Biomedical Engineering Reference
In-Depth Information
as the limiting reagent, and poising the potential where the less
noble element will underpotentially deposit only on the more
noble element. A classic example is CdTe formation [27], where the
solution contains Cd
2+
and HTeO
2+
, usually at pH 2. The potential is
set to reduce HTeO
2+
to Te on the surface at a limiting rate, while
Cd
is reduced on the Te at an underpotential, a potential where no
bulk Cd is formed. Cd
2+
ions are present in a large excess, to deposit
quantitatively on Te as it is formed, resulting in stoichiometric CdTe.
Although the structure and morphology of codeposited compounds
are variable, some having been described as “cauliflower” like,
high-quality deposits have been formed [32]. There are a number
of papers in the literature concerning the formation of compound
semiconductor diodes by electrodeposition, the most popular
structure being a CdS-CdTe-based photovoltaic. CdS was generally
deposited first on an ITO/glass substrate, followed by a layer of
CdTe, usually by codeposition [33-39]. Sailor and Martin et al.
grew an array of CdSe-CdTe nano-diodes in 200 nm pore alumite
[40-42] using a compound electrodeposition methodology called
sequential monolayer electrodeposition [43]. A commercial
process is being developed by BP Solar to form CdTe-based
photovoltaics using codeposition. Relatively rapid deposition
rate has been achieved by codeposition and it is presently the
most practical compound semiconductor electrodepostion
methodology. Codeposition holds great promise if greater control
can be achieved. At present, the main parameters of control are
solution composition and deposition potential. There have been
a number of attempts to improve the process by using variations
in reactant concentration, pH [44,45] and the potential program
[43,46-50]. In most cases, the deposits are improved by annealing.
In the application of photovoltaic applications, annealing is used to
convert CdTe from the as-deposited n-type material to the desired
p-type [39,51]. Ideally lattice-matched semiconductor substrates
could be used to form deposits. For instance, InSb is lattice-
matched with CdTe and could be used as a substrate. Good-quality
deposits of CdSe have been formed on InP and GaAs substrates
using codeposition [52,53]. This clearly shows the applicability
of high-quality commercial compound semiconductors wafers as
substrates for compounds electrodeposition.
2+
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