integration technology
rhombus4
Tube lasers prepare to light
up silicon circuits
Differences in polarity, lattice constant and thermal expansion hamper the unification of compound
semiconductor light emitting structures and silicon ICs. But Zetian’s Mi’s team from McGill
University can avoid all these issues by turning to a novel micro-tube laser architecture that
suspends the device just above the wafer surface. Richard Stevenson reports.
S
ilicon has a major weakness. It’s a lousy light leakage current path that causes local heating. These
emitter, and this means that it cannot be used to problems are compounded by defects that stem from
build monolithic ICs incorporating lasers or LEDs. So to strain in the epilayers caused by lattice mismatch.
address this Achilles heel, many researchers are trying to Some researchers, including those from Intel, sidestep
find ways to unify compound semiconductor light sources these problems by growing lasers on a native platform,
with silicon. If a cost-effective technology for high-volume and then bonding their devices to silicon. But this
manufacturing could be found, then many would welcome approach cannot eliminate stresses that are caused by
it. The electrical interconnects that are currently being thermal expansion coefficient differences. What’s more,
used for chip-to-chip data transfer have very little this type of laser cannot be scaled to the sub-micron sizes Micro-tube
headroom left, and switching to an optical approach needed for realizing ultra-low power consumption laser fabrication
would alleviate a looming bottleneck. In addition, silicon alongside modulation speeds of hundreds of GHz. begins with the
ICs with opto-electronic functionality could spur MBE growth of
developments in biological sensing applications. Mi’s approach, which promises to deliver on all these an epistructure
fronts, begins by taking a GaAs substrate and depositing that features
Efforts at developing III-V-on-silicon light sources go back a strained structure with an active region onto it. The two quantum
a long way and progress has been hampered by several epitaxial layers are patterned into a U-shape, before a dot layers
major differences between the two types of material. In sacrificial layer is removed. The strain in the remaining embedded in
fact, it might actually make more sense to keep them structure causes it to roll into a tube, which can then a GaAs film
slightly apart, rather than putting these unhappy be transferred to silicon with the aid of a solvent (see Credit: Owen
bedfellows together. Figs. 1 and 2). Egan
One researcher holding precisely that view is Zetian Mi
from McGill University, Montreal. He is leading a team
that’s developing a free-standing tube laser that sits a few
hundred nanometers above the silicon platform. This
GaAs-based novel architecture promises to provide the
circuit with a light source operating at incredibly fast
modulation speeds, while consuming very little power.
Mi wants to keep the III-Vs and silicon apart due to their
incompatibility – these materials have significant
differences in polarity, lattice constant and thermal
coefficients of expansion. “When you grow GaAs on
silicon, in some regions the gallium atoms attach to silicon
first, and in other regions arsenic attaches to silicon first.
So as you grow more and more layers there will be a
boundary, called the anti-phase domain boundary.” This
interface, which stems from differences in polarity, creates
a high density of dislocations in the material, leading to a
November / December 2009 www.compoundsemiconductor.net 17
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