Environmental Engineering Reference
In-Depth Information
The second step is to use the highest-purity chemicals available
for the preparation of the ionic solutions. It is usual practice to
use 5N or 6N (99.9999%) purity chemicals in all semiconductor
growth techniques today. Electrodeposition, however, has an added
advantage over other semiconductor growth techniques, which is
the self-purification capability. Once the electrolyte solution has
beenpreparedusingthehighest-puritychemicalsavailable,theself-
purification process can remove any background impurities from
the electrolyte. An application of slightly lower voltage than the
usual growth voltage over a suitable period prior to growth will
enablesubsequentgrowthofhigh-puritysemiconductingmaterials.
Thepurityofsemiconductinglayersimprovesasthemateriallayers
are plated from a given electrolyte due to the removal of impurities
from the solution.
Hydrogen passivation and sulphur passivation are well-known
phenomena in the growth of silicon and surface preparation of
GaAs. The electrodeposition from aqueous solutions has a built-in
hydrogen passivation mechanism since H
+
ions are also attracted
to the surface of the cathode and discharged together with other
semiconductor elements. Any dangling bonds formed during the
formation of the semiconductor are immediately passivated by the
most active hydrogen atoms produced after the discharge of H
+
ions. On various occasions, sulphur compounds are added to the
electrolyte and, therefore, the passivation could also take place due
to the incorporation of sulphur in these layers.
3.3.3
Extrinsic and Intrinsic Doping
Oncethebackgroundimpuritylevelhasbeenreducedtoaminimum,
it is an easy task to apply extrinsic doping. A calculated amount
of external dopant can be added to the electrolyte before the
growth of any semiconducting layer. This has been applied to
electrodeposition of ZnSe layers in recent research [15]. The
conduction type of the thin films grown was determined using
photoelectrochemical(PEC)cellmeasurements,asshowninsection
3.4.3.
InthecasesofII-VIandI-III-VI
2
materials(CuInSe
2
,forexample),
intrinsicdopingispossiblebythevariationofitscomposition.Inthe
