Environmental Engineering Reference
In-Depth Information
of water. The use of non-aqueous solvents, such as ethyleneglycol
and dimethylsulfoxide (DMSO), expands this temperature range to
20-170
◦
C. Because of the low growth temperature involved, the
thin-film materials obtained are usually amorphous, nano- or mi-
crocrystalline in nature. Crystalline materials are also possible only
in special cases like ZnSe [15]. For electronic device applications
such as photovoltaics, the electrodeposited layers must undergo a
post-deposited annealing step in order to improve the crystallinity,
electronicandopticalproperties.Thiscanbeseenasadisadvantage,
butinsemiconductorprocessingterms,heattreatmentat
∼
400
◦
Cis
still a comparatively low-temperature process.
Impurities in semiconductors play an important role in the
development of electronic materials and devices. Therefore, the
impurity levels should be controlled at parts per million (ppm)
levelsinmanycasesandpartsperbillion(ppb)levelsinsomecases.
Theimpuritysourcesofelectrodepositiontechniqueincludevessels,
solvents, electrodes, and the chemicals used in the electrolytic cell.
As discussed in earlier sections, leaching of sodium and silicon
from glass ware in acidic media is a disadvantage but can be easily
avoided by using Teflonware to contain the electrolyte. In the case
of aqueous solutions, water can be purified adequately through de-
ionisation and distillation processes. Non-aqueous solvents can be
purified using electrodeposition itself prior to the preparation of
solutions. Electrodes used need to be made out of inert elements
such as platinum or high-purity graphite. The use of reference
electrodessuchasstandardcalomelorAg/AgCl,maybeproblematic
since these are sources of ions such as Hg
+
and Ag
+
. These ions
can be beneficial for certain materials but could poison the bath
for some material systems. For example, Hg
+
ions are beneficial
in improving cell performance [21] but Ag
+
ions have completely
detrimental effects by reducing cell e
ciencies for solar cells based
on electrodeposited CdTe [22]. For this reason, electrodeposition
has been tested successfully using the two-electrode system by
avoiding the reference electrode [16, 17].
Another concern in electrodeposition is that the material layers
grow on the cathode in the presence of other ionic species, such as
OH
−
,SO
−
4
,Cl
−
or NO
3
. These ions could also be incorporated in
thin films depending on the thermodynamic and kinetic conditions,
