Luminescence properties of electron-hole-droplets in pure and doped germanium
<p>This work contains 4 topics dealing with the properties of the luminescence from Ge.</p> <p>The temperature, pump-power and time dependences of the photoluminescence spectra of Li-, As-, Ga-, and Sb-doped Ge crystals were studied. For impurity concentrations less than abou...
| Summary: | <p>This work contains 4 topics dealing with the properties of the
luminescence from Ge.</p>
<p>The temperature, pump-power and time dependences of the photoluminescence
spectra of Li-, As-, Ga-, and Sb-doped Ge crystals were
studied. For impurity concentrations less than about 10<sup>15</sup>cm<sup>-3</sup>, emissions
due to electron-hole droplets can clearly be identified. For
impurity concentrations on the order of 10<sup>16</sup>cm<sup>-3</sup>, the broad lines in
the spectra, which have previously been attributed to the emission from
the electron-hole-droplet, were found to possess pump-power and time
dependent line shape. These properties show that these broad lines cannot
be due to emission of electron-hole-droplets alone. We interpret
these lines to be due to a combination of emissions from (1) electron-hole-
droplets, (2) broadened multiexciton complexes, (3) broadened
bound-exciton, and (4) plasma of electrons and holes. The properties
of the electron-hole-droplet in As-doped Ge were shown to agree with
theoretical predictions.</p>
<p>The time dependences of the luminescence intensities of the
electron-hole-droplet in pure and doped Ge were investigated at 2 and
4.2°K. The decay of the electron-hole-droplet in pure Ge at 4.2°K
was found to be pump-power dependent and too slow to be explained by
the widely accepted model due to Pokrovskii and Hensel et al. Detailed
study of the decay of the electron-hole-droplets in doped Ge were
carried out for the first time, and we find no evidence of evaporation
of excitons by electron-hole-droplets at 4.2°K. This doped Ge result
is unexplained by the model of Pokrovskii and Hensel et al. It is
shown that a model based on a cloud of electron-hole-droplets generated
in the crystal and incorporating (1) exciton flow among electron-hole-droplets
in the cloud and (2) exciton diffusion away from the cloud is
capable of explaining the observed results.</p>
<p>It is shown that impurities, introduced during device fabrication,
can lead to the previously reported differences of the spectra of
laser-excited high-purity Ge and electrically excited Ge double injection
devices. By properly choosing the device geometry so as to
minimize this Li contamination, it is shown that the Li concentration
in double injection devices may be reduced to less than about 10<sup>15</sup>cm<sup>-3</sup>
and electrically excited luminescence spectra similar to the photoluminescence
spectra of pure Ge may be produced. This proves conclusively
that electron-hole-droplets may be created in double injection
devices by electrical excitation.</p>
<p>The ratio of the LA- to TO-phonon-assisted luminescence intensities
of the electron-hole-droplet is demonstrated to be equal to the
high temperature limit of the same ratio of the exciton for Ge. This
result gives one confidence to determine similar ratios for the
electron-hole-droplet from the corresponding exciton ratio in semiconductors
in which the ratio for the electron-hole-droplet cannot
be determined (e.g., Si and GaP). Knowing the value of this ratio
for the electron-hole-droplet, one can obtain accurate values of
many parameters of the electron-hole-droplet in these semiconductors
spectroscopically.</p> |
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