Indirect bandgap material

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... 

In an indirect bandgap material such as Si and Ge, a transition at the bandgap energy must be accompanied by a phonon to supply the needed momentum to reach the conduction band at its lowest point, as illustrated in Figure 20.5. Such transitions are possible but are not as likely as the direct transitions possible in a direct bandgap material. This difference is reflected in the absorbance at the band edge as seen in Section 20.6. 

Photons may induce direct or vertical transitions from any occupied band to a higher energy band that is not completely full. Such transitions are called interband transitions and contribute to the absorption spectra. So even in an indirect bandgap material, valence electrons can be promoted vertically to the conduction band by adsorbing photons of sufficient energy. Once in the conduction band, the hot electrons become thermalized through collisions and will eventually move to the lowest point in the band. 

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. 

Figure 4.5. Comparison of (a) direct bandgap e.g., GaAs) and (b) indirect bandgap e.g.. Si, Ge) materials. Reproduced with permission from Kasap, S. O. Principles of Electronic Materials and Devices, 2nd ed., McGraw-Hill New York, 2002.

Another interesting material is silicon because of its indirect bandgap. The question is whether the indirect gap develops with size in the same way as the direct gap. According to quantitative studies performed by Brus et al., the bandgap and lumines-... 

Cubic boron nitride has high thermal conductivity, high dielectric constant, great hardness, and good chemical stability. The material can be doped n-type with Si and p-type with Be to form p-n junctions. While cubic boron nitride (c-BN) has been successfully doped p- and n-type to produce the first UV-LEDs, it is an indirect bandgap semiconductor which will ultimately limit emission efficiency. Relatively few studies have been performed on this material system. ECR-LPCVD techniques [23, 24] and LPCVD [25] have had the most success informing BN films. As with other specialty materials there is a lack of BN substrates. In order to produce the c-BN phase, high deposition temperatures often are combined with assisted techniques. 

Recent developments in solid state solutions of AlN/SiC/InN/GaN open up the possibility of a new generation of heterostructure devices based on SiC. Single crystal epitaxial layers of AlN/SiC/InN have been recently demonstrated by Dmitriev [4]. A whole range of solid state solutions has been grown. Recently Dmitriev et al [5] reported on an (AlNx-SiC,.x)-(AlNySiC,.y) p-n junction. Solid state solutions of AlN-SiC [6,7] are also expected to lead to direct gap ternary materials for UV and deep blue optoelectronics, including the development of visible lasers. The direct to indirect bandgap transition is predicted to occur at between 70 and 80 % of AIN in SiC. 

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