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hole pairs in the Ge nanocrystals has several radiative relax-
ation procedures, emitting different luminescent bands.3,11,14
The recombination may be within the Ge nanocrystals, or at
the interface between the nanocrystals and the matrix, or at
the defects of the SiO2 matrix formed by implantation and
annealing.3,11,14 Further work is underway in order to under-
stand the radiative relaxation procedures for the 2.1 and 1.6
eV bands.
Many causes can lead to the differences between the EL
and PL spectra. An important cause is the difference in the
mechanisms for the population of exciting states between
electrical and optical pumping. Another important factor is
that the photoluminescent and electroluminescent processes
take place in different regions of the oxide films. In the pho-
toluminescent process, the luminescence mainly comes from
the pump absorption in the layer below the surface. The
weak intensity of 2.1 and 1.6 bands in PL spectra is a result
of the lack of Ge nanocrystals in the layer. In the electrolu-
minescent process, the luminescence is controlled by both
the carrier motion and the electric field distribution in the
oxide film.
serve bias͒. On the other hand, when the EL device shows
the 3.1 eV luminescent band, the average field strength in the
film is about 106 V/cm. In fact, the electric field in some
regions is above 106 V/cm, because the presence of nano-
crystals in the SiO2 film induces inhomogeneous distribution
of the electric field.16 Thus, we speculate that the mechanism
for excitation of the GODC defect is related to impact ion-
ization by hot electrons in the high field, similar to the elec-
troluminescence in ZnS:Mn.5,14
In summary, Ge nanocrystal-based electroluminescent
devices have been fabricated by using Geϩ implantation and
thermal annealing, a method compatible with a conventional
silicon integrated circuit process. The PL and EL spectra
display three luminescent bands peaked at 3.1, 2.1, and 1.6
eV. The 3.1 eV luminescent band is related to the GODC
defect, and its electroluminescent excitation is related to the
impact ionization by hot electrons. The 2.1 and 1.6 eV lumi-
nescent bands are related to Ge nanocrystals, their electrolu-
minescent excitation is related to the recombination of the
hole-electron pairs.
This work was supported by the National Natural Sci-
ence Foundation of China.
In general, there are two kinds of electroluminescent
mechanisms. One is impact ionization by hot carriers, and
the other is the recombination of electron-hole pairs. To ex-
plore the electroluminescent mechanism, the carrier motion
in the Ge-implanted SiO2 films should be studied. Electron
injection in SiO2 film prevails over hole injection due to
energy barrier height asymmetries of the contacting metal or
the semiconductor with the oxide ͑the electron barriers are
lower than the hole barriers͒.5,14–16 The presence of Ge
nanocrystals inside the SiO2 film allows direct carrier tunnel-
ing between nanocrystals.14–16 In the moderate electric field
range, the presence of Ge nanocrystals in the layer of the
SiO2 film at the Si/SiO2 interface enhances carrier tunneling
from the Si substrate into the SiO2 film.14–16 Therefore, we
propose the motion of electrons and holes as follows: when
the forward voltage is applied on the MIS structure, electrons
are injected into the film from the Au/SiO2 interface. Mean-
time, hole tunneling from the Si substrate into the film is
enhanced due to the presence of Ge nanocrystals in the layer
at the Si/SiO2 interface. The carriers then transport from
nanocrystals to nanocrystals and recombine via different pro-
cesses, emitting the 2.1 and 1.6 eV luminescent bands. The
inhibition of these two bands under reserve bias is because
Ge nanocrystals are lack in the layer at the Au/SiO2 interface
͑the layer is near the positively biased electrode under re-
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Appl. Phys. Lett., Vol. 71, No. 17, 27 October 1997 Zhang, Wu, and Bao 2507
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