A high-precision study of excited-state transitions of shallow donors in semiconductors

Submillimeter laser and carcinotron sources have been used to study Zeeman transitions originating and terminating on the excited states of shallow-donor impurities in a variety of III–V and II–VI semiconductors. Although weak compared to transitions from the ground state, these transitions have advantages in precision of measurement and in the accessibility of states parity forbidden from the ground state. In GaAs these studies show that the difference in energy between 2p, 2s, and 3d states can be described to an accuracy of better than 1 % by variational calculations based on effective-mass theory. The results for the more polar material CdTe show deviations amounting to as much as 5% which in part can explained by the well-established “polaron pinning” and by a “polaron-induced Lamb shift” between the 2p and 2s states.