Integrated Optics - Applied Optics Fundamentals and Device Applications - page 237

signal paths. It is important that the waveguide building blocks for an opti-
cal circuit be designed within a system framework that is compatible with
the eventual monolithic integration of laser sources, photodetectors, micro-
wave devices, and electronic elements.
The approach using epitaxial growth of AlGaAs optical and microwave
devices on a semi-insulating Cr-doped GaAs substrate offers excellent com-
patibility with the requirements of both optical and microwave integrated
circuits. The low-loss transmission of microwave signals via metallic strip-
line waveguides fabricated on Cr-doped GaAs substrates has been well
documented and is widely used even in commercially available monolithic
microwave integrated circuits (MMICs). The Cr-doped substrate has a resis-
tivity of approximately 10 7 or 10 8 Ω cm −1 and behaves essentially as a high
field strength, low-loss dielectric. Microwave losses and leakages are thus
minimized. The use of such a substrate in an optical integrated circuit com-
plicates the design somewhat. The semiconductor substrate cannot be used
as a return ground path for the electrical current through lasers, photodi-
odes, and modulators. This problem can be solved rather easily, however,
by incorporating a buried n + layer and by designing optical device struc-
tures that have all their electrical terminals on the top surface of the wafer.
Efficient lasers, modulators/switches, and detectors of this type have been
demonstrated [27]. Standard photolithographic metallization techniques can
then be used to interconnect devices and power sources as in a conventional
electronic integrated circuit. Metallization lines can be deposited directly
onto the semi-insulating Cr-doped substrate without any intervening oxide
layer because of the high substrate resistivity and dielectric strength. A
deposited oxide or nitride layer can be used where intersections (crossovers)
of metal lines occur, as long as the insulating layer is thick enough (typically
1.0 μm) to reduce capacitive coupling to an acceptable level.
Monolithic optical integrated circuits are commonly fabricated in AlGaAs
epitaxial grown layers on GaAs substrates. The energy bandgap and index
of refraction can be conveniently adjusted through control of compositional
atomic fractions to produce efficient light emitters and detectors as well as
low-loss waveguides, couplers, and switches. This technology is well devel-
oped, along with improved techniques of patterned epitaxial growth and
etching to define the circuit elements in development.
Monolithic microwave integrated circuits are fabricated on semi-insulating
GaAs substrate in epitaxially grown layers of GaAs to take advantage of the
high electron mobility and scattering limited velocity in these materials.
Microwave generators such as IMPATT and Gunn diodes can be fabricated
as well as FETs for amplification and switching. Passive elements such as
capacitors and inductors for filtering and impedance matching can be fab-
ricated by metal film deposition directly onto the semi-insulating substrate.
The circuit elements are defined by conventional photolithographic tech-
niques. This MMIC technology is all basically compatible with that used
for OIC fabrication. Care must be taken in the design of OMMICSs to avoid