Electronic Structure and Electron Transfer at Surfaces

In most semiconductors, there are, in addition to the allowed energy levels for electrons in the conduction and filled bands of the ideal crystal, discrete levels with energies in the forbidden gap which correspond to electrons localized at impurity atoms or imperfections. In zinc oxide, such levels arise when there are excess zinc atoms located interstitially in the lattice. At very low temperatures the interstitial zinc is in the form of neutral atoms. However, the ionization energy of the interstitial atoms in the crystal is small and at room temperature most are singly ionized, their electrons being thermally excited into the conduction band. These electrons give rise to the observed A-type conductivity. 

The band structure of a semiconductor is often plotted with the total energy of an electron as the ordinate and the distance through the crystal as the abscissa. A typical example is shown in Fig. 1. Energy levels 

2 electron volts. Not shown in Fig. 1, although used in subsequent illustrations is the energy n, which is the electrochemical potential, or 

Proposed energy-level diagram for zinc oxide. 

Fermi level, of the electrons. If ( (N) is the free energy of a crystal containing N electrons, then 

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