2.0,2.0, and 1.7 eV, respectively. 

The apparent discrepancy between the bandgaps of CuS (both indirect bandgap values) found from these two studies should be noted. The former found a considerably larger value, even though the crystal size was slightly larger and the film was not annealed. A comparison of the two transmission spectra does not... 

Figure 7.6 Schematic representation of fundamental absorption processes in (a) direct bandgap and (b) indirect bandgap semiconductors. Phonon emission and phonon absorption processes are marked in red. (Adapted from Yacobi [211)...

The n parameter equals 1 for direct bandgap semiconductors or 4 for indirect bandgap semiconductors in the case of allowed fundamental transitions [22], Other values of n, 2 or 3, are valid only for forbidden transitions. The proper transformation allows estimation of the bandgap energy, Eg, for both types of crystalline semiconductors. Figure 7.7 presents the procedure of Eg evaluation. 

In indirect bandgap semiconductor crystals both the emission and absorption of phonons are allowed to preserve the momentum (see Figure 7.6.). Therefore two contributions to the overall absorption spectrum should be considered aa and ae, associated with phonon absorption and emission, respectively [21] ... 

Figure 7.9 Recombination of electrons and holes in the case of (a) direct bandgap semiconductor and (b) indirect bandgap semiconductor. The energy E is a function of the wave vector k...

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