Biomedical Engineering Reference
In-Depth Information
advantage of this technique lies in the precise external manner by which
doping of materials can be controlled. Any known species can be introduced
into a given material, and the dopant concentration is not limited by ordi-
nary solubility considerations. (However, the concentration of dopant atoms
occupying substitutional lattice sites is generally limited by the solid solu-
bility.) The dosage can be monitored accurately and the depth profile of the
implanted ions can be controlled by adjusting the implant energy. This per-
mits realization of dopant profiles that could not be achieved by diffusion or
any other technique. Also, implantation of materials can be performed over
a wide range of temperatures, allowing unique interactions of the implanted
ions. Precise device configuration is achievable due to the parallel develop-
ment of high resolution GaAs lithography and masking, and through proper
control and selection of the ion beam energy and fluence and the substrate
doping level.
Ion implantation can produce a change in the refractive index of a crys-
talline semiconductor material through various physical mechanisms [81].
These mechanisms include damage to the crystal lattice, ultimately resulting
in an amorphous region; introduction of dopant atoms into the lattice, caus-
ing a change in the polarizability of the unit cell; localized regions of stress
in the lattice due to damage and the presence of a large number of dopant
atoms in these regions; and the compensation of the free carriers in suitably
doped materials. This last mechanism has been the primary method used to
form optical waveguides in semiconductor materials through ion implanta-
tion. Of course, in any actual implementation the mechanisms are interre-
lated, and the change in the index of refraction of the implanted materials is
due to the total effect of these processes. It is possible, however, to accentuate
one of the mechanisms and to minimize the effects of the others. The most
common ion species utilized for GaAs optical device fabrication is the pro-
ton (H+). These ions produce the least amount of damage to the crystal and
minimize the contributions to the optical properties arising from the physi-
cal processes other than free-carrier compensation. Protons also are unlikely
to occupy substitutional sites in the lattice and have the greatest projected
range by direct energetic penetration without relying on any secondary or
defect diffusion.
5.10.2 Ion-Implanted Semiconductor Annealing
As discussed in Laser Annealing of Semiconductors [82], the abrupt recrystal-
ization process upon annealing ion-implanted silicon in the temperature
range 500°C-600°C is in contrast to the more complex multistage process in
the solid phase annealing of ion-implanted GaAs. Several publications [83]
illustrate that damage removal of ion-implanted damage in GaAs occurs over
a broad temperature range.
The nature of the solid phase annealing process was further revealed [84]
as shown in Figure 5.24. Measured GaAs disorder, obtained from Rutherford
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