AN ACCOUNT OF BOUND EXCITONS IN SEMICONDUCTORS

The term “bound exciton,” used in connection with semicon-ductors, is generally taken to describe a state in which a hole and an electron is localized at some defect or impurity in the crystal. Often such states are shallow, that is, a free exciton is bound to the impurity by only a few millielectron volts, but it is quite logical to include much deeper states, for there is no difference in kind between deep and shallow states. In addition, one often thinks of bound excitons as being attached to point defects in the solid. However, it is becoming increasingly clear that pairs of nearest neighbor impurities play important roles in semiconductor physics, and there seems to be no reason for excluding excitons bound to such centers from the present classification. Having gone this far, one does not want to exclude excitons bound to next nearest neighbor pairs of defects, and so one ends up including effects due to electrons bound to donors interacting with holes bound at acceptors, even though the donors and acceptors are far apart.