Some Speculations on Exciton-Associated Experiments in Germanium

Until quite recently, exciton-associated physics in semiconductors was almost entirely in terms of optical measurements. High resolution optical absorption measurements for cuprous oxide as well as for cadmium sulfide made it possible to assign a hydrogen atom-like energy level picture for the electron-hole binding.1).z) Emission lines -resulting from the recombination of excitons, either free or bound to impurities, are observed in silicon as well as in germanium with association of phonons.)-) Existence of excitonic molecule in silicon has been concluded also from a systematic optical measurement. ) Exciton lines are also clearly observed in magneto-optical absorption experiments.). ).s) Since the exciton in itself is electrically neutral, it cannot be an object of usual transport study involving an electric field. Recent modern techniques of achieving various limiting conditions, however, may enable us to deal with some sort of transport experiment even for neutral excitons. Achievement of very high density excitons by a photopulse, for example, furnishes the condition for the Mott transition, which guarantees a metallic conduction.>' 10 ) Meanwhile, some past works, taking only free carriers into account, should be reconsidered with the presence of excitons, especially when one excites the carriers by an intrinsic light. u)-) New type experiments, more or less connecting excitons with transport problems, will now be introduced and discussed in conjunction with the existing experimental results. Though the present treatise is deployed on the ground of germanium medium, it can, in principle, be applicable also to silicon as well as to many other semiconducting materials.