Infrared absorption of free carriers produced in semiconductors by photoeffect

Abstract

IN the following paper will be found the results of the infrared absorption of free carriers produced in semiconductors by the inner photoelectric effect in the range of the fundamental lattice absorption. The number of electrons and holes is equal because of the electrical neutrality of the crystal. The carrier absorption is modulated by the use of chopped light and detected by a tuned amplifier. Absorption measurements on injected carriers have been carried out in the case of germanium but with contradictory resu1ts.o~3) The following investigation with photoelectricallyproduced excess carriers tries to clear up these results for germanium and gives the absorption cross-section for electron-hole pairs in silicon.* Because the investigation method is independent of the macroscopic structure of the sample and is independent of an electrical contact or barrier layer it is hoped to extend the measurements to other polycrystalline semiconductors or photoconductors. The following questions are connected with the production of carriers by photons: (1) If the energy of the photon is greater than the energy gap, the carriers may have a large kinetic energy. The question of the decay of this energy is connected with the dependence of the effective mass on the kinetic energy. On the other hand, it is possible that the relative population of overlapping bands is changed. (2) If one has a sufficiently high trap density one might have an optical regeneration of the carriers out of these traps and find the corresponding absorption band. In this case the trap level is given by the wavelength of the absorption maximum.