Optically detected cyclotron resonance in Ge and Si exchange interaction in impact dissociation

We have investigated optically detected cyclotron resonance (ODCR) and ordinary cyclotron resonance (CR) under the same condition, in Ge and Si, which include both high-purity and doped samples. In impact dissociation processes, which are the origin of ODCR, donorbound excitons have larger cross section for impinging electrons than for holes, and conversely, acceptor- bound excitons have larger cross section for holes than for electrons. Moreover, the ratio of impact dissociation cross section for holes to that for electrons varies with the number of excitons bound to an impurity. These phenomena are understood primarily in terms of exchange interaction between impinging carriers and constituents of bound excitons. In addition, it is found that the relative intensity of hole cyclotron resonance against electron resonance is larger in ODCR than in CR. This is understood in terms of exchange interaction by taking the many valley nature of the conduction band into account.