Pure crystals excitation band

In ultra pure crystalline silicon, there are no extra electrons in the lattice that can conduct an electric current. If however, the silicon becomes contaminated with arsenic atoms, then there will be one additional electron added to the silicon crystal lattice for each arsenic atom that is introduced. Upon heating, some of those "extra electrons will be promoted into the conduction band of the solid. The electrons that end up in the conduction band are able to move freely through the structure. In other words, the arsenic atoms increase the conductivity of the solid by providing additional electrons that can carry a current when they are promoted into the conduction band by thermal excitation. Thus, by virtue of having extra electrons in the lattice, silicon contaminated with arsenic will exhibit greater electrical conductance than pure silicon at elevated temperatures. 

It is reported that the band structure of ZnS doped with transition metal ions is remarkably different from that of pure ZnS crystal. Due to the effect of the doped ions, the quantum yield for the photoluminescence of samples can be increased. The fact is that because more and more electron-holes are excited and irradiative recombination is enhanced. Our calculation is in good correspondence with this explanation. When the ZnS (110) surface is doped with metal ions, these ions will produce surface state to occupy the valence band and the conduction band. These surface states can also accept or donate electrons from bulk ZnS. Thus, it will lead to the improvements of the photoluminescence property and surface reactivity of ZnS. 

In the previous section we summarized the chemical evidence that oxide ions in a state of low coordination can act as electron donors. At the same time, spectroscopic evidence has been accumulated which shows that highly dispersed alkaline-earth oxides have optical absorption bands that are not present in the pure single crystal. This is surprising at first because the energy required for electronic excitation of bulk MgO corresponds to a frequency in the vacuum ultraviolet. In order to understand this we must look at the absorption process more closely. 

These processes give rise to the electronic absorption bands of lowest energy observed in the pure undamaged single crystals which occur at 7.68 eV for MgO and 6.8 eV for CaO (142). Defects within the crystal structure are associated with optical absorption bands at reduced energies [for example, the anion vacancy band in the alkali halides (143)] because of the lower Madelung potential. The energy is still absorbed by the processes described in Eqs. (27) and (28), but the exciton is now bound to a defect and is equivalent to an excited electronic state of the defect. 

As well known from semiconductor physics, in non-metals electrons are, at finite temperatures, excited from the highest occupied band to the lowest unoccupied band to form excess electrons in the conduction band and electron holes in the valence band. Owing to long-range order each crystal possesses a certain amount of free electronic carriers. The mixed conductor which exhibits both ionic and electronic conductivity, will play an important role in this text, since it represents the general case, and pure ionic and electronic (semiconductors) conductors follow as special cases. 

Figure 12.9 Lewis structures for pure and doped silicon crystals, (a) Pure silicon showing excitation of two elcclron-hole pairs, (b). Si doped with P. an electron donor. Electrons can be excited from ihc donor band lo ihc coiuluclion band to form a n-type semiconductor, (c) Si doped wilh 15, an clci. iron acceplor. I dcclrons excited from the valence band leave positive holes which enable p type lomiui iivily.

The band gap of pure crystalline germanium is D1 X 10 J at 300 K. How many electrons are excited from the valence band to the conduction band in a 1.00-cm crystal of germanium at 300 K Use the equation given in the preceding problem. 

Pure silicon is an intrinsic semiconductor. It has very high electrical resistivity, especially at low temperatures, because few electrons are thermally excited across the energy gap into the conduction band. However, incident x-rays can cause excitation and thereby create a free electron in the conduction band and a free hole in the valence band. As shown later, the absorption of one x-ray quantum creates about a thousand electron-hole pairs. If a high voltage is maintained across opposite faces of the silicon crystal, the electrons and holes will be swept to these faces, creating a small pulse in the external circuit. 

Turning now to the intensity of this absorption band in the [Ti(H20)6]3+ ion, we note that it is extremely weak by comparison with absorption bands found in many other systems. The reason for this is that the electron is jumping from one orbital that is centrosymmetric to another that is also centrosymmetric, and that all transitions of this type are nominally forbidden by the rules of quantum mechanics. One-electron transitions which are allowed have intensities that give molar absorbance values at the absorption peaks of 104. If the postulate of the crystal field theory, that in both the ground and the excited states the electrons of the metal ion occupy completely pure d orbitals that have no other interaction than a purely coulombic one with the environment of the ion, were precisely correct, the intensity of this band would be precisely zero. It gains a little intensity because the postulate is not perfectly valid in ways that will be discussed on page 578. It will also be noted that the band is several thousand cm"1 broad, rather than a sharp line at a frequency precisely equivalent to A0. This too is a general phenomenon that will be discussed in detail below. 

Y. L. Sandler Westinghouse Research Laboratories)-. In view of the magnitude of the optical gap in ZnO ( 3 e.v.), it seems very unlikely that illumination by means of an incandescent lamp as used in Professor Schwab s experiments (Lecture 24) would cause any appreciable electronic excitation from the valance band to the conduction band in a pure ZuO crystal. It seems more likely that the electrons come from impurity levels due to the presence of water. We have recently demonstrated that the reduction of silver ions in aqueous solution can be photocatalyzed in presence of pure Ti02 or Si02 by light of wavelengths not absorbed by these oxides when in a dry state. 

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