Shallow-Deep Instabilities of Donor Impurity Levels and Excitons in Many-Valley Semiconductors

Two recent theories are compared quantitatively in order to describe the intervalley mixing effect on donor impurity levels and excitons in many-valley semiconductors. Deep levels are predicted for a screened point-charge potential, regardless of the position at which the potential is centered. Altarelli-Hsu predict that the same potential has a deep level only if centered at an interstitial site. It is shown that this basic discrepancy is a direct consequence of a key Altarelli-Hsu approximation.