Integrated Optics - Applied Optics Fundamentals and Device Applications - page 239

with lower carrier mobility and increased optical scattering. Also, the sur-
face polishing normally provided by the suppliers of GaAs wafers or elec-
tronic and microwave integrated circuits may not be smooth enough for
the fabrication of optical components with low scattering loss. Additionally
chemo-mechanical polishing to remove surface damage generally found in
“polished” wafers may be necessary.
5.6.3 Epitaxial Growth
One must determine whether it is better to use oxide or nitride masking
techniques to define the lateral dimensions of devices during epitaxial
growth, or to grow a layer over the entire wafer and then mask and etch
away unwanted portions of the layer. The desired percentages of constituent
elements and dopants in each layer must be determined to control index of
refraction, optical absorption, and scattering for optical devices, along with
regard for carrier concentration, mobility, and lifetime for electronic and
microwave devices. Fortunately, many of the requirements are the same for
all of these devices. For example, low defect densities in the epi-layer which
are necessary for minimal optical scattering, are also required for high car-
rier mobility and scattering-limited velocity in microwave and electronic
devices. Accurate control of dopant densities, which is needed to produce
low background carrier concentration in order to minimize free-carrier
absorption, also results in reduced microwave loss and permits the fabrica-
tions of microwave IMPATT and Gunn diodes.
5.6.4 Metallization
The metallization material and deposition technique chosen should be com-
patible with microwave, electronic, and optical devices. Most of the metalli-
zation requirements are the same for all these devices, that is, high electrical
conductivity, good adherence to GaAs and related III-V ternary and quater-
nary materials, no adverse chemical reactions, accurate pattern definition,
and ease of deposition. In some optical devices it may be desirable to utilize a
semitransparent metal film. Thus the chosen metal should be one that can be
deposited in very thin layers (∼1000 Å) for transparency as well as in layers of
several microns thickness to produce high conductance. The most likely met-
allization system to satisfy these requirements on GaAs is the widely-used
chrome-gold technique (a very thin layer of Cr to provide reliable adherence,
capped with a thicker layer of Au for high conductance). Other potentially
useful metallizations such as aluminum may also be considered.
5.6.5 Thin-Film Insulators
Thin-film insulators may be required at various points to provide electri-
cal insulation for metallic crossovers. The material chosen should have