Integrated Optics - Applied Optics Fundamentals and Device Applications - page 238

any undesired stray interactions between devices. For example, an IMPATT
oscillator should not be located directly next to a laser diode because they are
both high power heat generating devices that represent a “hot spot” on the
wafer. Devices should also be separated far enough to prevent stray coupling
through electro-magnetic fields. Spacings greater than 1 μm should be suf-
ficient because both the optical and microwave devices are fabricated in thin
layers (∼10 μm), so fringing fields do not extend far beyond the device.
MMIC and OIC fabrication technologies are compatible to the extent that
fabrication of an OMMIC on a GaAs substrate does not require any extensive
modification of the processes already in place. Several examples of OMMICs
have already been demonstrated [28]. Furthermore, the heterojunction wave-
guide structure is basically the same as that for a double heterojunction laser,
that is, the device with a reverse bias operates as a modulator but, with a
forward bias, it operates as a laser. Monolithically integrated FETs have been
used to modulate AlGaAs laser diodes on the same substrate at GHz fre-
quencies [29], and a monolithic integrated circuit consisting of microwave
FETs, an AlGaAs laser, and a photodiode has been used to produce a feed-
back-stabilized laser light source [30].
5.6 ProcessingandCompatibilityConstraints
5.6.1 Introduction
The fabrication process should be designed so that it is compatible with
electronic and microwave device requirements as well as those for opti-
cal devices. The various steps of the fabrication process must be compat-
ible in terms of temperatures required, chemical interactions, and structural
strength considerations.
5.6.2 Substrate Specifications
The first step in the process design procedure should be the determination
of specific requirements for the substrate material. It is clear that Cr-doped
semi-insulating gallium arsenide will provide electrical isolation between
devices and will make possible microwave stripline interconnections for
compatibility with the eventual incorporation of monolithic microwave
devices. Additional constraints on the substrate material will have to be
determined. For example, ideally, the substrate material should contain just
enough chromium atoms to compensate the residual background dopant
concentrations (usually about 10 15 cm −3 Si atoms in the highest purity GaAs,
which acts as donors). Excess Cr atoms can diffuse into the epitaxial layer
during the growth producing a nonuniformly doped compensated layer