integration technology
rhombus4
Tube pioneers
Mi did not invent micro-tube technology – this accolade
goes to Victor Prinz from the Institute of Semiconductor
Physics (Siberian Branch) at the Russian Academy of
Sciences. He developed the technology for producing a
micro-tube structure in the late 1990s and reported his
results in 2000.
Since then trailblazing has transferred to Germany. Efforts
by Oliver Schmidt and co-workers at the Max Planck
Institute for Solid State Research in Stuttgart, along with
Tobias Kipp’s team at the University of Hamburg have led
to an improved understanding of the formation of these
structures, and observations of the optical emission from
GaAs-based devices at very low temperatures and
SiO2/silicon heterostructures at room temperature. Mi’s
two recent, major contributions to this field are the
demonstration of coherent emission - and ultimately lasing
- from III-V micro-tubes; and the development of a process
to transfer these structures onto a silicon substrate.
The McGill academic employs MBE for the growth of his
epiwafers, which comprise a 50 nm-thick AlAs sacrificial
layer, a 20 nm thick In0.18Ga0.82As layer, and a 30 nm thick
GaAs cap that incorporates two In0.5Ga0.5 As quantum
dots layers. Some of these structures were grown in
Pallab Bhattacharya’s group at the University of Michigan,
but Mi has plans to move production in-house after his
team has conditioned a recently purchased MBE tool that
is dedicated to arsenide deposition.
Mi says that his active region contains quantum dots,
rather than quantum wells, because they offer superior
carrier confinement. “The resulting near-discrete density of
states promises both large gain and large differential gain
for laser operation, compared to quantum wells.” In
addition, the dots provide strong carrier localization that
greatly reduces non-radiative recombination associated
with surface defects.
A U-shaped GaAs-based structure is defined by
photolithography and subsequent etching into the InGaAs
layer. Self-rolling of these epilayers is then initiated by
selective etching of the AlAs sacrificial layer with
hydrofluoric acid. This eventually leads to the formation of
“fully released” quantum dot micro-tube structures on a
GaAs substrate. Figure 1: Fabrication of micro-tube lasers begins by taking a GaAs
substrate, and using MBE to deposit a strained arsenide-based
Other groups have already developed methods for the epistructure that features a quantum dot active region onto this platform. A
transfer of III-V devices onto alternative substrates, but Mi U-shaped structure is etched into this surface (a), before the AlAs
says that these processes are incompatible with his sacrificial layer is etched away. Strain drives the rolling-up of this structure
micro-tubes. “The reason that we don’t use dry printing is to form a free-standing micro-tube (b), which can be transferred to a
that our structures are hollow and fragile, and you cannot silicon surface in the presence of a solvent (c). The micro-tube binds to
press on the surface. Solution casting works very well, but the silicon surface due to the gravitational force induced by the solvent in
you can’t position these micro-tubes where you want to.” and around the tube (d)
November / December 2009 www.compoundsemiconductor.net 19
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