This work contains 4 topics dealing with the properties of theluminescence from Ge.
The temperature, pump-power and time dependences of the photoluminescencespectra of Li-, As-, Ga-, and Sb-doped Ge crystals werestudied. For impurity concentrations less than about 1015cm-3, emissionsdue to electron-hole droplets can clearly be identified. Forimpurity concentrations on the order of 1016cm-3, the broad lines inthe spectra, which have previously been attributed to the emission fromthe electron-hole-droplet, were found to possess pump-power and timedependent line shape. These properties show that these broad lines cannotbe due to emission of electron-hole-droplets alone. We interpretthese lines to be due to a combination of emissions from (1) electron-hole-droplets, (2) broadened multiexciton complexes, (3) broadenedbound-exciton, and (4) plasma of electrons and holes. The propertiesof the electron-hole-droplet in As-doped Ge were shown to agree withtheoretical predictions.
The time dependences of the luminescence intensities of theelectron-hole-droplet in pure and doped Ge were investigated at 2 and4.2°K. The decay of the electron-hole-droplet in pure Ge at 4.2°Kwas found to be pump-power dependent and too slow to be explained bythe widely accepted model due to Pokrovskii and Hensel et al. Detailedstudy of the decay of the electron-hole-droplets in doped Ge werecarried out for the first time, and we find no evidence of evaporationof excitons by electron-hole-droplets at 4.2°K. This doped Ge resultis unexplained by the model of Pokrovskii and Hensel et al. It isshown that a model based on a cloud of electron-hole-droplets generatedin the crystal and incorporating (1) exciton flow among electron-hole-dropletsin the cloud and (2) exciton diffusion away from the cloud iscapable of explaining the observed results.
It is shown that impurities, introduced during device fabrication,can lead to the previously reported differences of the spectra oflaser-excited high-purity Ge and electrically excited Ge double injectiondevices. By properly choosing the device geometry so as tominimize this Li contamination, it is shown that the Li concentrationin double injection devices may be reduced to less than about 1015cm-3and electrically excited luminescence spectra similar to the photoluminescencespectra of pure Ge may be produced. This proves conclusivelythat electron-hole-droplets may be created in double injectiondevices by electrical excitation.
The ratio of the LA- to TO-phonon-assisted luminescence intensitiesof the electron-hole-droplet is demonstrated to be equal to thehigh temperature limit of the same ratio of the exciton for Ge. Thisresult gives one confidence to determine similar ratios for theelectron-hole-droplet from the corresponding exciton ratio in semiconductorsin which the ratio for the electron-hole-droplet cannotbe determined (e.g., Si and GaP). Knowing the value of this ratiofor the electron-hole-droplet, one can obtain accurate values ofmany parameters of the electron-hole-droplet in these semiconductorsspectroscopically.
