Semiconductor indirect type

Trigonal selenium is variously called metallic gray or black selenium and occurs in lustrous hexagonal crystals, which melt at 220.5 °C. Its structure, which has no sulfur analogue, consists of infinite, unbranched helical chains. Its density, 4.82 g cm , is the highest of any form of the element. Trigonal selenium is a semiconductor (intrinsic p-type with a rather indirect transition at about 1.85 eV [5]), and its electronic and photoelectric properties are the basis for many industrial uses of this element. 

MoS2-type materials are indirect band-gap semiconductors. The energies of the indirect (momentum forbidden) and direct (momentum allowed) band-gap transitions are given in Table 1. The electronic structure of these materials may be qualitatively understood in terms of the crystal structure. 

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. 

NiO 3.47 A p-type semiconductor with indirect gap optical transition. 371, 372... 

YFeOs 2.58 N-type semiconductor with an indirect optical transition. 428... 

The optical band-gap of the semiconductor (Section 1.2) is an important parameter in defining its light absorption behavior. In this quantized process, an electron-hole pair is generated in the semiconductor when a photon of energy hv (v = frequency and hv > Ef) is absorbed. Optical excitation thus results in a delocalized electron in the CB, leaving behind a delocalized hole in the VB this is the band-to-band transition. Such transitions are of two types direct and indirect. In the former, momentum is conserved and the top of VB and the bottom of CB are both located at /c = 0 (A is the electron wavevector). The absorption coefficient (a) for such transitions is given by [202]... 

The compound Tl2Tc3 is a p-type semiconductor in which the thallium atoms are located in channels between puckered Te layers. A ° T1 and jvjmr study of this material has revealed that significant indirect exchange coupling occurs between the nuclei by overlap of the thallium electron wave functions, mainly across the Te atoms. Good agreement was found between the NMR conclusions and the calculated electronic stmcture and density of states in this semiconductor (Panich and Doert 2(XX)). 

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. 

The experiments hitherto described dealt with catalytically active electrons and positive holes released by light. They allow only indirect conclusions regarding thermal catalysis. It is felt that direct observations are necessary in the present stage more than ever. Some work along these lines has been mentioned in the Introduction. Other observations on semiconductors of the ferrite type (d) have shown that the carbon monoxide oxidation, a donor reaction, is catalyzed best by inverse spinels, in which ferric ions, situated in octahedral positions, chemisorb carbon monoxide. Zinc ferrite, in which all the occupied octahedral positions carry ferric ions, showed a... 

W. Kautek, H. Gerischer, and H. Tributsch, The role of carrier diffusion and indirect optical transitions in the photoelectrochemical behavior of layer type J-band semiconductors, J. Electrochem. Soc. 127 (1980) 2471-2478. 

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