Chemistry Reference
In-Depth Information
differ greatly in structure and properties due to the diversity of electrochemical condi-
tions under which an anodic oxide can be formed. The information on anodic oxides
in this section is thus presented according to the specific conditions under which they
are formed rather than being dealt with in a general fashion. A brief review of the basic
electrical properties of thermal oxides is provided to serve as the baseline to which
those of a specific anodic oxide can be related.
Thermal Oxides.
The electrical properties of silicon to silicon oxide are asso-
ciated with the various types of charges and states (or traps) in the system as shown in
Fig. 3.26.
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They include fast surface states located at the silicon/oxide interface and
slow states which include charges and states within the oxide due to mobile impurity
ions, such as sodium ions, or traps ionized by radiation or fixed surface state charge
near the interface. The interface contains defects which result in the interface
states and charges. The defects in and at the interface whose charge state depends
on the dc bias are called interface traps or surface states. Those independent of gate
bias are classified as charges which are subdivided into fixed and mobile for as-grown
oxides.
The fast surface states, or interface traps (Fig. 3.26a), arise from the disruption
of the periodicity of the Si lattice at the surface. Because they are located on the surface,
these states are in good electrical communication with the semiconductor bulk and
can act as surface recombination centers. The density of fast surface states on a
clean silicon surface, i.e., a cleaved surface, is on the order of one fast state for every
surface atom, resulting in a density of about
by a thermal oxide film have densities of surface states that are 5 or 6 orders of mag-
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The silicon surfaces passivated
nitude less than this. Also, a silicon surface covered with an adsorption layer may
have a density of fast surface states on the order of due to the stabi-
lization of the dangling bonds on the surface. The surface states, which can be donor
or acceptor, are distributed over the energy gap of silicon. The main effects of surface
charges or interface traps are to decrease surface mobilities of mobile carriers across
the interface, to change the surface potential at the interface under a given bias condi-
tion, and to act as recombination or generation centers for minority carriers at the
surface.
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The impurity ionic charges (Fig. 3.26b) with a density distribution of
are
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mobile under an electric field, resulting in a rearrangement of the density distribution.
The presence of impurity ions in the oxide is the result of contamination during pro-
cessing. Due to their mobile nature they affect the flatband potential by the redistribu-
tion of the ionic charge under field resulting in drift of the flatband potential and cause
a shift of the
C-V
curve along the voltage axis with time.
The positive charge at the interface (Fig. 3.26c) may occur by irradiation with X
rays, rays, or electrons. During irradiation, electron-hole pairs are created in the oxide.
If no electric field is present in the oxide, the electrons and holes will recombine result-
ing in no net charge building up in the oxide. However, the electrons and holes tend to
separate if there is a field in the oxide. If this field corresponds to the positive gate
voltage, the holes are pulled toward the oxide/silicon interface gradually building up a
space charge.
Finally, the surface state charge or the fixed oxide charge illustrated in Fig. 3.26d
is a result of an oxide growth process that has the following characteristics:
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