Chemistry Reference
In-Depth Information
required for the anodization of
-type silicon in the dark relative to that under illumi-
nation. The loss of photosensitivity at a certain oxide thickness has a twofold meaning:
(1) it indicates that above 380 Å the rate-determining step changes from inside silicon
to inside oxide and (2) the field at the oxide/Si interface is large enough to satisfy the
supply of minority carriers for the growth of the oxide on
n-
Si
.
The photosensitivity
also appears to be influenced by the mode of anodization. The critical thickness at which
sensitivity to light disappears was found to be about 240 Å in one case
106
and 350 Å in
another.
370
n
3.3.6. Electroluminescence
Electroluminescence is observed to occur during anodization on both
n-
and
p-
type materials. The luminescence on
p
type is uniform on the sample surface, whereas
that on
n
type is highly nonuniform.
370
It occurs only when the oxide reaches a certain
thickness as shown in Fig. 3.14.
230
No light emission is observed below a thickness of
15 nm. For greater than 25 nm thick, the intensity of emitted light increases expo-
nentially, the exponential factor being 10nm as shown in Fig. 3.14.
The spectrum of emitted light consists of two different regions, namely, a low-
energy region (LE) and a high-energy region (HE), as shown in Fig. 3.15.
230
The LE
region has a broad peak corresponding to an energy of 1.93 eV and the HE region
exhibits two peaks of 3.04 and 2.76 eV. In another study, the illumination was found to
be centered in two regions with LE at about 2 eV and HE at 3.44 eV.
370
The intensity
of both peaks increases with oxide thickness from a few nanometers to about 100 nm.
At larger thicknesses the intensity of LE still increases with increasing thickness but
HE becomes relatively constant.
230
Addition of water in the ethylene glycol solution of
reduces the intensities of both peaks whereas addition of a low concentration of
KCl, 0.001 M, enhances significantly the LE peak.
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