Optical absorption in heavily doped silicon

Optical-absorption measurements at 300 and 4 K on a series of heavily doped Si and Si samples are reported. The interband contribution is isolated and confronted with the predictions of an electron-gas calculation. Disorder effects are observed and impurity-derived states are found to play a significant role, invalidating the electron-gas model at concentrations lower than 1020 cm-3. The gap shrinkage follows a critical behavior, going to zero at the insulator-metal transition and varying approximately linearly with concentration at high doping. The discrepancy between device-based and optical determinations of the gap shrinkage is discussed. © 1981 The American Physical Society.