Chemistry Reference
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4K
20K
(b)
50K
100K
150K
Fig. 10.5
Sample temperature dependence of PL intensities after annealing at 900
◦
C for 30min:
(
a
) PL spectra and (
b
) integrated PL intensities
The PL intensities after high-temperature annealing cannot be attributed to the
Ge nanodots since the Ge nanodots decayed after high-temperature annealing as
shown in Fig.
10.3
b. We have not observed any dot size dependence of the PL peak
position of
∼
.
8 eV. They can originate from radiative defects such as Ge-Si-O or
Si-O complexes in Si capping layers near the Ge nanodot layer [
20
-
23
]. The high-
temperature annealing produced these complexes and decreased defects for non-
radiative carrier recombination, which resulted in the enhancement of PL intensities.
Figure
10.5
shows sample temperature dependence of (a) PL spectra and (b) inte-
grated PL intensities in 0.75-0.9 eV range after annealing at 900
◦
C for 30min. PL
intensities decreased with the increase of the sample temperatures. The integrated
intensity profile can be fitted by a formula of
a
0
, where
a
and
b
are some constants,
E
a
is the activation energy for thermal quenching of PL
intensity. We obtained about 30meV for
E
a
. This indicates that carriers escape from
the radiative defect sites having 30meV barrier before radiative recombination and
are non-radiatively recombined.
We made metal electrodes (Au and Al) on front and back sides of the annealed
sample and performed electroluminescence (EL) measurement. Figure
10.6
shows
sample temperature dependence of EL intensities at sample voltage of 6.8 V and
sample current of 48 mA. We obtained similar EL spectra to the PL ones with high
intensities in the energy region of
/
{
1
+
b
exp
(
−
E
a
/
kT
)
}
8 eV. The integrated EL intensity profile was
similar to the PL one (Fig.
10.5
b) and the activation energy for thermal quenching
of EL intensity was about 20meV. This indicates that the EL was caused by the
same mechanism as the PL one. EL intensity, however, became very low at room
temperature.
We formed multilayer structures of Ge nanodots (five layers) by repeating Ge
nanodot and Si film growths on p-type Si(001) substrates [
24
]. To fabricate light-
emitting diodes (p-i-n), plasma etching was applied to form 0
∼
0
.
m high and
0.3mm diameter mesa structures. The sample was thermally oxidized at 900
◦
Cto
grow 300 nm thick SiO
2
films. Then SiO
2
etching, P
+
ion implantation and metal
deposition were performed to make Au and Al electrodes (the inset in Fig.
10.7
a).
.
6
μ
