378 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 60 



2. Interstitials, when a carbon atom is displaced tx) some location between 

 the atoms of an already complete lattice. The diamond lattice offers 

 a relatively large amount of space for these interstitials. 



3. Impurities, when a foreign atom replaces the normal carbon atom at one 

 of the lattice sites. 



If these defects form a small percentage of the possible lattice sites, 

 then the energy diagram is represented as in figure 7. An energy 

 level like the one at F in the diagram might be due to an impurity 

 atom, such as areenic which has five valence electrons instead of four. 

 Four of these electrons will be found in the covalent bonds between 

 neighboring atoms, while the fifth will be rather loosely bound to the 

 nucleus. A small increment of energy will remove this electron, and 

 since there are no bonds which it can enter, it will constitute a mobile 

 negative charge. The band picture places the impurity level F fairly 

 close to the conduction band. A small amount of additional energy 

 is sufficient to raise it to the conduction band, when it will be free to 

 move through the crystal. It leaves behind it a positively charged 

 center which, in due time, may capture another electron and return 

 to its original condition. An impurity of this kind, which may lose 

 an electron and thus become a positive "trapped" hole, is called a 

 donor impurity. 



There are also acceptor impurities which have impurity levels 

 near the valence band. Such impurities may be boron, for example, 

 which has only three valence electrons and readily captures a fourth 

 to complete the covalent bonds. It requires only a small amount of 

 energy to raise an electron from the valence band into an acceptor 

 level. The empty level left behind in the valence band is a mobile 

 hole and the electron has become "trapped" at t\\Q impurity. This 

 electron may fall back into the valence band again, recombine with 

 a hole, and so complete the cycle. In some materials the donor and 

 acceptor levels may be so close to the band edges that thermal 

 energy even at room temperature is sufTicient to ionize them. The 

 donora give up electrons to the conduction band and the acceptors 

 liberate holes in the valence band so that electrical conduction is 

 possible. The material is no longer an insulator but a semiconductor. 

 In the type 26 diamonds, it is the acceptors which are close to the 

 valence band and which make the material a semiconductor. If the 

 impurity levels are more than about one electron volt from the band 

 edges, then thermal effects are ineffective in inducing conductivity. 



With these intermediate energy levels and the possibility of trap- 

 ping electrons and holes at the impurity sites, a great flexibility is 

 introduced into the theory of the properties of solids. Many of the 

 optical properties can be explained. The energy at which absorption 

 will occur is no longer determined by the width of the gap between 

 the valence and conduction bands, but by the kind and number of 



